 rigid, closed-system eye model. A closed system can be pressurized—an open system cannot. Although the pressure response differs between the two systems,both experimental methods generate measurements that help characterize flow rates during vitrectomy.Another important difference to note is the composition of aspirated materials. The heterogenous properties of vitreous drastically differ from BSS irrigating solution.Vitreous is a semisolid mixture of water, collagen fibers, and hyaluronic acid. The complex nature of vitreousis unpredictable during vitrectomy and can sometimes generate a high standard deviation due to vitreous flow obstructions. Clear fluid flow tests were repeatable, predictable, and more accurately reflect Poiseuille’s law of flow. Despite differences in the composition of aspirated fluids, a comparison between clear fluid and vitreous helps characterize vitrectomy flow behavior.
In summary, the 50% duty cycle mode generated an increase in vitreous flow at high cut rates suggesting that high cut rates generate less flow resistance.Constant vitreous flow in the core duty cycle mode across all cut rates implies that the high flow effects of the core duty cycle are offset by the increased resistance to vitreous flow associated with larger bite sizes.In the shave mode, vitreous flow increased at high cut rates as the frequency of constant-volume bites increased with high cut rates. Overall, the operation of23-gauge UltraVit probes at high cut rates yielded reduced resistance to flow at high cut rates and more effective aspiration of the vitreous body than lower cut rates.
The purpose of the second experiment was to determine the effects of cut rate on peak flow velocity and peak acceleration of clear fluid and microbeads through25+-gauge dual-pneumatic cutters.
A Constellation Vision System was used to test an UltraVit probe in a closed-system, test chamber. Flow was evaluated at 5000 cpm, 650 mm Hg vacuum and at2500 cpm, 575 mm Hg vacuum to achieve equivalent flow rates of approximately 7 cc/min. All testing was performed under 30 mm Hg infusion pressure and 50%duty cycle.
The test chamber was designed to simulate the closed system of a compliant human eye while allowing for visualization of fluid entering the probe port. The test chamber was a 1x1x2-inch open box made of clear acrylic. A square 1.5-inch silicone sheet of 0.005-inch thickness with a reading of 55A durometer was used to cover and seal the box (McMaster-Carr, Santa FeSprings, CA). The silicone sheet was secured over the top of the acrylic test chamber using a metal clamping mechanism (Figure 12). The flexible properties of the silicone cover simulated the dampening effect of compliants clera. The test chamber was filled with aqueous solution and 75 μm micro beads were injected to help visualize fluid motion during aspiration.
To replicate fluid input and output during vitrectomy,the test chamber was modified to incorporate to two trocar cannula entry ports—one for BSS fluid infusion and a second port for the vitrectomy probe. A 0.02-inch diameter hole was drilled in the side wall of the test chamber. A 25+-gauge trocar cannula (Alcon Laboratories Inc.) was secured in the drilled hole using Loctite 4014 (Henkel, Berkeley, CA). A 25+ infusion cannula(Alcon Laboratories Inc.) was inserted into the25+-gauge trocar cannula and supplied BSS infusion to the test chamber. A second port was created by inserting a 25+-gauge trocar cannula into the silicone cover using the 25+-gauge trocar blade. The vitrectomy probeto be tested was secured vertically over the test chamber using a three-screw adjustable ring mount (EdmundOptics, Barrington, NJ). The probe tip was inserted into the test chamber through the second entry port of the silicone cover and centered in the test chamber.
Two different camera perspectives were required for3-D motion analysis. Two Phantom high-speed cameras(Vision Research, Wayne, NJ) were oriented orthogonally around the test chamber. Each camera was secured to an Edmund Optics X-Y-Z axis metric stage for precise position adjustment. A Computar 10X magnification lens was fastened to each camera and focused on the vitrectomy probe port. Two fluorescent back light panels illuminated the test chamber.
Both cameras were tethered to one another in a master and slave configuration to synchronize recording.An external pickle switch was connected to both cameras in order to manually trigger video recording at the same time. Both cameras were directly connected to a personal computer with an Ethernet cable. High-speed video was recorded at 1000 frames per second. The video resolution for each camera was 640x480 pixels.Brightness, contrast, and exposure time was adjusted to clearly define micro beads around the probe port.
Prior to high-speed recording, the appropriate Constellation Vision System settings were chosen. An infusion pressure of 30 mm Hg was applied to the test chamber. Video capture was triggered with the pickles witch after the cutter was engaged and fluid flow stabilized. Recording was stopped after approximately20 seconds of recording and each camera saved an audio video interleave (AVI) file.
