TSD and Ethnicity
In this study, in reflection of Bolton’s standard, the results show that Malay subjects have a significantly greater prevalence TSD than the Chinese or the Indians. Table 2 show that there was a very high significant interaction between ethnic group of the anterior ratio and overall ratio (p = 0.000). Post hoc test described that there is significant difference observed in anterior ratio between Malay and Chinese subjects (p = 0.001) and also very high significant differences in overall ratio between Malay and Chinese (p = 0.001) and between Malay and Indian (p = 0.029).
In comparison of Bolton standards with Malaysian subjects (Table 3), Malaysian subjects do not show significant difference in both anterior and overall ratios. However, when comparing the ethnic groups in this study with the Bolton findings, it was found that Malay subjects show a highly statistical and clinically differences (p = 0.000) in both anterior and overall ratios. Because the type of malocclusion does not predispose a patient to a TSD problem, the differences between our results of Malay mean and Bolton’s cannot be explained by the orthodontic samples used. Bolton’s anterior and overall ideal ratios were significantly smaller than of the estimates in the current study of Malay subjects, except those for Chinese and Indian subjects. This explained the fact that Bolton’s original sample which has been composed primarily of Caucasian samples and implies that the Bolton ratio is only applicable to Malaysian in general but it is not applicable to Malay population in particular.
A few other similar studies are in agreement with the findings of the present study, indicating specific tooth size standard for their populations; Spanish [15], Japanese [11], and Iranian-Azari [16] of both the anterior and overall ratios. Ta et al., [3] found that Bolton’s standard is only applicable to their Class I occlusion of Southern Chinese but not to those of Class II and Class III occlusion. Whilst Smith et al., [17] who studied was on Bolton’s applicability on three populations; Black, Hispanics and White, found that Bolton ratios apply to white females only; the ratios should not be indiscriminately applied to white males, blacks, or Hispanics. They also concluded that interarch tooth size relationships are population and gender specific. In this study we found no significant difference for gender (Table 1).
Frequencies of Bolton TSD
Proffit [18] quoted that prevalence of TSD in the general population as being 5%. Othman and Harradine [19] noted that the basis for this prevalence was not explained and maybe defined as the proportion of cases that will fall outside 2SD from Bolton’s mean ratios. Based on this definition, sample of this study consist of 20.7% of anterior ratio TSD and 4.7% prevalence of overall ratio TSD. It was found to vary among different findings of different populations.
There was higher incident of Malaysian subjects that fall beyond 2SD of Bolton anterior ratios as compared to overall ratio. Our findings are in agreement as that found by Crosby and Alexander [20] when comparing (3–3) anterior and (6–6) overall ratios, the authors found that in every malocclusion group, there is a greater percentage of patients with anterior mesial-distal TSD greater than 2SD from Bolton’s mean as compared with patients with overall discrepancies. The higher prevalence of anterior TSD seems to be quite agreeable to most authors [8, 20–23].
This is true whether we are looking at maxillary or mandibular tooth size excesses and results from the present study are in agreement with findings from Crosby and Alexander [20]. This could be explained by the fact that anterior teeth, especially incisors, have a much greater incidence of tooth size deviations. The greatest variables in mesio-distal tooth width occur in the anterior region.
In the study, it remains questionable which deviations of the overall and anterior values proposed by Bolton affect the final treatment outcome. Peg shaped lateral incisors are easily detected and express most often as TSD. Smith et al. [17] has similarly suggested that maxillary lateral incisors have previously been shown to be quite variable in size and are frequently the reason for a TSD between arches. And in this matter, aesthetic correction (crowns, veneers, etc) is often the first choice of treatment. Hidden TSD from a generalized discrepancy in tooth width between upper and lower teeth is less detectible at first sight and may cause a less favorable treatment outcome. Because the present sample consisted of patients who were in the orthodontic waiting list with high orthodontic treatment need, the presence of a larger percentage of TSD than that in Bolton’s sample seem reasonable.
It is possible that the population of patients may be more diverse in the city of Kuala Lumpur with high migration rate from all over the country concentrated in the capital of Malaysia of which majority of the center’s orthodontic patients came from. Thus, the findings of this study may also represent the whole population in the country. It would seem logical that the percentage of patients that present with TSD may be somewhat dependent on the selection process or the characteristics of the population from which the subjects are drawn as observed in this study.
Discrepancy in Millimetres as a Measure of Clinical Significance
Threshold of 2 standard deviations (SD) of Bolton’s mean ratios as clinically significance have been well accepted and documented [8, 19–22, 24]. In this present study, 20.7% of Malaysian samples fall beyond the suggested clinically significant 2 SD.
