Hydroxyapatite-coated acetabular components. Histological and histomorphometric analysis of six cups retrieved at autopsy between three and seven years after successful implantation.

Background: Important questions remain regarding the use of hydroxyapatite-coated acetabular components in total hip arthroplasty. What is the relation of resorption of the hydroxyapatite coating to enduring fixation? Will unresorbed or dislodged hydroxyapatite particles cause adverse tissue reactions? Retrieval studies of clinically well-functioning acetabular components should help to answer these questions.

Methods: We examined six clinically successful hydroxyapatite-coated cementless acetabular components that were retrieved at autopsy between 3.3 and 6.6 years after implantation. All components were of the same design. The prostheses and the surrounding bone were prepared for qualitative histological and quantitative histomorphometric analysis. The percentage of bone growth onto the implant, the relative bone area around the implant, the extent of residual hydroxyapatite coating, and the coating thickness were measured.

Results: All of the cups showed bone ongrowth, with a mean bone-implant contact (and standard deviation) of 36.5% +/- 13.5%. The contact area was the same in all three zones delineated by DeLee and Charnley. The extent and thickness of the hydroxyapatite layer were much reduced in the specimens from older patients and in those associated with a longer duration of implantation. Degradation of the hydroxyapatite coating by osteoclasts was observed. We did not observe loose hydroxyapatite granules far from the coating, nor did we note any adverse tissue reaction to these granules. In contrast, polyethylene debris was noted in approximately half of the empty screw-holes.

Conclusions: Cell-mediated hydroxyapatite resorption seems to be the main reason for loss of hydroxyapatite coating. The area of bone ongrowth was within a certain range (20% to 50%) of the measured surfaces, and it was independent of the amount of hydroxyapatite residue. The hydroxyapatite coating showed a slow rate of resorption with time, without any adverse tissue reactions.