Utilization of the AO LockingRatNail in a novel rat femur critical defect model.

BACKGROUND: Our objective was to utilize a commercially available rodent locked intramedullary nail in a rat femur diaphyseal defect. This model is needed for future studies where materials in the critical defect could be modified with agents to fight infection (antibiotics) or promote osteogenesis. METHODS: After unsatisfactory attempts to develop a reliable femur critical size defect model utilizing various forms of fixation, a locked intramedullary nailing system (AO LockingRatNail) was employed in 105 male Sprague Dawley rats. A 5 mm critical size mid-diaphyseal femoral defect was created using a pneumatic sagittal saw. The intramedullary nail was placed in the femur in a retrograde manner. A prefabricated polymethyl-methacrylate (PMMA) spacer was utilized to fill the defect. Once adequate alignment was achieved, two locking pegs were placed (one distal, one proximal) to provide stable fixation. RESULTS: The technique was successful in 90% of femurs (95 of 105). The majority of complications centered on failure of the placement of locking pegs (7 of 10). One rat presented with migration of the nail out of the knee. Two rats presented with fractures not recognized intraoperatively. These complications occurred early in the study and decreased as surgical experience increased. Surgery was tolerated well by the rats as reflected by significant weight postoperative gain (p < .001). CONCLUSIONS: The AO LockingRatNail is a novel, reproducible, and successful method for stabilization of critical size femoral diaphyseal defects in the rat. This model has future value in the examination of the biological processes involved in the healing of critical bone defects.