Bolton’s Tooth Size Discrepancy in Malaysian Orthodontic Patients: Are Occlusal Characteristics Such as Overjet, Overbite, Midline, and Crowding Related to Tooth Size Discrepancy in Specific Malocclusions and Ethnicities?

Introduction

An optimal orthodontic outcome is dependent on the relative sizes of maxillary and mandibular teeth to each other which has been referred to as the “seventh key” in addition to Andrew’s six keys to optimal occlusion.^[1] An ideal ratio between upper and lower teeth was deduced by Wayne Bolton from his study of 55 excellent occlusions,^[2,3] which was 77.2% for anteriors (“anterior ratio”) and 91.3% for all teeth (“overall ratio”). Higher or lower ratio values indicate the presence of a tooth size discrepancy (TSD) between maxillary and mandibular arches. Proffit and Fields^[4] suggest tooth width discrepancy larger than 1.5 mm be included in the problem list since it may cause difficulties in treatment. Bolton suggested that a discrepancy greater than ± 1 standard deviation (SD) could create clinical problems, but Crosby and Alexander,^[5] Endo et al.,^[6] Freeman et al.,^[7] Othman and Harradine,^[8] and Santoro et al.^[9] have deduced values ± 2 SD outside Bolton’s mean to be clinically significant.

Bolton also hypothesized that a relationship existed between occlusal features such as overbite and tooth size but failed to find any significant correlation between the two in his study.^[3] However, Akyalçin et al. found significant correlation between anterior ratio and midline shift in Angle’s Class II cases. Overall ratio correlated significantly with overjet in Class I cases, overbite in Class II cases, and lower incisor inclination in Class III cases.^[10-12]

While assessing difference in ratio values among different categories of malocclusion, Crosby and Alexander,^[5] Freeman et al.,^[7] Akyalçin et al.,^[10] and Johe et al.^[13] reported no differences between malocclusion groups’ and Bolton’s ideal values which were derived from ideal occlusion samples. Anterior and overall ratio values of Class I, Class II, and Class III malocclusions were not significantly different from each other according to some authors,^[5,10,14,15] whereas others found higher mean anterior ratio values in Angle’s Class III individuals.^[16-18] Araujo and Souki^[18] also found significantly greater prevalence of tooth size discrepancies in Angle’s Class I and Class III individuals. Mandibular tooth size excess, smaller maxillary teeth, and larger mandibular teeth have also been found in Class III malocclusions.^[19-22]

Reports of ethnic variation in ratio values have been diverging with some studies showing significant differences from Bolton’s ideal in certain ethnicities^[23-27] and others with no difference.^[28-33] Studies have also found significant interethnic differences between Cacuasians, African-americans and Hispanics^[33] and between Peruvian and Spanish.^[34] African-americans were found to have the highest overall ratios, Caucasians the smallest, and the Mongoloids and Hispanics the intermediate values.^[21,33]

Since tooth size, occlusal features, and ethnicity are so inseparably interlinked, there exists a need to comprehensively investigate their interrelationships. The Malaysian population which consists of three main ethnic groups of Malays, Chinese, and Indians lacks such studies on it. Hence, we conducted this study with the objectives to (1) determine the Bolton’s anterior and overall ratio for Malaysians with malocclusion and for each malocclusion and ethnic subgroup and compare them with Bolton’s ideal values, (2) compare the anterior and overall ratios within malocclusion and ethnic subgroups, (3) assess the level of clinically significant discrepancy in anterior and overall ratios, and (4) determine if there is a correlation between occlusal characteristics, Bolton’s ratios, and ethnicity.

Materials and Methods

Pretreatment study models of 273 orthodontic patients visiting the Department of Orthodontics for treatment were assessed for inclusion in our retrospective, explorative study. Ethical approval for this study was obtained from the Institutional Research Ethics Committee. The criteria for being included were that the patients had to be Malaysian citizens and had to have available good quality pretreatment study models where all permanent teeth from first molar to first molar were present, fully erupted and with intact mesiodistal dimensions without any tooth deformities or abnormal morphology. Exclusion criteria were previous history of orthodontic treatment, any missing teeth from first molar to first molar, over-retained deciduous teeth or supernumerary teeth, proximal caries, restorations, or any abnormal tooth morphology affecting mesiodistal width of the tooth.

Based on these inclusion and exclusion criteria, 112 study models were selected and molar relationship, overjet, overbite, midlines, maxillary and mandibular crowding, and/or spacing were recorded. Mesiodistal widths of all teeth from first molar to molar in both arches were measured using the technique described by Moorrees and Reed^[35] using electronic digital calipers, Insize^© (Series 1108, Resolution 0.01 mm/0.0005”). Bolton’s anterior and overall ratio were calculated as sum mandibular 3-3/sum maxillary 3-3 and sum mandibular 6-6/sum maxillary 6-6, respectively. Three examiners (AMI, JNR, MBNN) were trained and calibrated by the expert and principal investigator (MPS). Each examiner’s measurements on 30 study models were compared with those of the expert’s using Pearson’s correlation coefficient. For intraexaminer reliability assessment, 10 study models were selected at random after 6 weeks, and a second set of measurements was obtained each examiner to compare with their first set. Every variable was recorded by all three examiners and an average of the three was recorded as the final reading. From the data collected, frequency, percentage, range, mean, SDs, and confidence intervals were calculated. One-sample t-test was to assess the difference between anterior and overall ratio values of our study and Bolton’s study, and ANOVA was used to compare differences in TSD between malocclusion groups and ethnicities. Correlation was assessed using Chi-squared test and Spearman’s rho correlation coefficient. All the statistical analyses were carried out using Epi Info™ 6.0, and statistical significance was set at P < 0.05.

