Abstract
In diaphyseal femoral fractures, an accurate quantification of the biomechanical differences between implants can help the surgeon make his choice. This study uses the finite element (FE) method to characterize the differences between intramedullary nails and osteosynthesis plates at different stages of femoral fracture healing, and assesses how these differences are affected by the size and consolidation of the fracture. Simulations were performed using a femur model with a 1 and 3 mm diaphyseal fracture at two stages of consolidation. The fracture was stabilized with an osteosynthesis plate or an intramedullary nail made of stainless steel (SS), titanium (Ti) or a composite material. The Ti implants bore lower von Mises stresses (σvM) than the SS implants, and σvM in the implants decreased with fracture consolidation. Nails tended to bear higher stresses than plates, though these differences were reduced in unstable fractures. This change in trend proves that fixation choice is critical for weak fractures. Both the dilatational stress σdil and the octahedral shear strain ∊oct of the fractures varied significantly with the type of fracture and fixation device. In accordance with the literature, our results suggest that plates induce fracture healing through intramembranous ossification without fracture callus formation. Nails, on the other hand, induce endochondral ossification with fibrous tissue formation. The composite implants have mechanical limitations, but increasing their yield stress could overcome these drawbacks. (Journal of Applied Biomaterials & Biomechanics 2005; 3: 157–67)