Adolescence is an age when an individual is vulnerable to overdependence and addiction to smartphone use owing to the relative physical, psychological, and social immaturity when compared with adulthood [1].

Smartphone overdependence refers to a state that when a person experiences issues arising from increased prominence to smartphone use; this is typically caused by the overuse of smartphones and a low degree of self-control [2].

A smartphone overdependent risk group is relatively intolerable to withdrawal symptoms from overuse and their daily life is affected [3]; their weakened immune system because of smartphone overuse may be a cause of chronic oral disease [4]. Brushing teeth helps in removing plaque and controlling diseases such as periodontitis and dental caries [5]; thus, humans have been brushing teeth to improve oral health [6].

Raising awareness and taking efforts to improve oral health behavior in adolescents is important for improving oral health in adulthood [7].

Therefore, this study was performed to investigate the correlation between the duration of smartphone use and overdependence on smartphones to oral health behaviors of Korean adolescents, and to contribute to guide appropriate smartphone usage for improvement of oral health of adolescents. We used data from Korea Youth Risk Behavior.

1. Subjects

This study used raw data from the 16th Korea Youth Risk Behavior Survey conducted in 2020 at Korea Disease Control and Prevention Agency. We extracted the 16^th Korea Youth Risk Behavior sample class from the order “study group stratification, sample distribution, and sample extraction” to obtain sample that represented middle school and high school students nationwide. Inclusion criterion was all students who attended school, whereas the exclusion criteria were all students who were absent from school for a long period of time, special children, and students with disorders related to reading texts. As of April 2020, 2,631,888 middle and high school students were enrolled in schools, of which, 57,925 were selected for the present study.

2. Methods

A total of 54,948 students participated in the survey (parti-cipation rate, 94.9%), and an anonymous self-administered survey was conducted.

Only sex and grades were used as general characteristics, and the subjects were categorized as “middle school students” and “high school students.”

A survey item related to oral care behavior named “How many times did you brush your teeth yesterday?” was added, and the options provided were as follows: not at all, once, twice, and three times or more. In addition, as an answer to the question “How often did you brush your teeth after lunch at school in the last 7 days?” the answers “always” “mostly,” and “sometimes” were changed to “Yes”, and the answer “not at all” was changed to “No”. With the item related to Regarding experience of sealant use, the answers were classified as “Yes” or “None.”

The answer for a question related to smartphone usage behavior was classified as “Yes” and “None” for weekdays and weekends, respectively. Among subjects who have experience with using a smartphone, the usage hours were reorganized as “<2 hours,” “≥2 hours and <4 hours,” and “≥4 hours” as of next day for weekdays and weekend.

Overdependence to smartphone was evaluated using 10 questions from smartphone overdependence screening questionnaire (Table 1), and each score of 10 questions were summed up and classified as high-risk for smartphone overdepen-dence. Of the total 40 points, participants who received 23-30 and ≥31 were classified in the potential- and high-risk groups, respectively. Furthermore, both the potential- and high-risk groups were classified as smartphone overdependence group. In this study, ≥23 points was classified as part of the “smartphone overdependence group.”

Table 1 . Smartphone overdependence screening questionnaire

Items	1. Strongly disagree	2. Disagree	3. Agree	4. Strongly agree
1) Every time I try to reduce the amount of time I use my smartphone, I fail.
2) I have difficulty in controlling the duration of smartphone use.
3) I have difficulty in maintaining an appropriate duration for smartphone use.
4) I have difficulty in focusing on other things when my smartphone is next to me.
5) I cannot stop thinking about my smartphone.
6) I feel a strong urge to use a smartphone.
7) I have had health problems because of smartphone use.
8) I once had a strong disagreement with my family because of my smartphone use habits.
9) I have experienced severe conflicts in social relationships with friends, colleagues, or friends because of the use of smartphones.
10) I have difficulty in performing tasks (Studies, jobs, etc.) because of smartphones.

Regarding questions related to experience of oral disease, in case when the one has experience of “Toothache when drinking or eating cold or hot drinks or food,” “Toothache and throbbing pain,” and “Gingiva hurts or bleeds” in the recent 12 months, the responses were reorganized as “Yes” and for those who had not experienced symptoms of oral disease, the responses were reorganized as “None.”

