[Characteristics and correlative factors of posterior corneal surface changes after laser in situ keratomileusis]

To assess the changes of the posterior corneal surface and the factors influencing these changes after laser in situ keratomileusis (LASIK). These factors included the preoperative refraction, preoperative intraocular pressure (IOP), preoperative corneal thickness, spherical equivalent correction and one-week postoperative corneal thickness.

The posterior corneal radius of curvature for the posterior best fit sphere (PBFS), irregularity of posterior surface, and anterior chamber depth (ACD) were obtained with the scanning slit topography system before and 1 week, 1 month and 3 months after LASIK in 64 eyes of 34 patients with myopic refractive errors of -2.25 to -12.5 diopters. Multiple regression analysis was used to assess the factors that affecting these changes of posterior corneal surface. The difference of radius PBFS and posterior elevation difference (PED) was used as an indicator for the forward shift of posterior corneal surface.

The ACD before the surgery and 1 week, 1 month and 3 months after the surgery was (3.2448 +/- 0.2806), (3.2770 +/- 0.2810), (3.2579 +/- 0.2932) and (3.2651 +/- 0.2875) mm, respectively. The PBFS at that time was (6.5095 +/- 0.2177), (6.3731 +/- 0.2127), (6.4257 +/- 0.2358) and (6.4354 +/- 0.2266) mm, respectively. The posterior corneal irregularity at 5 diameters zone was 0.622 +/- 0.142, 0.978 +/- 0.244, 0.884 +/- 0.207 and 0.881 +/- 0.174, respectively The preoperative posterior corneal radius of curvature for PBFS, the irregularity of the posterior surface and the ACD were significantly different from that of postoperative data at each time point (P < 0.01). There was no significant difference in these indexes between 1 month and 3 months postoperatively. Multiple regression analysis showed that factors relevant to the changes of posterior corneal surface were the corrected preoperative refraction (regression coefficient r = 0.0173, P = 0.005) and 1 week postoperative corneal thickness (mm) (regression coefficient r = -1.495, P = 0.001).

The posterior corneal surface after LASIK become steeper and more irregular and remains stable in three months after the surgery. Eyes with thinner corneas and higher myopia requiring more ablation are more predisposed to the changes of posterior corneal surface. The results of the present study should not be used in extremely high degree myopia. A study using a larger group of patients with a longer observation period is required.