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Associations between self-rated oral health, subjective oral conditions, oral health behaviours and oral health-related quality of life among college students: A study using structural equation modelling

Background: This study aimed to evaluate the relationship between self-rated oral health, subjective oral conditions, oral health behaviours, and oral health-related quality of life (OHRQoL) in Chinese college students. Methods: We conducted an online cross-sectional survey inviting college students from eastern China. A total of 1,708 participants were included. We constructed a structural equation model to explain and assess the associations among self-rated oral health, subjective oral conditions, oral health behaviours, and OHRQoL. Results: Self-rated oral health had a direct positive effect on subjective oral conditions and OHRQoL. Oral health behaviours had a direct negative effect on subjective oral conditions and OHRQoL, while oral health behaviours had a direct negative effect on tooth condition perception and oral health interventions. Subjective oral conditions had a direct positive effect on OHRQoL. There was a positive correlation between oral health behaviours and self-rated oral health. In addition, subjective oral conditions partially mediated both the effect of oral health behaviours on OHRQoL and the effect of self-rated oral health on OHRQoL. Conclusion: There were inuential associations between self-rated oral health, subjective oral conditions, oral health behaviours, and OHRQoL among college students in eastern China.

Epidemiological studies often use self-rated oral health as an indicator of a subjective experience that has an important impact on people's well-being and quality of life [8][9][10][11]. A study by Yamane et al. pointed out that 63.2% of Japanese college students were found to have fair or even poor self-rated oral health, and self-rated oral health had a signi cant impact on OHRQoL [12]. Self-rated oral health re ects an individual's subjective experience of perceptions of physical and mental health, treatment needs, functioning, clinical signs and symptoms of adverse conditions [13]. It is one of the patient's most immediate responses, covering multiple aspects and determinants of oral health [14], and can sometimes even replace clinical disease [14][15][16]. Conducting a self-rated oral health can help society to plan and monitor health services as well as health promotion interventions to improve overall health [10,11,14,[17][18][19]. Previous studies have shown that the monitoring of young people's oral health should also include information on testing self-rated oral health [20,21]. The main factors associated with self-rated oral health include clinical examination oral factors as well as subjective oral factors. Clinical examination oral factors included tooth loss, tooth decay, bleeding gums, and dental care, while subjective measures included reported overall health, oral appearance, and dental pain [14]. Subjective oral conditions are primarily derived from individual judgements, that is, an individual's subjective perception of oral conditions [22]. Currently, subjective oral conditions are used more often in dental care to determine the severity of a patient's disease rather than the only traditional objective oral conditions indicators [23]. It has been shown that an individual's subjective feelings have a more signi cant impact on how they feel about themselves, and it can make a large difference in a patient's mood and condition [24]. A study by Bernabé E et al. reported that about a quarter of adolescents would have an impact on their quality of life due to poor oral conditions [25]. The physical and psychological changes of adolescents are quite rapid, and their health problems are gradually shifting from their parents' to their own responsibility [26]. But their oral health awareness is limited [27]. Attention to the subjective oral conditions of college students is of great importance for both their physical and mental development.
Studies has reported that poor oral health behaviours may adversely affect OHRQoL [28,29]. Oral health behaviours include proper brushing, use of uoride toothpaste, ossing, reducing intake of high-sugar foods, and regular oral examinations. A study by Hsu KJ et al. found that oral health behaviours and oral habits affect dental health [30]. Study has pointed out that as college students adjust to a new environment, they will also face greater stress and therefore they tend to neglect oral health care [31]. Thereby, college students may be inattentive to adopt appropriate oral health care behaviours. A study by Crabtree R et al. found that proper oral health behaviours among college students were an important factor in overall health and well-being [31]. This may further have an impact on their oral health-related quality of life .
In China, there are few studies on the associations between self-rated oral health, subjective oral conditions, oral health behaviours and OHRQoL. In addition, there are not many studies that combine oral health, personal behaviour, and OHRQoL. Therefore, this study aimed to construct structural equation modelling (SEM) to explore the complex associations between them. The results of our study will help to intervene and improve the oral health of college students.

