Collateral Ligament Pathology

Updated: Apr 10, 2021
  • Author: Michael P Nogalski, MD; Chief Editor: Thomas M DeBerardino, MD, FAAOS, FAOA  more...
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Practice Essentials

The medial and lateral collateral ligaments of the knee are 2 distinct entities, are injured by different mechanisms, and often generate different algorithms for treatment. Treatment of these structures, when the injury is isolated, often is conservative and involves brief protection and functional rehabilitation. In lateral-sided injuries, other structures, such as the entire posterolateral complex, the anterior cruciate ligament (ACL), and the posterior cruciate ligament (PCL), can be injured as well. [1, 2]

If untreated, injuries to the collateral ligaments can result in functional instability of the knee in daily activities, work, and sports. This is often noted in association with other ligament injuries of the knee. Management of these injuries depends on an understanding of the biology, anatomy, and mechanical function of these structures. Over the past 2 decades, research into the basic science of ligaments, mostly in animal models, has improved the understanding of the injury and repair mechanisms. Each injury is considered with respect to isolated injuries, combined injuries of the ACL and PCL, and combined injuries in knee dislocations.

Isolated lateral collateral ligament (LCL) injuries are rare. More commonly, this ligament is injured as a component of a posterolateral injury of the knee. Instead, LCL or posterolateral complex injury often occurs with a PCL injury or with an ACL/PCL injury.

An isolated LCL injury is treated in much the same way as a medial collateral ligament (MCL) injury (usually one of low grade). Attention should be focused on the entire posterolateral corner of the knee when a lateral injury to the knee is suspected.

Given the excellent results with conservative treatment of isolated collateral ligament injuries, controversy has waned in this area. Injuries of the ACL and MCL still generate some discussion, but most authors favor conservative management of MCL injuries in this setting, with ACL reconstruction for appropriate patients. Primary repair of the MCL in this setting has few proponents currently.

Lateral complex injuries are less well defined, and numerous approaches are possible. Timing of the repair of isolated lateral injuries is also still debated. Continued improvement in understanding of the lateral side of the knee and defining the role that the lateral side plays in PCL injuries is ongoing.


Relevant Anatomy

Medial collateral ligament injuries of the knee

Dissection and anatomy studies have shown that the MCL has 2 primary components: a more superficial ligamentous structure, the superficial MCL; and a deeper capsular MCL complex. The deep layer is attached to the medial meniscus and transitions into the posterior oblique ligament (POL) just posterior to the posterior edge of the superficial MCL. [3, 4] The superficial MCL attaches to the medial epicondylar region and to an area well inferior to the joint line, posterior to the insertion point of the pes anserine bursa. The POL has 3 portions: the tibial, superficial, and capsular arms. Grood et al found that the MCL is the primary static restraint providing valgus stability at 25 degrees of flexion, and that the ACL also contributes significantly to valgus stability. [5]

Grade 1 or 2 injuries involve a portion of the ligament and are partial-thickness injuries. Grade 3 ligament injuries are complete tears of the superficial and deep layers of the MCL. Bony avulsions can occur but are unusual. If an avulsion is observed, pathologic bone at the avulsion site should be considered.

Lateral collateral ligament injuries

The LCL extends from the lateral epicondyle to the fibular head and is the primary restraint to varus stress of the knee. Below the LCL is the rest of the posterolateral complex, made up of the arcuate complex, the posterolateral capsule, and the popliteus tendon. The arcuate complex, which is associated with the posterior-lateral capsule, attaches as well to the fibular head. The lateral capsule is thick in its middle third and is analogous to the deep fibers of the MCL. Deep to the capsule, the popliteus tendon winds around to insert on the lateral condyle intra-articularly.

The LCL limits lateral joint opening with varus stress on the knee. The posterolateral complex has been shown to be most effective in controlling external rotation of the tibia on the femur at 30 degrees of knee flexion. [5]

Injuries to the LCL complex probably involve a spectrum of injuries, from an isolated LCL injury, which would come from a pure varus stress, to an injury to the LCL and posterolateral ligament complex. This combined injury usually involves more energy and rotational forces.



In animal studies, the medial collateral ligament heals with fibroblast proliferation in the hematoma/plasma exudate that occurs in the zone of injury. Similarities to tendon healing in the collateral ligament healing process have been cited by investigators, as opposed to the cruciate ligaments, which heal with fibrocartilaginous cells.

Functional treatment with protection from valgus stress has been found to improve healing and orientation of the collagen fibers in the healed ligament. Some animal studies have shown elongation of the ligament, but, as noted below, clinical studies have not found this to be a significant problem in isolated MCL injuries. [6, 7, 8, 9, 10] Clinical studies, which started with the work of Indelicato, have also documented slight laxity after healing, but minimal functional significance has been identified. [11, 12, 13, 14]



Medial collateral ligament injuries of the knee

A valgus stress with or without a combined rotational stress to the knee most commonly causes this injury. The foot or lower leg usually is held in a fixed position, and the upper leg and body moves or twists in relation to the lower leg. MCL tears can also be noncontact injuries. The MCL may also be injured in conjunction with tears of the ACL, PCL, and/or lateral complex. [15, 16, 17]

Lateral collateral ligament injuries

A direct blow to the medial knee usually is the mechanism of injury that results in isolated LCL tears. Injury to the posterolateral structures of the knee come from medial blows to the knee with the knee in flexion and from rotational forces placed on the knee at the same time. Wrestling is thought to be a sport that can generate pure LCL injuries. [18]

In a study of athletes in the 2016 Olympic Games who underwent at least one knee MRI, ACL sprains had a significant association with medial and lateral meniscal tears and bone contusions. [19]



Despite the relatively common nature of medial collateral ligament injuries in particular, the frequency of collateral ligament injuries is not well defined in the literature.

In a longitudinal cohort study, Roach and colleagues examined the epidemiology of isolated MCL sprains that occurred at the United States Military Academy between 2005 and 2009. During 17,606 student person-years over the study period, 128 cadets sustained isolated MCL injuries, resulting in an incidence rate of approximately 7.3 per 1000 person-years. Males had a 44% higher incidence rate than females. Contact sports such as wrestling, hockey, judo, and rugby were associated with the highest injury rates. [20]

A survey of sports-related knee injuries in US high school students found that medial collateral ligament injury was the most common, being reported in 36.1% of cases; the lateral collateral ligament was involved in 7.9% of knee injuries. [21]

During the 2016 Olympic Games, 74.4% of athletes who underwent MRI for knee pain were diagnosed with anterior cruciate and medial collateral ligament (ACL/MCL) sprains.  Althletes participating in wrestling, hockey, track and field and judo competitions represented the majority of the diagnoses. [19]

A study of 2460 knee injuries among students playing football at American universities found MCL was the most common injury (1389; 56.4%) followed by ACL (522; 21.2%). [22]   



Outcome is related directly to the severity of the injury and the functional rehabilitation possible. Patients with isolated injuries typically do well if the injuries are managed safely but aggressively. Injuries to the ACL and MCL usually are associated with early return of full extension and patellar mobility and subsequent strength. Results of other multiple ligament injuries are best reviewed in the literature regarding knee dislocations.