Radiologic diagnosis of traumatic occipitovertebral dissociation: 1. Normal occipitovertebral relationships on lateral radiographs of supine subjects.

OBJECTIVE: The objective of this study was to establish a reliable method of determining the normal occipitovertebral relationship seen on lateral radiographs of the cervical spine in supine subjects in order to recognize alterations of normal that characterize occipitovertebral dissociation.
MATERIALS AND METHODS: We define the rostral extension of the posterior cortex of the axis body as the posterior axial line, the distance between the basion (tip of the clivus) and the posterior axial line as the basion-axial interval, and the distance between the basion and the rostral tip of the dens as the basion-dental interval. The basion-axial interval was measured on horizontal-beam lateral radiographs of the cervical spine obtained at a 40-in. (1-m) target-film distance in 400 adults who had no occipitovertebral abnormalities. The basion-dental interval was measured in 374 (94%) of the same cohort of adults in whom the superior cortex of the dens could be identified. All radiographs were obtained with the subjects supine. The excursion of the basion referable to the posterior axial line was determined on lateral flexion and extension radiographs of 25 of the same cohort of adults. The basion-axial interval only was measured on radiographs of 50 children 2-13 years old who had no occipitovertebral abnormalities.
RESULTS: In 392 (98%) of the 400 adults, the basion-axial interval did not exceed 12 mm. In eight adults (2%), the basion was situated 1-4 mm posterior to the posterior axial line. The excursion of the basion in flexion and extension ranged from 0 to 10 mm, but did not exceed the 12-mm limit of normal. The basion translated posterior to the posterior axial line in six (24%) of the 25 adults in whom excursion was measured. Of the 400 adults, the basion-dental interval ranged from 2 to 15 mm (mean, 7.5 +/- 4.3 mm [2 SD]); the 95% accuracy range was 11.8 mm. In all 50 children (100%), the basion was within the normal basion-axial interval of 12 mm, and in no instance was the basion posterior to the posterior axial line.
CONCLUSION: In adults, the occipitovertebral junction can be considered normal when both the basion-axial interval and basion-dental interval are 12 mm or less. In children less than 13 years old, the basion-dental interval is not reliable because of the variable age at which complete ossification and fusion of the dens occur. The normal basion-axial interval in children did not exceed 12 mm. This simple, anatomically based method of recognizing normal occipitovertebral relationships facilitates identification of occipitovertebral dissociation.