Biomechanical Effects of Femtosecond and Microkeratome-based Flap Creation: Prospective Contralateral Examination of Two Patients

PURPOSE: To examine flap morphology, corneal topography, and aberrometry after flap creation with a femtosecond laser in one eye and a mechanical microkeratome in the fellow eye. Comparative outcomes after subsequent refractive laser correction were also investigated. METHODS: Two patients (4 eyes) were enrolled in a clinical study in which LASIK flaps were created using the Intralase laser set at 90 or 100 μm in one eye and the Moria M2 microkeratome with a 90-μm head in the fellow eye. Pre- and postoperative flap data at 1 day, 1 week, and 1 month were collected before lifting the flap to perform a customized laser ablation (Alcon LADARVision4000 CustomCornea System). Postoperative laser data were collected at 1 week, 1 month, 3 and 6 months, and 1 year. Visual acuity, pachymetry, high-frequency flap ultrasound profilometry (Artemis Ultralink), topography, aberrometry (LADARWave), and refraction were examined and compared between eyes. RESULTS: Postoperative flap comparisons showed a hyperopic refractive shift in all four eyes with a shift of up to 1.00 diopter (D) in the microkeratome eyes and <0.50 D in the Intralase eyes. Corneal topography difference maps showed peripheral steepening that was more pronounced in the microkeratome versus Intralase eyes (2.00 D vs 0.50 to 1.00 D, respectively). Spherical aberration by wavefront variably increased by 50% to 100% in the microkeratome eyes and remained virtually unchanged in the Intralase eyes. At 3-month postoperative laser treatment, visual acuity was 20/15 for both eyes in patient 1 and 20/15 in the right eye (IntraLase) and 20/25 in the left eye (microkeratome) in patient 2. Aberrometry revealed a two-fold greater level of higher order aberrations in microkeratome eyes than in IntraLase eyes throughout the postoperative laser period. CONCLUSIONS: Femtosecond laser and mechanical methods of flap creation lead to subtle biomechanically induced aberrations, which appear more prominent with the mechanical Moria M2 microkeratome in this small case study. Further study with a larger patient population is warranted to determine whether these differences are statistically significant.