with 2 Scheimpfug cameras. J Cataract Refract Surg. 2018;44(4):471–8.

measured values of total corneal astigmatism for toric intraocular lens power calculation. J Refract Surg. 2017;33(12):794–800.

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# **The A-Scan Biometer 16**

### Maya C. Shammas and H. John Shammas

A-scan ultrasonography has been used for diagnostic purposes since the 1960s. Biometry was then limited to measuring eyes with deformities affecting the axial length (AL), i.e., congenital glaucoma, axial myopia, and phthisis bulbi. Around the mid-1970s, the use of intraocular lenses during cataract surgery gained in popularity and many intraocular lens (IOL) theoretical formulas were published to determine the IOL power. All these formulas required an AL measurement, and A-scan biometry was the only way to accomplish the task. The original units required manual measurement of the ultrasound travel time with a caliper from a Polaroid picture of the A-scan and converting it to millimeters. The measurement was then entered into a calculator to obtain the IOL power needed for emmetropia. Through the years, the ultrasound biometer evolved with the introduction of electronic gates, automatic calibration, and computerized capabilities. Most available A-scan biometers are now compact, effcient, computerized, and complete with IOL power calculation capabilities.

Routine use of A-scan ultrasound biometry has been largely replaced by the more accurate, precise, and reproducible optical biometry. However, the use of optical biometry is limited

M. C. Shammas

Shammas Eye Center, Lynwood, CA, USA

H. J. Shammas (\*) Ophthalmology, The Keck School of Medicine of USC, Los Angeles, CA, USA

when measuring eyes with a mature cataract or other vitreoretinal pathology. A-scan biometry is needed in these cases. A-scan biometry can be achieved by contact or immersion techniques. The contact technique applanates the probe against the cornea to obtain the measurements; errors can occur with axial length measurements from excessive indentation of the probe against the cornea. With the immersion technique, the probe is immersed in a gonioscopic solution or balanced salt solution (BSS) contained within a scleral shell; because it does not cause indentation of the cornea, the results are more reliable and therefore the preferred method whenever possible.

### **Basic Technology**

An ultrasound unit is composed of four basic elements: *the pulser*, *the receiver*, and *the display screen* all contained within the same chassis and connected to *the transducer*, located at the tip of the probe by an electrically shielded cable (Fig. 16.1a). The pulser produces electrical pulses at a rate of 1000 pulses/s. Each pulse will excite the electrodes of the piezo-electric crystal of the transducer, generating sound waves. The returning echoes are received by the transducer and transformed into electrical signals. These signals are processed in the receiver and demodulator and then displayed on the screen.

© The Author(s) 2024 289

J. Aramberri et al. (eds.), *Intraocular Lens Calculations*, Essentials in Ophthalmology, https://doi.org/10.1007/978-3-031-50666-6\_16

**Fig. 16.1** (**a**) An ultrasound unit is composed of four basic elements: the pulser, the receiver, and the display screen all contained within the same chassis and connected to the transducer, located at the tip of the probe by an electrically shielded cable. (**b**) The piezoelectric principle: changes in the polarity of an electric current passing through a quartz crystal will cause changes in the shape and size of the crystal, and vice versa. This in turn will transform the electrical energy into mechanical energy in the form of sound waves. When the sound waves return to the probe, the mechanical energy will modify the thickness of the crystal and produce electrical energy. (**c**) A crystal with a fat surface emits a non-focused beam, essential for biometry. (**d**) A crystal with a concave surface emits a focused beam essential for B-scan echography

This process is based on the piezo-electric principle (Fig. 16.1b); changes in the polarity of an electric current passing through a quartz crystal will cause changes in the shape and size of the crystal, and vice versa. This in turn will transform the electrical energy into mechanical energy in the form of sound waves. When the sound waves return to the probe, the mechanical energy will modify the thickness of the crystal and produce electrical energy. The performance of the crystal depends mainly on its shape and thickness. A flat surface emits a non-focused beam (Fig. 16.1c) essential for biometry; a concave surface emits a focused beam essential for B-scan echography (Fig. 16.1d).

#### **Physical Principles of A-Scan Ultrasound**

*Ultrasound* refers to sound waves beyond the range of human hearing. In order to make this defnition more precise and to explain the properties of ultrasound, we must frst explain *sound*. Consider knocking on a door. When the knuckles of the hand strike the door's surface, the molecules of which the door is made are temporarily forced closer together. The compression or "mechanical disturbance" has thus *moved* or "propagated" deeper into the "medium" which is the material of the door. Disturbances which propagate in this way are generally called *waves*; hence, we speak of "sound waves."

A *point source* of sound creates a spherical wavefront, with sound propagating in all directions away from the source. A piston-like sound source creates a quasi-planar wavefront, with sound propagating mostly in a single direction, as shown in Fig. 16.2.

When the piston is moving repetitively back and forth at a constant *frequency,* we can plot a graph of the piston's position against time and we would obtain a curve as shown in Fig. 16.3. Such a curve is called *periodic* because it repeats itself continuously. The smallest repeated portion is called a *cycle*, and the length of time required for one cycle is called the *period*. The study of periodic sound provides the theoretical basis of ophthalmic ultrasound. Frequency is measured in cycles per second, also called *Hertz* (after Heinrich Hertz, a German physicist who studied wave phenomena at the end of the nineteenth century), which is abbreviated Hz. One *kilohertz*, abbreviated kHz, equals 1000 cycles/s. One *megahertz*, abbreviated MHz, equals 1,000,000 cycles/s. The healthy human ear can detect sound frequencies in a range of about 20 Hz to as much as 20 kHz. *Ultrasound* is sound at frequencies well above the 20 kHz.

Real-world ultrasound equipment generates sound pulses whose energy is confned to a lim-

**Fig. 16.2** Quasi-planar wavefronts created by a broad, fat sound source (piston)

ited range or *band* of frequencies. The size of the range is called the *bandwidth*. The midpoint of the range is called the *center frequency*. Both are typically measured in MHz. A typical A-scan biometry system, for example, might have a center frequency of 10 MHz and a bandwidth of 4–6 MHz.

The amount of distance corresponding to one cycle (Fig. 16.4) is called the *wavelength*, and it depends on both the *frequency* of the sound and the speed or *velocity* at which it propagates through the medium, according to the formula:

$$\mathcal{A} = \nu / f$$

where "*λ*" represents wavelength, "*v*" represents velocity, and "*f*" represents frequency. Most ocular ultrasound images work at frequencies of 8–10 MHz. The average velocity of sound in human tissue is about 1550 m/s. 10 MHz sound in human tissue has a wavelength of 155 μm (millionths of a meter).

#### **Major Components of a Biometer**

Four different components are herein discussed including the probe and its transducer, the sensitivity setting, the velocity setting, and data analysis with IOL calculation.

#### **The Probe and Its Transducer**

The probe is connected to the main chassis of the biometry by an electronically shielded cable and contains a transducer at its tip [1–3]: The original solid probe (Fig. 16.5) has been designed for standardized A-scan echography using the Kretztechnik 7200 MA ultrasound unit. This probe can also be used to measure the axial length through an immersion technique. The newer probe (Fig. 16.6) is thinner and designed specifcally for biometry.

The transducer emits the ultrasound beam. Ultrasound consists of high-frequency sound waves over 20,000 cycles/s, which is the highest frequency audible to the human ear [4]. The ultrasound beam is formed of ultrasound waves that display different characteristics depending on the ultrasound frequency, wavelength, velocity, and direction.

The *frequency* [5] is the number of hertz (Hz) or cycles per second. Higher frequencies provide

**Fig. 16.5** The Kretztechnik 7200 solid probe is used for diagnostic standardized A-scanning and for biometry

**Fig. 16.6** Smaller solid probe designed for biometry

a higher resolution while lower frequencies provide better penetration but a reduction in the resolution. To obtain the high resolution needed for axial length measurement, biometry units use ultrasound frequencies ranging between 8 and 25 MHz (1 MHz = 1 megahertz = 1 million cycles/s).

The *wavelength* [6] is the distance between two particles in the same phase of oscillation. Within the ocular tissues, the wavelength is approximately 0.19 mm if an 8 MHz probe is used and 0.15 mm if a 10 MHz probe is used.

The *velocity* [7] is the speed of sound propagation and is expressed in meters per second (m/s). The velocity varies according to the medium through which sound propagates; within the eye, the ultrasound velocity is 1532 m/s in aqueous and vitreous, 1640 m/s in a clear lens, and 1550 m/s in solid tissues. During an accurate measurement of the different eye components, the proper sound speed must be used for each of these entities.

The *direction* of the ultrasound beam [8, 9] affects the display of the tissues under examination. During biometry, the emitted sound beam will meet multiple interfaces. At each interface, part of the sound beam is refected toward the probe and the remainder of the sound beam keeps propagating deeper into the tissues. This process will generate echo spikes from the different interfaces that have been intersected, i.e., anterior surface of the cornea, posterior surface of the cornea, anterior surface of the lens, posterior surface of the lens, anterior surface of the retina, and anterior surface of the sclera. When the ultrasound beam reaches the orbital tissues, it is attenuated until it loses all its energy. The sound beam returns to the transducer that also acts as a receiver. The pulses are then processed within the biometer to display "echo-signals" on the screen.

#### **The Sensitivity Setting**

The sensitivity setting controls the height of the echo spikes displayed on the screen.

The axial length is more accurately measured at a lower system sensitivity that allows a better pattern recognition of the anterior and posterior corneal surfaces, anterior and posterior lens surfaces, and anterior retinal surface.

#### **The Velocity Setting**

The velocity setting controls the speed of sound propagation. The velocity, measured in meters per second (m/s), varies according to the medium through which sound propagates. Most units use an average velocity of 1548–1556 m/s in a cataractous eye and 1532 m/s in an aphakic eye. Newer units measure each ocular compartment at its correct velocity; the anterior chamber depth is measured with a velocity of 1532 m/s; the cataractous lens is measured with an average velocity of 1640 m/s; the vitreous cavity's depth is measured with a velocity of 1532 m/s like the aqueous. These measurements are then computed within the instrument to display one axial length reading.

#### **The Electronic Gates**

Electronic gates allow ultrasound units to provide an electronic read-out of the axial length in millimeters. The gates will measure the travel time between the leading edges of the spikes (Fig. 16.7) or the peaks of the spikes (Fig. 16.8). Biometers are equipped with 2, 4, or 5 gates. The two main gates are the "corneal gate" placed in

**Fig. 16.7** The four gates of this horizontal caliper lights (arrows) measure the distances between the leading edges of the anterior corneal surface, anterior lens surface, posterior lens surface, and the retina

**Fig. 16.8** Five vertical gates (dotted lines) measure the distances between the peaks of the anterior cornea (*C*1), the posterior cornea (*C*2), anterior lens surface (*L*1), posterior lens surface (*L*2), and retina (*R*)

the region of the anterior corneal spike and the "retinal gate" placed in the region of the retinal spike. Such instruments measure the travel time between the anterior surface of the cornea and the anterior surface of the retina and use an average sound velocity for the measurement of the axial length.

Instruments equipped with four gates (Fig. 16.7) allow the positioning of these gates over the leading edges or the peaks of the echoes generated from the anterior surface of the cornea, the anterior surface of the lens, posterior surface of the cornea, the anterior surface of the lens, the posterior surface of the lens, and the anterior surface of the retina. A measurement of the anterior chamber depth, lens thickness, and the total axial length is displayed on the screen.

Instruments equipped with fve gates (Fig. 16.8) will additionally locate the posterior corneal surface and include a measurement of the corneal thickness.

#### **Data Analysis and IOL Calculation**

Most biometers will analyze the measurements and will display the axial length, anterior chamber depth, and lens thickness. IOL power calculations are provided using available modern formulas. Many will be able to provide a print out or connect directly to an imaging system or Electronic Medical Record. These programs are also able to store information, compare results, review data, and refne the ELP constants.

#### **Choosing the Appropriate Ultrasound Biometer**

Most biometers provide reproducible and accurate measurements and are programmed with popular formulas. However, each biometer is characterized by specifc features and components that have been discussed in length in this chapter. Here are some essential features to look for in a biometer:

Some biometers are available with diagnostic B-scan, ultrasound biomicroscopy (UBM), and/ or pachymetry within the same chassis. Practices that handle vitreoretinal pathology and/or perform corneal surgery will enjoy the added features in one compact unit.

#### **Commonly Used Ultrasound Biometers**

There are many excellent ultrasound machines available. Here is a list of some of the currently available machines. All the companies offer models that complete axial length measurements utilizing contact or immersion technique, measure anterior chamber depth and lens thickness, and have formulas pre-programmed in their system (Table 16.1).

#### **Table 16.1** Commonly used ultrasound biometers

#### **References**

Pro

1. Buschman W. Special transducer probes for diagnostic ultrasonography of the eyeball. In: Goldberg R, Sarin L, editors. Ultrasonics in ophthalmology. Philadelphia: W.B. Saunders Co.; 1967. p. 87–101.

