In this cross sectional study, the eruption times of the permanent teeth and the anthropometric measurements were evaluated in a group of Ugandan school children aged between 4 to 15 years in Kampala. The data comprised of 1041 children with an almost equal distribution according to sex (520:521/boys: girls; Table 1). In some cultural practices indigenous to each region, data related to the date of birth might be misleading, especially if no central registration boards or no precise proof of age exist . In the present study, the age of the child was ascertained by asking the child the date of birth and then confirmed with school records. The two sources of information corroborated.
The eruption or emergence of a tooth is the biological process that follows the formation of the dental crown and is essentially the penetration of the covering oral mucosa by any part of a tooth . In the present study, clinical assessment of penetration of the oral mucosa by any part of the tooth was so obvious and did not require any reliability assessment of the examiners . However, the reliability test of anthropometric measurement was done and gave an almost perfect agreement  with no evidence of systematic error observed.
The present study was school based with no radiological facilities available and consequently, we were not able to determine the level of congenitally missing teeth. Previously, Holman and Jones  had discussed the impact of congenitally missing teeth on the mean eruption times and concluded that estimates of eruption time without considering congenitally missing teeth were biased upward, but in any case less than 1%. Moreover, they stated that for adequate sample sizes, agenesis does not lead to substantially biased estimates. The present study recruited a moderate sample size of children (n = 1041) implying that the influence of congenitally missing teeth may not be significant.
Researchers in previous studies [1, 4, 28] postulated that there was a role of the endogenic, exogenic and/or environmental factors in tooth eruption time. Socioeconomic or environmental factors have been reported to directly influence nutrition with an impact on child development including tooth eruption [7, 8], although Friedlaeder and Bailit  expressed a relative unimportance of environmental influence on eruption times of permanent teeth. Tooth eruption times in Uganda was last studied more than 40 years ago . Over that period many environmental and socioeconomic changes have taken place, which included civil wars leading to internal migration and disruption of socioeconomic activities. However, with peaceful environment in the subsequent years, this was followed by an impressive economic growth  that improved the socioeconomic development and presumably, the nutritional status of individuals. It could be argued that these changes may partly explain the later eruption times in the present compared to the previous study  (Tables 5 and 6). Assessment of the role of the endogenous, exogenous or socioeconomic and environmental factors in tooth eruption was beyond the scope of the present study. Other factors that could lead to observed differences in the eruption times between the two studies are that the previous study  was a national estimate based on a smaller sample of children (n = 622).
Based on partial correlation analyses in the present study, the mean tooth eruption times were directly related to child height, although the relationship was generally not statistically significant (Table 3). On the other hand, eruption times were generally directly related to the weight of the child, but significant in 50% of the teeth (Table 4) indicating that the influence of weight on eruption time is non-conclusive. In a previous comparative study  among the Japanese children in Hiroshima, tooth eruption times were found to be directly influenced by height and weight.
In further analysis based on Pearson linear correlation, we did not observe any specific relationship between tooth eruption times and BMI probably due to conflicting outcomes of weight and height. Among the Pakistani children in Karachi, Khan  previously observed that tall children exhibited delayed tooth eruption irrespective of their weight while heavy and short children had early tooth eruption. He also found the eruption time of different teeth to be either directly or indirectly related to the BMI. In other studies involving first molars and incisors of Saudi male children in Riyadh and Jeddah, Khan et al.  observed a non significant correlation between BMI and eruption times except for tooth #32. The eruption times were generally inversely related to the BMI.
In the present study, we found a sex difference in tooth eruption times; being an average of 0.8 (range, 0–1.5) years earlier in girls (Table 2). Similarly, in the previous survey in Uganda , eruption times were lower in girls than boys with a difference of an average of 0.5 (range, 0–1.6) years. Additionally, we found tooth #25, #32 and #42 to have erupted earlier in boys than girls while in the previous study  tooth #11, #16, #26 and #46 erupted earlier in boys than girls. The reason for the differences in tooth eruption times between boys and girls is poorly understood. It is assumed that the earlier onset of the permanent dentition is part of the different sexual maturity of both sexes at a given age .
In the present study, about 4% and 4.4% of the boys and girls, respectively, had lost teeth due to caries. By assumption, the prevalence of extracted teeth due to caries in the deciduous dentition could have been at the same low level in these Ugandan children. Retained deciduous teeth till physiological shedding could retard the eruption of permanent teeth .
We found no significant differences in the mean eruption times between the teeth in the right and left side of the jaws in these Ugandan children, which corroborates findings in the previous reports [10, 33].
Generally, there were no substantial differences between the mean eruption times of the children in the present study and other studies involving African populations (Tables 5 and 6). However, our results showed lower values when compared with studies from outside Africa. The major differences in eruption times among girls in both the maxillary and mandibular teeth were with the Iranian children  (Table 5) while among the boys, bigger differences were with the Australian  and Iranian children  in the maxillary teeth and Pakistani children  in the mandibular teeth (Table 6). The impact of ethnicity on the eruption process was reported in previous surveys [2, 4–6] and meta-analysis .