To introduce correction target of clivo-axial angle (CXA) in quantitative surgical reduction of basilar invagination (BI). The exact mechanism of BI with or without atlantoaxial dislocation is still unclear. Sagittal deformity is a key feature of BI. Correction of CXA directly influences subaxial cervical lordosis (increase of CXA equals the decrease of cervical lordosis). However, a quantitative reference for correction surgery has not been established.

CXA was divided by Chamberlain line into clivus tilt (CT) and axial tilt (AT). Patients diagnosed with BI were retrospectively included. Patients with degenerative cervical spine diseases or vascular diseases (without BI) were included as controls. CT, AT, and other common parameters were measured and analyzed with t test and multiple linear regression. Demonstration case was presented.

A total of 42 BI patients and 23 controls were included. Normal references for CXA, AT, and CT were 162.3±7.1, 93.8±6.5, and 68.6±3.8 degrees, respectively. BI patients had a 30.3-degree smaller CXA, a 15.5-degree smaller AT, and a 14.9-degree smaller CT. Basal angle (P=0.002) independently had a significant influence on extent of BI, while CT and AT did not. Demonstration cases showed that CT was fixed and correcting AT to an ideal 94 degrees was optimal for an individual patient.

Proper quantitative correction of CXA needs to be individualized in consideration of CT and AT. The difference between actual AT and its ideal value (about 94 degrees) is the optimal target of CXA correction to decompress neural elements ventrally and recover better subaxial cervical lordosis.