Correct placement of the femoral and tibial tunnels is the key to rapidly restoring motion and maintaining stability after reconstruction of the anterior cruciate ligament (ACL). Incorrect tunnel placement causes graft failure, which can lead to recurrent instability, stiffness, and prolonged recovery that cannot be prevented by adjusting other variables such as the graft material and size, rehabilitation program, timing of return to sport, and bracing. Unfortunately, there is currently no universally agreed on definition for proper tunnel placement.
The most important finding from the Level-I study by Meuffels et al. is that two surgeons, both highly experienced in ACL surgery, had “considerable and unexpected lack of precision in tibial and femoral tunnel placement,” with 40% of tibial tunnel and 60% of femoral tunnel positions being outliers when they randomized the use of conventional and computer-assisted surgical instrumentation for placing the tunnels. It is both enlightening and disconcerting that highly experienced surgeons have poor precision in drilling the tunnels in the intended target. Even with use of “more sophisticated” computer-assisted technology, imprecision and outliers are currently unavoidable.
One question that the study did not address is “What target should be used for placing the tunnels?” The authors used well-accepted targets—for the tibial tunnel, an “average” position of 44% of the anterior-to-posterior length of the tibial plateau as defined by Stäubli and Rauschning; and for the femoral tunnel, an “average” position in the most posterior and most proximal quadrant with respect to the lateral condyle and the Blumensaat line as defined by Bernard et al. However, is an average tunnel placement the best target for each individual patient?
We have suggested that the placement of the ACL graft and tunnels should be customized on the basis of the patient’s anatomy and the graft diameter1-3. We follow the principle of Markolf et al. that the tunnels should be placed so that the tension pattern of the ACL graft mimics that of the intact ACL: a modest increase in tension in terminal full extension and flexion, with no or low tension in between4-6. In the sagittal plane, our target for placing the tibial tunnel is just posterior to the Blumensaat line with the knee in extension, to avoid impingement of the ACL graft against the intercondylar roof in terminal extension2. In the coronal plane, our targets are to widen the space between the lateral femoral condyle and the posterior cruciate ligament (PCL) until it matches the diameter of the ACL graft, to place the tibial tunnel between the tibial spines, and to place the femoral tunnel midway between the base and apex of the notch with no more than a 1-mm back wall to avoid impingement of the ACL graft on the PCL with knee flexion1,3,6.
Meuffels et al. suggested that their use of the transtibial technique, which was the gold standard and the most commonly used technique at the time they initiated their study in 2007, might be considered a limitation by those who prefer the currently popular anteromedial portal technique, whereby the femoral tunnel can be placed lower toward the posterolateral bundle footprint of the native ACL. We believe that there are substantial anatomical variations from patient to patient and that one technique may not be suitable for all. Furthermore, it is possible with any drilling technique to incorrectly place the tunnel. The tension in extension of an ACL graft placed in the posterolateral bundle location is higher than that of the intact ACL, which may cause loss of extension or failure of the graft4,5. Of greater concern is the recent report of the Danish ACL registry that the revision rate for failed reconstruction for the anteromedial portal technique is two times greater than that for the transtibial technique7. For these reasons, we suggest caution in the unbridled adoption of the anteromedial technique.
One final lesson from the study by Meuffels et al. and our analysis of our own studies and those of others is that we should be sensible and cautious about criticizing the placement of tunnels by other surgeons until instrumentation for placing the tunnels with greater precision is available and until the intended targets have been universally agreed on.