What we have learned about long-term cementless fixation from autopsy retrievals

The current authors summarize insights gained from their experience studying uncemented porous-coated femoral and acetabular hip replacement components retrieved at autopsy. For femoral components, autopsy studies showed that osseointegration occurs over an average 35% of the porous surface with the most predictable ingrowth occurring near the termination of the porous coating. The bone ingrowth pattern causes a predictable bone remodeling pattern: an overall decrease in bone mineral content (mean, 23% loss) occurring on a gradient with the most loss proximally and the least distally. This pattern occurs regardless of the implant's level of coating, mirrors the gradient of strain reduction, and because it is recognized easily, can be used by orthopaedists to distinguish between components with and without bone ingrowth on standard radiographs. Studies also have shown that femoral bone loss is more related to the characteristics of the implanted femur than any other variable. On the acetabular side, bone ingrowth is more unpredictable and occurs randomly. Although the amount of acetabular bone ingrowth averaged 32%, it ranged from 3% to 84%. Unfortunately, a consistent pelvic remodeling pattern has not been associated with an implanted porous-coated uncemented cup. Consequently, orthopaedists cannot determine radiographically whether an acetabular component truly is bone ingrown. Overall, autopsy studies have contributed to a basic understanding of the histologic and radiographic appearance of osteointegrated porous-coated implants and have confirmed the durability of this three-dimensional fixation even in the presence of osteolysis.