The best age for the examination of MIH has been given as 8 years when the permanent first molars and permanent incisors have all erupted (15). However, the current study used 8–11 year-olds, like in other previous studies (8. 9) had done, and only included opacities 2 mm or greater for the diagnosis of MIH (16).
Prevalence of MIH
The prevalence of MIH in the current study was 20.1%, which was higher than the global average prevalence of 16% (9), but comparable to results reported of 19.3% from Finland (17), 21,8% from Spain (18), and 20.0% from Thailand (19). Alternatively, when the current results were related to those reported from other African countries, they were higher than the 17.7% from Nigeria (9), 13.7% from Kenya (10), and the 2.9% from Libya (8). Globally, the researches that has reported the least prevalence of MIH were from Hong Kong (8) and Libya (20) of less than 3%, and the highest were from Brazil (11) and Denmark (23).
Association of MIH with Age and gender
The current study has shown no significant statistical difference with the occurrence of MIH in relation to age (p = 0.231), which is consistent with results reported by Yodeled et al. (9), and Ghanim et al., (21), but not for results by Koch et al. (22), and Da costa-Silva et al. (23) that showed significant increase with age. In terms of gender, the current study showed boys had slightly higher MIH prevalence than girls, although this was not statistically significant (p = 0.675), just as some studies have indicated before (24, 29) and contrary to results from other studies where the prevalence were high for girls (10, 20, 24,) and for boys (8, 25).
Number and distribution of MIH-affected teeth
The mean number of MIH-affected teeth in a child in the current study was 3.3, consistent with that reported for Denmark of 3.6 (11), Brazil with 3.3 (26) and Nigeria with 3.5 (27), but lesser than that reported in China of 2.6 (20) and Italy of 2.0 (28), and contrasted those recorded in Bosnia and Herzegovina of 5.59 (24), Germany with 4.8 (25) and India with 4.31 (29). The variations in the mean number of affected teeth could suggest that regardless of the MIH prevalence in the population, the mean number of teeth affected appear to be almost similar, a denotation of a common characteristic of the defect. The majority of the children in the current study had one and two molars affected rather than three or four molars, the same finding was reported in a study in Finland (17) and in Ankara, Turkey (30). In contrast other studies have reported most of children have four molars affected (7, 11).
The risk of incisor involvement in the current study appeared to increase significantly with the number of molars affected, a situation similar to the findings in a studies conducted by Wogelius et. al. (11) and Da costa-Silva et. al. (23), but in contrast to the that found that the risk was insignificant (18).
In the current study population, the permanent first molars were more frequently affected than incisors, just as reported in other studies (14, 32), but this result was lower than that obtained in Denmark in 2008, where the affected incisors were more than the molars (11). Further, in the current study, the affected maxillary teeth were more than the mandibular teeth, a situation consistent with the findings by Leppaniemi et al. in 2001 (17) and Temilola et al. in 2015(27). Nonetheless, studies by Cho et al (20) in 2008, Ghanim et al in 2011 (13), found equal distribution of defects in the upper and lower jaws. The current study also showed that the mandibular and maxillary molars were equally affected (P = 0.218), in line with what Weerheijm et al reported in 2001 (33) and Cho et al in 2008 (20), but contrary to the results from Sweden (32), Lithuania (14), Jordan(34) and Gujarat, India (35) that found more mandibular than maxillary molars being affected. Greek (24) and Iraqi (13) studies found more maxillary than mandibular molars were affected.
The maxillary incisors in the current study were more frequently affected, a result that mirrored the findings by Parikh et al in 2012 (13), Ghanim et al in 2011 (29) and Muratbegovic et al in 2007) (35), but contrary to th results of a Turkish study by Sönmez et al in 2013 (30), who reported more mandibular than maxillary incisors being affected. The current study also showed the upper central incisors (right: 11.2%, left: 9.0%) were the most frequent affected as was also reported by Lygidais et al in 2008 (24), Zawaideh et al in 2011 (14), Jasulaityte et al in 2008 (34) Allazzam et al in 2014 (31).
Pattern of MIH defect in hypomineralized teeth
The current study found that the most common pattern of MIH defects were demarcated opacities. The white/creamy demarcated opacities were more frequent than yellow/brown opacities, which was in agreement with previous studies by Wogelius et al in 2008 (11) and Mitta et al in 2015 (36), but just the opposite to the findings by Ghanim et al in 2014 (21) which revealed that yellow/brown opacities were the most common form of MIH defects. The prevalence of post-eruptive breakdown in the present study was 28% which was comparable to the 28.2% obtained from Lithuania (14) but higher than thE 8.4% reported in Finland (17) and 2.0% FROM Brazil (7). This variation may be explained by the inclusion of older age group children in the current study, as demarcated opacities may tend to break down over time (11). The most severe MIH defects (PEB, AR and extractions due to MIH) were present on the first permanent molars, unlike the permanent incisors where PEB was rare possibly due to less masticatory forces placed on these teeth (13, 37, 23)