EDITOR'S NOTE: The following is a small sample of the Interactive Knee CD in the Primal Pictures 3-D Anatomy CD ROM series.
The cruciate ligaments are two in number. They are named anterior and posterior with regard to the positions of their attachments on the tibial plateau: the anterior cruciate ligament being attached to the anterior intercondylar area of the tibial plateau and the posterior cruciate being attached to the posterior intercondylar area of the tibial plateau. They are named cruciate ligaments because they cross each other (like the limbs of the letter X). Both cruciate ligaments are situated within the capsule of the knee joint. However, they are not within the synovial cavity of the knee joint. The cruciate ligaments receive a sensory innervation from the genicular branches of the tibial, common peroneal and obturator nerves. Sensations subserved by these sensory nerves include both pain and proprioception, and correspondingly, both pain receptors and mechano-receptors have been identified within the cruciate ligaments.
The cruciate ligaments are vascularized structures; the blood supply of the cruciate ligaments being derived from the genicular branches (principally the middle genicular branch) of the popliteal artery. Thus, hemarthrosis is an important clinical feature of cruciate rupture.
The function of the anterior cruciate ligament is to resist posterior displacement of the femur on the tibia. The function of the posterior cruciate ligament is to resist anterior displacement of the femur on the tibia.
The anterior cruciate ligament lies entirely within the capsule of the knee joint but extrasynovially. Its inferior attachment (i.e., tibial attachment) is to a facet on the medial part of the anterior intercondylar area of the tibial plateau. Its superior attachment is to a facet on the posterior part of the medial surface of the lateral femoral condyle. Thus, the anterior cruciate ligament runs obliquely upwards, posteriorly and laterally from its tibial attachment to its femoral attachment. However, the fibers arising most anteriorly on the tibial plateau are attached most posteriorly on the lateral femoral condyle, and the fibers arising most posteriorly on the tibial plateau are attached most anteriorly on the femur. As a result of this arrangement, the anterior cruciate ligament is slightly twisted about its long axis. Because of this helical structure, in a functional sense the anterior cruciate ligament appears to consist of two bands, although morphologically it is a single structure. These are referred to as the anteromedial and posterolateral bands. Of the two, the posterolateral component makes up the greater part of the ligament.
Knee flexion is associated with a greater degree of tautness in the anteromedial component, while extension results in a greater degree of tautness in the posterolateral component. Another consequence of the spiral nature of the anterior cruciate ligament is that tension in the ligament is increased during internal rotation of the joint, while external rotation of the joint results in a decrease in ligament tension.
In the adult, the anterior cruciate ligament is 38mm long and 10mm wide, on average.