Evaluation of the stiffening effect of riboflavin/UVA treatment in different depth of the cornea

Abstract

Abstract: : Purpose: Treatment of keratoconus with riboflavin/UVA is based on a significant stiffening of the cornea. Aim of the study was to examine to which depth of the cornea the stiffening effect is present biomechanically. Methods: 20 enucleated porcine eyes were treated with riboflavin as photosensitizer and UVA (370 nm, 3 mW/cm², 30 min) and 20 eyes served as controls. Two consecutive flaps of 200 µm thickness were cut with a microkeratom from each. Each flap was divided into stripes of 5 mm width and 7 mm length. Stress–strain curves were measured with a material tester to characterize the stiffening effect. For statictics ANOVA was used. Results: Stress at 5 % strain was 261±133x10³ N/m² for the superficial treated flaps in comparison to 104±52x10³ N/m² corresponding control flaps. This difference was statistically significant (P=0.0001). The deep treated flaps showed a stress of 105±56x10³ N/m² compared to the corresponding control flaps with 103±62x10³ N/m². This difference was not statistically significant (P=0.94). The stiffness of the superficial flap was statistically significant increased in comparison to the deeper flaps (P = 0.001).The stiffness between the superficial and deep control flaps was not statistically significant different (P=0.9). Conclusions:Treatment of the cornea with riboflavin/UVA leads to a significant stiffening only in the anterior 200 µm of the cornea. This locally limited stiffening effect might be explained by the absorption behaviour of the riboflavin–treated cornea for UVA. 65 – 70 % of UVA–irradiation is absorbed in the first 200 µm and only 20 % in the next 200 µm.

Keywords: cornea: basic science • cornea: clinical science • keratoconus