Anterior Cruciate Ligament (ACL)

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Introduction

File:Knee.png

The anterior cruciate ligament (ACL) is a band of dense connective tissue which courses from the femur to the tibia. The ACL is a key structure in the knee joint, as it resists anterior tibial translation and rotational loads.

Attachments

Origin

Arises from the posteromedial corner of medial aspect of lateral femoral condyle in the intercondylar notch. This femoral attachment of ACL is on posterior part of medial surface of lateral condyle well posterior to longitudinal axis of the femoral shaft.  The attachment is actually an interdigitation of collagen fibers & rigid bone thru transitional zone of
fibrocartilage and mineralized fibrocartilage.

Orientation

It runs inferirorly, medially and anteriorly.

Insertion

Anterior to the intercondyloid eminence of the tibia, being blended with the anterior horn of the medial meniscus.  The tibial attachment is in a fossa in front of & lateral to anterior spine, a rather wide area from 11 mm in width to 17 mm in AP direction.

Nerve Supply

The ACL is innervated by posterior articular branches of the tibial nerve.

Vascular Supply

The ACL is vascularized by branches of the middle genicular artery.

Composition

The ACL has a microstructure of collagen bundles of multiple types (mostly type I) and a matrix made of a network of proteins, glycoproteins, elastic systems, and glycosaminoglycans with multiple functional interactions.

Bundles

There are two components of the ACL, the smaller anteromedial bundle (AMB) and the larger posterolateral bundle (PLB), named according to where the bundles insert into the tibial plateau.

The anteromedial bundle is tight in flexion and the posterolateral bundle is tight in extension.  In extension both bundles are parallel; in flexion the femoral insertion site of the posterolateral bundle moves anteriorly, both bundles are crossed, the anteromedial bundle tightens and the posterolateral bundle loosens.

With the knee extended, resistance to anterior tranlation of the tibia, Lachmans Test, is by the bulky posterolateral bundle. With the knee flexed, resistance to anterior translation of the tibia, the Anterior Drawer Test, is by the anterior medial bundle.

Rupture of the posterolateral bundle causes increase in hyperextension, anterior translation (extended knee), increase in external and internal rotation (knee extended), and increases in external rotation with the knee in mid flexion; Rupture of the anteromedial bundle causes anterolateral instability with an increase in anterior translation in flexion, minimal increase in hyperextension, and minimal rotational instability.

Function

The ACL provides approximately 85% of total restraining force of anterior translation. It also prevents excessive tibial medial and lateral rotation, as well as varus and valgus stresses. To a lesser degree, the ACL checks extension and hyperextension. Together with the posterior cruciate ligament (PCL), the ACL guides the instantaneous center of rotation of the knee, therefore controlling joint kinematics. While the anteromedial bundle is the primary restraint against anterior tibial translation, the posterolateral bundle tends to stabilize the knee near full extension, particularly against rotatory loads.