Biomechanical Testing of the Proximal Femoral Epiphysis: Intact and Implanted Condition

There is renewed interest in resurfacing hip prostheses. While stemmed prostheses have been extensively studied in the past, little is known about the biomechanics of epiphyseal prostheses. Our aim was to develop a combined experimental-numerical tool to study the intact and operated epiphysis. Bone and implant stress, relative micromotion and failure mode in the intact and implanted bone were investigated. Twelve pairs of cadaver human femurs were studied intact, to fully characterize the proximal epiphysis. Four were then implanted with a commercial resurfacing prosthesis. They were tested in the elastic range, while strains were measured with 15 rosettes. Implant micromotions were measured in the operated condition. A total of 7 loading scenarios were simulated to cover the range of typical motor tasks. Additionally, Finite Element (FE) models were built using a validated procedure for assigning inhomogeneous material properties based on CT data. To allow extensive validation of the FE model, additional measurements were taken in vitro: bone deflection in various points, indirect measurement of load application point, digitizing of the bone surface and gauge locations. The FE models were also used to identify the most critical load scenario to recreate in vitro spontaneous head-neck fractures. Strain measurements were successfully obtained in intact and implanted femurs, providing the natural strain pattern, and indicating moderate stress-shielding in the operated condition. Results on the 6 femurs that were modeled showed that FE can predict overall displacements with an accuracy of 0.4mm, and principal stress with an accuracy of 10% (Root Mean Squared, RMSE). In vitro failure tests were successful: all specimens fractured, with a variety of failures ranging from sub-capital to trans-trochanteric. This confirms the suitability of this test model to replicate spontaneous fractures in elderly subjects. In conclusion, an experimentally validated FE method was developed, that run in parallel with an optimized in vitro simulation. These tools can successfully predict the stress distribution and the failure mode in the proximal femur both in its natural condition and with a resurfacing prosthesis.