Differential fracture healing resulting from fixation stiffness variability: a mouse model.

Background: The mechanisms underlying the interaction between the local mechanical environment and fracture healing are not known. We developed a mouse femoral fracture model with implants of different stiffness, and hypothesized that differential fracture healing would result.
Methods: Femoral shaft fractures were created in 70 mice, and were treated with an intramedullary nail made of either tungsten (Young's modulus = 410 GPa) or aluminium (Young's modulus = 70 GPa). Mice were then sacrificed at 2 or 5 weeks. Fracture calluses were analyzed using standard microCT, histological, and biomechanical methods.
Results: At 2 weeks, callus volume was significantly greater in the aluminium group than in the tungsten group (61.2 vs. 40.5 mm(3), p = 0.016), yet bone volume within the calluses was no different between the groups (13.2 vs. 12.3 mm(3)). Calluses from the tungsten group were stiffer on mechanical testing (18.7 vs. 9.7 N/mm, p = 0.01). The percent cartilage in the callus was 31.6% in the aluminium group and 22.9% in the tungsten group (p = 0.40). At 5 weeks, there were no differences between any of the healed femora.
Conclusions: In this study, fracture implants of different stiffness led to different fracture healing in this mouse fracture model. Fractures treated with a stiffer implant had more advanced healing at 2 weeks, but still healed by callus formation. Although this concept has been well documented previously, this particular model could be a valuable research tool to study the healing consequences of altered fixation stiffness, which may provide insight into the pathogenesis and ideal treatment of fractures and non-unions.