Unmasking the subtle clues of hip dislocation: Air bubble and notching sign as CT-based indicators.

Background: Spontaneously reduced hip dislocation or reduced hip dislocation with a missing reduction history are challenge to the treating surgeon as the signs are usually subtle in such cases. The purpose of this study is to investigate and report on the incidence of the signs of femoral head notching and the presence of intracapsular air bubble in the computed tomography (CT) scans of hip dislocation cases in our center.

Methods: Cases of traumatic hip dislocation, either without associated acetabular fractures or with acetabular fractures that did not require surgery from 2002 to 2021 were included retrospectively. Their CT scan films were analyzed to look for the presence and direction of femoral head notching and appearance of intracapsular air-bubble.

Results: 30 hips with traumatic hip dislocation and 28 hips with acetabular posterior wall fracture without dislocation were included in this study. We noted notching on the femoral head in the CT axial scans of 23 of 30 hips (76.7 %). 17 cases of notching were noted in association with posterior dislocation, and there were 6 cases associated with anterior dislocation (p = 0.543). We observed intracapsular air bubbles in the CT scans of 28 of the 30 hips in our series (93.3 %). The sensitivity and specificity of notching sign were 56.7 % (95 % CI 37.4∼74.5 %) and 100 % (95 % CI 87.7∼100 %), respectively. For the bubble sign, the sensitivity and specificity were 90 % (95 % CI 73.5∼97.9 %) and 100 % (87.7∼100 %), respectively.

Conclusion: We recommend an early CT scan in patients presented with a history of suspected hip dislocation without an obvious finding on plain radiography. The presence of femoral head notching or intracapsular air-bubble would strongly suggest a history of hip dislocation.

Level Of Evidence: Diagnostic Level III, Retrospective cohort study.