the incision, usually into the capsular bag within the posterior chamber (in-the-bag implantation). sometimes, a sulcus implantation—in front or on top of the capsular bag but behind the iris—may be required because of posterior capsular tears or zonulodialysis. this requires a modification of refractive power because of the more-anterior placement on the optical axis. other designs of multifocal intraocular lens that focus light from distant and near objects, working with similar effect to bifocal or trifocal eyeglasses, are available. preoperative patient selection and good counselling is necessary to avoid unrealistic expectations and post-operative patient dissatisfaction, and possibly a requirement to replace the lens. acceptability of these lenses has improved and studies have shown good results in patients selected for expected compatibility. one model of accommodating lens has two hinged struts on opposite edges that displace the lens along the optical axis when an inward transverse force is applied to the haptic loops at the outer ends of the struts—the components transferring the movement of the contact points to the device—and it recoils when the force is reduced. it is implanted in the eye's lens capsule, where the contractions of the ciliary body, which would focus the eye with the natural lens, are used to focus the implant. iols used in correcting astigmatism have different curvature on two orthogonal axes, as on the surface of a torus, and are called toric lenses. the staar surgical intraocular lens was the first-such lens developed in the united states; it may correct up to 3.5 dioptres. alcon created a different model of toric lens that may correct up to 3 dioptres of astigmatism. to achieve the most benefit from a toric lens, the surgeon must place the lens with the axes of curvature to suit the patient's astigmatism. intraoperative aberrometry can be used to assist the surgeon in toric lens placement and minimize astigmatic errors. monofocal iols provide accurately focused vision at one distance only; far, intermediate, or near. patients who are fitted with these lenses may need to wear glasses or contact lenses while reading or using a computer. these lenses usually have uniform spherical curvature. cataract surgery may be performed to correct vision problems in both eyes; if both eyes are suitable, patients are usually advised to consider monovision. this procedure involves inserting into one eye an iol that provides near vision and one that provides distance vision in the other. although most patients can adjust to having monofocal iols with differing focal length, some cannot compensate and may experience blurred vision at both near and far distances. an iol optimised for distance vision may be combined with an iol that optimises intermediate vision instead of near vision as a variation of monovision. the first aspheric iols were developed in 2004; these have a flatter periphery than the middle of the lens, improving contrast sensitivity. the effectiveness of aspheric iols depends on a range of conditions and they may not always provide significant benefit. some iols absorb ultraviolet and high-energy blue light. the natural crystalline lens of the eye filters these potentially harmful frequencies. a cochrane review of 2018 found there is unlikely to be a significant difference in distance vision between blue-filtering and plain lenses, and was unable to identify a difference in contrast sensitivity or colour discrimination. the light-adjustable iol was approved by the u.s. food and drug administration (fda) in 2017. this type of iol is implanted in the eye and then treated with ultraviolet light to alter the curvature of the lens. in some cases, it may be necessary or desirable to insert an additional lens over the already implanted one, also in the posterior capsule. this type of iol placement is called "piggyback" iols and is usually considered when the visual outcome of the first implant is not optimal. in such cases, implanting another iol over the existing one is considered safer than replacing the initial lens. this approach may also be used in patients who need high degrees of vision correction. the appropriate refractive power of the iol must be selected, much like a spectacle lens prescription, to provide the desired refractive outcome. preoperative measurements, including corneal curvature, axial length, and white-to-white measurements are used to estimate the required power of the iol. these methods include several formulae, including hagis, hoffer q, holladay 1, holladay 2, and srk/t. free online calculators use similar input data. a history of lasik surgery requires different calculations to take this into account. refractive results using power calculation formulae based on pre-operative biometrics leave patients within 0.5 dioptres of target (correlates to visual acuity of 6/7.5 (20/25) when targeted for distance) in 55% of cases and within one dioptre (correlates to 6/12 (20/40) when targeted for distance) in 85% of cases. developments in intraoperative wavefront technology have demonstrated power calculations that provide improved outcomes, yielding 80% of patients within 0.5 dioptres (7/7.5 (20/25) or better). cost is an important aspect of these lenses. although medicare covers the cost of monofocal iols in the united states, patients will have to pay the price difference if they choose more expensive lenses. preoperative evaluation an eye examination or pre-operative evaluation by an eye surgeon is necessary to confirm the presence of a cataract and to determine the patient's suitability for surgery. the patient must fulfil certain requirements: the degree of reduction of vision due largely to the cataract should be evaluated. while the existence of other sight-threatening diseases, such as age-related macular degeneration or glaucoma, does not preclude cataract surgery, less improvement may be expected in their presence. the eyes should have a normal pressure, or any glaucoma should be adequately controlled with medications. in cases of uncontrolled glaucoma, a combined cataract-glaucoma procedure (phaco-trabeculectomy) can be planned and performed. the pupil should be adequately dilated using eyedrops; if pharmacologic pupil dilation is insufficient, procedures for mechanical pupil dilatation may be needed during the surgery. patients with retinal detachment may be scheduled for a combined vitreo-retinal procedure, along with pciol implantation. patients taking tamsulosin (flomax), a common drug for enlarged prostate, are prone to developing a surgical complication known as intraoperative floppy iris syndrome (ifis), which must be correctly managed to avoid the complication posterior capsule rupture; prospective studies have shown the risk is greatly reduced if the surgeon