INTRODUCTION

Hypodontia is one of the most common dental anomalies in permanent dentition.1–4 It is a dental disability that affects a patient's dental functions and esthetics.5 When describing the phenomenon of congenitally missing teeth, hypodontia is the term most frequently used.6 Another term that appears in the literature to describe a reduction in the number of teeth is oligodontia. Oligodontia is a condition in which six or more teeth are congenitally missing, excluding third molars.7

The prevalence of hypodontia varies from 2.63% to 11.20%, depending on the race of the orthodontic population.8–10 Mild hypodontia is relatively common (absence of one to five teeth), but severe hypodontia (oligodontia, or the absence of six or more teeth) is more rare. The prevalence of hypodontia and oligodontia in the Turkish orthodontic population was reported as 2.63% and 0.13%, respectively.9 Hypodontia is the most common dental anomaly in the Turkish orthodontic population.9

Several studies have examined the relationship between hypodontia and craniofacial morphology. Most authors2,3,7,11–25 found that patients with hypodontia had significant differences between the normal population in the sagittal dimension; however, no significant differences were found by other authors.26–29 Some authors found that patients with hypodontia had a smaller ANB angle and were susceptible to Class III malocclusions.3,12,13,15,17,18,25,30 On the other hand, some authors have found no significant differences in the relationship between the maxilla and the mandible.2,16,26,28 In addition, Dermaut et al.14 reported that the prevalence of Class I skeletal relationships was significantly higher in the hypodontia group than in the control group.

In the vertical dimension, some authors found the anterior face height significantly shorter in hypodontia patients,13,25 while others found no difference between hypodontia patients and the normal population.2,3,14,15,17,18,26–28 The mandibular plane angle was found to be significantly smaller in hypodontia patients by several authors2,12,13,15,25,27,30; however, others have found no difference between hypodontia patients and the normal population.14,17,26,28 In addition, Chung et al.18 found a significant increase in the maxillomandibular plane angle in hypodontia patients.

The severity of hypodontia is an important factor in craniofacial morphology. However, only a few authors have investigated the effects of severity of hypodontia on craniofacial morphology.12,15,17,27,30

Besides the severity, the locations of the missing teeth are also an important factor on craniofacial morphology. Endo et al.3 found different morphologic features between patients with missing teeth in the anterior region and patients with missing teeth in the posterior region. However, to the best of our knowledge, there is no study in the literature evaluating the effect of the location of missing teeth on jaws (on maxilla or mandible). This situation and the conflicting results obtained by these studies suggest the need for well-designed controlled studies to evaluate clearly the effects of severity of hypodontia and the location of missing teeth on craniofacial morphology.

Therefore, the aim of this study was to evaluate the effects of severity of hypodontia and the location of missing teeth on craniofacial morphology.

MATERIALS AND METHODS

This study was performed with a hypodontia patient group and a control group with all teeth other than the third molars present. All hypodontia patients were examined in Suleyman Demirel University, Faculty of Dentistry, Department of Orthodontics, between 2005 and 2009 by the same examiner. Hypodontia was diagnosed by radiographic and clinical observations.

The inclusion criterion was:

• having two or more congenitally missing teeth.

The exclusion criteria were:

• having any associated syndrome;

• unexplained, previous loss of teeth;

• caries or restorations on mesial or distal surfaces;

• unerupted or partially erupted teeth; and

• having previous orthodontic treatment.

Cephalometric radiographs of 154 patients (98 girls, 56 boys) (mean age: 13.28 ± 3.92 years; range 9–21 years) with two or more congenitally missing teeth (excluding third molars) were obtained. Patients were divided into two groups according to the severity of their hypodontia. Group I (mild) consisted of 118 patients (78 girls, 40 boys) with two to five missing teeth. Group II (severe) consisted of 36 patients (20 girls, 16 boys) with six or more missing teeth.

The control group (group III) consisted of a selection of patients from the University of Suleyman Demirel, Faculty of Dentistry, Department of Orthodontics, who had Angle Class I relationship with no missing teeth (excluding third molars). Group III consisted of 50 patients (31 girls, 19 boys) with mean age of 14.26 ± 1.90 years.

