Changes in the refractive index of the human corneal stroma during laser in situ keratomileusis:

Purpose

To compare the refractive index and calculated stromal bed hydration lifting the flap before and after photoablation during uneventful laser in situ keratomileusis (LASIK) using a microkeratome (M2, Moria) or a femtosecond laser (IntraLase, IntraLase Corp.) to create the flap.

Setting

Vissum-Instituto de Oftalmológico de Alicante, Alicante, Spain.

Methods

Uneventful LASIK was performed in 76 eyes of 49 patients. Flaps were created using a microkeratome (57 eyes) or a femtosecond laser (19 eyes). On lifting the flap, the refractive index of the stroma was measured using a customized manual Abbé refractometer. The measurement was repeated immediately after photoablation. Treatment time was noted. Hydration was calculated using the Fatt-Harris equation.

Results

Before LASIK, the mean refractive index was 1.366 ± 0.0049 (SD) in the microkeratome group and 1.374 ± 0.0047 in the femtosecond group. After LASIK, it was 1.382 ± 0.0066 and 1.391 ± 0.0102, respectively. The increase after ablation was statistically significant in both groups (P<.001). The increase in the refractive index linearly correlated with treatment time (microkeratome: r = 0.355, P = .007; femtosecond, r = 0.506, P = .027). Before photoablation, the refractive index was significantly lower in the microkeratome group than in the femtosecond group (P<.001). There was no significant difference in age between the 2 groups.

Conclusions

Photoablation increased the refractive index of the stroma, and the increase was influenced by treatment time. Hydration of the stroma was 1.8 mgm/mgm greater in the microkeratome group than in the femtosecond laser group.