Introduction  

A flattened cup was designed to create a more physiological load transfer to the pelvic bone compared to hemispherical cups, and to allow more bone contact compared to low-profile’ spherical cups. To investigate these theoretical advantages and the potential influence of the quality of the acetabular bone, a clinical study was performed in patients with osteoarthritis (OA) and inflammatory arthritis (IA). The aims of the study were (1) to evaluate the fixation of the cup, postoperatively and later when osseous integration should have taken place, (2) to assess perioperative complications such as acetabular fractures and (3) to monitor the polar gap, a potential risk factor for osteolysis.

Patients and methods  

A prospective study was performed on all consecutive OA and IA patients with an indication for primary total hip arthroplasty (THA). Three hundred and nine OA patients (340 hips) and 65 IA patients (76 hips) were included. The acetabular component was the flattened press-fit EPF-PLUS^® cup, the femoral component the tapered cementless Zweymueller SL-PLUS^® stem. All revisions and complications were recorded.

Results  

The incidence of early loosening of the cup was 0 out of 340 in the OA group and 1 out of 76 in the IA group. The incidence of acetabular fractures was 7 out of 340 in the OA group and 3 out of 76 in the IA group. Failure rate for the acetabular component due to aseptic loosening or osteolysis after 6–10 years was 0% in the OA group and 4.8% in the IA group. In all cases available for follow-up the polar gap had disappeared and full osseous integration had taken place in both the groups.

Interpretation  

This study shows that the flattened press-fit acetabular component creates adequate initial mechanical stability to allow osseous integration and that the cup can be safely used in both OA and IA patients. However, after 6–10 years, in the IA group failure of the cup due to aseptic loosening occurred once and failure due to osteolysis occurred three times, while these type of failures did not occur in the OA group.