Vertical Reduction Using Atlantoaxial Facet Spacer in Basilar Invagination with Atlantoaxial Instability

Case 1 History and examinationA 45-year-old male patient was admitted for diffuse neck pain developed from 3 years ago.There was no history of special trauma or rheumatoid arthritis.In the physical examination, abnormal findings were not detected, and in the neurologic examination, motor weakness or sensory changes were not detected.In cervical spine X-ray and computed tomography (CT), the atlanto-dental interval (ADI) was observed to be increased by 10 mm at the neutral and flexion position, the protrusion of the tip of odontoid to the McRae line 11) just above, which was the finding suspicious to be BI, and thus various radiologic criteria diagnosing BI were measured (Fig. 1A).The odontoid tip was located below the Chamberlain line 2) , the apex of odontoid tip was located above the McGregor line 10) by 3.5 mm, which did not meet the criteria of BI.Nonetheless, in the Clark station 3) , the anterior ring of the atlas was located in the station II, and it was 32 mm by the Redlund-Johnell criterion 14) , 11 mm by the Ranawat criterion 13) , and findings satisfying the criteria of BI were observed.In cervical spine magnetic resonance imaging, on T2 weighted images, high signal changes of the spinal cord at the C1-2 level was not shown, but behind the spinal cord, compression finding was detected.In CT angiogram, the findings of the high riding vertebral artery, abnormal course of vertebral artery, and narrow C2 pedicle were not observed.Evaluat-