SAG Test: Purpose, Performance, and Clinical Significance for PCL Injuries

What is the SAG test used for?

The SAG test, formally known as the Posterior Sag Sign or Gravity Sag Sign, is a crucial orthopedic assessment used to identify potential injuries to the Posterior Cruciate Ligament (PCL) of the knee.

Understanding the SAG Test

The knee joint is a complex structure stabilized by a network of ligaments, including the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). While the ACL prevents the tibia from sliding too far forward relative to the femur, the PCL is the primary restraint against posterior displacement of the tibia. It is a strong ligament, making PCL injuries less common than ACL tears, but they can occur due to significant trauma.

The SAG test is a clinical examination technique that leverages gravity to reveal instability caused by a compromised PCL. It is a visual assessment that requires keen observation by the examiner.

Primary Purpose: Detecting Posterior Cruciate Ligament (PCL) Injury

The fundamental purpose of the SAG test is to assess the integrity of the Posterior Cruciate Ligament (PCL). A positive SAG sign strongly suggests a complete or significant tear of the PCL.

PCL injuries often result from:

• Direct blow to the anterior aspect of the tibia (e.g., a dashboard injury in a car accident, falling onto a flexed knee).
• Hyperextension of the knee.
• Rotational forces applied to the knee.

When the PCL is torn, its ability to hold the tibia in its proper anterior position relative to the femur is compromised. Gravity then pulls the unsupported tibia posteriorly, creating the visible "sag" that characterizes a positive test.

How the SAG Test is Performed

While not a detailed instructional guide, understanding the basic setup of the SAG test helps illustrate its mechanism:

1. Patient Positioning: The patient lies supine (on their back) on an examination table.
2. Knee and Hip Flexion: The examiner flexes the patient's hip to approximately 45 degrees and the knee to 90 degrees.
3. Foot Placement: The patient's foot is placed flat on the table, supporting the lower leg.
4. Observation: The examiner observes the knee from the side, specifically looking at the tibial tuberosity (the bony prominence just below the kneecap) in relation to the femoral condyles (the rounded ends of the thigh bone).

The test relies on the force of gravity acting on the lower leg when the PCL is unable to provide adequate support.

Interpreting the Results

The interpretation of the SAG test is straightforward:

• Negative SAG Test: If the PCL is intact, the tibial tuberosity will appear aligned with or slightly anterior to the femoral condyles. There will be no visible posterior displacement or "sag" of the tibia.
• Positive SAG Test: A positive test is indicated by a visible posterior displacement or "sag" of the tibia relative to the femur. The tibial tuberosity will appear to drop posteriorly, creating an indentation or step-off just below the kneecap. This is often compared to the appearance of a "dropped" or "sunken" knee.

A positive SAG sign is a strong indicator of a PCL injury, often a complete tear, as partial tears may not produce a noticeable sag.

Clinical Significance and Limitations

The SAG test is a valuable initial screening tool for PCL injuries, particularly in acute trauma settings where a quick assessment is needed.

Clinical Significance:

• High Specificity: When a sag is clearly visible, it is highly specific for a PCL injury, meaning it rarely gives a false positive.
• Early Detection: Can help guide further diagnostic steps and appropriate management.

Limitations:

• Subjectivity: The visual nature of the test means it can be subjective and may require an experienced eye to detect subtle sags.
• Acute Swelling/Pain: In acute injuries, significant swelling or muscle guarding may obscure the sag or make proper positioning difficult.
• Not Definitive Alone: While highly indicative, the SAG test is rarely used in isolation for a definitive diagnosis. It is typically complemented by other specific PCL tests, such as the Posterior Drawer Test (where the examiner manually pushes the tibia posteriorly) and the Quadriceps Active Test (where the patient contracts the quadriceps, which can reduce the sag if the PCL is torn).
• Imaging Confirmation: For a conclusive diagnosis, especially to determine the grade of the tear and assess for concomitant injuries, imaging studies like Magnetic Resonance Imaging (MRI) are typically performed.

Beyond the PCL: Associated Injuries and Considerations

While the SAG test primarily points to PCL damage, clinicians also consider the possibility of other injuries that might accompany a PCL tear, especially in high-energy trauma. These can include:

• Meniscal tears: Damage to the cartilage pads in the knee.
• Collateral ligament injuries: Tears to the medial or lateral collateral ligaments.
• Capsular injuries: Damage to the joint capsule.
• Fractures: Bony injuries around the knee.

A comprehensive orthopedic examination is always necessary to rule out these associated injuries.

Conclusion

The SAG test is an essential clinical maneuver for evaluating the integrity of the Posterior Cruciate Ligament. By observing the gravitational displacement of the tibia relative to the femur, clinicians can quickly identify potential PCL tears. While a highly specific indicator, it is always used as part of a broader diagnostic process involving other physical tests and advanced imaging to ensure an accurate diagnosis and guide appropriate treatment, whether conservative management or surgical intervention.