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LASIK Hinge Placement Can Make a Difference to Procedure's Success: Presented at ARVO

By Mike Fillon

FORT LAUDERDALE, FL -- April 26, 2004 -- Creation of either a superior-nasal or a superior-temporal hinged flap with a microkeratome induces peripheral steepening during laser in situ keratomileusis (LASIK), consistent with a biomechanical model of corneal response.

"This known pattern of response may be important in planning customized ablations and predicting outcomes in the next generation of ablation profile development," said lead researcher Cynthia Roberts, PhD, department of ophthalmology and biomedical engineering, Ohio State University, Columbus, Ohio.

Results of the study were reported here April 25th in a poster presentation at the Association for Research in Vision and Ophthalmology Annual Meeting.

In the study, a total of 25 eyes in 15 patients scheduled for LASIK surgery were recruited at the Cornea and Refractive Surgery Unit, Instituto de Microcirugía Ocular, Barcelona, Spain. The Moria M2 130 microkeratome (Microtech, France) was used to create either a superior-nasal hinge in the right eye or a superior-temporal hinge in the left eye. Corneal thickness was measured using an ultrasound DGH pachymeter in the center and at 4 cardinal points–superior, inferior, nasal, and temporal–placed at 3 mm from the center, both before and after the cut onto corneal stroma. The flap thickness of each sector of the cornea was calculated by subtracting the average of the postflap corneal thickness from the average of the preflap corneal thickness.

The researchers also used the same microkeratome to create superior nasal hinges in the right eyes and superior-temporal hinges in the contralateral eyes in another 10 eyes of 5 patients. Using the Keratron Scout, corneal topography was acquired prior to ablation both before and immediately after the flaps were created. The topographic exam results were exported to custom software for analysis. A Zernike decomposition was performed on the topographic elevation data. The surface was iteratively reconstructed, and the curvature map calculated.

When analyzed as a group, the creation of the LASIK flap produced a significant increase in Z7, the vertical coma term (P <.0187), indicative of the superior location of both hinge positions. No other higher order terms were affected. A small but significant (P <.0251) decrease in the magnitude of theZ4 or second order defocus term was also observed, which is indicative of central flattening or a hyperopic shift, Dr. Roberts said.

"In comparison between hinge position groups, only Z8 or horizontal coma differed between groups," said Dr. Roberts. "This is indicative of the temporal-nasal difference in hinge position between eyes. No other terms showed a significant difference. Tangential topography maps demonstrated a pattern of inferior steepening indicative of oblique coma oriented as a function of hinge position."


[Study title: The Influence of Hinge Placement on the Corneal Shape Response to the Creation of a Flap in LASIK. Abstract B181.]

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