Recent findings have shown RS is a three-dimensional spatial definition [10, 11]. The RS was analyzed in the CBCT to avoid the error of X-ray film[18]. This study aimed to use CBCT to reconstruct 3D model and find out the relationship between M3 and RS.
Patients we included were adults aged 18 to 40 years with non-vertical growth. Zhao Z et al found RS had a maximum in the hypodivergent group and was twice as large as in the hyperdivergent group[19]. Research reports the missing rate of M3 in patients with vertical skeletal craniofacial pattern was higher [20] Evidence in the literature supported our selection of patients. In the current study, we found that with the increase in average age, the M3 tends to PG-I within the classification of ramus relationship. Possibly as a result of the eruption of the M3 increases the eruption space and promotes the further growth of the mandibular angle[21].
In addition, previous studies used OP to measure the amount of tooth movement[10.11.19]. The tooth movement of malocclusion patients in orthodontic treatment is likely to influence the position of OP[22]. The findings confirmed the WALA ridge arch can represent the alveolar arch the dental arch and WALA ridge arch have high matching[23, 24]. Hence, this study fitted the WP as a reference plane by referring to the least square method and it was also the innovation of this study[25]. We found that WP had high stability in the present study by comparing the standard deviation of < FH-WP and < FH-OP. It is suggesting that WP can be the reference plane. WL and WP were highly correlated and the result was supported by Gupta[24]. This may reflect the fact that the selected measurement datum line is also scientific. In our study, the OP was used as an auxiliary to illustrate the reliability of the results obtained by the WP.
In recent years, three-dimensional digital technology with high efficiency, high accuracy, and high maneuverability can help dentists to simulate orthognathic surgery, reconstruct 3D structure of the airway, analyze organizational change in orthodontic treatment and provide effective means for personalized orthodontic treatment[26]. With the development of digital orthodontics, digital model, invisible appliance and personalized appliance have been widely used. In this study, digital technology was also used to fit the plane, which is an innovative method, hoping to help the follow-up orthodontic research work.
Because of certain Angle between the reference planes, it had noticeable differences in RS which were obtained by OP and WP in this study. The consistent results with Kim were that RS on the crown side was bigger than on the root side and RS had a gradual reduction from the crown to the root tip[10]. Thus, More attention should be paid to the distal movement of roots clinically. During distal movement, the molars will tilt when the root tip touches the cortical bone. This is consistent with many previous studies[27, 28]. Otherwise, RS had no significant difference in gender and Angle classification. In the age classification, the older group has larger RS (especially in the root tip), and B angle. The finding by Choi[11] that the available space at the posterior boundary of molars is influenced by age supports our results. From this, the influence of age on RS should be considered in orthodontics. And the influence of age on RS may be caused by periodontal disease or physiological alveolar ridge absorption.[29]
The connection between M3 and RS is controversial, Previous studies[11, 19] analyzed the RS with or without the M3 and found no notable difference. However ,many literatures reported that the existence of the third molar would increase the available space of the posterior segment of the dental arch[30]. But these scholars did not classify M3 in detail. Therefore, This study conducted an in-depth classification study and found that RS were significantly different under different classifications. In Pell-Gregory classifications, the RS presented a gradual reduction from PG-A to PG-C. Similarly, RS gradually decreased from PG-I to PG-III. With respect to angle classification ,the smaller A angle, the smaller RS. And no significant difference existed in the B-angle classification. In this study,we also confirmed that the degree of mesial impaction increased with the increase of the degree of impaction in PG-A/B/C and PG-I/II/III classification. B-angle has no significant difference in Pell-Gregory classifications. In agreement with our results, Tsai H confirmed that posterior molar space was related to the mesial angle of impacted teeth[31]. Consequently, RS was affected by the depth of impacted M3, the degree of mesioangular impaction and the distance between the anterior edge of ramus and the second molar. No matter which classification criterion, the RS decreased with the deepening of M3' s impaction degree.RS can be initially estimated by observing the depth, mesial angle, or posterior space of M3. Our findings could be used to help some primary hospitals without large dental facilities predict RS by observing panoramic or lateral radiograph, which will be beneficial to the design of orthodontic plans to push molars distally.
The deficiency of this study is that the thickness of the buccolingual bone plate and the width of the Mandibular alveolar arch have not been considered and the application of digitalization is relatively superficial. Now that the digital technology has matured, people have invented the manipulator to bend the arch wire[32] .In future research, we can focus on digital technology and artificial intelligence, applying them in oral research to better help our clinical work.