Introduction

Hypodontia is a congenital absence of teeth or congenital dental aplasia. It is one of the most common dental anomalies, which can adversely affect both aesthetics and function. It is one of the results of alterations during the early stages of development (1).

The etiology of hypodontia is multifactorial. The most supported theory suggests a polygenic mode of inheritance, with genes and environmental factors that exert some influence on the phenotypic expression of the genes involved (2).

The hypodontia, in our study, was evaluated from panoramic radiographs as in many other studies (3, 4).

It can be said that there is great variability in the figures obtained regarding the percentage of prevalence of hypodontia by different authors ranged from 4.28% to 8.5% (5-7).

The most frequently affected teeth are the second lower premolars, followed by the upper lateral incisors or the upper second premolars (8).

Based on gender, hypodontia is more frequent in women (9, 10).

This study aimed to evaluate the prevalence of hypodontia in a sample of children aged 7–11 at the University Alfonso X el Sabio (Madrid).

Materials and methods

The main objective of this retrospective study was to evaluate the prevalence of hypodontia in 2500 children between seven and eleven years of age, which included 500 seven-year-old patients, out of which 247 were females and 253 males; 500 eight-year-old patients, out of which 242 were females and 258 males; 500 nine-year-old patients, 232 of whom were females and 268 males; 500 ten-year-old patients, out of which 249 were females and 251 males; and finally 500 eleven-year-old patients, of which 265 were females and 235 males.

The panoramic radiographs which were less than 10 years old were randomly evaluated according to the following criteria:

The following were the inclusion criteria: patients born between the years 2004–2009, current patients of the university clinic of the Alfonso X El Sabio University who were born between 2014–2016, patients with panoramic radiography performed at the Alfonso X El Sabio University, and patients with no history of extractions because of decay or dental trauma.

The following were the exclusion criteria: patients with craniofacial syndromes or developmental disorders, patients with a history of extractions according to dental records, patients without panoramic x-ray taken at Alfonso X El Sabio University, and radiographs in poor condition.

The study evaluated the teeth, the maxilla, the side and the gender where agenesis is more prevalent among the studied patients aged seven to eleven. The differences in the proportions were analyzed in contingency tables by means of the χ2 statistics, or the Fisher's exact test. Statistical analyses were performed using SPSS software for Windows (version 20; IBM, Armonk, NY).

The study was performed at the university clinic of the University Alfonso X El Sabio located in Madrid.

X-rays were evaluated by the same operator (RAS). No ethical approval was considered to be necessary because of the nature of the study.

Results

The sample of 2500 patients studied consisted of 1235 females, out of which 41 presented hypodontia (3.31%) and 1265 males, out of which 46 had at least a dental agenesis (3.63%).

There was no significant difference in proportion of hypodontia between males and females (p> 0.05).

40 patients (1. 6%) had agenesis on the upper quadrants, 39 (1. 56%) on the lower and 9 (0.36%) on both; no differences were found between upper and lower (p> 0.05); there was a higher proportion of hypodontia in only one jaw than in both jaws (p<0.0001).

56 patients (2.24%) had unilateral hypodontia and 43 of them had (1.72%) bilateral, but the difference was not significant (p>0.05) (Figure 1-5{ label needed for fig[@id='f2'] }{ label needed for fig[@id='f3'] }{ label needed for fig[@id='f4'] }{ label needed for fig[@id='f5'] }).

Figure 1 Percentages of hypodontia in men and women.

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Figure 2 Percentages of bilateral and unilateral hypodontia in women and men.

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Figure 3 Total hypodontia percentage of the 2500 patients studied.

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Figure 4 Percentage of single and multiple hypodontia in both women and men.

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Figure 5 Percentages of hypodontia per piece found in the patients studied.

48 (1.92%) patients had single agenesis, whereas 40 (1.6%) had multiple agenesis. Again, the difference was not significant (p>0.05).

Table 1 shows that the teeth more affected by agenesis were lower second premolars, followed by upper lateral incisor.

No hypodontia of the upper and lower first molars, upper and lower second molars, and lower canines has been found.

In this study, no dental agenesis was found in temporary dentition.

Discussion

Hypodontia is the object of study in many universities and in many specialized clinics, since it is a very frequent anomaly in humans.

This study has some limitations, since we didn’t consider the ethnicity of each patient and the fact is that many articles found in the literature compare the difference in prevalence of hypodontia between races (11).

In our study, syndromic patients were also ruled out; hence we did not obtain the knowledge of hypodontia’s prevalence in them, which is an interesting issue for future research (12).

We did not take into account the prevalence of hypodontia in the third molars, mainly because of patients’ age; the prevalence of hypodontia excluding third molars was 6.2% (women 8% and men 4.2%) according to Abed Al Jawad (13) and in another study, the prevalence was 9.7% (14). Both are higher than our results, which showed a prevalence of 3.48%, being 3.63% in males and 3.31% in females, also excluding third molars.

In other studies found in the literature, hypodontia is more frequent in women, although the differences found by gender are not significant (15, 16).

Nevertheless, in our study, there were no significant differences in the hypodontia between men and women.

Observations from literature show that in the upper arch the teeth that have the highest percentage of hypodontia have been the lateral incisors, followed by the second premolars. In the lower arch, it is the second premolars, followed by lateral incisors, in contrast to what occurred in the upper arch (17). This is completely in accordance with the results of our study.

In general, according to different authors (18-20), the most frequently affected teeth are the second lower premolars, followed by the upper lateral incisors and the second upper premolars. Another study shows a greater absence of lower lateral incisors and lower second premolars (21).

Some of the percentages found in the literature on dental agenesis are: second lower left premolar (5.9%), second lower right premolar (5.1%), upper left lateral incisor (4.0%), second upper right premolar (3.1%), and second upper left premolar (3%) (22). Other percentages are: lower second premolar (20.7%) and upper second premolar (10.2%) (23).

The available literature shows that the number of hypodontia in the upper arch (5.3%) was higher than that in the lower one (3.5%) (24).

In some previous studies, a greater number of bilateral hypodontia (68.9%) than unilateral hypodontia (31.1%) has been observed (25-27). It is in contrast to what happened in our study, where there was a higher prevalence of unilateral hypodontia than bilateral hypodontia in men and women.

In our study, there was 54% of hypodontia in the left side and 46% in the right side; we found 45.7% of single hypodontia and 54.3% of multiple hypodontia, whereas in other studies there has been a greater number of hypodontia on the right side than on the left side (28) and more single hypodontia than multiple ones (11).

Other studies refer to oligodontia (6 or more missing pieces) observed in 0.8 and 1.4% of patients (29, 30). In our study, only 0.04% of patients had at least 6 missing pieces.

Conclusions