Prediction of width of un-erupted incisors, canines and premolars in a Ugandan population: A cross sectional study

Accurate prediction of the space available to accommodate the un-erupted canines and premolars forms an important part of an orthodontic assessment in the mixed dentition [1‐4] as it is reported to assist dental practitioners to determine the treatment options for the patients [2]. Different methods have been employed to predict space for un-erupted teeth [3‐17]. The most commonly used methods are Moyers’ probability tables [5] and the prediction equation of Tanaka and Johnston [14]. However, these methods were developed on Caucasian populations and their predictive accuracy on populations from other races is doubtful [17‐23]. Consequently, this led to development of prediction equations and probability tables for different populations [19, 20, 24‐26]. So far we have not come across any information regarding development of formulae for predicting widths of un-erupted teeth in a Ugandan population. The purpose of this study was to develop a formula for predicting the widths of un-erupted canines and premolars in a Ugandan population and to compare the predicted values with those obtained from methods developed by Moyers [5], and Tanaka and Johnston [14].

The study population comprised of 220 children aged 12–17 years (mean, 14.6 ± 3.5 years). The width of maxillary and mandibular canine and premolars was significantly higher in the boys as compared to girls (p < 0.05, t test; Table 1). There was a significant correlation between the mandibular incisors, and sum of the maxillary canine and premolars, and the sum of the mandibular canine and premolars (p < 0.05, t test; Table 2). The overall regression coefficient of mandibular canine and premolars was 0.83 and for maxillary canine and premolars was 0.78 (Table 2). The standard error of estimate ranged from 0·30 to 0·52 mm with the errors smaller in girls as compared to boys (Table 2). The values of regression coefficient b ranged from 0.54 to 0·68 and were all significantly different (p < 0·05, t test). Tables 3 and 4 show the differences between the sizes of canine and premolars widths obtained using the regression equations from the present study and the predicted widths of canine and premolars for mandibular and maxillary arches derived from Moyers’ probability tables [5] at different probability levels, respectively. There were no statistically significant differences between the sizes predicted by the equations and the predicted widths from Moyers’ probability tables [5] at the 65% level for the boys and 75% level for the girls in the mandibular arch (p > 0.05, t test; Table 3). In the maxillary arch, no significant difference between the sizes predicted by the equations and the predicted widths from Moyers’ probability tables [5] at 75^th level in boys and at 75^th and 95^th levels in the girls (p > 0.05%, t test; Table 4). The difference (mm) between the regression values derived using the equations from the present study of the sum of the canine and premolars of Ugandan children and those predicted from Tanaka and Johnston equations [14] for same subjects in the mandibular and maxillary arches is presented in Table 5. There were statistically significant differences between predicted widths using equations from the present study compared to Tanaka and Johnston regression equations [14] (p < 0.05, t test; Table 5).

Prediction of the width of un-erupted teeth in a population based on values from another race is likely to provide inaccurate estimates [18, 19, 23, 24]. This may be attributed to racial differences in tooth sizes [25]. Data to provide such equations for a Ugandan population for the first time were generated in the present study. The correlation coefficients between the width of the mandibular incisors, mandibular and maxillary canines and premolar teeth ranged from 0.77 to 0.84 (Table 2) using the derived prediction formulae. Our correlation coefficients were higher compared to those from Hong Kong Chinese (r = 0.58 to 0.66) [19], Jordanian (r = .60 to 0.66) [28], Syrian (r = 0.58 to 0.66) [26] and the Pakistan population (r = .64 to 0.67) [30]. Comparable to our findings, a study from a Brazilian population [31] found a correlation value of 0.81. The differences in correlation values may be due to the genetic influence on the tooth sizes [25, 30]. We also found a sex difference in the correlation values which may partly be attributed to the size of the canines and premolars in the maxillary and mandibular arches (Table 1) as previously reported [30].

In the present study, the slope of the simple linear regression equation ranged from 0·54 for the maxillary teeth in the boys to 0·68 for the mandibular teeth in the girls (Table 2). These values are comparable to those reported for the African American [17, 32], Thai [33], Hong Kong Chinese [19], Senegalese [34], Saudi [18] and Jordanian populations [28].

Moyers [5] recommended using the 75th percentile level of probability in his tables to predict mesio-distal widths of un-erupted permanent teeth, however, previous studies showed that Moyers’ probability tables are not an accurate method for the prediction of mesio-distal widths of un-erupted permanent teeth in different populations [18‐28, 30‐36].

In the present study there were no statistically significant differences between the sum of the sizes of the canines and premolars predicted by our equations and the predicted widths from Moyers’ tables [4] at the 65^th and 75^th percentile levels of probability in boys and girls, respectively, in the mandibular arch (Table 3). Furthermore, we observed no statistical significant differences between the sum of the sizes of the maxillary canine and premolars predicted from our equation and the predicted width from Moyers’ tables [5] at 75^th percentile level of probability in boys as well as 75^th and 95^th levels in girls (Table 4). This is an indication that Moyers’ probability tables can be used to predict un-erupted tooth widths in a Ugandan population at those particular percentile levels of probability. In a previous study in the Jordanian population [28], it was found that Moyers’ probability tables [5] for prediction of sizes of un-erupted permanent teeth can be applied at 65^th and 75^th levels in male and at the 85^th level in female subjects for both maxillary and mandibular arches. However, in the Saudi Arabian population [24], the most accurate width of un-erupted permanent canine and premolars was predicted at 50^th percentile level of probability as compared to the most commonly used 75^th level when both sexes are combined. These variations in percentile levels of probability used in predicting the most accurate tooth widths call for the need to develop regression equations for the different populations.

It is evident from the findings of the present study (Table 5) that the Tanaka and Johnston technique [14] overestimates the actual size of the Ugandan tooth widths. This overestimation can partly be explained by racial differences between the present study and that of Tanaka and Johnston [14]. However, our finding is in support of some previous workers [18, 19, 33] who also reported an overestimation of the size of un-erupted canines and premolars when using the Tanaka and Johnston prediction equations [14]. On the other hand, other workers [28] reported an underestimation of the actual mesio-distal widths of un-erupted permanent teeth when using the same technique. It can therefore be concluded that because of the differences in mesio-distal widths of mandibular incisors, mandibular and maxillary canines and premolars among different racial groups, data collected from one ethnic group for the purpose of predicting the size of un-erupted permanent teeth might generally, not be applicable to another [18].

Moyers’ probability tables could accurately be used to predict tooth widths of Ugandan population at the 65^th and 75^th percentile levels of probability in boys and girls, respectively, in the mandibular arch, and at 75^th level in the boys as well as 75^th and 95^th levels in girls in the maxillary arch. However, the Tanaka and Johnston prediction equation overestimates the actual size of tooth widths of the Ugandan population.