Anterior cruciate ligament

The anterior cruciate ligament (ACL) extends from the femur to the tibia. The ACL is primarily responsible for resisting anterior tibial translation and rotational loads.[1] The anterior cruciate ligament is attached to the depression in front of the intercondyloid eminence of the tibia, being blended with the anterior extremity of the lateral meniscus; it passes upward, backward, and lateralward, and is fixed into the medial and back part of the lateral condyle of the femur.

The ACL consists of two major bundles: the anteromedial (AM) bundle and the posterolateral (PL) bundle. These bundles are named based on their tibial insertion sites. The AM bundle is located anteriorly and medially, while the PL bundle is positioned posteriorly and laterally. The AM bundle tightens during knee flexion, whereas the PL bundle tightens during knee extension.[2]

The femoral attachment of the ACL is oval-shaped and located on the posterior part of the lateral femoral condyle. The tibial attachment is broader and fans out, forming a "C"-shaped footprint on the anterior intercondylar area of the tibia.[3-4] The ACL is not isometric; its length changes with knee movement, with the AM bundle lengthening and the PL bundle shortening during flexion.[1]

The ACL's microstructure includes collagen fibers (primarily type I), a matrix of proteins, glycoproteins, elastic systems, and glycosaminoglycans, which provide the ligament with the ability to withstand multiaxial stresses and varying tensile strains.[1] The ligament is innervated by the posterior articular branches of the tibial nerve and vascularized by branches of the middle genicular artery.[1]

MRI features of ACL:

• The ACL typically exhibits a "bow tie" shape on MRI, with distinct morphological features at different points along its length. The femoral insertion of the ACL appears semicircular, with a slightly straight anterior side and a convex posterior side, while the tibial insertion is kidney bean-shaped. The midsubstance of the ACL, particularly at midsubstance 4, is characterized by an oval isthmus, which is the narrowest and most balanced part of the ligament.

• MRI is also crucial in differentiating between acute and chronic ACL tears. Acute tears are often associated with edema, while chronic tears may present with variable appearances, including intact bands with low signal intensity due to fibrous scarring

Note: The crucciate ligaments are called cruciate because they cross each other somewhat like the lines of the letter X; and have received the names anterior and posterior, from the position of their attachments to the tibia.