A Retrospective Study of Dental Changes after Facial Contouring Surgery

Background: The purpose of this study was to conrm whether long-term dental changes after people were aged beyond their 20s would be affected by undergoing facial contouring surgery, such as angle and zygoma reduction. Additionally, we intended to verify whether these changes would be signicantly different depending on the type of contouring surgery. Methods: We included 77 subjects who underwent contouring surgery from December 13, 2011, to December 3, 2018. Of these subjects, 73 were female, and 5 were male. We followed up with these patients from 11 to 82 months after surgery. The average follow-up period was 33.6 months. The age at surgery was 18 to 48 years old. The average age was 29.5 years old. To assess dental changes, we measured variables such as the extrusion of a maxillary anterior and posterior tooth, angle from point A to nasion to point B (ANB°) change, and overjet/overbite change between pre- and postoperative periods using computed tomography. Results: There was a positive linear correlation between the extrusion of the maxillary teeth and the follow-up period. The correlation coecients of incisors and molars were 0.454 and 0.461, respectively (p = 0.000 for incisors and 0.000 for molars). With age, there was also a negative correlation in extrusion. The correlation coecient was -0.442 for incisors and -0.394 for molars (p = 0.000 for incisors and 0.000 for molars). We also conrmed signicant differences in incisor extrusion and ANB° change depending on the type of surgery (p = 0.018 for incisor extrusion and 0.003 for ANB°). Conclusion: We observed dental changes in subjects who underwent facial contouring surgery, and these changes were affected by the type of operation.


Background
Maxillofacial surgeons generally assumed that patients' growth does not continue to change when performing facial contouring surgeries after a person is aged beyond their 20s. The authors have also established a surgical plan on the premise that tooth movement and facial contour surgery are not related. However, since 2011, the authors have performed computed tomography (CT) analysis of all patients before and after contouring surgery. Most of the patients who underwent follow-up (F/U) observations for more than six months were able to con rm tooth extrusion regardless of orthodontic treatment. Therefore, the authors reviewed the literature to con rm whether this was caused by surgery or a physiological phenomenon.
Until the mid-20th century, scientists had thought that growth stopped after puberty. However, studies using a cephalometric X-ray reported that growth continues in adulthood [1][2][3][4][5]. They revealed that the facial bones become longer after a person is aged beyond their 20s. They also found maxillary teeth extrusion and posterior rotation of the mandible in women. Other studies have also reported that these changes are related to changes in the dentoalveolar complex [2,[6][7][8][9].
However, no studies have proven whether dental changes are prevented or promoted by facial contouring surgery. Therefore, this study aimed to con rm whether a later dental change occurring after a person is aged beyond their 20s would also be found in subjects who underwent facial contouring surgery.
The authors were also able to review the studies showing that mandibular angle surgery can affect masticatory muscle, and occlusal changes occur when Botox is injected [10,11]. Thus, compared to the cases in which zygoma surgery or angle surgery were performed individually, when the surgeries are performed simultaneously, it is assumed that masticatory muscles such as masseter muscle or temporal muscle would be more greatly affected. Hence, we tried to determine if there was a difference in dental changes between the groups.

Study design and sample
Patient selection -All the patients in this study had undergone facial contouring surgery, such as angle reduction, zygoma reduction, and genioplasty. They underwent the operation between December 13, 2011, and December 3, 2018. Among the patients, we enrolled 89 subjects who had F/U sessions 11 months after surgery or later with pre-and postoperative CT images. Among the subjects, we kept the exclusion criteria as follows: 1. if he/she had a maxillary incisor or molar prosthesis that distorted the CT image, 2. if he/she had undergone orthodontic treatment during the postoperative F/U period, 3. if he/she had resorption of the condyle, and 4. if he/she had undergone a revision operation.
As a result, we included 77 patients in this study. Of these subjects, 72 were female, and 5 were male. All study protocols were approved by the Public Institutional Bioethics Committee (P01-202006-21-029). The study was performed according to the Declaration of Helsinki guidelines on experimentation involving human subjects. Table 1 shows the type of surgery, the number of cases, the age at the operation, and the observation period for each surgery. To evaluate the effect of contouring surgery on dental changes, we compared two surgery groups: one group (group 1) had undergone angle and zygoma surgery simultaneously. The other group (group 2) underwent only one operation for angle and zygoma reduction or underwent genioplasty without angle and zygoma surgery ( Table 2). All male patients belonged to group 1. Angle: angle reduction, zygoma: zygoma reduction, genio: genioplasty Group 1 underwent angle and zygoma surgery at the same time. Group 2 underwent only one surgery for angle and zygoma reduction or underwent genioplasty without angle and zygoma surgery.

