This study is the first 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 confirmed the change in ANB° as measured as 0.58 ± 0.63° in women [1]. We verified 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 significant 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 first five 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 five years, measurements in females increased by 0.35 ° but by 0.17 ° for the next five 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 five 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 floor to the incisal tip and from the nasal floor 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 significant differences according to the type of surgery. The dentofacial change was physiologic, but contouring surgery might influence 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 find 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.