The medial collateral ligament (MCL) is a flat band of connective tissue that runs from the medial epicondyle of the femur to the medial condyle of the tibia. Its role is to provide valgus stability to the knee joint. MCL injuries often occur in sports, especially in skiing; in fact, 60% of skiing knee injuries involve the MCL. 
The mechanism of injury may involve abrupt turning, cutting, or twisting. MCL injuries can also result from direct blows to the lateral knee that cause an extreme valgus stress. Injuries to the MCL can be isolated but more commonly will occur in conjunction with injuries to other knee structures. For example, the “unhappy triad” consists of concomitant injuries to the MCL, anterior cruciate ligament (ACL), and medial meniscus. 
Patients may report acute or chronic medial knee pain. In acute cases, the patient will usually be able to describe a specific recent incident that resulted in the onset of pain or swelling, such as a sporting event. The patient may hear or feel a pop at the time of injury. They may or may not have difficulty walking and complain of knee instability.
Examination of the knee in MCL injury is most revealing at the time of injury and is often diagnosed on the sidelines by physical exam. However, diagnosis can still be made when the patient later presents in the emergency room or office setting.
Inspection may reveal a joint effusion and ecchymosis either at the lateral knee from direct trauma or at the medial knee from the ligament injury. Effusion should be localized around the MCL and will rarely be a generalized knee effusion, as this would be more likely due to intraarticular injuries. Gait is frequently normal, though patients can sometimes present with an antalgic or vaulting gait.
Palpation should be performed along the full length of the MCL. Tenderness specifically at one attachment site indicates the injury likely occurred there. Mid-substance tears can cause tenderness at the medial joint line, which can be confused with a medial meniscus injury. Distal MCL tears can cause tenderness at its attachment to the medial tibial condyle, which can be confused with pes anserine bursitis.
Valgus stress testing is the best way to test the integrity of the MCL directly. The patient should be positioned supine with the hip abducted on the affected side so that the leg is unsupported off the table. The knee should be brought into 30 degrees of flexion. The examiner should grasp the ankle with one hand and push the ankle laterally while applying a valgus force to the knee with the other hand. Laxity should be noted and then graded per the following classification:
Grade 1 – pain along the MCL with valgus stress but little to no joint opening
Grade 2 – some opening of the joint but with a firm endpoint
Grade 3 – significant joint opening, no endpoint
The exam should be repeated with the knee in full extension. The posterior cruciate ligament (PCL) and posterior joint capsule contribute to knee stability in full extension, so if the test is positive at full extension, then it is likely that there is more than just an isolated MCL injury.
Imaging should include plain radiographs to evaluate for occult fractures or avulsion fractures. A Pellegrini-Stieda lesion, in which there is ossification of the MCL near its attachment to the femoral epicondyle, suggests an old avulsion injury of the MCL. Stress radiographs may also be performed, especially in skeletally immature patients. However, the imaging test of choice is magnetic resonance imaging (MRI) without contrast. In addition to direct evaluation of the MCL, it can also provide valuable information about other soft tissue structures about the knee and whether concomitant injuries occurred. Occasionally, MR arthrography may be used when meniscal or capsular tearing is suspected. Ultrasound evaluation can be considered as a faster, more portable, and lower cost alternative to MRI. A sonographic exam was able to identify injury location and severity in 94% of patients with MCL injury and offers the added benefit of performing the dynamic valgus stress test.
Treatment in most cases is conservative. Grade I to II injuries are treated with a conservative approach unless there is an associated injury that is more severe and warrants surgery. Non-steroidal anti-inflammatory drugs (NSAIDs) may be used to help control pain and swelling. A knee immobilizer and crutches may also be used short-term after injury, with gradually less reliance on these as pain and swelling subside, and the patient can participate adequately in physical therapy. Therapy exercises should include quadriceps strengthening, cycling, and progressive resistance exercises. Patients should gradually progress through a return-to-play protocol that involves increasing the difficulty of the exercise and sports-specific maneuvers. Patients with grade I injuries typically can return to play within ten to 14 days, while those with grade II injuries have more variable timelines for return and should wait until both lower extremities display equal strength, and there is no pain elicited by valgus stress. Recovery for grade I and II injuries with conservative treatment only has shown effectiveness in 98% of athletes.
Grade III injuries may be treated conservatively or operatively. The operative route is especially common for athletes because this severity of the injury can lead to lasting rotational instability. Grade III injuries are also often accompanied by associated injuries that require surgery, such as concomitant ACL tear. Acute tears are typically able to be repaired, while chronic tears may require reconstruction using allograft or autograft. After surgery, the patient should wear a hinged brace locked at 30 degrees of flexion and is to be toe-touch weight bearing for about three weeks. The range of motion exercises may be performed up to 90 degrees, and strengthening exercises should be done while in the brace. After three weeks, weight-bearing can be advanced to full, and the brace can be unlocked to allow for full range of motion. The patient should continue to advance to closed kinetic chain exercises and higher resistance strength exercises.
Complications of MCL injuries are rare, especially when detected early and treated. Recurrence of rupture is unlikely. Untreated cases or those with poor adherence to rehab programs may develop ossification at the injury site, as in a Pellegrini-Stieda lesion.
MCL injuries are best managed by a multidisciplinary team that includes an orthopedic nurse and a physical therapist.
MCL injuries are quite common in sporting individuals. However, it is important for clinicians to remember that this is one injury that can heal with conservative treatment. The patient should be encouraged to enter a rehabilitation program after the acute symptoms have subsided. Return to sports is possible in most athletes. However, professional athletes may want to consider surgery as the repair is more durable.
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