A2000

2. Coleman DJ, Carlin B. Transducer alignment and electronic measurement of visual axis dimensions in the human eye using time amplitude ultrasound. In: Oksala A, Gernet H, editors. Ultrasonics in ophthalmology. Basel: S. Karger; 1967. p. 207–13.

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# **ZEISS IOLMaster 700 17**

Oliver Klaproth

#### **Introduction**

The ZEISS IOLMaster 700 with SWEPT Source Biometry® is the latest optical biometer in the more than 20-year long history of biometry innovation from ZEISS (Fig. 17.1). It combines all measurements required for modern IOL power calculation formulas at very precise levels [1]. It combines those with innovative technologies to support the improvement of refractive outcomes such as Total Keratometry (TK®), which enables the use of posterior corneal curvature measurements in IOL power calculation [2–5], the cornea-to-retina scan, the unique fxation check [6], and central topography. The ZEISS IOLMaster 700 is an integral part of the ZEISS Cataract Workfow, which enables remote IOL power calculation and surgical planning, IOL ordering, and more, in combination ZEISS EQ Workplace® for ZEISS FORUM® and ZEISS Veracity® Surgical software. It also enables markerless toric IOL alignment in combination with ZEISS CALLISTO eye® for surgical microscopes OPMI LUMERA® or ARTEVO® 800. It adheres to applicable DICOM standards and can therefore be networked with EMR and PACS systems. Overall, the ZEISS IOLMaster 700 builds on more than 20 years of experience in optical biometry, delivers precise and reliable measurements, and helps optimizing clinical cataract workfows [2, 3, 7]. It is designed to increase patient throughput and for getting fewer refractive surprises.

O. Klaproth (\*) ZEISS Ophthalmology, Berlin, Germany

e-mail<EMAIL_ADDRESS>
© The Author(s) 2024 297 J. Aramberri et al. (eds.), *Intraocular Lens Calculations*, Essentials in Ophthalmology, https://doi.org/10.1007/978-3-031-50666-6\_17

#### **SWEPT Source Biometry®**

#### **OCT Technology**

The ZEISS IOLMaster 700 is a SWEPT Source OCT Biometer that operates at a wavelength from 1035 to 1080 nm with a scanning rate of 2 kHz and 44 mm scan depth, to generate a 2D OCT cornea-to-retina cross-section scan of the eye in six meridians (0°, 30°, 60°, 90°, 120°, and 150°). Each meridional scan is averaged from three single scans. This technology is used to derive all axial biometry measurements, including **axial lengths**, **central corneal thickness**, **anterior chamber depths** from epithelium and endothelium, and **lens thickness**, at very precise levels (Table 17.1) [4, 5, 9–17].

The seamless cornea-to-retina cross-section scan furthermore aids surgeons in detecting unusual eye geometries. In contrast to A-Scan biometers, it furthermore enables to verify the correctness of measurements by displaying the anatomical structures and overlaying the segmentation used for axial measurements (Fig. 17.2).

The ZEISS IOLMaster 700 is designed to optimize workfow effciency, even when handling dense cataracts.

The combination of SWEPT Source OCT 6-meridian scan pattern and the use of approx. 1055 nm wavelengths (rather than, for example, 1300 nm, which has a higher energy absorption rate in the vitreous) enables a cataract penetration rate of up to 99% [18].

#### **Unique Fixation Check**

The ZEISS IOLMaster 700 also utilizes the SWEPT Source OCT to take a horizontal cross section image of the central 1 mm of the macula, which is averaged from 4 scans (Fig. 17.2). This enables users to verify whether the axial lengths and keratometry measurement were taken during correct fxation, to reduce the risk of refractive surprises due to incorrect measurements caused by undetected poor fxation.

**Fig. 17.1** ZEISS IOLMaster 700

**Table 17.1** ZEISS IOLMaster 700 technical data

**Fig. 17.2** ZEISS IOLMaster analysis screen, showing measurement values, SS-OCT images, central topography maps, telecentric 3-kone keratometry spots, and CD/reference images, and plausibility checks

#### **Distance Independent Telecentric 3-Zone Keratometry**

**Telecentric Keratometry** utilizes projection of keratometry points on the cornea via a parallel ray system, rather than refecting divergent light directly from LEDs, thus simulating an infnite light source incident on the precorneal tear flm. The light spots are then diffusely refected by the tear flm. Due to an aperture stop placed in the focus of the objective lens in the observation beam path, only the rays parallel to the optical axis are detected by the sensor. A slight change of distance between the cornea and the objective thus has no effect on the measurement.

Keratometry values are generated in 3 zones out of 18 spots in an area of about 3.2 mm for an 8 mm radius cornea. The system is designed to sample local irregularities in the central zone of the cornea into the fnal keratometry, which would not be possible with a ring measurement only. This 3-zone approach also allows for measurement of the central topography (see below). The keratometry measurement is repeated 15 times, with an algorithm detecting and erasing outliers and providing warnings to users in case of inconsistencies between single measurements or the different measurement rings. This telecentric approach to keratometry has proven to be precise and repeatable in many studies. [4, 5, 9–17]

The **keratometric index** on the IOLMaster 700 can be chosen by the user to match individual requirements. The choice of keratometric index has no effect on IOL power calculation though, as for the internal calculation algorithms, only directly measured radii are used, which are unaffected by the keratometric index.

#### **Additional Measurements**

#### **Angle Alpha- and Kappa-Chords**

The ZEISS IOLMaster 700 measures the Subject Fixated—Coaxially Sighted Corneal Light Reflex (SF-CSCLR) as described by Chang and Waring [19], or corneal vertex, which gives an approximation of the position of the corneal intercept of the visual axis. It then displays the **chord of angle alpha** and the **chord of angle kappa**, as the angles (in degrees) cannot be measured directly. The chords are defined as the distance between the corneal vertex and the limbus center (alpha)/ the pupil center (kappa). They are displayed either in cartesian (*Ix*/*Iy* and *Px*, *Py*, for alpha and kappa, respectively) or polar coordinates (alpha (chord) and kappa (chord)/CW-Chord, respectively).

#### **Corneal and Pupil Diameter**

The horizontal corneal diameter and pupil diameter are measured from the image of the eye taken at 860–880 nm.

#### **Total Keratometry: Replacing Assumptions with Measurements**

#### **Principle of Total Keratometry**

A known limitation of classic keratometry is that the posterior surface of the cornea is not measured but considered via a keratometric index only. However, several studies have confrmed that posterior corneal astigmatism magnitude and axis orientation cannot be adequately predicted by measuring the anterior corneal curvature alone [20–22]. Therefore, nomograms and mathematical models have been created in order to predict the posterior surface astigmatism and optimize toric IOL power calculation [23–25]. Yet, these methods are based on theoretical assumptions of posterior corneal astigmatism and, therefore, generally cannot fully account for outliers and irregularities.

This led ZEISS to the development of technology able to measure, not estimate, the posterior curvature: **Total Keratometry (TK®)** [22]. TK® considers measured corneal thickness and measured posterior corneal curvature in addition to the anterior corneal curvature measurements. It combines telecentric 3-Zone Keratometry of the ZEISS IOLMaster 700 with its SWEPT Source OCT cornea-to-retina scan [4, 11, 26].

TK® has been designed to match the Gullstrand ratio in normal eyes. However, it still can detect the impact of posterior astigmatism in individual eyes, such as eyes with post corneal laser vision correction. An additional signifcant advantage of this approach is that TK® can be directly incorporated in classic IOL power calculation formulas, while using existing optimized IOL constants, such as ULIB and IOLCon [27, 28].

#### **Precision of Total Keratometry Measurements**

Goggin and LaHood frst published data confrming that the ZEISS IOLMaster 700 is capable of measuring the posterior corneal surface [22]. Savini et al. [29] assessed the repeatability of TK® and standard keratometry measurements provided by ZEISS IOLMaster 700. They conclude that TK® measurements offer high repeatability in unoperated and post-excimer laser surgery eyes.

#### **Clinical Results with Total Keratometry in Non-toric and Toric IOL Power Calculation**

While TK® values are equivalent to K values in normal eyes, they will differ in eyes with an unusual ratio of anterior-to-posterior corneal curvature or in patients with an unusual posterior astigmatism. In these cases, the classic posterior corneal astigmatism nomograms cannot detect these outliers, while TK® can. Therefore, one can expect that TK® and K will overall perform relatively similar in terms of mean refractive outcomes after cataract surgery in normal eyes. However, TK® will be able to help surgeons avoid outliers or refractive surprises in the unusual cases mentioned above. Published studies by Fabian and Wehner or Srivannaboon and Chirapapaisan confrm this behavior with respect to spherical equivalent and cylinder prediction errors [27, 28].

#### **Clinical Performance of Total Keratometry in Post-corneal Refractive Surgery Eyes**

Eyes after refractive corneal laser surgery are the most prominent example of unusual anterior and posterior corneal curvature ratio, as the anterior surface has been altered. In these eyes, TK® becomes very benefcial, as it does not rely on assumptions on the posterior surface but is a measurement of total corneal power taking actual posterior corneal curvatures into consideration.

Wang et al. have shown, for example, that TK*®* can be used in classic IOL power calculation formulas such as the Haigis formula, resulting in overall similar results like specifcally designed post-LVC formulas as the Barrett True K without taking historical refraction data into account [30].

Lawless et al. have shown in their publication that when using the Barrett True K TK® formula, which was specifcally designed for TK®, it outperformed any other non-history formula in postmyopic LASIK eyes evaluated in this study [31] (Fig. 17.3). They also confrm, that Haigis with TK provides similar results as Barrett True K with K and no history.

Yeo et al. analyzed in this open-access paper 64 eyes with previous myopic laser refractive surgery by comparing the prediction error on different formulas [32]. In their analysis, EVO with TK® followed by Barrett True-K TK® and Haigis with TK® achieved the highest percentages of patients with absolute prediction error within 0.50 and 1.00 D (Fig. 17.4).

**Fig. 17.3** Percentage of eyes within 0.25, 0.50, 0.75, and 1.00 D of absolute prediction error; no-history formulas followed by formulas using TK. (Taken from Lawless et al. 2020)

**Fig. 17.4** Percentage of eyes within 0.50, 0.75, and 1.00 D of absolute prediction error. (Taken from Yeo et al. 2020)

#### **Central Topography: Starting Your Workfow with More Insights**

Central topography combines keratometry data from the 3-Zone Telecentric Keratometry with data of the corneal thickness measurement of the SWEPT Source OCT to create a **total axial power map** from the anterior and posterior corneal surface. This provides surgeons with more information on the central corneal shape right from the start without changing workfow or taking more time (Fig. 17.2).

Wang et al. compared central topography maps to topographic maps from a Placido-dual-Scheimpfug Topographer. This study included 105 eyes with various corneal conditions such as regular/ irregular corneas, previous corneal refractive surgery, and keratoconus or pellucid marginal degeneration. In 68.6–89.5%, similar overall shape was observed which leads to the same decision for premium IOL selection in 75.2–97.1% of cases [33].

**Fig. 17.5** Example of IOLMaster 700 reference image with planned toric IOL, predicted refraction, formula information, and implantation axis information based on TK®

#### **Markerless Toric IOL Alignment**

The ZEISS IOLMaster 700 is able to acquire a redfree (520 nm) image of the limbal vessels and iris structures during the keratometry measurement.

Both reference image and keratometry data (Fig. 17.5) can be transferred to ZEISS CALLISTO® eye, e.g., together with your surgical planning from VERACITY® Surgical or EQ Workplace. During surgery, the image is then used for intraoperative matching with the live eye image. All data needed is displayed in the eyepiece or 3D screen of the surgical microscopes OPMI LUMERA® or ARTEVO® 800 from ZEISS. Preoperative corneal marking and additional measurements for Toric IOL alignment thus become obsolete. Solomon and Ladas conclude that within their study, the use of ZEISS CALLISTO® eye yields less remaining refractive cylinder than Toric IOL placement guided by intraoperative aberrometry [3].

#### **IOL Power Calculation**

The ZEISS IOLMaster 700 offers a broad range of formula to calculate IOLs for a wide range of eyes, from long to short eyes, including toric IOL power calculation (Fig. 17.6) and post-corneal refractive surgery IOL power calculation, including post-RK cases (Table 17.2).

The ZEISS IOLMaster 700 supports import of IOL power steps and ranges as well as constants from the IOLCon.org website. Download of IOL data directly from IOLCon.org is recommended to always have access to the latest and best optimized IOL data directly from the respective IOL manufacturer.

**Fig. 17.6** ZEISS IOLMaster 700 toric IOL power calculation report

**Table 17.2** IOL power calculation formulas available on ZEISS IOLMaster 700, ZEISS EQ Workplace, and ZEISS Veracity Surgical (June 2021)

#### **ZEISS Cataract Workfow**

ZEISS offers a selection of software solutions to complement the ZEISS IOLMaster 700 and help cataract surgeons to streamline their cataract workfow. From a wider selection of formulas, remote IOL power calculation, remote surgery planning, constant optimization, IOL ordering to postop quality assessment. Those will be addressed in the following chapter: ZEISS Cataract Workfow.

