Acetabular and Femoral Anteversions in Standing Position are Outside the Proposed Safe Zone After Total Hip Arthroplasty

Background

Although most hip dislocations occur in either standing or sitting position, the safe zone for implant position is defined for the supine position. Our goal was to determine preoperative and postoperative pelvis and hip orientations and whether the safe zone defined in supine position can be used to assess standing radiographs.

Methods

Preoperative and postoperative three-dimensional EOS images were assessed in 66 total hip arthroplasty patients. None of the patients had dislocation within the follow-up period (12-36 months). The acetabular anteversion (both anterior pelvic plane APP] and patient functional plane) and the femoral anteversion were measured. The sacral slope, pelvic version, pelvic inclination, and pelvic incidence were also measured.

Results

Acetabular anteversion increased postoperatively in both APP and patient functional plane (P <.001). Femoral neck anteversion decreased postoperatively (P =.0942). Sacral slope was 42.4° (?25.9° to 24°) preoperatively compared with 40.3° (?4.1° to 64.2°) postoperatively (P =.013). Pelvic version changed from 15.2° (?10.4° to 43.8°) to 17.2° (?6° to 46.7°; P = 0.008). Pelvic inclination was 1.12° (?25.9° to 24°) before total hip arthroplasty and ?1.2° (?40.7° to 23.4°) postoperatively (P =.005).

Conclusion

The acetabular and femoral implant orientations in standing position reside out of the safe zone in most patients. The APP is not vertical in standing position in most patients due to anterior or posterior pelvic tilt. The proposed safe zone in supine position may not be a useful measure in the assessment of standing radiographs of patients with significant anterior or posterior pelvic tilt.

Level of Evidence

Level IV, therapeutic case series study.