Ultrasonically determined elasticity and cortical density in canine femora after hip arthroplasty

Effects of canine hip replacement (with a porous-coated femoral component) on the material properties of surrounding cortical bone were evaluated. The hypotheses were: (1) after four months of implantation, mechanical properties of the cortex would change, and (2) a collared implant would be associated with smaller changes than a collarless design. Unilateral total hip arthroplasty was performed in 15 mixed-breed dogs. Nine received a collared and six received a collarless femoral component. Four months after implantation, longitudinal ultrasonic wave propagation velocities and bone mineral densities (from dual energy X-ray absorptiometry) were measured in harvested femora and used to calculate the axial elastic constitutive coefficients for the cortex surrounding the implants. Results showed no difference in bone elasticity or bone density between collared and collarless designs. Significant velocity decreases from control values (p<0.0001) were noted in all implanted femora at four months. Bone mineral densities also displayed decreased values after four months of implantation (p<0.0145). Elastic coefficients were consistently less after four months of implantation when compared to control values (p<0.0001). This alteration in material properties would affect load transfer into the implanted femur via the increased disparity between implant and bone stiffnesses regardless of the component design. Significant differences in the elastic coefficients between implanted and control femora support hypothesis 1. However, no group differences were found between collared and collarless implantations; thus, the study does not support hypothesis 2.