Modular necks in primary hip arthroplasty without anatomical deformity: no clear benefit on restoration of hip geometry and dislocation rate. An exploratory study.

Modular necks were introduced in total hip arthroplasty (THA) to improve restoration of hip-geometry and reduce dislocation-rates. This presumed advantage was evaluated retrospectively for patients with arthritis in otherwise anatomically normal hips. Restoration of hip-geometry was assessed on preoperative and postoperative calibrated radiographs in 95 consecutive primary THAs with a modular neck design and compared with 95 match controlled THAs with a similar monoblock stem. No significant differences were seen in restoration of body moment arm, leg length and cupangle. Offset restoration revealed a borderline significant difference (P = 0.48) with higher values for the monoblock stem. In both groups 4 dislocations within one year were encountered. In this study modular necks did not reveal a clear benefit in restoring hip geometry and dislocation rate after straightforward THA.