Review Question - QID 9

Placing the center of the tibial tunnel 10mm anterior to the anterior border of the posterior cruciate ligament (PCL) will result in the most anatomically accurate graft placement during anterior cruciate ligament reconstruction (ACLR).

Anatomic tunnel placement in ACLR creates knee kinematics closer to that of an intact knee, better restoring anterior translational stability, as well as rotational stability. Inappropriate tunnel placement is the leading cause of residual knee dysfunction and need for revision ACL reconstruction. Positioning the tibial tunnel too anterior leads to tightness in flexion and impingement of the graft against the roof of the intercondylar notch in extension, limiting full knee range of motion. Conversely, a tibial tunnel that is placed too posterior can result in laxity in both flexion and extension, as well as impingement of the graft against the PCL.

Hutchinson et al. performed a cadaveric study evaluating the tibial insertion of the ACL in 42 knees. The most reproducible anatomic landmark was the PCL. The center of the ACL insertion was consistently 10.7mm anterior to the anterior border of the PCL. The posterior edge of the ACL insertion was located 6.7mm anterior to the anterior border of the PCL. Measurements based on the medial tibial eminence of the anterior horn of the lateral meniscus (AHLM) were particularly erratic and therefore, should not be used to guide tibial tunnel placement.

McGuire et al. performed a retrospective study of 20 patient who underwent ACLR with bone-patellar tendon-bone autograft, followed by a post-operative CT scan. The distance from the posterior aspect of the tibial tunnel to the anterior edge of the PCL insertion, defined as “backset”, was 6.2mm. The backset value was found to vary with knee size, as measured by the anteroposterior (AP) diameter of the tibial plateau. For small knees (AP diameter < 50mm) a backset of 5mm is recommended, while for medium knees (AP diameter 50-60mm) and large knees (AP diameter > 60mm) backset values of 6mm and 7mm, respectively, are advised.

Illustration A is a cadaveric dissection of the tibial plateau demonstrating the relationship between the ACL and the AHLM (labelled LARA for lateral anterior root attachment), medial tibial eminence (MTE) and PCL.

Incorrect Answers:
Answers 1 + 2: The AHLM is a less reliable landmark for determining tibial tunnel placement; however, if used, the center of the ACL is most often found medial and slightly anterior to the posterior border of the AHLM. Placing the center of the tibial tunnel at either of these positions would result in an inappropriately posterior tunnel.
Answer 3: Placing the center of the tibial tunnel only 5mm anterior to the anterior border of the PCL would result in an inappropriately posterior tunnel.
Answer 5: The medial tibial eminence is less reliable landmark for determining tibial tunnel placement; however, if used, the center of the ACL is most often found ~5mm anterior to the medial tibial eminence. Placing the center of the tibial tunnel at this position would result in an inappropriately anterior tunnel.