Enamel is the hardest tissue of the human body and is structurally made up of millions of highly mineralised prisms. It is translucent, and is even more translucent as it becomes mineralised. Chemically, it is made up of an organic (1-2%) and inorganic matrix (95%) with water (3-5%), (1). The disorders that arise during the initial matrix secretory stage in the process of amelogenesis can lead to quantitative structural defects that manifest as dental hypoplasia while those affecting the maturation or mineralisation stages lead to hypomineralisation or qualitative defects (2).
Mineralisation or maturation problems of the enamel occur in the area of the tooth that corresponds to the developmental stage, since the hard dental tissue cannot be repaired (3). It is thought that the origin of mineralisation problems are disturbances in the resorption capability of the organic matrix and the inhibition of the proteolytic enzymes, which entails the retention of proteins and interferes with the formation of crystals as there is not enough space for depositing minerals (4).
The term “molar-incisor-hypomineralisation” was proposed by Weerheijm and co-workers in 2001 (4). This description was accepted at the EAPD meeting in Athens in 2003 as a definition. It affects first permanent molars (FPM) and occasionally permanent incisors (PI). At present, its unknown etiology presents a challenge to the clinician (5).
Histologically, the microstructure is preserved and ameloblasts will function normally during the secretion stage. However, the crystals appear less compact and organized in the porous areas, which points to a disturbance in the maturation phase. The mineral concentration of the enamel is reduced from the amelodentinal junction to the subsurface area of the enamel in the areas affected, which is contrary to what is observed in normal enamel (6). The enamel is porous and the morphology of the prisms is disturbed, hence the mechanical properties in this enamel disorder are similar to dentin as the hardness and elasticity are reduced (7).
Generally, MIH develops during the first three years of life when the mineralisation process of the crowns of the FPM and PI takes place (8). Clinically, it can be observed how, in demarcated areas of the enamel, abnormalities will appear in the translucent area due to a loss of mineral content that does not affect its thickness. Asymmetric opacities can be observed in white, cream, yellow or brown colour that may affect only one molar or all four molars. They vary in extension and severity without affecting the gingival area (9). These opacities appear mainly in the cusp or incisal third of the crown of the affected teeth and they are characterised by well-defined borders between the normal and affected enamel (4, 10). Jälevik and Norén (8) concluded that the opacities that vary in colour from yellow to a yellow-brown colour are due to irreversible damage of the ameloblasts.
So far, most of the reports on the prevalence of MIH have been carried out in Europe. More recently, there has been an increased interest in the prevalence of MIH worldwide.
Currently, depending on the different studies reviewed, there is a wide range of prevalence from 2.8% to 40.2% (11).
The severity of MIH varies and it does not vary only from patient to patient but also between different teeth in the same patient. Not all FPM are compromised to the same degree even when their tooth buds have been affected by the same systemic disturbance (12-14). In PI the enamel defects seem to show less of a tendency to fracture and are most commonly found on the buccal surface (15). In extreme cases the enamel disintegrates following eruption favouring the onset of caries, which will have a significant effect on the need for treatment (10, 16).
Materials and methods
The research protocol (P15/022) for this cross-sectional study on the prevalence of MIH was approved by the Governmental Health Service Ethics Committee.
A power calculation to assess the sample size needed, for at least 80% power and α = 0.05 indicated a minimum of 600 children. The study sample consisted of 705 Spanish school children, 344 boys (48.72%) and 361 girls (51.28%) aged 6-14 years and 11 months old. Children were recruited from two towns. They were randomly selected from the central region of the province of Barcelona, Spain, composed of 51 towns.
The oral examinations were performed as part of the governmental health control programme after informed consent had been obtained for each child.
The children were examined by one pediatric dentist (MH) following a series of calibration exercises. This was completed using a chart of 50 clinical pictures of affected teeth covering all the degrees of MIH and other lesions such as hypoplasia, amelogenesis imperfecta and fluorosis. Using Kappa factor analysis, an intra-examiner agreement of 97.6% was found.
Full mouth inspection of cleaned and wet teeth was completed using the EAPD 2003 criteria for diagnosis of MIH5. Children were examined in a dental surgery/office with good lightning under direct vision with the aid of a dental mirror. All teeth had been previously cleaned and were wet5. General enamel defects such as opacities less than 2mm were excluded. Incompletely erupted teeth were considered as if the visible part of them was less than a third and were also excluded to avoid an overestimation of the prevalence of MIH and to favor the reproducibility and comparison of with different studies. Data were entered into IBM SPSS Statistics for Windows, version 22.0 (Armonk, NY: IBM Corp.) and a Chi square (χ2) test was used to determine differences in MIH prevalence regarding gender.