#### **References**

in cataract surgery. Graefe's archive for clinical and experimental ophthalmology. Albrecht Von Graefes Arch Klin Exp Ophthalmol. 2019;257:2677–82.

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

## **Digital Solutions in the Cataract Workfow from ZEISS 18**

Melanie Schüle, Kyle Smith, Stephan Dreyer, and Jeremiah Elliott

#### **Introduction**

Cataract surgeons in today's competitive business environment demand solutions that enhance effciency, cost-effectiveness, and clinical outcomes. Patients, at the same time, are demanding more from their surgeons. They want to maintain an active lifestyle into their later years, performing their everyday activities mostly without visual aids. Cataract surgeons must be thoughtful business people, striving for maximum effciency in the clinic while taking the time to manage the concerns and expectations of every individual patient. Most patients expect a great deal more from their cataract surgery than they did 20 years ago, in part because modern technology has brought cataract surgery outcomes close to what corneal refractive surgery provides. With modern IOL calculation formulas, more sophisticated and accurate diagnostic devices, many highquality toric and presbyopia-correcting IOLs on the market, and constantly improving surgical techniques, patients can expect excellent results from their surgery. This advanced technology creates more challenges for the busy surgeon, especially when patients are willing to pay extra for premium results. Surgeons understand that

M. Schüle (\*) · K. Smith · S. Dreyer · J. Elliott ZEISS, Oberkochen, Baden-Württemberg, Germany e-mail<EMAIL_ADDRESS><EMAIL_ADDRESS><EMAIL_ADDRESS><EMAIL_ADDRESS>
outstanding surgical results are directly related to the quality of the planning done prior to the surgery itself. The voluminous data available for every patient and the rapidly expanding treatment options make surgical planning a time-consuming prospect if the surgeon must process all that information with traditional manual methods.

For these reasons, cataract surgeons are focusing more intently on achieving excellent outcomes through meticulous surgical planning and more precise alignment of toric lenses during surgery. Thankfully, ZEISS has created several powerful digital solutions to support cataract surgeons and their teams by integrating the entire process of cataract surgery—from the offce to the OR and back.

#### **ZEISS FORUM**

FORUM® from ZEISS is a scalable and fexible ophthalmic data management solution. It streamlines practice workfows by connecting involved devices and providing access to all patient examination data, allowing healthcare professionals to make confdent decisions at a glance.

#### **Streamline the Workfow**

ZEISS FORUM enables a fully electronic workfow that automatically sends patient demograph-

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J. Aramberri et al. (eds.), *Intraocular Lens Calculations*, Essentials in Ophthalmology, https://doi.org/10.1007/978-3-031-50666-6\_18

ics to diagnostic devices. It facilitates a completely paperless workfow that saves storage space and reduces the cost of printing.

#### **Decide with Confdence**

ZEISS FORUM facilitates clinical case visualization by preparing ophthalmic data and images in a way that lets healthcare professionals make decisions with confidence. ZEISS FORUM effectively supports patient care with unique features and workplaces designed specifically for glaucoma, retina, and cataract surgery.

#### **Simplify the Environment**

ZEISS FORUM integrates conveniently with the healthcare providers' existing IT infrastructure using established standards as well as customized tools. Its centralized storage of all examination data ensures patient record consistency across the network, including EMR, HIS, DICOM, and non-DICOM instruments—those created by ZEISS as well as instruments from other manufacturers. FORUM's multi-site solution ensures that patient data is consistently available and shared across multiple sites.

#### **ZEISS EQ Workplace**

The EQ Workplace® software from ZEISS is a cataract surgery planning software based on ZEISS FORUM allowing cataract surgeons to streamline and automate processes in the cataract workfow. The software gives users the ability to access and review ZEISS IOLMaster and other diagnostic data in one place, calculate, select, and order IOLs remote of the measurement device, and prepare surgery for CALLISTO eye® from ZEISS. It also generates personalized IOL constants for future improvement of outcome prediction.

#### **Preoperative Surgical Planning**

#### **Data Review**

ZEISS EQ Workplace enables automatic data transfer and auto-population of data, saving cataract surgeons valuable time preoperatively. Biometry and diagnostic data can be accessed and reviewed remotely in the workplace or from any FORUM workstation, supporting the complex decision of selecting the right IOL for the patient.

#### **IOL Calculation**

With all relevant data for IOL calculation already pre-populated, ZEISS EQ Workplace allows a quick and easy calculation and selection of the IOL for surgery. It uses state-of-the-art IOL power estimation formulas to determine the proper IOL power and automatically uses the right calculator depending on the eye status and within the surgeon's preferred formulas. Available formulas include the Barrett Suite and Haigis Suite, Holladay I and II, Z CALC, and more. Users can compare multiple IOLs and formulas, can change and add IOL models, select a different formula or power, and see the results update immediately.

#### **IOL Ordering**

ZEISS EQ Workplace can send an e-mail order of the planned IOL directly from the workplace to pre-defned recipients. The essential data related to the IOL planned in EQ Workplace automatically transfers to the IOL in order to avoid errorprone manual transcription.

#### **Surgery Preparation**

ZEISS EQ Workplace allows users to set all relevant surgical assistance parameters in the workplace. It then automatically transfers these parameters, the planned IOL, and reference image to ZEISS CALLISTO eye.

#### **Documentation**

ZEISS EQ Workplace produces a planning PDF that can be printed, saved, forwarded, or archived as DICOM in ZEISS FORUM. Surgical teams may reference this document to be sure everyone is familiar with the plan.

#### **Individual Constant Personalization**

ZEISS EQ Workplace uses post-op refractions entered into the post-up screen of the workplace to personalize the surgeon's constants for each IOL model. Surgeons can use their own personalized optimized constants, or optimized constants imported from IOLCon (https://iolcon.org).

#### **Intraoperative Assistance.**

#### **Surgery Preparation and Digital Toric Alignment**

The surgical plans created in EQ Workplace can be imported by ZEISS CALLISTO eye in the operating room. This means that the OR team will have the details of every patient's surgical plan available in the device used for toric alignment without additional manual data entry.

During surgery and to further protect against never-events, the IOL at hand can be verifed by comparing it with the IOL selected prior to surgery as displayed in ZEISS CALLISTO eye.

#### **ZEISS VERACITY Surgery Planner**

ZEISS VERACITY Surgery Planner is only available in the USA and Canada.

ZEISS created **VERACITY Surgery Planner** to address the challenge of time-consuming surgical planning for a variety of treatment options based on traditional manual methods. ZEISS VERACITY Surgery Planner represents an entirely new category of medical software, a tool that serves as the hub for all information related to ophthalmic surgery, a web-based digital assistant that brings the relevant data together in one location and automates much of the planning process, intraoperative documentation, and outcome analysis. The primary objective is to save time and prevent errors that could adversely affect outcomes.

#### **Preoperative Surgical Planning**

#### **Confgurability**

Even though surgical teams can begin using ZEISS VERACITY Surgery Planner with minimal initial setup, the tool is highly confgurable fexible—so each team can continue working without disruption of its normal workfow.

#### **Electronic Medical Record (EMR) Integration**

Surgeons and their team members spend a great deal of time entering patient information into their EMR systems as part of the normal clinic workfow. ZEISS VERACITY Surgery Planner exploits that effort by importing that data directly from the EMR system, so team members are not duplicating effort. VERACITY Surgery Planner can import data from most EMR systems in the USA. This automated data transfer typically includes patient demographics, refractions, visual acuities, problem lists, medications, allergies, and prior surgical procedures.

#### **Diagnostic Device Integration**

ZEISS VERACITY Surgery Planner directly interfaces with most of the diagnostic devices commonly used in planning cataract surgery: optical biometry, corneal topography, and OCT. This includes devices made by ZEISS (IOLMaster 700, ATLAS topography, CIRRUS OCT) and many non-ZEISS devices: Lenstar (Haag-Streit), OPD-scan III (Nidek), Pentacam (Oculus), Cassini (Cassini), iTrace (Tracey Technologies), and more. VERACITY Surgery Planner imports the discrete data generated by these devices as well as the images they produce.

#### **Patient Questionnaire**

ZEISS VERACITY Surgery Planner incorporates answers from a confgurable patient questionnaire that can be sent to patients in advance of their appointments via text message. This digital process automatically captures the patient's concerns, desires, and expectations for surgery. The text message can also include embedded patient education videos unique to each practice.

#### **Data Validation and Alerts**

ZEISS VERACITY Surgery Planner analyzes the imported data and alerts the surgical team to data inconsistencies, abnormal data, and other patient-related issues that could be of concern for the cataract surgeon.

#### **Suggestions**

ZEISS VERACITY Surgery Planner processes the available patient data acquired from the EMR, diagnostic devices, and the patient questionnaire and then generates a suggested treatment plan focused on the patient's desired outcome, and consistent with the surgeon's confgured preferences.

#### **IOL Calculations**

ZEISS VERACITY Surgery Planner uses some of the most well-respected IOL power estimation formulas and toric calculators to determine the proper IOL power. These include the ZEISS AI IOL Calculator, Barrett Suite, Kane, Holladay II, the Abulafa-Koch toric calculator, and more. The system automatically uses the Barrett True-K calculator for eyes with prior corneal refractive surgery. ZEISS VERACITY Surgical displays the net astigmatism and the predicted fnal refraction for every surgical plan. When users change the IOL model or power, they see the results update immediately. ZEISS VERACITY Surgical also calculates the proper powers for a series of back-up lenses.

#### **Automated Constant Optimization**

ZEISS VERACITY Surgery Planner uses postop refractions imported from the EMR system to optimize the constants automatically for each IOL model. Surgeons can use community-based optimized constants or their own personal optimized constants.

#### **Arcuate Incisions**

ZEISS VERACITY Surgery Planner also includes formulas for calculating the appropriate arcuate incisions for astigmatism reduction when a toric IOL is not possible.

#### **Surgical Plan**

The surgical plan ZEISS VERACITY Surgery Planner produces is much more than an IOL calculation. It includes documentation of the preferred surgical method (standard phaco vs. femtosecond laser), supplemental procedures (i.e., MIGS), supplemental techniques (i.e., ZEISS miLOOP, capsular tension ring), anesthesia method, and many other details.

#### **Surgery Scheduling**

ZEISS VERACITY Surgery Planner assists with scheduling procedures, including non-cataract eye surgeries, so the entire surgical team can plan and work effciently in the same digital environment.

#### **Documentation**

ZEISS VERACITY Surgery Planner generates a planning document for each case that can be sent to the ASC in advance of the surgery. Some surgical teams use it to be sure everyone is familiar with the plan.

#### **Intraoperative Assistance**

#### **Digital Toric Alignment**

The surgical plans created in ZEISS VERACITY Surgery Planner can be imported by ZEISS CALLISTO eye in the operating room. This means the OR team will have the details of every surgical plan available in the device used for toric alignment without additional manual data entry.

#### **OR Display**

ZEISS VERACITY Surgical generates a display of the surgical plan appropriate for use in the OR so every member of the surgical team is aware of the essential information for that case, including the proper IOL model and power, supplemental procedures or techniques planned, and any other patient-related concerns (Flomax, allergies, etc.).

#### **Automated Operative Note**

When the procedure has been completed, ZEISS VERACITY Surgical automatically generates a comprehensive operative note that includes the relevant details of the procedure. The system comes with preconfgured notes for virtually every type of cataract surgery and the most common supplemental procedures, but users can edit these templates as needed.

#### **Postoperative Data Analytics**

ZEISS VERACITY Surgery Planner imports post-op refractions from the EMR for use in its data analytics tool. Surgeons can view graphical displays of their surgical results in multiple formats depending on flter settings. Without any manual data entry in ZEISS VERACITY Surgery Planner, surgeons can see how often their patients' post-op refraction is within 0.5 D of target, or see which formula is providing the best results, or see how many premium lenses they are implanting each month.

#### **ZEISS CALLISTO eye**

Surgeons understand that toric IOLs are superior to arcuate incisions in the management of astigmatism. A correctly aligned toric IOL is the key to reduce postoperative astigmatism because a malrotation of only 3° will reduce the astigmatism correction by as much as 10% [1]. For these reasons, surgeons who desire excellent surgical outcomes require precise toric IOL alignment, which requires the appropriate adjustment for cyclotorsion. Historically, surgeons have turned to marker-based technologies, intraoperative aberrometry, and markerless computer-guided alignment to add precision to their toric alignment during surgery.

With ZEISS CALLISTO eye markerless alignment, manual marking steps can be skipped altogether for an effcient [2] and precise [3, 4] toric IOL alignment to reduce residual astigmatism. ZEISS CALLISTO eye imports surgical plans created in EQ Workplace or VERACITY Surgery Planner, or IOL selections made in ZEISS IOLMaster 700, and uses that information to generate intraoperative overlays for computer-guided cataract surgery. This effcient technology saves time for the OR team by eliminating the need for manual preoperative marking, manual data transfer, and manual intraoperative marking.

