Direct measurement of microkeratome gap width by electron microscopy

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Abstract
Purpose: To perform an accurate direct measurement of the microkeratome gap width using scanning electron microscopy (SEM). Setting: Electron Microscope Unit, University of Hong Kong, Hong Kong, China. Methods: The Cambridge Stereoscan S440 scanning electron microscope was used to measure the gap width of 4 SCMD microkeratomes with high accuracy (±1.5 μm). Results: The manufacturer's gap specification for the 4 microkeratomes was 150.0 μm. The gap width measurements using SEM were 164.7 μm, 190.0 μm, 200.6 μm, and 145.9 μm and the respective errors, 9.8%, 26.7%, 33.7%, and 2.7%. Two of the 4 microkeratomes had more than a 25% error in gap width from the specification. Conclusions: The great variation in gap width from the manufacturer's specification for the 4 SCMD microkeratomes was beyond the standard of tolerance normally accepted in laser in situ keratomileusis (LASIK). Many unexpected LASIK-related keratectasia and corneal perforations may be related to substandard microkeratome manufacturing and calibration. All new microkeratomes and blades should be validated before use to avoid keratectasia and other flap problems in LASIK.

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