Table 1 shows the distribution of the participants in the study. Out of 705 examined children 56 (22 boys and 34 girls) showed MIH to a greater or lesser degree, which implies a population prevalence of 7.94%; 6.39% for boys and 9.41% for girls. MIH lesions were more commonly observed in girls (60.71%) than boys (χ2 = 4.9, p= 0.023) with a male/female ratio of 1:1.54.
The distribution of MIH, shown in Table 2, for affected teeth by gender and tooth by tooth, with χ2 test, p values and for different ratios are demonstrated. The total number of teeth with MIH was 215 (137 maxillary and 78 mandibular); 86 (40%) in boys and 129 (60%) in girls (χ2 = 5.8, p= 0.014) with a ratio boys/girls of 1:1.5. The data for the 137 maxillary teeth recorded 56 were in boys (40.9%) and 81 (59.1%) in girls with a boy/girl ratio of 1:1.44. The results for the 78 affected mandibular teeth were 30 (38.5%) in boys and 48 (61.5%) in girls with a boy/girl lower jaw ratio of 1:1.6. The maxilla to mandibular ratio was 1.86:1 in boys and 1.68:1 in girls.
The FPM most affected was found in the maxilla with 28.6% of tooth #26 in boys and the 27.2% of tooth #16 in girls. Concerning the incisors 19.7% of tooth #21 was in boys while the corresponding finding for girls was 19.7% of tooth #11. In the mandible, the most affected molars were 35.4% of tooth #46 in girls and 33.3% in boys. When the data for mandibular lateral incisors was considered it was noted that the prevalence was 12.5% in girls for tooth #32 while the prevalence in boys was 16.7% for tooth #42.
Table 3 shows the distribution of FPM and PI affected with MIH by gender group. It was noted that girls showed a higher prevalence in all the groups, even though in the group with all four FPM affected the differences are lower. It should be emphasized that 7.14% of children did show any defect in their permanent incisors.
MIH studies have become generalised and investigations are being conducted in many countries around the world. A wide disparity in the prevalence rate of MIH has been reported (11) ranging from as low as 2.8% (17) to as high as 40.2% (18). These data reflect the variability of the different diagnostic criteria as well as the experience, training and calibration of the examiners. Variations in sample size, age and dental records may result in errors in the estimation of MIH prevalence.
These variations in prevalence can be attributed to methodological differences such as sampling, because the studies do not always consider children of the same age, and uniform methods of clinical examination and recording systems have not been used. Various methods such as those established by Koch in 1987 (19), Alaluusua in 1996 (20), the Modified Developmental Defects of Enamel Index (mDDE), (1) in 2001 or the EAPD, (5) criteria for evaluation of MIH in 2003 have been proposed.
The simplicity of EAPD criteria has aided in the reproducibility of clinical examinations for the recording of enamel defects. Epidemiological studies before the establishment of the EAPD criteria in 2003 probably did not reveal the true prevalence of MIH due to the disparity of criteria used at the very beginning of the period of MIH investigations. At the EAPD congress held in Athens in 2003, it was confirmed that dental examinations should be performed on wet teeth and this has been the case in most of the MIH studies since. There is a higher MIH prevalence recorded in studies with teeth examined dry teeth than wet (5, 11). Following the example of previous authors, we considered teeth as ‘not erupted’ if any visible part of the enamel crown was less than a third (5, 21).
The prevalence of MIH using EAPD 2003 criteria herein was found to be similar to other studies evaluating children in different geographic locations. A prevalence of 7.94% was found in this Spanish population sample, which was within the range of those found in other child populations (11, 21-38). The MIH prevalence found in this study in Barcelona was the lowest reported as other Spanish authors have recorded prevalences of 21.8% in a sample of eight-year-old children and 17.8% using transillumination (25, 32).
The age distribution of children in this study ranged from 6 years 6 months to 14 years 11 months, although the EAPD guidelines state that the most convenient age for carrying out an examination is at age 8. This age range was chosen for the present cross-sectional study due to the sample size identified as necessary.