In a single-center study (*n* = 57 eyes), the mean overall surgical time for toric IOL patients treated with ZEISS CALLISTO markerless was signifcantly shorter than for patients using manual marking (727.2 ± 198.4 s versus 1110.0 ± 382.2 s; *P* < 0.001). The mean deviation from the target induced astigmatism was also signifcantly lower in the digital (CALLISTO eye) group (0.10 ± 0.08 D versus 0.22 ± 0.14 D; *P* = 0.008) [2]. The authors conclude that the "digital tracking approach for toric IOL alignment was effcient and safe to improve refractive outcomes [and] image-guided surgery helped streamline the workfow in refractive cataract surgery" [2]. A single center, randomized, contralateral, controlled study (*n* = 104 eyes) that compared the outcomes of markerless toric lens alignment (ZEISS CALLISTO eye) with intraoperative aberrometry (Alcon ORA system) demonstrated a residual astigmatism of 0.29 ± 0.22 D for CALLISTO eye group and 0.46 ± 0.25 D for ORA group (*P* = 0.00039). More than 25% of the patients in CALLISTO eye group had no postoperative astigmatism, whereas only four patients (8%) in ORA group showed no postoperative astigmatism. 92.2% of patients had <0.5 D in CALLISTO eye group and 76.5% in ORA group. 100% of patients were below 1.0 D in CALLISTO eye group and 96.1% in ORA group. The median absolute error in predicting cylindrical correction was reported to be similar for both groups: 0.35 D for CALLISTO eye group and 0.39 D for ORA group (*P* = 0.91). The authors concluded that "intraoperative markerless computer-assisted registration and biometric guidance summarily yielded less remaining refractive cylinder than toric IOL placement guided by intraoperative aberrometry" [5].

The assistance functions provided by ZEISS CALLISTO eye are surgeon-controlled via foot control panel or hand grips. CALLISTO eye's Z ALIGN® feature facilitates rotational alignment of toric IOLs and centration of IOLs on the visual axis (from data provided by IOLMaster 700). ZEISS CALLISTO eye's incision assistant helps to position incisions (optionally on the step axis and opposite clear cornea incisions when needed) and paracentesis. The device's rhexis assistant helps surgeons to size and shape the capsulorrhexis and center it on the visual axis provided by the ZEISS IOLMaster. And the LRI assistant guides surgeons in performing limbal relaxing incisions when a toric IOL is either insuffcient or inappropriate for astigmatism management.

ZEISS CALLISTO eye helps surgeons address increasingly lofty patient expectations by projecting valuable information directly into the surgeon's view through the microscope.

#### **Summary**

ZEISS supports cataract surgeons with two cataract planning solutions, EQ Workplace and VERACITY Surgery Planner, and the intraoperative surgical guidance system, ZEISS CALLISTO eye, all created to improve effciency and reduce errors while adapting to the surgeon's existing workfow.

ZEISS EQ Workplace supports cataract surgeons and their staff by streamlining the entire cataract surgery planning process. It saves valuable time during preoperative processes, reduces the risk of an IOL selection error, further protecting against never-events, and gives surgeons access to relevant data from anywhere in the clinic. By connecting ZEISS IOLMaster via FORUM to the ZEISS CALLISTO eye in the OR, EQ Workplace builds a secure data trail into the OR by remotely preparing all the surgical assistance functions for ZEISS CALLISTO eye. This digitization improves patient safety and effciency in the clinical workfow.

ZEISS VERACITY Surgery Planner is a tool conceived by surgeons and perfected over the years by gathering feedback from surgeons using the product in actual clinical settings. It was designed to save time, and it does. In a recently published, prospective study, ZEISS VERACITY Surgical signifcantly reduced surgical planning time when compared with traditional paper-based methods (*P* < 0.00001) [6]. Improved effciency is but one advantage. The automated data transfer and data validation ZEISS VERACITY Surgery Planner provides can help prevent errors that could result in poor patient outcomes. And the automated data analytics tool provides valuable insights that could lead to improvements in techniques and surgical decision-making.

#### **References**

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

**Biometry Measurements Using a New Large Coherence Length 19 Swept-Source Optical Coherence Tomography**

Clinical Experience with the Argos Biometer

H. John Shammas and Maya C. Shammas

Optical coherence tomography (OCT) presents several advantages over other techniques to evaluate biometry [1]. It is noninvasive and its high speed allows the collection of two- or threedimensional data in hundreds of milliseconds with high lateral resolution and axial resolution. Most of the previously proposed swept-source OCTs have a depth range that is defned by a coherence length ranging around 2 mm, far below the measurement range required for the axial length of the eye. The coherence length was improved by using a swept-source technology that implements quasiphase continuous tuning (QPCT) combined with multiple beam expanders at a swept rate of 2.5 kHz, which is about 5 to 10 times larger than what can be achieved in current systems. This swept-source OCT enables simple measurements of the axial length of the eye, where you need to only divide the obtained distance by the known refractive index. This technology has been the foundation of developing the Argos biometer [2], allowing a high-speed measurement (~30× faster than optical biometry), with two-dimensional

The Keck School of Medicine of USC, Los Angeles, CA, USA

M. C. Shammas Shammas Eye Center, Lynwood, CA, USA imaging of the eye and measuring all 9 parameters in a fraction of a second.

Recently presented systems with extended axial range allow the capturing of the anterior segment or even the full eye. OCT systems based on swept-source technology provide an extended imaging axial range without compromising the axial resolution. Furthermore, the use of OCT 2-D data should improve the success ratio in measuring the axial length, as well as improve the repeatability of its measurements.

The Argos uses a 1060 nm wavelength and 20 nm bandwidth swept-source technology to collect 2-D OCT data of the full eye [1]. The device provides 3 OCT images in every acquisition to measure not only the axial length (AL) and the anterior chamber depth (ACD) but also the central corneal thickness (CCT), aqueous depth (AD), lens thickness (LT), pupil size (PS), and the corneal diameter (CD). An automatic algorithm evaluates all the biometry parameters, and the optical distances are converted into geometric distances using the standard refractive indices of 1.376 for the cornea, 1.336 for the aqueous and vitreous, and 1.410 for the lens (Fig. 19.1); this is in contrast of other biometers that use different proprietary functions to convert the optical path length into millimeters (Fig. 19.2).

H. John Shammas (\*)

© The Author(s) 2024 313

J. Aramberri et al. (eds.), *Intraocular Lens Calculations*, Essentials in Ophthalmology, https://doi.org/10.1007/978-3-031-50666-6\_19

**Fig. 19.1** Argos uses a segmented method to measure the AL using multiple indices of refraction. A specifc refractive index is used for each segment, where: AL = CCT/1. 375 + AD/1.336 + LT/1.41 + VIT/1.336 − RT

**Fig. 19.2** The Lenstar and IOLMaster 500 biometers use proprietary calibration functions to convert the optical path length into millimeters

To minimize measurement errors, manual adjustment of the parameters from the OCT images is possible and is recommended in the presence of outliers. Keratometry (fattest and steepest meridians and astigmatism) is obtained from OCT information in combination with a ring LED; the OCT information locates the eye in space, and this information is introduced to the equations that allow evaluating the curvature of the anterior cornea. The unit displays the anterior corneal radius of curvature (*R*) at the fattest and steepest meridians along with the average value (*R*AV) and the *K* readings using a 1.3375 corneal index of refraction.

Argos also contains a double-checking system for those cases where the patient is not fxating correctly: the camera provides a panoramic view of the eye and allows alignment of the patient eye with respect to the pupil center, and a manual adjustment of the parameters provided by the OCT images is included to minimize the impact of possible errors in the distances provided by the automatic algorithm. While the former is used in the acquisition process, the latter is used to reprocess (manually adjust) the eyes identifed as outliers. An alert system is activated if any unsuccessful measurement or a higher-than-normal standard deviation is detected; it urges the user to check the plausibility in analysis mode, and it suggests manual adjustment if necessary.

We have been using the Argos biometer since 2014, and I would like to share my clinical experience with this biometer.

#### **Repeatability and Reproducibility of the Argos Measurements**

The repeatability and reproducibility of the Argos measurements have been tested by means of variation analysis study, and our study clearly demonstrated that the new OCT biometer produces precise and reproducible measurements [1].

The repeatability of the Argos measurements was analyzed as the average, standard deviation (SD), and range of the standard deviations of the biometric parameters (AL, ACD, CCT, AD, LT, PS, CD, and *R*AV) obtained from the 3 images provided by the instrument in every acquisition. The repeatability analysis of the measurements was performed on the 3 OCT data images provided by Argos in a single acquisition. The intraset average difference was 0.01 mm for AL, 0.01 mm for CCT, 0.01 mm for ACD, 0.01 mm for AD, 0.02 mm for LT, 0.05 mm for PS, 0.11 mm for CD, and 0.01 mm for *R*AV.

The reproducibility of the Argos measurements was analyzed by means of variance analysis using the data provided from 3 sets of measurements and each set containing 3 images. Realignment was performed between measurements in all patients. To measure the reproducibility of the measurements, the average and standard deviation of the variation of the 9 images were calculated for every parameter. The obtained average of standard deviations of the 9 images were 0.01 mm for AL, 0.01 mm for the CCT, 0.01 mm for the AD, 0.01 mm for the ACD, 0.03 mm for LT, 0.10 mm for PS, 0.14 mm for CD, and 0.02 mm for *R*AV. No statistically signifcant differences in paired *t* test (*p* < 0.01) were found in the data provided.

#### **Comparing the Argos Measurements to the IOLMaster 500 and Lenstar Biometers**

We compared the AL, ACD, and *R*AV measurements to the results obtained with the IOLMaster 500 and the Lenstar LS900 biometers, while CCT, AD, LT, PS, and CD are also compared to those results provided by the Lenstar LS900 biometer. Three different examiners, one for each instrument, performed the measurements in a randomized manner, and without knowledge of the results of the other two instruments. Measurements were performed under natural conditions (no dilation drops were used) using the artifcial ambient light in the clinic. For each measurement, the subjects were stabilized using the forehead and chin rests of each biometer and alignment was achieved with the subjects fxating on a light projected at optical infnity. For the IOLMaster and Lenstar measurements, the procedures from their respective manuals were followed and the result printouts were used for the study.

AL is defned as the measurement between the anterior corneal surface and the retinal surface; ACD is the measurement between the anterior corneal surface and the anterior lens surface; *R*AV is the average anterior corneal radius of curvature; CCT is the measurement between the anterior and posterior corneal surfaces; AD is the measurement between the posterior corneal surface and the anterior lens surface; PS and CD measure the pupil size and the corneal diameter, respectively, taken from a horizontal section.

In one study [1], there was general agreement between the AL measurements taken by the OCT unit and those taken by the PCI unit and the ones taken the OLCR unit with a correlation coeffcient of 1.00 compared to both instruments, with an average difference of −0.01 mm when compared to the IOLMaster and 0.01 mm when compared to the Lenstar.

The clinical relevance of these measurement differences is insignifcant when performing IOL power calculation in an average eye. All commonly used third-generation formulas, including the Hoffer Q [3], Holladay I [4], and SRK/T [5], base their calculations on AL and K measurements. A 0.01 mm longer AL decreases the calculated IOL power by less than 0.05 D depending on the AL and keratometry of the eye. We always recommend personalizing formula constants when any measurement or surgical technique is modifed; however, initial calculations with the new OCT unit can be accurately performed using the same ACD constant for the Hoffer Q, surgeon factor for the Holladay 1, and A constant for the SRK/T formula used with the PCI unit. The Haigis formula [6] uses preoperative ACD measurements in addition to AL values. In our study, the OCT biometer measured on average a 0.17 mm deeper ACD. Clinically, the deeper ACD increased the IOL power by 0.1 D when the standard Haigis constants are used. We recommend a small decrease of approximately 0.02 in the a0 constant when the Haigis formula is frst used with the OCT unit until all three constants in the Haigis formula are properly personalized.

#### **Acquisition Rate**

The patient group in this study [1] included many eyes with advanced cataracts. The AL could not be measured in 14 cases by one or more biometer; two patients had mature white cataracts and could not be measured by all three instruments. In the case of Argos, 54 out of the 56 eyes (96%) could be measured for all parameters and only the 2 cases with the mature cataracts were discarded due to no visibility of the retina. In the case of the IOLMaster, the success rate for AL measurement was 77% (43/56 eyes) and 13 eyes could not be successfully measured; these included the 2 mature cataracts, 2 cases with stage 5 nuclear sclerosis with posterior subcapsular changes, and 9 cases of stage 2 to stage 3 nuclear sclerosis with stage 3 posterior subcapsular changes. Finally, for the AL measurements by Lenstar, the success rate was 79% (44/56 eyes) and 14 eyes could not be successfully measured; these included the 2 mature cataracts, 3 cases with stage 4 cortical changes, and 7 cases with stage 3 posterior subcapsular changes.