The patients with hypodontia were divided into three groups according to the location of their missing teeth in the dental arches. Group I (anterior) consisted of 45 patients (31 girls, 14 boys) with missing teeth in only the anterior region (incisors and canines). Group II (posterior) consisted of 57 patients (37 girls, 20 boys) with missing teeth in only the posterior region (premolars and molars). Group III (anteroposterior) consisted of 52 patients (30 girls, 22 boys) with missing teeth from both the anterior and posterior regions.

The patients with hypodontia were also divided into three groups according to the location of missing teeth between the jaws. Group I (maxilla) consisted of 37 patients (18 girls, 19 boys) with missing teeth in only the maxilla. Group II (mandible) consisted of 32 patients (28 girls, 4 boys) with missing teeth in only the mandible. Group III (maxilla-mandible) consisted of 85 patients (52 girls, 33 boys) with missing teeth in both the maxilla and mandible.

Before the study, all subjects gave their informed consent after receiving an explanation of the aim of this study. The study protocol was approved by the Ethical Committee of the University of Suleyman Demirel, Faculty of Medicine.

Lateral cephalograms of the patients were taken when the teeth were in centric occlusion. Anatomic landmarks were identified on tracing paper and transferred to a personal computer. Twenty-one angular and linear measurements were performed on the lateral cephalograms. Measurements were calculated using the Dolphin version 9 software package (Dolphin Imaging and Management Solutions, Chatsworth, Calif).

Thirty of the cephalometric films were selected randomly, and measurements were repeated after 3 months to estimate the repeatability of the measurement technique. The correlation values did not reveal any systematic measurement error (correlation coefficients were over 0.990).

RESULTS

The cephalometric comparison of the groups according to severity of hypodontia is presented in Table1. Anterior (P < .001) and posterior (P < .05) face heights were shorter in each hypodontia group than in the control group. There were no significant differences in anterior cranial base and midface lengths (P > .05).

Table 1. Cephalometric Comparison of the Groups According to Severity of Hypodontia

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There were no significant differences in SNA, SNB, or ANB angles among the groups (P > .05). Ramus height was shorter in the hypodontia groups than in the control group (P < .01). The SN-GoGN angle was significantly smaller in the severe hypodontia group than in the control group (P < .05).

Each hypodontia group showed more retrusive upper (P < .01) and lower (P < .001) incisors than the control group. In the severe group, excessive retrusion of the lower incisors was found (P < .001). In the hypodontia groups, the interincisal angle was greater than that of the control group (P < .05).

No significant difference was found in soft tissue measurements except for the soft tissue convexity angle. The soft tissue convexity angle was significantly smaller in the severe hypodontia group than in the control group (P < .05).

A cephalometric comparison of the groups according to the location of missing teeth in the dental arches is presented in Table2. Significant differences were found in anterior and posterior face heights, ramus height, interincisal angle, SN-GoGn (degrees), L1-NB (degrees), L1-NB (mm), and upper lip-E plane measurements between the hypodontia groups and the control group (P < .05).

Table 2. Cephalometric Comparison of the Groups According to Location of Missing Teeth in the Dental Arches

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A cephalometric comparison of the groups according to the location of the missing teeth between the jaws is presented in Table3. Significant differences were found in anterior and posterior face heights, ramus height, interincisal angle, U1-NA (mm), L1-NB (degrees), L1-NB (mm), and upper lip-E plane measurements between the hypodontia groups and the control group (P < .05). There were no significant differences between the hypodontia groups, except for upper lip-E plane measurement. Upper lip-E plane length was significantly greater in the mandible group than in the maxilla group (P < .01).

Table 3. Cephalometric Comparison of the Groups According to Location of Missing Teeth in the Jaws

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DISCUSSION

Although genetic regulation of odontogenesis has recently been studied, the etiology of hypodontia is still unknown.28 Hypodontia can be regarded as a multifactorial condition, and it is a very common and relatively nonspecific finding in multiple congenital anomaly syndromes, eg, Down syndrome, ectodermal dysplasias, and Wolf-Hirschhorn syndrome.31,32 Since these syndromes can severely affect craniofacial morphology, in this study we excluded the patients who had any associated syndrome. The aim of this study was to evaluate the effects of severity and location of nonsyndromic hypodontia missing teeth on craniofacial morphology.

