[Acetabular shape and cementless cups. Comparison of osteoarthritic hips and implant design]

The anatomy of the hip must be taken into account in order to ensure primary stability of cementless acetabular implants. In this study we analyzed the shapes of osteoarthritic acetabula and compared these to normal non-degenerative acetabula and to pressfit implants. We measured 92 acetabula with osteoarthritic deformations and 35 non-degenerative acetabula. Bone tissue samples from 50 osteoarthritic acetabula were microradiographically analyzed. Furthermore, the size of the entrance plane of 37 pressfit cups was determined. The craniocaudal and ventrodorsal diameters of osteoarthritic acetabula correlate strongly ( r=0.87). In craniocaudal direction, the acetabular diameter correlates significantly to both the radius of the lunate surface ( r=0.42) and the acetabular base ( r=0.54). Osteoarthritic acetabula have a deeper shape as degeneration increases and the entrance plane becomes significantly more circular ( p<0.05). When comparing osteoarthritic and non-arthrotic acetabula, the following values differ significantly (p<0.05): craniocaudal radius of the acetabular base, craniocaudal and ventrodorsal radius of the lunate surface, and ventrodorsal divergence between lunate surface and acetabular base. To reconstruct an acetabular offset which concurs with central points of the femoral head and the radius of the lunate surface, the level of the insert's entrance plane must be outside the entrance planes of the cup and acetabulum. The rims of hemispheric cups need to be trimmed to prevent these cups from extending beyond the acetabular rim.