Is the transverse acetabular ligament a reliable cup orientation guide?

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° (SD 10, range -12 to 35). The OA group (least-square mean 16°, 95% confidence interval (CI): 14-18) had larger anteversion than the ON group (least-square mean 6.2°, CI: 3.8 - 7.5). Females (least-square mean 20°, CI: 17-23) had larger anteversion than males (least-square mean 7.0°, 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°, 39% of OA hips and 9% of ON hips had more than 10° variance from the target anteversion, which was 15°.

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.