Is the transverse acetabular ligament a reliable cup orientation guide? Computer simulation in 160 hips

Background and purpose It is controversial whether the transverse acetabular ligament (TAL) is a reliable guide for determining the cup orientation during total hip arthroplasty (THA). We investigated the variations in TAL anatomy and the TAL-guided cup orientation. Methods 80 hips with osteoarthritis secondary to hip dysplasia (OA) and 80 hips with osteonecrosis of the femoral head (ON) were examined. We compared the anatomical anteversion of TAL and the TAL-guided cup orientation in relation to both disease and gender using 3D reconstruction of computed tomography (CT) images. Results Mean TAL anteversion was 11 degrees (SD 10, range -12 to 35). The OA group (least-square mean 16 degrees, 95% confidence interval (CI): 14-18) had larger anteversion than the ON group (least-square mean 6.2 degrees, CI: 3.8 - 7.5). Females (least-square mean 20 degrees, CI: 17-23) had larger anteversion than males (least-square mean 7.0 degrees, CI: 4.6-9.3) in the OA group, while there were no differences between the sexes in the ON group. When TAL was used for anteversion guidance with the radiographic cup inclination fixed at 40 degrees, 39% of OA hips and 9% of ON hips had more than 10 degrees variance from the target anteversion, which was 15 degrees. Interpretation In ON hips, TAL is a good guide for determining cup orientation during THA, although it is not a reliable guide in hips with OA secondary to dysplasia. This is because TAL orientation has large individual variation and is influenced by disease and gender.