Corneal crosslinking without epithelial debridement reduced the biomechanical effect by approximately one fifth compared with standard crosslinking, probably because of restricted and inhomogeneous stromal distribution of riboflavin. The cytotoxic damage was restricted to 200.0 microm stromal depth, which is an advantage over the standard method. Therefore, C3-R is not recommended for the routine treatment of keratoconus but primarily for cases with a corneal thickness less than 400.0 microm in which standard crosslinking cannot be used without serious risk to the endothelium.

ABSTRACT.Purpose: Scleral biomechanical weakness and thinning is known to be one of the main factors in the pathogenesis of progressive myopia. We tried to strengthen rabbit sclera by cross-linking scleral collagen using ultraviolet A (UVA) and the photosensitizer riboflavin. Methods: Circumscribed 10 · 10 mm sectors of the posterior -equatorial sclera of six chinchilla rabbit eyes were treated in vivo using a UVA double diode with 4.2 mW/cm 2 UVA at 370 nm and applying 0.1% riboflavin-5-phosphate drops as photosensitizer for 30 min. 1 day postoperatively biomechanical stressÀstrain measurements of three treated scleral strips were performed using a microcomputercontrolled biomaterial testing device and compared to non-treated contralateral control sclera. In addition, three treated eyes were examined histologically by light microscopy, TUNEL staining and electron microscopy to evaluate side-effects. Results: Following the cross-linking treatment, the ultimate stress was 11.87 -1.8 MPa versus 3.63 -0.40 in the controls (increase of 227.9%, p = 0.014), Young's modulus 27.67 -4.16 MPa versus 4.9 -2.15 MPa in the controls (increase of 464.7%, p = 0.021) and ultimate strain 92.2 -7.43% versus 165.63 -19.09% in the controls (decrease of 54.52%, p = 0.012). Histologically, serious side-effects were found in the entire posterior globe with almost complete loss of the photoreceptors, the outer nuclear layer and the retinal pigment epithelium (RPE). Conclusions: Our new method of scleral collagen cross-linking proved very effective in increasing the scleral mechanical strength; the new treatment may represent an option for strengthening scleral tissue in progressive myopia. However, serious sideeffects were observed in the outer retina. In future studies these side-effects could be avoided by reducing the irradiation dose below the cytotoxic level of the retina. Before its clinical application, the new method should be tested in a myopia animal model.

. Purpose:  Photodynamic riboflavin/ultraviolet‐A (UVA)‐induced collagen cross‐linking, which increases the biomechanical stiffness of the human cornea by about 300%, has been introduced recently as a possible treatment for progressive keratoconus. The present study was undertaken to evaluate the longterm biomechanical effects of this new cross‐linking treatment as a necessary prerequisite to its clinical success. Methods:  The corneas of the left eyes of nine male rabbits were cross‐linked. The contralateral eyes served as controls. After removal of the central 7 mm of the epithelium, the corneas were treated with the photosensitizer riboflavin and UVA irradiation for 30 mins with an irradiance of 3 mW/cm2 using a 370‐nm UVA double diode. Groups of three animals were killed immediately after treatment and at 3 and 8 months, respectively. Biomechanical stress–strain measurements were performed using a microcomputer‐controlled biomaterial tester on 4 × 10‐mm corneal strips. Results:  Corneal thickness in the treated rabbit cornea was 408 ± 20 μm. A constant and significant increase in ultimate stress (of 69.7–106.0%), Young’s modulus of elasticity (of 78.4–87.4%) and a decrease in ultimate strain (of 0.57–78.4%) were found over a time period of up to 8 months after cross‐linking treatment. Conclusions:  Riboflavin/UVA‐induced collagen cross‐linking leads to a longterm increase in biomechanical rigidity which remains stable over time. These data support our previous longterm clinical observations and give hope that this new treatment will halt progressive keratoconus definitively.

After riboflavin/UVA cross-linking of rabbit cornea, a complete cell loss occurs in the irradiation area with an irradiance of 3 mW/cm(2). The cytotoxic damage is repaired by repopulation after approximately 4-6 weeks. A combination of cross-linking with other procedures such as the implantation of intracorneal rings should be performed only after a sufficient time interval of approximately 2 months, allowing cellular regeneration.

. Purpose:  Scleral crosslinking by the photosensitizer riboflavin and ultraviolet A (UVA) has been shown to increase significantly the scleral biomechanical rigidity and might therefore become a possible sclera‐based treatment modality for progressive myopia. In the present study, the long‐term effect of the new crosslinking method on biomechanical properties was investigated in the rabbit sclera. Methods:  A 10 × 10 mm sector of the equatorial sclera of nine Chinchilla rabbit eyes was treated in vivo using a UVA double diode of 370 nm with a surface irradiance of 3 mW/cm2 and application of 0.1% riboflavin‐5‐phosphate drops as photosensitizer for 30 min. Three days, 4 months and 8 months postoperatively, biomechanical stress–strain measurements of the treated scleral strips were performed and compared to contralateral control sclera using a microcomputer‐controlled biomaterial tester. In addition, routine histological controls were performed. Results:  Following the crosslinking treatment, Young’s modulus was increased by 320% after 3 days, 277% after 4 months and 502% after 8 months, and ultimate stress by 341% after 3 days, 131% after 4 months and 213.8% after 8 months versus the controls. The decrease in ultimate strain was between 24% and 44.8%. On histology, no tissue damage was detected. Conclusion:  Our new method of scleral collagen crosslinking proved very effective and constant over a time interval of up to 8 months in increasing the scleral biomechanical strength. Therefore, the new treatment might become an option for strengthening scleral tissue in progressive myopia and other conditions associated with weakened sclera. There were no side‐effects on the retina or retinal pigment epithelium. The new crosslinking treatment could now be tested in a suitable myopia model (like the tree shrew) and finally in human eyes.