Persistent synchondrosis of the second cervical vertebra simulating a hangmanʼs fracture in a child. Report of a case.

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Abstract
This case report emphasizes the difficulties involved in the evaluation of roentgenograms of the cervical spine in a child who has sustained an injury to the neck. Fractures of the cervical spine are unusual in children but, when they occur, they tend to involve the cephalic part of the cervical spine [6]. Traumatic spondylolisthesis of the axis, or hangman's fracture, has been described in children [7,8,15]. It is unusual to find any neural deficit associated with this fracture, which usually unites with non-operative treatment. Variations in the roentgenographic appearance of the cervical vertebrae may simulate fractures and even instability [1,2,4]. Pseudosubluxation [2], laminar defects [9], and synchondrosis [10,12,14] must be distinguished from traumatic lesions. An eighteen-month-old boy fell from a height of approximately two and one-half meters (eight feet), landing head first in shrubbery and sustaining facial abrasions and contusions. At the local emergency department, there was no evidence of a neural deficit. The lateral roentgenogram of the cervical spine (Fig. 1-A) was interpreted by the attending neurosurgeon and radiologist as showing an acute fracture of the posterior elements of the second cervical vertebra. Computerized axial tomograms showed a bilateral cleft of the posterior arch, which supported the diagnosis of a fracture. With the patient under general anesthesia, the neck was immobilized in a halo vest. The boy fell again one month later, during normal play with his brother. The right occipital and frontal pins of the halo penetrated the calvarium. Roentgenograms made at that time showed no signs of healing, so the pins were replaced at new sites with the patient under general anesthesia. Five months after the initial injury, cellulitis developed on the right side of the face from an infection at the site of a pin. The child was referred to the University of California at San Francisco for further management. We reviewed the original roentgenograms and computerized axial tomography scans and thought that the defects were, in fact, a synchondrosis of the posterior arch of the second cervical vertebra at the junction of the pedicles with the body of the axis. The halo vest was removed. Lateral roentgenograms of the cervical spine in flexion and in extension did not show any instability. A soft collar was provided for comfort and to allay parental concerns; within a few days, the child discarded the collar. Roentgenograms made six months after the initial injury (Fig. 1-B) and computerized axial tomography scans made two years after the injury (Fig. 2) showed no change in the appearance of the synchondrosis. Five years after the fall, roentgenograms of the cervical spine showed some ossification of the synchondrosis (Figs. 3-A and 3-B). The boy was active and asymptomatic at the most recent follow-up examination, five and one-half years after the initial injury. The axis develops from five primary centers of ossification, which are usually present at birth [1]. There is one center for the body of the axis, two for the odontoid process (which represents the body of the atlas), and two for the vertebral arch. The neurocentral synchondrosis, the cartilaginous structure that joins the body to the two posterior centers of ossification, ossifies between the ages of three and six years. Spondylolysis of the axis has been reported in adults [10] and may represent a persistent neurocentral synchondrosis. With increasing ossification, the radiolucent image of the synchondrosis becomes narrow.