Changes in the posterior cornea after laser in situ keratomileusis and photorefractive keratectomy

PURPOSE: To study the changes in posterior corneal elevation after laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) using Scheimpflug topography with the Pentacam anterior segment imaging system (Oculus, Inc.). SETTING: Department of Ophthalmology, Albany Medical Center, and a private clinical practice, Albany, New York, USA. METHODS: In this prospective study, 121 consecutive myopic eyes (103 LASIK and 18 PRK) were evaluated preoperatively and postoperatively with the Pentacam to determine elevation changes in the posterior corneal surface. Changes in posterior elevation were calculated by comparing the best-fit sphere preoperatively and postoperatively to a fixed reference sphere using the central 9.0 mm preoperative cornea. Statistical and graphic analyses were performed. RESULTS: The 103 LASIK eyes had a mean correction of -3.76 diopters (D) and a mean ablation depth of 62.1 mu m. The mean estimated residual bed thickness (RBT) (329 mu m) demonstrated a mean posterior displacement of 2.64 +/- 4.95 mu m. The 18 PRK eyes had a mean correction of -2.69 D and a mean ablation depth of 53.2 mu m. The mean estimated RBT (464 mu m) had a mean posterior displacement of -0.88 +/- 4.64 mu m. The difference in the mean posterior corneal displacement between the LASIK and the PRK eyes was not statistically significant (P >.05, Student t test). CONCLUSIONS: There was no statistically significant difference in posterior corneal displacement between the LASIK and PRK patients. The changes in PRK and LASIK eyes appeared to be within acceptable measurement variation. Contrary to previous reports, ectatic changes to the posterior corneal surface did not routinely occur after LASIK surgery.