Laser Thermokeratoplasty (LTK): Biomechanical properties and ir-laser induced forces in porcine cornea

Purpose. A biomechanical model s developed to quantify the influence of laser parameters and exposure geometry in laser thermokeratoplasty. Therefore the mechanical properties and the laser induced alterations of the porcine cornea as a model eye in LTK have to be investigated. Methods. Uniaxial tensile tests and relaxation measurements were performed on specimens of porcine cornea to determine their nonlinear stress-strain relation and their viscoelastic behaviour. Laser induced forces have been measured while corneal strips were exposed with a pulsed Cr:Tm:Ho:YAG laser (λ=2.12 μm) and a cw diode (λ≈1.87 μm) laser system. Results. The forces observed correlated qualitatively well with the radiant exposure close to the threshold. For larger pulse energies the stress saturated after a few pulses, and further laser application could neither increase nor decrease the level of stress. After the end of the last laser pulse the specimens relaxed within several minutes down to a level of residual stress, which is probably to be held responsible for the refractive change in LTK. Conclusions. Material properties and laser induced forces of the porcine cornea have been investigated and can be provided for a biomechanical model. The laser induced forces observed support the hypothesis that the average temperature rise during laser exposure is responsible for the increase of tension in the stroma resulting in a change of corneal curvature.