Residual bed thickness and corneal forward shift after laser in situ keratomileusis

Purpose: To prospectively assess the forward shift of the cornea after laser in situ keratomileusis (LASIK) in relation to the residual corneal bed thickness. Setting. Miyata Eye Hospital, Miyazaki, Japan. Methods: Laser in situ keratomileusis was performed in 164 eyes of 85 patients with a mean myopic refractive error of -5.6 diopters (D) +/-2.8 (SD) (range -1.25 to -14.5 D). Corneal topography of the posterior corneal surface was obtained using a scanning-slit topography system before and 1 month after surgery. Similar measurements were performed in 20 eyes of 10 normal subjects at an interval of 1 month. The amount of anteroposterior movement of the posterior corneal surface was determined. Multiple regression analysis was used to assess the factors that affected the forward shift of the corneal back surface. Results: The mean residual corneal bed thickness after laser ablation was 388.0+/-35.9 mum (range 308 to 489 mum). After surgery, the posterior corneal surface showed a mean forward shift of 46.4+/-27.9 mum, which was significantly larger than the absolute difference of 2 measurements obtained in normal subjects, 2.6+/-5.7 mum (P<.0001, Student t test). Variables relevant to the forward shift of the corneal posterior surface were, in order of magnitude of influence, the amount of laser ablation (partial regression coefficient B=0.736, P<.0001) and the preoperative corneal thickness (B=-0.198, P<.0001). The residual corneal bed thickness was not relevant to the forward shift of the cornea. Conclusions: Even if a residual corneal bed of 300 mu m or thicker is preserved, anterior bulging of the cornea after LASIK can occur. Eyes with thin corneas and high myopia requiring greater laser ablation are more predisposed to an anterior shift of the cornea.