The adverse consequences of impingement,
dislocation,
and implant wear have stimulated increasing interest in accurate component orientation in total hip arthroplasty and hip resurfacing [1].
Hip dislocation is a frequent mode of failure of THA with 0.5% to 10% dislocation rates reported for primary THA and 10% to 25% after revision surgery.
The dislocation risk is multifactorial.
In addition to infection,
wear,
issues with the quality of the surrounding soft tissue,
and the type of prosthesis,
the position of the implants plays a major role in the mechanical stability of a THA [2].
Correct anteversion of the acetabular component in THA is essential for prosthetic stability and to minimize wear.
[2,
3,
4]
The optimal cup position and intra- and post- operative measurement methods are still under discussion.
The CT scan is the current gold standard for assessing the position of the implants in case of dislocation [5,
6].
Some surgeons prefer to align cup according to the concept of the “safe zone”,
while others use anatomical landmarks.
The orientation of the native acetabulum did not match the "safe zone" for acetabular component placement described by Lewinnek [3,
7].
Hypothesis of Beverland,
et al.
is that,
in the normal acetabulum,
the transverse acetabular ligament and labrum form a plane that comes just beyond the equator of the acetabulum,
unlike the bony acetabulum,
which is less than a hemisphere.
If the hemispherical “cup sizer” (reamer) is positioned so as to be cradled by the transverse acetabular ligament and orientated so as to sit parallel to and just deep to the line formed by the transverse acetabular ligament and the remaining posterior labrum this is the ideal location for the cup by Beverland,
et al [13].
(Fig 1.)
Fig. 1: The concept of Beverland et al. [13] The central diagram is the ideal position with the cup parallel to the transverse acetabular ligament and just deep to it. The top left is too anteverted. The top right is too retroverted. The bottom left is too deep and the bottom right is too high.
References: Beverland D, The Transverse Acetabular Ligament: Optimizing Version ORTHOPEDICS September 2010;33(9):631. DOI: 10.3928/01477447-20100722-22
Alignment of the acetabular component with the TAL and the posterior labrum might reduce the variability of acetabular component placement in total hip replacement.
(Fig.
2-7)
Fig. 4: Intraoperative view. Patient is in the lateral decubitus position. The hip is dislocated and the femoral head resected. An intraoperatively acetabular exposure showing the various retractors and a black arrow pointing to the transverse acetabular ligament. Acetabular component is too anteverted.
Fig. 5: Axial CT-scan. AAA=34.7° Cup is in excessive anteversion.
Fig. 6: Intraoperative view. Patient is in the lateral decubitus position. The hip is dislocated and the femoral head resected. An intraoperatively acetabular exposure showing the various retractors and a black arrow pointing to the transverse acetabular ligament (partial view up to excessive retroversion). Acetabular component is too retroverted.
Fig. 7: Axial CT-scan. Acetabular version angle=-15.2° Cup is in retroversion.
The advantages of the transverse acetabular ligament are many.
It is independent of patient positioning.
The cup version can be individualized by the patient.
The surgeon can avoid estimating version angle of 15° to 20° intraoperatively.
It is easy to teach and consistently present.
It is valuable in minimally invasive surgery.
Using the transverse acetabular ligament provides an acceptable dislocation rate with the posterior approach.
If the cup is cradled by the transverse acetabular ligament (Fig 2,
3),
it helps restore acetabular joint center [8,
9,
13].
Fig. 2: Acetabular component is in optimal position. Intraoperative view. Patient is in the lateral decubitus position. The hip is dislocated and the femoral head resected. An intraoperatively acetabular exposure showing the various retractors and a black arrow pointing to the transverse acetabular ligament. Acetabular component is positioned so as to be cradled by the transverse acetabular ligament and orientated so as to sit parallel to and just deep to the line formed by the transverse acetabular ligament and the remaining posterior labrum. This is the ideal location for the cup by Beverland et al[13].
Fig. 3: Axial CT-scan. AAA=10.8° Cup is inside the “safe zone”.