INTRODUCTION

In contrast to the progress that has been made in recent decades in the field of orthodontic treatment, especially in fixed appliances, diagnostic aspects have not undergone such extensive development. Essential diagnostic elements such as tooth-size harmony and cephalometric analysis have not been the focus of research, and this has led to few studies being published in this field.

The proportional relationship between the sizes of upper and lower teeth is accepted as an important index by which the orthodontist can determine the possible functional and esthetic limits of the treatment, especially regarding the finishing phase.1 The early pioneers of orthodontics realized the importance of the harmonious relationship between the teeth in the same arch and between arches,2 and recent studies have demonstrated interarch tooth-size discrepancy for various groups of malocclusion.3–5

More than a century ago, Black6 developed tables for average tooth sizes. In the research that followed, different approaches were developed for interarch tooth-size analysis and harmonious values.7–12 Most of these previous studies were performed on casts of Caucasians or regardless of the ethnicity or sex of the subjects.

Subsequent investigations such as the Michigan University study,13 the study by Sanin and Savara14 or a Chinese study5 differentiated these analyses by sex. Smith et al,15 who examined the validity of Bolton ratios for different ethnic groups, recently concluded that Bolton's ratios apply only to white women and should not be applied indiscriminately to white men, blacks, or Hispanics. Thus, the generalized use of the Bolton analysis and the proposed values for a harmonious dentition are under discussion and might not be valid for other populations.

The aim of this study, which is part of the national plan for standardizing international norms in the field of medicine and dentistry in Syria, was to calculate both the anterior and overall ratios for a sample of harmonious Syrian dentitions and to compare these ratios with the original data from Bolton2 and the Michigan studies13 that differentiated the analysis by sex.

MATERIALS AND METHODS

Plaster models of 55 patients (35 male subjects, 20 female subjects, ages 11–22 years) were randomly chosen from patients at the Department of Pediatric Dentistry, Damascus University, according to the following inclusion criteria:

• Neutral occlusion of the permanent dentition (Angle Class I molar and canine relationship),

• Harmonious overjet and overbite (2 ± 0.5 mm),

• Caries/filling-free teeth and no other reduction of mesiodistal tooth width,

• No extractions.

In 48 patients, orthodontic treatment had already been completed.

The mesiodistal widths of each tooth from incisors to the first permanent molars were measured in both arches in two separate measurements with vernier calipers (0.02-mm accuracy) according to the method described by Seiple16 and Moorrees and Reed.8 The mean value of the two measurements was used in further calculations.

In the statistical data analysis (PC/SPSS 10.7), the anterior ratio (Σ width of six lower anterior incisors/Σ width of six upper anterior incisors × 100) and overall ratio (Σ width of lower 12 teeth/Σ width of upper 12 teeth × 100) were calculated according to Bolton.2 The individual data were summarized as ranges and mean values of these ratios. The data were checked for normal distribution. Variation was analyzed as coefficients of variation, standard deviations, and standard errors of the mean values.17 These data were compared with results from Bolton2 and the Michigan University studies.13

RESULTS

In the Syrian sample, the ratios of the upper and lower incisors were normally distributed, and they ranged from 75.4 to 83.00 with a mean value of 78.99 and a low standard deviation of 2.18 (Table 1). The overall ratio ranged from 88.15 to 95.90 with a normal distribution and a mean value of 92.26 ± 2.06, also indicating low variability.

TABLE 1.  Range, Mean Values, and Measurements of Variability for the Anterior and Overall Ratio of Upper and Lower Teeth Ac cording to the Bolton Analysis

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The mean values for the anterior and overall ratios for male and female subjects were very similar and did not differ significantly (P > .48, Table 2). Thus, the norm values for the Syrian population would be 78.99 ± 2.18 for the anterior ratio and 92.26 ± 2.06 for the overall ratio.

TABLE 2.  Arch Length in Millimeters for Maxillary/Mandibular Front (3–3) and Dentition (6–6), and Anterior and Overall Ratio in the Syrian Sample by Sex

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The comparison with the original data from Bolton showed minimally higher ranges and mean values for the anterior and overall ratio (Table 3). The degree of variation (standard deviation, standard error, coefficients of variation) was nearly equal in both populations.

TABLE 3.  Comparison of the Anterior and Overall Ratio in the Syrian Sample, the Original Data from Bolton,² and a Sex-Specific Analysis from the Michigan University Study¹³

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The sex-specific analysis performed in the Michigan University study13 also demonstrated no relevant differences between male and female subjects (Table 4). The mean values of the anterior ratio in the Michigan University study were low compared with the results of the Syrian sample and Bolton's data, whereas the overall ratios in all three studies were very similar.

TABLE 4.  Comparison of the Anterior and Overall Ratio in the Syrian Sample, the Original Data from Bolton,² and a Sex-Specific Analysis from the Michigan University Study¹³

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The standard deviations in the Michigan University study were relatively high compared with Bolton's or the Syrian data.

DISCUSSION

The narrow range of values in the Syrian sample demonstrates low variability. This might be attributed to the strict selection of harmonious casts and to the high accuracy of the vernier calipers (0.02 mm) used in this study. The low degree of variation between the highest and lowest values for the proportions of tooth size between lower and upper teeth allows the definition of values for a harmonious dentition. This confirms the findings of Freeman et al,3 who showed that a large percentage of the orthodontic patients had discrepancies in upper and lower tooth size greater than two standard deviations from Bolton's values. Other studies in the United States and China45 also detected considerable tooth-size discrepancies in various groups of malocclusion. Therefore, these studies suggest the inclusion of Bolton's tooth-size analysis before orthodontic treatment planning.

The data from the Syrian sample—representing an Arabian population—are similar to Bolton's original data2 from an American population. This is quite surprising because the Michigan University study13 found lower anterior ratios for another American sample, and Smith et al15 detected variation for different ethnic groups within the United States. On the other hand, the absolute magnitude of the ethnic differences in the mean overall ratio (whites 92.3%, Hispanics 93.1%, blacks 93.4%) exists despite their relatively low statistical significance.15 The mean overall ratio in American whites15 and the Syrian sample were identical, indicating the generalized application of the Bolton analysis from Caucasian samples to an Arabian population.

In summary, this study indicates that the analysis and the original data from Bolton can also be used on an Arabian or at least a Syrian population. It also confirms the results of the Michigan University study13 and a Chinese study5 that no relevant sexual dimorphism exists and that these harmonious values are valid for both male and female subjects.

CONCLUSIONS

This study, which applied Bolton's tooth-size analysis to a sample of harmonious Syrian models, found values similar to the original data of an American population. Therefore, the analysis of and ideal values for a harmonious dentition developed by Bolton can also be used on an Arabian or at least a Syrian population.