Thermomechanical behavior of collagen-cross-linked porcine cornea

Purpose: Collagen cross-linking using combined riboflavin/ UVA treatment has been shown to increase the biomechanical rigidity of the cornea and has been used successfully for the treatment of progressive keratoconus. From morphological and biochemical investigations, a different degree of cross-linking for the anterior and posterior stroma by the treatment is suggested. The present study was undertaken to better evaluate this effect by testing the thermomechanical behavior. Methods: Ten 10 ! 5 mm corneal strips from porcine cadaver eyes enucleated within 5 h post mortem were crosslinked using the photosensitizer riboflavin and UVA irradiation (370 nm, irradiance = 3 mW/cm(2)) for 30 min and compared to ten untreated corneal strips and ten corneal strips cross-linked with 0.1% glutaraldehyde. The temperature in a water bath was raised from 60 to 95 degreesC with temperature increments of 1 degrees C per minute. The hydrothermal shrinkage of the corneal strips was measured in 2.5 degreesC steps using a micrometer. In addition, six 10-mm whole corneal buttons were cross-linked with riboflavin/ UVA and immersed into water at 70 or 75 degreesC. Results: The maximal hydrothermal shrinkage for the untreated control specimens and the posterior portion of the riboflavin/ UVA-treated corneas was at 70 degreesC, for the anterior portion of the cornea cross-linked by riboflavin/UVA at 75 degreesC and for glutaraldehyde-cross-linked cornea at 90 degreesC. In the cross-linked corneal buttons, a typical mushroom-like shape was observed at 70 degreesC and a cylinder shape at 75 degreesC. Conclusions: The different degree of collagen cross-linking in the corneal stroma after riboflavin/ UVA treatment is reflected by the differences in the maximal shrinkage temperature of the anterior and posterior portion. Therefore, in the corneas cross-linked with riboflavin/ UVA a higher shrinkage temperature was observed for the anterior portion of the cornea (75 degreesC) compared to the posterior stroma ( 70 degreesC) due to the higher degree of cross-linking of the anterior stroma. The anterior localization of the cross-linking effect is advantageous for the endothelium and for the preservation of the anterior corneal curvature. Copyright (C) 2004 S. Karger AG, Basel.