Selection of an animal model for resurfacing hip arthroplasty

This study was done to determine whether sheep provide a better model of resurfacing hip arthroplasty than dogs. Eighteen sheep were subjected to unilateral resurfacing arthroplasty. Fifty-eight percent developed femoral loosening by 10 months. This reflects the clinical situation in humans: loosening has been the leading cause of failure. By contrast, reports of experiments with dogs describe very low loosening rates. Sheep provide a more stringent test of hip arthroplasty than dogs. The critical difference appears to be activity level. Sheep allowed free activity on a farm more closely simulate the situation of active patients than do dogs housed in small enclosures in conventional research facilities. New techniques of prosthetic hip arthroplasty that lend themselves to animal models should be studied in sheep before being studied in humans.