Sag Test: Purpose, Procedure, and PCL Injury Diagnosis

What is the Sag Test For?

The Sag Test, also known as the Posterior Sag Sign or Godfrey's Test, is a clinical diagnostic examination used to assess the integrity of the posterior cruciate ligament (PCL) in the knee joint, specifically identifying posterior displacement of the tibia relative to the femur.

Introduction to the Sag Test

The knee joint, a complex articulation designed for both mobility and stability, relies heavily on its ligamentous structures. Among these, the posterior cruciate ligament (PCL) plays a critical role in preventing posterior translation of the tibia on the femur. When a patient presents with a knee injury, especially following trauma, a thorough clinical examination is paramount. The Sag Test is a foundational component of this examination, serving as a rapid and effective screening tool for potential PCL tears. Its simplicity and visual nature make it an invaluable first-line assessment in sports medicine and orthopedics.

Understanding the Posterior Cruciate Ligament (PCL)

To fully appreciate the Sag Test, it's essential to understand the PCL's anatomy and function. The PCL is one of the four main ligaments of the knee, running from the posterior intercondylar area of the tibia to the lateral aspect of the medial femoral condyle. Its primary function is to resist forces that push the tibia posteriorly on the femur, particularly when the knee is flexed.

Common mechanisms of PCL injury include:

• Dashboard Injury: Occurs when the tibia is forced posteriorly, such as in a car accident where the shin hits the dashboard.
• Direct Blow to the Proximal Tibia: A fall onto a flexed knee with the foot plantarflexed.
• Hyperextension Injury: Excessive straightening of the knee, though this often involves other ligaments as well.

Unlike ACL injuries, PCL tears often present with less acute swelling and pain, making a precise diagnostic test like the Sag Test crucial for accurate identification.

Purpose of the Sag Test

The primary purpose of the Sag Test is to visually detect a posterior subluxation or displacement of the tibia in relation to the femur, which is indicative of a compromise or rupture of the posterior cruciate ligament. When the PCL is torn, it can no longer effectively restrain the tibia from sliding backward, allowing gravity and the weight of the lower leg to pull the tibia posteriorly.

Specifically, the test aims to:

• Identify PCL Insufficiency: Determine if the PCL is unable to perform its function of stabilizing the tibia against posterior translation.
• Differentiate from Other Ligamentous Injuries: While other tests assess different ligaments (e.g., Lachman for ACL, varus/valgus stress for collateral ligaments), the Sag Test is highly specific to PCL integrity.
• Guide Further Diagnostic Steps: A positive Sag Test strongly suggests a PCL injury, prompting further investigation through imaging (e.g., MRI) and potentially other clinical tests.

How the Sag Test is Performed

The Sag Test is a passive, non-invasive examination performed by a clinician.

Steps for performing the Sag Test:

1. Patient Positioning: The patient lies supine (on their back) on an examination table.
2. Knee and Hip Flexion: Both hips are flexed to approximately 45 degrees, and both knees are flexed to 90 degrees. The patient's feet should be flat on the table, supporting the lower legs.
3. Observation: The examiner stands at the foot of the table and observes the contour of both knees from the side. The key is to compare the injured knee with the uninjured knee.
4. Visual Assessment: In a healthy knee, the tibial tuberosity (the bump on the front of the shin below the kneecap) should appear relatively prominent, and the anterior contour of the knee should be smooth. If the PCL is torn, the tibia on the affected side will "sag" or drop posteriorly due to gravity, making the tibial tuberosity appear less prominent or even concave when viewed from the side.

This posterior displacement is often subtle but becomes more apparent when comparing the injured leg to the uninjured leg, highlighting the difference in the anterior step-off of the tibia.

Interpreting the Results

Interpreting the Sag Test is straightforward:

• Negative Sag Test: If the anterior contour of the tibia on the injured side appears normal and is symmetrical with the uninjured knee, indicating no significant posterior displacement, the Sag Test is considered negative. This suggests the PCL is likely intact.
• Positive Sag Test: If the tibia on the injured side visibly "sags" or drops posteriorly, creating a loss of the normal anterior tibial step-off, the Sag Test is considered positive. This finding strongly indicates a tear or significant laxity of the posterior cruciate ligament. The degree of sag can sometimes correlate with the severity of the PCL injury (e.g., complete rupture vs. partial tear).

Clinical Significance and Next Steps

A positive Sag Test carries significant clinical implications:

• Confirms PCL Injury Suspicion: It provides strong clinical evidence supporting a diagnosis of a PCL tear.
• Informs Treatment Strategy: The diagnosis of a PCL injury guides subsequent management, which can range from conservative approaches (rest, ice, physical therapy, bracing) for isolated or partial tears to surgical reconstruction for more severe or combined ligamentous injuries, especially in active individuals.
• Prevents Long-Term Complications: Undiagnosed or untreated PCL injuries can lead to chronic knee instability, pain, and accelerated degenerative changes, including osteoarthritis, due to altered knee kinematics and increased stress on other joint structures.
• Guides Imaging: A positive test often necessitates confirmatory imaging, typically a magnetic resonance imaging (MRI) scan, which provides detailed visualization of the ligaments and other soft tissues of the knee.

Limitations and Considerations

While the Sag Test is a valuable tool, it's important to consider its limitations:

• Acute Swelling: In the very acute phase of injury, significant knee effusion (swelling) can sometimes obscure the subtle posterior sag, making the test less clear.
• Chronic Injuries: In chronic PCL injuries, compensatory mechanisms or muscle guarding might reduce the visible sag.
• Part of a Battery of Tests: The Sag Test should never be performed in isolation. It is always part of a comprehensive knee examination that includes palpation, range of motion assessment, and other specific ligamentous stability tests (e.g., posterior drawer test, quadriceps active test, reverse pivot shift test) to provide a complete clinical picture and rule out concomitant injuries.
• Patient Relaxation: The accuracy of the test relies on the patient's ability to relax their hamstring muscles, as hamstring contraction can pull the tibia posteriorly and mimic a sag, or mask a true sag.

Conclusion

The Sag Test is a cornerstone in the orthopedic and sports medicine assessment of knee injuries, specifically designed to identify damage to the posterior cruciate ligament. Its ability to quickly and visually detect posterior tibial displacement makes it an indispensable initial screening tool. A positive Sag Test serves as a critical indicator, guiding clinicians towards further diagnostic investigation and ultimately enabling the development of an appropriate and timely treatment plan to manage PCL injuries effectively and preserve long-term knee function.