Research on the correlation between the size of condyle and occlusion plane in skeletal Class II malocclusions

Abstract Objectives This study was designed to investigate the relationship between the morphological structure of condyle and occlusal plane in skeletal Class II malocclusions by imaging measurement. Materials and Methods This study included 65 skeletal Class II adult patients (18–35 years old) who met the criteria, and all were taken with cone beam computed tomography (CBCT) images (skeletal Class II high angle 38 cases, average angle 18 cases, and low angle nine cases). The statistical methods of mean standard deviation, Pearson correlation, and analysis of variance were used to study the correlation between the size of the condyle and occlusal plane in skeletal Class II malocclusion. Results The FMA and SN‐OP between the groups in skeletal Class II malocclusion are considered statistically significant, p < .05 high angle group > average angle group > low angle group, whereas there are significant correlations between FMA, FH‐OP, SN‐OP, and the medial–lateral diameter (MLD) of the condyle, p < .05, showing a negative correlation. The anteroposterior diameter of the condyle has no significant correlation with these angles, and the high‐angle group size is smaller than the other groups. Conclusion In patients with skeletal Class II high angle malocclusion, the MLD and anteroposterior diameters of condyle were smaller than those of average angle and low angle groups, and negatively correlated with the FMA and SN‐OP. That is the steeper occlusal plane, the smaller MLD of the condyle. It suggests whether orthodontists can promote the stability of the morphological structure of the condyle by changing the inclination of the occlusal plane during the orthodontic process.


| INTRODUCTION AND BACKGROUND
Skeletal Class II malocclusion is a common type of malocclusion in the clinic, which is characterized by the sagittal upward relationship of the maxilla and mandible, accompanied by vertical malocclusion.
However, the relationship between malocclusion and the temporomandibular joint (TMJ) is still in conflict. Abnormal occlusion may cause temporomandibular disorders (TMD) has been basically recognized, and the influence of occlusal relationship on the morphology of TMJ is still controversial. Some studies try to reveal the correlation between occlusal factors and mandibular joint morphology, but others believe that there is no correlation between them. However, the changes in the internal morphology and structure of the TMJ can affect the growth and development of the mandible by affecting the condyle. The condyle is an important growth and reconstruction center of the mandible, and it is a key part of transmitting bite force. When an abnormal occlusal relationship exists, the condyle will undergo adaptive reconstruction after being stimulated by stress, showing different morphological structures and sizes, which is very likely to affect the morphological development of the TMJ, and then cause abnormal mandibular development (AI Taki et al., 2015). In recent years, many scholars have conducted research on the location of the condyle and found that facial growth, occlusal changes, and maxillofacial functional or pathological changes can affect the location of the condyle (Paknahad & Shahidi, 2017;Paknahad et al., 2016;Weiping & Haiyuan, 2015;Yin et al., 2017). A large number of articles have also studied the position and structure of the TMJ in different occlusal relationships and skeletal relationships (Ueki et al., 2008;Vitral et al., 2004). Scholars have explored the causes of this type of malocclusion from different angles. However, most scholars have focused on the study of the morphological characteristics of the submandibular joints in patients with different skeletal malocclusions, and there are few studies on the influence of the steep slope of the occlusal plane on the size of the condyle (Di Venere et al., 2016;Osborn, 1987).
In this study, adult patients with skeletal Class II malocclusion were selected as the research object, and the high angle, average angle, and low angle skeletal Class II malocclusion were grouped as the research object to explore whether the condyle size of adult patients with skeletal Class II malocclusion was related to the occlusal plane. To provide definite guidance for orthodontists in clinical examination, scheme design, and prognosis judgment.

