## 4 Discussion
The results suggested the association between mandibular asymmetry and the condylar process. [PERSON] et al. also found that lateral imbalance of masseter muscle activity led to inhibition of chondrogenesis and induced asymmetric formation of the condylar cartilage during the growth period [28], which supported our results. So, we speculated that an asymmetrical activity of masticatory muscles between the scissors bite side and non-scissors bite sides. Moreover, in our study, no significant difference was found in the coronoid process and mandibular angle. The coronoid unit is affected by the temporalis muscle. Previous studies found that the volumes of the temporalis muscles were not significantly different between the two sides in patients with facial asymmetry [29; 30]. This seemed to be basically in accordance with our study that no significant difference was found in the coronoid process. We speculated that mandibular asymmetry is possibly caused by the differences between the skeletal units, which may be related to the surrounding muscles. The asymmetrical muscle activities could transfer to the mandible. However, considering the complex role of the muscle, the exact causal relationship between the skeletal units and the muscles has not been fully clarified, and more research is required. Therefore, although certain trends were obvious, specific bone morphology may not necessarily be related to specific occlusal characteristics. The deep mechanism called for deep exploration. Scissors bite cannot be corrected spontaneously and will gradually deteriorate as the maxillary teeth continue to erupt. The lateral movement of the mandible is limited in patients with unilateral scissors bite, which can induce temporomandibular joint disorders for some susceptible persons [24]. Therefore, researchers suggested early intervention for younger patients because of the growth potential [6]. Various non-surgical treatments and appliances have been advocated, such as maxillary contraction [31] and mandibular lingual transforce appliance [2]. However, the correction plans tend to be more complex and invasive in adulthood [32]. Implant anchorage is frequently used to correct the scissors bite on the one hand. On the other hand, it can also prevent the elongation of the locked teeth after correction [4; 33]. However, severe scissors bite may only be corrected with orthognathic treatment to narrow the maxillary arch or widen the lower arch [34]. Previous researchers have used traditional two-dimensional radiographs to analyze the symmetry of the mandible. [PERSON] et al. [35] evaluated mandibular symmetry of 60 pre-orthodontic patients with unilateral posterior crossbite via panoramic radiographs. The heights of mandibular condylar, mandibular ramal, and mandibular condylar were measured to calculate Habets' asymmetry index [36]. More asymmetric mandibular condylar and mandibular ramal on the crossbite side were observed. Apart from two-dimensionalradiographs, there were also studies based on CBCT. [PERSON] et al. [37] assessed the mandibular asymmetry of patients with unilateral posterior crossbite by CBCT images. The results showed that the length of the condyle and mandibular body on the crossbite side was reduced. Different from the above research, which adopted linear measurement, our study used the 3D distance mapping technique to analyze the surface. The results also demonstrated the asymmetry condylar process, mandibular ramus, and mandibular body. In addition, alveolar and chin were found affected. The mandible is an irregular surface, but lines are just chords of curved surfaces, and cannot fully represent the true shape of the mandible. The previous methods described above simply measured linear distances and angles, ignoring the contours of the mandible. Therefore, the method that can calculate the surface can better reflect the morphological characteristics of the mandible. 3D technology can reconstruct models, which are helpful in quantifying the displacement with more precise measurements [38; 39].