STRAIN DISTRIBUTION IN THE PROXIMAL FEMUR WITH FLEXIBLE COMPOSITE AND METALLIC FEMORAL COMPONENTS UNDER AXIAL AND TORSIONAL LOADS

This study investigated strain distribution changes in the proximal femur after implantation of a flexible composite femoral component (carbon composite material, modulus of elasticity = 18.6 GPa), a titanium alloy implant (E = 100 GPa), and a stainless steel implant (E = 200 GPa). Transverse as well as longitudinal strain was measured using bipolar strain gauges at eight locations on the proximal femur under both physiologic axial (1000 N and 2000 N) and physiologic torsional (10 N-m and 20 N-m) loads. Under axial load, longitudinal compressive strain at the calcar region was significantly greater in intact femurs and the carbon composite stem specimens than in the two metal stem specimens. The difference between intact femurs and the carbon composite stem specimens was not significant. Stress shielding in the proximal lateral region of the femur, however, was still apparent even in the carbon composite stem specimens. Without seating of the stem collar on the femoral neck, longitudinal compressive strain was not generated at the calcar region, and transverse tensile strain at this region was increased. With conventional implant design, the stem collar was still necessary even in the flexible composite stem to provide near normal longitudinal compressive strain in the calcar region. Under torsional load, proximal strain in intact femurs was small and the proximal strain levels observed after either carbon composite or titanium alloy stem implantation were greater than strain levels before implantation. It seemed unlikely that torsional stress relief played a significant role in proximal bone loss after total hip arthroplasty. Both longitudinal and transverse strains at the calcar region under torsional load were significantly greater in the carbon composite stem specimens than in both intact femurs and the titanium alloy stem specimens, suggesting that these abnormally high proximal stresses may cause high proximal micromotion of the implant, and even bone fracture. Proximal implant design seems to be of paramount importance with flexible composite femoral components to avoid excessive proximal stress concentration under torsional load and to provide near normal strain distribution in the proximal femur.