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

Abstract

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-R<sup>2</sup> = 0.17, R<sup>2</sup> = 0.15, R<sup>2</sup> = 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- R<sup>2</sup> = 0.80, p < 0.001). Regarding the C0–2 angle, OI and OT were independent predictors in the univariate regression analysis (adjusted-R<sup>2</sup> = 0.08, R<sup>2</sup> = 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.

Clinical Impact and Correlations of Odontoid Parameters Following Multilevel Posterior Cervical Fusion Surgery

“…Table 2 summarizes the cervical measurements compared with normative data from asymptomatic subjects. 13 Symptomatic patients had larger C2S, cSVA, and T1S-CL and smaller OT values than asymptomatic subjects. Preoperatively, the odontoid parameters showed statistically significant correlations with established cervical parameters (Table 3).…”

“…20 Likewise, the morphology of the dens differs among individuals, and the clinical impact of the C2S may differ. 13 At a given C2S, patients with a larger OI have a smaller clinical impact than patients with a smaller OI (Fig. 5).…”

“…10 The extent of the T1S-CL mismatch can be represented by the sagittal malalignment of the dens, which can be meticulously described with odontoid parameters, as OI is an anatomical feature unique to each individual, regardless of its position, and C2S and OT are inversely related. 13 Recent studies have reported multiple cutoff values for the optimal T1S-CL. In one study, a cutoff value of 20° for the T1S-CL predicted moderate clinical disability according to the NDI score following multilevel cervical fusion, 18,19 and another demonstrated that moderate NDI could be predicted if the C2S exceeds 17°.…”

“…However, each individual has a unique morphology of the dens. 13 PI reflects the relative position of the pelvis. Subsequently, patients with low PI have a low sacral slope and a low reservoir of pelvic retroversion or PT.…”

“…A recent study proposed a novel concept of odontoid parameters, analogous to the pelvic parameters, as an adjunct to C2S. 13 However, the clinical and prognostic postoperative correlations of these parameters have not been demonstrated.…”