Anterior Cruciate Ligament (ACL) Injury

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 going down stairs

General Considerations

• ACL tears are common with sporting injuries among young adolescents and middle-aged patients

• 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 maneuvers 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

• Stability tests assess the amount of laxity of the knee (Table 43–6)

• Pivot shift test

  • Used to determine the amount of rotational laxity of the knee

• The Lachman test

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

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

• The anterior drawer test

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

  • Sensitivity, 48%; specificity, 87% for ACL laxity compared with the other side