Introduction
To achieve proper occlusion, practitioners must consider tooth-size discrepancies between the jaws. Previous studies have shown considerable differences in tooth sizes between sexes, ethnicities, and malocclusion categories. The aim of this study was to compare mean tooth-size statistics between these groups, specifically determining a maxillary or a mandibular excess tooth-size discrepancy in clinically relevant cases.
Methods
This study involved 306 subjects of varying sex, ethnicity, and malocclusion category, randomly chosen from the treatment population of the orthodontic clinic at the New Jersey Dental School, University of Medicine and Dentistry of New Jersey. The prevalence of discrepancies (±1 and 2 SD) between all groups and within groups was measured.
Results
Fifty percent of the subjects had anterior Bolton tooth-size discrepancies, and 41% had overall Bolton tooth-size discrepancies of ±1 SD. Tooth-size ratios compared with analysis of variance (ANOVA) showed no significant correlation between and among the sexes, ethnicities, and malocclusion groups. Compared with Caucasian and Hispanic patients, African-American patients had significantly greater odds of having a clinically significant (±2 SD) anterior ratio. When we compared the numbers of subjects above or below the clinically significant ratio, there was equal distribution of maxillary and mandibular excess in Class II and Class III patients. Caucasian and African-American patients had equal distributions of maxillary and mandibular excess, whereas Hispanic patients displayed a higher bias toward mandibular excess.
Conclusions
Tooth-size discrepancies are common in orthodontic populations and are evenly distributed among sex, ethnicity, and malocclusion category, with some exceptions.
When completing orthodontic treatment, specific dimensional relationships must exist between the maxillary and mandibular teeth to ensure ideal conditions of interdigitation, overjet, and overbite. For proper occlusion, the teeth must be proportional in size. Although natural teeth match well in most patients, approximately 5% of the population has some disproportion among the sizes of individual teeth. This is defined as a tooth-size discrepancy. Patients with interarch tooth-size discrepancies typically require special finishing steps, such as removal (interproximal reduction) or addition (composite buildups or porcelain veneers) of tooth structure to correct this discrepancy so that the teeth interdigitate properly.
Practitioners should be concerned with tooth-size discrepancies because of their high incidence in orthodontic patient populations. Bolton reported tooth-size discrepancies greater than ±1 SD in 29% of the patients in his private practice, and Richardson and Malhotra reported similar discrepancies in 33.7% of their patients. Crosby and Alexander stated that a tooth-size discrepancy had to be greater than ±2 SD—eg, 3 to 4 mm of deviation—to influence orthodontic treatment. In their study, 22.9% of the subjects showed anterior ratios that significantly deviated in this amount from the Bolton analysis mean. Freeman et al found that 30% of their 157 subjects had an anterior tooth-size discrepancy ratio greater than ±2 SD from the Bolton mean. In a more recent study, Santoro et al reinforced the findings of Crosby and Alexander, observing that 28% of Dominican Americans had a discrepancy greater than ±2 SD.
Although most of these studies noted statistical significance regarding prevalence of tooth-size discrepancies of ±2 SD, the studies rarely specified whether the subjects were above or below the chosen standard deviations; this isolates the discrepancy to the arch in question and aids in proper treatment. If the subject’s mean is above the set standard deviation, the discrepancy is a mandibular excess, and if it is below the standard deviation, it is a maxillary excess.
Sex differences have been reported in the literature and can have clinical relevance. According to Seipel there are fewer sex differences in the deciduous dentition than in the permanent dentition. Men’s teeth are generally recognized to be larger than women’s. In both the deciduous and permanent dentitions, the maxillary canines and central incisors have the greatest sex differences, whereas the maxillary lateral incisor and the mandibular central incisor are the most homogenous. Agreement is lacking regarding sex differences in relation to the tooth-size proportions between the maxillary and mandibular anterior teeth.
