Introduction
The present study aimed to analyze possible factors involved in irreversible (IRR) ectopic eruption (EE) of the first permanent molar and explore potential predictors for the IRR outcome.
Methods
Children aged 4-11 years, with at least 1 EE and who took their first panoramic radiograph before the age of 8 years, were selected in this study. The subjects were assigned to the self-correcting (SC) and IRR groups. Patients’ age, sex, distribution of EE, and accompanying dental anomalies were recorded. Eruptive angulation (EA) of the first permanent molar, the grade of root resorption in the second deciduous molar, the magnitude of impaction index (MOII), and horizontal distance were measured on the panoramic radiographs. Chi-square tests and independent-sample t test were used for nominal and continuous variables, respectively. The receiver operative characteristic curve was used to determine the critical value.
Results
A total of 406 children with 634 first permanent molars, presenting EE, were enrolled, with 61.3% of the teeth in the SC group. Sex of children with EE and distribution of EE were not relevant to the IRR outcome. The presence of supernumerary teeth might be a protective factor for the IRR outcome. The increasing severity of root resorption in the second primary molar indicated an IRR outcome. A higher MOII and a larger EA suggested an IRR outcome with moderate-to-high quality. The horizontal distance exhibited debatable results, with a low predictive quality.
Conclusion
Close monitoring and early intervention would benefit children with increasing severity of distal atypical resorption in the second primary molar, higher MOII, and larger EA.
Highlights
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This study explored outcome predictors for ectopic erupted first permanent molar.
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This study is the first to apply the magnitude of impaction on panoramic radiographs.
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The magnitude of impaction index tended to be a high-quality predictor.
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The existence of supernumerary teeth is possibly related to self-correcting outcome.
First permanent molars can sometimes erupt too far mesially, causing atypical resorption of the distal root of the second primary molar. The prevalence of this situation is 0.75%-6% and is referred to as ectopic eruption (EE) of the first permanent molar. EE is thought to have a multifactorial etiology. Previous studies have reported that maxillary undergrowth, an oversized first permanent molar, cleft lip and palate, , supernumerary teeth, and weight and height deficiency are associated with the onset of EE. A familial tendency has also been observed.
Bjerklin and Kurol distinguished 2 types of EE in 1981: self-correcting (SC) and irreversible (IRR). The SC type can release itself from the distal contour of the second primary molar at the age of 7-8 years, and no interceptive treatment should be initiated. The IRR type can remain locked after the age of 8 years, and if left untreated, it can lead to the premature loss of the second primary molar and subsequent decrease in the dental arch length, shifting of the first permanent molar, and supraeruption of the opposing molar. Early prediction of IRR outcome and early initiation of interceptive treatment should be undertaken to avoid harmful consequences.
Previous research has also explored the predictive factors for IRR outcome. , , , Dabbagh et al analyzed the clinical and radiographic predictors for the IRR outcome and reported that the magnitude of impaction of the first permanent molar measured on bitewing radiographs was positively correlated with the IRR outcome and was the most reliable predictor among the predictors assessed. Bjerklin and Kurol studied the lateral cephalograms, panoramic radiographs, and casts of 192 patients with or without EE, reporting that the eruptive angulation (EA) and crown width of the first permanent molar were 2 crucial factors in the outcome of the EE for this tooth. However, a recent study by Sun et al suggested that the EA was correlated with the onset of EE; however, no significant difference was detected concerning the IRR outcome. Conflicting and limiting results underline the importance of further investigations into the predictors for the IRR outcome.
Radiographic examination is helpful in the diagnosis and treatment planning of dental diseases, and the EE is no exception. Among the techniques used in the radiographic examination, cone-beam computed tomography (CBCT) is the most precise one. However, it is costly and can expose the patient to a relatively high radiation dose. Thus, it should be applied with caution in pediatric practice. Panoramic radiography, in contrast, only delivers no more than 10% of the radiation dose produced by CBCT and can provide an overview of the teeth and jaws with 1 examination. , Nonetheless, because it produces a 2-dimensional image, there are concerns about distortion and overlapping of teeth. In addition, there are concerns about distortion caused by the movement of young children, and an additional holder and a proper tell-show-do technique are necessary for managing their behavior. It was reported that angular distortion of the molar region on panoramic radiographs is the least affected by head positioning compared with other teeth in the dentition. Cast studies also revealed its precision in detecting the crown-to-root ratio. If correctly conducted, the panoramic radiography technique can be a reliable tool for measuring tooth angulation. , Thus, it would be a helpful tool for predicting the outcome of EE.
This study aimed to analyze the prevalence of the EE of the first permanent molar to explore the possible correlation factors for the IRR outcome and evaluate the predictors of high quality in panoramic radiography for the IRR EE of the maxillary first permanent molar tooth.
Material and Methods
This retrospective study was approved by the Institutional Review Board of West China Hospital of Stomatology (WCHSIRB-D-2016-166). This research strictly followed the Declaration of Helsinki, and written consent was obtained from all subjects. The medical history and panoramic radiography of children visiting the Department of Pediatric Dentistry of our hospital from November 2013 to November 2015 were reviewed for case selection. A review of all the radiographic records of the selected patients taken before April 2018 was also conducted.
The study can be divided into 3 parts: collecting the general demographic data of the patients with the EE of first permanent molars, analyzing factors associated with the outcomes of EE, and exploring the predictive factors of SC EE of first permanent molars on panoramic radiographs. For different purposes of the study, different inclusion criteria were established. More detailed information about patients and teeth included in different parts is described in the following paragraphs and flow chart in Figure 1 .
