In total hip arthroplasty (THA), excessive retroversion is associated with posterior instability, anterior impingement, and resultant groin pain. Excessive anteversion can lead to anterior instability and posterior impingement. The transverse acetabular ligament straddles the inferior limit of the bony acetabulum. It is a strong load-bearing structure and, in the normal hip, in association with the labrum, provides part of the load-bearing surface for the femoral head. It is our hypothesis that the transverse acetabular ligament defines normal version for the acetabulum. In Belfast, we found that using the transverse acetabular ligament helped reduce our primary dislocation rate from 3.7% to 1%. The key is good intraoperative exposure. A grading of 1 to 4 was based on 1000 consecutive cases: (1) normal transverse acetabular ligament easily visible on exposure of the acetabulum, 49%; (2) covered by soft tissue, 35.1%—cleared by blunt dissection; (3) covered by osteophytes, 15.6%—cleared using an acetabular reamer; (4) no transverse acetabular ligament identified, 0.3%. As can be seen, the transverse acetabular ligament is only immediately visible in 49% of cases. In the other 51%, soft tissue or bone must be cleared to define the ligament. 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, it helps restore acetabular joint center. However, the transverse acetabular ligament does not help with inclination. We recommend 35° of operative inclination when using the posterior approach.
One of the most widely referenced papers on acetabular component orientation was published in 1978 by Lewinnek et al.1 With respect to cup version, the authors recommended a safe range of 15° of anteversion (±10°) based on the radiographic analysis of 9 dislocations out of 300 total hip arthroplasties (THAs) done by 5 different surgeons. The safe range quoted in this article should be viewed with caution because of the small numbers involved and the diversity of the group.
In contrast to version, we have more proven boundaries when it comes to a safe zone for cup abduction (inclination) angles. It is widely acknowledged that radiographic abduction angles >50° are to be avoided. Patil et al2 demonstrated increased polyethylene wear with abduction angles >45°. De Haan et al3 found increased metal ion levels in large metal-on metal-articulations with inclinations >55°.
Compared to cup abduction, anteversion is both more difficult to control during surgery and to measure on the postoperative radiograph. Furthermore, there is more individual variation with anteversion than with abduction angle.
At our institution, Archbold et al4 reported on 25 consecutive patients, who were being investigated for labral tears with a magnetic resonance image (MRI) arthrogram of the hip. The use of contrast allowed us to visualize the plane formed by the transverse acetabular ligament and labrum. Perhaps not surprisingly, answers similar to the quoted literature were found in that the average operative anteversion of the transverse acetabular ligament-labrum plane was 23° and the average abduction was 45.6°. However, the range for anteversion was 5.3° to 36.1° (>30°), whereas the range for abduction was only 38.4° to 50.3° (<12°).
As can be appreciated from Figure 1, orthopedic surgeons are generally not good at judging 20°. Particularly with the lateral decubitus position, it is difficult to ensure correct patient position at the start of surgery compounded by the potential for the patient to move intraoperatively. Even if we could achieve 20°, would this be ideal for every patient? Based on our MRI results,4 the authors think not.
Figure 1: Demonstrates the difficulty in judging which 2 lines are 20° to each other (7=35°, 63=0°, 5=25°, 4=20°, 3=15°, 2=10°, 1=5°).
With respect to cup version, the ideal solution would be a patient-specific landmark that was easy to find, easy to use effectively, and independent of patient position on the table. The transverse acetabular ligament, in combination with the labrum, provides at least 1 viable solution. Figure 2 shows an anatomical dissection that clearly illustrates how the transverse acetabular ligament and labrum form a natural plane; but if you’re going to use the transverse acetabular ligament for version you must see it as clearly as this. The key to using the transverse acetabular ligament is good exposure (Figure 2).
Figure 2: An anatomical dissection illustrating the transverse acetabular ligament and labrum forming a recognizable plane. It also emphasizes the importance of good exposure intraoperatively if using the transverse acetabular ligament.
The authors use the posterior approach with the patient in the lateral decubitus position. The hip is dislocated and the femoral head resected. A caliper is then used to measure femoral head diameter. If for example femoral head diameter was 48 mm, then the acetabular cup size should be a maximum of 4 mm larger or 52 mm.