Dense Cataract with Grade 3 or higher(21 eyes)

**Fig. 19.3** In dense cataracts, the acquisition rate is much higher with the Argos biometer compared to the Lenstar and the IOLMaster 500. The acquisition rate increases with the use of the ERV mode

The high success rate of acquiring the axial length measurement in eyes with dense cataracts is due to two main differences between the Argos and other biometers: the wide scanning beam in OCT bypasses the cataract region allowing the light reaching the retina not to be blocked; furthermore, the OCT in the Argos unit uses a longer wavelength centered at 1060 nm which penetrates deeper in the cataract tissue compared to the PCI and OLCR units whose wavelengths are centered at 840 nm for the Lenstar LS900 unit and 780 nm for the IOLMaster 500 unit.

In very dense cataracts (Grade 3 or higher) (Fig. 19.3), the acquisition rate of the Lenstar biometer dropped to 62% and the IOLMaster 500 to 67%. The Argos biometer maintained a high 81% acquisition rate, which could even improve to 91% with the use of the ERV mode (Enhanced Retinal Visualization).

#### **The Value of Using Multiple Indices of Refraction**

The Argos® swept-source optical coherence tomographer measures the optical path length (OPL) of each segment of the eye and uses a specifc refractive index (SRI) for each of these segments (cornea, anterior chamber, lens, and vitreous). As such, when there are variations in the relative lengths of these components, the axial length calculation is appropriately adjusted. In this new study [7], we compared the AL measurements obtained with the Argos biometer with its multiple indices, one for each segment of the eye (ALmultiple) to a simulated axial length that uses a single index of refraction for the entire eye (ALsingle). We noticed that the use of a single index of refraction for the entire eye yielded longer measurements in the long eyes and shorter measurements in the short eyes (Fig. 19.4).

This is consistent with the notion that a single refractive index is developed based on a normative dataset, effectively presuming a fxed ratio of eye segments in the total axial length. In cases where this ratio is less likely to be observed (e.g., short eyes, long eyes), the use of different refractive indices for each ocular segment would be more reliable.

The difference in axial length measurements based on multiple specifc refractive indices for each segment of the eye to those obtained using a single refractive index for the entire eye had subsequent effects on IOL power calculation.

We evaluated the results in 595 eyes undergoing cataract surgery where biometry and IOL power calculations were based on axial length calculated with multiple specifc refractive indices (ALmultiple) versus those with a simulated axial length based on using a single refractive index (ALsingle). The expected residual refractions based on different IOL formulas were calculated for both single and multiple groups. Formulas were then optimized, and the mean prediction errors (MPE) and mean absolute prediction errors (MAE) were calculated, based on the difference between the (optimized) expected value and the actual refractive outcome. In nearly all cases, the average MPE in the ALmultiple group was lower than that for the ALsingle group across all axial lengths and formulas (Fig. 19.5). When larger differences in MAE were present, the multiple group results were more often lower (better).

Two other studies [8, 9], compared axial length measurements from an OLCR biometer using a single refractive index to calculate AL measurements using multiple refractive indices for each ocular segment, in reverse of the present study. Both studies found that the single

**Average of AL-SOS and AL-Sim**

**Fig. 19.5** The use of the sum-of-segments method (AL-SOS) using multiple indices improved the prediction results compared to the simulated method (AL-SIM)

across the entire range of the axial length with the Barrett 2 formula (**a**), Haigis (**b**), Hoffer Q (**c**), Holladay1 (**d**), and SRK/T (**e**)

**Fig. 19.5** (continued)

319

**Fig. 19.5** (continued)

index AL measurements taken from the OLCR biometer were on average too short in short eyes and too long in long eyes, when compared to the calculated measurements based on multiple refractive indices. Wang et al. [8] found the refractive accuracy using multiple indices of refraction to calculate AL and IOL power in 4992 eyes to be improved in short eyes with Hoffer Q and Holladay 1 formulas and in long eyes with all formulas except the Olsen formula.

Using multiple indices instead of a single index to calculate AL in 1442 eyes, Cooke and Cooke [9] improved predictions for formulas designed on US data (SRK/T, Holladay 1, Holladay 2, Hoffer Q, and Haigis) although predictions were worse with the Barrett and Olsen formulas. Both studies agree with our study in that most of the accuracy improvements are noted in short and in long eyes.

#### **References**

1. Shammas HJ, Ortiz S, Shammas MC, Kim SH, Chong C. Biometry measurements using a new largecoherence-length swept-source optical coherence tomographer. J Cataract Refract Surg. 2016;42:50–61. https://doi.org/10.1016/j.jcrs.2015.07.042.

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# **Argos Verion Image-Guided System 20**

Raiju J. Babu and Jessica Voegtle

Cataract surgery has evolved with improved technologies, advanced biometers, and increasing patient expectations to achieving complete spectacle independence. The goal of the cataract surgeon is to implant an intraocular lens (IOL) with an appropriate IOL power to compensate for the refractive error and leave the patient emmetropic (plano target). Based on the patient need, the surgeon may chose a target refraction of emmetropia at distance or, in certain instances, target a myopic outcome to address spectacle independence at near. Occasionally, a mono vision or mini mono vision approach where one eye is targeted for distance and the other eye is targeted for myopic outcomes is also seen in practice [1].

The planning and execution of the process involve several steps:

• A preoperative diagnostic examination of the eye captures all parameters necessary for the selection of the optimal spherocylindrical power. In addition, for patients needing astigmatism correction, the pre-op evaluation also determines the ideal toric IOL orientation relative to reference landmarks that will be visible to the physician at the time of surgery.

#### **Astigmatism Correction**

Accurately determining the power of an IOL can be done using online web-based calculators or those supplied by the IOL manufacturer. The post-surgical refractive error is dependent upon the parameters used in the IOL power calculation. New generation calculators account for posterior corneal astigmatism (PCA), anterior chamber depth (ACD), effective lens position (ELP), and surgically induced astigmatism (SIA), in addition to the usual biometry parameters of keratometry and axial length [2]. If the patient has preoperative astigmatism, the surgeon can correct it using a toric IOL or other methods such as Limbal relaxing incisions [3]. When aligning the axis of a toric IOL, it is essential to accurately determine the steep axis of the cornea. This should be done with the patient in the seated position, to account for cyclotorsion that would occur when the patient is supine. Manual corneal

R. J. Babu (\*) · J. Voegtle Alcon Vision LLC, Fort Worth, TX, USA e-mail<EMAIL_ADDRESS><EMAIL_ADDRESS>
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J. Aramberri et al. (eds.), *Intraocular Lens Calculations*, Essentials in Ophthalmology, https://doi.org/10.1007/978-3-031-50666-6\_20

**Fig. 20.1** Figure displays the reference image captured by the Argos Biometer and the Alcon Vision planner used to create the surgical plan including IOL power calculation

marking consists of preoperative marking of the horizontal axis, intraoperative alignment of the reference marks with the degree gauge of the fxation ring, and intraoperative marking of the target axis. The marking may be done with a skin-marking pen with a thin slit beam, a weighted thread, a pendulum marker, or a bubble marker [4]. Femtosecond laser-assisted corneal and capsulotomy marking provides more permanent markings for postoperative assessment of IOL position [5]. Image-guided systems capture preoperative digital photography of iris landmarks and conjunctival, scleral, or limbal blood vessels (Fig. 20.1). Based on the features of the eye, an intraoperative registration with the surgery image allows displaying the preoperative calculated toric implantation axis in the microscope.

Intraoperative wavefront aberrometry provides refnement of IOL selection and axis rotation by providing intraoperative aphakic refractive information to the surgeon to determine the correct IOL power and pseudophakic refractive data to correctly align the axis of a toric IOL [6]. With respect to toric IOL axis determination, risks associated with manual reference marking include smudging or smearing, irregular or thick markings, parallax error, corneal abrasions, signifcant learning curve, intersurgeon variability, and anterior chamber bacterial contamination [4, 7, 8]. In one study, they showed that 30% of the ink markings were poorly visible due to washout [9]. Anterior stromal puncture offers the benefts of precise marking with no smudging [4].

#### **Argos and Verion Digital Marking**

The Argos Biometer with Image Guidance by Alcon is an integrated biometer that provides image guidance to the surgeons (Fig. 20.2). The image guidance is provided by having hardware (Verion Digital Marker) to provide overlays in the operating room and also at the Alcon LenSx Laser. The Verion Reference Unit is also an integrated keratometer with image guidance capability at Alcon LenSx system and Operating room. Verion Reference Unit is not capable of providing biometry; therefore, another biometer would be required to input the information into the Vision Planner.

The Digital Marker exists in 2 variants:

Digital Marker L (DML) and Digital Marker M (DMM)

The core of the Digital Markers is the Digital Marker Panel PC running software for image processing (registration and tracking) and displaying of tracking results during surgery. The Panel PC is placed at the LenSx Laser or next to the surgeon's microscope and receives the image signal of the microscope. Via established network connection or USB stick, preoperative data from

**Fig. 20.2** The Argos Biometer with Image guidance by Alcon

the Argos Biometer or Verion Reference Unit is loaded into the planning screen of the system. If required, with user interaction, the planned surgery parameters can be modifed.

The Digital Marker L imports patient information and provides this to the Alcon LenSx Laser. Additionally, the eye image from the Argos Biometer is provided to the Digital Marker L to determine the cyclotorsion angle between reference image and image from the digital laser microscope. Based on the determined cyclotorsion angle and planned centration options for arcuate incisions and capsulotomy, positions of treatment patterns are proposed to the operator.

The Digital Marker M consists of the Panel PC, Microscope Integrated Display (MID), foot pedal, and optionally the VERION Link (established connection to Alcon Centurion phaco system). The Microscope Integrated Display (MID) is mounted into the surgery microscope and connected via communication cable to the Digital Marker M. The MID acquires digitally a microscope image, passing this to the Panel PC for the image processing, and injects context information received by the Digital Marker M into the surgeons optical microscopes view (Fig. 20.3). After potential adjustment and confrmation of the registration angle, the tracker will be initialized. A live image with the planned tracking overlay is shown. If the eye is moving and/or rotating, the difference to the new position of the eye will be determined and the overlay adjusted. According to every surgery step on the Digital

**Fig. 20.3** The fnal registration angle determined by the Digital Marker L and provided to the LenSx Laser for adjustment of the treatment parameters, i.e., incisions and centration of Capsulotomy and Lens fragmentation

Marker M, an equivalent overlay will be displayed at the MID. With the footpedal (connected via USB) user interactions with the Digital Marker can be controlled, without physical touch of the computer screen (e.g., getting into live mode, start, and confrmation of registration, and to toggle through the different surgery steps). Overlays may show incision locations, capsulorrhexis size and location, IOL centration, toric alignment axis, or IOL centration + toric alignment axis (Fig. 20.4). During surgery, the displayed overlay can be adjusted with appropriate buttons on the screen. Optional Aphakic, Pseudophakic, and Lens Axis Marker phases are available to trigger measurements with the ORA VerifEye Lynk System (Intraoperative aberrometer) or display corresponding information (e.g., measurement data) on the screen or in the MID.

In detail, the Digital Marker provides:

The benefts of image-guided systems include accurate alignment of the toric axis relative to anatomic landmarks in photographs of the iris and limbal vessels [4, 6–8, 10–13] and minimizing marking errors and improving postoperative alignment [11, 13]. In a randomized controlled trial [14], the IOL misalignment was signifcantly less with image-guided system compared to manual marking. This has implications with respect to

**Fig. 20.4** Figure displays the toric overlay providing thereby image guidance with respect to IOL implantation axis based on the preoperative plan

astigmatic outcomes, as 1 degree of IOL misalignment can translate to 3.3% reduction in effectiveness of astigmatic correction [15] using one method. In another method where the vector difference between the target and achieved astigmatic outcomes, an error of 4.9 degree would result in remaining astigmatism magnitude of 17% of the preoperative astigmatism [16]. The randomized control trial did not fnd the difference in Uncorrected VA or residual astigmatism, although a statistically signifcant difference was seen in degree of IOL misalignment [14]. The key differentiation is that image-guided systems eliminates or accounts for the possible change in head position when keratometry and steep axis is determined impacts ability to account for cyclotorsion and thereby affecting precision [9, 17]. In addition, image-guided systems reduce the risk of anterior chamber bacterial contamination [7]. A risk of image-guided systems is that the eye tracker may disengage during surgery and a repeat registration may be required [4]. Additional risks include intraoperative changes in the appearance of the limbal vessels, including conjunctival chemosis, ballooning, and bleeding, which may interfere with intraoperative registration [4, 10]. Furthermore, registration may not be possible in extremely uncooperative patients or for diffcult orbital anatomy, including extremely deep-set eyes or narrow palpebral apertures [4]. Image-guided technologies primarily help to reduce any source of error and variability from manual processes. It also helps in eliminating transcription errors as the data is integrated. An additional beneft is that the Vision Planner that is part of the image-guided system helps the surgeon to move away from additional tools and calculators for astigmatism correction for toric alignment axis and arcuate incision and Global IOL constants by providing personal optimization algorithms for constants and SIA which is intended to improve outcome.

