Introduction
The purpose of this study was to evaluate the prevalence and patterns of tooth agenesis in subjects with Angle Class II Division 2 malocclusion compared with general orthodontic patients in Japan.
Methods
Panoramic radiographs, dental casts, and cephalograms of 76 patients with Class II Division 2 malocclusions (52 female, 24 male) and 270 orthodontic patients as the control group (168 female, 102 male) who were 14 years of age or older were selected. The prevalences of tooth agenesis in this cohort and in each tooth type were calculated and compared between the groups with the chi-square test. Odds ratios and corresponding 95% confidence intervals were also calculated.
Results
The prevalence of tooth agenesis excluding the third molars was significantly higher in the Class II Division 2 group (22.4%) than in the control group (11.9%) ( P <0.05); the odds ratio was 2.14 (95% CI, 1.12-4.12). A higher prevalence of tooth agenesis excluding the third molars was observed in the Class II Division 2 group for the mandibular second premolar ( P <0.05) and the maxillary lateral incisor ( P <0.01). The prevalence of third molar agenesis was also significantly higher in the Class II Division 2 group (42.1%) compared with the control group (26.7%) ( P <0.05); the odds ratio was 2.00 (95% CI, 1.18-3.40).
Conclusions
Compared with the general orthodontic patient population, permanent tooth agenesis was observed approximately 2 times more frequently, and a distinctive agenesis pattern was found in the Class II Division 2 group in Japan.
Highlights
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Tooth agenesis was about twice as prevalent in Class II Division 2 malocclusions.
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Tooth agenesis prevalence was significantly higher in mandibular second premolars.
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Tooth agenesis prevalence was significantly higher in maxillary lateral incisors.
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Third molar agenesis was higher for the subgroup lacking all 4 third molars.
According to Angle, Class II Division 2 malocclusion is characterized by a distal position of the mandibular arch with retroclination of the maxillary incisors; this is the result of longer contact with the lips during normal respiratory function of the oral environment, rather than genetic factors. Consequently, lip pressure has been found to be significantly greater in patients with Class II Division 2 malocclusion than in those with Class I malocclusion.
However, research studies, a review article, and case reports of twins and triplets have suggested the heritability of Class II Division 2 malocclusion. Peck et al identified a characteristic pattern of heritable skeletal and tooth-size features in patients with Class II Division 2 cover-bite malocclusion and suggested a strong genetic influence on the formation of this pattern.
Subsequently, Basdra et al evaluated 267 Class II Division 2 patients in Germany and found a higher prevalence of heritable dental anomalies, including tooth agenesis, compared with other epidemiologic studies of general populations in Europe and the United States. In addition, the researchers observed a high prevalence of agenesis of the maxillary lateral incisor.
In another recent study in Portugal, Pereira et al examined 115 Class II Division 2 subjects without maxillary lateral incisor agenesis and classified them into 2 groups on the basis of maxillary incisor retroclination. These researchers found a higher prevalence of tooth agenesis in the group with retroclination of all 4 maxillary incisors than in the group with retroclination of bilateral maxillary central incisors. The authors suggested that different etiologic factors contribute to the different manifestations of Class II Division 2 malocclusion.
Developmental dental anomalies, including tooth agenesis, have been suggested to be an anatomic characteristic of patients with Class II Division 2 malocclusion. Therefore, an analysis of the prevalence and patterns of tooth agenesis may provide additional evidence of possible genetic factors contributing to Class II Division 2 malocclusion. In a recent review article, Hartsfield et al reported an increased risk of Class II Division 2 malocclusion in first-degree relatives of Class II Division 2 probands compared with the general population. Their findings suggested that the Class II Division 2 malocclusions resulted from a polygenic, complex etiology rather than from variations in a single gene. They also suggested the need to investigate possible common etiologic factors, including genes associated with tooth development and tooth agenesis in patients with Class II Division 2 malocclusion and environmental etiologies. However, a direct comparison of the prevalence and patterns of tooth agenesis between patients with Class II Division 2 malocclusion and those with general malocclusion has not been performed until now.
The prevalence of Class II Division 2 malocclusion has been studied worldwide, and wide geographic variations have been observed. The wide variations in prevalence among geographic regions suggest the possible influence of genetic background on the etiology of Class II Division 2 malocclusion. However, in Japan, previous studies of Class II Division 2 malocclusion were limited to evaluation of the skeletal and dental morphologies, and few studies have investigated the heritability of this malformation.
