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Ligaments and Injuries

Anterior cruciate ligament injuries ACL injuries of the knee are the most common injuries to the knee; the loss of an ACL not only produces abnormal kinematics but also frequently results in major degenerative changes in the knee.

The human ACL is a complex structure at every level. The ligament is designed to act as a stabilizer while allowing normal joint motion throughout the functional range of motion. The ACL is a band of regularly oriented connective tissue that connects the femur and tibia (Figures 11.6 and 11.7). It has an average weight of 20 g (0.7 oz) and an average length of 35 mm (1.4 in). It is narrow in the middle, fanning out inferiorly and to a lesser extent superiorly. The ACL attaches to the posterior aspect of the medial surface of the lateral
femoral condyle. The femoral attachment is in the form of a circle. Distally the ACL is attached to a fossa in front and lateral to the area in the middle of the knee, the anterior tibial spine. The tibial attachment is somewhat broader than the femoral attachment.
The ACL consists of an anteromedial band which is taut with the knee in flexion and relaxed when the knee is in extension and a posterolateral bundle which is tight in extension and relaxed in flexion. An intermediate band may be identifiable, which is tight through the whole range of motion from extension to flexion.The ACL is an intra-articular ligament surrounded by synovium. It is well vascularized, and contains nerve endings which may have a proprioceptive function.

Biomechanics and function
The ACL is the second strongest ligament in the knee with a maximum load of around 500 lb (2200 N). At extreme extension, the anteromedial band is slack and the posterolateral band is tight; with increasing flexion, there is tightening of the anteromedial band and increased laxity in the posterolateral band.
The ACL prevents the anterior movement of the tibia in relation to the femur. The ligament takes up 75% of the anterior force in full extension, 87% at 30° of flexion, and 85% at 90° of flexion. Other restraining factors are the iliotibial band, the medial and lateral capsule, and the medial and lateral collateral ligaments.
In ACL-deficient (malfunctioning) knees, the medial extra-articular structures resist anterior (forward)translation and valgus (to the outside) rotation at all flexion angles, while the lateral collateral ligament and the posterolateral structures resist anterior translation in extension only. The medial meniscus also resists
anterior translation at all flexion angles. The ACL is the main stabilizer for anterior translation of the tibia in relation to the femur. Together with the PCL, the ACL resists and limits hyperextension (overstraightening), hyperflexion (overbending), and internal rotation.

Mechanism of injury
Isolated injuries of the ACL can occur with a twisting impact, either in internal rotation and hyperextension,or in external rotation and valgus. In alpine skiing there are two typical injury mechanisms: the bootinduced ACL mechanism and the ‘phantom foot’ mechanism.

– The results of ACL reconstructive surgery are good: active people can return to the preoperative level in their sport after an acute ACL injury in around 80–90% of cases, and in about 70–80% of chronic cases.
– The management of chronic injuries follows the same surgical principles as acute injuries. The procedure is often a little more extensive, with excision of more bone (‘notchplasty’) to give room for the ligaments. There is often more articular cartilage damage and meniscal injury. The prognosis is, therefore, not as good as for an acute injury, but the results are still acceptable. Whether the surgery in the long run will prevent major degenerative disease is still unknown, but it seems that surgery at least stops further major deterioration of the joint. The patients most commonly experience good results from surgery and are very grateful.

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