In this study, we found a lower prevalence (1.8%) of OMLs in rural Yunnan than that in the previous study  and the most common OML was oral ulcer. Preschoolers had a greater risk of developing OMLs than did school-aged children. However, no other risk factors (including sex, caregiver, parental education, and last-month household income) for developing OMLs were found. These are major findings since OMLs in children in remote and rural areas are generally neglected and may impair oral health-related quality of life . However, this can improve with proper prevention and early intervention.
There are a few studies on OMLs among children reporting prevalence ranging from 4.1–69.5%, despite some variations in different studies [8, 17]. In our current study, the most prevalent OML was oral ulcer, which was consistent with Kleinman’s study  and de Oliveira’s study , although the rate was relatively low (0.4%) in children aged 4–5 and 7–8 years. Kleinman et al. reported a 1.23% prevalence of oral aphthous ulcers in children aged 5–17 years . A study from de Oliveira reported that ulcers were the most prevalent OMLs (29.4%) in children aged 5 years . Nevertheless, recurrent aphthous stomatitis was the 2nd most prevalent OML (1.64%) in the Third National Health and Nutrition Examination Survey among 2–17 year-old children and youth . According to Vieira-Andrade’s study , the most prevalent OMLs were coated tongue (23.4 %), melanotic macules (14.4 %), and oral ulcers (11.8 %) in children aged 0–5 years. Of the children with oral ulcers, 65.6% belonged to the 3–5 year age group. Bessa et al. reported aphthous ulcerations as the 5th most prevalent OML in children aged 0–4 years (1.47%) and the 6th most prevalent in children aged 5–12 years (1.72%), whereas the most common lesion was the geographic tongue . It is possible that diagnostic criteria, training of examiners, calibration of examination procedures, and enrolled participants could influence the results [20, 21]. The reason for the low occurrence of OMLs in this study might be partly because this region was covered by the National Oral Health Comprehensive Intervention Program for Children in China. Some aims of this program were to improve people’s knowledge, influence their attitudes, and modify their behavior toward oral health . This might benefit children and reduce the prevalence of OMLs.
Previously, in terms of risk factors, a study also found that age between 3 and 5 years was the determining factor for OMLs  and another study from the US found that 5–11 year-old children had a significantly higher prevalence of OMLs than children aged 12–17 years . Bessa et al. reported that older children (5–12 years) had a significantly higher occurrence of OMLs than younger children (0–4 years) group . It is notable that children around 5 years of age are more vulnerable to OMLs. The reason could be milestone children reaching 5 years of age. Oral ulcers and herpetic infections are among the most common types of OMLs in children [8, 15]. Oral ulcers can be associated with psychological and mechanical stress . For example, children around 5 years of age might struggle to adapt to new environments and be anxious or stressed about separation from their caregivers. As a result, anxiety or stress leads to the occurrence of oral ulcers. In addition, herpetic infections occur in children between 6 months and 5 years of age . Due to its communicable nature, the incidence of herpetic infection could reach its peak in children aged around 5 years, which occurs during their preschool years.
It is the age that social factors shape health . SES is associated with a variety of children’s health due to differences in access to medical services, social support, and reactions to stress . Evidence shows that OMLs are associated with a low socioeconomic status . However, we found this was not the case and our findings were consistent with those of a study by Bessa et al. . An explanation could be that all children lived in the town, not in villages. The residents have all been successfully lifted out of poverty. As a result, they can obtain basic healthcare, social resources, and support to adjust to stress. In addition, the effects of SES on the occurrence of OMLs might not be too evident in children. All these factors might contribute to the lack of association between OMLs and SES.
One strength of our study on the prevalence of OMLs in children is that it was conducted in a rural area, as such studies are rare. Another strength is that we enrolled a large sample size. Our study, however, has some limitations due in part to the study design. Oral ulcers and herpetic infections are two recurrent types of OMLs that can heal without treatment. These were not present in children when we performed the examination. As a result, our study may have underestimated the number of children with these two lesions because of its cross-sectional nature. Furthermore, some information on SES was obtained from self-reported data. For example, some caregivers might have falsely reported lower last-month household income due to fear of losing welfare, such as the poverty allowance. It is possible that we had an increased number of households with an income of less than 6000 Yuan/month. Consequently, a regression analysis of household income could be unprecise.
With economic growth and social changes, children are vulnerable to OMLs . Treatment and prevention of OMLs to make children free of pain, fear, and anxiety is the job of specialists in oral medicine. However, the number of specialists is inadequate to meet the demand for oral medicine care, especially in rural areas. Consequently, it is easy to miss good opportunities to diagnose and treat OMLs. In such circumstances, our data on OMLs among children in rural Yunnan can help inform oral health prioritization necessary to prevent and treat oral diseases.