Frequency of Bolton Tooth Size Discrepancies Outside 2 Standard Deviation of the Bolton’s Mean Among Orthodontic Patients

Frequency of Bolton Tooth Size Discrepancies Outside 2 Standard Deviation of the Bolton’s Mean Among Orthodontic Patients
BaburAshraf Quraishi1, Sheeraz Hussain2, FaizaAnsari3, Fareeha Zeeshan4

How to CITE:

J PakDentAssoc. 2011 (4) : 250 - 253


To evaluate, the mesiodistal dimensions of permanent teeth, frequency of Bolton ratio & the presence of Bolton discrepancy outside 2 SD of the Bolton’s mean in an orthodontic population, irrespective of the type ofmalocclusion.


The study was carried out at the department of orthodontics Fatima Jinnah Dental College & Hospital, Karachi. The sample comprised of 150 pairs of good quality pre- treatment study models with fully erupted & complete permanent dentition from first molar to first molar, which were selected from the orthodontic patient pool. Dental casts having grossly carious teeth, prosthesis or anomalies of teeth were not included. The mean, range and standard deviation were calculated for the mesiodistal dimensions of the teeth, anterior ratio and overall ratio.A2- sample t- testwas used to test for the statistical difference betweenmeans.


In general there was no difference between themesiodistal dimensions of themales and females so the subgroupswere combined for further calculations.Mean overall ratiowas found to be 91.54%(SD2) and the value ranged from 86.42%to 96.70%.Mean anterior ratio was found to be 78.85%(SD 2.3) and the values ranged from 72.49%to 83.14%. This study found that 13 subjects (9.1%) had overall ratio& 22 subjects (14.7%) had anterior ratio outside 2 SDofBolton’smean.


With such a high number of significant discrepancies, it is important to routinely perform Bolton tooth size analysis before starting orthodontic treatment asTSDis prevalent among orthodontic patients


Bolton tooth size discrepancy,Bolton ratio, tooth size discrepancy (TSD), orthodontic population


For proper alignment & good interdigitation at the end of orthodontic treatment, the tooth size should be in harmony in both the dental arches .According to Proffit approximately 5% of the population has some degree of discrepancy among the size of individual teeth. In 1902 G.V.Black was the first to develop means & set up tables for mesiodistal dimensions of teeth. Several other authors likeBallard,Neff, Steadman and Lundstrom after that followedBlack’s studywithmodifications

Bolton in 1958 developed formula relating the maxillary dentition to the mandibular dentition which became a gold standard in orthodontics. He took 55 white females with excellent occlusion & developed two ratios using themesiodistalwidth. The anterior ratio (77.2%+/- 1.65) was obtained by measuring the mesiodistal width from canine to canine in both arches & overall ratio (91.3 %+/- 1.91) whichwas obtained bymeasuring mesiodistal width from molar to molar in both arches. The formulas derived by Bolton are as follows:

Bolton concluded that if there is discrepancy in the tooth size ratio then it will be difficult to achieve ideal overjet, overbite and interdigitation at the end of treatment. Stifter applied all the tooth dimensions studies of that time on Class I occlusion cases and came to the conclusion that the anterior ratio is more important for excellent finishing.

Evidence exists in the literature that tooth size shows racial, ethnic & sex differences Lavelle compared Negroids, Caucasoids andMongoloids and found greater overall & anterior ratios in Negroids when compared to both Caucasoids and Mongoloids, and the overall ratio was greater inmales than in females for all the three races. Smith et al reported that the Bolton’s ratio is only applicable forwhite females&it’s not applicable forwhite males and other races. Crosby & Alexander reported a large number of orthodontic patients presented with a significantBolton tooth size discrepancy.

The objectives of this study were to identify possible sexual dimorphism in mesiodistal dimensions, evaluate anterior & overall ratios and the percentage of patients who presented with a significant tooth size discrepancy greater than 2 SD from Bolton’s means, at Fatima Jinnah Dental College& Hospital.


The data for the present study was obtained from the records taken at Fatima Jinnah Dental College. One hundred & fifty pre-treatment study casts were selected from a pool of 950 patients presenting for orthodontic treatment irrespective of their molar classification. There were 109 females ( 72.7%) & 41 males ( 27.3%) and they were selected on the basis that all permanent teeth were present from 1st molar to 1st molar in both maxillary & mandibular arches, no previous orthodontic treatment with age range of years. The dental casts were of good quality and there were no mesiodistal cavities, fillings, fractured teeth or prosthetic replacements. There were no obvious inter-proximal or occlusal wear. Casts having missing, impacted teeth or gross abnormalities were rejected

Mesiodistal dimensions of the teeth were measured using digital calipers with sharpened points (Sontax Co., Japan) up to 0.01 decimal at contact points. The mesiodistal width was obtained, by measuring the maximum distance between the mesial and distal contact points of the tooth on a line parallel to the occlusal plane, as described by Morrees . A single investigator (F.A) measured the casts and not more than 10 casts were done in a day to avoid eye fatigue. The measurements were inserted in the Bolton formula and values for the anterior and overall ratiowere obtained.

