The "gull sign" in acetabular fractures revisited - is the dome impacted or elevated?

The gull sign, representing superomedial dome impaction in acetabular fractures, was first described approximately 20 years ago by Anglen and co-workers. They concluded that this sign equates with poor outcomes after open reduction and internal fixation (ORIF), terming it a "harbinger for failure". Since then, the presence of the gull sign has frequently influenced surgical decision-making in geriatric acetabular fractures. The aim of this radiological descriptive study was to revisit the accuracy of the gull sign seen on pelvic radiographs in predicting dome impaction on computed tomography (CT).

In a retrospective study, conventional pelvic radiographs and CT scans of n = 201 patients (mean age±SD: 68±17y, 75 % male) with acetabular fractures treated surgically between 2009 and 2020 were analyzed. The presence of the gull sign was assessed on anteroposterior pelvic radiographs. CT scans were assessed for true impaction ("brick sign") with focus on the acetabular surface and compared to the findings according to Anglen`s description.

The gull sign was noted on pelvic radiographs in 49 of 201 cases (24 %). In 28 out of these 49 cases (57 %) a dome impaction was noted on CT. In the remaining 21 cases (43 %), CT revealed no actual impaction but rather an elevated dome following displaced fracture fragments. Conversely, among the 152 patients (76 %) without a gull sign, CT identified previously undetected dome impactions in 41 cases. Overall, the gull sign had a sensitivity of 41 %, a specificity of 84 %, and a positive predictive value of 57 % for detecting dome impactions.

The gull sign is an unreliable predictor for dome impaction in acetabular fractures for the following reasons: in the presence of the gull sign nearly half of the cases an elevated fragment only (not an impacted fragment) was noted on CT; despite an absence of the gull sign in nearly one-third of these cases dome impactions ("brick sign") were present on CT. Consequently, routine preoperative CT imaging is essential to accurately differentiate true dome impactions ("brick sign") from dome elevation, thereby guiding appropriate surgical decision-making between "disimpaction versus reduction" and in general between the "fix or replace" debate.