The purpose of this study was to evaluate the prevalence and patterns of permanent tooth agenesis in Japanese patients with Class II Division 2 malocclusion compared with general orthodontic patients in Japan and with previous studies in other countries.
Material and methods
For our Class II Division 2 group, the relationship of the first molars and the inclination of the maxillary incisors were evaluated by clinical intraoral and dental cast observations from a total population of approximately 3000 patients at the Nippon Dental University Hospital and private clinics in metropolitan Tokyo, and from a total population of approximately 1000 students at Nippon Dental University and other colleges for dental hygienists and dental technicians. One hundred four subjects with Angle Class II Division 2 malocclusion (Class II molar relationship, end to end or worse on at least 1 side in maximum intercuspation, and retroclination of bilateral maxillary central incisors with deep overbite) and no syndromes were selected by intraoral examinations or dental cast observations. Subjects with extracted permanent teeth or uncertain dental history were excluded. Subsequently, panoramic radiographs, cephalograms, and dental casts were evaluated and further selected according to the following inclusion criteria: U1-SN less than 90°, overbite deeper than 3 mm, no previous orthodontic treatment, no prosthetic crowns, and age 14 years or older. Tooth agenesis was evaluated by 2 dentists (K.O. and K.A.); in case of a divergence of opinion between the evaluators, the subject was excluded. Overall, 76 subjects with a mean age of 21.0 ± 6.9 years, including 52 female subjects with a mean age of 21.4 ± 6.5 years and 24 male subjects with a mean age of 20.6 ± 8.0 years, were selected as the Class II Division 2 group. The maximum age in this group was 41.9 years. The protocol of this study (NDU-T2011-34) was approved by the ethics committee of Nippon Dental University.
For the control group, panoramic radiographs and dental casts from the diagnostic records of approximately 1000 orthodontic patients from private clinics in metropolitan Tokyo were evaluated according to the following inclusion criteria: no previous orthodontic treatment, no prosthetic crowns, and age 14 years or older. Subjects with extracted permanent teeth or uncertain dental history were excluded. In addition, if the evaluations of tooth agenesis by the 2 evaluators did not agree, the subject was excluded. As the control group, 270 orthodontic patients with no syndromes and a mean age of 18.2 ± 4.8 years (168 female subjects with a mean age of 18.5 ± 5.0 years, 102 male subjects with a mean age of 17.3 ± 4.7 years) were selected. The maximum age in this group was 44.0 years.
The numbers of subjects with tooth agenesis and their tooth types were evaluated. The numbers of absent teeth were counted for each tooth type in the maxilla and the mandible. The prevalence of tooth agenesis among all subjects and the prevalence of agenesis for each tooth type were calculated and compared between the 2 groups using the chi-square test ( P <0.05). The results are presented as odds ratios with the corresponding 95% confidence intervals (CIs).
The prevalences of agenesis of at least 1 third molar in both groups and agenesis in the maxillary and mandibular arches were calculated and compared between the 2 groups using the chi-square test ( P <0.05). The results are presented as odds ratios with the corresponding 95% CI.
Additionally, the prevalence of agenesis was compared between the maxillary and mandibular arches within each group using the chi-square test ( P <0.05). Furthermore, subjects with agenesis of at least 1 third molar were divided into 4 subgroups according to the number of absent third molars for each subject, and the prevalence for each subject was compared between the 2 groups using the chi-square test ( P <0.05).
Sample size estimation (2 × 2 for independent samples, chi-square test) was performed using the SPSS Sample Power version 3 in the SPSS statistics package (version 21.1; IBM, Armonk, NY). Based on a previous study in which the prevalence values of tooth agenesis between different Class II Division 2 malocclusion groups were 19% and 41%, with power greater than 80% and an alpha of 5% significance (2-tailed), it was determined that a sample size of 70 would be required in each group.
To confirm the consistency of the subjects of the Class II Division 2 group, the prevalences of tooth agenesis excluding the third molars and third molar agenesis were compared between the orthodontic patients (n = 59) and the dental students (n = 17) using chi-square tests, and no statistically significant difference was observed (chi-square value = 1.418; P = 0.234; and chi-square value = 0.008, P = 0.930, respectively).
