As we have known, methods to measure the morphology of the teeth include periapical films, panoramic radiographs, and CBCT. CBCT scans might offer precise measure and analysis information and could help answer numerous questions within the morphological characteristics of the teeth and alveolar bone in three-dimensional form of patients with closed deep overbite. In this experiment, the ALARA principle (As Low As Reasonably Achievable principle) was adhered to in consideration of the potential risks associated with each examination. The exposure duration was minimized and patients were provided with shielding. CBCT is only suitable for orthodontic treatment that changes orthodontic mechanics, and is not suitable for all Class II division 2 malocclusion. Previous studies have shown that bone is difficult to be clearly visualized on CBCT (voxel size 0.38 mm) when the thickness of alveolar bone was less than 0.6 mm22. Reducing CBCT voxel size can increase the accuracy of measurement24. In our study, the voxel size of CBCT used was 0.25 mm, thereby effectively enhancing the precision of linear distance measurement pertaining to alveolar bone.
Our study also has some limitations. The root of the anterior teeth is in close proximity to the labial alveolar bone in patients with closed deep overbite occlusion, the labial bone plate was thin or even absent, therefore, there may be a large discrepancy between the measured data of labial bone plate and the actual data. The root apex of lateral incisors and canines were found to exhibit incomplete closure in a minority of patients, as observed in this study. The measurement of root length for these patients may contain certain inaccuracies. In the future, more in-depth research should be carried out to provide stronger evidence for exploring the causes of anterior teeth crown-root morphology and alveolar bone structure abnormalities in patients with closed deep overbite malocclusion.
The crown-root ratio of teeth is often concerned by doctors and researchers during orthodontic treatment. Certain scholars have suggested that an abnormal crown-root ratio may exert an impact on the long-term stability of the orthodontic treatment as well as root resorption during tooth movement, complicating orthodontic treatment planning25.Related studies have shown that, normal root/crown (R/C) ratio in healthy teeth have been reported in the order of 1.55 for females and 1.63 for males26. Short Root Anomaly (SRA) is defined as the crown-root ratio exceeding 0.9126. In this study, the maxillary central incisors crown-root ratio of Angle Class II division 2 was 0.96 ± 0.14. This suggests that maxillary central incisors in patients with close deep bite occlusion may be more prone to SRA. The study also revealed a statistically significant disparity between genders in the Class II division 2 group, wherein the crown-root ratio exhibited a significantly higher value among females (1.02 ± 0.15) compared to males (0.90 ± 0.10). This suggests a potential inclination towards the occurrence of SRA among females27,28. The etiology of SRA is still unclear, however, and most scholars believe that it may be likely influenced by a combination of genetic and environmental factors26,29. For example, the female predilection30, the prevalence among family members30 suggest genetic influence outweighs other factors, such as developmental disturbances31. The results of this experiment demonstrated that the thickness of apical labial alveolar bone in the Class II division 2 group was comparatively lower than that observed in both the Class II division 1 group and individuals with a normal jaw, the thickness of the labial alveolar bone is the thinnest. The abnormal crown-root ratio may be attributed to the retroclination of the anterior teeth, which leads to proximity between the roots and both the labial cortical bone and labial bone plate, the limited space available at the root apex of the tooth hinders further growth, consequently impacting root development32.
However, patients with closed deep overbite malocclusion often exhibit an abnormal ratio between the crown and root of their anterior teeth, orthodontic treatment is not contraindicated for teeth with higher crown-root ratio. The utilization of CBCT is recommended for monitoring the alterations in crown-root ratio throughout orthodontic treatment. The status of the anterior crown-root ratio in patients with closed deep overbite malocclusion, particularly female patients, should be carefully monitored during treatment to mitigate the medical risk of tooth loosening and loss.
