الفهرس 
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Abstract
Stability of the knee joint is maintained by the shape of the condyles and menisci in combination with passive supporting structures. These are the four major ligaments, the anterior cruciate ligament (ACL), the posterior cruciate ligament (PCL), the medial collateral ligament (MCL), and the lateral collateral ligament (LCL). In addition, significant contributions are also made by the posteromedial and posterolateral capsular components and the iliotibial tract. Moreover, the muscles acting over the joint provide secondary dynamic stability.
As general rule, instability resulting from ligament injury may result from direct or indirect trauma. The most frequent mechanism is “non-contact,” involving twisting, jumping, and sudden deceleration.
Assessment of instability begins with a detailed history, including a description of the injury. The timing of an effusion (acute hemarthrosis usually occurs within two hours) and hearing or feeling a “pop” (highly suggestive of an ACL injury) are significant events. Moreover, chronic instabilities present with mechanical symptoms such as locking, catching, clicking, or giving way, particularly with twisting movements. Age, occupation, lifestyle, level of sporting activity, and past history are all factors considered in subsequent management.
Initially a physical examination may be difficult because of swelling, pain, or muscle spasm.
However, the specific physical examination includes looking for a localized bruise or swelling or localized tenderness and applying a gentle valgus and varus force with the patient’s knee in 15 to 20 degrees of flexion. The degree of medial and lateral joint opening compared with the uninjured knee is a measure of damage to the MCL and LCL. A difference of only 5 mm indicates substantial structural damage. If the knee is stable in full extension, it can be safely assumed that the posterior oblique ligament has no significant damage. Therefore, investigations must include plain radiographs of the knee. These may show fractures, avulsions, osteochondral fragments, or the fluid level of a hemarthrosis.
In general, if a clear diagnosis is made, a specific treatment can be started. Conversely, if an adequate examination is possible, but diagnosis is inconclusive, an expectant policy of mobilization, physiotherapy, and reevaluation in about two weeks may be adopted. On the other hand, if adequate examination is not possible because of pain, spasm, etc, the options available are reevaluation, magnetic resonance imaging (MRI), or examination under anesthesia and arthroscopy. MRI is particularly useful because of its noninvasive nature.
In the past decade, several advances have occurred in the understanding, evaluation, treatment, and rehabilitation of knee instabilities. Despite these advances, an unstable knee still poses many challenges to treating clinicians because of the complexity of its nature and the demands of the patients, who are usually young and active sport enthusiasts.