Integrated Cranio-Cervical Organization in Adults: Association Between Morphocranial Pattern and Segmental Cervical Alignment After Orthodontic Treatment

Objective: This study aimed to examine the association between the study-specific Cl-Na/Cl-Ba cranial-base angle and segmental cervical organization in adults, considering both inclination and sagittal spatial position, and to explore whether longitudinal changes in cervical inclination differed among morphocranial groups over the observed orthodontic treatment interval. Materials and Methods: Pre-treatment and post-treatment lateral cephalograms from 297 adult orthodontic patients obtained in natural head position were analyzed. Cranial-base morphology was operationally classified using the Cl-Na/Cl-Ba angle into brachycephalic (<115°), mesocephalic (115–124°), and dolichocephalic (>124°) groups. Segmental cervical inclination (C2–C5) and horizontal and vertical vertebral coordinates were recorded. Intergroup differences were assessed using Kruskal–Wallis tests with Dunn–Holm post hoc comparisons and effect-size estimates. Associations between the continuous cranial-base angle and cervical variables were evaluated using Spearman correlations. Longitudinal changes in cervical inclination were examined using linear mixed-effects models adjusted for sex, age, cervical segment, and repeated measurements. Results: The continuous cranial-base angle was inversely correlated with all segmental inclination variables (ρ = −0.213 to −0.333; p ≤ 0.002). Spatial coordinates showed larger associations, particularly for vertical positioning (C2 Y: ρ = −0.653; p < 0.001). Morphocranial groups differed in segmental inclination and spatial coordinates, with the largest effect sizes observed for upper cervical vertical variables. In the mixed-effects model, time was significant (p = 0.014), and morphocranial pattern remained independently associated with cervical inclination (p < 0.001). No significant time × morphocranial-pattern interaction was observed (p = 0.184). Conclusions: In this retrospective adult orthodontic sample, the study-specific cranial-base angle was associated with segmental cervical inclination and sagittal spatial coordinates, with the largest associations observed in the upper cervical region. The longitudinal model provided no evidence of differential longitudinal changes in cervical inclination among morphocranial groups over the observed interval. These findings are observational and do not establish a causal cranio-cervical mechanism or a treatment-specific effect. Segmental angular and spatial variables may be complementary descriptive measures when interpreting cranio-cervical cephalograms.