## 4 Discussion
Cervical curvature was classified into four categories within each group. It is well accepted that the physiological cervical curvature is lordotic in a natural head posture. However, our results showed that only 19.6% of the samples had lordotic cervical curvature. A straight cervical curvature was the most common (48.9%), followed by kyphotic (25.0%), lordotic (19.6%), and sigmoid (6.0%) curvatures. In previous studies, lordotic cervical curvature was not dominant. [PERSON] et al. [4] conducted a study on 100 healthy adults and reported that approximately one-third of the population had a lordotic cervical spine, one-third had a straight spine, and the remaining third had either kyphotic or sigmoid curvatures. [PERSON] et al. [3] reported that 28% of young asymptomatic Chinese adults have lordotic cervical spines, whereas 45% have straight spines. Nonetheless, compared to previous findings [3; 4], our results indicate a lower proportion of lordotic cervical curvature. This is the first study to describe the prevalence of each type of cervical curvature in different vertical facial patterns. In the hypodivergent and hyperdivergent groups, the straight curve was most prominent, whereas in the normovergent group, the lordotic curve was predominant. [PERSON]'s exact test indicated an association between the vertical facial pattern and the type of cervical curve. The normovergent group had a higher frequency of lordotic curves and fewer straight curves, the hypodivergent group had a predominance of straight curves, and the hyperdivergent group had fewer lordotic curves. Compared with previous studies [3; 4; 36], the distribution of cervical curvature types in the normovergent group was similar, but the hypodivergent and hyperdivergent groups showed a higher proportion of non-lordotic cervical curvatures. In this study, although there were no significant differences in the mean values of cervical curvature measurements, the distribution of cervical curvature types showed a significant difference between the three groups. This may be linked to the observation that the hypodivergent group had the highest proportion of straight curves and the lowest proportion of kyphotic curves, whereas the normovergent group had an even distribution for each curvature type. As expected, there was a greater prevalence of non-lordotic cervical curvature in the hyperdivergent group, which tended to show a forward head posture. However, the hypodivergent group showed a greater prevalence of straight curvature. [PERSON] et al. [37] reported that subjects who changed from a neutral position to a military posture (backward craniocortical posture) often experienced the loss of cervical lordosis, leading to a straight cervical alignment. Variations in muscle tension around the shoulder and neck areas may contribute to this phenomenon [38]. In addition, craniofacial development is closely linked to stomatogathic functions, such as mastication, swallowing, and respiration, all of which influence craniocortical posture [39; 40]. Further research is needed to investigate their biomechanical impact on cervical alignment, with biomechanical analysis and longitudinal studies providing further validation of these relationships. In this study, we investigated the correlations among NSL/ML, craniocortical posture, and cervical curvature. The Pearson correlation suggests that the NSL/ML showed a moderate correlation with craniocortical posture, implying that head and cervical postures may vary according to the vertical facial pattern, aligning with the aforementioned soft-tissue stretching hypotheses. Thus, an increase in the mandibular plane angle was associated with a more anterior positioning of the head and neck. Our results showed a weak negative correlation between NSL/ML and cervical curvature measurements.