Positive Sag Test: Understanding PCL Injuries, Diagnosis, and Management

What is a positive sag test?

A positive sag test, also known as the posterior sag sign or gravity sag test, is a clinical finding indicating a tear or significant insufficiency of the Posterior Cruciate Ligament (PCL) in the knee, characterized by the tibia appearing to sag or drop posteriorly relative to the femur when the knee is flexed.

Understanding the Sag Test: An Overview

The sag test is a fundamental orthopedic assessment maneuver used by clinicians to evaluate the integrity of the posterior cruciate ligament (PCL), one of the four primary ligaments stabilizing the knee joint. It is a passive test, relying on gravity to reveal instability, and is often among the first observations made when a PCL injury is suspected. Recognizing a positive sag sign is crucial for guiding further diagnostic steps and appropriate management.

Anatomy at Play: The Posterior Cruciate Ligament (PCL)

To understand the sag test, it's essential to grasp the role of the PCL. The PCL is the strongest and largest ligament in the knee, originating from the medial femoral condyle and inserting onto the posterior aspect of the tibia. Its primary function is to prevent the tibia from translating too far posteriorly on the femur. It also acts as a secondary stabilizer against external rotation and hyperextension of the knee. Unlike the Anterior Cruciate Ligament (ACL), which prevents anterior translation, the PCL is less commonly injured but its damage can lead to significant functional limitations.

Mechanism of PCL Injury

PCL injuries typically result from high-energy trauma. Common mechanisms include:

• Direct Blow to the Anterior Tibia: Often referred to as a "dashboard injury," where a flexed knee strikes a dashboard during a car accident, forcing the tibia backward.
• Hyperextension Injury: Excessive straightening of the knee, pushing the tibia posteriorly.
• Hyperflexion with Axial Load: Landing hard on a flexed knee, such as during a fall or sports injury, where the foot is plantarflexed.

PCL injuries can range from mild sprains (Grade I) to partial tears (Grade II) to complete ruptures (Grade III), with the severity influencing the degree of posterior instability observed.

Performing the Sag Test (Posterior Drawer Test in Sag)

The sag test is performed with the patient lying supine (on their back).

• The hip of the affected leg is flexed to approximately 45 degrees, and the knee is flexed to 90 degrees.
• The patient's foot should be resting flat on the examination table, with the examiner supporting the lower leg.
• The examiner observes the knee from a lateral perspective, comparing it to the uninjured knee.

The key is to allow gravity to act on the tibia. No external force is applied initially; the observation is purely visual.

Interpreting a Positive Sag Test

A positive sag test is indicated by the following observation:

• Visual Posterior Displacement: When the knee is positioned at 90 degrees of flexion, the tibia appears to "sag" or drop posteriorly relative to the femoral condyles. The normal contour of the patella and quadriceps tendon may appear flattened or sunken when viewed from the side, as the tibia has shifted backward.
• Loss of Normal Contour: In a healthy knee, the anterior border of the tibia should align with, or be slightly anterior to, the femoral condyles. With a PCL tear, this alignment is lost, and the tibia's superior aspect appears to recede posteriorly.

This posterior displacement occurs because the PCL, which normally restrains such movement, is no longer intact or sufficiently taut to hold the tibia in its correct anatomical position against the force of gravity. The extent of the sag can sometimes correlate with the severity of the PCL injury.

Clinical Significance and Associated Symptoms

A positive sag test is a strong indicator of PCL insufficiency, suggesting either a partial or complete tear. Patients with a PCL injury may experience:

• Pain: Often localized to the posterior aspect of the knee, especially with activity.
• Swelling: Acute injuries usually present with effusions.
• Instability: A feeling of the knee "giving way," particularly during activities requiring deceleration or descending stairs.
• Difficulty with Weight-Bearing: Pain and instability can make walking, running, or jumping challenging.
• Functional Limitations: Problems with kneeling, squatting, or activities that load the knee in flexion.

Over time, chronic PCL insufficiency can lead to degenerative changes within the knee joint, including patellofemoral pain and osteoarthritis, due to altered biomechanics.

Diagnosis and Next Steps

While the sag test is a valuable initial screening tool, it is part of a comprehensive clinical assessment. Other tests, such as the posterior drawer test (where the examiner actively pushes the tibia posteriorly) and the quadriceps active test (where the patient contracts the quadriceps to reduce the sag), help confirm the diagnosis and rule out other injuries.

For definitive diagnosis and to assess the extent of the damage, Magnetic Resonance Imaging (MRI) is the gold standard. An MRI can visualize the PCL directly, identify associated injuries (e.g., meniscal tears, collateral ligament damage), and provide crucial information for treatment planning. Referral to an orthopedic surgeon or sports medicine specialist is typically warranted following a positive sag test and suspected PCL injury.

Rehabilitation and Management

Management of a PCL injury depends on its severity, the patient's activity level, and the presence of concomitant injuries.

• Conservative Management: Many isolated PCL injuries, especially lower-grade sprains, can be managed non-surgically. This typically involves:
  • Rest and Immobilization: Initially, to reduce pain and swelling.
  • Physical Therapy: A cornerstone of rehabilitation, focusing on strengthening the quadriceps muscles, which act as a dynamic stabilizer against posterior tibial translation. Proprioception and balance training are also critical.
  • Bracing: Functional bracing may be used to provide support during activity.
• Surgical Reconstruction: Surgical intervention is usually reserved for high-grade PCL tears, chronic instability, or when other knee ligaments are also injured. The goal is to reconstruct the PCL using a graft (autograft or allograft) to restore stability. Post-operative rehabilitation is lengthy and intensive.

Conclusion

A positive sag test is a key clinical sign that strongly suggests a PCL injury. It is a simple yet effective observation that highlights the loss of the PCL's critical role in preventing posterior tibial translation. While this test provides valuable initial information, a definitive diagnosis requires a thorough clinical examination combined with advanced imaging techniques like MRI. Early and accurate diagnosis is essential for guiding appropriate treatment strategies, whether conservative or surgical, to optimize patient outcomes and restore knee function.