Objective: The purpose of this study was to describe a method for recognizing acute traumatic occipitoatlantal dissociation that uses the basion-axial interval and basion-dental interval and to compare the accuracy of this method with the accuracies of two other methods: the Powers ratio and the x-line method.
Materials and methods: Lateral radiographs of the cervical spine of 37 patients in whom the diagnosis of occipitoatlantal dissociation had been made on the basis of the relationship of the basion to the tip of the dens, the Powers ratio, and/or the x-line method were reviewed. Retrospectively, the occipitoatlantal junction of each was reassessed by using the the Powers ratio, the x-line, and the basion-axial interval-basion-dental interval methods. Independently, the neurologic findings at admission and the final neurologic diagnosis at discharge were obtained from the hospital records and were compared with the radiologic findings to determine the degree of radiologic-clinical correlation.
Results: Three groups of patients were identified by analyzing the basion-axial and basion-dental intervals of the occipitovertebral junction and related clinical findings. Twenty-three patients (group 1) had frank occipitoatlantal dislocation. Eight patients (group 2) had incomplete occipitoatlantal dissociation, which was defined as occipitoatlantal subluxation. The remaining six patients (group 3) had normal radiologic and clinical findings. Four patterns of occipitovertebral dissociation were identified: purely anterior (4/31, 13%), purely distracted (6/31, 19%), concomitantly anterior and distracted (20/31, 65%), and purely posterior (1/31, 3%). Regardless of the magnitude or direction of occipitoatlantal dissociation, the basion-axial interval-basion-dental interval method correctly identified the abnormality and the type of each. Positive clinical correlation was found in 13 (57%) of the 23 patients in group 1 and in 100% of the eight and six patients in groups 2 and 3, respectively. Neither the Powers ratio nor the x-line method could be applied in 17 (46%) of 37 cases, either because the opisthion could not be detected on the radiographs or because fusion of the posterior arch of C1 had not occurred. In the remaining 20 patients in whom the Powers ratio and the x-line method were applicable, the type of occipitoatlantal dissociation was correctly identified by the Powers ratio in 12 (60%) and by the x-line method in four (20%). Neither the Powers ratio nor the x-line method was applicable in three (50%) of the six patients in whom analysis by the basion-axial interval-basion-dental interval method and clinical findings showed no occipitoatlantal abnormality. Normal occipitovertebral anatomy was correctly identified by the Powers ratio in the remaining three patients (50%) and by the x-line method in two (33%).
Conclusion: Direct measurement of occipitovertebral skeletal relationships altered by occipitoatlantal dissociation using the basion-axial and basion-dental intervals provides the most accurate radiologic assessment of this injury.