Is transverse acetabular ligament a reliable guide for aligning cup anteversion in total hip arthroplasty? A measurement by CT arthrography in 90 hips

Background Transverse acetabular ligament (TAL) has been used as a landmark for aligning cup anteversion. The use of TAL as a guide is based on the assumption that TAL version is distributed within the safe zone of acetabular cup. However, there was rarely reported to compare anteversion between TAL and acetabulum using direct measurement methods. The purpose of this study was to measure the anteversion of TAL in computed tomography arthrography (CTA) and compare it with Lewinnek's safe zone and anteversion of bony acetabulum. Methods 81 patients (90 hips) were selected among 204 patients (228 hips) who received CTA for hip pathology evaluation between March 2010 and June 2013. The anteversion of TAL measured at the lowest level of the acetabular notch and the anteversion of the acetabulum was measured at the level of femoral head center. Results The mean TAL anteversion was 11.8° (SD 4.5, range 0–22.2). In eight hips (8.8%), TAL anteversion was outside the safe zone (15° ± 10°) as defined by Lewinnek. The mean acetabular anteversion was 13.3° ± 4.4° (range −1.0° to 22.6°). There was a strong correlation between TAL anteversion and acetabular anteversion (Pearson's correlation coefficient; 0.908, p < 0.001). Conclusions TAL anteversion has a large individual variation, and considerable portion of hips have TAL anteversion outside the safety zone of cup anteversion. TAL anteversion is influenced by acetabular anteversion. In hips with retroverted or pauci-anteverted acetabulum, TAL should be used cautiously because there is a risk of cup malposition.