INTRODUCTION

Hypodontia is one of the most common dental anomalies in the permanent dentition.12 Numerous studies have been published on the prevalence of hypodontia (third molars excluded) in various populations.3 The reported hypodontia rates range from 3.7% in an American population2 to 10.1% in a Norwegian population.4 Almost all studies have reported higher occurrences in females than in males.3–11 The majority of previous studies dealing with Caucasian populations have revealed that the most commonly congenitally missing tooth is the mandibular second premolar, followed by either the maxillary lateral incisor1212 or the maxillary second premolar.46–810 Some studies have shown that ethnicity strongly influences the prevalence of hypodontia.89 The hypodontia rate in Japanese populations reportedly ranges from 7.4%11 to 8.5%,3 and the mandibular lateral incisor is more commonly missing in Japanese than in any other ethnic groups.311

Several studies have associated hypodontia with smaller cranial base length1314 and angle,1315 more retrognathic15–18 and shorter maxilla,13141719 more prognathic mandible,131420 smaller mandibular plane141520 and sagittal jaw relationship angles,1516 straighter facial convexity,151718 greater retroclination of maxillary13151618 and mandibular incisors,131516 larger interincisal angle,131518 and shorter lower anterior facial height.1415 Some other studies have concluded that hypodontia has little or no effect on craniofacial morphology.2122

A few studies have been conducted on the effect of the pattern of hypodontia or the distribution of congenitally missing teeth in the dentition on craniofacial morphology.14171822 Wisth et al17 revealed more retrognathic and shorter maxilla and more proclined maxillary incisors in Norwegian children with up to six congenitally missing teeth and found that these deviations were independent of whether the congenitally missing teeth were located in the maxilla or in the mandible. Yuksel and Ucem22 classified Turkish orthodontic patients with tooth agenesis into three groups according to the location of missing teeth and found no statistically significant differences in craniofacial morphology between the patients with anterior tooth agenesis and those with posterior tooth agenesis. In an investigation of the effects of the distribution of congenitally missing teeth on craniofacial morphology in Israeli orthodontic patients, Ben-Bassat and Brin18 demonstrated that anterior tooth agenesis predominantly influenced craniofacial morphology compared with posterior tooth agenesis. Woodworth et al14 investigated craniofacial morphology in individuals with bilateral maxillary lateral incisor agenesis and found shorter maxillary and mandibular lengths and a tendency for forward mandibular rotation and shorter upper and lower anterior facial heights.

No literature on the association of the distribution of congenitally missing teeth with craniofacial morphology in Japanese patients was found in a PubMed search on the Internet. The purpose of the present study was to explore the relationships between hypodontia patterns and variations in craniofacial morphology in Japanese orthodontic patients.

MATERIALS AND METHODS

Subjects

A total of 50 Japanese girls with hypodontia (the total group), excluding third molars, were selected as the subjects from the files of orthodontic patients who had been treated at our clinics in The Nippon Dental University Niigata Hospital (Niigata, Japan). Boys were not included in this study so as to avoid skewing cephalometric measurements with sexual differences in craniofacial morphology. The subjects were selected on the basis of the following criteria: a dentition showing the eruption of second molars or within approximately 6 months after full eruption of all four second molars, no premature loss of deciduous teeth, no previous orthodontic or prosthodontic treatment, and no craniofacial anomalies. The mean age of the subjects was 13 years (SD 1 year 8 months, Table 1).

TABLE 1. Sample Description of Patients per Group

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Hypodontia was diagnosed by using orthodontic records, which included orthopantomograms, cephalograms, and anamnestic data. A tooth was identified as a congenitally missing tooth when there was no evidence that it had been extracted and when no mineralization of the tooth crown could be recognized on orthopantomograms.37 The anamnestic data were used as reference material to avoid wrong diagnoses. Longitudinal orthopantomograms were examined to exclude the registration of late mineralized teeth as congenitally missing teeth. A final orthopantomogram examination was performed on the subjects of 13 years of age and older. The criteria for the finals were based on the finding by Aasheim and Ogaard,7 who reported that apart from third molars no tooth had been found mineralized after the age of 12 years. Third molars were excluded from the present study. The same investigator reexamined each orthopantomogram, and a reproducibility of 100% was obtained in the identification of hypodontia.

