The effect of stem stiffness on femoral bone resorption after canine porous-coated total hip arthroplasty.

Bilateral noncemented total hip arthroplasty (THA) was produced in dogs to determine the effect of stem stiffness on stress-related bone resorption. Two porous-coated femoral implants of substantially different stiffnesses were designed for direct comparison. One was manufactured from cobalt-chromium (CoCr) alloy, the other from titanium alloy. The titanium stem was hollowed out to a wall thickness of 1 mm to further reduce its stiffness. The cumulative stiffness differences were about 5.4-fold axially and 3.6-fold in bending and torsion. Staged bilateral THA was performed on eight dogs. Each dog received a stiff CoCr stem on one side and a flexible titanium stem on the other. After death, the femora were removed and processed for undecalcified thin-section histology. Bone ingrowth and remodeling were quantified by computer-aided image analysis and compared between stem designs. All femoral specimens showed bone ingrowth fixation of both stiff and flexible stems along the implant length. Tetracycline labeling indicated active bone turnover in the femoral cortex and in regions of ingrowth. However, gross differences in femoral bone remodeling were observed both roentgenographically and histologically. Femora with the flexible stems consistently showed much less bone resorption than those with the stiff stems. Quantitative analysis of paired cross-sections indicated an average of 25%-35% more cortical bone area in the femora with flexible stems. Severe resorption of the cortex in the midstem region occurred in three of the femora with the stiff stems but in none with the flexible stems. Stem stiffness strongly influences bone remodeling. The flexible stem results in more uniform load transfer and less stress shielding.