The present study evaluated third molar development in Ugandan adolescents and young adults resident in the cosmopolitan city of Kampala, which is the capital city of Uganda. Kampala is a commercial hub and has a day population of 3.5 million people with a well representation of all tribes in Uganda. The study population represented urban Ugandans who live on both refined and non-refined (fibrous) diets. The findings in the present study showed no big differences in timing of the third molar root development and other Black populations except South African Blacks who tended to present with higher ages when the root development is at half-length and apex half-closed stages. These differences could have been due to varying study methods or the cosmopolitan nature of the South African population. On the whole, however, studies on dental development in sub-Saharan populations are too few to give a good comparison.
On the other hand, this study found notable differences with other ethnic groups, especially among White British, White Americans, Germans, and the Japanese. The differences ranged from a few months to 2 years earlier throughout the third molar root development stages among Ugandans. Generally, these findings were in agreement with other studies on third molar development among Blacks taking place earlier than other populations. The observed racial differences have been associated with palatal dimensions, where larger dimensions have been observed in Africans compared to other races.
Age estimation based on third molar root development
Third molar root development is considered an important biological marker for growth during the late adolescence because at this time, other markers would have achieved adult morphology. For instance, in the 15–18-year age group, the ossification of the hand-wrist bones is unreliable marker because these bones would have matured and their epiphyses fused (Schmeling A, 2006). Similarly, the onset of secondary sexual characteristics would have occurred at this time (Rosenfield, Lipton, & Drum, 2009). This makes the third molar an ideal marker for age estimation in late adolescence; therefore, this study sets out to establish age estimates based on distal root development of the mandibular third molar tooth.
Homologous pairs (left-right) of third mandibular molar
The left-right symmetry in root development for the mandibular third molars was very high in this present study (Table 3). There were no significant differences in the mean age of root development observed in all the stages of homologous pairs of third molars in either males or females (P ≥ 0.05). The highest mean difference between the left and right mandibular third molars for all root stages was 0.1 year at stages D, E, and F (Table 3). Since these differences were not statistically significant and were not consistent throughout the root stages, moreover, neither side was considered more advanced compared to the other; it follows that any of the mandibular third molar could be used in age assessment. In the present study, the left mandibular third molar (tooth 38) was used to establish age estimation in the Ugandan population. These findings have been reported among Americans (Mincer, Harris & Berryman, 1993), Austrians (Meinl, Tangl, Huber, Maurer & Watzek, 2007), and Black Africans (Olze et al., 2006). Meinl et al. (2007) proposed that in instances when asymmetry is observed, the average dental age of the left and right molars could be considered.
Sex differences in mandibular third molar root development
In the present study, there were sex differences in root development where females had development of root stages earlier compared to males (Tables 4 and 5). The mean age differences between the females and males ranged between 0.9 and 1.4 years for all the root development stages. These findings are in support of reports from other African populations (Olze et al., 2006). However, the sex difference was not significant, which is in contrast to other studies among Arabs and Africans of Sudanese origin (Elamin, Hector, & Liversidge, 2017), Botswanese (Cavrić, Vodanović, Marušić, & Galić, 2016), Turkish (Sisman et al., 2007), and Brazilians (de Oliveira, Capelozza, Lauris, & de Bullen, 2012), although exceptional traits have been observed in many other populations, where third molar development occurs earlier among the males compared to females for American Whites (Mincer et al., 1993), French Canadians (Levesque et al., 1981), Indians (Kanmani, Srinivasan, & Daniel, 2012), Bangladeshi (H. M. Liversidge, 2008), Turkish (Sisman, Uysal, Yagmur, & Ramoglu, 2007), Austrians (Meinl et al., 2007), and Spanish (Prieto, Barbería, Ortega, & Magaña, 2005).
Mean age at end of crown mineralization/root initiation
The age of end of crown mineralization (based on Demirjian stage D) and root initiation (based on MFH and Haavikko stage Ri) was found at 12.6 (95% CI, 12.07–13.15) years for females and a few months later at 13.5 (95% CI, 12.86–14.18) years for males (Tables 4 and 5).
