The SAG test, also known as the posterior sag test, is used to assess the integrity of the posterior cruciate ligament (PCL) in the knee.
Understanding the SAG Test
The test is designed to identify whether the tibia (shin bone) has dropped backwards relative to the femur (thigh bone). This posterior movement, or "sag," can indicate a tear or insufficiency of the PCL.
How the SAG Test Works
Here's a breakdown of how the test is performed and interpreted:
- Positioning: The patient lies on their back (supine position) with their knees bent at a 90-degree angle and their hips flexed.
- Observation: The examiner observes the patient from the side to see if the tibia drops or sags backward in relation to the femur.
- Interpretation:
- Normal: In a healthy knee, there should be little to no posterior sagging of the tibia.
- Positive Test: A visible backward drop or sag of the tibia indicates a potential PCL tear. According to the referenced video, the sag should not be noticeable in a healthy knee.
Why is the SAG Test Important?
- PCL Injuries: The PCL is a key ligament in the knee, and injuries to it can cause instability and pain. The sag test is a useful tool for diagnosing PCL injuries.
- Differentiating from ACL Injuries: While both PCL and anterior cruciate ligament (ACL) injuries can cause knee instability, the sag test helps differentiate between them by assessing posterior translation of the tibia.
- Initial Assessment: This test is often performed as part of an initial assessment of knee injuries to help determine if further diagnostic tests are needed.
Summary
| Test Name | Purpose | Positive Finding |
|---|---|---|
| Posterior Sag Test | Assess PCL integrity | Posterior Sag of the Tibia |
| Identifies PCL Tear or Insufficiency |
The SAG test provides valuable information during a physical examination to assess the possibility of a PCL injury by observing posterior translation of the tibia.