3. Statistical analysis

This study analyzed the general characteristics of subjects, oral health behavior, and smartphone use behavior through frequency analysis. The difference in oral disease symptoms between general characteristics and duration of smartphone use and dependence on smartphones was analyzed through Chi-square test, and a dichotomous logistic regression analysis was performed to confirm the relationship between smartphone use, time, and dependence on smartphones.

For statistical analysis, SPSS statistics version 19.0 (IBM Corp, IL, USA) was used. The statistical analysis significance consideration was set at the significance level (α) of 0.05.

1. General characteristics

Results related to analyzing the difference in the presence or absence of oral disease symptoms according to the general characteristics of the subjects are presented in Table 2. Of the 52,948 subjects, 46.1% (25,332 subjects) had experienced oral disease symptoms in the last 12 months. The percentage of female students with oral disease symptoms (51.5%) was higher than that of male students (41.1%; p<0.001). Further-more, the percentage of high school students (49.3%) was higher than that of middle school students (43.2%; p<0.001; Table 2).

Table 2 . Differences in oral disease symptom experience according to general characteristics

Variable	Oral disease symptoms		p
	No	Yes
Total	29,616 (53.9)	25,332 (46.1)
Sex			<0.001
Boys	16,709 (58.9)	11,644 (41.1)
Girls	12,907 (48.5)	13,688 (51.5)
School year			<0.001
Middle school	16,438 (56.8)	12,523 (43.2)
High school	13,178 (50.7)	12,809 (49.3)

Data analysis used Chi-squared test.

2. Differences in oral disease symptoms by oral care behavior

As a result of analyzing the difference in the presence or absence of oral disease symptoms according to oral care behavior, we observed the highest rate of oral disease symptoms was observed in 59.3% of those who did not brush their teeth the day before responding to the survey, 47.1% of those who did not brush after lunch at school, and 56.8% of those who had sealant experience (p<0.001; Table 3).

Table 3 . Differences in oral disease symptom experience according to oral care behavior

Variable	Oral disease symptoms		p
	No	Yes
Number of toothbrushes			<0.001
Not	194 (40.7)	283 (59.3)
Once	1,773 (46.3)	2,058 (53.7)
Twice	12,800 (52.8)	11,438 (47.2)
≤3 times	14,849 (56.2)	11,553 (43.8)
Tooth brushing after lunch			<0.001
No	13,081 (52.9)	11,663 (47.1)
Yes	16,535 (54.8)	13,669 (45.3)
Experience of Sealant			<0.001
No	23,061 (58.0)	16,714 (42.0)
Yes	6,555 (43.2)	8,618 (56.8)

Data analysis used Chi-squared test.

3. Differences in oral disease symptoms by smartphone usage behavior

Analysis of the presence or absence of oral diseases according to the smartphone use behavior of the subjects revealed that 96.4% (52,978 subjects) of the total subjects used smartphones during the week and 96.7% (53,148 subjects) used smartphones on weekends. Additionally, 49.1% of the subjects used smartphones for more than 4 hours during the week and 48.0% of the subjects used smartphones for more than 4 hours on weekends experienced symptoms of oral disease.

Analysis of whether there is a risk of dependence on smartphones and symptoms of oral disease revealed that 25.1% (13,775 subjects) of all subjects were dependent on smartphones and 61.3% of adolescents in the risk group of dependence on smartphones experienced symptoms of oral disease (p<0.001; Table 4).

Table 4 . Differences in oral disease symptom experience according to smartphone usage types

Variable	Oral disease symptoms		p
	No	Yes
Smartphone use (Weekday)			<0.001
No	1,205 (61.2)	765 (38.8)
Yes	28,411 (53.6)	24,567 (46.4)
Smartphone use (Weekend)			<0.001
No	1,104 (61.3)	696 (38.7)
Yes	28,512 (53.6)	24,636 (46.4)
Daily hours of using smartphone (weekday)			<0.001
No use	1,205 (61.2)	765 (38.8)
<2 hours	3,099 (58.7)	2,178 (41.3)
2-4 hours	10,131 (56.6)	7,767 (43.4)
>4 hours	15,181 (50.9)	14,622 (49.1)
Daily hours of using smartphone (weekend)			<0.001
No use	1,104 (61.3)	696 (38.7)
<2 hours	1,477 (60.3)	973 (39.7)
2-4 hours	5,680 (59.0)	3,951 (41.0)
>4 hours	21,355 (52.0)	19,712 (48.0)
Dependence on excessively smartphone			<0.001
No	24,281 (59.0)	16,892 (41.0)
Yes	5,335 (38.7)	8,440 (61.3)

Data analysis used Chi-squared test (p<0.05).

