Femoral Tunnel Creation Using the Zimmer Biomet SwitchCut Femoral Aimer

Many factors influence the outcome of anterior cruciate ligament (ACL) reconstruction. Technical factors including surgical method, tunnel positioning, graft type, graft tension, and fixation, as well as clinical factors including rehabilitation and patient selection, all play an important role in the overall outcome.

The ACL consists of two bundles: the anteromedial (AM) bundle and posterolateral (PL) bundle. The AM bundle is responsible for anteroposterior stability, whereas the PL bundle is responsible for rotational stability. Overall stability and proper kinematics of the knee are controlled by the combined action and interaction of the AM and PL bundles.

The ACL femoral footprint forms a segment of a circle on the posterior aspect of the medial surface of the lateral femoral condyle. The anterior border of the footprint is straight, and the posterior border is convex. The anterior border of the footprint is located directly behind the lateral intercondylar ridge. The posterior border of the footprint is located 3.5–4 mm from the posterior articular surface of the lateral femoral condyle. In the coronal plane the femoral origin of the ACL is low in the notch. In the sagittal plane the ACL origin is posterior in the notch. Using a clockface method described in the study by Heming et al., the center of the femoral footprint averaged a position of 10:49 (left knee). The tibial footprint of the ACL is located on the fovea, just anterior to the tibial eminence.

Four main techniques for creation of an ACL femoral tunnel or socket are transtibial (TT), AM transportal (either with a rigid reamer or with a flexible reamer; AMP), outside-in (OI), and a modification of OI technique using retrograde drilling (RD).

Historically, nonanatomical TT has been the most widely used and reproducible technique. In the TT technique, location of the femoral tunnel is determined by the orientation of the tibial tunnel. Over the past several years there has been a movement away from using TT technique, due to creation of a vertically oriented graft resulting in graft impingement, rotational instability with persistent pivot shift, and abnormal knee kinematics.

The goal of anatomical ACL reconstruction is to restore the physiological function of the native ACL by replicating native dimensions, collagen orientation, graft tension, and insertion site size. Accurate femoral tunnel placement is one of the most important factors for obtaining good clinical results in ACL reconstruction. An ideal femoral tunnel is located relatively low in the intercondylar notch and is oblique in its course, providing resistance to anterior translation and rotatory moment of the tibia.

Transtibial, AM transportal, and OI techniques are all accepted ACL reconstruction techniques. Femoral tunnels created by AMP and OI techniques have demonstrated larger femoral PL bundle coverage than tunnels created by TT technique. A more obliquely placed femoral tunnel created by either AMP or OI technique resists rotational forces better than a vertically placed tunnel. Comparison of AMP and OI techniques in multiple studies suggests that OI technique is less technically difficult and is associated with fewer complications.

There are multiple technical disadvantages to the AMP technique. The AMP technique requires a high angle of knee flexion (>110 degrees) for anatomical placement of the femoral tunnel. Hyperflexion results in shorter femoral tunnels or sockets (<30 mm) that may compromise or limit the type of fixation used. Knee hyperflexion may be difficult or impossible in obese population. A high degree of knee flexion limits the visibility of femoral insertion during drilling. Without knee hyperflexion, there is a risk of damage to the posterior wall of the lateral femoral condyle (posterior wall blowout). Lastly, AMP technique requires that a medial portal be placed low, risking injury to the anterior horn of medial meniscus.

In comparison, the OI technique has significantly fewer disadvantages, making it a better choice for both primary and revision ACL reconstruction procedures for some surgeons. Using this technique the femoral tunnel is completely housed within the lateral femoral condyle, with very low incidence of posterior wall blowout. With OI technique, the femoral guide is freely positioned at the anatomical center of the ACL footprint. It is not inhibited by the flexed position of the knee nor the anatomy of the lateral wall. Coronal obliquity of the femoral tunnel is not dependent on hyperflexion of the knee during surgery. In addition, the OI technique provides unobstructed visualization of the medial wall of the lateral femoral condyle throughout the case. Low and oblique placement of the ACL graft using this technique rarely requires notchoplasty. Addition of an RD device, such as the SwitchCut reamer (Biomet, Warsaw, Indiana), to the OI technique provides an option for minimally invasive bone-sparing femoral socket creation. In addition, OI RD technique allows for all epiphyseal drilling in skeletally immature patents, measurement of femoral intraosseous distance before socket creation, minimization of fluid leakage, and ability to tension graft after passage with the use of adjustable graft-loop buttons.

In summary, although multiple techniques for ACL reconstruction are currently in use, some surgeons feel that the OI technique with use of an RD device provides the most anatomically accurate femoral socket creation and has many technical advantages with equal or better clinical outcomes when compared with other techniques.

The Zimmer Biomet SwitchCut reamer is designed to allow access to any point on the medial or lateral wall of the femur. The handle and arm are designed to allow firm ergonomic grip of the aimer, which reduces the possibility of the drill missing its mark. The rigidity of the 4.5-mm drill also allows for superb tactile feel during the drilling process ( Fig. 45.1 ).