Corneal elasticity and ocular rigidity in normal and keratoconic eyes

The elastic properties of a given tissue may be characterized by Youngs Modulus (Y) which relates the stress (force per unit cross sectional area) and the resultant strain (the relative linear deformation). Cornea and sclera react to stress by a biphasic viscoelastic response consisting of a quick immediate deformation followed by a further slow deformation. Thus, it is necessary to distinguish between Y determined from the immediate elastic response (Yi) and Y determined in steady state (Ys). In a normal (n = 29) and a keratoconic group (n = 27) the ocular rigidity (E) was determined as estimate of the Yi of the ocular tunics. Further based on measurements of intraocular pressure, corneal diameter, corneal shape and thickness profile that value of Ys which represents elastic minimum energy content in the actual corneal membrane was estimated in the two groups. E as well as Ys were found to be significantly lower in keratoconic eyes compared with normals and uncorrelated to each other in the two groups.