What are the different parts of the Anterior Cruciate Ligament (ACL)?

From the Guidelines

The anterior cruciate ligament (ACL) is a complex structure consisting of two main bundles, the anteromedial and posterolateral bundles, which play crucial roles in knee stability and function, and understanding these components is essential for effective management of ACL injuries, as highlighted in the 2023 clinical practice guideline summary by the American Academy of Orthopaedic Surgeons 1. The ACL is composed of dense connective tissue with collagen fibers, primarily type I, arranged in a hierarchical structure of fascicles, and it contains mechanoreceptors that provide proprioceptive feedback about knee position and movement. Key aspects of the ACL include:

• The anteromedial bundle, which is tighter during knee flexion and provides primary resistance to anterior tibial translation
• The posterolateral bundle, which tightens during knee extension and helps control rotational stability
• The origin of the ACL from the posteromedial aspect of the lateral femoral condyle and its insertion on the anterior intercondylar area of the tibial plateau
• The blood supply from the middle genicular artery and innervation from branches of the tibial nerve Considering the importance of these anatomical components, ACL reconstruction can be considered to lower the risk of future meniscus pathology or procedures, particularly in younger and/or more active patients, as well as to improve long-term pain and function, as recommended in the 2023 clinical practice guideline summary by the American Academy of Orthopaedic Surgeons 1. In the context of ACL injuries, understanding the distinct anatomical components is vital for proper diagnosis and treatment, as different bundles may be affected differently in partial tears, and surgical reconstruction techniques often aim to restore the function of both bundles for optimal knee stability.

From the Research

ACL Structure

• The anterior cruciate ligament (ACL) consists of two major fiber bundles, namely the anteromedial and posterolateral bundle 2.
• The anteromedial bundle is located proximal and anterior in the femoral ACL origin, while the posterolateral bundle starts in the distal and posterior aspect of the femoral ACL origin 2.
• The anteromedial bundle insertion is in the anterior part of the tibial ACL footprint, and the posterolateral bundle insertion is in the posterior part 2.

Bundle Functionality

• The anteromedial bundle is the primary restraint against anterior tibial translation, while the posterolateral bundle tends to stabilize the knee near full extension, particularly against rotatory loads 2.
• The bundles are not isometric, with the anteromedial bundle lengthening and the posterolateral bundle shortening during flexion 3.

ACL Anatomy

• The ACL is a band of dense connective tissue that courses from the femur to the tibia, with a mean length of 32 mm and a width of 7-12 mm when the knee is extended 3.
• The ACL has a microstructure of collagen bundles and a matrix made of proteins, glycoproteins, elastic systems, and glycosaminoglycans, allowing it to withstand multiaxial stresses and varying tensile strains 3.
• The ACL is innervated by posterior articular branches of the tibial nerve and is vascularized by branches of the middle genicular artery 3.

Double Bundle Structure

• The double bundle structure of the ACL consists of an anteromedial and posterolateral bundle, which are oriented near parallel with the knee extended and twist around each other as the knee is flexed 4.
• The double bundle model facilitates restoration of the original footprint anatomy and biomechanics, and is essential for anatomic ACL reconstruction 4.