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Key Features

  • An injury involving an audible pop when the knee buckles

  • Acute swelling immediately (or within 4 hours)

  • Instability occurs with lateral movement activities and going down stairs

  • ACL tears are common with sporting injuries

  • Can result from both contact (valgus blow to the knee) and non-contact (jumping, pivoting, and deceleration) activities

Clinical Findings

  • Acute swelling of the knee, causing difficulty with motion

  • After the swelling has resolved, the patient can walk with a 'stiff-knee' gait or quadriceps avoidance gait because of the instability

  • Patients describe symptoms of instability while performing side-to-side maneuvers or descending stairs

Diagnosis

  • Stability tests assess the amount of laxity of the knee while performing side-to-side maneuvers or descending stairs

    • The Lachman test is performed with the patient lying supine and the knee flexed to 20–30 degrees (Table 41–7)

    • The anterior drawer test is performed with the patient lying supine and the knee flexed to 90 degrees (Table 41–7)

    • The pivot shift test is performed with the patient lying supine with the knee in full extension and is used to determine amount of rotational laxity of knee (Table 41–7)

  • Plain radiographs are usually negative in ACL tears but are useful to rule out fractures

  • MRI is the best method to diagnose ACL tears

Table 41–7.Knee examination.

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