From the Guidelines
The anteromedial (AM) bundle of the anterior cruciate ligament (ACL) is primarily responsible for rotatory stability of the knee. The ACL consists of two functional bundles: the anteromedial and posterolateral bundles. While both contribute to knee stability, the anteromedial bundle plays the dominant role in controlling rotational movements, particularly during flexion. This bundle remains taut throughout the knee's range of motion, especially when the knee is flexed, providing crucial resistance against internal tibial rotation. The AM bundle's anatomical position and orientation allow it to effectively counteract the rotational forces that occur during pivoting movements. Understanding this functional anatomy is important in ACL reconstruction surgeries, where recreating both bundles (double-bundle technique) may provide better rotational control than single-bundle techniques, as suggested by the American Academy of Orthopaedic Surgeons clinical practice guideline summary 1. The AM bundle's role in rotatory stability explains why patients with ACL injuries often report instability during cutting or pivoting movements, as these actions particularly challenge rotational stability. Key points to consider in ACL reconstruction include:
- The importance of the anteromedial bundle in rotatory stability
- The potential benefits of double-bundle techniques in recreating both bundles for better rotational control, although single-bundle or double-bundle techniques can be considered as outcomes are similar 1
- The need to understand the functional anatomy of the ACL in order to provide effective treatment and rehabilitation for patients with ACL injuries.
From the Research
Anterior Cruciate Ligament (ACL) Bundles and Rotatory Stability
The anterior cruciate ligament (ACL) is composed of two main bundles: the anteromedial (AM) bundle and the posterolateral (PL) bundle. Research has shown that these bundles play different roles in maintaining knee stability, particularly in terms of rotatory stability.
Role of the PL Bundle in Rotatory Stability
- The PL bundle is tight near knee extension and has a role in controlling tibial rotational laxity 2.
- Studies have found that the PL bundle is more important than the AM bundle in controlling rotational movement, particularly near full extension 3, 4.
- The PL bundle's oblique orientation in the coronal plane allows it to resist tibial rotation more effectively than the AM bundle 2.
Role of the AM Bundle in Rotatory Stability
- The AM bundle is more important for resisting tibial anterior drawer, which is the primary function of the ACL 2.
- The AM bundle is close to isometric when the knee flexes, meaning it remains relatively tight throughout the range of motion 2.
- Some studies have found that the AM bundle has a larger tension than the PL bundle during a simulated pivot shift, suggesting it may play a role in controlling rotatory stability in certain situations 2.
Comparison of AM and PL Bundles in Rotatory Stability
- A study found that the double-bundle technique, which reconstructs both the AM and PL bundles, increases AP translation and rotational stability in internal rotation more than the single-bundle technique 5.
- Another study found that fixation of the PL bundle significantly improved knee laxity during internal-external rotation at 30° of knee flexion, and that the addition of the AM bundle further improved knee laxity during the Lachman and anterior drawer tests 6.