REMODELING AND INGROWTH OF BONE AT 2 YEARS IN A CANINE CEMENTLESS TOTAL HIP-ARTHROPLASTY MODEL

Remodeling and ingrowth of bone in association with the use of uncemented femoral components were examined at two years in a canine total hip-arthroplasty model. Twenty-two dogs received a unilateral uncemented femoral stem that was made of Ti6Al4V and was covered with one of three types of titanium porous coating: fiber-metal, beads, or plasma flame-spray. The amount and distribution of ingrowth of bone differed somewhat among the groups at two years, but the patterns of remodeling of bone in the medullary canal and cortex were similar. In general, about 15 to 18 per cent of the cortical bone was lost adjacent to the levels of the stem that were covered with the porous coating. Most of the loss of cortical bone was due to subperiosteal resorption proximally and endosteal resorption at the middle and distal levels of the stem. Increased cortical porosity accounted for only a small fraction of the loss of cortical bone. The amount of medullary bone increased proximally and distally, so that the loss of total bone mass was significant only at the mid-part of the stem. The amount of loss of cortical bone was similar to that observed in a previous six-month study, suggesting that a steady state was achieved in the present model. CLINICAL RELEVANCE: Loss of cortical bone proximally due to stress-shielding is a matter of concern in patients who have been treated with cementless total hip arthroplasty. The present study indicates that the cortex atrophies and the medullary bone hypertrophies proximally. This adaptive response occurs when the length of the stem is covered with a fiber-metal, bead, or plasma flame-sprayed porous coating. The adapted osseous structure appears to provide adequate support for the prosthesis. Although the stems in this study filled the canal substantially, they did not do so as completely as many currently used stems. Thus, there should still be concern about cortical atrophy, particularly in light of the trend toward the use of stiff, canal-filling stems.