Posterior sag sign
- Fysiobasen
- Dec 18, 2025
- 3 min read
The Posterior Sag Sign, also known as Godfrey’s test, is a passive orthopedic test used to detect posterior cruciate ligament (PCL) rupture. It is especially relevant after high-energy trauma, such as a dashboard injury in car accidents, where a posterior force drives the tibia backward¹.
This test evaluates posterior tibial translation relative to the femur. It is considered a passive variant of Müller’s test, providing a visual indication of tibiofemoral instability.
Clinical background
The first known description of posterior sag was made in 1903 by Dr. A. W. Mayo Robson, who reported how the tibia would drop backward in a miner with cruciate rupture and could be repositioned manually². Today this observation is recognized as the clinical foundation of the posterior sag sign.
Clinical anatomy
The PCL is the strongest ligament of the knee, attaching from the posterolateral tibia to the anterolateral surface of the medial femoral condyle³.
It prevents posterior translation of the tibia relative to the femur.
When ruptured, tibial stability is lost, allowing the tibia to "sag" backward in flexion.
This posterior drop is visible when the quadriceps relax in supine position⁴.
Test execution
Starting position
Patient lies supine.
Hip is flexed to 45°, knee flexed to 90°.
Heel rests on the table, quadriceps relaxed.
Observation
Compare tibial step-off bilaterally.
Normally, tibia sits ~1 cm anterior to femoral condyle.
In PCL rupture, the tibia on the injured side sags posteriorly – creating a “hanging” appearance⁵.
Interpretation
Positive finding: Visible posterior sag of the tibia compared to the femur.
Clinical meaning: Strongly indicates full-thickness PCL rupture, particularly in high-grade injuries.
Use: Especially helpful when other tests are too painful in acute settings.
Diagnostic accuracy
Test | Sensitivity | Specificity |
Posterior drawer (low-grade) | 70 % | 99 % |
Posterior drawer (high-grade) | 97 % | 100 % |
Posterior sag sign | 79 % | 100 % |
Specificity: Very high (up to 100 %)⁷.
Sensitivity: Moderate (higher in severe injuries).
Best use: To confirm PCL rupture rather than to screen.
According to Rubinstein et al., the sag sign substantially increases diagnostic certainty when combined with the posterior drawer test⁷. Malanga et al. concluded that it enhances diagnostic accuracy, especially in subacute and chronic phases⁸.
Differential diagnosis and related tests
Posterior drawer test – gold standard dynamic test.
Quadriceps active test – reveals posterior tibial shift corrected by quadriceps contraction.
Dial test – assesses posterolateral rotatory instability.
False negatives may occur with massive effusion or strong quadriceps activation.
Summary
The Posterior Sag Sign (Godfrey’s test) is a simple, passive, and highly specific clinical test for detecting posterior cruciate ligament rupture. By observing posterior tibial displacement at 90° knee flexion, clinicians can confirm instability without stressing the joint.
Its high specificity makes it a valuable tool for confirming PCL rupture, particularly when combined with the posterior drawer test.
Sources:
Robson AM. VI. Ruptured crucial ligaments and their repair by operation. Ann Surg. 1903;37(5):716.
Gjennomgått - Trukket
Malanga GA, Andrus S, Nadler SF, McLean J. Physical examination of the knee: a review of the original test description and scientific validity of common orthopedic tests. Arch Phys Med Rehabil. 2003;84:592–603.
Rubinstein RA, Shelbourne KD, McCarroll JR, VanMeter CD, Rettig AC. The Accuracy of the Clinical Examination in the Setting of Posterior Cruciate Ligament Injuries. Am J Sports Med. 1994;22(4):550–557.
