Unilateral cervical facet joint dislocation (UCFJD) is the most frequently missed cervical spine injury on plain radiographs. If left untreated, UCFJD can progress to bilateral cervical facet joint dislocation. Given the complexity of cervical facet joint dislocations, radiologists rely on metaphorical signs to identify them on radiographs. The “Bowtie” and “laminar space” signs represent UCFJD on plain radiographs. The “reversed hamburger”, “naked facet” and “headphones” signs represent cervical facet joint dislocations on axial cross-sectional imaging. Illustrating these signs in an engaging manner facilitates pattern-based recognition, which can benefit trainees and radiologists. Moreover, pattern-based recognition can be applied to machine learning.
In the normal anatomical relationship between the cervical facet joints, the inferior articulating facet of the superior vertebral body lies posterior to the superior facet of the inferior vertebral body. This anatomical relationship can be upset in a cervical facet joint dislocation.
Unilateral cervical facet joint dislocation (UCFJD) is one of the most commonly missed cervical spine injuries on plain radiographs [1, 2, 3], and it can have dire consequences. If left untreated, a UCFJD can progress to a bilateral cervical facet joint dislocation (BCFJD) , which is an unstable cervical spine injury. Owing to the serious implications of a missed UCFJD, it is imperative to perform computed tomography scans whenever there is clinical suspicion of a cervical spine injury, even when initial plain radiographs may not suggest any abnormality .
Given the complexity of cervical facet joint dislocations, radiologists rely on metaphoric imaging signs to identify these injuries . In this review, we highlight some of the classical radiological signs which can facilitate the diagnosis of cervical facet joint dislocations.
The stability of the cervical spine is mainly contributed by its supporting ligaments . Ligamentous injuries in facet joint dislocation often result from flexion-distraction (primarily hyperflexion) injuries with a rotational component about an axis anterior to the vertebral body [8, 9]. The inferior articular facet of the upper vertebra projects anterior to the superior articular facet of the lower vertebra and becomes locked, giving rise to UCFJD . Likewise, when both facet joints are involved and locked, it results in BCFJD .
These signs are associated with UCFJD and best seen on plain radiographs. On the lateral radiograph, one can observe up to 25% translation of the superior vertebral body onto the anteroposterior diameter of the inferior vertebral body .
The laminar space—which is the distance between the spinolaminar line and the posterior surface of the articular pillars—changes abruptly at the level of the injury due to sudden rotational forces from the injury . This abrupt change in the laminar space is referred to as the “laminar space sign”. Displacement of the interfacetal joint with rotational deformity produces a “bow-tie” appearance at the level of injury; hence it is referred to as the “bow-tie sign” . In addition, this produces a “double-appearance” of articular facets above the level of the injury.
According to Allen and Ferguson, the traumatic cervical facet joint dislocation can be summarized into four stages :
Stage I: Subluxation of the facet with increased gap distance between interspinous ligaments. This indicates compromise to the integrity of the posterior ligamentous complex.
Stage II: UCFJDs, with up to 25% translation of the superior vertebral body onto the anteroposterior diameter of the inferior body.
Stage III: BCFJDs, with up to 50% translation of the superior vertebral body onto the anteroposterior diameter of the inferior body.
Stage IV: Nearly 100% translation of the superior vertebral body onto the inferior body, resulting in the appearance of a “floating vertebra” (Figure 2).
Healthy facet joints appear as “hamburgers” on axial CT scans of the cervical spine, where each facet represents half of the “hamburger bun”. Facet joint dislocations upset this relationship and reverse the orientation of the “half buns”, giving rise to the “reverse hamburger bun sign” .
The headphones sign is best appreciated on the axial CT scan. The concentric appearance of bilateral uncovertebral joints in a healthy vertebra resembles a pair of headphones on its wearer’s head . In a fracture or dislocation of the uncovertebral joint, the superior vertebral body is no longer supported by the uncinate process of the inferior vertebra. Thus, the superior vertebral body is translated anterior to the inferior vertebra, and it rotates towards the normal side. This appearance gives rise to the unilateral positive “headphones sign”, which is commonly associated with UCFJD . On axial CT scans, the “headphones sign” and “reverse hamburger bun sign” are reliable indicators of cervical facet joint dislocations.
The classic radiological signs seen in patients with traumatic cervical facet joint dislocation are inspired by nature. These signs correlate with the anatomical abnormalities presented in cervical spine injuries, thus facilitating the diagnosis of the injuries. Trainees and radiologists may both benefit from this pattern-based approach of learning and practising radiology. Additionally, this pattern recognition approach of learning radiology may be useful in future machine-based learning applications.
The authors have no competing interests to declare.
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