Micromotion of the acetabular component and periacetabular bone morphology.

The quality of the periacetabular bone might be an explanation for the increase in the rate of socket loosening seen radiographically in patients with rheumatoid arthritis as compared with patients with osteoarthrosis. Early implant micromotion, as measured by roentgen stereophotogrammetric analysis, is of predictive value with regard to long time retention. For uncemented porous sockets, the initial fixation might be decisive for the degree of bone ingrowth. Roentgen stereophotogrammetry was used to study cemented Charnley acetabular components in 32 hips with rheumatoid arthritis and 30 hips with osteoarthrosis, and uncemented Harris-Galante acetabular components in 19 hips with osteoarthrosis. Micromotions as long as 24 months after surgery were related to the periacetabular cancellous bone quality, as assessed by histomorphometric methods from samples taken during surgery. Acetabular components migrated more in hips with rheumatoid arthritis than in those with osteoarthrosis (p < 0.04). Hips with rheumatoid arthritis had approximately 4 times more nonmineralized bone than hips with osteoarthrosis (p < 0.0002). However, within each diagnostic group, no correlation was found between migration and the degree of mineralization (r < or = 0.24, p > or = 0.07). Migration of uncemented acetabular components did not correlate with the histomorphometric variables (r < or = 0.20, p > or = 0.42). Histomorphologic characteristics of the periacetabular trabecular bone do not seem to be of importance for acetabular component micromotion.