Reliability of the transverse acetabular ligament as a landmark for functional cup anteversion in total hip arthroplasty

Background: The transverse acetabular ligament (TAL) can be a useful and reproducible landmark in the orientation of the acetabular cup in total hip arthroplasty (THA). Its role in guiding cup orientation when aiming to implant in a functional anteversion orientation is unclear. The aim of this study was to assess the relationship between the TAL and the planned acetabular cup anteversion when implanted in a function orientation. Material and Methods: In a retrospective study the anteversion of the TAL in the contralateral un-replaced hip was measured in CT scans of patients undergoing THA and compared to the functional cup anteversion using the patient-specific spinopelvic parameters. Comparative measurements of the native acetabular version were made from the superior rim to the inferior rim and at 10 mm intervals between the 2, all in reference to the anterior pelvic plane. Results: 96 hips were measured. The mean TAL anteversion angle was 17.2 degrees +/- 4.5 degrees. The mean planned acetabular cup anteversion angle was 26.3 degrees +/- 4.7 degrees. Pearson's correlation coefficient of this measurement with the TAL was -0.03 (p = 0.769). There was a significant difference between the planned acetabular cup anteversion and the measurement of the TAL (p < 0.0001). Conclusions: If cups are implanted parallel to the TAL, almost 80% will be >5 degrees different to targeted functional cup version. It should be aimed to implant cups with more anteversion than the TAL indicates.