PURPOSE. To investigate the effect of the LASIK flap hinge on corneal topography. METHOD. Corneal topography was obtained before and after LASIK surgery on 145 eyes of 79 patients. Patients were grouped according to microkeratome. In 60 eyes, a superior hinge was created with the Hansatome. In 85 eyes, the SKBM was used to produce a nasal hinge. From the difference map, data was assessed along the horizontal and vertical axis in 1 mm increments (ranging up to 3.0 mm) from the center of the ablation zone. Data points equidistant from the center of the ablation zone were compared to find asymmetry along either axis relative to the hinge location. Axial power and sagittal height data were recorded.
RESULTS. In Hansatome eyes, neither power nor sagittal height were significant in the axis perpendicular to the hinge. Parallel to the hinge plane both power and sagittal height were significant. Temporal points showed a greater flattening and less lowering compared to nasal points. In SKBM eyes, areas closest to the hinge has a higher sagittal height compared to points farthest from the hinge. Axial power at points 1 mm away from the ablation center showed significant flattening closer to the hinge. SKBM eyes were not significant at any points parallel to the hinge plane.
Results of stratification into low and high diopter groups were similar to pooled data for the low attempted corrections only.
Variations in the ablation profile were observed. Significant differences in nasal compared to temporal treated areas were found for power in both Hansatome and SKBM eyes.
CONCLUSIONS. The lamellar flap in LASIK may influence postoperative corneal topography. Results suggest that modern microkeratomes produce fairly homogeneous flaps, reducing concerns of hinge placement. However, an understanding of flap mechanics is critical in the develpoment of wavefront guided ablation systems.