Many of the authors found that patients with hypodontia had smaller ANB angles than the control group.3,12,13,15,17,18,25,30 However, others have reported that hypodontia had little or no effect on the ANB angle.2,16,26,28 We have found that patients with hypodontia have insignificantly smaller ANB angles than the control group, and the effect of hypodontia increased in patients with severe hypodontia. Lack of posterior teeth might have forced the mandible to rotate forward. A cephalometric comparison of the groups according to the location of missing teeth in the dental arches showed no significant differences in ANB angles. Similar to these findings, Yüksel and Üçem26 found no significant differences between these measurements. On the contrary, Endo et al.3 reported a significantly smaller ANB angle in the anteroposterior missing teeth group compared with the control group.

Some authors showed that patients with hypodontia had a smaller SNA angle2,15,17 and a larger SNB angle3,12,13,27 than a healthy control group. However, others have reported that hypodontia had little or no effect on SNA3,12–14,16,18,25–28,30 or SNB14–18,25,26,28,30 angles. Similar to these studies, we have found no significant difference in SNA and SNB angles. Different results between the studies may be due to adolescent's period of growth and the large age range of patients. There were no significant differences found between the anterior missing teeth group and the posterior missing teeth group. These results were in accordance with other studies.3,26

The finding of significantly smaller SN-GoGN angles and shorter anterior and posterior face heights in the hypodontia groups was in agreement with previous studies.2,12,13,15,25,27,30 However, some authors showed no significant differences between the groups on SN-GoGN angle.14,17,26,28 We suggest that the reduction in SN-GoGN angles and shorter anterior and posterior face heights is related to a developmental deficiency of alveolar bone due to the absence of the teeth. When teeth fail to develop, so does the associated alveolar bone, and this could result in skeletal discrepancies. In the present study, we also found that patients with hypodontia had more retrusive upper and lower incisors. Most authors were in agreement with these findings,3,13,15,17 while some were not.18,28,30 Retrusion of the lower incisors was more excessive in the severe hypodontia group. The retrusion of upper and lower incisors could be due to retroclination of the anterior teeth to the excessive space created by the lack of permanent teeth; however, there were no significant differences found between the anterior missing teeth group and the posterior missing teeth group.

Woodworth et al.25 reported that patients with hypodontia had shorter mandibles than the control group. On the other hand, others have found no relationship between hypodontia and the size of the mandible.2,3,12–18,26–28 Similar to these studies, we found no significant difference between groups in mandibular corpus length. However, we did find a significant reduction in ramus height. This finding is consistent with the reduction in facial height, but incompatible with the anterior rotation of the mandible. Surprisingly, there were no significant differences found according to the location of hypodontia.

In this study, the hypodontia groups showed no significant differences in almost all soft tissue measurements; this finding was in agreement with several studies.17,26,28 However, Ogaard and Krogstad15 claimed that, in cases of hypodontia, the upper and lower lips were more retrusive than those of the control group. Bondarets and McDonald12 showed that patients with hypodontia had hypodivergent face types and old face views due to a reduction in anterior face height. In agreement with these studies, we found larger convexity angles in the hypodontia groups. The increase in convexity may be due to shorter anterior face heights. There were no significant differences found between the anterior missing teeth group and the posterior missing teeth group. These results were in accordance with other studies.2,3

To the best of our knowledge, this is the first study comparing the cephalometric measurements of the hypodontia patients according to the location of missing teeth between the jaws; therefore, we were unable to compare the results of this study with the results of others. There were no significant differences found between those missing teeth only in the maxilla and those missing teeth only in the mandible, except in upper lip-E plane measurements. Surprisingly, upper lips were more retrusive in patients with missing teeth in the mandible. This could be explained by larger SNB angles and longer mandibular corpus lengths, which might have caused a forward position of the chin and affect the E plane measurement.

Different distribution of the patients in subgroups was the weakness of the study. Because oligodontia is rarer than hypodontia, the hypodontia subgroup was greater than the oligodontia subgroup. On the other hand, the maxilla-mandible subgroups were twice as large as the other subgroups.

CONCLUSIONS

• Hypodontia did not cause any differences in cranial base, maxillary, or mandibular skeletal relationships. Hypodontia caused reductions in the vertical dimensions of the face.

• Smaller mandibular plane angles were found in patients with hypodontia. Hypodontia caused retrusion of the upper and lower incisors.

• Patients with hypodontia had more concave profiles than the normal population. These differences were more excessive in patients with severe hypodontia.

• Upper lips were more retrusive in the group with missing teeth from the mandible.

• Patients with congenitally missing teeth have different skeletal features. The severity and location of missing teeth have a significant effect on the craniofacial morphology of the patients.