Introduction: Craniocervical instability is commonly seen in patients with Down syndrome, connective tissue disorders, and in patients with Chiari 1 malformation following suboccipital decompression. Radiographic measurements have been described to help quantify the degree of instability of these patients, including the clivoaxial angle (CXA) (also known as the clivus canal angle), the clival slope (CS), and the sagittal axis (XS). However, there is a paucity of normative data on these angles in healthy individuals to use as a standard for evaluating pathologic states.
Methods: We retrospectively reviewed cervical spine CT scans of 250 normal patients over a six-year period. Patients ranged in age from 4 to 65 years old. CXA was determined from the angle at the intersection of the lines delineating the dorsal slope of the clivus (Wackenheim’s line) and the posterior spinal line of the C2 body (the posterior axial line). The CS was measured as the angle between the dorsal surface of the clivus and a horizontal line drawn from the basion. The XS was determined by the angle between a line drawn horizontally from the basion with a second line drawn vertically from the posterior aspect of the odontoid process. The CXA was considered the sum of the CS and the XS.
Results: The mean values for CXA, CS, and XS in 250 normal patients were 157.52°±11.09°, 57.0°±?10.7°, and 100.7°±?11.19°, respectively.
Conclusions: The average CXA in normal patients is around 157°. This is the sum of the CS and XS which are on average in normal individuals 57° and 100°, respectively. These values can be used as standards when evaluating patients with craniocervical instability pre- and post-operatively following occipitocervical fusion.
Patient Care: A better understanding of normal and abnormal craniocervical measurements may aid in the surgical decision making process of patients with instability.
Learning Objectives: 1. To understand how to measure the clivoaxial angle in normal individuals on CT.
2. To understand how to measure the clival slope in normal individuals on CT.
3. To understand how to measure the sagittal axis in normal individuals on CT.