Retrospective investigation was conducted over images of dental radiographs, panoramic radiographs, and cone-beam computed tomography (CBCT) in child patients with mesiodens who visited our hospital for their first visit from November 2017 to March 2022. Such patients without any disease that affected mesiodens were included; on the contrary, those with hereditary disease or other diseases that affected mesiodens were excluded. As a result, 128 patients participated in the investigation. Gender composition of the patients, age of their first visit, and the number and shape of the mesiodens (supplemental, conical, tuberculate, and molariform) were surveyed. Existence or absence of the corresponding complications in the adjacent permanent teeth such as displacement, retarded eruption/low tooth germ position, and rotation were also investigated.

The patients were divided into two groups for each complication in the adjacent permanent teeth; displacement (D) and without displacement (WOD) groups, retarded eruption/low tooth germ position (R/L) and without retard eruption/low tooth germ position (WOR/L) groups, or rotation (R) and without rotation (WOR) groups. Patient age was compared between the groups with and without each complication.

Values in the text and Tables are shown as mean ± standard deviation (SD). The significance of differences between two groups was analyzed using the unpaired Student’s t-test (for equal variance) or Welch’s correction (for unequal variance). A p-value < 0.05 was considered significant.

The ages of the patients’ first visit ranged from three to ten years old (Figure 1). The patient population consisted of ninety-seven males and thirty-one females (Figure 2).

Ninety-seven patients had one mesidens, while thirty-one had two. For shape of mesiodens, one-hundred and twenty were conical, thirty-four tuberculate, one molariform, and four supplemental (Figure 3).

For posture, seventy-three mesiodens were vertical, seventy-eight inverted, and eight horizontal (Figure 4). Since thirty-one patients had two mesiodens, the total number of the mesiodens did not equal the number of the patients.

The presence or absence of displacement, retarded eruption/low tooth germ

position, or rotation were investigated based on the age of the patients. The two common kinds of displacement among the patients were diastema and labiogression. More than 67% of the patients at all ages had displacement (Figure 5).

As shown in Figure 6, all the three-year-old patients had low tooth germ position. The incidences of retarded eruption/low tooth germ position in four to six-year-old patients were around 68%, with a decreasing incidence in seven- and eight-year-old patients, and no eruption defects in ten-year-old patients.

Rotation was observed eight and less-year-old-patients, but not in nine and ten years old (Figure 7). Incidences of retarded eruption/low tooth germ position

in four to six-year-old patients were around 68%, with a decreasing incidence in seven- and eight-year-old patients, and no eruption defects in ten-year-old patients.

Rotation was observed eight and less-year-old-patients, but not in nine and ten years old (Figure 7).

Since some complications of mesiodens were affected by age, dentition [9] [10], and oral development [11], patients’ age was compared between the D and WOD, R/L and WOR/L, or R and WOR groups. As shown in Table 1, age of the patients in the WOD group was significantly higher than that of the D group (p < 0.05). Age of the patients in the WOR/L group was also significantly higher than that in the R/L group (p < 0.01).

Upper incisor region is well known for preferred site of dental caries. Hence,

**p < 0.01, *p < 0.05; D: The group in which displacement was found in the adjacent permanent teeth. WOD: The group in which displacement was not found in the adjacent permanent teeth. R/L: The group in which retarded eruption/low tooth germ position was found in the adjacent permanent teeth. WOR/L: The group in which retarded eruption/low tooth germ position was not found in the adjacent permanent teeth. R: The group in which rotation was found in the adjacent permanent teeth. WOR: The group in which rotation was not found in the adjacent permanent teeth.

dentists take radiographs of that region in their clinical practices. So, it was considered that mesiodens was accidentally found through dental treatment or checkup in three- to five-year-old patients, resulting in finding mesiodens. Upper central incisors erupt approximately one year after the lower central incisors’ eruption. The upper central incisors’ eruption is known for being retarded by mesiodens [12] [13]. So, it was considered that in the cases mesiodens interfered with the eruption of the adjacent upper central incisor, patients consulted their dentist for retarded eruption of the upper central incisor around the ages of their eruption; from six to seven years old.

Previous studies reported that male/female ratio was approximately 3:1 [14] [15] [16], supporting our result. Though the ratio varied by paper [17] [18] [19], these previous reports concluded that the number of patients with supernumerary teeth is higher in men than in women.

For shape, conical was previously reported to be the commonest [4] [20] [21] and tuberculate occupied 14% - 20% [22] [23] [24], supporting our results.

Kim et al. (2018) [25] and Nam et al. (2015) [26] reported that the most common direction of mesiodens in Korean children was inverted (59.5% and 56.2%, respectively). Zhao et al. (2021) [27] also concluded in their investigation of mesiodens in Chinese children that invert posture was the most frequent, all supporting our result. On the other hand, Gündüz et al. (2008) [21] and Altan et al. (2019) [22] reported in their respective investigations among children in Turkey or black sea area that vertical position was the most frequent in mesiodens. These present and previous reports suggested that invert mesiodens was the most frequent in east Asian children, and the most frequent posture of the supernumerary tooth may vary by region or race.

In previous studies, displacement, retarded eruption, and rotation in adjacent teeth are the major complications of mesiodens [14] [16] [21] [24] [28]. The complications were determined by the positional relationship between the mesiodens and the adjacent teeth, and the relationship could be affected by oral and maxillofacial growth, hence the patients’ age was compared between groups with and without each complication. The present study indicated that the patients with displacement or retarded eruption/low tooth germ position visited our hospital younger than those without such complications by eleven months. The difference suggested that if the patients recognized displacement or retarded eruption/low tooth germ position in the upper permanent incisors either by radiograph or by actually looking inside the oral cavity, they wished to consult their dentist as soon as possible for functional or aesthetic reasons. Especially, incidence of retarded eruption/low tooth germ position was low in eight- and nine-year-old patient and got zero in the patients at 10 years old, thus it was suggested that most of the patients having retarded eruption/low tooth germ position visited their dentist and finish the treatment before they got 10 years old. On the other hand, no age difference was found regardless of existence or absence of rotation. Notably, in this investigation, all the participating patients accompanying rotation had displacement, retarded eruption/low tooth germ position, or both of the complications, too. Therefore, rotation was considered to be associated with displacement and retarded eruption/low tooth germ position. Taking the results into consideration, it was suggested that all the patients with rotation might have consulted their dentist for treatment of mesiodens and corresponding multiple complications at the similar ages as the patients with displacement or retarded eruption/low tooth germ position did.

There was a report on supernumerary tooth in Japanese deciduous dentition [29]; however, radiographs were not taken in all patients in the survey. In the present study, investigation was conducted using dental radiographs, panoramic radiographs and CBCT images, that enable us detecting both unerupted mesiodens and complications existing in the maxillary bone.

Mesiodens causes serious complications that inhibit development of oral function. Moreover, mesiodens itself may turn into cystic lesion. The present report will contribute to knowing and diagnosing mesiodens in Japanese children better.

This investigation suggested that invert mesiodens was the most frequent in east Asian children, and the most frequent posture of the supernumerary tooth may vary by region or race. It also suggested that rotation was accompanied by displacement or/and retarded eruption. However, present report couldn’t provide their supporting evidence, so further study is needed to conclude the suggestions with evidence.