Dependent variable
To assess dental changes, we measured variables such as the extrusion of a maxillary anterior and posterior tooth, the angle from point A to nasion to point B (ANB°) change, and the overbite (OB)/overjet (OJ) change between the pre-and postoperative periods. Of the 77 patients, we did not measure molar extrusion in one patient who lost six posterior teeth. We also excluded ve patients with dual bites from ANB° and OB/OJ measurements.

Measurement method
CT scans were performed using cone-beam CT (Alphard-3030, ASAHI ROENTGEN, Kyoto, Japan) at the habitual intercuspal position. We used C-mode to obtain an adequate exposure area. The data were processed using InVivoDental 3D imaging software (Anatomage, Inc., San Jose, CA). In the superimposition view, we opened the pre-and postoperative scans concurrently and superimposed them.
In the sagittal plane, we measured the extrusion of the maxillary incisor, the change in ANB degree, and the OB/OJ ratio. Fig. 1 shows how these variables were measured. If the sagittal plane overlapped the interproximal space of the maxillary incisor, we moved the reference plane to the incisal tip. With this adjustment, we measured the extrusion of the incisor. When the tooth was extruded, we assigned it a positive value. In the coronal plane, we measured the extrusion of the distal buccal cusp of the rst maxillary molar. We averaged the change of both sides (Fig. 2).

Reliability of the measurements
The cross-sectional CT image was enlarged to a predetermined size and measured twice using a length measuring instrument embedded in the InVivoDental program. The CT image was viewed at 5× magni cation during measurements (adjusted so that the scale bar of 2 cm on display is 10 cm for the actual ruler). We repeated the analysis until the value of two successive measurements had a discrepancy error less than 0.2 mm or 0.2 degrees.

Statistical analysis
The descriptive statistics of the pre-and postoperative measurements were processed using the SPSS for Windows version 21 software program (IBM Corp., Armonk, NY). The normality test (Kolmogorov-Smirnov) was performed for all variables. Accordingly, we selected parametric (T test) or nonparametric (Mann-Whitney U test) methods to compare the differences between the groups. To con rm the correlation between the variables, we chose parametric (Pearson) or nonparametric analysis (Spearman) as appropriate. If normality and equality of variance were veri ed, we performed an analysis of covariance (ANCOVA) using age and period as covariates. Differences were considered to be signi cant at p < 0.05.

Results
The pattern of dental change after contouring surgery There was a positive linear correlation between the extrusion of the maxillary teeth (both incisors and molars) and the F/U period (Fig. 3). The correlation coe cients of incisors and molars were 0.454 and 0.461, respectively. When the F/U period was less than 18 months, the extrusion of the maxillary incisor and molar was 0.19 mm, but when the patient was observed for more than 55 months, the amount increased to 0.62 mm in the incisor and 0.55 mm in the molar.
There was also a negative linear correlation between the extrusion of the maxillary teeth and the age at surgery (Fig. 4). When we operated on subjects under 22 years old, the extrusion of the maxillary incisor and molar was 0.54 mm. When the subjects underwent surgery after 36 years of age, the amount of extrusion was only 0.21 mm for the incisor and 0.19 mm for the molars. The correlation coe cient was -0.442 for incisors and -0.394 for molars. Age at surgery and OB also showed a signi cant negative correlation (correlation coe cient = -0.279). We con rmed that the longer the F/U period, the higher the amount of extrusion, and the younger the surgery, the higher the amount of extrusion.
Differences in dental change according to the combination of surgery type We performed three kinds of surgery: angle reduction, zygoma reduction, and genioplasty. According to how these three surgeries were combined, one patient could undergo one of seven surgery types (Table   1). To check if there is a statistically signi cant difference according to these surgery types, we grouped patients in various ways based on surgery type. As a result, we con rmed the statistically signi cant difference between the two groups ( Table 2).
We analyzed the differences with the Mann-Whitney U test in the age that the patient underwent surgery and the period between the two groups, but no signi cant differences were identi ed (p = 0.572 for age, 0.641 for the period) ( Table 2). In group 1, we found incisor extrusion by 0.43 mm and molar extrusion by 0.34 mm. In group 2, these values were increased by 0.19 mm and 0.25 mm, respectively. The ANB°i ncreased by 0.31° in group 1 and 0.09° in group 2. We found statistically signi cant differences in the variables (p = 0.018 for incisor and 0.003 for ANB°) ( Table 3).

