Central corneal thickness alterations may cause residual refractive errors following laser in situ keratomileusis (LASIK). This study reports associations between central corneal thickness alterations and residual refractive error following uncomplicated LASIK. Ninety-one myopic patients with a mean refractive correction of -3.91+/-3.2 DS / -0.66+/-0.3 DC were evaluated. Central corneal thickness was measured prior to, during and following surgery and 2 months later using ultrasound pachometry Results indicate increased tissue removal (94+/-33 microm; mean +/- SD) compared to the nominal Nidek value (52+/-24 microm, P<0.001). Twenty-four hours later the tissue removal was 46+/-27 microm. There was no association between altered central corneal thickness and ablation depth (r = 0.058, P = 0.454). Central corneal thickness change was inversely proportional to residual refractive error (r = -0.364, P<0.01). Increased tissue removal may occur due to rapid stromal dehydration. Central corneal thickness changes between 24 h, and 2 months after surgery were constant over a range of ablation depths, which may partly explain the stability of LASIK procedures over a range of corrections.

For a constant laser energy output, lower water content materials ablated to a greater extent than higher water content materials. This model provides a simple way to assess the effect of water content and dehydration on myopic laser in situ keratomileusis.

BACKGROUND: The predictability of LASIK refractive surgery is important both to the patient and the surgeon in obtaining an optimal result and in reducing the need for enhancement surgery. Some instruments, large ablation zones and possibly other variables may increase hyperopic overcorrection. METHODS: A retrospective study was undertaken of 345 myopic LASIK eyes (175 patients) treated with a Nidek EC-5000. The need for additional surgery in the form of an enhancement was determined after the patient had a stable refraction. The variables measured in the study were the patient's refractive correction, corneal curvature using an Alcon EH-290 topographer, the patient's age and Nidek excimer laser ablation optic and transition zone size. The same nomogram was used for all eyes and where possible bilateral surgery was conducted on all patients. The effect of ablation sizes, refractive errors, patient age and corneal curvature on the enhancement surgeries was evaluated using SPSS 6.0. RESULTS: The most significant variable that precipitated a LASIK enhancement was an optic zone of 6.5 mm with a transition zone of 7.5 mm (paired t-test, p < 0.0025). Multivariate analysis indicates that the older the patient and the larger the refractive error, the greater the risk of not achieving a residual refractive error of +/- 0.50 D at three months. The steeper pre-operative corneas have a greater chance of enhancement (mean of sample 44.48 +/- 1.47 D and mean of enhancements 45.30 +/- 1.65 D, p = 0.01, independent sample test). The smaller optic zone was associated with a smaller refractive over-refraction after LASIK surgery (mean for 5.5 mm optic zone, +0.71 +/- 0.29 D; mean, for 6.5 mm optic +1.27 +/- 0.50 D, paired t-test p < 0.0001). CONCLUSIONS: In this study, the 5.5 mm optic zone appears to dampen or reduce the undesirable refractive results. The optic zone size of choice with the Nidek EC-5000 laser should be 5.5 mm if the patient has small pupils and steep corneas. In this sample, the targeted emmetropic refractive state (range plano to +0.50 D sphere and plano to -0.50 D cylinder) was achieved in 93.3 per cent of cases after three months.