Musculoskeletal · Knee Injuries
The facts most likely to be tested
The most common mechanism of injury for a Posterior Cruciate Ligament (PCL) tear is a dashboard injury involving a high-energy direct blow to the proximal tibia with the knee in flexion.
The posterior drawer test is the most sensitive and specific physical exam maneuver for diagnosing a PCL tear.
A sag sign (or posterior sag sign) is present when the tibia appears to sag posteriorly relative to the femur when the knee is flexed at 90 degrees.
The PCL is the primary restraint to posterior tibial translation relative to the femur.
Isolated PCL tears are typically managed non-operatively with quadriceps-strengthening exercises and functional bracing.
Magnetic Resonance Imaging (MRI) is the gold standard diagnostic modality to confirm a PCL tear and evaluate for concomitant ligamentous injuries.
Multi-ligament knee injuries involving the PCL often require surgical reconstruction due to the high risk of long-term patellofemoral and medial compartment osteoarthritis.
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A 28-year-old male is brought to the emergency department after a motor vehicle collision where his knee struck the dashboard. He reports generalized knee pain and difficulty walking. On physical examination, there is a large knee effusion. With the patient supine and the knee flexed to 90 degrees, the tibia sags posteriorly compared to the contralateral side. A positive posterior drawer test is elicited. There is no significant laxity with valgus or varus stress testing.
What is the most likely diagnosis?
Posterior Cruciate Ligament (PCL) tear
The vignette describes the classic 'dashboard injury' mechanism and the pathognomonic physical exam findings of a posterior sag sign and positive posterior drawer test, which are diagnostic for a PCL tear.
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Etiology / Epidemiology
Commonly caused by dashboard injuries or high-energy trauma involving a direct blow to the proximal tibia with a flexed knee.
Clinical Manifestations
Patients present with posterior sag sign and a positive posterior drawer test; often associated with multi-ligamentous knee injury.
Diagnosis
The MRI is the gold standard for definitive diagnosis; clinical diagnosis relies on the posterior drawer test.
Treatment
Management is primarily conservative physical therapy focusing on quadriceps strengthening; surgery is reserved for multi-ligamentous instability.
Prognosis
Most isolated tears have a good functional prognosis, but chronic instability leads to early-onset osteoarthritis.
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Epidemiology & Etiology
PCL tears are significantly less common than ACL tears, typically occurring in motor vehicle accidents where the tibia strikes the dashboard. They are also frequently seen in contact sports involving a fall onto a flexed knee with the foot in plantar flexion. High-energy trauma often results in associated injuries to the posterolateral corner.
Pertinent Anatomy
The PCL is the primary restraint to posterior tibial translation relative to the femur. It originates from the lateral aspect of the medial femoral condyle and inserts into the posterior aspect of the tibial plateau. Its strength is approximately twice that of the ACL, requiring significant force to rupture.
Pathophysiology
The classic mechanism is a direct anterior force applied to the proximal tibia while the knee is flexed, known as the dashboard injury. This force drives the tibia posteriorly, stressing the PCL beyond its tensile limit. Isolated tears are rare; clinicians must evaluate for concomitant injuries to the posterolateral corner or other cruciate ligaments.
Clinical Manifestations
Patients often report a vague sense of knee instability or pain in the popliteal fossa. The posterior sag sign is pathognomonic, where the tibia sits posteriorly relative to the femur when the knee is flexed at 90 degrees. Neurovascular compromise, specifically of the popliteal artery or peroneal nerve, must be ruled out in high-energy trauma cases.
Diagnosis
The posterior drawer test is the most sensitive clinical exam, performed with the knee flexed at 90 degrees. A positive posterior drawer test is defined by the absence of a firm endpoint and increased posterior translation. MRI is the gold standard imaging modality to confirm the tear and assess for associated meniscal or ligamentous damage.
Treatment
Isolated PCL tears are managed with physical therapy emphasizing quadriceps rehabilitation to compensate for posterior laxity. Avoid hamstring strengthening in the early phases as it increases posterior tibial translation. Surgical reconstruction is indicated for multi-ligamentous injuries or persistent symptomatic instability despite conservative management.
Prognosis
Isolated injuries generally have a favorable outcome with non-operative management. However, chronic PCL deficiency significantly increases the risk of medial compartment osteoarthritis and patellofemoral joint degeneration over time.
Differential Diagnosis
ACL tear: positive anterior drawer test and Lachman test
MCL tear: positive valgus stress test
LCL tear: positive varus stress test
Meniscal tear: positive McMurray test and joint line tenderness
Posterolateral corner injury: positive dial test