Stability of the knee depends on the shape of the condyles and menisci in combination with ligamentous and muscular structures. The four major ligaments are the anterior cruciate (ACL), the posterior cruciate (PCL), the medial collateral (MCL), and the lateral collateral (LCL). Kakarlapudi reviews the diagnosis and management of knee ligament injury.

Assessment of knee instability begins with a detailed history that includes a description of how the injury occurred. Important factors can include timing of effusion (acute hemarthrosis usually occurs within two hours), hearing or feeling a “pop”(highly suggestive of an ACL injury), and the presence of mechanical symptoms such as locking, catching, clicking, or giving way (usually associated with chronic instabilities). A physical examination and plain radiographs of the knee should follow.

Radiography may reveal fractures, avulsions, osteochondral fragments, or the fluid level of a hemarthrosis. If a clear diagnosis is made, treatment can be initiated. If the diagnosis is inconclusive, an expectant regimen of mobilization, physiotherapy, and reevaluation in two weeks may be used. Magnetic resonance imaging (MRI) or examination under anesthesia and arthroscopy are options when adequate examination is not possible because of pain or spasm.

MCL injuries usually occur as a result of a direct blow to the lateral aspect of the knee in a slightly flexed position. Diagnostic clues include localized bruising, swelling or tenderness, and medial joint opening with the application of gentle valgus force while the knee is in flexion. Management is conservative, with Rest, Ice, Compression, and Elevation (RICE) during the first 48 hours, followed by temporary immobilization and the use of crutches. Early mobilization and physiotherapy as soon as weight bearing is tolerated are useful, and the patient is usually able to return to normal activities within about six weeks.

ACL injuries, often resulting from hyperextension, are diagnosed by the Lachman and pivot shift tests. Plain radiographs may show evulsion of the ACL insertion. Acute repair is associated with poor results. Initial treatment is based on reduction of pain and swelling and the early restoration of normal joint movement. If the ACL instability becomes intolerable, ACL reconstruction can be considered. Highest risk activities in patients with an ACL-deficient knee include those with a high likelihood of an unanticipated twisting injury, such as high-level skiing, basketball, football, or volleyball. The role and usefulness of knee bracing are uncertain.

LCL injury is rarely isolated and usually involves the PCL or the ACL. Diagnosis is made using the varus stress test or by noting increased external knee rotation with flexion. MRI is useful in these patients to detect damage to other structures in the region, including tendon or nerve injury. Management is early surgical repair of all injured structures. PCL injury is most commonly caused by hyperflexion or a fall on the flexed knee. The posterior sag test and the posterior drawer test are performed at 90 degrees of flexion. Plain radiograph may show a PCL avulsion. Reconstruction is usually not necessary, and aggressive physiotherapy following an initial period of RICE and the use of an extension splint for three to four weeks is generally successful.