Distal Femoral Physeal Fixation: Are Smooth Pins Really Safe?

Background: Distal femoral physeal fractures have a high incidence of physeal arrest. Several factors have been postulated to contribute to this high incidence, including fracture type, displacement, the undulating nature of the physis, and fracture reduction/fixation. The purpose of this study was to determine whether the position of percutaneous smooth pins across the physis contributes to physeal bar formation. Methods: The previously validated New Zealand white rabbit model was used. Power analysis determined that 30 animals were required. All animals had a constant 0.045 smooth Kirschner (K) wire placed under fluoroscopic guidance from the distal lateral femur across the physis centrally. A second 0.045 K-wire was placed in a cross-pin configuration from the medial side in one of 2 positions: zone 1-crossing the physis centrally or zone 2-crossing the physis peripherally. Pins were removed after 4 weeks and micro computed tomography was performed at 8 weeks to assess for physeal bar formation. Histologic analysis was performed to confirm bar formation. Results: Two physeal bars (7%) were seen after removal of the constant (lateral pin). The peripheral pin resulted in bar formation in 2 animals (13%) and the central pin in 1 animal (7%). A chi(2) test was performed; there was no statistically significant difference between zones in terms of bar formation (P=0.5428). Conclusions: Injury to the growth plate after distal femoral fracture may be unavoidable. Treatment is aimed to minimize further injury to the physis. Cross-pinning with smooth K-wires results in a low rate of physeal injury. Pins that cross the physis both centrally and peripherally appear to have the same risk for physeal bar formation. Clinical Relevance: This study reveals that physeal bar formation can be seen with smaller than previously reported cross-sectional damage to the distal femoral physis. This study highlights the need to carefully select and perform fixation of the distal femoral physis with as little additional trauma to the physis as possible.