These findings for the Ugandan females are corroborated by earlier studies done among Ugandans (Mwesigwa et al., 2019) and Botswanese (Cavrić et al., 2016) where stage D was found at 12.6 and 12.4 years, respectively. Similarly published tables from Moorrees’ original work by Harris and Buck (Harris & Buck, 2002) showed stage Ri was within the same age range at 12.9 years for North American White children. Similarly, various other populations like South African Blacks (13.6 years) (Olze et al., 2006), Turkish (13.6 years) (Sisman et al., 2007), and sub-Saharan Africans (13.7 years) (H. M. Liversidge et al., 2017) stage D were just a few months later in comparison with the present Ugandan population, while Africans of Sudanese origin (14.2 years) (Elamin et al., 2017), Indians (14.8 years) (H. M. Liversidge et al., 2017), Japanese (14.7 years) (Arany, Iino, & Yoshioka, 2004), Austrians (15.4 years) (Meinl et al., 2007), Spanish (15.1 years) (Prieto et al., 2005), and UK Whites (15.0 years) (H. M. Liversidge et al., 2017) were more than a year late.
Likewise, Ugandan males also displayed results similar to other Black populations including Botswanese at 12.6 years (26), South African Blacks (13.4 years) (Olze et al., 2006), and sub-Saharan Africans (14.1 years) (H. M. Liversidge et al., 2017), although among others like Black Africans from Sudan (14.5 years) (Elamin et al., 2017) and American Blacks (14.4 years) (Blankenship, Mincer, Anderson, Woods, & Burton, 2007) there were a few months difference observed. Other populations including the Turkish (Sisman et al., 2007) and Brazilians (de Oliveira et al., 2012) at 12.9 years, Iranians at 13.4 years (Jafari et al., 2012), and Indians (14.1 years) (H. M. Liversidge et al., 2017) were also within the same age range as males in this present study. Various other populations including Arabs (14.7 years) from Sudan (Elamin et al., 2017), Japanese (14.8 years) (Arany et al., 2004), Spanish (15.0 years) (Prieto et al., 2005), and White children from the UK (14.7 years) (H. M. Liversidge et al., 2017) were a few months later in comparison with the Ugandan population. Although American Whites (15.7 years) (Blankenship et al., 2007), Austrians (16.1 years) (Meinl et al., 2007), Germans (16.3 years), and Japanese (18.2 years) (Olze et al., 2004) attained this stage at remarkably much more advanced ages, this variation could be due to racial differences.
Mean age when root is half-way developed
The root at half-length (stage F) based on Demirjian et al. (1973) occurred at 16.3 years for females (Tables 4 and 5), which is comparable to 16.8 years in Botswanese (Cavrić et al., 2016), 16.1 years in Black Americans (Blankenship et al., 2007), 16.6 years in Brazilians (de Oliveira et al., 2012), and 16.8 years in Spanish (Prieto et al., 2005). However, the Ugandan females had their root developed half way earlier than South African Blacks (Olze et al., 2006), White Americans (Garn, Lewis, & Bonné, 1962b), and Iranians (Jafari et al., 2012).
The Ugandan males had their root half way developed at 17.4 years, which corroborates findings in Japanese (Arany et al., 2004) and American Whites (Mincer et al., 1993). Additionally Ohio-born White Americans were 16.7 years (Garn et al., 1962a); Iranians, 16.8 years (Jafari et al., 2012); Botswanese, 16.6 years (Cavrić et al., 2016); American Blacks, 16.6 years; (Blankenship et al., 2007); Brazilians, 15.9 years (de Oliveira et al., 2012); and the Spanish, 16.4 years (Prieto et al., 2005). However, higher values were reported among South African Blacks (18.6 years) (Olze et al., 2006) and Japanese (20.4 years) (Olze et al., 2003).
