Characterization of the bone phenotype and fracture repair in osteopetrotic incisors absent rats.

Osteopetrotic patients possess a genetic condition that leads to a deficiency in osteoclast number or function. Patients have a high bone density and suffer from an increased risk of fracture. The lack of normal osteoclast activity has the potential to impede repair by complicating orthopedic fixation and/or by affecting the biology of fracture healing. The naturally occurring incisors absent (ia/ia) rat was adopted as a rodent model of congenital osteopetrosis. A detailed phenotypic analysis of the ia/ia rat indicated that some functional recovery occurred between 7 and 9 weeks. Consequently a fracture repair study was undertaken using 5-week-old rats. Closed femoral fractures were generated in ia/ia rats and control ia/+ and +/+ rats using an Einhorn apparatus. Fracture healing was examined radiologically and histologically at 1-3 weeks. No difference was seen in bridging between ia/ia and control rats at any time point. The ia/ia rats showed no delay in cartilage removal but showed a significant delay in hard callus remodeling. This is consistent with an essential role for osteoclasts in only the latter stages of endochondral bone repair. This delay in hard callus remodeling was offset by an increase in moment of inertia.