Our hypodontia subjects (the total group) were categorized into three groups according to the distribution of congenitally missing teeth in the dental arches. The anterior group consisted of 25 patients with hypodontia in the anterior region only (incisors and canines). The posterior group consisted of 15 patients with hypodontia in the posterior region only (premolars and molars). The anterior-posterior group consisted of 10 patients with hypodontia in both anterior and posterior regions. The numbers and mean ages of patients, the distribution of patients by the number of missing teeth, and the distribution of different missing teeth in each group are shown in Tables 1–3.

Cephalometric Analysis

A single investigator prepared and assessed lateral cephalograms, which were taken with the same cephalostat and with the standardized settings. Seventeen reference points were marked, and 11 reference lines were manually drawn on each tracing paper (Figure 1). For each tracing, 26 linear and 17 angular measurements were made with a vernier type of micrometer and a protractor (Table 4). Eleven linear measurements (eg, U1e-x, U1e-y) were made by a coordinate system with the x-axis parallel to the Frankfort horizontal plane and the y-axis perpendicular to the Frankfort horizontal plane through the sella turcica (Figure 1, Table 4). The linear and angular measurements were estimated to the nearest 0.1 mm and 0.5°, respectively.

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TABLE 4. Definitions of Measurements Used

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RESULTS

The means and standard deviations for all the measurements and the level of significant difference are shown in Table 5.

TABLE 5. Craniofacial Morphology in Each Hypodontia Group and Control Group

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DISCUSSION

Some studies showed that the types of the most commonly missing teeth differed from one ethnic group to another.489 In our study, the most commonly missing tooth was the mandibular second premolar, followed by the maxillary second premolar in the total and posterior groups. These findings were consistent with the results published by the majority of other researchers who used Caucasian populations.5–810 The prevalence of mandibular lateral incisor agenesis was higher than that of maxillary lateral incisor agenesis in the total and anterior groups. This finding of ours was in disagreement with those of several other studies that found the prevalence of maxillary lateral incisor agenesis to be higher than that of mandibular lateral incisor agenesis in Caucasian populations14–71012 and was consistent with the previous studies that investigated the prevalence of hypodontia in Japanese311 and Chinese9 populations.

The findings of significantly shorter anterior and overall cranial base lengths in the total, anterior, and posterior groups were in agreement with the previous studies.1314 The significantly shorter maxillary length shown in every hypodontia group might have resulted from the interaction of anterior and posterior growth deficiency, when the ANS-y and Ptm-y dimensions were measured. In particular, the shorter maxillary length in the total and posterior groups was mainly attributed to the posterior growth deficiency caused by the significantly larger Ptm-y dimension and might have resulted from inadequate apposition to the tuberosity area in posterior tooth agenesis.13 Some other investigators demonstrated that the shorter maxillary length was mainly caused by an anterior growth deficiency in individuals with hypodontia.15–1721

None of the mandibular measurements showed any significant differences between the anterior and posterior groups and the control group, whereas the significant prognathism of the mandible was observed in the total and anterior-posterior groups compared with the control, anterior, and posterior groups. This prognathic mandible in the total and anterior-posterior groups might imply a predominant influence of the widespread distribution of congenitally missing teeth on the mandible. Yuksel and Ucem22 also observed a more prognathic mandible in the bilateral anterior tooth agenesis group than in the control group without tooth agenesis or malocclusion. Similar findings to ours were published by other researchers investigating individuals with severe hypodontia1320 and with bilateral lateral incisor agenesis.14 Therefore, not only the number but also the distribution of missing teeth should be taken into consideration when performing orthodontic treatment of patients with hypodontia.

