THE BONE-IMPLANT INTERFACE OF FEMORAL STEMS WITH NON-CIRCUMFERENTIAL POROUS COATING - A STUDY OF SPECIMENS RETRIEVED AT AUTOPSY

A histological study was performed of the bone-implant interface of fi fteen titanium-alloy femoral stems with porous coating limited to thre e proximal areas that did not cover the full circumference of the devi ce. The specimens were obtained at autopsy from ten cadavera at a mean of forty-sis months (range, one to eighty-nine months) after the impl ant had been inserted without acrylic cement. The volume fraction of b one within the porous spaces (the percentage of the porous space that was filled with bone) and the extent of bone ingrowth (the percentage of the porous-coated surface covered with ingrown bone that was more t han one-half fiber-diameter deep, as measured from the outer surface o f the porous coating), were determined with histomorphometric methods. Eleven of the fifteen stems had bone within the porous coating that w as in continuity with the surrounding medullary bone. The mean volume fraction of bone ingrowth in these specimens was 26.9 per cent (range, 12.2 to 61.0 per cent), and the mean extent of bone ingrowth was 64.3 per cent (range, 28.6 to 95.2 per cent). Both of these parameters inc reased with time. in the other four stems, the bone lacked continuity with the surrounding trabecular bed. Two of these stems had a limited amount of bone within the porous coating, and two stems (from one pati ent) had no bone ingrowth, Periprosthetic membranes surrounded by a sh ell of trabecular bone covered the uncoated surfaces of the stems. The membranes of implants that had been in situ for eight months or more demonstrated polyethylene wear debris, and other particles generated a t the level of the joint, within histiocytes throughout the length of the femoral stem. CLINICAL RELEVANCE: The findings in this study are r elevant to the utilization and mechanisms of failure of femoral stems inserted without cement. Bone ingrowth and the resulting stability of the implant can be achieved with porous-costed stems. However, the ext ent of the surface that is porous-coated must be sufficient to prevent trabecular fracture as a secondary mechanism of loosening. Interrupti ons in the circumferential extent of the porous surface are associated with the formation of periprosthetic membranes, which provide a pathw ay for migration of particulate wear and corrosion products to the dis tal part of the stem. A circumferential coating may retard the access of particles and thus decrease the possibility of diaphyseal osteolysi s.