4. Relationship between smartphone use, time, overdependence on smartphone, and symptoms of oral disease

The results of examining the factors affecting symptoms of oral disease are as presented in Table 5. There was a significant correlation between smartphone use during weekdays and symptoms of oral disease in adolescents (p=0.036).

Table 5 . Relationship between smartphone usage, time, overdependence on smartphone, and symptoms of oral disease

Variable	Oral disease symptoms		p
	OR	95% CI
Smartphone use (Weekday)	1.777	1.011-1.372	0.036*
Smartphone use (Weekend)	2.777	0.984-1.351	0.079
Daily hours of using smartphone (weekday)
No use	0.941	0.873-1.015	0.116
<2 hours	0.911	0.874-0.950	<0.001***
Daily hours of using smartphone (weekend)
No use	0.865	0.783-0.954	0.004**
<2 hours	0.880	0.834-0.928	<0.001***
Smartphone dependence	2.194	2.108-2.284	<0.001***

CI: confidence interval, OR: odds ratio. Data analysis using logistic regression. *p<0.05, **p<0.01, ***p<0.001.

Adolescents using a smartphone for ≥2 hours during weekdays experienced 0.94 times more symptoms of oral disease than those who did not use smartphones (p=0.036), and adolescents who used smartphones for ≥2 hours during weekends experienced 0.89 times more symptoms of oral disease than those who used smartphones for <2 hours (p<0.001).

Depending on overdependence of smartphones, the group with overdependence experienced 2.19 times of symptoms of oral disease than the non-dependent group (p<0.001).

In this study, 54.2% and 74.7% adolescents in Korea used their smartphones for ≥4 hours during weekdays and weekends, respectively.

Of the adolescents in whom smartphone use of ≥4 hours during weekdays and weekends was the highest, the proportion of adolescents with experience of symptoms of oral disease was the highest, and it was evident that a longer duration of smartphone use on weekdays and weekends was associated with a higher possibility of the occurrence of oral disease symptoms than the adolescents who did not use smart-phones. This is from the result that long-time usage of smartphone leads to the decrease of physical activities [8], and the decrease of physical activities are related to the basic physical activity, tooth brushing, which results in the increase of risk to symptoms of oral disease [9].

According to a study on the overdependence on smartphone in Korea, the proportion of overdependence on smartphone has been increasing every year from 8.4% in 2011 to 19.1% in 2018 [10]. This study also confirmed that the proportion of individuals with a high risk of overdependence has been increasing annually from 25.1%.

Among adolescents with experience with using smartphones, 61.3% had experienced symptoms of oral disease, and this can be seen as a result of research that shows that excessive use of the Internet through smartphones causes mental health problems and ultimately reduces ganglion behavior [11].

Consequently, since the correlation between smartphone usage time has been reported in the past, overdependence, and symptoms of oral disease, the possibility of oral disease caused by smartphone addiction should also be considered in the oral health assessment of adolescents.

A limitation of this study is that factors analyzed by the online Korea Youth Risk Behavior survey, a cross-sectional survey, may have limitations related to generalization of the relationship between adolescents' smartphone usage time and the risk of overdependence and symptoms of oral diseases. Therefore, in the future, it is imperative to more thoroughly clarify the causal relationship by conducting a prospective investigation on the health-related factors of adolescents.

This study was conducted to investigate the relationship between symptoms of oral disease and smartphone usage status of adolescents in Korea by using statistical data from the 16th Korea Youth Risk Behavior Survey.

The results are as follows:

1.Adolescents using smartphones for ≥2 hours during the weekdays experienced symptoms of oral disease 0.94 times higher than those who did not (p<0.001).

2.Adolescents using smartphones for ≥2 hours during the weekends experienced symptoms of oral disease 0.88 times higher than those who used smartphones for <2 hours (p< 0.001).

3.Regarding smartphone overdependence, adolescents group with overdependence experienced symptoms of oral disease 2.19 times more than those who in non-dependence group (p<0.001).

In conclusion, it was found that the increase in smartphone usage time and dependence on smartphones among Korean adolescents were related to symptoms of oral disease.

No potential conflict of interest relevant to this article was reported.