The retroclination of the maxillary incisors in the total and anterior-posterior groups and the retroclination and elongation of the mandibular incisor in every hypodontia group were especially remarkable compared with the control group. The interincisal angle increased significantly in every hypodontia group, resulting from the retroclination of the maxillary and mandibular incisors. These dental deviations were consistent with the observations of some previous investigators13151618 and contradictory to others.2122 The retroclination of incisors found in our study might have resulted from a disturbance in a tongue-lip pressure balance and a reduced lingual support caused by anterior tooth agenesis. Differences in significance level between maxillary and mandibular incisor retroclination in each hypodontia group indicated that disturbing influences of hypodontia on dental deviations were probably greater in the mandible than in the maxilla. This could be due to a higher prevalence of lateral incisor agenesis in the mandible than in the maxilla and the compensation of mandibular incisors for the significantly prognathic mandible. The upward-positioned mandibular incisors in our present study might have been caused by the interplay of the retroclination of the mandibular incisors and the dental compensation for the prognathic mandible, resulting in a counterclockwise-rotated occlusal plane, which was a significant difference between the total and anterior-posterior groups and the control group.

The skeletal and dental deviations were more remarkable in the total and anterior-posterior groups than in the anterior and posterior groups. Moreover, the anterior and posterior groups were much the same in the degree of skeletal and dental deviations. These findings were consistent with the results obtained by Yuksel and Ucem22 and inconsistent with those reported by Ben-Bassat and Brin.18 The sample in this study, as in the study by Yuksel and Ucem,22 is based mainly on patients with only a few missing teeth, whereas the sample reported by Ben-Bassat and Brin18 was based on multiple congenitally missing teeth. The findings of Ben-Bassat and Brin,18 similar to those of Nodal et al,20 indicated that the influence of hypodontia on the skeletodental pattern became greater as the number of missing tooth increased. The inconsistency of our results with those of Ben-Bassat and Brin18 might have been because of a basic difference in the composition of the samples.

Homeobox genes have a critical role in regulating tooth and craniofacial morphogenesis. In mice, it is said that the expression of MSX 1 and MSX 2 is required for direct epithelial-mesenchymal interactions that initiate tooth formation.25 The MSX 1 homeodomain missense mutation (arg31pro) has been reported to cause selective agenesis of the second premolars and third molars in an American family.2627 Another mutation (ser105stop) in MSX 1 has been regarded as responsible for orofacial clefting and tooth agenesis in a Dutch family.28 From these different pieces of evidence of the mutation, it can be said that there is some genetic difference of the different populations in tooth agenesis. It has been reported that MSX 1–deficient mice exhibited a cleft secondary palate (a deficiency of alveolar mandible and maxilla) and abnormalities of the nasal, frontal, and parietal bones and of the malleus in the middle ear in addition to tooth agenesis,29 which might suggest that tooth agenesis was genetically related to the development of cranium and maxillary complex. These observations were in accord with our findings that craniofacial morphological deviations occurred in every hypodontia group. PAX 9 is also associated with tooth agenesis. A frameshift mutation in PAX, resulting in an alternation in the paired domain of PAX 9, is associated with autosomal dominant oligodontia, which involves the normal primary dentition and the lack of most of the permanent molars.30 A published study has shown that TGFA played a role in tooth agenesis in a Brazilian population, and statistically significant evidence was afforded to indicate that the MSX 1 and PAX 9 interact in tooth agenesis, though the interaction between MSX 1 and TGFA was not fully corroborated.31 The genetic heterogeneity for hypodontia might be responsible for the variability of the association between hypodontia patterns and craniofacial morphology.

CONCLUSIONS

• Every hypodontia group showed shorter anterior and overall cranial base lengths, shorter maxillary length, greater retroclination and elongation of mandibular incisors, and smaller interincisal angle than the control group.

• The total and anterior-posterior groups, especially, showed more prognathic mandible, greater retroclination of maxillary incisors, and a more counterclockwise-rotated occlusal plane.

• These skeletal and dental deviations were more remarkable in the anterior-posterior group than in either the anterior or posterior group.

• Anterior hypodontia exerted as much influence on craniofacial morphology as posterior hypodontia.