is informed of the patient's history with the drug, and has appropriate alternative techniques prepared. a cochrane review of three randomized clinical trials, including over 21,500 cataract surgeries, examined whether routine preoperative medical testing resulted in a reduction of adverse events during surgery. results showed performing preoperative medical testing did not result in a reduction of risk of intraoperative or postoperative medical adverse events, compared to surgeries with no or limited preoperative testing. operation procedures antibiotics may be administered pre-operatively, intra-operatively, or post-operatively. frequently, a topical corticosteroid or nonsteroidal anti-inflammatory drug (nsaid) is used in combination with topical antibiotics post-operatively. most cataract operations are performed under local anaesthetic, allowing the patient to go home the same day. the use of an eye patch may be indicated, usually for some hours and while sleeping, after which the patient is instructed to use anti-inflammatory eyedrops to control inflammation, and antibiotic eyedrops to prevent infection. lens and cataract procedures are commonly performed in an outpatient setting; in the united states, 99.9% of lens and cataract procedures in 2012 were done in an outpatient setting. occasionally, a peripheral iridectomy may be performed to minimize the risk of pupillary block glaucoma. surgical iridectomy, an opening through the iris can be fashioned manually or with a neodymium-doped yttrium aluminium garnet (nd:yag) laser. laser peripheral iridotomy may be performed either before or following cataract surgery. the iridectomy hole is larger when done manually than when performed with a laser. when the manual surgical procedure is performed, some negative side-effects, such as the opening of the iris being visible to others, and the passage of light into the eye through the new hole, creating visual disturbance. in the case of visual disturbance, the eye and brain often learn to compensate and ignore the effects over two months. sometimes, the peripheral iris opening can heal and the hole ceases to exist. the most-commonly used procedures are phacoemulsification and manual small incision cataract surgery (msics). in either of these procedures, it can sometimes be necessary to convert to eccs to deal with a problem better managed through a larger incision. phacoemulsification main article: phacoemulsification the surgical procedure in phacoemulsification for removal of cataract is typically performed under an operating microscope. either topical, sub-tenon, peribulbar, or retrobulbar local anaesthesia is used, usually causing the patient little or no discomfort. topical anaesthetics are most commonly used; these are placed on the globe of the eye as eyedrops before surgery or in the globe during surgery. local-anaesthetic injection techniques include sub-conjunctival injections and injections behind the globe (retrobulbar block) to block regional nerves and prevent eye movement. intravenous sedation may be combined with the local anaesthetic. general anaesthesia and retrobulbar blocks were historically used for intracapsular cataract surgery, but local and topical anaesthesia are commonly used for small incision surgery and phacoemulsification. the operation site is prepared using disinfection; this includes the area around the eye and the rest of the face, and exposure of the eyeball using an eyelid speculum; entry into the eye is made through a minimal incision; the incision for cataract surgery has evolved along with the techniques for cataract removal and iol placement. in phacoemulsification, the size depends on the requirements for iol insertion. a more-posterior incision simplifies wound closure and decreases induced astigmatism but is more likely to damage blood vessels. with foldable iols, it is sometimes possible to use incisions smaller than 3.5 mm (0.14 in). the shape, position, and size of the incision affect the capacity for self sealing, the tendency to induce astigmatism, and the surgeon's ability to manoeuvre instruments through the opening. one or two smaller side-port incisions at 60-to-90 degrees from the main incision may be needed to access the anterior chamber with additional instruments. ophthalmic viscosurgical devices (ovds), also known as viscoelastics, are injected into the anterior chamber to support, stabilize, and protect the eyeball to help maintain eye shape and volume, and to distend the lens capsule during iol implantation. in capsulorhexis, a circular opening is made on the front surface of the lens capsule to access the lens within. in phacoemulsification, an anterior continuous curvilinear capsulorhexis is used to create a round, smooth-edged opening through which the lens nucleus can be emulsified and the intraocular lens implant inserted. the cataract's outer (cortical) layer is then separated from the capsule by a gentle, continuous flow or pulsed dose of liquid from a cannula, which is injected under the anterior capsular flap along the edge of the capsulorhexis opening. this step is called hydrodissection. in hydrodelineation, fluid is injected into the body of the lens through the cortex against the nucleus of the cataract, which separates the hardened nucleus from the softer cortex shell by flowing along the interface between them. the smaller hard nucleus can then be more-easily phacoemulsified. the posterior cortex serves as a buffer, protecting the posterior capsule membrane. the smaller size of the separated nucleus requires shallower and less-peripheral grooving, and produces smaller fragments after cracking or chopping. the posterior cortex also maintains the shape of the capsule, which reduces the risk of posterior capsule rupture. after nuclear cracking or chopping (if needed), the cataract is destroyed using ultrasound and the remaining lens cortex (outer layer of lens) material from the capsular bag is carefully aspired and in capsular polishing, the remaining epithelial cells from the capsule are removed if necessary. the folded intraocular replacement lens is implanted, usually into the remaining posterior capsule, and correct unfolding is ensured. aligning the iol in the correct axis to counteract astigmatism is necessary for a toric iol. ovds that were injected to stabilize the anterior chamber, protect the cornea from damage, and distend the cataract's capsule during iol implantation are removed from the eye to prevent post-operative viscoelastic glaucoma, a severe intra-ocular pressure increase. this is done via suction from the irrigation-aspiration instrument and replacement by buffered saline solution (bss). removal of ovds from behind the implant reduces the risk and magnitude of postoperative pressure spikes or capsular distention. in the final step,