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 met hod of determining the normal occipitovertebral relationship seen on l ateral radiographs of the cervical spine in supine subjects in order t o recognize alterations of normal that characterize occipitovertebral dissociation. MATERIALS AND METHODS. We define the rostral extension o f the posterior cortex of the axis body as the posterior axial line, t he distance between the basion (tip of the clivus) and the posterior a xial line as the basion-axial interval, and the distance between the b asion and the rostral tip of the dens as the basion-dental interval. T he basion-axial interval was measured on horizontal-beam lateral radio graphs of the cervical spine obtained at a 40-in. (1-m) target-film di stance in 400 adults who had no occipitovertebral abnormalities. The b asion-dental interval was measured in 374 (94%) of the same cohort of adults in whom the superior cortex of the dens could be identified. Al l radiographs were obtained with the subjects supine. The excursion of the basion referable to the posterior axial line was determined on la teral flexion and extension radiographs of 25 of the same cohort of ad ults. The basion-axial interval only was measured on radiographs of 50 children 2-13 years old who had no occipitovertebral abnormalities. R ESULTS. 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 posterio r axial line in six (24%) of the 25 adults in whom excursion was measu red. 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-ax ial 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 1 3 years old, the basion-dental interval is not reliable because of the variable age at which complete ossification and fusion of the dens oc cur. The normal basion-axial interval in children did not exceed 12 mm . This simple, anatomically based method of recognizing normal occipit overtebral relationships facilitates identification of occipitovertebr al dissociation.