The effect of hinge position on corneal sensation and dry eye after LASIK

Purpose: To investigate the effect of hinge position on corneal sensation and dry eye syndrome after laser in situ keratomileusis (LASIK). Design: Prospective, randomized, self-controlled trial. Participants: Fifty-two patients greater than or equal to18 years of age undergoing bilateral LASIK. Intervention: Patients underwent bilateral LASIK with the superior-hinge Hansatome microkeratome in one eye and the nasal-hinge Amadeus microkeratome in the other eye. In all eyes, the flaps were 160 mum thick, with a diameter of 9.5 mm. Main Outcome Measures: Masked Cochet-Bonnet esthesiometry was performed centrally before surgery and at 1 week, 1 month, 3 months, and 6 months after surgery. Dry eye was evaluated at the same time intervals with lissamine green corneal and conjunctival staining, Schirmer testing with anesthesia, and tear-film breakup time. Subjective evaluation of dry eye sensation was performed at 3 and 6 months after surgery. Results: Corneal sensation was reduced in eyes with either superior- or nasal-hinge corneal flaps at 1 week, 1 month, and 3 months after surgery (P < 0.001). Compared with preoperative values, a significant reduction in corneal sensation remained at 6 months in corneas with superior-hinge flaps (P < 0.001) but not in corneas with nasal-hinge flaps (P = 0.263). Mean corneal sensation was greater in corneas with a nasal-hinge flap compared with corneas with a superior-hinge flap at all postoperative visits (P < 0.001). The loss of sensation was greatest at 1 week and showed improvement at each subsequent time interval up to 6 months. Overall, dry eye signs and symptoms were greatest during the immediate postoperative period and improved at all subsequent time intervals. Dry eye signs and symptoms were generally greatest in the eyes with a superior-hinge flap and milder in eyes with a nasal-hinge flap. Conclusions: The long posterior corneal nerves, which innervate the cornea, enter the eye at 3- and 9-o'clock. A superior-hinge flap transects both arms of the neuroplexus, whereas a nasal hinge transects only the temporal arm. LASIK results in a significant reduction in corneal sensation. Corneal sensation and dry eye signs and symptoms decreased immediately after LASIK and improved at all time periods between 1 week and 6 months in eyes with both a nasal-hinge flap and a superior-hinge flap. However, the loss of corneal sensation and presence of dry eye syndrome were greater in eyes with a superior-hinge flap than in eyes with a nasal-hinge flap.