Distal femoral fractures in the elderly: biomechanical analysis of a polyaxial angle-stable locking plate versus a retrograde intramedullary nail in a human cadaveric bone model.

INTRODUCTION: Compromised bone quality and the need for early mobilization still lead to high rates of implant failure in geriatric patients with distal femoral fractures. With the newest generation of polyaxial locking plates and the proven retrograde femoral nails today two minimally invasive surgical procedures have been established. Indications for both procedures overlap. This study attempts to define the strength and failure mode of both surgical procedures.

MATERIALS AND METHODS: A standardized fracture model was established to simulate an unstable AO/OTA 33-A3 fracture. Eight pairs of human cadaver femora (mean age 79 years, range 63-100 years) with compromised bone quality were used. Osteosyntheses with eight retrograde femoral nails and eight locking plates were randomly performed. A materials testing machine (Instron 5566) was used to perform cyclic stress tests according to a standardized loading protocol, up to a maximum load of 5,000 N.

RESULTS: All specimens survived loading of at least 2,500 N. Three nail and one plate construct survived a maximum load of 5,000 N. The mean compressive force leading to failure was 4,400 N (CI 4,122-4,678 N) for nail osteosynthesis and 4,429 N (CI 3,653-5,204 N) for plate osteosynthesis (p = 0.943). Proximal cutting out of the osteosynthesis was the most common reason for interruption in the nail and plate osteosyntheses. Significant differences between the retrograde femoral nail and plate osteosyntheses were seen under testing conditions for plastic deformation and stiffness of the constructs (p = 0.002 and p = 0.001, respectively).

CONCLUSION: Based on our results, no statements regarding the superiority of either of the devices can be made. Even though the load to failure values for both osteosyntheses were much higher than the loads experienced during normal walking; however, because only axial loading was applied, it remains unclear whether both osteosyntheses meet the estimated requirements for postoperative full weight-bearing for an average heavy patient with a distal femoral fracture.