Determination of Normal Values of the Basal Angle in the Era of Magnetic Resonance Imaging

BACKGROUND: The flexion of the skull base (basal angle [BA]) is the inclination between the anterior fossae and the basilar ramp of the occipital and sphenoid bones. An increased BA, termed platybasia, is usually associated with basilar invagination. BA reference values in the magnetic resonance imaging (MRI) era and the diagnosis of platybasia are of clinical importance. The transnasal approach has been the surgical technique of choice to remove the odontoid process in cases of ventral brainstem compression in patients with platybasia. The knowledge of normal BA values has been influenced by technological image acquisitions. The aim of this study was to determine the normal BA values in normal subjects in the MRI era. METHODS: For the determination of normal BA values, the literature reporting these values in normal individuals was reviewed and a meta-analysis of pertinent studies was performed. RESULTS: A total of 8 articles were included in this study, evaluating a total of 667 individuals. A summary measure of the results was obtained. The data obtained in this analysis provided a final basal angle value of 116.5 degrees (95% confidence interval, 104.39-128.7). The normal basal angle ranges between 104 and 129 degrees. CONCLUSION: Platybasia can be defined as a value >129 from the basal angle.