Introduction
Supernumerary teeth (ST), one of the most common dental anomalies, are defined as teeth or odontogenetic structures that exceed the normal number of 20 primary teeth or 32 permanent teeth [
1]. Mesiodentes are the most common type of ST and are located in the maxillary central incisor region [
2]. The prevalence of mesiodentes varies between different racial groups, and there is a higher frequency in the Asian population of approximately 3% [
3]. People of any age can have ST, while ST are commonly found in children with mixed dentition. In addition, males are more affected than females (ratio of 3:1) [
4]. To date, the aetiology of ST remains unclear. Several scientific hypotheses have been put forward to explain the aetiology and development of ST including atavism, dichotomy of the tooth bud, hyperactivity of the dental lamina, and genetic factors [
5]. It has been reported that genetics and heredity play a key role in the occurrence of ST, especially in patients with a syndrome or family history [
6]. Environmental factors will increase individual genetic susceptibility [
7].
It has been reported that the prevalence of ST among permanent and primary dentition is 1.5-3.5% and 0.3-0.8%, respectively [
4,
8]. The reported prevalence of ST varies in different studies due to the different sample populations and diagnostic tools used [
9]. A meta-analysis indicated that full-mouth radiographic evaluation was critical for supernumerary tooth identification [
10]. However, considering medical ethics, radiographic examination should not be used as a conventional tool for supernumerary tooth screening in the general population [
11,
12]. Recently, epidemiological studies of ST have utilized hospital populations who have undergone radiographic evaluations [
2,
13,
14]. Although these studies were from hospital populations, the incomparability of results could be caused by the use of different radiographic tools. The two major radiographic tools adopted in the current related literature are panoramic radiography (PR) and cone-beam computed tomography (CBCT). PR is a 2-dimensional (2D) radiographic technique that requires a much lower dose than CBCT [
15]. However, this traditional 2D imaging modality fails to provide highly accurate information on ST and the spatial relationships between ST and neighbouring structures and is thus sometimes insufficient for optimal treatment planning and surgery risk assessment [
14]. According to the current guidelines, CBCT images should only be obtained when a lower-dose radiological examination, such as panoramic imaging, cannot provide adequate information for clinical diagnosis and treatment [
16]. Generally, the information provided by PR is sufficient for some erupted ST, and CBCT images are commonly applied to determine the 3D information of nonerupted ST [
14]. The baseline population in many recently published epidemiological reports on ST generally consists of patients who underwent only CBCT imaging [
13,
14]. It is supposed that the supernumerary tooth eruption rate assessed by CBCT is lower than the true level [
13].
For the treatment of erupted ST, it is generally recommended that ST should be extracted as soon as possible to prevent associated complications, including impacted or delayed eruption, median diastema, displacement or rotation, cyst formation, and root resorption of the adjacent teeth [
17‐
19]. However, the timing of nonerupted supernumerary tooth extraction remains controversial with respect to permanent tooth germ development and potential injury risks, especially for children with mixed dentition [
3]. Based on the 25% eruption rate of ST in the literature [
14], we supposed that the extraction of some nonerupted ST with eruption potential in appropriate cases can be delayed until self-eruption, and that minimally invasive treatment can be performed for these ST. Therefore, the evaluation of supernumerary tooth self-eruption potential is crucial. The eruption rate of ST has been extensively investigated in many studies [
9,
20,
21]. Nevertheless, rather limited attention has been given to the study of the factors associated with supernumerary tooth eruption. Very few related studies have been undertaken to research these factors among 3- to 12-year-old children in Guangzhou, a southern central city of China.
The purpose of this study was twofold, as follows: (i) to present the clinical and 3D characteristics of nonsyndromic ST in Chinese children aged 3–12 years based on medical records and radiographs; and (ii) to analyse the factors related to the supernumerary tooth eruption status based on CBCT images and discuss the optimal extraction time for nonerupted ST.
Discussion
The precise causes of ST are unclear. Several hypotheses have been proposed in previous studies, such as atavism, dichotomy of the tooth bud, hyperactivity of the dental lamina, and genetic factors [
22]. It is believed that genetic factors are an important cause of ST [
23‐
25]. McBeain et al. found that 20.5% of patients with ST had first-generation relatives who also had ST [
15]. The prevalence of ST in the Caucasian population is lower than that in the Mongolian population [
3]. Asian countries or regions such as India and Hong Kong have a higher prevalence of ST (1.40-2.97%) [
26‐
28], and the prevalence of ST in Australia and Rome is generally lower (0.28-0.66%) [
15,
28,
29]. The present study covered 13,336 participants in a baseline population and revealed a 6.67% prevalence rate of ST among the total population. The prevalence rates among males and females were 9.39% and 3.46%, respectively, which are greater than those in previous studies [
2,
13,
14]. One reason for this difference could be that most subjects have mixed dentition. Previous studies have indicated that the prevalence of ST among children with mixed dentition is higher than that among subjects in other age groups [
30,
31].
Because panoramic films are usually used for initial supernumerary tooth screening [
10], patients who have undergone panoramic imaging in dental hospitals were selected as our surveyed subjects. A Hong Kong study reported that the supernumerary tooth prevalence rate among 1,093 randomly sampled 12-year-old students in school undergoing PR was 2.7% [
28]. The prevalence rate of ST among 12-year-old patients in the present study was similar (2.61%, 42/1608).
The supernumerary tooth prevalence rate increased first, reached a maximum value at approximately 7 years, and then gradually declined with increasing age. For patients aged 3–7 years, the supernumerary tooth prevalence rate increased gradually with age, which could be due to the following reasons. On the one hand, ST gradually begin to erupt or cause related complications with increasing age, thereby increasing the demand for medical treatment. On the other hand, the development time and location of supernumerary tooth germs are uncertain, and late-developing ST will appear with age [
32].
