Bone ingrowth on a smooth-surfaced hydroxyapatite-coated acetabular cup.

We studied 29 retrieved smooth-surfaced hydroxyapatite (HA)-coated acetabular cups that had been in situ for an average of 54.9 months. The outer surface of all cups carried two circular grooves dividing the smooth surface. The extent of bony in-growth and hydroxyapatite absorption was calculated using a computer imaging system (analySIS-pro 3.0, Soft Imaging System GmbH, Germany). Absorption of HA was seen in all 29 cups. The extent of absorption was, on average, 60.5%. Twenty-five cups showed bony in-growth covering an average of 13.8% of the cup surface. Thirteen cups showed bony in-growth on the smooth surface as well as the circular grooves. On 11 cups, the bony in-growth was confined to the grooves. We found that bony in-growth on the smooth-surfaced HA-coated acetabular cup was minimal. It occurred along the two circular grooves rather than on the smooth surfaces. We conclude that a macro-structure surface enhancing bony in-growth is necessary for the long-term survival of the studied cup.