Effect of excimer laser keratectomy on the mechanical performance of the human cornea

Enucleated human eye globes were set-up for measurements of pressure (2-100 mmHg) induced tangential epithelial and endothelial side corneal strain, and for measurements of central corneal thickness and central radius of curvature. In one experimental group (N = 6), a shallow ablation (15-20 microns deep) removing Bowman's layer was performed in the central 7 mm diameter zone of the cornea. The fellow eye served as control (N = 6). In another experimental group (N = 6) a 70% deep keratectomy was performed in the central 7 mm diameter zone. The pressure induced central corneal strain was 5-10% higher in corneas without Bowman's layer compared with intact corneas. The epithelial side tangential corneal strain in eyes with a 70% deep keratectomy increased to twice as much as that of intact corneas, but the endothelial side strain was similar. In all groups of eyes Young's modulus of elasticity increased rapidly with the corneal stress level up to testing loads of 25 mmHg. At higher loads the degree of stress-stiffening was smaller. The differences in average corneal strain between the 3 groups could be explained by the induced changes in central corneal thickness, as there were no statistically significant differences in stress-normalized Young's modula of elasticity between the 3 groups. The central radius of curvature did not change significantly with increasing pressure in intact eyes or in eyes without Bowman's layer. In deep keratectomized corneas the corneal radius of curvature decreased approximately 1% when the intraocular pressure was increased from 2 to 100 mmHg.(ABSTRACT TRUNCATED AT 250 WORDS)