Thermomechanical behavior of the cornea

Shrinkage of corneal collagen is used during thermokeratoplasty, a method to remodel the corneal curvature. The goal of our investigations was to determine the optimal temperature range for maximal shrinkage of the collagen fibers with minimal damage. By means of a commercially available stress-strain-measuring device including a paraffin oil bath at temperatures varying from 35 degrees to 120 degrees C, strips of pig cornea measuring 5 mm in width and 9 mm in length were investigated in the stress range of sigma = (0.5-12.5) x10(4) N/m2 by stress-strain, stress-relaxation, and creep measurements. The biomechanical properties of the cornea remained unchanged in the temperature range of 30-50 degrees C. Starting at 60 degrees C, shrinkage occurred that increased up to a temperature of 90 degrees C. The maximal rate of shrinkage of 57 +/- 12% was determined at temperatures of 75 degrees and 80 degrees C. At above 100 degrees C the shrinkage effect decreased because of the destruction of intermolecular bonds between the collagen fibers. The stress-strain curves generated for shrunken corneas were flatter than those generated for native corneas, which means that Young's modulus is significantly reduced. For the achievement of optimal shrinkage during thermokeratoplasty, temperatures of 70-85 degrees C should be reached in the tissue to be coagulated. Higher temperatures are capable of causing a shrinkage effect but also induce the destruction of tissue.