Patients aged 7 years had the highest prevalence rate, which may be due to the onset of maxillary incisor eruption at this age and a number of ST-related complications, such as delayed eruption of neighbouring teeth. Among patients aged 7–12 years, the supernumerary tooth prevalence rate decreased slowly with age. The reason may be that erupted ST or ST with complications were extracted in the early dentition stage. The remaining ST were mainly without complications or buried in the bone and were incidentally discovered by radiological examination.
Noticeably, there was no significant difference in the prevalence of ST between the primary dentition (6.1%, 157/2584) and mixed dentition (6.8%, 733/10,752) populations (P > 0.05). In this study, the prevalence of ST in the primary dentition population was far more than the 0.3 − 0.8% previously reported in the literature [
4]. The difference is mainly attributed to the diagnostic methods. Previous studies on ST in primary dentition mainly used visual diagnosis, resulting in a large number of missed diagnosis [
11,
12].
The occurrence of ST was related to sex, and males were diagnosed more often than females. This result is consistent with the findings in the literature [
15,
33,
34]. In the present work, the prevalence of ST among males relative to females was 2.7:1. Recently, many studies on the genes and family epidemiology related to ST have been published [
24,
35]. Autosomal or sex chromosome heredity has been proposed to be an important aetiological factor [
36]. Our results further supplement the epidemiological data of ST.
In the present study, the number of ST ranged from 1 to 4. A single supernumerary tooth (68.2%) was the most common, followed by 2 ST (31.2%), 3 ST (0.3%) and 4 ST (0.2%). Multiple ST (≥ 5 ST) are very rare and are usually associated with a syndrome, such as cleidocranial dysplasia [
6]. A similar study was reported by Bereket, and the results suggested that 77.4% of patients had one supernumerary tooth, 18.4% of patients had 2 ST, and 4.2% of patients had 3 or more ST [
37].
The location distribution of ST was different in the upper and lower jaws. The location of maxillary ST was basically symmetrical. ST were distributed mainly near the central incisor area, followed by the lateral incisor area, canine area, premolar area, and molar area. Mandibular ST were most often found in the premolar area, followed by the canine and incisor areas, while ST were not detected in the molar area. It is worth noting that a rare case of ST in the mandibular central incisor region was observed, as presented in Fig.
1. It is usually believed that ST rarely occur in the mandibular central incisor area [
38].
It has been reported that 47.6-88.5% of ST cause complications in patients [
38,
39]. Our results show that 71.4% of ST had related complications. Notably, the manifestations of the ST-associated complications were different in each age group. Considering that most ST were located in the maxillary incisor area and that the eruption period of the maxillary incisors is approximately 7–9 years of age [
4], it can be concluded that most cases of ST in the 7- to 8- and 9- to 10-year age groups included related complications owing to the effect of ST on the eruption of maxillary incisors. In the 7- to 8-year age group, the main complication of ST was median diastema, followed by displacement/rotation and failed eruption of adjacent teeth. The most common complication in the 9- to 10-year age group was displacement/rotation of adjacent teeth, followed by abnormal eruption and median diastema. Delayed/impacted eruption of adjacent teeth was the most common complication in the 11- to 12-year age group, followed by other rare complications (Fig.
5). It is suggested that the later the intervention is performed, the more difficult the subsequent treatment will be [
3].
Previous studies have reported that approximately 1/4 of ST can erupt [
14,
40]. According to the PR and medical records of the 890 patients, 482 of the 1,180 ST erupted, for a rate of 40.9%, which is higher than the rate previously reported in the literature [
40]. However, among the 598 patients who underwent CBCT, the supernumerary tooth eruption rate was 25.3%, which is consistent with the eruption rate of 25.0% that was previously reported in the literature [
13,
14]. The reason for the difference in the eruption rate could be related to the inconsistency of the diagnostic tools. In general, CBCT is mostly used for nonerupted ST in the clinic. Some studies [
13,
14] only included supernumerary tooth patients who underwent CBCT, which could cause the supernumerary tooth eruption rate to be far lower than the actual rate.
Generally, while erupted ST can be extracted, extraction is not always suitable for nonerupted ST, especially in children with permanent tooth germ development in their jaws [
41]. Currently, the best timing for the extraction of nonerupted ST in this population is still controversial. Alsani et al. suggested that the early extraction of ST (before 6 years) can effectively prevent the related complications of ST and can maximize the eruption potential of permanent teeth [
42]. It is emphasized that ST should be removed as soon as they are found. However, considering that the developing permanent tooth germ could be damaged during the operation, some authors advocate delaying extraction until the patient is 8 to 10 years old, when the roots of the adjacent teeth are basically developed [
3]. However, the disadvantage is that delayed extraction could cause adjacent teeth to lose their eruption potential. Then, secondary surgery and/or orthodontic interventions could be needed in later stages [
43].
The present study showed that six-year-old patients had the highest eruption rate (57.80%) and that the eruption rate of ST was strongly negatively correlated with age. Meanwhile, after the age of six, ST-associated complications were more likely to occur. Therefore, the age of 6 years old may be a suitable time for the extraction of nonerupted ST. The results of this study also showed that the 3D characteristics (orientation and position) of ST were associated with the eruption status of the ST, suggesting that the CBCT evaluation of nonerupted ST for accurate planning is recommended. However, the cross-sectional design of this study had some limitations, and longitudinal studies should be conducted in the future to further confirm the results.
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