To evaluate the efficacy of the smooth surface hydroxyapatite (HA)-coated acetabular cup in total hip arthroplasty.

From March 1992 to June 1998, 289 smooth surface HA coated acetabular cups were implanted. This study included 29 retrieved cups that had been in situ for an average of 54.9 months. The three dimensional acetabular cups were converted into two dimensional figures using a digital camera, and the ratio of bone ongrowth and HA absorption were calculated using a computer imaging analysis system (analySIS-pro 3.0, Soft Imaging System GmbH, Germany). The central hole and the peripheral 12 holes were excluded from total surface area of the cup, which was divided into 3 surfaces by 2 circular grooves.

Bone ingrowth was noted in 26 of the 29 cases; in 11 cases (37.9%) on the central surface, 12 cases (41.3%) on the middle surface and 8 cases (27.5%) on the peripheral surface. Bone ongrowth was noted in 22 cases (75.8%) on the medial groove and in 21 cases (72.4%) on the lateral groove. The average rate of bone ongrowth was 13.8 percent. HA absorption was noted in all cases and averaged 60.5 percent.

Bone ongrowth on the smooth surface HA-coated acetabular cup was very scant. Bone ongrowth was seen more on the grooves than on the surfaces. We believe that a rough surface would enhance bone growth on HA-coated acetabular cup.