Biomechanics of LASIK Flap and SMILE Cap: A Prospective, Clinical Study

PURPOSE: To analyze the acute effect of flap cut in laser in situ keratomileusis (LASIK) eyes and cap cut in small incision lenticule extraction (SMILE) eyes on corneal biomechanical properties of patients undergoing surgery. METHODS: This was a prospective, interventional, longitudinal case series. Forty-eight eyes of 24 patients underwent contralateral LASIK and SMILE. Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) measurements were performed preoperatively, intraoperatively, and 1 week and 1 month after surgery. In LASIK eyes, the flap was cut but not lifted before intraoperative measurements. In SMILE eyes, the cap and side cut incision were made before intraoperative measurement. Thirty biomechanical variables were analyzed, assuming multiple comparisons. RESULTS: In LASIK and SMILE eyes, 36.7% and 13.3% of the total number of variables detected biomechanical weakening after flap and cap cuts ( P = .02), respectively. Further, 13.3% and 40% of the total variables detected no biomechanical changes after flap and cap cut, respectively ( P = .03). These acute biomechanical effects of flap and cap cuts did not influence 1-week and 1-month measurements ( P > .05) because both LASIK and SMILE eyes showed similar biomechanical weakening. CONCLUSIONS: Flap and cap cuts induced biomechanical weakening in patient corneas. The flap caused more weakening than the cap intraoperatively. However, biomechanical differences between LASIK and SMILE eyes were similar after removal of tissue and ongoing wound healing. [ J Refract Surg . 2019;35(5):324–332.]