Image Quiz

Cervical Spine Injury in a Six-Year-Old Boy (continued)

Answer: Atlantoaxial dislocation secondary to a displaced chondrum terminale.


Fig. 1		 Fig. 1 Lateral radiograph of the cervical spine, made on the day of injury, demonstrating atlantoaxial dislocation.

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Fig. 2-A
Fig. 2-B
Fig. 2-C
Fig. 2-A T2-weighted sagittal magnetic resonance imaging scan of the upper portion of the cervical spine, showing the displaced chondrum terminale. The transverse ligament is interposed between the chondrum terminale and the stalk of the odontoid.
Fig. 2-B Drawing showing the position of the structures shown in Fig. 2-A, including the transverse ligament (A), the chondrum terminale (B), the apical ligament (C), the tectorial membrane (D), the anterior atlanto-occipital membrane (E), the lower portion of the cruciate ligament (closely applied to dura) (F), the anterior arch of the atlas (G), and the posterior atlanto-occipital membrane (H).
Fig. 2-C T2-weighted sagittal magnetic resonance imaging scan of the upper portion of the cervical spine, showing the transverse ligament continuing out laterally to insert on the tubercle of the atlas.

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Discussion

Fracture-dislocation through the chondrum terminale may be more prevalent than is reported in the literature. Freiberger et al., Ricciardi et al., and Fielding have all reported traumatic cases of atlantoaxial dislocation in children with no associated fractures. For an atlantoaxial dislocation to occur without fracture, rupturing of the transverse ligament must take place. The transverse ligament is nonelastic and strong, while the open, immature chondrum terminale physis is relatively weak.

We hypothesize that, in the case of our patient, a sudden forward flexion and distraction force to the upper cervical spine transposed the transverse ligament up and over the tip of the odontoid. This movement was unrestrained by the unfused chondrum terminale, and the transposing ligament pushed the chondrum terminale anteriorly at the level of its physis. It is possible for this type of injury to reduce spontaneously, but alternatively, as in the case of our patient, the ligament can remain transposed between the chondrum terminale and the remainder of the odontoid, causing the patient to present with an atlantoaxial dislocation.

In 1980, Hakuda et al. reported on traumatic atlantoaxial dislocation in a five-year-old child for whom radiographs demonstrated an "oval, radiopaque mass" just posterior to the anterior arch of the atlas. A magnetic resonance imaging scan was not acquired. In our opinion, this mass was most likely the ossiculum terminale (a partially ossified chondrum terminale). This mass moved forward with the atlas when the neck was flexed. The child was managed with transoral resection of the "cicatrized ligamentous remnants," during which an attempt to fuse the lateral atlantoaxial articulation failed. A second decompression was done, this time posteriorly, but the child ultimately died of pneumonia.

Sherk and Nicholson reported on a child with Down syndrome in whom an ossiculum terminale was associated with progressive atlantoaxial dislocation. The child first had symptoms of torticollis when she was six years old, and an upper cervical fusion was recommended. The parents refused treatment and, by the time the child was fifteen years old, the dislocation was irreducible. She became quadriplegic and died. At the time of autopsy, the apex of the odontoid process was a separate ossicle (ossiculum terminale), which was displaced anteriorly with respect to the rest of the odontoid. The transverse ligament was noted to have "jumped" the stalk of the odontoid and prevented reduction of the dislocation.

An os odontoideum, a developmental or posttraumatic anomaly of the axis, is an oval "ossicle" that has a circumferential cortex of uniform thickness and smooth borders and that is separated from the base of the odontoid by an appreciable gap. According to Fielding et al., "The apical ossicle (ossiculum terminale) is never large enough to be confused with the whole of the os odontoideum." The cause of os odontoideum is still a matter of debate. Our described mechanism of dislocation supports trauma as at least one cause of an os odontoideum. The blood supply to the base of the odontoid is lost with trauma, but the blood supply to the chondrum terminale continues by way of the apical arcade. It is theorized that the proximal portion of the odontoid resorbs, and the terminale is left to grow into an os.

Unossified cartilage in the upper cervical spine of young children makes accurate diagnosis of atlantoaxial dislocation secondary to displaced chondrum terminale a challenge. The injury mechanism of our patient should be kept in mind when evaluating a child who has sustained a cervical injury. When this type of injury has been confirmed, we recommend halo traction until adequate reduction of the dislocation is achieved. It is our opinion that halo traction is warranted until enough callus has formed at the physeal junction of the chondrum terminale to allow further mobilization in a halo vest. Halo-vest treatment should be maintained until full osseous healing of the odontoid has taken place. Atlantoaxial stability should be documented on flexion-extension radiographs before halo treatment is discontinued.

Reference

1. Hammerstein J, Russo S, Easton K. Atlantoaxial dislocation in a child secondary to a displaced chondrum terminale. A case report. J Bone Joint Surg Am. 2007;89:413-7.