Measurement of extent of bone ongrowth and hydroxyapatite absorption in retrieved acetabular cups.

BACKGROUND

To understand why hydroxyapatite (HA)-coated acetabular cups did not produce satisfactory results, we measured the extent of bone ongrowth and HA absorption of the retrieved cups.

METHODS

Between March 1992 and June 1998, a total of 289 patients underwent total hip arthroplasty with an HA-coated ABG (Anatomical Benoist Giraud) prosthesis in our hospital. We revised 71 cups in our hospital, 43 of which were included in this study. The cups were in situ for an average of 80.4 months. We revised 21 cups for polyethylene wear and osteolysis, 20 for loosening, and 2 for recurrent dislocation. The outer surfaces of the cups were subdivided by two circular grooves into three areas: central, middle, peripheral. Bone ongrowth and HA absorption was measured in terms of area and thickness. The areas were graphically calculated using a computer imaging analysis system. The thickness of the HA coating remaining on the retrieved cup was measured by an x-ray coating thickness measurement instrument.

RESULTS

Of the 43 cups, 37 had bone ongrowth. Bone ongrowth was observed on the central surface in 23 cups, on the middle surface in 27 cups, and on the peripheral surface in 15 cups; 6 of the 37 cups had bone ongrowth only in the groove. Altogether, 37 cups showed bone ongrowth on 6.43% of the outer acetabular surface. HA absorption appeared in all 43 cups, and an average 63.21% of the HA coating area was absorbed. there was a proportional relation between the rate of HA absorption and implantation duration, but there was no consistent trend between the thickness of the HA coating and implantation duration.

CONCLUSIONS

We suggest that progressive HA absorption and minimal bone ongrowth are related to the failure of HA-coated acetabular cups at long-term follow-up after total hip arthroplasty.