Refractive index change in bovine and human corneal stroma before and after lasik: a study of untreated and re-treated corneas implicating stromal hydration.

PURPOSE To measure the refractive index (RI) of the mammalian corneal stroma in relation to hydration in vitro and human corneal stroma before and after LASIK. METHODS RI at the anterior stromal surface of bovine corneal buttons was measured after the epithelium was scraped away. Samples were weighed and oven dried to calculate hydration. RI of the stromal bed surface was measured with a modified hand-held Abbé refractometer immediately before and after excimer laser photoablation. Thirty-one untreated persons (group 1: 44 corneas; age range, 19-65 years) and eight re-treated patients (group 2: 10 corneas) were examined. RESULTS RI of bovine stromal surface was significantly associated with hydration (H) (RI = 1.4067 - 0.00599H, r = -0.9079, P < 0.001). Photoablation significantly increased the RI of the midstroma (group 1: 1.3721 +/- 0.0041 to 1.3839 +/- 0.0050; group 2: 1.3717 +/- 0.0038 to 1.3819 +/- 0.0039). Differences between groups were not significant. In group 1 (n = 31), change in RI (DeltaRI) was significantly related to preoperative RI (DeltaRI = 1.155 - 0.833RI, r = 0.595, P < 0.001) and RI was significantly related to age (x) (RI = 1.3634 + 0.00026x, r = 0.603, P < 0.001). CONCLUSIONS Mammalian corneal stromal RI correlates with hydration. LASIK significantly increases the refractive index of the treated stromal bed, and this equates to an average change in hydration from 4.3 to 2.9. For individual cases, change in RI is associated with the pre-op RI. The lack of any difference between untreated and re-treated corneas suggests that with time hydration returns back to normal levels. The RI in the older corneal stroma is slightly higher relative to the RI in the younger corneal stroma.