**Disclaimer** Raiju Babu and Jessica Voegtle are employee's of Alcon Vision LLC.

#### **References**

RMMA. Image-guided system versus manual marking for toric intraocular lens alignment in cataract surgery. J Cataract Refract Surg. 2017 Jun;43(6):781–8.

lens implantation in cataract and refractive surgery. J Cataract Refract Surg. 2011 Aug;37(8):1394–402. https://doi.org/10.1016/j.jcrs.2011.02.024.

17. Xiang W, Chen W, Liu R, et al. Ocular cyclorotation and corneal axial misalignment in femtosecond laser-assisted cataract surgery. Curr Eye Res. 2019;44(12):1313–8.

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# **Optical Biometer OA-2000 21**

Naoko Hara, Kathrin Benedikt, and Hirofumi Owaki

## **Introduction**

Optical biometer OA-2000 (Tomey Corporation, Nagoya Japan) can measure all biometric parameters needed for pre-cataract surgery: axial length, anterior chamber depth, lens thickness, corneal radius of curvature over a 2.5 mm and 3.0 mm diameter, corneal topography, central corneal thickness, pupil diameter, and Corneal Diameter (Fig. 21.1 and Table 21.1). It is done almost automatically by one measurement in short time. The touch screen enables intuitive operation, measurement, data checking, IOL calculation, and data output within one compact

**Fig. 21.1** Optical biometer OA-2000

N. Hara · K. Benedikt · H. Owaki (\*) Tomey Corporation, Aichi, Japan e-mail<EMAIL_ADDRESS><EMAIL_ADDRESS>

**Table 21.1** Specifcations of OA-2000

a Optional, depending on sales area

b Optional

unit. The OA-2000 supports doctors to reduce stress on patients and accurate and smooth preoperative examination and planning for high-quality cataract surgery by these features on daily practice [1].

#### **Axial Length Measurement**

The touch-alignment system is employed on OA-2000. It can provide a stable measurement and a high reproducibility between operators because OA-2000 aligns itself automatically to patient eye once an operator only touches the center of the pupil on the screen. Operators can measure all data easily without any special skills (Fig. 21.2) [2].

OA-2000 achieves a high signal–noise ratio (SNR) and high transmitting on opacity parts by using swept-source coherence tomography method. In addition, long coherence length allows high penetration and high success rate on long axial length myopic eyes (Fig. 21.3) [3]. Additionally, OA-2000 obtains not only A-scan wave form but also B-scan image of 1 mm width on retina by 2D scanning. On a measurement result of normal eye (Fig. 21.4), SNR is high and standard deviation (SD) is very low because the measurement light isn't reduced as the crystalline lens is clear. It means the measurement is very stable.

On the other hand, there are some cases where it is diffcult to obtain the signal around the center of the retina as shown in Fig. 21.5a. In such case, OA-2000 measures the axial length by detecting a stronger signal from peripheral area automatically. When the retinal signal cannot be detected even with that way, V scan (Fig. 21.6) is performed instead of horizontal scan to fnd less opacity position so that the retinal signal can be

**Fig. 21.2** Touch alignment, easy operation. http://vimeo.com/259629303?width=640&height=480

**Fig. 21.3** Results of cataract eye with long axis length

**Fig. 21.4** Measurement result of normal eye

**Fig. 21.5** Wave forms and B-scan images on retina of cataract eyes. (**a**) Lens central opacity. (**b**) Multi-peaks wave form

detected. High success rate is achieved by these techniques in combination.

In the case of dense cataract with low SNR, multi-peaks retinal waveforms may appear as Fig. 21.5b shows. OA-2000 detects retinal pigment epithelium (RPE) automatically by an original algorithm based on signal appearance ratio and peak analysis results. It is recommended to judge it comprehensively including the results of other examinations, when it is suspected if the

**Fig. 21.7** Caliper function

detected retinal peak is correct or not, for example, miss-detecting epiretinal membrane as RPE. Caliper function (Fig. 21.7) allows operators to select RPE position manually if the auto-detected RPE is not correct.

For converting optical path lengths (OPLs) to geometrical distances, an original conversion formula was established by clinical dataset of ultrasound biometer and OA-1000, how to be performed based on the way by Haigis [4, 5]. OPLs of OA-2000 are in good agreement with OA-1000, which is the previous model; OA-2000 also uses the same conversion formula.

#### **Clinical Cases**

Clinical cases of cataract eyes are introduced in Fig. 21.8.

OA-2000 includes the function of wireless connection with ultrasound biometer AL-4000 (Fig. 21.9). It is useful when measurement is diffcult by an optical biometer due to subject eyes conditions. The measurement results of AL-4000 can be transferred to OA-2000, and it can be used for IOL calculation on OA-2000.

**Fig. 21.8** Clinical cases of cataract eyes, Provided by Dr. Hitoshi Tabuchi, Chief director of Department of Ophthalmology, Tsukazaki Hospital. (**a**) Hypermature cataract eye. The nuclear sclerosis is Emery class V. SDs for axial length, ACD, and lens show small numbers even with relatively low SNRs; measurement result is stable. (**b**) Cortex and posterior subcapsular cataract eye. The nuclear sclerosis is Emery class III. SDs for axial length, ACD, and lens show small numbers even with relatively low SNRs; measurement result is stable.. (**c**) Cortex and posterior subcapsular cataract eye. The nuclear sclerosis is Emery class II-III. The retinal signal is very low because the opacity of the cortex and the posterior subcapsular is strong. However, SDs are small enough and the measurement result is stable

**Fig. 21.8** (continued)

**Fig. 21.9** Ultrasound biometer AL-4000

## **Measurement of Corneal Radius of Curvature and Topography**

OA-2000 employs a Placido disc for measuring corneal radii and topography, which has been proven being an accurate measurement method and used for many years [6]. The Placido-based topography can capture image in a single shot; therefore, it is hard to be affected by eye motion, when performed at the same time as the axial length measurement. Figure 21.10 shows examples of comparison of topography map between TMS-4N and OA-2000 for same eyes. The measurement range of OA-2000 is 5.5 mm which is narrower than TMS-4N. However, it is enough for evaluating visual function, and they have good consistency in that range.

**Fig. 21.10** Comparison between TMS-4N and OA-2000 on topography. (**a**) Topography of with-the-rule astigmatism eye. (**b**) Topography of keratoconus eye

### **Important Indices of Cornea for Cataract Surgery**

OA-2000 provides not only the corneal radius of curvature results but also important indices of KAI (Kerato-Asymmetry Index) and KRI (Kerato-Regularity Index) (Fig. 21.11). These indices are ranked with A (Low)/B (Slightly high)/C (High) which show possibility to be irregular corneal astigmatism. When KAI shows high value in B or C, the eye is suspected to be a deformed cornea such as in keratoconus. When KRI shows high value, the eye is suspected to be a corneal transplant or with CL-induced problems, etc. It can call to attention checking the topographic maps so that doctors can prevent postoperative troubles. In the case that these values are high, there is higher risk of insuffcient visual recovery and it should be carefully considered to choose multifocal IOLs.

CEI (Corneal Eccentricity Index) is shown in the topographic screen (Fig. 21.12). When CEI indicates positive number, the corneal shape is prolate which is normal eye. On the other hand, when CEI indicates negative number, the corneal shape is oblate which can be observed in typical post-LASIK eyes. By checking this value, even in the case that the surgical history is unknown, doctors can realize the possibility being post-LASIK eye and adopt post-LASIK IOL formula so that they can avoid refractive surprise after surgery.

**Fig. 21.11** Kerato-Asymmetry Index (KAI) and Kerato-Regularity Index (KRI)

**Fig. 21.12** Corneal Eccentricity Index (CEI) on post-LASIK eye

#### **IOL Calculation and Toric Calculation**

OA-2000 includes different kinds of IOL formulae, which are standard IOL formulae, toric IOL formulae, and post-Lasik IOL formulae (Table 21.1 and Fig. 21.13). In addition, function for optimization of IOL constants is included in OA-2000 and it can support to calculate personal lens constant for each surgeon (Fig. 21.14).

Toric planning function to support with axis registration is available (Fig. 21.15). It allows doctors to mark the target axis based on the reference axis of iris pattern or conjunctival blood vessels.

**Fig. 21.13** IOL calculation screen

**Fig. 21.14** Optimization of IOL constants

**Fig. 21.15** Toric Planning screen

#### **References**

ferometry and optical low-coherence interferometry. Am J Ophthalmol. 2017;173:64–9.

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# **ANTERION Swept-Source OCT Biometer 22**

Jana Schröpfer, Richard Cornwell, Sandro Gunkel, Melanie Polzer, and Steven Thomson

#### **Background: Swept-Source OCT Imaging for the Anterior Segment of the Eye**

Optical coherence tomography (OCT) has become a standard for diagnostic imaging and management of various ocular conditions. Since its introduction, OCT has gained prominence in imaging the posterior segment of the eye and developed into a relevant tool in the clinical evaluation of the cornea and anterior segment. As advances to the technology have improved the acquisition speed and enhanced the resolution of images, the impact of anterior segment OCT imaging on clinical practice has increased [1]. Anterior segment OCT imaging allows for the visualization and assessment of the cornea, conjunctiva, sclera, rectus muscles, iridocorneal angle, lens, and other ocular features [2].

With the commercial introduction of spectraldomain OCT (SD-OCT) technology, imaging of the anterior segment at high speeds with good axial resolution became feasible [1]. However, most commercial SD-OCT devices use relatively

S. Thomson (\*)

Heidelberg Engineering GmbH,

Heidelberg, Germany

short-wavelength light sources (820–880 nm), resulting in limited image depth range and a low penetration of deeper anterior ocular structures [3]. More recently, swept-source OCT (SS-OCT) was introduced with refnements made to the illumination source and detection system. Combining SS-OCT technology with a longer wavelength light source results in an optimized approach for anterior segment image acquisition and analysis. The longer wavelength permits increased penetration depth while the SS-OCT technology ensures minimal sensitivity roll-off at this depth. This combination and the short acquisition time help reduce motion artifacts to generate high-defnition images of the entire anterior chamber [1, 2].

The ability to image anterior ocular structures with high clarity and contrast provides the basis for generating clinically relevant data, such as corneal topography, corneal tomography, anterior segment analysis, and biometry. SS-OCT with a long-wavelength light source can further serve as a vital tool to measure the axial length of the human eye and has been shown to have better tissue penetration compared to partial coherence interferometry (PCI) technology [4–7]. The inherent characteristics of long-wavelength SS-OCT thus provide clinicians with the biometric data considered essential to conduct intraocular lens (IOL) power calculations that can result in accurate refractive prediction [8, 9].

J. Schröpfer · R. Cornwell · S. Gunkel · M. Polzer ·

e-mail<EMAIL_ADDRESS><EMAIL_ADDRESS><EMAIL_ADDRESS><EMAIL_ADDRESS>
© The Author(s) 2024 339

J. Aramberri et al. (eds.), *Intraocular Lens Calculations*, Essentials in Ophthalmology, https://doi.org/10.1007/978-3-031-50666-6\_22

### **Clinical Applications of Anterior Segment OCT as Implemented on ANTERION**

The ANTERION® from Heidelberg Engineering is a multimodal platform optimized for the anterior segment. It makes use of the technological advantages of long-wavelength SS-OCT and combines it with proprietary features that increase image clarity, thereby enabling the generation of precise measurements needed in cataract and anterior segment surgery. Acquiring high-resolution OCT scans at a relatively long wavelength of 1300 nm, ANTERION is well suited for imaging structural details in the anterior segment as well as performing corneal topography, tomography, anterior segment biometry, axial length measurements, and IOL calculations. By combining these measurements and examinations in one upgradable device, ANTERION caters to multiple clinical applications. The platform is designed to increase patient care by streamlining clinical workfows, saving on space in the examination room and minimizing patient chair time. ANTERION's most common application areas to date include cataract surgery with IOL power calculations, refractive surgery, cornea diagnostics, structural imaging for anterior chamber angle evaluation, and anterior segment imaging for various ocular conditions. To adapt to the workfow needs of each clinical discipline, ANTERION can be confgured with different "Apps": Imaging App, Cornea App, Cataract App, and Metrics App.

The **imaging application (Imaging App)** is included in every confguration of the device. It acquires OCT scans with an axial resolution of less than 10 μm, a lateral scan length of up to 16.5 mm, and a scan depth range of 14 ± 0.5 mm. ANTERION's eye tracking technology on the corneal vertex increases the imaging capabilities as it offers geometric alignment of OCT scans along the fxation axis. This also allows for automated quality checks such as eye movement, blinking, and surface segmentation (see Table 22.1 for more technical specifcations).

The resulting high-resolution images allow for the evaluation of the anterior segment, with the ability to visualize all structures of interest in

**Fig. 22.1** ANTERION image of a healthy eye

one image (see Fig. 22.1). The Imaging App includes customizable scan patterns and can also be used for corneal, scleral, iridocorneal angle, and peripheral imaging, supporting the diagnosis of diseases in these locations.