Significant gender differences were found (p= 0.023) as MIH lesions were seen more often in girls (60.7%) than boys. In the present study, MIH was 1.54 times greater than in boys, a finding that is common in many MIH prevalence studies and also the higher prevalence in the maxilla coincided with the findings of other previous studies (11). Table 2 shows the percentage of affected maxillary and mandibular teeth for boys and girls. The maxilla to mandible ratio in boys (1.86:1) was slightly higher than in girls (1.68:1). However these results differ from other authors who have found a higher prevalence in boys. Oyedele et al. (21) reported a prevalence of 6.8% in girls and 10.9% in boys in a Nigerian sample although the difference was not statistically significant. Allazzam et al. (23) also found a higher prevalence in boys (9.7%) than in girls (7.5%) in a Saudi Arabian population.
Both maxillary molars (#16 and #26) presented MIH in 53.6% in boys and the 50.6% in girls, while the mandibular data (#36 and #46) recorded 63.3% in boys and 64.6% in girls. This finding is in agreement with the observations of most previous researchers (3, 12, 22, 28) but disagrees with the observations of studies on Dutch (4), Australian (15) and Chinese (17) populations that found a similar distribution of molar hypomineralisation in both arches.
In the present study the distribution of the affected teeth coincided with the reports of other authors (17, 24, 25, 33, 34) who found that FPM were the most affected teeth with a frequent association with maxillary incisors. However, the previous findings did not agree with the observations of the study herein, which reported that mandibular incisors are rarely involved. A prevalence of 36.6% of affected mandibular permanent incisors in boys and 35.4% in girl was found. A higher number of maxillary incisors than mandibular was observed, a finding which is in accordance with those of most studies on different populations (3, 12, 17, 22, 34).
Table 2 shows the most commonly affected FPM and PI in the maxilla and the mandible. Our findings are consistent with the studies of different authors (17, 22, 35) who claimed that the most frequently affected teeth are the FPM, and #46 being more frequently affected. Regarding incisors, the most affected were #11 and #21, but those findings disagree with those reported by Martínez et al. (32) who found the #16 as the FPM to be the most commonly affected in their sample.
On the left side of the jaws, we found that the percentage of affected teeth was higher in boys with the exception of maxillary and mandibular left lateral incisors. On the right side, girls had a higher prevalence than boys except for the #42, which showed a higher prevalence in boys (16.7%) than girls (8.3%).
A 23.2% of our population had one molar affected, which is in agreement with other authors (22, 23, 27) who reported a prevalence of 23%, 21.7% and 21.4% respectively. However our findings were in disagreement with Jing et al. (34) who reported a prevalence of 46.7%, although their sample was made up of three distinct races (Chinese, Malayan and Indian).
Some studies have not separated data for boys from that for girls, and by doing so higher differences in the results may have resulted. Different studies have found greater disparities in prevalence’s, for example 53.7% in Iranian school children (31), 41.1% in Spanish children (32), 23% in a child population from India (22) and 5.8% in a group of Italian school children (33).
In our sample, 52 out of 56 children (92.8%) examined also had lesions in one to eight PI. Only 7.1% of them had lesions in permanent molars without the incisors involved. Ghanim et al. (31) reported 81.5% of mild defects in incisors while Wogelius et al. (37) stated that permanent incisors were 2.5 times more frequently affected among children with one or more affected FPM. Balmer et al. (38) examined 3,233 children in Northern Ireland and found a strong positive correlation (p=0.037) between the prevalence of MIH and incisor hypomineralisation.
Many authors (4, 12, 17, 32) have performed calibration exercises using clinical photographs although there is little information in previous papers concerning the way it was performed. In more recent studies (27, 29, 38), the index chosen was the modified Developmental Defects of Enamel (mDDE) which scored tooth surfaces of index teeth (FPM and PI) for the presence of demarcated, diffuse, or hypoplastic defects or combinations of them. A collection of clinical pictures, with special emphasis on hypoplasia, genetic defects, fluorosis stains and opacities from white to brown-yellowish was used to calibrate the examiner in this study, who achieved a higher diagnostic agreement (0.97 Kappa factor).
This study found a MIH prevalence of 6.39% in boys and 9.41% in girls. The rate among girls was 1.54 times greater than in boys, a common finding in most MIH prevalence studies using the EAPD criteria. In the present study, there was a higher prevalence of MIH in maxillary teeth and the maxilla/mandible ratio in boys (1.86:1) was larger than in girls (1.68:1). Future studies on the prevalence of MIH should take into consideration the 2003 recommendations of the EAPD and the severity classification of MIH in order to adopt a uniform protocol, which would facilitate the comparison between different studies.