There are also tooth-size differences among various ethnic groups, and it is reported that people with African ethnic backgrounds have larger teeth than do Caucasians. Studies including Hispanic populations reported significant differences in relation to Caucasians but similarities to African-Americans. Currently, there is little information regarding tooth-size discrepancy prevalence among ethnicities. Sameshima found that the frequency of subjects with greater than 2 SD of tooth-size discrepancy was not significantly different among ethnic groups or sexes, but that the overall tooth-size ratio for Africans was significantly larger than that of Asians and Hispanics. Meanwhile, no significant differences among the groups in the anterior ratio were found, and the discrepancy was attributed to larger mandibular posterior teeth.
Recently, a correlation among tooth-size discrepancies and malocclusion groups was reported. Lavelle studied subjects for anterior tooth sizes and showed a tendency for Angle Class III patients to have smaller maxillary teeth compared with those classified as Class I or Class II. Moreover, the study indicated that mandibular teeth were larger in Class III than in Class I and Class II subjects, with the inference that a Bolton discrepancy is greater in Class III patients than in the other malocclusion groups. Sperry et al, studying the prevalence of the tooth-size discrepancy in malocclusion groups, found that Class III subjects showed greater mandibular tooth-size excess than did the Class I and Classs II groups. Xia and Wu found no significant difference for tooth-size ratios between malocclusion groups and a normal occlusion group, whereas Qiong and Jiuxiang compared 5 malocclusion groups and reported no statistical differences between groups, but they noted a tendency toward a Bolton discrepancy in the Class II and Class III malocclusions. Cua-Benward et al studied the prevalence of missing teeth in various malocclusion groups. They found a greater prevalence of maxillary tooth deformities in Class III patients and more mandibular tooth deformities in Class II subjects.
In this study, we aimed to compare mean statistics between the sexes, malocclusion groups, and ethnicities in the overall tooth ratio and the anterior ratio in patients at an orthodontic clinic. We explored the prevalence of significant discrepancies (±1 and 2 SD) between and within groups, and specified whether the discrepancies were above or below the chosen standard deviations. We investigated the correlation between malocclusions and the prevalence of such maxillary or mandibular excess discrepancies.
Material and methods
Three hundred six pretreatment dental casts of patients in treatment at the orthodontic clinic at the New Jersey Dental School were selected and evaluated based on predetermined exclusion and inclusion criteria. This study was approved by the institutional review board of the University of Medicine and Dentistry of New Jersey. Once the dental casts were deemed acceptable for the study, coincident lateral cephalograms and patient demographics pertaining to sex and ethnicity were obtained and recorded, and the remaining criteria were evaluated.
The selection criteria were (1) good-quality pretreatment models; (2) complete permanent dentition from first molar to first molar in both arches; (3) no tooth deformities; (4) no partially erupted teeth; (5) no size alterations of teeth; (6) no mesiodistal and occlusal abrasion, caries, or Class II restorations; and (7) equivalent dental and skeletal classifications.
The rejection criteria were (1) gross restorations, buildups, crowns, onlays, Class II amalgams, or composite restorations that affected the tooth’s mesiodistal diameter; (2) congenitally defective or deformed teeth; (3) interproximal or occlusal wear to teeth; and (4) congenitally missing teeth or any missing permanent tooth from first molar to first molar.
Our subjects included 182 female and 124 male patients. Malocclusion category was assigned based on coincident Angle classification (Class I, Class II, or Class III, based on occlusal relationship), and skeletal relationship was based on the Steiner ANB angle: Class I, 0 <ANB <5°; Class II, ANB >5°; and Class III, ANB <0°.
The sample consisted of 146 Class I subjects, 94 Class II subjects, and 66 Class III subjects. There were 62 African-Americans, 90 Caucasians and 154 Hispanics. Ethnicity was recorded based on patient’s response.