Acetabular exposure is shown in Figure 3. The first Charnley pin enters the ischium followed by a second Charnley pin into the posterior wall of the acetabulum, being careful that it does not enter the acetabulum. The anterior retractor goes anteriorly and superiorly so as not to go over the anterior wall. Placing this retractor over the anterior wall can damage the anterior wall and increase the risk of neurovascular injury because of the proximity of the structures. The exposure is then completed by placing an inferior teardrop retractor beneath the transverse acetabular ligament (Figure 3).
Figure 3: An acetabular exposure intraoperatively showing the various retractors and a green arrow pointing to the transverse acetabular ligament.
As reported by Archbold et al,5 the authors have created a classification of transverse acetabular ligament based on 1000 consecutive cases (Table). In half the cases (49%), the transverse acetabular ligament is immediately visible on exposing the acetabulum. But this means that in the other half, the surgeon has to make an effort to find and expose it. Approximately one-third (35.1%) are grade II, which means the transverse acetabular ligament is covered by soft tissue. In this situation the soft tissue can be easily cleared by a combination of blunt and sharp dissection. The remainder (15.6%) are grade III, and in this grade the transverse acetabular ligament is covered by osteophyte. This can be removed to expose the transverse acetabular ligament by using a small diameter reamer which is directed caudally. This needs to be done slowly and carefully so as to avoid transverse acetabular ligament damage. The surgeon has to alternate between reaming and looking until the fibers of the transverse acetabular ligament start to appear. There is also the rare (0.3%) grade IV transverse acetabular ligament, which cannot be found. In this situation, the surgeon has inadvertently destroyed it. The greatest risk of damage to the transverse acetabular ligament occurs when it is covered by osteophytes (grade III).
Having exposed the acetabulum and transverse acetabular ligament, attention can then focus on the labrum. In almost 95% of cases the posterior labrum is present and relatively undamaged as reported by Archbold et al.5 The posterior labrum can usually be retained, as it does not obstruct preparation of the acetabulum. Its medial surface can be shaved with the scalpel, leaving the distal free margin, that in continuity with the transverse acetabular ligament, provides an aid to alignment. In contrast, the anterior and superior labrum, if still present, are frequently torn and need to be excised, as they impede and obstruct acetabular preparation. Also, any remains of the ligamentum teres is removed from the floor of the acetabulum, although it is our practice to leave the fat pad intact and not remove it.
Having exposed the acetabulum, an acetabular reamer that is the same size as the original femoral head is then used as a cup sizer. It is our hypothesis 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. In this position there should be no anterior overhang; joint center in terms of offset and height should be restored and version should be correct. The aim for us is to put the definitive cup in the same location. The only difference is that the abduction angle is reduced.
In contrast to this technique, much traditional teaching has been to ream the acetabulum down to true floor, but if this is done, the cup will sit too deep relative to the transverse acetabular ligament and joint center will be medialized resulting in a decreased acetabular offset. This is usually also associated with the cup sitting higher than the original joint center, thus offset and height are both incorrect with respect to restoration of joint center.
Initial reaming begins with a reamer that is smaller than the original head size, except in cases of protrusio, when a reamer equal to or even bigger than original head diameter is used to avoid any further deepening of the acetabulum. When reaming, the reamer is kept parallel to the transverse acetabular ligament except for the final reamer, which is moved eccentrically just before it is removed. This is because normally the teeth on acetabular reamers do not move as far as the equator; therefore, unless the reamer handle is moved eccentrically, the reamed acetabular cavity will not be hemispherical. The acetabulum should be reamed size for size or a maximum of 1 mm smaller. At the completion of reaming, bleeding subchondral bone should be visible and the fat pad should still be intact. Even when the acetabulum has been deepened because of mild dysplasia, reaming will not have gone as far as the true floor. Clearly in the presence of significant dysplasia, the acetabulum may have to be reamed down to true floor, but this is uncommon in our practice.
When it comes to cup insertion the cup will initially obscure the transverse acetabular ligament. This can be counteracted by initially reducing the abduction angle (in lateral decubitus the surgeon lowers his/her hand). This then brings the transverse acetabular ligament into view and the cup can be orientated so as to be parallel to the transverse acetabular ligament. The surgeon can then lift his/her hand to the chosen abduction angle and then impact the cup. Based on work soon to be published by Hill et al6 from our institution, our recommendation when using the lateral decubitus position is to aim for 35° of operative inclination as defined by Murray.7 In lateral decubitus, this means that the cup introducer is held at 35° relative to the floor. Once the cup is in place, the liner can then be inserted. It is critical that when the cup is inserted, there is no anterior overhang. If the acetabular component is inserted parallel and just deep to the transverse acetabular ligament, then there should be no anterior overhang and the cup should be flush with the psoas valley. In most cases, when using a cementless component, the external surface of the cup will be exposed superiorly and posteriorly as shown in Figure 4.