Results
The prevalence of tooth agenesis excluding third molars in the Class II Division 2 group (22.4%, n = 17/76) was significantly higher than in the control group (11.9%, n = 32/270) according to the chi-square test (chi-square value = 5.396, P <0.05) ( Table I ); the odds ratio was 2.14 (95% CI, 1.12-4.12).
The highest prevalence of tooth agenesis excluding third molars was found in the mandibular second premolar, mandibular lateral incisor, and maxillary lateral incisor in the Class II Division 2 group and in the mandibular lateral incisor, mandibular second premolar, and maxillary second premolar in the control group. A significantly higher prevalence was found in the mandibular second premolar and maxillary lateral incisor in the Class II Division 2 group than in the control group ( Table II ).
| Tooth type | Class II Division 2 group, n = 76 | Control group, n = 270 | Statistical comparison between the groups | |||
|---|---|---|---|---|---|---|
| Number of subjects (%) | Number of teeth (%) | Number of subjects (%) | Number of teeth (%) | Chi-square value | P value | |
| Mn I2 | 6 (7.9) | 9 (28.1) | 12 (4.4) | 15 (25.9) | 1.432 | 0.232 NS |
| Mn P2 | 7 (9.2) | 9 (28.1) | 8 (3.0) | 12 (20.7) | 5.582 | 0.018 ∗ |
| Mx P2 | 3 (3.9) | 4 (12.5) | 7 (2.6) | 11 (19.0) | 0.388 | 0.533 NS |
| Mn I1 | 0 | 0 | 5 (1.9) | 6 (10.3) | – | – |
| Mx M1 | 0 | 0 | 3 (1.1) | 7 (12.1) | – | – |
| Mx C | 0 | 0 | 2 (0.7) | 4 (6.9) | – | – |
| Mx P1 | 0 | 0 | 1 (0.4) | 1 (1.7) | – | – |
| Mn P1 | 1 (1.3) | 1 (3.1) | 1 (0.4) | 1 (1.7) | 0.922 | 0.337 NS |
| Mx I2 | 4 (5.3) | 7 (21.9) | 1 (0.4) | 1 (1.7) | 9.969 | 0.002 † |
| Mn M1 | 1 (1.3) | 1 (3.1) | 0 | 0 | – | – |
| Mx M2 | 1 (1.3) | 1 (3.1) | 0 | 0 | – | – |
| Total | 32 (100) | 58 (100) | ||||
The prevalence of at least 1 absent third molar in the Class II Division 2 group (42.1%, n = 32/76) was significantly higher than in the control group (26.7%, n = 72/270) according to the chi-square test (chi-square value = 6.724, P <0.05) ( Table I ); the odds ratio was 2.00 (95% CI, 1.18-3.40).
The prevalences of third molar agenesis in the maxillary and mandibular arches were higher in the Class II Division 2 group than in the control group ( Table III ).
| Class II Division 2 group, n = 76 | Control group, n = 270 | Chi-square value | P value | |
|---|---|---|---|---|
| Maxilla | 27 (35.5%) | 58 (21.5%) | 6.313 | 0.012 ∗ |
| Mandible | 19 (25.0%) | 35 (13.0%) | 6.524 | 0.011 ∗ |
| Chi-square value | 1.995 | 6.872 | ||
| P value | 0.158 NS | 0.009 † |
Compared with the Class II Division 2 group, the prevalence of third molar agenesis was higher in the maxillary arch than in the mandibular arch in the control group ( Table III ).
The prevalence of third molar agenesis was significantly higher in the Class II Division 2 group than in the control group for the subgroup lacking all 4 third molars ( Table IV ).
| Group | Number of absent third molars in each subject | |||
|---|---|---|---|---|
| One | Two | Three | Four | |
| Class II Division 2 group, n = 76 | 8 (25.0%) | 12 (37.5%) | 5 (15.6%) | 7 (17.1%) |
| Control group, n = 270 | 25 (34.7%) | 30 (41.7%) | 9 (12.5%) | 8 (11.1%) |
| Class II Division 2 vs control | χ 2 = 0.110 P = 0.740 NS |
χ 2 = 1.217 P = 0.270 NS |
χ 2 = 1.609 P = 0.205 NS |
χ 2 = 5.582 P = 0.018 ∗ |
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