In the 198 severe skeletal class III patients included in this study, approximately 64.1% showed protrusive mandibles, 18.7% showed a retrusive maxilla, and 8.1% showed a combination of protrusive mandible and retrusive maxilla. Our data indicate that a protrusive mandible is the main cause of skeletal class III malocclusion in almost 72.2% of the cases. From the cephalometric analysis, we can see that the maxillary incisors has a significant proclination and the mandibular incisors has a significant retroclination, which indicate that maxillary incisors labially inclined and mandibular incisors lingually inclined for compensation respectively in these severe skeletal class III subjects.
For severe skeletal Class III patients, orthodontic camouflage treatment always can’t achieve a good therapeutic effect, orthodontists tend to choose combined orthodontic and orthognathic treatment. Previous studies revealed that in skeletal class III malocclusion, the alveolar bone around incisors is originally or developmentally thinner than normal occlusion, and there was still further absorption during presurgical treatment, particularly the alveolar bone around the mandibular anterior teeth [5, 6, 12–14]. The mandibular incisors, which are lingually inclined for compensation in skeletal Class III patients, are more susceptible to the recession of labial gingival and the decrease of alveolar bone thickness (ABT) and height during presurgical orthodontic treatment [7, 15]. Preoperative orthodontic treatment aims to decompensate the maxillary and mandibular incisors to obtain normal and healthy tooth axial inclinations within their alveolar bone base. As is known, teeth must move through the alveolar bone house, consequently, orthodontists need to pay special attention to the morphology of the anterior alveolar bone and take a safe treatment plan for each patient to achieve a balance between the health of the alveolar bone and the outcome of orthognathic surgery. Therefore, we need a indicator to evaluate the inclination of the mandibular incisors then we can known the necessary amount of incisor decompensation and the already expressed compensation could be eliminated appropriately to facilitate surgical movements.
The angle between the inclination of the mandibular incisors and the mandibular plane (IMPA) was traditionally used to evaluate the saggital axial inclination of mandibular central incisors. Tweed [16] indicated that the IMPA was essential for facial esthetics and tooth stability. In this study, the IMPA of the untreated severe skeletal Class III patients were generally significantly lower than normal value to compensate for the negative overjet. With the increase of SN-MP, the mean of IMPA decreases significantly (Table 3). Studies showed that the alveolar bone around incisors becomes thinner with the discrepant growth of jaws and the developmental compensation of teeth [6]. As is shown in Fig. 3 and Fig. 4a, during preoperative treatment in severe skeletal Class III cases, decompensate completely based on the index IMPA to guarantee that the lower incisors standing upright in the mandible might cause the incisors to exceed the alveolar bone house, leading to more severe alveolar bone loss, fenestration, gingival recession and other unacceptable side effects [7, 17]. As is known to us all, the combination of dental implants and alveolar bone is osseointegration, mechanical force is transferred to the support bone. Studies showed that angle of force application and implant offset on supporting bone effected the stress to supporting bone significantly. Changes in the angle of force application resulted in greater stress to supporting bone. The least stress in the supporting bone was found with vertical loading of the no-offset implants [18, 19]. In Fig. 3 and Fig. 4a, after completely decompensation, the lower central incisor axis stays away from the long axis of the mandibular union. It is unfavorable for the transferring of occlusal force. However, in skeletal class Ⅲ patients treated by force transmission technique for camouflaging skeletal deformity, as is shown in Fig. 4b, the consistency between the lower incisor axis and the long axis of the mandibular union was maintained after treatment, though the lower incisor was more lingually inclined evaluated by IMPA. Light force induced physiological reconstruction of the alveolar bone, which is beneficial for the transferring of occlusal force.
In this study, the low angle group and normal angle group showed the mean of MIA were 3.70°and 3.52° respectively. The mean of MIA (2.33°) was smaller in the high angle group. It showed no statistically differences among the low angle, normal angle and high angle groups (Table 3). We can concluded that in the untreated physiological conditions, though the lower incisors lingually inclined for compensation, the long axis of the mandibular incisors are highly consistent with the long axis of the mandibular union, which was more obvious in the high angle subjects. Compared with the angle between the inclination of the mandibular incisors and the mandibular plane, we should pay more attention to that whether the mandibular incisors stand upright in the mandibular union in skeletal class III malocclusion. From a physiological and functional point of view, MIA reflects the relationship between mandibular incisors and their alveolar bone house better. During preoperative orthodontic treatment, we should not only concern about whether the lower incisor standing upright in the mandible, more often we need to pay special attention to the morphology of the anterior alveolar bone and maintain lower incisor’s lingual inclination to some extent, which is more practical and safety.
The 2-dimensional X-ray lateral cephalograms limit cephalometric analysis to linear and angular measurements between landmarks superimposed onto a single plane of space, often leading to distortion errors. Relatively speaking, investigators can visualize and measure the true 3-dimensional anatomy of patients from 3-dimensional CBCT scans, which avoiding the intrinsic weaknesses of 2-dimensional imaging (distortion, superimposition, investigators) [20, 21].Studies have been done to evaluate the accuracy and reliability of CBCT measurement, Leung et al21 reported that there is no significant difference between CBCT linear measurement measurement and physical measurement and Timock et al [22] presented that CBCT can be used to quantitatively assess alveolar bone height and thickness with high precision and accuracy. Therefore, except cephalometric analysis, we also selected 11 subjects’ CBCT images randomly and measured their MP-SN、FH-MP and MIA in comparison with the cephalometric measurements to invalidate the reliability of the cephalometric analysis. Paired-samples t Test indicated that there were no statistically differences in MP-SN、FH-MP and MIA between the cephalometric measurements and CBCT measurements. The reliability of the cephalometric analysis has been invalidated to a certain extent (Table 4)
The major limitation of the study is the sample size. The conclusions of this study are limited to 198 severe skeletal Class III malocclusion, and we just selected 11 samples randomly and measured their CBCT for invalidation. Further studies should expand the sample size.