Effect of flexibility of the femoral stem on bone-remodeling and fixation of the stem in a canine total hip arthroplasty model without cement

The purpose of this study was to compare, with regard to fixation of the im plant and femoral bone resorption, two fully porous-coated stems of differe nt stiffnesses in a canine total hip arthroplasty model. A bilateral arthro plasty was carried out with insertion of a titanium-alloy stem (which had s tiffness Properties comparable,vith those of the canine femur) on one side and with insertion of a composite stem (which was three to fivefold more fl exible 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, int racortical porosity, and bone-remodeling. Despite the markedly greater flexibility of the composite stems, no signifi cant difference could be defected (with the numbers available), with regard to the overall degree of femoral stress-shielding, cortical area, or corti cal 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 e ighteen months, the values for bone ingrowth were 9.7 +/- 5.38 percent (mea n and standard deviation) for the composite stems compared with 28.1 +/- 5. 31 percent for the titanium-ahoy stems (p = 0.003). Furthermore, the histol ogical sections from the femora containing a composite stem showed radiopaq ue 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 ef fective limit beyond which increasing the stiffness of the femoral stem rel ative 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 decreas e in the extent of bone ingrowth (p = 0.003), These findings seem to confir m the theory that excessive flexibility of the stem may jeopardize interfac e stability. This information may be helpful in the design of femoral prost heses for clinical use.