Purpose: To determine rabbit cornea thermal tolerance and evaluate the effects of ultrasound (US) on this tissue after applying defined US heat doses.
Setting: Eye Clinic; Anatomy Histology and Forensic Medicine, University of Florence, Florence, Italy.
Methods: Hyperthermia was induced in rabbit corneas using US, simulating a phacoemulsification procedure. The US power was set at 100% in continuous mode, and temperature values were reached within 10 seconds of the onset of US treatment. Corneal surface temperatures were continuously monitored and recorded by thermographic registration. The eyes of 16 rabbits were examined: 4 controls, 8 treated at 40 degrees C for 10 seconds, 8 treated at 50 degrees C for 10 seconds, and 12 treated at 60 degrees C for 10 seconds. All 32 corneal buttons were removed and prepared for light microscopic evaluation with hematoxylin and eosin staining, trichromic staining, and zinc iodide-osmium tetroxide solution. The 12 corneas treated at 60 degrees C for 10 seconds were also processed for immunohistochemical analysis.
Results: Corneas at 40 degrees C for 10 seconds were grossly and histologically normal and were not different from control corneas. Corneas at 50 degrees C for 10 seconds showed initial stromal damage with collagen disorganization, mild stromal edema, and initial signs of keratocyte damage. Half of the corneas at 60 degrees C for 10 seconds were examined at time 0 and the other half after 1 week. At time 0, massive corneal damage with epithelial cell edema, collagen disorganization, severe stromal edema, intrastromal vacuole formation, plump keratocyte nuclei, and endothelial cell detachment were found, as was a severely impaired nerve plexus. At 1-week follow-up, corneas showed persistent stromal and endothelial cell edema with an increase activated keratocytes and mitotic features in the stroma and the epithelial layer.
Conclusions: Rabbit corneas showed a considerable tolerance to US damage up to 50 degrees C. Higher thermal doses produced severe histological damage, even though corneas showed a considerable plasticity due to their regenerative capacity.