A summary of cup position is shown in Figure 5. As can be seen from Figure 5, staying parallel to the transverse acetabular ligament can control version. In addition, by not reaming too deep to the transverse acetabular ligament, acetabular offset is maintained. If the cup is deep to the transverse acetabular ligament, then when using a cementless cup, an offset liner can be used to compensate. Making sure the cup is cradled by the transverse acetabular ligament ensures that the cup is not left too high.
Figure 4: An acetabular cup in place showing the exposed external surface of the cup both superiorly and posteriorly, but with no anterior overhang.
Figure 5: 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.
For the concept to be effective, the surgeon has to ensure that the cup is parallel to the transverse acetabular ligament. As we have already seen in Figure 1, it is not easy to judge 20°, but how good are we at knowing when 2 lines are parallel? The human eye is able to discern when 2 lines are out of parallel by only 1° (Figure 6).
Figure 6: A photograph demonstrating that in contrast to the difficulty we have in judging 20°, the human eye is good at judging whether or not 2 lines are parallel. The central operative photograph shows the face of the cup lying parallel to the transverse acetabular ligament. In the diagrams, 5=10°, 4=5°, 3=3°, 2=2°, 1=1°.
The transverse acetabular ligament is consistently present, patient specific, independent of patient position on the operating table, easy to use, and easy to teach. Previous work published from our institution by Ogonda et al8 has shown that it is also of value in minimally invasive surgery of the hip using the posterior approach. Most specifically, it helps to control version and restore joint center, but not abduction angle. When using the posterior approach, we recommend that the surgeon aim for 35° of operative inclination.
- Lewinnek GE, Lewis JL, Tarr R, Compere CL, Zimmerman JR. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am. 1978; 60(2):217-220.
- Patil S, Bergula A, Chen PC, Colwell CW Jr, D’Lima DD. Polyethylene wear and acetabular component orientation. J Bone Joint Surg Am. 2003; 85 Suppl 4:56-63.
- De Haan R, Pattyn C, Gill HS, Murray DW, Campbell PA, De Smet K. Correlation between inclination of the acetabular component and metal ion levels in metal-on-metal hip resurfacing replacement. J Bone Joint Surg Br. 2008; 90(10):1291-1297.
- Archbold H, Slomczykowski M, Crone M, Eckman K, Jaramaz B, Beverland DE. The relationship of the orientation of the transverse acetabular ligament and acetabular labrum to the suggested safe zones of cup positioning in total hip arthroplasty. Hip Int. 2008; 18(1):1-6.
- Archbold HAP, Mockford B, Molloy D, McConway J, Ogonda L, Beverland D. The transverse acetabular ligament, an aid to acetabular component placement during total hip arthroplasty. J Bone Joint Surg Br. 2006; 88(7):883-886.
- Hill J, Gibson D, Pagoti R, Beverland D. Photographical measurement of acetabular cup inclination in total hip arthroplasty using the posterior approach. J Bone Joint Surg Br. In press.
- Murray DW. The definition and measurement of acetabular orientation. J Bone Joint Surg Br. 1993; 75(2):228-232.
- Ogonda L, Wilson R, Archbold P. A minimal-incision technique in total hip arthroplasty does not improve early postoperative outcomes. A prospective, randomized controlled trial. J Bone Joint Surg Am. 2005; 87(4):701-710.
Dr Beverland is from The Primary Joint Unit, Musgrave Park Hospital, Belfast, United Kingdom.
Dr Beverland’s research unit receives financial support from Biomet and DePuy.
Presented at Current Concepts in Joint Replacement 2009 Winter Meeting; December 9-12, 2009; Orlando, Florida.
Correspondence should be addressed to: David Beverland, MD, FRCS, The Primary Joint Unit, Musgrave Park Hospital, 20 Stockman’s Ln, Belfast BT9 J7B United Kingdom (firstname.lastname@example.org).