Is the transverse acetabular ligament a reliable cup orientation guide?

Abstract

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.

Figure 1

Definition of TAL and TAL-guided cup placement. Step 1: The opening plane (OP), which was fitted to the opening plane of the acetabulum, was settled. This plane contained two lines. A. The first line (white line on the coronal plane) was drawn from the tear drop to the upper acetabular rim edge at the level of the femoral head center. The second line (white line on the axial plane) was drawn from the anterior edge to the posterior edge of acetabular rim at the level of the femoral head center. B. The OP was rotated to be parallel with the y-axis in the coronal plane and parallel with the z-axis in the axial plane. C. The acetabular notch (AN) was identified at the inferior acetabulum. Step 2: The TAL (the black line) was defined as the AP line between the anteroinferior edge; point A and posteroinferior edge; point P of the acetabular rim in the craniocaudal view (D) and lateral view (E). F. Intersection; point R of acetabular rim edge with the vertical plane containing the TAL was settled in the anteroposterior view of the TAL. Line RM, the guide for cup inclination, was drawn from the TAL midpoint M to point R. The TAL plane containing lines AP and RM was settled. G. 3D image of the TAL and the TAL plane. Step 3: The equatorial plane of the cup was virtually placed so as to be parallel with line AP in the craniocaudal view (H), lateral view (I), and line RM in the anteroposterior view (J). K. 3D image of the cup placement. Step 4: Anatomical anteversion of the TAL in the ACS and the FCS and radiographic cup orientation guided by the TAL in the ACS and the FCS was measured in the axial view (L) and the coronal view (M).

Figure 2.

Verification of the TAL alignment measurement method. We verified the measurement method using cadaveric hips. A. The TAL and acetabulum were exposed. B. The beads were attached at the anterior and posterior insertion of the TAL. C. CT images before and after attachment of the beads on the posteroinferior and anteroinferior edges of the acetabular rim were obtained. We compared the anteversion of the line between the bony points on the first CT and the line between the beads on the second CT.

Figure 3.

Scatter diagram of cup orientation measurements in the ACS and FCS. In the ACS, 78 of 80 OA hips (98%) and 49 of 80 ON hips (61%) had a more than 10° difference from our target cup orientation (dotted line area). In the FCS, 78 of 80 OA hips (98%) and 45 of 80 ON hips (56%) had a more than 10° difference from the target cup orientation.