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G. A. Markakis, C. J. Roberts, R. G. Lembach, C. B. Call; Effect of Depth of Cut on Central Corneal Thickness and Curvature in Human Cadaver Eyes Using a Microkeratome.. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1764.
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© ARVO (1962-2015); The Authors (2016-present)
Clinically significant differences between actual and predicted corneal topography after refractive surgery are not fully explained by conventional models used to derive ablation algorithms. Customized algorithms may be improved once the corneal biomechanical response to surgical alteration can be quantified and predicted. The purpose of this study was to examine the immediate post-operative effect of microkeratome flap incisions on corneal thickness and curvature as a function of depth of cut.
20 eyes from 10 donors (ages 39-69 years, mean=55±12 years, 5 male, 5 female) were studied. After chart review and slit lamp examination for exclusion criteria (ocular disease/pathology, postmortem time >12 hours, refrigeration), eyes were placed in a custom mount and intraocular pressure (IOP) was maintained at 10±3mmHg. Central corneal thickness (CCT) and curvature were obtained in triplicate for pre- and post-operative states. CCT and flap depth were measured by retinal OCT (averaged central 50 A-scans from each of 3 linear B-scans, length 1.13mm, refractive index correction factor=1.36/1.376). Corneal curvature was measured by the PAR CTS with 1 drop 0.25%/0.4% fluorescein-benoxinate, and maps were constructed using 4mm, 6mm, and 8mm zone best-fit spheres (BFS). Microkeratome flaps were cut using an ALK microkeratome with settings of 160µm or 450µm in fellow eye pairs. Order was randomized. Statistical comparisons between pre- and post-operative CCT and between cut depths in fellow eyes were performed using paired and unpaired T tests (2 tail, p<0.05), respectively. Curvature changes for BFS between pre- and post-operative states for a given cut were analyzed with ANOVA (2 factor, replication, p<0.05).
Measured flap depths were: 149±38µm and 405±71µm (difference= 256±86µm, p=4.63x10-9). Postoperative CCT increased by 20±21µm for the "160µm" cut (p=0.016), but was unchanged for the "450µm" cut. Post-operative BFS curvature for the "450µm" cut steadily increased towards the periphery (4mm to 6mm to 8mm fits, p=0.041), to a maximum increase of 1.87±2.19D at 8mm (p=0.024), indicating peripheral steepening. No changes in BFS were observed with the "160µm" cut (p=0.301). IOP was constant throughout the experiment.
Microkeratome flaps create immediate changes in corneal thickness and curvature that are depth dependant.
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