Prevalence of malocclusion
In the present study, 62.0% of the preschool children showed some type of malocclusion. The prevalence of malocclusion in this general age group is reported to be diverse throughout the world, ranging from 45.5% to 83.9% [2-12, 29]. Additionally, excessive overjet and deep overbite recorded the highest prevalence in the present study, which was consistent with our previous study and previous studies globally (excessive overjet: 10.2%–46.1%, deep overbite: 6.05%–41.5%) [2-12, 29]. However, no children exhibited transverse malocclusion in the present study, although several children had a lateral edge-to-edge bite, which was not included in the malocclusion criteria. These findings were consistent with our previous study [29]. Although Asians generally have a low prevalence of posterior crossbite and scissors bite [6, 30], further investigation is necessary to clarify the detailed prevalence in larger samples.
Dentition traits
In the present study, apart from the criteria for malocclusion, mandibular midline deviation and unspaced dentitions were investigated as dentition traits. Mandibular midline deviation was observed in 40.4% of children, which was higher than in previous studies (21.9%–26.6%) [2, 3, 6], and was not related to the four oral habits in the present study (data not shown). Interestingly, right-side deviation recorded significantly higher frequency than left-side deviation. Midline deviation is generally caused by lateral mandibular deviation related to posterior crossbite, tipping and/or drifting of the incisors, arch asymmetry, or any combination of these factors [37]. Although the cause of the results in the present study remains unclear, further investigation considering oral functions such as habitual chewing side may be needed. Unspaced dentitions were in observed in about 35% of children. In most previous studies, space analysis in the dentition was divided into the maxilla and the mandible, or primate space and developmental space, making comparison with the present study difficult [38-40]; however, unspaced dentitions in both the maxilla and mandible were more prevalent in the present study than in the report by Otuyemi et al. (18.1%) [41]. This may be related to ethnic differences or may be evidence of an increasing trend. Additionally, unspaced dentitions were found to have significantly higher frequency in girls than boys (P < 0.01). This tendency was consistent with previous studies [38, 39], and is considered to be universal.
Oral habits and nose and throat conditions
27.8% of children exhibited ILS in the present study, which was similar to a previous study in Japan [27]. However, ILS did not show a significant increasing trend with age in the present study, which was inconsistent with the previously mentioned study [27], probably because of the limited age range (3–6 years). The prevalence of nail biting (18.9%), finger sucking (7.8%), and lip sucking or biting (3.0%) was also similar to previous studies (nail biting: 15.2%–23.0%, finger sucking: 5.6%–25.0%, lip sucking or biting: 4.7%–5.6%) [2, 19-22], and no significant age trend was observed in the present study. It is thought that the prevalence of these oral habits remains constant in the preschool years, and then decreases with age [42]. However, chin resting on a hand recorded significantly higher frequency in boys than in girls (P < 0.05), with a significantly increasing trend with age from 3 years to 6 years. (P < 0.001). Chin resting on a hand is considered to exert lateral pressure on the jaws [43, 44]. The increasing trend in this habit may be related to an increase in sedentary behavior among Japanese children depending on age.
Approximately 30% of children tended to have nasal obstruction, and 17% had been diagnosed with allergic rhinitis in otolaryngology and pediatric clinics. Additionally, allergic rhinitis occurred significantly more frequently in boys than girls (P < 0.05), with a significantly increasing trend with age from 3 years to 6 years (P < 0.001). Allergic rhinitis has been reported to show a moderate increasing trend and be more prevalent among boys during childhood [45, 46]. Although nasal obstruction is a symptom of allergic rhinitis, it can be associated with various diseases, such as non-allergic rhinitis, chronic sinusitis, and severe septal deviation, resulting in a higher frequency than allergic rhinitis [47]. Furthermore, 3.6% of the children had been diagnosed with palatine tonsil hypertrophy in otolaryngology and pediatric clinics, with a significantly higher frequency in boys than girls (P < 0.01). Tonsil size significantly increases during the first 3 years of life, with only a moderate and not significant increase from 3 years to 12 years [48]. However, the lingual tonsils comprise Waldeyer’s ring with the palatine tonsils, and their hypertrophy is more frequently observed in patients with allergic rhinitis; therefore, palatine tonsil hypertrophy, as well as allergic rhinitis, may have recorded sex differences [49].
Related factors and characteristics of malocclusion
Based on the above sample characteristics of the present study, the following analysis was performed. Crude analysis revealed that ILS (P < 0.01), especially severe ILS (P < 0.001), and palatine tonsil hypertrophy (P < 0.05) recorded significantly higher frequencies, and nail biting (P < 0.05) recorded significantly lower frequency, in malocclusion than in normal occlusion. Additionally, the results of adjusted analysis showed a significant association of ILS (P < 0.01) and nail biting (P < 0.05) with malocclusion. Although ILS was reported to be a factor in malocclusion in some previous studies [27, 50-52], our findings revealed that malocclusion in preschool children was strongly associated with ILS in the cross-sectional study in the present study. Open mouth posture is considered to be associated with a slower pattern of maxillary growth, a narrow maxillary dental arch, and increased facial height [52, 53]. Nail biting was found to be a negative factor for malocclusion in the present study. Additionally, as mentioned above, nail biting was not related to mandibular midline deviation. The effect of nail biting on malocclusion is unclear and is not backed up by clinical or statistically significant evidence [24]. The results of the present study suggest that nail biting may not necessarily be a deleterious habit for the occlusion, but may be a guide for edge-to-edge incisal contact and attrition, resulting in the low prevalence of sagittal and vertical anomalies in preschool childhood.
No significant association with malocclusion was found for nose and throat conditions. Although it is thought that upper airway obstruction can cause mouth breathing and can contribute to a narrow maxillary dental arch and increased facial height, no consensus has yet been reached about an association with malocclusion [52-55]. Further research including accurate diagnosis and assessment of the degree of severity of these conditions is needed.
Limitations
The present study had some limitations. Our study sample was derived from one kindergarten in a local area of Japan and the sample size was small. Therefore, the data cannot represent the prevalence of malocclusion in all Japanese preschool children. Additionally, because the definition of malocclusion was based on the classification constructed by the Japanese Society of Pediatric Dentistry, we did not include some criteria such as distal steps of the second primary molars and primary canine relationships. Furthermore, it was impossible to statistically analyze the characteristics by each occlusion group separately in the present study, because 86 out of 503 children exhibited two or more malocclusions. However, there is a large variation in the characteristics of each malocclusion group as shown in Table 6. Future research should analyze the influencing factors focusing on specific types of malocclusion. With respect to oral habits, it is necessary to consider the intensity, long-term frequency, daily frequency, and duration when examining the impact on malocclusion. As for nasal and throat conditions, allergic rhinitis and palatine tonsil hypertrophy may have been under-reported because they were based on a questionnaire. However, it should be noted that early detection of oral habits and nasal and throat disease and minimizing their impact are important for proper maxillofacial development in early childhood.