The imaging capabilities assist clinicians in the diagnosis of anterior segment anomalies and provide visual confrmation of any measured parameters. ANTERION thus provides precise eye measurements as well as additional information for surgical planning and follow-up, such as the visualization of phakic lenses, IOLs, ICLs, or corneal rings.

**Fig. 22.2** ANTERION Cornea App: Corneal topography and tomography maps as well as corneal imaging for an eye with astigmatism and cataract. **Image Courtesy:** Damien Gatinel, MD, PhD, Paris, France

ANTERION's **cornea application (Cornea App)** scans the cornea in detail, providing a comprehensive analysis that is required in cornea diagnostics, anterior segment surgery, and IOL power calculations. The cornea data is acquired using 65 radial OCT scans (256 A-scans per B-scan), with an acquisition time of <1 s. This provides a total of 16,640 data points that contribute to the calculation of corneal maps and reports within an 8-mm zone. ANTERION's optimized SS-OCT technology considers both the anterior and the posterior corneal surface, providing important corneal topography and tomography data. The maps in the Cornea App include anterior and posterior axial curvature, tangential curvature, elevation, pachymetry, posterior/anterior corneal curvature radii ratio, total corneal power, as well as anterior and total corneal wavefront. Parameters such as pupil diameter, angle kappa, corneal vertex, thinnest point, or minimum radius can be overlaid onto the corneal maps. Due to the high-resolution imaging of the cornea, ANTERION also offers the possibility to verify the segmentation of the corneal surfaces in the accompanying OCT images. The Cornea App templates can be customized to display all clinical information: operators can select their preferred maps and data in a multi-view template, conduct a comparison of both eyes with differential maps, and use a layout for follow-up examinations that automatically calculates progression analysis for selective measurements (see Fig. 22.2 for a customized Cornea App template). This diagnostic tool will support clinicians in the investigation of various keratopathies and ectatic disease along with refractive and other corneosurgical involvement. Beyond that, ANTERION's comprehensive corneal data is used to augment refractive cataract surgery and populate IOL power calculations.

For assessing **anterior chamber biometry and angle metrics**, ANTERION offers the **Metrics App.** With one acquisition, it provides six OCT images of the anterior chamber in a radial view (each B-scan consisting of 768 A-scans). These high-resolution OCT images provide the basis for freehand measurements and for calculating relevant angle metrics. The segmentation lines for the corneal surfaces, lens surfaces, and the iris are automatically displayed but can also be adjusted by the operator. The ability to acquire high-contrast images of the anterior chamber allows for the qualitative visualization of its architecture and the quantitative assessment of all relevant parameters. Besides iridocorneal angle assessment (anterior chamber angle, angleopening distance, trabecular-iris space area, and scleral spur angle), the Metris App also provides measurements of the anterior chamber, cornea, and lens. Among these values are anterior chamber volume, spur-to-spur and angle-to-angle distance, central corneal thickness, corneal diameter, lens thickness, and lens vault. The ability to measure these structures while visualizing the anterior chamber at various angles can serve as a complementary tool to gonioscopy considering that the OCT technique offers the additional beneft of being non-contact [10]. The ANTERION Metrics App can therefore support in the assessment and monitoring of anterior chamber and angle closure disease. Furthermore, it can generate information useful in the evaluation of cataract surgery, anterior and posterior chamber phakic lens implantation, and other surgical procedures (see Fig. 22.3).

Finally, ANTERION offers the **Cataract App** for the **streamlined planning of cataract surgery and IOL calculation**. It combines key biometric measurements with a suite of IOL power calculation methods. The optimized SS-OCT technology provides accurate axial and surface measurements and offers visual confrmation with high-resolution images. The ability to identify eyes that have unusual geometry and to integrate total corneal power into the IOL prediction further supports the selection of the most suitable IOL.

The following chapter section presents the Cataract App in detail and summarizes all functionalities that make ANTERION a valuable tool for optical biometry and complex IOL power calculations.

**Fig. 22.3** ANTERION Metrics App: Evaluation of the same eye before (**a**) and after (**b**) cataract surgery including selected measurement overlays for anterior chamber angles, spur-to-spur distance, and lens vault. **Image Courtesy:** Damien Gatinel, MD, PhD, Paris, France

#### **Biometry and IOL Power Calculations with ANTERION**

ANTERION serves as a relevant tool for optical biometry and IOL power calculations due to its SS-OCT technology combined with highdefnition topography and tomography.

Effective IOL power calculations require accurate biometry, with axial length (AL) and keratometry being two of the primary components. Improved refractive prediction accuracy can be achieved when additional variables such as anterior chamber depth (ACD), lens thickness (LT), and corneal indices are considered. ANTERION calculates ACD from both central corneal thickness (CCT) and aqueous depth (AQD). Clear OCT images of the anterior segment signifcantly contribute to improved refractive accuracy by facilitating precise preoperative measurements. ANTERION generates all relevant parameters based on its precise SS-OCT imaging and sets itself apart by providing a much more comprehensive corneal analysis. Keratometry measurements, for example, are based on 16,640 data points over an 8-mm zone and detail both anterior and posterior corneal curvature (see Table 22.2 for more ANTERION parameters and features for IOL power calculation).

It is well accepted that postoperative refractive errors in IOL power calculations are typically ascribed to inaccurate preoperative AL measurements [11]. Rozema et al. [12] presented that the threshold of the AL and ACD to change the IOL power by 0.250 D for cataract surgery is 0.074 and 0.6 mm, respectively, when applied to the Haigis calculation. With the Cataract App, ANTERION measures AL with a high number of scans and calculates their standard deviation to provide clinicians with an objective rationale for evaluating the patient's fxation quality and overall reliability. The AL measurement is displayed alongside high-quality OCT section images. This includes an A-scan graph that shows the OCT signal intensity of the cornea, lens, and retina. The retinal pigment epithelium peak refection is displayed and can be manually adjusted by the user. Furthermore, the software provides a comprehensive data display for both standard and premium IOL selection with additional anterior segment values. This includes corneal diameter, lens thickness, pupil dimeter, pupil center (kappa angle), as well as spherical aberration and higher order aberration summary. Any asymmetry between the right and left eye is automatically displayed, thus can help to identify errors and irregularities (see Fig. 22.4 for data displayed in the Cataract App).

One of the major benefts of ANTERION is the ability to combine essential parameters with an integrated IOL calculator menu. The spherical and toric IOL calculators provide various calculation methods, with IOL constants populated

**Table 22.2** ANTERION parameters and features for IOL power calculation

*ACA* anterior chamber angle; *AOD* angle-opening distance; *TISA* trabecular-iris space area; *SSA* scleral spur angle

from either *IOL Con* or *ULIB.* Alternatively, this information can be entered manually with the preferred constants of the surgeon. Importantly,

**Fig. 22.4** ANTERION Cataract App: Both eyes (OU) view showing anterior axial curvature maps, OCT section images and intensity graphs, axial length diagrams, and parameters for cataract surgery planning

the patient's eye status can be edited to consider histories of refractive surgery, previous IOLs, aphakia, or vitreous surgery.

Traditionally, toric IOL calculations were based on keratometry measurements of the anterior surface of the cornea. More recently, estimation algorithms have been introduced that consider the posterior surface curvature, resulting in signifcant improvements in toric IOL power calculation [13–17]. ANTERION offers a toric IOL calculator, taking the incision location and surgically induced astigmatism (SIA) into account, as well as enabling the surgeon to choose either corneal astigmatism derived from anterior corneal curvature or total corneal power (see Fig. 22.5). Using preoperative crystalline lens measurements to predict potential postoperative tilt of toric IOLs may provide additional refractive improvements [18]. Ray tracing models can also be employed with these measurements to improve the postoperative refractive outcomes of toric IOLs [3]. ANTERION offers various approaches to IOL power calculation in an attempt to provide an interface that can adapt to new developments as they become available. Within its IOL power calculation section, for example, ANTERION provides an interface to the *OKULIX* IOL ray tracing application. The *OKULIX* prediction utilizes both anterior and posterior corneal measurements and considers ACD and LT as important variables when calculating IOL power. Recent studies have suggested that *OKULIX,* populated with ANTERION data, is capable of providing surgical outcomes at a high level [19].

It should be noted that the high-resolution SS-OCT images from ANTERION provide the basis for accurate biometric measurements while aiding in the visualization of the anterior segment (including the crystalline lens). The images can be used to confrm the postoperative IOL location, including posterior chamber phakic lenses. Furthermore, visualizing and documenting cataracts with SS-OCT has been deemed a useful approach to identify those eyes symptomatic of having haze, glare, or haloes. Surgeons performing anterior cortical cataract cases may fnd this additional information particularly relevant [3].

The combination of ANTERION's multiple tools and apps offers clinicians and surgeons a precise evaluation of the patient's individual eye geometry and a dedicated tool for complex IOL

**Fig. 22.5** ANTERION Cataract App: OU toric IOL calculator accounting for corneal astigmatism, incision location, and surgical-induced astigmatism

calculations. Numerous studies (detailed below) have further confrmed the precision of ANTERION and its agreement to established devices.

#### **Study Results**

Various studies have reported on the repeatability of ANTERION in both healthy eyes and in eyes undergoing cataract surgery (see Table 22.3). The repeatability of a device to provide precise measurements cannot be overlooked.

Several authors have evaluated the ANTERION in healthy eyes. Montés-Micó et al. [22] measured 69 Caucasian eyes for corneal diameter (CD), angle-to-angle (ATA), spur-tospur (STS), and lens vault distances. Eyes were measured fve times with both horizontal and vertical meridians. Repeatability was good for the variables evaluated. Within-subject standard deviation (Sw) values were low and ranged from 0.01 to 0.07. Coeffcient of repeatability (CoR) values showed a similar pattern being larger for those metrics measuring angles. Coeffcient of variation (CoV) values were reported as very small for CD, ATA, and STS distances (0.16– 0.57%). Intraclass correlation coeffcient (ICC) values for all parameters analyzed were >0.97. An ICC < 0.75 indicates a poor correlation, whereas an ICC > 0.90 indicates a high correlation between the measurements obtained [25]. The study found no statistically signifcant difference in any of the repeated measurements. While

**Table 22.3** Repeatability of ANTERION measurements across selected studies

*Sw* within-subject standard deviation; *CoR* coeffcient of repeatability; **CoV** coeffcient of variation; *ICC* intraclass correlation coeffcient; *SimK* simulated keratometry; *Kmin* fat axis keratometry; *Kmax* steep axis keratometry; *TCP* total corneal power; *AL* axial length; *AQD* aqueous depth; *CD* corneal diameter; *LT* lens thickness; *CCT* central corneal thickness (in μm)

the authors did fnd lens vault distance about 10% compared to CD, ATA, or STS distances about 0.5% (CoV), indicating these measurements were more variable, the values were clinically negligible.

Tañá-Rivero et al. [23] prospectively evaluated 74 phakic eyes (74 patients) and considered average, steep and fat keratometry (K), astigmatism for anterior, posterior, and total at 3 mm, average K and astigmatism at 6 mm, anterior and posterior eccentricity, higher order and spherical aberration, and anterior and posterior best ft sphere (BFS) at 8 mm. All eyes had fve consecutive measurements taken over the course of the same session. Subjects in this study had a baseline mean spherical equivalent of −0.43 ± 1.43 D (range, 1.50 to −4.50 D). Sw values were <0.09, varying from 0.035 (posterior average K at 6 mm) to 0.0878 (anterior fat K at 3 mm). CoV values were also low and were similar among most parameters (from 0.08% to 0.21%), except for anterior, posterior, and total astigmatism (from 2.25% to 8.46%). The study concluded that corneal measurements with ANTERION are highly repeatable and, in some cases, superior to other devices.

Ruiz-Mesa et al. [24] prospectively evaluated 74 healthy eyes, analyzing corneal thickness (central and at 2, 4, and 6 mm diameters), AQD, LT, anterior chamber volume (ACV), AL, and pupil (diameter and position) in fve consecutive measurements taken during one visit. In this evaluation, there were no statistically signifcant differences between repeated measurements (*P* > 0.05). The mean difference for corneal thickness was between −0.08 and 0.28 μm. For AQD and LT, the difference was 0.004 and −0.004 mm, respectively. The mean ACV difference was −0.03 mm3 , and the mean AL difference was 0.001 mm. Pupil diameter and position mean differences ranged between −0.008 and 0.009 mm. Overall, most measurements had a Sw < 1 and a CoR < 2 in their respective units, and an ICC > 0.92, again indicating good repeatability for different ocular biometric measurements.

**Table 22.4** Comparison between ANTERION and other optical biometers

*LoA* limits of agreement; *ICC* intraclass correlation coeffcient; *MAD* mean absolute difference; *SimK* simulated keratometry; *Kmin* fat axis keratometry; *Kmax* steep axis keratometry; *AL* axial length; *ACD* anterior chamber depth; *LT* lens thickness; *CD* corneal diameter; *CCT* central corneal thickness (in μm)

There are several studies to date that compared ANTERION to other biometers (see Table 22.4).

