The sag test, also known as the posterior sag sign test, is used to assess the integrity of the posterior cruciate ligament (PCL) in the knee.
Understanding the Posterior Sag Sign
The test aims to identify a "sag" or concavity just below the patella, which indicates that the tibia has shifted backwards relative to the femur. This posterior displacement is a sign of potential PCL damage. According to the YouTube video reference, a concavity observed distal to the patella suggests a positive posterior sag sign, meaning PCL injury is likely.
How the Test Works:
The patient lies on their back with their hips and knees bent at 90 degrees. A trained healthcare provider will observe the knee, looking for the aforementioned concavity below the patella, and the level of the tibial tuberosity relative to the femoral condyles.
- Positive Sign: If a "sag" or concavity is present, and the tibial tuberosity appears to be lower compared to the femoral condyles when viewed from the side, the posterior sag test is positive, indicating a possible posterior cruciate ligament (PCL) tear or laxity.
- Negative Sign: If the tibial tuberosity sits at a normal level without a concavity, the test is negative, which suggests that the PCL is likely intact.
Clinical Significance
A positive sag test is a strong indicator of a PCL injury. The severity of the posterior displacement can give insight into the extent of the ligament damage. It is essential that this test be part of a thorough knee examination and that findings are corroborated with other tests and imaging for an accurate diagnosis.
Sag Test Summary
| Feature | Positive Sag Test | Negative Sag Test |
|---|---|---|
| Observation | Concavity/sag distal to the patella | No concavity below patella |
| Tibial Position | Tibia displaced posteriorly, tibial tuberosity appears lowered | Tibial tuberosity appears at normal level |
| Indication | Possible PCL tear or PCL laxity | PCL likely intact |
| Confirmation | Other tests and imaging usually needed | Usually requires no further testing |
The referenced video emphasizes that if a posterior sag is observed, the patient should slowly extend their knee for further observation.
