Confocal microscopy evaluation of stromal ablation depth after myopic laser in situ keratomileusis and photorefractive keratectomy

Abstract

Purpose: To compare the measured ablation depth after myopic laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) with the predicted ablation depth. Setting: Mayo Clinic, Rochester, Minnesota, USA. Methods: Twenty-five eyes of 15 patients had PRK and 25 eyes of 15 patients had LASIK to correct refractive errors between −1.50 diopters (D) and −11.00 D. The corneas were examined by in vivo confocal microscopy before and 1 month after both procedures. Thickness measurements were obtained from digital-image analysis of confocal scans. The measured ablation depth, an estimate of the actual photoablation depth, was obtained as the surgically induced stromal thinning between the preoperative and the 1-month post-PRK or post-LASIK central stromal thickness. The predicted ablation depth was recorded from the laser's software program. Results: In LASIK, the measured ablation depth (81 μm ± 34 [SD]) was 25% greater than the predicted ablation depth (65 ± 13 μm, P = .007) and the difference between the measured and predicted ablation depths was positively associated with the mean ablation depth ( r = 0.81, P<.001). In PRK, there was no difference between the measured ablation depth (48 ± 19 μm) and the predicted ablation depth (47 ± 18 μm, P = .84). Conclusion: Significantly more tissue than predicted was removed by LASIK than by PRK excimer photoablation with the laser system used in this study.