Comparison of mechanical properties of keratoconus and normal corneas

In order to measure more directly the physical properties of the corneoscleral shell in keratoconus, mechanical stress-strain tests were performed on strips of human cornea. Rectangular human corneal samples were glued to the arms of an extensiometer, a technique that avoids the problems of slippage and compression found with conventional clamps. Strain was measured using an electromagnetic displacement probe. The entire apparatus was immersed in a thermoregulated mineral oil bath to maintain constant corneal hydration. The resultant data obey the relation σ = A[e αϵ −1]. Stiffening constants α ranged from 34 to 82. We found that there are significant differences in the parameters for the two groups, with keratoconus corneas yielding lower α and higher A values than the normal controls. However, within the physiological range these differences offset so that there is no difference in the overall elastic behaviour of the keratoconus and normal corneas. Creep tests at constant load were also performed on normal and keratoconus corneas.