Prior to video analysis, 3-D measurements of TEMAMotion Software (Photo-Sonics Inc, Burbank, CA) were calibrated. Relative camera orientations were manually entered into the program. Six points on a calibration target visible from both cameras were identified. The software translated 2-D measurements from each camera into 3-D motion. During analysis, AVI video files ofeach camera perspective were imported into the TEMA software program. Each file was synchronized and viewed simultaneously in the analysis window.Individual microbeads were manually selected from each perspective and the software tracked microbead motion into the vitrectomy probe port frame-by-frame.Velocity and acceleration data was exported from the program as a Microsoft Excel file. Peak velocity andpeak acceleration values of five tracked microbeads were averaged for each cut rate. All data are presented as mean ± standard deviation. Velocities and accelerations were compared using a two-tailed, Student’s t-test with statistical significance of P<.05.
Figure 10 and 11 depict peak velocity and peak accelerationgraphs of microbeads entering a 25+-gauge UltraVit cutter at 2500 cpm and 5000 cpm, 50% duty cycle. Each data point on Figure 10 illustrates the average peak velocity of 5 microbeads during aspirationinto the vitrectomy probe port. Each data point inFigure 11 illustrates the average peak acceleration offive microbeads. The time axis of Figures 10 and 11 canrelate to microbead distance from the port and rangefrom 0.1 to 0.25 for clear illustration of velocity andacceleration profiles. Error bars represent 95% confidenceintervals.
At 2500 cpm, the average peak velocity ofmicrobeads increased as the beads approached theport. Just immediately before entering the probeport, the average maximum peak velocity was 52.98± 11.89 mm/s. With 5000 cpm cut rate and vacuumpressure of 650 mm Hg, the average peak velocity alsoincreased with time. The average maximum peak velocitybefore aspiration into the probe port was 42.06± 10.51 mm/s. Statistical evidence suggests that peakvelocities during aspiration at 2500 cpm were significantlyhigher than 5000 cpm (P<.05). On average, peakvelocities occurring at 2500 cpm were approximately24.28% faster than 5000 cpm peak velocities.
For 2500 and 5000 cpm cut rates, the average peakacceleration increased as the beads approached the port.Maximum peak acceleration of microbeads reached5442.46 ± 1479.84 mm/s2 prior to entering the probeport. At 5000 cpm, peak accelerations reached an average value of 2909.88 ± 538.82 mm/s2. Statistical evidence suggeststhat aspiration at 2500 cpm generated significantlyhigher average peak accelerations than beads aspirated at5000 cpm (P<.05). Average acceleration for 2500 cpm cutspeeds generated approximately 34.80% higher peak accelerationvalues than 5000 cpm cut speeds.
Aspiration with the 25+-gauge UltraVit probe demonstratedan increase in velocity and accelerationas beads approached the port for both 2500 and 5000cpm. The 2500 cpm cut rate generated faster peak flowvelocities and higher peak accelerations than 5000 cpm.
This study demonstrated the effects of cut rateon peak velocity and peak acceleration of fluid in aclosed-system test chamber. We evaluated the motionof 5 microbeads into an UltraVit 25+-gauge probeat 2500 cpm, 575 mm Hg vacuum and 5000 cpm,650 mm Hg vacuum. All testing was performed under50% duty cycle and 30 mm Hg infusion.
During a cut cycle, the velocity of fluid increases anddecreases according to port open and close times. Whenthe port is fully open, fluid reaches peak velocity. Thisvelocity can be directly related to flow using a basicprinciple of volume flow rate: Flow rate = orifice area xfluid velocity. Reduced peak velocity at high cut ratestranslates into reduced clear fluid flow at high cut rates.Under surgical conditions, reduced peak flow velocitiesand flow rates of BSS suggests that operation at 5000cpm may reduce pulsatile disturbance of tissue and fluidin the eye. The data shows fluid and microbeads aspiratedat 2500 cpm approached the probe port with 24.28%faster peak velocity than 5000 cpm.
According to Newton’s second law relating accelerationand inertial mass to force (Force = mass x acceleration),lower acceleration leads to reduced forces on fluidand tissue around the port. As high cut rate yields loweraspiration forces at the probe port, surgeons can preciselyoperate closer to the surface of tissue and membranes,reduce vitreoret