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.

Basilar Invagination: A Craniocervical Kyphosis.

BACKGROUND: For more than a century, odontoid process prolapse, which compresses cranial base structures, has been the hallmark of basilar invagination (BI). The angulation of the whole skull toward the cervical spine may be considered to contribute to BI and odontoid prolapse. The objective of this study was to evaluate the craniocervical angular craniometry of patients with BI compared with patients with Chiari malformation (CM) and normal subjects. METHODS: Angular craniocervical relationships among midline magnetic resonance imaging scans from patients with BI, patients with CM, and normal subjects were evaluated and compared. Angles were measured by Meazure 2.0 software from digitalized images. RESULTS: Using data from 121 images, 42 patients with BI (14 type I BI, 28 type II BI), 47 patients with CM, and 32 control subjects were evaluated. The BI group had a more acute kyphotic angle than the CM group (P < 0.01) and control group. The kyphosis difference was 17.57 ± 2.3° (P < 0.01) between the BI and CM groups and 21.19 ± 2.3° (P < 0.01) between the BI and control groups. Basal angles were significantly larger in the BI group compared with the other 2 groups. A strong correlation was found with kyphosis and CLV and the clivus-canal angle and cervical lordosis angle (P < 0.01). CONCLUSIONS: Craniocervical kyphosis in patients with BI was approximately 20° greater than in normal subjects and patients with CM. Craniocervical kyphosis should be considered a pathophysiologic condition in BI.