Causes / typical injury mechanism: The medial knee consists of a complex of layered capsular and ligamentous structures that includes the medial collateral ligament (MCL) and posterior oblique ligament, both of which provide resistance to valgus stress on the knee. The MCL has 2 main components: the superficial MCL and the deep MCL. The MCL is one of the most commonly injured ligaments of the knee, involved in approximately 20% of knee injuries. Injury frequently results from either direct valgus pressure to the knee or abduction / rotation of the lower leg in relation to the thigh, resulting in a sprain or tear of the MCL.

Classic history and presentation: Patients will often present acutely with pain and swelling of the medial aspect of the knee, often with point tenderness at the femoral insertion (medial epicondyle) of the MCL. Some patients report a popping or tearing sensation at the time of injury. Mild MCL injuries may not result in significant valgus laxity, but patients with more severe lesions often report instability with lateral and pivoting maneuvers. Significant instability may indicate the involvement of additional surrounding structures or other knee ligaments being compromised (multiligamentous knee injury).

Prevalence: Contact and collision sports (wrestling, hockey, soccer, football, skiing, etc) are frequently implicated; therefore, MCL injuries frequently occur in younger, active populations.

• Age – Teenage years to fifth decade.
• Sex / gender – MCL injuries predominantly affect men more than women.

Grade / classification system: The grade of an MCL injury is determined by the amount of medial joint line opening with valgus stress:

• Grade 1 – Less than 5 mm
• Grade 2 – 5-9 mm
• Grade 3 – 10 mm or more