Harvey's Garden

Knee ligament injuries

2 min read

Cruciate ligament injuries

Lateral femoral condyle to anterior tibia: ACL. Medial femoral condyle to posterior tibia: PCL. LAMP.

Anterior Cruciate Ligament (ACL) Injury

  • Function
    • Primarily prevents anterior translation of the tibia relative to the femur.
    • Provides rotational stability to the knee, resisting internal tibial rotation.
  • Mechanism of Injury
    • Most common knee ligament injury.
    • Typically non-contact pivoting or twisting motion with the foot planted (e.g., soccer, basketball, skiing).
    • Sudden deceleration or hyperextension.
  • Clinical Presentation
    • Pt often reports hearing or feeling a distinct “pop”.
    • Rapid development of hemarthrosis (joint swelling due to bleeding) within hours.
    • Sensation of instability or the knee “giving way.”
  • Physical Examination
    • Lachman Test: Most sensitive test. With knee flexed at 30°, stabilize femur and pull tibia anteriorly. Positive if excessive anterior translation without a firm endpoint.
    • Anterior Drawer Test: With knee flexed at 90°, pull tibia anteriorly. (Less sensitive due to hamstring guarding).
    • Pivot Shift Test: High specificity; reproduces the instability event.
  • Associations
    • “Unhappy Triad” (O’Donoghue’s Triad): Result of severe lateral force to the knee. Classically involves damage to the:
      1. ACL
      2. MCL (Medial Collateral Ligament)
      3. Medial Meniscus (Note: Modern literature suggests lateral meniscus is more commonly injured acutely, but USMLE traditionally emphasizes the medial meniscus).
  • Diagnosis & Treatment
    • MRI: Gold standard for confirmation.
    • Tx: RICE, Physical Therapy. Surgical reconstruction (autograft/allograft) indicated for young, active patients or those with persistent instability.

Posterior Cruciate Ligament (PCL) Injury

  • Function
    • Primarily prevents posterior translation of the tibia relative to the femur.
    • Strongest ligament in the knee; acts as a central axis of rotation.
  • Mechanism of Injury
    • Much less common than ACL injuries.
    • “Dashboard Injury”: Direct posterior force applied to the proximal tibia with the knee flexed (e.g., knee hitting dashboard in MVA (motor vehicle accident)).
    • Severe hyperflexion or hyperextension.
  • Clinical Presentation
    • Vague posterior knee pain.
    • Instability (though often less pronounced than ACL).
    • Swelling is typically mild to moderate compared to ACL.
  • Physical Examination
    • Posterior Drawer Test: With knee flexed at 90°, push tibia posteriorly. Positive if excessive posterior translation (tibia “sags” back).
    • Posterior Sag Sign: With patient supine and hips/knees flexed to 90°, look for posterior subluxation of the tibia due to gravity.
  • Diagnosis & Treatment
    • MRI: Gold standard.
    • Tx: Usually non-operative (bracing, intense quadriceps strengthening) for isolated Grade I/II injuries. Surgery reserved for multi-ligament knee injuries or failed conservative management.

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