A digital caliper accurate to 0.01 mm was used for measurements. On each patient’s dental cast, each tooth from the maxillary and mandibular right first molar to the left first molar was measured at the largest mesiodistal dimension and recorded (to nearest 0.01 mm) on an Excel spreadsheet (Microsoft, Redmond, Wash) with the patient’s sex, malocclusion category, and ethnicity.
Each cast and tooth was measured by the same examiner (T.S.), who was calibrated by comparing measurements of 10 randomly chosen models with the measurements of the principal investigator (R.S.J.), who was considered an expert measurer. After every 100 models measured, the examiner and principal investigator again measured 10 randomly chosen models to assess accuracy and calibration. Several steps were taken to assess measurement agreement between the examiner and the principal investigator, who was considered the expert. A Pearson correlation coefficient was determined for the 2 examiners. One set of measurements was used as the dependent variable and the other set as the independent variable for a linear regression analysis; 95% CIs were constructed around the slope to determine whether it was significantly different from 1, and 95% CIs were constructed around the intercept to assess whether it was significantly different from 0.
Once all data had been collected, the Bolton ratio was computed for each subject as follows:
-
(sum of mandibular 12/sum of maxillary 12) × 100 = overall ratio (%)
-
(sum of mandibular 6/sum of maxillary 6) × 100 = anterior ratio (%)
To compare the study sample means for anterior and overall ratios with the sample means from Bolton’s original study population, large-sample z tests for equality of means from 2 independent samples were used. A 3-factor analysis of variance (ANOVA) was used to assess the association between the anterior ratio and the overall ratio and ethnicity, and the malocclusion category and sex to account for the unequal sample sizes.
To determine the percentage of tooth-size discrepancies in these groupings, a Bolton discrepancy of ±2 SD (using the original Bolton standard deviation) was defined as clinically significant and therefore abnormal. Frequencies of discrepancies ±1 and ±2 SD in each grouping were calculated and chi-square tests performed in each group to assess statistical significance. Whether these clinically significant subjects had a maxillary or mandibular excess was determined.
Logistic regression was used to determine the odds ratios (OR) and 95% CIs for the association of ethnicity categories and the anterior and overall Bolton discrepancies; for the anterior and overall Bolton discrepancies, the outcomes were categorized as normal vs abnormal at 1 and 2 SD. All analyses were completed by using SAS software (version 9.0, SAS Institute, Cary, NC), with significance set at alpha <0.05.
Results
The Pearson correlation coefficient between the examiner’s and the expert’s measurements was 0.999, indicating a strong linear relationship. The estimated slope and 95% CI from the regression equation for the examiner’s and the expert’s measurements was 0.997 (0.987, 1.009), suggesting that the slope was not significantly different from 1. The intercept and the 95% CI for the regression analysis was .042 (–.112, 0.053), indicating that the intercept was not significantly different from 0. Therefore, the measurements between the examiner and the expert had strong agreement.
The anterior and overall Bolton ratio data for the entire study population and by sex, ethnicity, and malocclusion category are given in Tables I and II . The mean anterior ratio of 77.5 (±2.67 SD) was not significantly different from the originally reported Bolton anterior ratio of 77.2 (±1.65 SD; P = 0.266); the mean overall anterior ratio of 91.6 (± 2.37 SD) was not significantly different from the original reported Bolton overall ratio of 91.3 (±1.91 SD; P = 0.303). There was no significant difference for anterior ratio or overall ratio by sex, ethnicity, or malocclusion category for the study sample based on the 3-factor ANOVA.
