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The purpose of this study was to compare, with regard to fixation of the implant and femoral bone resorption, two fully porous-coated stems of different stiffnesses in a canine total hip arthroplasty model. A bilateral arthroplasty was carried out with insertion of a titanium-alloy stem (which had stiffness properties comparable with those of the canine femur) on one side and with insertion of a composite stem (which was three to fivefold more flexible than the canine femur) on the contralateral side. Eight femora were evaluated at six months and eight, at eighteen months after the operation, to determine the extent of bone ingrowth, periprosthetic cortical area, intracortical porosity, and bone-remodeling. Despite the markedly greater flexibility of the composite stems, no significant difference could be detected (with the numbers available), with regard to the overall degree of femoral stress-shielding, cortical area, or cortical porosity, between these stems and the stiffer, titanium-alloy stems at either time-period. However, the composite stems had less bone ingrowth and more formation of radiopaque lines than did the titanium-alloy stems. At eighteen months, the values for bone ingrowth were 9.7 ± 5.38 percent (mean and standard deviation) for the composite stems compared with 28.1 ± 5.31 percent for the titanium-alloy stems (p = 0.003). Furthermore, the histological sections from the femora containing a composite stem showed radiopaque lines indicative of fibrous ingrowth approximately threefold more often than did those from the femora containing a titanium-alloy stem (p = 0.02). CLINICAL RELEVANCE: These findings demonstrate that there seems to be an effective limit beyond which increasing the stiffness of the femoral stem relative to that of the femur cannot prevent femoral stress-shielding. Use of a composite stem with a flexibility that was above this limit resulted in a negligible alteration in femoral bone-remodeling and a significant decrease in the extent of bone ingrowth (p = 0.003). These findings seem to confirm the theory that excessive flexibility of the stem may jeopardize interface stability. This information may be helpful in the design of femoral prostheses for clinical use.