Odontoid Incidence: A Novel Cervical Parameter Influencing Cervical Alignment From Top to Bottom.

OBJECTIVE: By using angulation of the axis itself, this study aims to define and analyze odontoid incidence (OI) and odontoid tilt (OT) as novel cervical alignment parameters and investigate their correlations with cervical alignment. METHODS: Novel and existing parameters were measured with whole-spine lateral plain radiographs and EOS images of 42 adults without cervical symptoms. The correlations of OI, OT, C2 slope (C2S), and T1 slope (T1S) were calculated. RESULTS: The OI, OT, and C2S showed significant correlations with C2-7 angle (r = 0.43, r = -0.42, r = 0.62, respectively) and C0-2 angle (r = -0.33, r = 0.48, r = -0.61, respectively). OI, OT, T1S were independent predictors of the C2-7 angle in univariate regression analysis (adjusted-R2 = 0.17, R2 = 0.15, R2 = 0.28, respectively). OI, OT, and T1S were independent predictors in the multivariable regression analysis with estimated standardized coefficients of 0.36, -0.67, -0.69, respectively (adjusted- R2 = 0.80, p < 0.001). Regarding the C0-2 angle, OI and OT were independent predictors in the univariate regression analysis (adjusted-R2 = 0.08, R2 = 0.21, respectively). CONCLUSION: OI, OT, and C2S had significant correlations with cervical alignment. As the pelvic incidence, the OI is the only anatomical and constant parameter that could be used as a reference point related to the cervical spine from the rostral end. The study results may serve as baseline data for further studies on the alignment and balance of the cervical spine.