Purpose: This study was undertaken to investigate the wound healing process of the first 6 weeks after photodynamic cross-linking treatment in the rabbit cornea, using the photosensitizer riboflavin and UVA.
Methods: After removal of the central epithelium, the right corneas of 8 Chinchilla rabbits were cross-linked with a photosensitizing 0.1% riboflavin solution and UVA light (370 nm; irradiance, 3 mW/cm2; dose, 5.4 J/cm2) for 30 minutes. Two animals were euthanized 3 days, 7 days, 4 weeks, and 6 weeks postoperatively. The corneas of the enucleated eyes were evaluated using 4-μm light microscopic sections with routine stains and avidin-biotin complex immunostaining with anti-α-smooth muscle actin.
Results: By day 3 after treatment, complete apoptotic damage and loss of the endothelial cells and the stromal keratocytes were found in the irradiated area through the entire thickness of the stroma. There was marked stromal edema (850 ± 66 vs. 332 ± 43 μm in the untreated controls; P < 0.01). The epithelium was already closed again. At the margins of the lesion, there was a mild inflammatory reaction with scattered macrophages, lymphocytes, and neutrophils. By day 7, the endothelium was already intact again, and keratocyte repopulation of the posterior stroma was noted. By week 4, the keratocyte repopulation of the anterior stroma was observed with some acellular areas between. By week 6, the cytoarchitecture of the cornea seemed normal again. By weeks 4 and 6, α-actin-positive keratocytes were identified, especially in the periphery of the irradiated area.
Conclusions: After riboflavin/UVA cross-linking of rabbit cornea, a complete cell loss occurs in the irradiation area with an irradiance of 3 mW/cm2. The cytotoxic damage is repaired by repopulation after ∼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 ∼2 months, allowing cellular regeneration.