Globally total tooth loss is estimated to affect of oral of burden 267 million individuals in 2017 [1], and has significant negative effects on the quality of life and oral and general health [2]. It leads to the deterioration of the basic functions of teeth such as mastication and speech and a loss of aesthetic sense [3], which further causes weight loss, nutritional loss, and systemic diseases. In severe cases, it also leads to death [4-7]. Therefore, maintaining oral health is essential to maintain systemic health. Tooth loss generally starts around the age of 40 and becomes rapidly prevalent after the age of 65 [8]. It is caused by various factors such as dental caries, periodontal disease, tumors in the jawbone, and trauma [9,10]. Functional loss caused by tooth loss can be restored through prosthetic treatment.

Edentulism refers to a condition in which all natural teeth have been lost without any prosthetic treatment. Although prevalence differs by country and region, it is generally prevalent in adults over the age of 50. It is also observed in younger age groups. Thus, it is one of the many health problems that requires care and management [11-13]. The prevalence of edentulism for the last 30 years has decreased in developed countries (e.g., United States); however, low-income families and the elderly still suffer from masticatory problems due to edentulism [13,14].

With the recent aging of the general population, the interest in the relationship between the elderly and oral health has increased worldwide. Research on the trend of edentulism prevalence is actively being conducted in many different countries, with comparisons between the countries as well. For example, Cardoso et al. [15] assessed the prevalence and trend of edentulism using data from Brazil’s national oral health survey, and Eklund & Burt [16] used the Unite States national survey data to investigate the prevalence of edentulism. Additionally, Elani et al. [17] compared the education-based social inequality for the prevalence of edentulism and tooth loss between five countries (Australia, Canada, Chile, New Zealand, and United States). In 2016, Jun & Ryu [3] reported the oral health behavior of the elderly according to tooth loss using raw data from the 6th National Health and Nutrition Survey in Korea. However, there is a lack of epidemiological studies on the prevalence of edentulism and rate of tooth loss in Korea even though the elderly over the age of 65 accounts for the 14.9% of the total population as of 2019.

Eklund & Burt [16] analyzed the trend of edentulism prevalence according to demographic variables, which are important predictors of edentulism prevalence, and reported that tooth loss was more frequent in the low-education and low-income groups despite the decrease in the overall prevalence of edentulism. In Korea, Ryu & Jeon [18] showed that low-income groups who are eligible for medical benefits have a lower rate of receiving implants that have been covered by insurance since 2014, compared to local/workplace subscribers. However, studies on the prevalence of edentulism according to demographic variables before and after insurance coverage are not reported.

Studies on the prevalence of edentulism can help assess and in the diagnosis of the oral health of the population and help establish policies for improvement. Therefore, the purpose of this study was to estimate the prevalence of edentulism in Korea according to demographic variables for each year using raw data from the 2007-2018 (4th-7th) KNHANES and to determine differences in the trend of edentulism prevalence change.

Tooth loss is an indicator of oral health; currently, studies are evaluating the trend of edentulism prevalence as well as making comparisons between different countries in this regard [8,17]. However, there is a lack of studies on the prevalence of edentulism in Korea, one of the main Asian countries. Therefore, this study was conducted to understand the trend of edentulism among adults over the age of 19 using raw data from the 2007-2018 (4th-7th) KNHANES.

