Chamberlain's Line Violation in Basilar Invagination Patients Compared with Normal Subjects: A Systematic Literature Review and Meta-Analysis

-OBJECTIVE: To compare the measured odontoid tip violation above Chamberlain's line described in the liter-ature to diagnose basilar invagination (BI) and to establish the normal placement of the dens tip defining individuals without BI (normal subjects).-METHODS: A systematic literature review was per-formed to identify clinical or radiological studies that expressed the amount of odontoid violation above Cham-berlain's line in patients with a BI diagnosis. In addition, a meta-analysis was performed to evaluate normal subjects' values of Chamberlain's line violation (CLV). -RESULTS: There were 23 studies included (13 radiological and 10 clinical). Most studies used computed tomography and/or magnetic resonance imaging. Eight different cutoff values were used to measure dislocated odontoid apexes above Chamberlain's line regardless of the radiological mo-dality. The mean measured amount of CLV was 3.95 mm (me-dian 5 mm; range, 0e9 mm). The meta-analysis included 8 studies (1233 patients) with a normal sample population with a mean normal CLV of L0.63 mm (below the line) (95% con-fidence interval [L0.8, 1.18 mm], random effects model). -CONCLUSIONS: Different values were found in the assessed studies used for CLV in BI diagnosis. This vari-ability is especially important for type B BI, as type A BI has other craniocervical diagnostic parameters. Consid-ering the results obtained in this meta-analysis, BI should be diagnosed in the case of any dens violation >1.18 mm.