Statistical analysis:

The mean, range and standard deviation were calculated for the tooth widths. To determine gender differences independent sample t- tests were performed using Statistical Package for Social Sciences (SPSS) version 13.0. The second part of the study was to evaluate the percentage of patients who had tooth size discrepancies which were within one, two or more than two standard deviations from Bolton’s mean. The mean, range & standard deviation were calculated for both the overall ratio&anterior ratio.


Toothmeasurements & gender

Table I reports the mean, range & standard deviation of the mesiodistal width of maxillary & mandibular teeth, from central incisor to 1st molar in both males & females. In this sample the mesiodistal dimensions of teeth for males were slightly larger than that of females but no sexual dimorphism was observed, hence both male & female subgroups were combined to evaluate the Bolton ratio.

Anterior and Overall ratio:

Table 2 summarizes the mean overall ratio, which was found to be 91.54 with a standard deviation of 2.0. The value ranged from a low of 86.42 to a high of 96.70 with the median as 91.55. The mean anterior ratio for the orthodontic sample was found to be 77.85 with a standard deviation of 2.3.The values ranged from72.49 to a high of 83.14.The overall&anterior ratioswere also compared to the Bolton’s original sample, though readings of the present studywere slightly higher for both the ratios but it was statistically insignificant.
91.30 is Boltonmean
89.39- 91.29&91.31- 93.21within 1 SD
87.48- 89.38&93.22- 95. 12within 2 SD
<87.47&>95.13 outside 2 SD.

TSD&itsClinical significance:

Table 3 gives the frequency of the tooth size discrepancies of 1, 2 and more than 2 SD from the Bolton mean for the anterior and overall ratios. . Of the 150 cases 13 (9.1 %) cases had overall ratios outside the 2 SD from theBolton’s mean (fig 1). About 22 (14.7%) subjects had anterior ratio that fell outside of 2 SD from the Bolton’s mean value (fig 2).

Comparison of present studywith other populations:

Table 4 compares the anterior and overall ratios of the present study with the other studies done exclusively on subjects selected irrespective of the type ofmalocclusion.

In this study, 150 study models were selected out of 950 patients meeting the inclusion criteria. Although the means of the present study and that of Bolton study were similar the ranges and standard deviations of the present study are significantly larger. This result has been supported by other studies and can be due to the fact that in our study all the malocclusions were present in contrast to Bolton who selected only excellent occlusion subjects. Bolton suggested that values outside 1SD from his reported mean requires consideration but many authors have considered a threshold of 2 SD to be a clinically significantBolton discrepancy.Again the reason for this difference of selection for standard deviation is that Bolton used cases with excellent occlusion and the other studies used subjectswithmalocclusion.

In a normally distributed population, 5% of subjects would fall 2 SD from the mean. The present study found that 9.1 % of the sample had overall tooth width ratios greater than 2 SD (5%greater than – 2 SD&4.1%greater than +2 SD) from Bolton’smean (fig 1) and similar results were found by others . The anterior ratio in the present sample was 14.7% greater than 2 SD (6%greater than – 2 SD & 8.7 % greater than + 2 SD) from Bolton’s mean. The results of the present study for the overall ratio were similar to Al- Omari , less than that of other studies and higher than Othman and Harradine . When the anterior ratio of the present studywas compared to the other studies, itwas found to be the least.Almost all studies had a higher percentage of TSD for anterior ratio.The reason for variation in results for these studies is variation in the composition, selection and number of examiners doing the measurements. Freeman et al1 had 24 different examiners and this can introduce errors in measurement as inter-examiner errors were not reported by them. Although Othman et al did perform intra and inter-examiner systematic and random errors but their sample sizewas very small having only 40 dental students from University of Malaya. The results from such small sample size may not be a true reflection of the Malaysian population, as a very high percentage (47.5 %) of the dental subjects had anterior TSD outside the 2SD Bolton mean.

The current study found no gender differences and this trend is supported by other studies , the sample was selected with strict selection criteria having subjects of Pakistani origin therefore representative of all malocclusions for our population. The findings of the present study suggest that a large number of orthodontic patients have a Bolton tooth size discrepancy outside of Bolton 2 SD. This may influence treatment goal and results so Bolton tooth size analysis should be performed on all patients prior to initiation of orthodontic treatment rather than only for cases with small lateral incisors or premolars as reported by Proffit.


About 14.7 % of the orthodontic patients had anterior tooth size discrepancy and 9.1%had overall tooth size discrepancy greater than Bolton 2 standard deviations. It is important to routinely perform Bolton tooth size analysis for all patients for good interdigitation &results at the end of treatment


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