The Influence of Retrograde Femoral Nail Removal With and Without Interpositional Fat Grafting on Distal Femoral Physeal Behavior: A Sheep Study

Background: Previous investigations have demonstrated that up to 7% of the distal femoral physis can be violated using a rigid, retrograde nail without growth inhibition or arrest. The purpose of this investigation was to evaluate the behavior of the distal femoral physis after retrograde femoral nail removal in a sheep model, with and without placement of an interpositional fat graft. Methods: Retrograde femoral nails were placed in 8 skeletally immature sheep. Implants were removed at 8 weeks, with the residual defects left open (n=4) or filled with autologous fat graft (n=4). Differences in femoral length between surgical versus contralateral control femurs were measured after an additional 3 (n=4) or 5 months (n=4) before sacrifice, and the physis was evaluated histologically. Results: When compared with control limbs, femoral length was significantly shorter in limbs sacrificed at 3 months (mean: 3.9±1.3 mm; range: 2.7 to 5.7 mm) compared with limbs at 5 months (mean: 1.0±0.4 mm; range: 0.4 to 1.2 mm) (P=0.005). No significant difference in mean shortening was appreciated in limbs without (2.4±1.6 mm) versus with fat grafting (2.5±2.3 mm) (P=0.94). Histologic analysis revealed no osteoid formation across the physis in sheep sacrificed at 3 months, whereas there was evidence of early osteoid formation across the physis in sheep at 5 months. All specimens demonstrated evidence of an active physes. Conclusions: Femurs undergoing retrograde implant placement were significantly shorter when compared with control limbs in sheep sacrificed at 3 months, whereas differences were nominal in sheep sacrificed at 5 months after retrograde implant removal, suggesting growth inhibition with nail removal improved with time. Fat grafting across the distal femoral physis did not result in a significant difference in femoral lengths. Histologic evidence at 5 months revealed early development of a bone bridge, emphasizing the importance of follow-up to skeletal maturity in patients treated with retrograde nailing across an open physis. Level of Evidence: Level IV.