Changes in the refractive index of the human corneal stroma during laser in situ keratomileusis. Effects of exposure time and method used to create the flap.

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.