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Knee instability: isolated and complex
  1. T K Kakarlapudi,
  2. D R Bickerstaff
  1. Specialist Registrar in Orthopaedic Surgery The Northern General Hospital Sheffield S5 7 AU
  2. Consultant Orthopaedic Surgeon Thornbury Hospital Sheffield S10 3BR United Kingdom email: drbickerstaff@uk-consultants

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    The past decade has seen several advances in the understanding, evaluation, treatment, and rehabilitation of knee instabilities. Despite these advances, an unstable knee still poses many challenges to the treating clinician because of the complexity of its nature and the demands of the patients who are usually young and active sport enthusiasts. This article presents an overview of the various aspects of knee ligament instabilities.

    Stability and instability

    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). Significant contributions are also made by the posteromedial and posterolateral capsular components and the iliotibial tract. The muscles acting over the joint provide secondary dynamic stability.

    Instability resulting from ligament injury may result from direct or indirect trauma. The most frequent mechanism is “non-contact” involving cutting, twisting, jumping, and sudden deceleration.


    This begins with a detailed history, including a description of the injury. The timing of an effusion (acute haemarthrosis usually occurs within two hours) and hearing or feeling a “pop” (highly suggestive of an ACL injury) are significant events. 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. Initial physical examination may be difficult because of swelling, pain, or muscle spasm. The specific physical signs are described below. Investigations must include plain radiographs of the knee. These may show fractures, avulsions, osteochondral fragments, or the fluid level of a haemarthrosis.

    If a clear diagnosis is made, a specific treatment can be started. If an adequate examination is possible, but diagnosis …

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