Ectopic eruption of the maxillary first permanent molar is a local disturbance in which the tooth erupts mesially to its normal path. The molar then causes an abnormal resorption of the distal root of the maxillary second primary molar [1]. Young [2] classified ectopic eruption into the jump type and the hold type. The jump type of ectopic eruption is a reversible eruption of the maxillary first permanent molar, and is characterized by a spontaneous self-correction, after which the tooth erupts in its normal position within the arch. The hold type of ectopic eruption is an irreversible eruption in which the maxillary first permanent molar is locked under the distal surface of the maxillary second primary molar in the cervical area.

Ectopic eruption of the maxillary first permanent molar is diagnosed on routine radiographic examination performed between the ages of 5 - 7 years. This condition is diagnosed when the radiograph reveals an impacted maxillary first permanent molar overlapping the distobuccal root of the proximal maxillary second primary molar [3] or shows pathological root resorption [4]. The diagnosis can also be made clinically. The distal cusp of the maxillary first permanent molar can erupt before the mesial cusp [5], or the eruption can be delayed unilaterally or bilaterally [2].

Bjerklin and Kurol [1] demonstrated that the maxilla tends to be shorter in children with an irreversible ectopic eruption of the maxillary first permanent molar than in children with a normal eruption of this tooth. Pulver et al. [6] suggested that ectopic eruption can be caused by a combination of factors, such as an abnormally short maxillary length and a posterior maxillary position in relation to the cranial base. Canut and Raga [7] suggested that a short and posteriorly positioned maxilla may affect the ectopic eruption of the maxillary first permanent molar.

The cause of ectopic eruption of the maxillary first permanent molar is considered to have a multifactorial etiology, and is not well known [1,6,7]. However, only a few studies have surveyed the association between a skeletal imbalance of the jaw and ectopic eruption of the maxillary first permanent molar based on cephalometric analysis. There have been even fewer such studies in Korean populations. Therefore, the present study investigated the types of skeletal malocclusion associated with ectopic eruption of the maxillary first permanent molar, in a Korean population, and assessed the presence of ectopic eruption on panoramic radiographs.

Previous studies have reported that the prevalence of ectopic eruption of the maxillary first permanent molar varies between 0.75% and 6% [11-13]; the large variation in the prevalence is affected by the sample sizes [14]. In addition, Carr and Mink [15] and Bjerklin et al. [16] showed that ectopic eruption of the maxillary first permanent molar in cleft children occurs more frequently when compared with non-cleft children. This study found that the prevalence of ectopic eruption of the maxillary first permanent molar was 11.8%, which was higher than that reported in previous studies.

This study investigated the association between the presence of ectopic eruption of the maxillary first permanent molar and skeletal malocclusion in children with mixed dentition. The SNA, SNB, and ANB angles are the most commonly used measurements for assessing the anteroposterior relationships of the jaw and to assess skeletal malocclusion [17,18]. However, Freeman [19] asserted that the sizes of the SNA, SNB, and ANB angles were variable, depending on the position of the nasion, even in the absence of changes in the jaw relationships. The present study additionally used the the A-B plane angle, the mandibular plane angle and the APDI for skeletal assessment, and the A to N-perpendicular distance was selected to analyze maxillary undergrowth.

Studies that have incorporated cephalometric analyses have demonstrated that ethnicity, sex, and region affected these measurements. Therefore, various analytical methods have been used in such studies in Korea to identify reference values for these measurements in Koreans. Park [20] studied the changes in the maxillofacial development of Korean children aged 6 - 9 years, using Ricketts analysis. Yang and Kim [9] performed Ricketts analysis on 9-year-old children. Suh et al. [10] performed cephalometric analyses on children with normal occlusion of primary dentition, and Kim et al. [21] reported normal cephalometric values for children with mixed dentition. In the present study, we classified skeletal malocclusion based on the cephalometric studies of Lee [8], Yang and Kim [9], and Suh et al. [10] in children with normal occlusion; however, this study is limited in that the average age of the children studied is differed from the average age of the children included in the present study.

Pulver [6] have reported that patients with ectopic eruption of the maxillary first permanent molar have short arches and small maxilla and that the maxilla is posteriorly positioned relative to the cranial base. Canut and Raga [7] have assessed this in Europeans, and their results supported the findings of Pulver [6]. Chintakanon and Boonpinon [13] measured the SNA, SNB, and ANB angles, soft tissue facial convexity, maxillary length (ANS-PNS), and mandibular length (Co-Pg), and found no significant differences in these measurements between the ectopic eruption group and the normal eruption group. Mucedero et al. [22] measured the maxillary arch lengths, the intercanine widths, and the intermolar widths, to assess the skeletal features of the maxillary arches of patients with ectopic eruption of the maxillary first permanent molar. Their findings indicated that the maxillary arches were shorter and narrower in the ectopic eruption group than those of the normal eruption group. These studies analyzed maxillary structures using different measurements than those used in the present study. However, they found that maxillary undergrowth can affect ectopic eruption of the maxillary first permanent molar, which is consistent with the findings of the present study.

This study has several limitations. This study was a cross-sectional study, and it is limited in that we did not distinguish between the cases with hold and jump types ectopic eruption of the maxillary first permanent molar in the study group. Furthermore, this study only examined children who visited the hospital for orthodontic diagnoses, which does not reflect the prevalence as a whole. Patients were classified based on only panoramic radiographs, without considering clinical test results. Furthermore, the measurements used in this study as a reference for classifying skeletal malocclusion were not an absolute representation of maxillary development. Thus, future studies should use other measurements to analyze the association between skeletal malocclusion caused by maxillary undergrowth and the eruption pattern of the maxillary first permanent molar.

In this study, the association between ectopic eruption of the maxillary first permanent molar and skeletal malocclusion was investigated. Based on our findings, we were able to determine that maxillary undergrowth has an impact on the ectopic eruption of the maxillary first permanent molar.