Results

One hundred and twelve pretreatment study models consisting of 84 females with average age 19.77 ± 4.86 years and 28 males with average age 20.07 ± 4.45 years were assessed. The distribution of ethnic groups was 50 Chinese, 32 Indians, and 30 Malays and malocclusion groups was 46.4% Class I, 33% Class II, and 20.5% Class III [Table 1]. Interexaminer and intraexaminer reliability analyzed using Pearson’s coefficient correlation and Spearman’s rho coefficient correlation, respectively, found that measurements for all 3 examiners were highly correlated, at statistically significant levels (P < 0.05).

Anterior ratio values in our study were Class I – 77.9% ± 2.47%, Class II – 78% ± 2.5%, Class III – 77.5% ± 2.96%, Chinese – 78.1 ± 2.23, Malays – 78.1 ± 3.12, and Indians – 77.2% ±2.49%. Anterior ratio of our entire study sample differed statistically significantly from Bolton’s ideal values (P = 0.01) and so did the Chinese subgroup [Table 2].

For overall ratio, all malocclusion categories and racial groups showed values slightly lower (91 ± 1.99) than Bolton’s ideal value of 91.3%. No statistically significant differences in overall ratios within malocclusion or ethnic subgroups or between subgroups and Bolton’s ideal values were detected (P > 0.05). Clinically significant ± 2 SD discrepancy for anterior ratio was seen in 19.6% individuals and for overall ratio was seen in 8% individuals [Table 3].

Among Class I individuals, significant positive correlation between anterior ratio and overbite (r_s = 0.30, P = 0.028), overall ratio and maxillary spacing (r_s = 0.30, P = 0.038), and significant negative correlations between overall ratio and maxillary crowding (r_s = −0.34, P = 0.014) were found [Table 4 and Figure 1].

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Figure 1

Scatter plot depicting positive correlation between anterior ratio and overbite in Class I malocclusion

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In Class II malocclusion, statistically significant negative correlations were detected between overjet and anterior ratio among all ethnic groups and in overall study sample (r_s = −0.53, P = 0.001) except in the Indians in whom the overall ratio was strongly negatively correlated with overjet (r_s = −0.57, P = 0.034). Significant positive correlations between anterior ratio and midline shifts and overall ratio and mandibular crowding were found in both overall study participants and the Chinese [Table 5 and Figure 2].

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Figure 2

Scatter plot depicting negative correlation between anterior ratio and overjet in Class II malocclusion

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In Class III malocclusion, highly significant positive correlations were found between both overall (r_s = 0.56, P = 0.006) and anterior ratios (r_s = 0.55, P = 0.007) and mandibular crowding. In addition, in Malays, strong negative correlations between anterior ratio and maxillary (r_s = −0.66, P = 0.027) and mandibular spacing (r_s = −0.64, P = 0.035) were observed [Table 4 and Figure 3]. No correlation between Bolton’s anterior and overall ratio and malocclusion categories or ethnicity was detected (P > 0.05) [Table 5].

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Figure 3

Scatter plot depicting positive correlation between anterior ratio and mandibular crowding in Class III malocclusion

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Discussion

Our study investigated the occlusal traits and tooth size discrepancies in Malaysian orthodontic patients and their interrelationships. The Malaysian population consists of three mainstream ethnic groups – Malays (63.1%), Chinese (24.6%), and Indians (7.3%)^[36] However, in our study, the distribution of patients was Chinese – 44.6%, Indians – 28.6%, and Malay – 26.8% indicating the pattern of seeking private dental services in our institution among ethnicities.

Conclusions

1. The mean anterior ratio value of Malaysian orthodontic patients – 77.8 ± 2.57 – was statistically significantly higher than Bolton’s ideal whereas overall ratio was lower than Bolton’s ideal (91 ± 1.99, P > 0.05)

2. Anterior ratio of Chinese was significantly higher than Bolton’s ideal values

3. Anterior ratios were in the order of Class II > Class I > Class III, and for overall ratio, Class I > Class III > Class II. Differences between malocclusion categories were not significant

4. Among ethnicities, for both anterior and overall ratios, values were in the order of Malay = Chinese > Indians, with differences not being statistically significant

5. Clinically significant ± 2 SD discrepancy occurred in 19.6% for anterior ratio and 8% for overall ratio

6. In Class I malocclusion, significant correlations of anterior ratio were found with overbite and of overall ratio with maxillary spacing and maxillary crowding

7. In Class II malocclusion, significant correlations of anterior ratio were found with overjet and midline shift and of overall ratio with overjet and mandibular crowding

8. In Class III malocclusion, significant correlations of anterior ratio were found with mandibular crowding and spacing in maxilla and mandible and of overall ratio with mandibular crowding.