Loosening of the cup after low-friction arthroplasty in patients with acetabular protrusion - The importance of the position of the cup

Between 1972 and 1990, we performed 168 primary low-friction arthroplasties in 125 patients with acetabular protrusion, Twelve hips were lost to follo w-up within eight years and eight which became infected were excluded from the final study. Of the 148 hips remaining, 62 with a mild protrusion were classified as group 1, 54 with moderate or severe protrusion as group 2 ana l, after 1985, 32 with moderate and severe protrusion which required bone g rafts as group 3, The mean follow-up was 18.3 years (3 to 24) for group 1, 17.4 years (8 to 22) for group 2 and ten years (8 to 13) for group 3, There were 31 revisions of the cup, 12 in group 1 and 19 in group 2, Accord ing to the Kaplan-Meier analysis the overall rates at 20 years were 21 +/- 10.79% in group 1 and 37 +/- 11.90% ist group 2. There have been 43 radiolo gical loosenings: 22 in group 1, 21 in group 2 and none so far in group 3, at ten years. The overall loosening rates at 20 years were 42 +/- 14.76% in group 1 and 49 +/- 19.50% in group 2, The grafts were well incorporated in all group-3 hips, and the hone structure appeared normal after one year. The distance between the centre of the head of the femoral prosthesis and t he approximate true centre of the femoral head was less in group 3 than in groups 1 and 2 (p < 0.01), According to the Cox proportional-hazards regres sion this wats the single most important factor ist loosening of the cup (o dds ratio 1.11; 95% CI 1.05 to 1.18/mm), Better results were obtained in mo derate and severe protrusions reconstructed with bone grafting than in hips with mild protrusion which were not grafted.