| Research object
A total of 65 healthy adult patients with normal permanent teeth and skeletal Class II were selected in our department. Cone beam computed tomography (CBCT) was taken after the patient and his family knew and agreed, and the ANB, FMA, SN-OP, and FH-OP angles were measured to group the study subjects. Meet the following inclusion criteria: 65 cases of young skeletal Class II malocclusion between 18 and 35 years old, 29 males and 36 females; The sagittal bone surface type was skeletal Class II (ANB ≥ 4.7°); the vertical bone surface type was the high angle in 38 cases (FH-MP ≥ 32°), and the average angle was 18 cases (22°< FH-MP < 32°), nine cases of low angle (FH-MP ≤ 22°); Normal permanent dentition, excluding the history of orthognathic surgery, cleft lip, and parotid, history of submandibular joint disease, maxillofacial trauma, tumor, and other diseases that can cause maxillofacial asymmetry deformity; The whole body is in good health.
The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the study are appropriately investigated and resolved. All procedures performed in the study involving human participants were as per the ethical standards of the institutional and/or national research committee (s) and the Helsinki Declaration (as revised in 2013). This study was approved by the Ethics Review Committee of our hospital. Radiographs and participation in the study were authorized by the patient's parents. Informed consent was obtained from the participant with a detailed description of the purpose and benefits of the study.

| Software and equipment
NewTom VG-AFP CBCT (Radiation posthumous 50 mSv, scanning thickness 0.15 mm). In this study, CBCT was used to measure and analyze the anteroposterior diameters and MLD of the condyle in adult patients with skeletal Class II malocclusion.

| RESULTS
Due to the small sample size, it can be seen that it is a roughly normal distribution according to the graphic method (histogram and Q-Q diagram) ( Figure 3). Excel 2010 is used to complete the data entry and sorting. In the next step, the mean standard deviation is used for description, Pearson correlation and analysis of variance, and other parameter tests.
As can be seen from Table 1, a grouping of research objects and measurement items.
As can be seen from Table 2, difference analysis in FMA and SN-OP and different angle groups were significant, p < .05, their sizes were high angle group > average angle group > low angle group; Difference analysis in MLD of the condyle and different angle groups were significant, p < .05. From the mean value of different angle groups, it can be seen that the MLD of the condyle in high angle group < average angle group < low angle group. The MLD and F I G U R E 1 Cephalometric Indexes. 1, ANB angle; 2, FH-MP angle; 3, FH-OP angle; 4, SN-OP angle. As can be seen from Table 3, there was a significant correlation between FMA, FH-OP, SN-OP, and the MLD of the condyle. The anteroposterior diameter of the condyle was not significantly correlated with FMA, FH-OP, and SN-OP.
As can be seen from As can be seen from Table 5 Different types of malocclusion deformity due to the different occlusal relationships, the stress that the condyle bears are different, resulting in an adaptive alteration of the shape of the condyle (Katsavrias & Halazonetis, 2005). found that the mandibular rotation  There was a significant difference between FMA and SN-OP in skeletal Class II malocclusion, p < .05, and their sizes were high angle group > average angle group > low angle group. It is suggested that the size of FMA in skeletal Class II malocclusion is highly consistent with the development trend of SN-OP. Growth and development are a concern for orthodontists. After Bjork (1969) proposed the concept of mandibular rotation, orthodontists began to realize the importance of mandibular rotation, the rotation of the lower jaw relative to the SN plane. At the same time, clinically, the author found that many The TMJ can be adaptively remodeled with changes in occlusion.
Occlusion interference and mandibular dyskinesia caused by longterm malocclusion can affect the shape and function of joints. Arnett and Gunson (2004) believes that the larger condyle is more stable when exercising the masticatory function, the matching degree of the condyle and the joint socket is higher, and this kind of condyle has a stronger ability to resist the occurrence of displacement. On the contrary, the small condyle does not provide enough support during occlusal changes, the connection with the joint socket is loose, and it is more likely to be displaced.

| CONCLUSION
The FMA in skeletal Class II malocclusion is highly consistent with the development trend of the SN-OP plane. The larger the FMA, FH-OP, and SN-OP among the groups, the smaller the MLDs of the condyle,