Comparative Study of Carbon Polymer Composite and Titanium Femoral Stems in Dogs Using Computed Tomography

Clinical evaluations of canal-filling, uncemented, femoral stems continue to reveal a loss of density and thickness of the cortex of the proximal femur, consistent with a stress-shielding process. This finding has stimulated interest in the implementation of more flexible carbon fiber-reinforced polymer (CFRP) composite devices. In this study, unilateral total hip arthroplasty was performed on 34 canines using CFRP or geometrically identical titanium alloy femoral endoprostheses for periods of 6, 12, or 24 months. The CFRP composite stems had approximately one-half the bending stiffness of the metallic controls. The collarless, straight stems had a canal-filling design developed expressly for dogs. The prostheses were uncoated, with a relatively smooth surface, and were implanted with a press-fit. Computed tomography (CT) was used to quantify the remodeling of bone around the implants. With calibration of the CT images, we were able to calculate material and structural properties of the bone around the implants and in the contralateral femora, including the bone cross-sectional area and apparent density. To date, 6 of the 34 animals suffered fracture of the proximal femur. In the present investigation, CT analyses were performed on the remaining animals that were sacrificed after 6 or 12 months (a total of 20 animals). The results of these analyses indicated marked new bone formation with an increase in the quantity of radiodense bone, especially in the femoral metaphysis, and around the distal end of the stem. However, the mean bone density decreased due to the lower density of the newly formed bone, with a trend of more normal bone density at 12 months in comparison to 6 months after surgery. The occurrence of fractures and the observed remodeling suggest that the stresses in the proximal femur were relatively high, most likely due to the smooth-surfaced, press-fit, canal-filling design of the endoprostheses. Twelve months after surgery, the bone remodeling around the CFRP prostheses was not significantly different than the remodeling around the titanium alloy devices. The remodeling response could best be described as a redistribution of bone that was probably not yet complete even after 12 months. Longer-term results might reveal differences in the homeostatic structure of bone around the metallic and more flexible composite stems.