Based on Solari and Abramovitch (2001) and Haavikko (1970) methods, the root development stages (R1/2 & R3/4) were attained at 15.7 and 16.9 years for females and at 17.0 and 17.6 years for males, respectively. When compared with other reports (Harris & Buck, 2002), females reached this root development stage at a comparable age: 15.8 and 16.4 years, respectively. However, in comparison with Moorrees’ original data (Harris & Buck, 2002), Ugandan males appeared to develop these stages close to 2 years later.
Mean age when root development is at complete length
The root development at complete length when the apex is half closed (Demirjian et al., 1973, Haavikko, 1970) occurred at 18.8 years for females and 19.0 years for males (Tables 4 and 5). The findings for Ugandan females are in support of other reports among Ugandans (Mwesigwa et al. (2019), the Finnish (Haavikko (1970), Brazilians (de Oliveira et al., 2012), and Botswanese (Cavrić et al., 2016). However, the same root development stages in the present study (Tables 4 and 5) occurred slightly earlier when compared with Austrians (Meinl et al., 2007) and Chinese (Zeng, Wu, & Cui, 2010).
On the other hand, the findings for the Ugandan males corroborated those among the Chinese (Zeng et al., 2010), Botswanese (Cavrić et al., 2016), Japanese (Arany et al., 2004), Iranians (Jafari et al., 2012), and Franco Canadians (Levesque et al., 1981). This was in contrast to 1–2 years earlier in comparison with other reports among Austrians (Meinl et al., 2007) and South African Blacks (Olze et al., 2006).
Based on Solari and Abramovitch (2001) and Haavikko (1970) methods, root development at complete length when the apex is half closed occurred at 18.9 and 18.2 years for females and 19.1 and 19.0 years for males (Tables 4 and 5). The findings for Ugandan females were comparable to Indian population (Liversidge et al., 2017), Africans of Sudanese origin (Elamin et al., 2017), and sub-Saharan Africans, although in contrast to British and Malaysians (Liversidge et al., 2017), which occurred a few months later than in the present study. Similarly, the findings for the Ugandan males corroborated reports among sub-Saharan Africans Indians (Liversidge et al., 2017), Africans of Sudanese origin (Elamin et al., 2017), and British and Malaysians (Liversidge et al., 2017), although much lower ages are reported by Moorrees et al. (1963).
Mean age when root development is at complete apex closure
In the present study, the mean age at apex closure (stages H and Ac) was at 19.8 for females and 20.1 years for males (Tables 4 and 5). These findings are comparable to other studies among Ugandans (Mwesigwa et al., 2019). Similar findings were also reported for males by Moorrees and co-workers (1963) and Nystrom and co-workers (2007). Despite the relatively similar values at root development stage H, the findings should be taken with caution because of the associated challenges in determining the end point of this stage (Roberts, McDonald, Andiappan, & Lucas, 2015). This is because tooth development is boundless at its upper limit, for instance different studies have enrolled participants with varying upper age limits ranging from 21 years, 22 years (Mwesigwa et al., 2019), 23 years (Cavrić et al., 2016), 24.9 years (Blankenship et al., 2007), and 25 to 26 years (Olze et al., 2004). Inadvertently, this affects the mean age for this stage, and furthermore, some teeth with open apices may be missed depending on the upper age limit set for the study. Nonetheless, in order to overcome these challenges, the present study used data from a previous Ugandan study (Mwesigwa et al., 2019) to set the upper age limit to 22 years. This was decided because in the previous study by 22 years, all the third molars had undergone complete tooth development and thus had their apices closed. Other studies excluded this stage arguing that once the root is mature, age cannot be estimated from development (Liversidge et al., 2017).
Limitations of the study
Radiographic identification of root development stages is subjective especially at stages G1 and A1/2 denoting apex half closed based on modified Demirjian method (Solari & Abramovitch (2001) and Moorrees et al. (1963). However, in order to minimize error, the two investigators (AKM and CML) trained and calibrated on criteria for recording root development before the study. Nevertheless, these challenges are not limited to the present study but have been reported by other workers (De Salvia A, Calzetta C, Orrico M, & De Leo D, 2004, Dhanjal et al., 2006). Furthermore, due to the different classifications of the root development stages employed in the various methods in the present study, the findings should be interpreted with caution.