Biomechanics of Two-Bundle PCL Reconstruction
abstract & commentary
Synopsis: A double-bundle technique for posterior cruciate ligament reconstruction reproduced knee stability and in situ ligament forces better than a single-bundle reconstruction.
Source: Harner CD, et al. Biomechanical analysis of a double-bundle posterior cruciate ligament reconstruction. Am J Sports Med 2000;28:144-151.
This article, winner of the 1999 cabaud award, compares the biomechanics of the single- vs. double-bundle techniques for posterior cruciate ligament (PCL) reconstruction. Ten cadavers were tested with a high-tech robotic system and comparisons were made between: a) intact knees, b) single-bundle reconstructed knees, and c) double-bundle reconstructed knees. Testing was done at five different flexion angles with a posteriorly directed force. Findings demonstrated that the single-bundle reconstruction was about 2 mm more loose throughout the range of motion. In addition, in situ forces in the graft were about 45% lower in the single-bundle graft as compared to the intact knee, suggesting that the secondary stabilizers of the knee were taking on more load in the knee with the single-bundle graft. The stability and in situ forces within the graft for the doublebundle reconstruction were basically equivalent to those for the intact knee. Thus, Harner and colleagues conclude that the double-bundle technique (Achilles tendon allograft for the anterolateral bundle and Semitendinosus tendon autograft for the posteromedial bundle) more closely restores the biomechanics of the intact knee throughout the entire range of motion.
Comment by Mark Miller, MD
This article suggests that the two-bundle graft more closely reproduces the biomechanics of the normal knee, a result that surprised even Harner! Although normal anatomy supports the concept of two portions or "bundles" of the PCL, most surgeons attempt to reconstruct only the stronger anterolateral bundle. This bundle has been shown to be important in flexion, and the PCL’s primary role is to resist posterior translation in flexion. It has been assumed that the posteromedial bundle has no role in flexion. This study suggests that reconstruction of both bundles improves PCL function in both extension and flexion. The question remains, however, whether this difference noted in the lab justifies making an already technically challenging operation even more difficult. The answer to this question can only be answered by clinical studies, which apparently are already under way at the University of Pittsburgh and elsewhere. Two other issues should also be considered: the effect of tibial inlay grafts and the issue of cyclic testing. In one recent study, inlay grafts reduced translation by 4 mm compared to transtibial techniques (which were used for tibial fixation in the present study). Additionally, cyclic loading may lead to stretching of the grafts and may affect the results that Harner et al have reported. Nevertheless, this study adds another substantial piece to the puzzle regarding a persistent challenge in knee reconstruction.
Biomechanical studies of the double-bundle PCL reconstruction, in comparison to the normal knee, suggest that:
a. the double- and single-bundle techniques are similar in both extension and flexion.
b. the double-bundle technique is better than the single-bundle technique in extension but both the double-bundle and singlebundle techniques are similar in flexion.
c. the double-bundle technique is better in both extension and flexion.
d. the single-bundle technique is better in flexion; the double-bundle technique is better in extension.
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