In this study, the trend of edentulism according to demographic variables showed a repeated trend of increase and decrease, with an overall decrease in prevalence by 2016-2018 compared to that in 2007. Moreover, edentulism prevalence increased with aging, and the magnitude in decrease of prevalence was also greater. We observed that in the 80s group, the prevalence increased by 9.57% from 2012 to 2013 and decreased by 13.89% from 2013 to 2014. This coincides with the time when implants started to be covered by health insurance for those citizens over the age of 75 in July 2014. Ryu & Jeon [18] reported that the rate of use of implant coverage increased from 1.2% in 2014 to 3.2% in 2015 and 4.9% in 2016. When there are not enough teeth to serve as abutment teeth for removable partial dentures, the remaining teeth are removed, and implants are installed in order to use removable partial dentures. In such cases, it is thought that insurance coverage for implants would have affected the decrease in the prevalence of edentulism. In 2016-2018, the magnitude of the decrease in edentulism prevalence in women was significant compared to that in 2007. However, in 2014 when implants started to be covered by insurance benefits, the prevalence of edentulism decreased in men while it increased in women. This may also be related to the higher rate of usage of implant coverage by men than by women [18]. When the prevalence was analyzed according to income level, the magnitude in the decrease of edentulism was greater in groups with a higher socioeconomic status than in those with lower socioeconomic status. Additionally, the groups with lower socioeconomic status had a higher prevalence of edentulism than those belonging to the higher status groups. This finding is consistent with the finding of Eklund & Burt [16] who reported that the elderly are highly influenced by socioeconomic factors of edentulism and that the chance of tooth loss in low-income families is high despite the decrease in the overall prevalence of edentulism. Moreover, our results are in line with those of Ryu & Jeon [18] who reported that the rate of usage of implant coverage by National Health Insurance was lower than that of local/work-based subscribers. Although the implant may be covered by insurance, there are many items of dental treatment that are not part of the medical benefits that would have affected the high prevalence of edentulism in low-income groups. Our finding on the prevalence of edentulism according to education level is similar to the results of a study by Elani et al. [17] on the comparison of the prevalence in five different countries. We observed that the prevalence of edentulism was lower in the high education level groups than in the low education level groups.

As an additional analysis, the teeth with severe mobility or root rest were treated as tooth loss and then edentulism was re-calculated. We obtained almost similar results. However, the prevalence of edentulism in 2016-2018 was 0.43% higher in elderly people over the age of 80 compared to the result of the present study. Older persons may be prone to have the dysfunctional teeth left untreated in their mouth (data not shown). It is thought to be related to the rate of limited oral function, which is caused by problems in the oral cavity, such as in the teeth and gums, and leads to mastication and speech discomfort. In fact, 2009-2019 KNHANES demonstrated that the rate of limited oral function increases with age [21].

We observed that a higher age and lower income and education level led to a higher chance of tooth loss and higher prevalence of edentulism. This finding is consistent with the results of previous studies conducted in other countries [8,22,23]. Although there are differences in the prevalence according to demographic variables, the overall decrease in prevalence over time is thought to have affected the expansion of the age for implants and the reduction in the self-pay rate. In addition, a previous study on the subjective oral health status in the elderly in Korea using data of KNHANES showed that the number of elderly persons who report bad subjective oral health is decreasing. It is possible that increased interest in oral health had positive effects on the prevalence of edentulism [24]. Among Organization for Economic Cooperation and Development countries, the health level of the Korean general population is comparable to that of other countries. However, in those having relatively fewer medical benefits with difficulty in access of medical care have less medical use as called Inverse Care Law, dental care system still needs to be improved in Korea [25]. Considering that there is still a gap in the prevalence of edentulism according to income and education levels, universal policies such as increased access to oral health and increased benefits for the low-income and low-educated groups would need to be continuously pursued.

There are several limitations in this study. Although data from KNHANES were used, the oral survey was a cross-sectional study with oral examinations and questionnaires. Therefore, the cause of the increase in the prevalence of edentulism in all groups, except the 60s, in 2013 could not be identified. Therefore, further studies would be required. In addition, other factors related to tooth loss, such as alcohol consumption and smoking, were not considered in this study. Lastly, the vulnerable population living in nursing homes, long-term care hospitals, and rehabilitation facilities were not included in KNHANES.

Nonetheless, this is the first study to analyze the trend of edentulism prevalence in Korea using large-scale samples of KNHANES, which represents the Korean population. Moreover, this study can be used as a reference material for the Korean population in future studies comparing the prevalence of edentulism among different countries.

In conclusion, the prevalence of edentulism in those over the age of 65 was generally decreased from 12.80% to 9.72% over time. There were also significant changes in the trend of edentulism in the different age groups according to demographic factors such as age, gender, income level, and education level. The prevalence of edentulism is higher in the elderly and in those with low income and education level. The introduction of expanded insurance benefit for implants in 2014 is expected to contribute to the decrease in edentulism. However, there are still other groups that suffer from edentulism and, thus, follow-up studies would be required. We expect that the study results could be used as basic data on oral health related to tooth loss and edentulism.