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RD Niederkohr, C Roberts, A Mahmoud; In Vitro Model of the Biomechanics of a Hyperopic Procedure: The Effect of Trephination Depth on Corneal Curvature . Invest. Ophthalmol. Vis. Sci. 2002;43(13):156.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To investigate the effect of depth of cut in a hyperopic pattern on central curvature change in an in vitro model. Method: 21 donor globes deemed unsuitable for human transplantation were obtained from the EyeBank with a mean of 19.3 hrs post-mortem. The mean age of the donors at time of death was 60.5 years. Eyes were secured in a custom-built mount and oriented with superior rectus muscle at the 90 degree position. The globes were infused with saline via a 25-gauge syringe into the vitreous chamber to achieve firm-to-touch pressure. Baseline Orbscan II corneal topography was obtained. A 6-mm diameter Hessburg trephine was used to create circular cuts at depths of 250, 375, 500 and 625 mm. Up to four successive cuts were made in the same donor globe, and Orbscan II exams were acquired after each successive cut without changing globe alignment. Pre-procedure curvature maps were subtracted from each post-trephine cut map. The maps were divided into three zones - central, mid-peripheral, and peripheral - for analysis. A new index was developed, the Flat Zone Location Magnitude Index (FLMI), which locates a potentially decentered flat circular zone on the difference map between post-cut minus baseline. The FLMI subtracts the area corrected average dioptric value outside the 2mm radius circle from the area corrected average dioptric value inside the circle. A negative value indicates that the difference map has flattened in the center and steepened in the periphery. Results: The FLMI was significantly different from the baseline value (p<0.05) at all cut depths. At 250 microns, FLMI was -2.003 ± 0.931 (p=0.0005). At 375 microns, FLMI was -1.848 ± 1.879 (p=0.0085). At 500 microns, FLMI was -2.802 ± 2.294 (p=0.0106). At 625 microns, FLMI was -3.519 ± 4.003 (p=0.0154). There was a trend toward increasing magnitude of FLMI with increasing depth of cut, indicating an increased dioptric difference between central and peripheral zones. Conclusion: Severing of corneal lamellae with a trephine produced central flattening and peripheral steepening without removal of tissue, which increased with increasing depth of cut. This demonstrates a biomechanical response to a pattern of lamellar cut that is analogous to a hyperopic procedure. This event may play an important role in refractive outcome and is not accounted for in current ablative algorithms.
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