Three-Dimensional Evaluation and Classification of the Anatomy Variations of Vertebral Artery at the Craniovertebral Junction in 120 Patients of Basilar Invagination and Atlas Occipitalization.

Abstract

Patients with basilar invagination and atlas occipitalization usually present abnormal anatomy of the vertebral arteries (VAs) at the craniovertebral junction (CVJ). To describe and further classify different types of VA variations at the CVJ with 3D visualization technology. One hundred twenty patients with basilar invagination and atlas occipitalization who had undergone 3-dimensional computed tomographic angiography (3D-CTA) were retrospectively studied. Imaging data were processed via the separating, fusing, opacifying, and false-coloring-volume rendering technique. Abnormal anatomy of the VA at the CVJ was categorized and related anatomic parameters were measured. Seven different types were classified. Type I, VAs enter the cranium after leaving VA groove on the posterior arch of atlas (26.7% of 240 sides); Type II, VAs enter an extraosseous canal created in the assimilated atlas lateral mass-occipital condyle complex before reaching the cranium (53.3%); Type III, VA courses above the axis facet or curves below the atlas lateral mass then enter the cranium (11.7%); Type IV, VAs enter the spinal canal under the axis lamina (1.3%); Type V, high-riding VA (31.3%); Type VI, fenestrated VA (2.9%); Type VII, absent VA (4.2%). Distance from the canal of Type II VA to the posterior facet surface of atlas lateral mass (5.51±2.17 mm) means a 3.5-mm screw can be safely inserted usually. Shorter distance from the midline (13.50±4.35) illustrates potential Type III VA injury during exposure. Decreased height and width of axis isthmus in Type V indicate increased VA injury risks. Seven types of VA variations were described, together with valuable information helpful to minimize VA injury risk intraoperatively.