A retrospective study on the relationship between altered native acetabular angle and vertical implant malpositioning.

PURPOSE: Acetabular cup positioning in extreme angles of vertical position affects both stability and long-term survivorship of total hip arthroplasty. The purpose of this study is to determine whether native Sharp's angle is associated with an increased abduction angle of the acetabular component.

METHODS: Consecutive patients who underwent primary total hip replacement between February 2012 and August 2015 were included. Vertical positioning of acetabular implant in the antero-posterior post-operative radiographs were measured. The proportion of implants positioned outside the safe zone (40° ± 10°) was calculated and through a multivariate analysis, Sharp's angle and other factors possibly associated with cup malpositioning were evaluated.

RESULTS: Five hundred twenty-eight hip arthroplasties were analyzed. Prevalence of cup malpositioning was 7.6% and 25 patients had an altered native acetabular angle. An altered pre-operative Sharp's angle was associated with a higher risk of vertical malpositioning of the acetabular component (OR 2.51 IC 95%: 1.17-5.39) (p = 0.02). Body mass index, surgeon's volume, size of the implant, gender, pre-operative diagnose and age, were not associated with the position of the cup.

CONCLUSIONS: The alteration of the Sharp's angle as an indicator of hip dysplasia in native hips increases the odds of acetabular cup malpositioning. Other factors explored did not correlate with the position of the acetabular prosthesis. Systematic assessment of Sharp's angle should be included in the pre-operative planning of primary hip arthroplasty.