The effect of hip stem material modulus on surface strain in human femora.

Human femora were used to compare the changes in bone surface strain resulting from decreasing the material modulus of a collarless hip stem to determine whether a highly elastic stem increased bone loading. Three substrate materials were tested: titanium (modulus of elasticity 110 GPa), carbon fiber composite (modulus of elasticity 52 GPa), and polymethylmethacrylate (PMMA, modulus of elasticity of 1.9 GPa). Two separate analyses were performed in which femora were implanted randomly with one of the three stem types. Results showed that assembly strains did not differ significantly among different materials. There was a large strain reduction in the proximal region of the femora for all stem substrates relative to the intact femur. Although there was statistically greater surface shear strain as the material modulus decreased, the PMMA stem did not substantially increase bone loading.