In vivo confocal microscopy through-focusing to measure corneal flap thickness after laser in situ keratomileusis

Purpose: To measure flap thickness in laser in situ keratomileusis (LASIK) patients using in vivo confocal microscopy through-focusing (CMTF) and compare measured versus intended flap thickness achieved by 2 microkeratomes, the Automated Corneal Shaper(R) (ACS) (Chiron Bausch & Lomb) and the Hansatome(R) (Bausch & Lomb). Setting: Department of Ophthalmology, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, USA. Methods: Twenty-seven eyes of 27 patients were examined by in vivo CMTF 3 to 12 months after LASIK was performed with the ACS (12 patients) or Hansatome (15 patients) microkeratome. The central cornea was scanned, and the epithelium, flap, stroma, and total corneal thickness were measured. Normalized flap thickness (NFT) was also calculated to account for changes in epithelial thickness that may have occurred postoperatively. Results: The mean posterior stromal thickness was 341.1 mum +/- 53.9 (SD) (range 233 to 431 mum) in the ACS group and 320.3 +/- 42.3 mum (range 258 to 382 mum) in the Hansatome group. The mean nonnormalized flap thickness was 132.7 +/- 12.5 mum (range 11 to 151 mum) in the ACS group and 167.4 +/- 21.4 mum (range 141 to 209 mum) in the Hansatome group. The NFT was 129.6 +/- 9.5 mum and 158.4 +/- 22.1 mum, respectively. Both microkeratomes cut significantly less than intended (P < .05); however, the ACS cut a thinner-than-intended thickness in all cases, and the Hansatome cut thicker than intended in 13% of cases. The Hansatome also showed significantly greater variability in flap thickness than the ACS (P < .05). Conclusions: A significant difference in precision was noted between the 2 microkeratomes. The findings emphasize the importance of performing thickness measurements and the usefulness of in vivo CMTF in making these determinations to ensure the safety and effectiveness of LASIK (C) 2002 ASCRS and ESCRS.