A biomechanical basis for laser thermokeratoplasty

Laser thermokeratoplasty (LTK) is a procedure in refractive surgery to correct hyperopia and astigmatism. The rise in refractive power of the eye is achieved by thermally induced shrinkage of collagen in the corneal periphery leading to an increase of the central corneal curvature. Biomechanical effects, which play an important role in LTK, were investigated in order to simplify the transfer of data from animal experiments to the situation in the human eye and to get a basis for improving the understanding of the influence of physical parameters involved in the surgical procedure. A 2D axially symmetric biomechanical LTK model of the cornea was developed using the finite element method (FEM). In this model the cornea is represented as a partial sphere under pressure fixed at the limbus. The coagulations were modeled by shrinking a corneal volume. The analysis showed the influence of the material stiffness, the corneal geometry, the amount of shrinkage and the ring diameter of an annular coagulation on the resulting corneal curvature. The model allows to analyze the influence of some biomechanical and of LTK parameters on the resulting corneal curvature, which is only possible with great effort or not at all by experiments. THe strong dependence of the results on Young's modulus points out that it would be useful to determine the patient's individual mechanical properties of the cornea before applying LTK.