Discussion
This study is the rst to verify long-term dental changes after contouring surgery by CT imaging. For this reason, it is necessary to prove whether this phenomenon is due to surgery or physiological responses to growth that can occur regardless of whether a person has a contouring operation.
Since the mid-20th century, scientists have accepted the concept of growth as the entire series of anatomical and physiological changes taking place between the beginning of prenatal life and becoming elderly [4]. Akgül et al. [9] said that this phenomenon favors the term change by aging or maturational process over the term growth because it includes not only an increase in dimension but also a decrease.
In this study, we chose the term dental change to describe the net result of dentoalveolar changes according to the progression of time regardless of the cause, whether iatrogenic or physiologic.
Many studies have reported that dental changes occur in adults without surgery or orthodontic treatment. Bondevik reported dental changes from ages 23 to 34 in subjects who did not receive any treatment [12]. Bishara et al. [1] also observed changes in untreated samples between the ages of 25 and 46. With a cephalogram, they con rmed the change in ANB° as measured as 0.58 ± 0.63° in women [1]. We veri ed that the direction of change was the same as that observed in their study. Our data showed that the change in ANB° was 0.31 ± 0.24° in group 1 and 0.09 ± 0.26° in group 2. However, there were signi cant differences in the amount of change between the surgery groups (p = 0.003).
Forsberg et al. [3] also observed skeletal changes in adulthood between the ages of 24 and 34 and assured posterior rotation of the mandible and an adjustment of the upper incisors to the new lower jaw position. During the rst ve years, the maxillary incisor increased vertically by 0.34 mm. The change was less than that of group 1 (0.43 mm) and more than that of group 2 (0.19 mm) in our study. They did not measure ANB°. Instead, the mandibular line (a tangent to the lower posterior border of the mandible through the gnathion) was measured, and over ve years, measurements in females increased by 0.35 °b ut by 0.17 ° for the next ve years.
Pecora et al. [5] analyzed craniofacial growth changes between the ages of 17, 47, and 57 years. They found that the anterior cranial base length increased by 1.6 mm from 17 to 47 years of age, and the nasion moved anteriorly. They reported that the maxillary teeth extruded by 0.3 mm. Both the length from the condylion to point A and the length from the condylion to the gnathion increased. The length in males increased by 4.1 mm, and that in females increased by 1.8 mm. Because of this, women had more vertical changes with posterior rotation of the mandible [5]. Therefore, ANB° increased by 0.7 ° in women up to the age of 47 and no increase was observed in men. The increase in ANB° in our study also seemed to be related to the fact that only 5 out of 77 subjects were males, especially in group 1 (0.31°). In all ve subjects, the ANB° changed less than the average (0.26° for males, 0.32° for females). However, in group 2, although all subjects were female, the change in ANB° was minimal.
Choi et al. [2] observed a change in the craniofacial complex and alveolar bone height for Asian females over four years with a lateral cephalogram. The length from the nasal oor to the incisal tip and from the nasal oor to the molar cusp increased by 0.7 mm and 0.6 mm, respectively, over four years [2]. In our study, extrusion of the incisor and molar between 43 and 54 months, corresponding to 4 years, was 0.57 and 0.46 mm, respectively. They reported that the ANB° increased by 0.5° over four years. In our study, the ANB° increased by 0.33° from 43 to 54 months. Overall, our research showed less change than that observed by Choi et al. [2] Whether this difference is due to an intervention in the physiological change induced by surgery or a cephalography measurement error making it inferior to CT analysis, more research is needed. However, the fact that the data in group 1 show more changes compared to the data in group 2 leads to the inference that surgery may not hinder physiological change; rather, surgery can exaggerate it.
Through a review of the above papers, we found that the extrusion of maxillary teeth and the increase in ANB observed may be related to a later dental change in physiological phenomena. However, we also found signi cant differences according to the type of surgery. The dentofacial change was physiologic, but contouring surgery might in uence the result. Therefore, we need to clarify how contouring surgery induces differences in physiological changes.