Schiano-Lomoriello et al. [21] assessed the repeatability of the ANTERION Cataract App to a Placido-disk corneal topography device (MS-39), using the IOLMaster 500 as the control in 96 healthy eyes (96 patients). Parameters analyzed included SimK average, keratometric astigmatism, posterior keratometry average, total corneal power (TCP), TCP astigmatism, central corneal thickness (CCT), corneal diameter, AQD, LT, and AL. Images were acquired three times for both the ANTERION and MS-39, and once for the IOLMaster 500 (or until a good quality measurement could be acquired). In this analysis, ICC was >0.98 for all variables except astigmatism (0.963) and all measurements (excluding astigmatism) showed a CoV < 1%. Repeatability improved signifcantly when only eyes with astigmatism >1.0 D were considered. This is a key point as keratometric astigmatism measurements are used to calculate toric IOLs that are usually not implanted in patients with low astigmatism. Importantly, it is noted that the only signifcant difference between measurements with the ANTERION and the IOLMaster 500 was in corneal diameter. These results add to fndings that the ANTERION has high repeatability and are among the frst to suggest the device also has interchangeability.

Shetty et al. [20] compared the repeatability of the ANTERION to the Lenstar LS 900 and the IOLMaster 700 to determine impact on predicted IOL power calculations. This study evaluated 127 eyes (76 patients) with established cataract. Repeatability of all measurements for a given device were excellent (ICC > 0.9, low CoV and Sw). The agreement of parameters between the biometers was very good (ranging from 0.93 to 0.99). The predicted IOL power differed statistically between the devices (*P* < 0.05), but the difference was clinically insignifcant between the three biometers (ICC > 0.99 for repeat calculation of IOL power). The best agreement between the biometers was obtained for AL and least for CD. Shetty et al. further found all scans had good penetration through the lens – even in cases of mature cataracts. The authors concluded that cataract surgery outcomes using ANTERION would be comparable to other commonly used biometers, even though other devices use different wavelengths for AL, different designs for keratometry measurements, and have different axial resolutions.

Fișuș et al. [26] evaluated 389 cataractous eyes (209 subjects) that underwent measurements (keratometry, CCT, ACD, LT, and AL) on the same day with both the ANTERION and the IOLMaster 700. Overall, the study found good agreement, with a minor offset for ACD and LT measurements. However, this group recommended that these two devices could not be used interchangeably, even although these key parameter differences were small. The mean absolute difference between the keratometry data of the two devices was 0.04 ± 0.05 mm (7.80 ± 0.26 mm for the IOLMaster 700 and 7.82 ± 0.26 mm for the ANTERION; *P* < 0.0001) for the steep meridian keratometry readings and 0.04 ± 0.04 mm (7.63 ± 0.26 mm and 7.65 ± 0.25 mm; *P* < 0.0001) for the fat meridian keratometry readings. For ACD and LT, the mean absolute difference was 0.07 ± 0.04 mm and 0.07 ± 0.04 mm. The mean absolute difference for AL was 0.02 ± 0.03 mm (23.55 ± 1.18 mm for the IOLMaster 700 and 23.54 ± 1.18 mm for the ANTERION; *P* < 0.0001). The mean difference in AL found in the Fisus study (0.01 mm) correlates to about 0.03 D refraction error, which is not considered clinically relevant.

Table 22.4 summarizes the fndings of Schiano-Lomoriello et al. [21], Shetty et al. [20], and Fișuș et al. [26], who compared ANTERION to other optical biometers. It indicates the agreement between the respective devices for biometric measurements and keratometry data.

Collectively, these studies confrm that ANTERION measurements show a high repeatability and a good agreement with those acquired by established devices. Future studies and publications that incorporate the ANTERION will allow for direct comparison of outcomes.

#### **Summary**

SS-OCT technology combined with a longer wavelength light source provides a strong basis for high-resolution anterior segment imaging, optical biometry, and other anterior segment measurements. It is possible that this combination will replace previous technologies and, in the future, further improvements to IOL power prediction and new application areas that enhance diagnostics and support clinical decision-making will emerge and evolve. The SS-OCT device ANTERION will help streamline cataract and refractive surgery and can also assist in the management of corneal diseases and glaucoma. This latest technology provides high-resolution images, precise biometric measurements, and comprehensive corneal data that can be used to augment refractive cataract surgery and populate IOL power calculations. Furthermore, it may prove particularly helpful in the selection of toric and multifocal IOLs and in assessing eyes with previous laser vision correction treatments. ANTERION presents an all-in-one solution which can substantially improve the workfows in busy practices and clinics by reducing the need for multiple devices. The examinations and calculations are performed at high speed and with a small footprint, thus facilitating improvements to effciency and logistics.

#### **References**

optical coherence tomography. Curr Eye Res. 2014;39(12):1178–86.

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## **The NIDEK Cataract Suite: The OPD-Scan III Multifunction 23 Diagnostic Device and the AL-Scan Optical Biometer**

Farrell C. Tyson, Stefan Pieger, and Harkaran S. Bains

The NIDEK Cataract Suite includes the OPD-Scan III multifunction diagnostic device and the AL-Scan optical biometer. In this chapter, we present the use of each of these devices for diagnostics, cataract surgery, premium intraocular lens (IOL) selection, IOL calculations, and postoperative assessment.

#### **OPD-Scan III**

The OPD-Scan III is a fundamental device for cataract diagnostics (Fig. 23.1). This device measures corneal topography, wavefront aberrations, autorefraction, keratometry, pupillometry, and pupillography on the same axis. This unique combination of measurements allows comprehensive preoperative and postoperative assessment of cataract surgery patients. The measurement of topography and whole eye wavefront allows separation of corneal and internal aberrations for rapid assessment of the refractive and optical effects of the cornea, physiologic lens, or an intraocular lens (IOL) (Fig. 23.2) [1].

S. Pieger Ingenieurbüro Pieger GmbH, Wendelstein, Germany e-mail<EMAIL_ADDRESS>
H. S. Bains Sight By Design, Edmonton, AB, Canada **Fig. 23.1** OPD-Scan III

The OPD-Scan III measures aberrations using a unique method called dynamic spatial skiascopy [1]. This method utilizes optically conjugate projecting and receiving systems to measure aberration data in refractive diopters. A slit of infrared light is projected into the eye and rotated at 1° increments over 360°. Simultaneously, photodetectors rotate at the same rate and meridian as the projecting system and the time difference to stimulate individual photodetectors is converted into refractive power data (OPD maps).

F. C. Tyson (\*) Tyson Eye, Cape Coral, FL, USA

© The Author(s) 2024 351 J. Aramberri et al. (eds.), *Intraocular Lens Calculations*, Essentials in Ophthalmology, https://doi.org/10.1007/978-3-031-50666-6\_23

**Fig. 23.2** Overview summary for preoperative evaluation of a patient with cataract. The top row presents the OPD map (top row left), axial corneal topography (top row, middle), and the Internal OPD map (top row, right).

On the OPD and Internal OPD maps, cooler colors indicate hyperopia power, warmer colors indicate myopia, and green indicates emmetropia

The refractive power is then converted to traditional wavefront data (wavefront maps). The advantage of this method is a high range of measurement (−20 D to +22 D) and the ability to measure highly aberrated eyes. The OPD-Scan III measures 2520 data points and plots aberrations for pupil diameters up to 9.50 mm, out to the eighth Zernike order. The diameter and Zernike order are selectable to address the variability in physiologic pupil size. Corneal topography is measured with Placido disk technology that uses 33 rings to cover the corneal surface utilizing 11,880 data points. Multiple studies have verifed the accuracy, repeatability, and reproducibly of the various functions of the OPD-Scan III [2–5].

Preoperatively, evaluation of the Placido mires during corneal topography allows the detection of subtle ocular surface abnormalities that may indicate dry eye. A pristine ocular surface is essential for accurate preoperative measurements to generate excellent postoperative outcomes. Distortions in the Placido mires can alert the surgeon to investigate for dry eye or other corneal pathology. A recent study from the US indicates that 80% of patients presenting for a cataract evaluation had objective signs of dry eye, yet only a small proportion had been previously diagnosed [6]. The OPD-Scan III includes a neural network module that screens the cornea for pathology such as keratoconus, keratoconus suspects, and pellucid marginal degeneration and classifes eyes that have undergone refractive surgery.

Corneal pathology such as anterior membrane dystrophy and Salzmann's nodules can cause irregular astigmatism that often warrants regularization of the cornea prior to IOL surgery. The combination of corneal, internal, and whole eye maps more readily facilitates patient discussion by showing the optical effects of corneal versus internal aberrations using the point spread function and Internal OPD maps. In patients with irregular corneal astigmatism undergoing IOL implantation, the PSF can be used to educate patients that there is preexisting pathology that is distorting vision and will not be corrected by cataract surgery and will prevent them from achieving optimal vision postoperatively (Fig. 23.3).

**Fig. 23.3** Point spread function (PSF) from the higherorder aberrations of the entire eye (right, PSF/OPD/HO), cornea (middle, PSF/Corn/HO), and internal aberrations (left, PSF/OPD/HO) of a patient with pellucid marginal

degeneration and cataract. The ocular and corneal PSF patterns are very similar and can used for patient education to explain the source of visual phenomena

The extensive data and multiple maps facilitate preoperative evaluation, surgical planning, and postoperative assessment from normal eyes to complex cases. The Overview summary presents information on refraction, corneal topography, the OPD (the whole eye), Internal OPD (everything behind the front corneal surface), pupil size, corneal astigmatism, and image quality (Fig. 23.1). This summary can be used as a screening tool for most patients. The Overview layout allows evaluation of whether corneal or internal pathology (cataract) is present and the effect on the whole eye. For example, in Fig. 23.1, the axial topography map indicates less than 0.25 D of corneal astigmatism and the Classifcation/Indices indicates a normal cornea. However, the OPD map and the Internal OPD maps are irregular with patterns that are similar to each other (Fig. 23.2). Additionally, the Internal OPD has −0.92 D of cylinder whereas the total and corneal astigmatism is relatively small (Fig. 23.2). Taken together, these observations indicate the source of irregularity and higher order aberrations are from the internal aspects of the eye, most likely a cataract. Clinical examination verifed a visually signifcant cataract. The changes in the Internal OPD can be used to follow the development of the cataract and used for patient education. The Internal OPD is an excellent way to evaluate the position or possible rotation of a toric IOL.

In addition to qualitative assessment, the OPD-Scan III generates multiple values that are required for IOL calculations, surgical planning, and astigmatism management. These values include refraction, simulated keratometry (simKs), corneal spherical aberration, and Corneal Diameter (Table 23.1). The fat and steep meridians of the cornea are marked for surgical planning, including toric IOL alignment, placement of the incisions, and placement of limbal relaxing incisions. To address the accuracy of keratometry in cases with irregular corneal astigmatism, the average pupillary power (APP) is a potential alternative to simK values. The APP is the average of all the keratometry values within the photopic or mesopic pupil or at a selectable diameter, whereas the simK value is an average of 2 orthogonal keratometry values. In postrefractive surgery cases, the effective central corneal power (ECCP), developed by Jack Holladay MD, corrects traditional keratometry values by using the central mean 4.5 mm corneal refractive power and data from the unchanged corneal periphery to estimate the amount of refractive correction. The ECCP avoids the keratometric refractive index error in cases of post-myopic refractive surgery cases. In post-myopic ablation cases, the average simK values are too high leading to selection of an IOL power that results in a hyperopic outcome. In post-hyperopic ablation cases, the simK values are too low resulting in a myopic outcome after IOL implantation. Hence if conventional IOL calculation formulas are used, the ECCP can be a more appropriate keratometry value for IOL selection in postrefractive surgery cases. ECCP should not be used in post-refractive surgery formulas such as

**Table 23.1** OPD-Scan III metrics used for intraocular lens selection and centration

Haigis L, Shammas PL, or Barrett True K as the keratometric index error is internally compensated for in these formulas and using ECCP would result in double compensation.

The advent of premium IOLs has resulted in stringent tolerances for centration and alignment. To address these criteria, the OPD-Scan III includes multiple IOL centration landmarks (shown on multiples map and images) including the photopic and mesopic angle kappa (Chord μ), angle alpha, and the photopic and mesopic line of sight (pupil center) (Table 23.1). Centration landmarks can assist with IOL selection. For example, patients with photopic angle kappa (Chord μ) values greater than 0.5 mm may be poor candidates for some multifocal IOL implants. Some have advocated angle alpha (LDist value on the OPD-Scan III, Table 23.1) as a better predictor of postoperative IOL centration.

Other indices and displays for IOL selection criteria include corneal spherical aberration, Corneal Diameter, pupillometry, and pupillography. For example, patients with small physiologic pupils or misshaped pupils may not be candidates for premium presbyopic IOLs. Corneal spherical aberration is routinely used to select the appropriate aspheric IOLs for implantation. In posthyperopic ablation cases, the increased negative spherical aberration generally indicates a spherical monofocal is more appropriate as implantation of an IOL with negative spherical aberration increases the overall magnitude of spherical aberration resulting in visual degradation akin to keratoconic corneas.