THERMOMECHANICAL BEHAVIOR OF THE CORNEA

Background Shrinkage of corneal collagen is used during thermokeratopl asty, a method to remodel the corneal curvature. The goals of our inve stigations were to determine the optimal temperature range for maximal shrinkage of the collagen fibers with minimal damage. Materials and M ethods By means of a commercially available stress-strain-measuring de vice with a paraffin oil bath of temperatures varying from 35 degrees C to 120 degrees C strips of pig cornea 5 mm in width and 9 mm in leng th were investigated in the physiological stress range from sigma = (0 ,5-12,5) . 10(4) N/m(2) by stress-strain, stressrelaxation and creep m easurements. Results Biomechanical properties of the cornea remain unc hanged in the temperature range from 30 to 50 degrees C. Starting at 6 0 degrees C shrinkage occures that increases up to 90 degrees C. The m aximal rate of shrinkage of (57 +/- 12)% was measured at temperatures of 75 to 80 degrees C. Above 100 degrees C this effect is reduced by t he destruction of intermolecular bonds between the collagen fibers. Th e stress-strain curves of the shrunk corneas are flatter than that of native corneas, which means, the Young's modulus is significantly redu ced. Conclusions In order to realize optimal shrinkage during thermoke ratoplasty temperatures of 65-85 degrees C should be achieved in the c oagulated tissue. Higher temperatures cause also a shrinkage effect bu t also a destruction of tissue.