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For further information, see CMDT Part 41-06: Musculoskeletal Injuries of the Knee

Key Features

Essentials of Diagnosis

  • An injury involving an audible pop when the knee buckles

  • Acute swelling immediately (or within 2 hours)

  • Instability occurs with lateral movement activities and descending stairs

General Considerations

  • 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

  • Prepubertal and older patients usually sustain fractures instead of ligamentous injuries

Clinical Findings

  • Acute swelling of the knee, causing difficulty with motion

  • Difficulty with weight bearing

  • Instability of the knee during side-to-side movement or while descending stairs

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


  • Plain radiographs

    • Usually normal in ACL tears

    • Useful to rule out fractures

    • A small avulsion injury can sometimes be seen over the lateral compartment of the knee ("Segond" fracture) and is pathognomonic of an ACL injury

    • An ACL injury that avulsed the tibial spine can be seen in radiographs

  • MRI

    • Best tool to diagnose ACL tears and associated articular and meniscal cartilage issues

    • Has > 95% sensitivity and specificity for ACL tears

Diagnostic Procedures

  • The Lachman test (Table 41–6)

    • performed with the patient lying supine and the knee flexed to 20–30 degrees

    • Sensitivity, 84–87%; specificity, 93%

  • The anterior drawer test (Table 41–6)

    • Performed with the patient lying supine and the knee flexed to 90 degrees

    • Sensitivity, 48%; specificity, 87%

  • The pivot shift test is used to determine the amount of rotational laxity of the knee (Table 41–6)

Table 41–6.Knee examination.

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