Differences in the corneal biomechanical effects of surface ablation compared with laser in situ keratomileusis using a microkeratome or femtosecond laser

PURPOSE: To compare the effects of different flap creation techniques on the biomechanical properties of the cornea in patients having myopic laser refractive surgery. SETTING: UCLA Laser Refractive Center of the Jules Stein Eye Institute, Los Angeles, California, USA. METHODS: In this retrospective case series, eyes that had myopic laser refractive surgery were categorized according to the type of flap creation: mechanical microkeratome (MK) LASIK (n=32), femtosecond laser (FSL) LASIK (n=32), or no flap creation (PRK) (n=33). The preoperative central corneal thickness, intraoperative flap thickness, and planned ablation depth (AD),and the preoperative and postoperative manifest refraction spherical equivalent, corneal hysteresis (CH), and corneal resistance factor (CRF) were recorded. RESULTS: The mean change in CH (DeltaCH) was 2.2 mm Hg, 1.9 mm Hg, and 2.3 mm Hg in the MK, FSL, and PRK groups, respectively. There were no significant differences in AD, DeltaCH, or DeltaCRF between the 3 groups. The correlation between AD and DeltaCH was significant in all 3 groups. The correlation was strongest in the FSL group (r=0.82, P<.0001) and weaker in the PRK group (r=0.47, P= .006) and MK group (r=0.46, P= .008). CONCLUSIONS: The biomechanical measures of CH and CRF decreased similarly after PRK and LASIK using laser or mechanical flap creation. However, LASIK using femtosecond laser flap creation caused a significantly more predictable change in corneal biomechanics, which correlated strongly with AD, than the change with PRK and LASIK with microkeratome flap creation.