Both cruciate ligaments are intracapsular but extra-synovial. The PCL does not move from its initial position. The ACL initially begins as a ventral ligament but migrates posteriorly in the knee joint as the intercondylar space develops. Both the posterior cruciate ligament (PCL) and ACL appear as well-oriented structures by nine weeks of gestation. A knee joint is identifiable starting at six weeks, and the ACL formation as a mesenchymal condensation in the fetal blastoma is observable between 6 to 7 gestational weeks. The knee starts to develop from a mesenchymal concentration at four weeks gestational age. The ACL may also serve as the secondary restraint to external rotation and varus-valgus angulation. demonstrated that an internal torque between 2 to 6 Nm resulted in the AMB strain when the knee was flexed at 90 degrees in vivo. In a study by Fleming et al., an internal torque of the tibia between 0-10 Nm resulted in the ACL strain in vivo.
The ACL also functions as the secondary restraint to internal rotation, especially when the joint is close to full extension. In cadaveric knees, the PLB plays an important role in stabilizing the anterior tibial translation at near-to-extension angles, whereas the AMB is more involved in stabilizing higher flexion angles. At 90 degrees of knee flexion, the forces of the muscle group actively stabilize against the anterior tibial translation. The ischio-crural muscle group, which includes the biceps femoris, the semitendinosus, and the semimembranosus, induces knee flexion by connecting the ischial tuberosity with the pes anserinus tibia and fibular head. In cadaveric knees with no active muscular forces, researchers observed that the highest increase in the anterior tibial translation was between 15 to 40 degrees of knee flexion. A study by Zantop et al. showed that the damaged ACL increased the anterior tibial translation by up to 10 to 15 mm at 30 degrees of flexion under the anterior load of 134 N. Patients with chronic ACL-deficient knees (grade 3 sprain) experience the anterior tibial movement relative to the femur that is about four times greater than those with healthy knees. With knee flexion, the anterior tibial translation can increase up to 3 mm while walking and up to approximately 6 mm under anterior load. When the knee is extended, the anterior tibial translation is low (maximum 2 mm) and supports the knee while standing. The ACL functions as the primary restriction to anterior tibial translation with respect to the femur.
It primarily stabilizes the anterior tibial translation and plays a small role in resisting internal rotation and prevents any excessive movements. The ACL is a crucial element in stabilizing the knee joint. The mean length of the ACL is 33 mm, and the mean mid-substance width is 11 mm. With knee extension, the PLB lengthens and tightens while the AMB remains tight but less so than the PLB. With knee flexion, the AMB lengthens and tightens while the PLB shortens and becomes lax. With knee motion, the length and orientation of the AMB and PLB bundles change. The PLB fascicles arise from the distal portion of the femoral attachment and attach to the posterolateral portion of the tibial attachment. The AMB fascicles arise from the most posterior and proximal site of the femoral attachment and attach to the anteromedial portion of the tibial attachment. The ACL is composed of two bundles: the anteromedial (AMB) and the posterolateral (PLB). The ligament is the narrowest near the femoral attachment and fans out and widens as it proceeds to the tibial attachment, approximately 15mm anterior to the PCL, a broader and oval-shaped footprint measuring 10mm by 30mm. From the femoral attachment, the ACL descends and attaches to the site anterior and lateral to the medial intercondylar tubercle. On the femoral side, the ACL attaches to the posterior aspect of the medial surface of the lateral femoral condyle, which is semi-circular in shape, measuring 20 mm by 10 mm. Fibroblasts make up for the cellular components embedded in a matrix of elastin (<5%) and proteoglycans. Mostly type I collagen (90%) and a small amount of Type III collagen(10%). They are predominantly made up of collagen fibers, which make up 70% of their dry weight. The ACL is a band of specialized connective tissue located in the knee joint that connects the femur and the tibia. This article presents the anatomy and function of the ACL to help readers better understand the injury mechanism and reduce the risk of ACL damage. It is one of the most commonly injured structures in sports medicine, and yet it, unfortunately, does not heal when damaged. The anterior cruciate ligament (ACL) is one of the two cruciate ligaments which stabilizes the knee joint by preventing excessive forward movements of the tibia or limiting rotational knee movements.