| n | Anterior ratio | Overall ratio | |||
|---|---|---|---|---|---|
| Mean (SD) | P value | Mean (SD) | P value | ||
| Ethnicity | |||||
| Caucasian | 90 | 77.52 (2.62) | 0.97 | 91.68 (2.22) | 0.05 |
| African-Americans | 62 | 77.44 (3.21) | 92.13 (2.57) | ||
| Hispanic | 154 | 77.55 (2.48) | 91.29 (2.35) | ||
| Sex | |||||
| Male | 124 | 77.75 (2.69) | 0.18 | 91.80 (2.30) | 1.10 |
| Female | 182 | 77.37 (2.66) | 91.42 (2.41) | ||
| Malocclusion class | |||||
| I | 146 | 77.64 (2.85) | 0.50 | 91.63 (2.34) | 0.91 |
| II | 94 | 77.30 (2.43) | 91.52 (2.32) | ||
| III | 66 | 77.57 (2.63) | 91.52 (2.54) | ||
| Ethnicity | Malocclusion class | n | Anterior ratio Mean (SD) |
Overall ratio Mean (SD) |
|---|---|---|---|---|
| African-American | I | 35 | 77.40 (3.48) | 91.96 (2.45) |
| II | 16 | 78.07 (3.14) | 92.75 (2.73) | |
| III | 11 | 76.67 (2.38) | 91.76 (2.81) | |
| Caucasian | I | 44 | 78.34 (2.82) | 92.16 (2.25) |
| II | 26 | 76.50 (2.18) | 91.07 (2.19) | |
| III | 20 | 77.06 (2.19) | 91.42 (2.04) | |
| Hispanic | I | 67 | 77.31 (2.43) | 91.12 (2.27) |
| II | 52 | 77.47 (2.24) | 91.37 (2.17) | |
| III | 35 | 78.15 (2.86) | 91.51 (2.77) |
As shown in Table III , 50% of the subjects in this study had anterior Bolton tooth-size discrepancies, and 41% had overall Bolton tooth-size discrepancies (greater than ±1 SD); 17% of this sample population had clinically significant anterior Bolton tooth-size discrepancies, and 12% had overall clinically significant Bolton tooth-size discrepancies (greater than ±2 SD).
| Anterior ratio | n | % | Overall ratio | n | % |
|---|---|---|---|---|---|
| <1 SD | 68 | 22.2 | <1 SD | 54 | 17.7 |
| Within 1 SD | 152 | 49.7 | Within 1 SD | 181 | 59.1 |
| >1 SD | 86 | 28.1 | >1 SD | 71 | 23.2 |
| <2 SD | 19 | 6.2 | <2 SD | 12 | 3.9 |
| Within 2 SD | 255 | 83.3 | Within 2 SD | 270 | 88.2 |
| >2 SD | 32 | 10.5 | >2 SD | 24 | 7.8 |
No statistical significance was found for prevalence of tooth-size discrepancies as a function of sex or malocclusion, except for ethnicity in relation to the anterior ratio greater than ±2 SD compared with normal ( P = 0.013) ( Tables IV and V ). In the anterior Bolton discrepancy category of ±2 SD, 29% of African-Americans 14% of Caucasians and 13% of Hispanics had clinically significant abnormal ratios. No significant difference was found when comparing all subjects above 2 SD and all subjects below 2 SD for the anterior Bolton category. However, Hispanics had a higher prevalence of the ratio greater than 2 SD as opposed to less than 2 SD, indicating mandibular excess anteriorly ( Fig 1 ).
| n | Anterior ratio (abnormal: <1 SD or >1 SD) |
Overall ratio (abnormal: <1 SD or >1 SD) |
|||
|---|---|---|---|---|---|
| % abnormal | P value ∗ | % abnormal |
P value ∗ | ||
| Ethnicity | |||||
| Caucasian | 90 | 47.8 | 0.54 | 0.22 | |
| African-American | 62 | 56.5 | 48.4 | ||
| Hispanic | 154 | 49.4 | 41.6 | ||
| Sex | |||||
| Male | 124 | 56.5 | 0.08 | 41.1 | 0.93 |
| Female | 182 | 46.2 | 40.7 | ||
| Malocclusion class | |||||
| I | 146 | 48.6 | 0.85 | 41.8 | 0.83 |
| II | 94 | 52.1 | 38.3 | ||
| III | 66 | 51.5 | 42.4 | ||
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