The crown-root ratio of maxillary anterior teeth and mandibular central incisors in this study was found to be higher in Angle Class II division 2 compared to Angle Class II division 1 and individual normal jaws. The crown-roots of adult patients with closed deep overbite were found to be significantly larger than those of juvenile patients, particularly in the maxillary and mandibular central incisors, as observed in the present experiment. The presence of a closed deep overbite occlusion in patients may result in excessive retroclination of the anterior teeth, accompanied by severe deep overbite, in severe cases, the lower incisor occludes on the palatal gingival mucosa of the maxillary incisor. This may be produced greater masticating stress during open-and-close movement and functional closure movement, the occlusal forces generated surpass the teeth and periodontal tissues’ adaptive capacity. Therefore, patients with closed deep overbite have different degrees of occlusal trauma in the anterior teeth region33. Adult patients are accompanied by prolonged chewing movements and occlusal trauma, certain inflammatory factors secreted by large occlusal trauma affect the normal development of the root and cause different degrees of root resorption. As a result, the crown-root ratio of the upper and lower incisor teeth of Angle Class II division 2 adult patients were large and statistically significant. Additionally, the labial-palatal alveolar bone height of Angle Class II division 2 was found to be significantly greater compared to the other two groups, suggesting that prolonged occlusal trauma is associated with periodontal supporting tissue loss34.
Therefore, it is the best age and developmental stage to release the closed deep overbite at adolescence. The occlusion relationship should be promptly adjusted to eliminate traumatic occlusion and facilitate the recovery and reconstruction of periodontal supporting tissues, in order to prevent further aggravation of periodontal disease caused by occlusal trauma.
The accurate expression of anterior teeth torque is crucial for achieving normal overbite and overjet, as well as ensuring satisfactory aesthetic outcomes and stable occlusal relationships. In the treatment of patients with anterior close deep bite occlusion, it is necessary to erect the lingual tilted anterior teeth first, which will cause a process of moving the crown lip and the root tongue. Therefore, the anterior torque control requirements are higher. The expression of anterior torque is affected by the shape of alveolar bone, the position of root in the alveolar bone and the crown-root morphology35. A lot of studies found that the height and thickness of local alveolar predominantly restricted the range of anterior teeth movement. In this study, the influence of tooth morphological characteristics and root-bone relation on tooth movement was investigated35. In our study, for the crown-root angle, our observation furtherly confirmed the widespread existence of the crown-root phenomenon in patients with anterior close deep bite occlusion. It can be found that patients with anterior close deep bite occlusion have the most obvious crown-root angulation. Especially the maxillary central incisor (175.46 ± 1.84°) and lateral incisor (175.91 ± 2.52°) in the anterior tooth region, the long axis of the crown and the long axis of the root are not in the same line, which is a common phenomenon in patients with close deep bite occlusion and notable lingual side bending of the long axis of root relative to long axis of crown in upper incisor. The crown-root angulation of canines varies greatly.
The presence of crown-root Angle affects the expression of anterior teeth torque during orthodontic treatment36. Combining the viewpoints of related studies on crown-root Angle, we suggest that the crown-root Angle would limit the labial inclination of the upper anterior teeth during orthodontic treatment. It also suggests that we should avoid excessive lip tilting of the upper front teeth, cause the root of the tooth is close to the lingual cortical bone, this increases the risk of bone dehiscence, fenestration, and root resorption.
In this experiment, there were significant statistical differences in crown-root Angle between different sex groups in Class II division 2. In the Class II division 2., the males sample showed significantly greater value of crown-root Angle as compared to the female. We speculate that the morphology was susceptible during development, for the genetic and environmental factors37. The direction of crown eruption can be changed by the forces from peroral muscles, mastication and so on. And the root grows in its original direction, results in the formation of crown-root angles. The cause of the crown-root Angle and differs among sex with crown-root Angle are still unclear, whether it is congenital factors such as growth signals or acquired changes in the oral environment, the mechanism of crown-root Angle formation needs to be further explored in our future studies.
According to the findings of this study, it is recommended that during the process of orthodontic treatment, especially in the design and treatment of patients with closed deep overbite occlusion, particular attention should be given to evaluating the crown-root ratio, crown-root angle and alveolar bone structure. The data and results obtained from this study can serve as valuable resources for future research endeavors.
