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Anterior Cruciate Ligament (ACL) Injury

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

Diagnosis

  • 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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Table 41–6. Knee examination.
Maneuver Description
Inspection Examine for the alignment of the lower extremities (varus, valgus, knee recurvatum), ankle eversion and foot pronation, gait, “SEADS” (swelling, erythema, atrophy, deformity, surgical scars).
Palpation Include important landmarks: patellofemoral joint, medial and lateral joint lines (especially posterior aspects), pes anserine bursa, distal iliotibial band and Gerdy tubercle (iliotibial band insertion).

Range of motion testing

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 Check range of motion actively (patient performs) and passively (clinician performs), especially with flexion and extension of the knee normally 0–10 degrees of extension and 120–150 degrees of flexion.
Knee strength testing Test resisted knee extension and knee flexion strength manually.
Ligament Stress Testing

Lachman test

image

Performed with the patient lying supine and the knee flexed to 20–30 degrees. The examiner grasps the distal femur from the lateral side, and the proximal tibia with the other hand on the medial side. With the knee in neutral position, stabilize the femur, and pull the tibia anteriorly using a ...

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