The three ethnics displayed similar trend out of their sample which fell beyond the 2 SD (19.2% of Malay, 20.4% of Chinese, 22.8% of Indians) as shown in Table 4. This figures of clinically significant are in agreement as reported by other studies [19] but the incidence of overall ratio that fall beyond 2SD in current study are found to be less in combined samples of Malaysian, in Malay and in Chinese sample than reported by this author which are between 5–14%. In this study, it was found that our sample displayed a lesser percentage of standard deviations of overall ratio in 1.9% in Malay and 3.7% in Chinese but a comparable amount as reported by [19] of 4.7% in Malaysian sample and 9.1% in Indian.
Endo et al. [24] mentioned the importance of TSD expressed in terms of both percentage and amount of millimetres required for correction. The authors quoted ‘ratios outside 2SD and TSD requiring more than 2 mm of maxillary and/or mandibular corrections are recommendable as the appropriate thresholds for clinical significance’. Othman and Harradine [19] had raised the issue of the fundamental of an absolute size of discrepancy thought to be incompatible with an acceptable occlusal fit.
Proffit suggested that 1.5 mm as a cut-off point to be clinically significant [25]. This has been accepted by Bernabe et al., [8] in his study as their limit of acceptable discrepancy. A TSD of less than 1.5mm is rarely significant, but larger discrepancies create treatment problems and must be included in the orthodontic problem list at treatment planning stage [25]. In the current study, subjects requiring more than 2.0 mm corrections of either arches were considered of clinically significant as recommended by recent studies [19, 24] as a threshold for clinically significant TSD.
Table 4–5 shows that for anterior ratio in Malaysian combined sample, higher percentage of corrections are required for maxillary deficiency (12%), indicating build ups or permanent prosthesis for increase tooth width and less correction required for mandibular excess (4%) hence tooth structure reduction like interproximal stripping are advocate.
Bernabe˘ and coworker [8] stated that the frequency of subjects with clinically significant TSD differs based on which arch is considered “normal”. If mandibular tooth-width is defined as normal, a tooth discrepancy would be described as a maxillary tooth-width deficiency or excess. From Table 4 illustrating percentage of subjects with clinically significant tooth-width discrepancies larger than 2.0 mm of Malaysian samples, it was found that in anterior as well as overall ratio, more subjects possessed a maxillary excess than other problems.
However, from Table 5 and Table 6, illustrating the clinically significant tooth width discrepancies according to ethnic group, it was found that Malay subjects present with more problem located in the maxillary arch due to maxillary excess in both anterior (26.9%) and overall (34.6%) ratios complemented by mandibular tooth deficiency in both arches (14.3% and 28.8% respectively).
As opposed to findings in Malay subjects, Chinese subjects presented with more problems in the mandibular arch, due to mandibular excess (20.4%, 29.7%) and maxillary deficiency (24.1%, 29.7%) in both anterior and overall ratios respectively (Table 4). Indian subjects on the other hand presented with equally fair distributions of all problem area with main problem of anterior ratio located 25% in the maxilla due to excess and locating overall ratio problem due to maxillary deficiency in 22.7% of subjects. McLaughlin et al. [26] found that there are more common to find an excess of tooth substance in the lower arch as found in the Chinese sample in this study. The authors mentioned that it is usual to reduce tooth mass in the lower incisors by inter-proximal enamel reduction and/or by addition of tooth mass with restorative materials in the opposing arch of upper incisors, commonly the laterals.
In the present study, high incidence of mandibular excess and maxillary deficiency in our Chinese samples could be contributed by high prevalence of Class III malocclusion found among Chinese. This is supported by Woon et al. [27] that there was a high percentage of edge to edge incisor relationship in Chinese (54%), followed by Malay (50%) whilst the Indian had 38%.
In this study we are comparing the malocclusion using BSI classifications which only relate the relationships between the upper incisors and lower incisors’ edges and found no correlation of TSD with malocclusion class (Table 2). This finding is in agreement with Laino et al. [28]. However, our study is in dispute with a study by Mulimani et al, [29] which found a significant correlation between malocclusion classes.
Malay samples displayed more maxillary tooth excess as presented in Table 4. It was reported [27] that Malay presented with the highest percentage of increased overjet problem (16.11%) as compared to Chinese (12.75%) and Indians (9.52%).
In clinical practice, clinicians often note the discrepancy of tooth size and skeletal size but seldom pay attention to TSD between maxillary and mandibular teeth. From this study, it might be reasonable for orthodontists to denote interproximal stripping or tooth extraction in the mandibular dentition for mandibular tooth size excess which is common in Class III malocclusion and in the maxillary dentition for maxillary tooth size excess, a common presentation of Class II malocclusion.
These results suggested that the Bolton analysis is important and should be considered when diagnosing, planning, and predicting prognosis in clinical orthodontics. With relatively high tooth excess with the opposing arch, it is another factor to consider for extractions decision. Bolton himself had stated the effects of extractions that can reduce the existing high anterior or overall ratios and this are supported by Saatci and Yukay [30].