Song et al. [10] have reported that alteration in bite force can lead to occlusal changes. They found that injecting Botox into both masseter muscles not only weakens muscle strength but also changes occlusion . Lo et al. [11] also reported that ostectomy of the mandibular angle could cause masseter muscle atrophy even without muscle resection. However, whether the decrease in volume correlates with the decrease in bite force has not been proven. There is a study that angle reduction does not affect the biting force between the period before the operation and six months afterward [13].
Tooth eruption may occur throughout one's whole life to compensate for occlusal surface wear [7,14]. This eruption may cause an open bite when there is no masticatory force strong enough to cause erosion of the teeth or to resist the eruption force. Therefore, although Tan et al. [13] reported no reduction in bite force after angle reduction surgery only, it may be estimated that there would be more reduction in bite force when the patient underwent angle and zygoma reduction simultaneously. However, to prove this, more research on the reduction of bite force after contouring surgery should be conducted.
We also focused on the physiological adaptation in which atrophic muscles return to their original length. In orthognathic surgery, maxillofacial surgeons have considered ramal inclination as a vital factor for postoperative dental change. When the ramus rotates clockwise, the muscle length becomes more extended, and relapse occurs due to physiological action that prompts it to return to the original length [15]. This reasoning may also be applied to contouring surgery. If the muscle atrophied due to the surgery, the muscle length would be shortened, and the physiological tendency to return to the original length would rotate the mandible clockwise. To adapt to this new mandible position, the alveolar process may extrude the teeth.
The advantage of using CT images to check dental changes over a lateral cephalogram is that the CT images can more accurately identify the differences through the superimposed data embedded in the 3D image software, rather than requiring X-rays of two periods to be traced to measure the amount of change. When measured by cephalogram, the error is more than 0.5 mm, so it is less accurate when analyzed using CT [16]. In addition, we can exclude the measuring error of the ANB° due to the dual bite.
In this study, we can see the discrepancy of the condylar position before and after an operation in 5 subjects, and we excluded these subjects from ANB°, OJ, and OB analysis. It is also possible to check whether the joints were absorbed with the CT images, but we could not nd condylar resorption during the study period in the subjects included in our study.
There are some limitations to this study. As a retrospective study, it does not have the impact of a randomized controlled trial. In addition, the F/U period was not constant. It seems that the amount of extrusion and the postoperative period are correlated. Therefore, more research is needed to exclude the effect of the postoperative period on our results. Therefore, more research is needed to evaluate the effect of horizontal movement on tooth extrusion.

Conclusions
1. This study investigated long-term dental changes after facial contouring surgery using cone-beam CT for the rst time.
14. Newman HN (1999   The correlation between the variables and the nal follow-up period after the operation. Incisor, the amount of maxillary incisor extrusion. When the incisor was extruded, we assigned it a positive value.
Molar, the amount of maxillary molar extrusion. ANB, calculated by subtracting the pre-ANB° from the post-ANB°. OJ, calculated by subtracting the pre-overjet value from the post-overjet value. OB, calculated by subtracting pre-overbite value from the post-overbite value. Overjet, the distance between the maxillary incisor and most labial and mandibular incisor edges parallel to the occlusal plane. Overbite, the distance between the maxillary incisor edge and mandibular incisor edge perpendicular to the occlusal plane. We con rmed a signi cant correlation with the period for both incisor and molar. *p value < 0.05 was considered statistically signi cant. The p value was calculated by Spearman analysis.