Stronger implant does not cause stress-shielding in the fixation of hip fractures - Validated finite element analysis and cadaver tests

Abstract

Background: A stable fixation by osteosynthesis is essential to allow an early mobilization of patients with hip fractures. When stronger implants are employed to prevent potential complications, it is feared that stress-shielding occurs. The hypothesis of this study was that a stronger intramedullary nail becomes less stressed during loading. It was specifically investigated if this would lead to more load on the bone and therefore reduce the likelihood of stress-shielding. Methods: Two intramedullary nails were compared, a Gamma3 long and a reinforced prototype with a larger diameter of the proximal shaft. The implants were analyzed by FEA in two fractures (AO 31A3.1/A3.3) and two load cases. To check the results by the FE models, analog experiments in human cadaver bones were carried out. Six pairs of human femurs (age 67±7) were osteomized and fixated with the two implants in a paired cross-over design. The specimens were tested in a servo electric testing machine to measure the construct stiffness and the strains on the nail by two strain gauges during maximum load. Results: The FE simulations resulted in 11-28% lower stresses on the nail. In the cadaver tests the stronger implant resulted in significant lower strains at the two strain gauges (p<0.001; p=0.016) and a comparable stiffness (p=0.446) Conclusion: The enhanced stability and stiffness of the implant by the reinforced proximal part of the nail shaft did not cause stress-shielding. The risk of fatigue failure was lower for the stronger implant because of the lower stresses and strains within the osteosynthesis. Thus, patients with expected delays of fracture healing may potentially benefit from the application of stronger implants.