Posterior Cruciate Ligament Rupture
A positive posterior drawer sign—excessive posterior translation of the tibia relative to the femur—is most suggestive of posterior cruciate ligament (PCL) rupture.
Diagnostic Reasoning
The posterior drawer test is the most accurate clinical examination for detecting PCL disruption 1, 2. When you push the proximal tibia posteriorly with the knee flexed at 90 degrees and observe excessive posterior translation, this directly tests the integrity of the PCL, which provides 95% of the strength to resist posterior tibial displacement 3.
Key Clinical Findings for PCL Injury
- Posterior sag sign: The most characteristic finding where the tibial tubercle appears to disappear in lateral inspection when the knee is flexed 90 degrees due to gravity-assisted posterior displacement of the tibia 3, 4
- Positive posterior drawer test: Performed at 90 degrees of flexion, this is sensitive for PCL tears though false negatives occur in acute cases 3, 2
- Severe posterior knee pain in >90° of flexion: A common complaint that should raise suspicion 4
Why Not the Other Options
Anterior cruciate ligament (ACL) rupture would show excessive anterior translation on the anterior drawer test or Lachman test, not posterior translation 5, 6. The Lachman test (performed at 20-30 degrees of knee flexion) has 84% sensitivity and 96% specificity for ACL tears when performed 4-5 days post-injury 5.
Lateral meniscus tear does not produce posterior drawer sign 7. MRI would be needed for definitive diagnosis, with 96% sensitivity and 97% specificity 7.
Tibial plateau fracture would be detected on plain radiographs and does not specifically cause isolated posterior drawer positivity 8.
Patellar tendon rupture presents with inability to extend the knee and a palpable defect in the tendon, not posterior tibial translation 4.
Clinical Pitfall
The PCL injury can masquerade as an ACL rupture because the posterior drawer test may appear as a positive anterior drawer test 1. Always assess the starting position of the tibia before performing drawer tests—if the tibia is already posteriorly subluxed, pulling it forward may seem like an anterior drawer when you're actually just reducing the posterior displacement 1, 3.
Associated Injuries to Evaluate
PCL ruptures frequently occur with other injuries, especially in high-energy mechanisms (up to 90% of cases) 1. Perform varus/valgus stress tests and dial test to assess for posterolateral corner or collateral ligament injuries 4. In polytrauma patients, PCL ruptures are frequently recognized late because clinicians don't consider this diagnosis 1.