Do ACL Reconstructions Prevent a Pivot?
Abstract & Commentary
Synopsis: Using biomechanical testing in a cadaver model, both hamstring and patella tendon ACL reconstructions effectively stabilized knees for anteriorly directed forces. However, for combined rotational forces such as experienced with a pivot shift, neither graft was effective.
Source: Woo SL, et al. J Bone Joint Surg Am. 2002;84-A: 907-914.
Both hamstring and patella tendons are commonly used to reconstruct the ACL with good clinical results. Most biomechanical studies have critically evaluated success with anteriorly directed loads. However, the functional instability from ACL tears involves a pivot shift, which is more of a rotational, anterior and valgus force. Woo and colleagues at the University of Pittsburgh used their well-established robotic/universal force-moment sensor testing system to address this issue.
Using 12 fresh human cadaver knees they tested the same knee with an intact ACL, a sectioned ACL, a hamstring reconstructed ACL, and a patella tendon reconstructed ACL. Each of these conditions were tested at varying degrees of flexion for an anterior load as well as a combined rotational load of valgus and internal tibial rotation in slight flexion to simulate a pivot shift. The amount of anterior translation and the in situ forces within the graft were measured.
Both the patella tendon and the hamstrings graft were effective in stabilizing the anteriorly directed forces. Interestingly, the patella tendon graft reduced translation to within 2.7 mm of the intact knee whereas the hamstring grafts reduced translation to only within 4.1mm of the intact knee. Unfortunately, for the combined rotational loads both grafts were not as effective. The amount of increased anterior translation for both reconstruction methods was within 2 mm of the amount of translation seen with the ACL deficient knee. Again, the patella tendon was slightly tighter than the hamstring grafts at 2.9 mm vs. 3.6 mm of translation compared to the intact knees. Woo et al conclude that current reconstruction techniques, which are fairly central within the knee and involve a single set of tunnels, may not accurately reproduce the anatomy and, therefore, may not adequately reconstruct the anterior cruciate ligament.
Comment by David R. Diduch, MS, MD
Rotational instability involving a pivot shift type motion for an ACL deficient knee is a difficult thing to quantify. Woo et al can be congratulated for the use of their robotic testing system to begin to quantify this symptomatic variable. Indeed, it is the pivot shift that patients complain of for functional instability and that we strive to prevent with our reconstructions. Woo et al raise an interesting point that current graft and reconstruction techniques do not adequately protect against this. They propose 2 bundle grafts or alternative reconstruction techniques as an answer.
What are we to do with this information? First, I think it is important to remember that functionally our patients really do well following ACL reconstruction. How can this be resolved? One possibility is that the rotational loads that they applied to the knees did not cause as much anteriorly directed force and translation that could be measured as the pure anteriorly directed force. Indeed, the amounts of translation for the ACL deficient knees were in the range of 18 mm for the anteriorly directed force but only 11 mm for the combined rotational loads. They concluded that because the reconstructed knees were only within 2 mm of the ACL deficient knees for anterior translation with rotation load that the grafts were not protective. However, they also concluded that roughly the same amounts of anterior translation for the reconstructed knees when subjected to an anteriorly directed force were protective. My concern would be that this testing system may not be sufficiently sensitive to detect differences for rotational loads when anterior translation is measured.
They also found that the in situ forces in the ACL grafts were extremely low under combined rotational loads at just 45-65% of that intact knee. They concluded that this was evidence that the grafts were not protective for this motion and indeed that may be the case. However, it still could also reflect the fact that this rotational load may not be sufficient to really translate the knees anteriorly and stress the graft.
Woo et al’s question of current reconstruction techniques bears further examination. Indeed, ACL reconstructions that are central in the knee with vertical tunnels would be poorly suited to resist rotational stresses. Recent work by Steven Howell and others has helped us to understand the importance of the angle of the tibial tunnel in the sagittal plane. By making this tunnel more obliquely at 70° or less from the tibial plateau we can have the graft oriented more horizontally towards a more lateral femoral tunnel. Comparing tunnel angles would be a logical step to use this robotic testing system next. Until then, I am not ready to give up on the good clinical results that I continue to see.
Dr. Diduch, Associate Professor, Department of Orthopaedic Surgery, University of Virginia School of Medicine, Charlottesville, VA, is Editor of Sports Medicine Reports.
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