After medial collateral ligament (MCL) injury in the knee joint due to misdiagnosis or failure to take appropriate treatment (e.g., rest, immobilization), the injured MCL heals in an elongated state, causing the medial knee structure to develop into obsolete laxity and knee instability, which will develop into osteoarthritis if not treated surgically, leading to pain and joint dysfunction. Therefore, early and correct diagnosis and reasonable treatment of MCL injury is of great importance. The pathogenesis of MCL injury is mostly caused by direct collision or self-torsion of the lateral knee joint with the injured person keeping the foot position still, resulting in excessive valgus, which is mostly seen in basketball and soccer sports such as collision, pick-up and landing. The medial knee is painful and swollen after the injury. Most MCL injuries are to the femoral stop or parenchyma and can heal completely on their own with conservative treatment; a few are to the inferior stop (tibial stop) and require surgical treatment. In addition, medial collateral ligament injuries are sometimes combined with meniscal and anterior and posterior cruciate ligament injuries, all of which require a physician’s examination.