Introduction: Recent studies showed that the position of the center of rotation and the prosthetic neck may infl uence implant fixation in hip arthroplasty. The purpose of this study is to evaluate the use of modular necks and their limits to restore hip geometry after the arthroplasty.

Methods: The study radiographically evaluates 117 cases of unilateral hip arthroplasty using a modular neck / head concept (Wright Medical). The analysis evaluated on a weight-bearing frontal pelvis view: center of rotation, horizontal abductor ratio, and vertical abductor index, comparatively to the controlateral hip using the student’t test. On the computerized templates of the association effectively used in each case was measured: neck length, lever arm and neck anteversion.

Results: In craniopodal the center of rotation averaged 0.19 in women and 0.23 in men. In mediolateral it averaged 0.26 in women and 0.32 in men. The mean horizontal abductor ratio was 0.65 in women and 0.70 in men. The mean vertical abductor index was 6:..4° in women and 6.5° in men. The mean neck length was 55.2 mm, the mean lever arm 39.3 mm, and the 15° ante or retroverted neck was noted in 10 %.

Discussion and conclusion: Restoration of the center of rotation was more accurate in mediolateral than in craniopodal, with a higher location as previously noted in the litterature. Abductor function was correctly restored excepted for the vertical index in women, probably due to the large variations of pelvis width. The limits of modular necks are large hip dysmorphy where neck length averages 60 mm, lever arm 45 mm and neck anteversion requires 30° of correction.