Generally, patients would be included in all the 3 parts of this study if (1) they had available and legible panoramic radiograph, with at least 1 first permanent molar exhibiting EE and if (2) their EE of first permanent molars achieved the stage VI according to Nolla method. Patients with no regular follow-up radiographs were considered as drop-outs for the analysis of correlation factors and possible predictors for the IRR outcome because it was impossible to determine whether the EE had to be categorized as IRR or SC. In addition, patients’ first panoramic radiographs taken after 8 years of age were not included in the analysis of predictive factors for IRR EE.
The exclusion criteria were as follows: (1) severe caries of the second primary molar and/or the first permanent molar, compromising the contact between the first permanent molar and the second primary molar or even premature loss of the second primary molar; (2) evidence indicating that root resorption of the second primary molars was caused by pulpitis because of dental caries; and (3) interventions were implemented to correct the EE of first permanent molars. In addition, patients with systemic diseases such as rickets and Treacher-Collins syndrome that possibly affected the structure of craniofacial bones and teeth were excluded for assessing the predictors in panoramic radiographs.
Age, sex, the chief complaint in the first appointment, the location and occurrence of EE of the first permanent molar, and systemic disease history were evaluated and recorded. To further understand whether other dental diseases or anomalies could affect the potential of IRR EE, the information on dental caries history, cleft lip and palate, supernumerary teeth, congenital hypodontia, and dilaceration of teeth were also collected.
Experienced radiologists took all the panoramic radiographs with the same x-ray machine (Veraviewepocs, Morita, Tokyo, Japan). Three guide lights were used for standardizing the head position: 1 to make sure that the Frankfort horizontal plane was parallel to the floor, 1 for placing the midsagittal plane perpendicular to the floor, and another to locate the maxillary canine. A 2.0-mm plastic occlusal bite was placed between the central incisors to open the bite, and a chin-holder was used to stabilize the position. All the parameters were measured in a picture-archiving and communication system (Marosis Enterprise PACS; Infinitt Healthcare, Seoul, Korea), and the quality of the image was 300 dpi.
The severity of root resorption of the second primary molar was determined through the following criteria introduced by Barberia-Leache in 2005: grade I, mild and limited resorption of the cementum or minimum dentin penetration; grade II, moderate resorption of the dentin without pulp exposure; grade III, severe resorption of the distal root, leading to pulp exposure; and grade IV, very severe resorption, involving the mesial root of the second primary molar.
Figures 2 and 3 present all the reference lines and points used for measurements. The midline was drawn as a longitudinal line through the nasal septum. The long axis of the maxillary first permanent molar is a longitudinal line from the midpoint of the crown and the apex of the palatal root in the maxilla. The occlusal plane was defined as an average plane of the occlusal contacts of posterior teeth. EA was defined as the mesial angle between the long axis of the first permanent molar and the occlusal plane. The horizontal distance was defined as the distance between the maximum convexity of the distal contour of the first permanent molar and the midline. The magnitude of impaction (MOI) of the first permanent molar was measured on the panoramic radiographs. As described by Barberia-Leache, a tangential plane was drawn up to the maximum convexity of the distal contour of the second primary molar perpendicular to the occlusal plane; MOI was defined as the distance between the mesial maximum convexity of the first permanent molar and the tangential plane. In this study, panoramic radiographs were used instead of bitewing radiographs. Therefore, to eliminate the influence of distortion on panoramic radiographs and consider the crown width for the evaluation of clinical outcomes, the magnitude of the impaction index (MOII) was developed through the following equation:
MOII=MOI of the first permanent molarcrown width of the second primary molar
One investigator measured the parameters. To assess the intrarater reliability, 20 panoramic radiographs were selected randomly and measured twice with a 2-week interval. Kendall’s Tb coefficient was calculated, which revealed proper consistency in the severity of root resorption in the second primary molars (Kendall’s Tb > 0.85, P <0.001). The intragroup correlation coefficient was used to determine the consistency of continuous variables, such as crown width, MOI, and horizontal distance. All the intragroup correlation coefficient were >0.75. All the continuous variables were measured 3 times, and a mean value was obtained.
Statistical analysis
Frequencies and percentages were calculated for descriptive variables; means and standard deviations were calculated for continuous variables. Other continuous variables were evaluated through the independent-sample t test. The receiver operator characteristic curve was used to determine the critical value of the crown width, EA, MOII, and the horizontal distance in predicting the outcome of the EE. Chi-square test or Fisher exact test was used to determine which factor was positively associated with the EE and IRR outcomes. Linear-by-linear association test was used to assess the correlation between the severity of root resorption in the second primary molar with IRR outcomes. Statistical tests were carried out using SPSS software (version 21.0; IBM, Armonk, NY).
Results
A total of 11,403 patients aged 4-11 years, who underwent panoramic radiography examinations for different chief complaints in our hospital from November 1, 2013 to November 30, 2015, were examined, of which 409 patients with 634 ectopically erupted first permanent molars met the inclusion criteria. The chief complaints of the subjects were mainly dental caries, orthodontic problems, routine examinations, supernumerary teeth, and retained primary teeth, with 28.4%, 26.9%, 15.9%, 4.9%, and 4.2% proportions, respectively. However, we failed to find the chief complaints from radiographic examination for 15.2% of the patients because of missing medical records. The occurrence of EE was 3.6% in the subjects, with 265 boys and 144 girls exhibiting EE. The EE of the first permanent molars was mostly seen in children aged 6-7 years ( Table I ).
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