Subjects and Design
We conducted a cross-sectional research survey of college students from 22 comprehensive colleges in Anhui Province, China between October 2019 and January 2020. An electronic questionnaire was sent to college students using an online survey. Verbal consent was obtained from all participants and there was no intervention throughout the information lling process. A total of 2,000 e-surveys were sent to these college students via a random sampling method within each college, and total returns were 1,817. The data analysis rejected 109 questionnaires and included a total of 1,708 valid questionnaires, with a response rate of 94.0%. The effective response rate was 85.4%. The exclusion criteria were as follows: (1) questionnaire missing values greater than 10%; (2) reading and comprehension di culties, and (3) logical inconsistency of answers.
Moreover, our study is based on a cross-section of the population of Anhui province. The sample size was calculated according to the formula N = Z²P(1-P)/d² (Z = 1.96, P = 60.1%, d = 0.05) [32]. Where N is the sample size. Z is the statistic, when the con dence level is 95%, Z = 1.96. P was calculated using the oral health knowledge rate of 60.1% of the population from a previous survey [33]. And considering 20% invalid response rate. The minimum sample size obtained was 441. We exceeded the minimum required sample size to ensure the credibility of our results.

Ethical Approval
The study was ethically approved by the Medical Ethics Committee of Bengbu Medical College, China (No. 2019-062). Prior to the start of the study, the details of the study and the processing of the results were explained to college students. Students who did not want to cparticipate in the study could refuse to ll out the questionnaire, and all participants did so voluntarily.

Hypotheses
According to our hypotheses (Figure 1), self-rated oral health and oral health behaviours have a positive effect on subjective oral conditions and OHRQoL. Oral health behaviours are also positively related to self-rated oral health. Also, subjective oral conditions are positively related to OHRQoL.

Survey Instruments
We designed our questionnaire based on reference literatures. The questionnaire included the following items: demographic characteristics, self-rated oral health, subjective oral conditions, oral health behaviours, and OHRQoL.

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The rst part is self-rated oral health, which consists of 5 questions. This part is divided into three categories. The rst category is the oral health perception, and use the question 'What is your judgment about your oral health?'. The answer used the 5-point Likert scale to calculate the score, '0 = very good', '1 = good', '2 = fair', '3 = poor' and '4 = very poor' [11]. Use 'What is your assessment of your tooth health' to determine the second category of tooth condition perception. The answer uses the 5-point Likert scale to calculate the score, '1 = excellent', '2 = very good', '3 = good', '4 = average', '5 = poor'. The third category is oral health interventions. The questions include 'When did you last visit the dentist (stomatologist)?'. The response options were '1 = never', '2 = last month', '3 = last three months', '4 = last six months' and '5 = one year ago'. 'What was your reason for going to the dentist?' and 'Have you received any oral health education (health education seminars) ?'. The response options are: '1 = yes' and '2 = no'.
The subjective oral conditions were evaluated using 9 questions such as 'Have you ever had bleeding gums?'; 'Have you ever had tooth pain?'; 'Did you ever feel TMJ pain when opening your mouth or chewing food?'. The answer uses the 3-point Likert scale to calculate the score, '0 = never', '1 = occasionally', '2 = frequently'.
Oral health behaviours include 9 questions. Participants lled in the questions according to their actual situation. This part is divided into four categories. The rst category is brushing. The questions include 'Do you brush your teeth ≥ 2 times a day?' (B1); 'Do you brush your teeth every time ≥ 3 minutes?' (B2); 'Do you change a toothbrush every three months?' (B3). The second category is tooth cleaning. The questions include 'Do you often use dental oss (or interdental brushes) to help clean interdental spaces?' (C1) [34]; 'Do you often gargle after meals?' (C2); 'Do you clean your teeth regularly (teeth washing)?' (C3); 'Is your brushing method the 'horizontal tremor brushing method' recommended by the Chinese Stomatological Association?' (C4). The third category is the oral health checkup. The question is 'Do you regularly perform oral health checkups?' [34]. The fourth category is the use of uoride toothpaste, the question is 'Do you often use uoride toothpaste?' [20]. The option scores of the questions are '1 = Yes' and '0 = No'.
The overall Cronbach's α for the questionnaire is α = 0.903. The internal consistency of this questionnaire is good.

Statistical analysis
We used IBM® SPSS® Statistics 20.0 and IBM® SPSS® Amos™ 24.0 for data analysis.
We used frequency, percentage and mean ± standard deviation to describe the demographic characteristics of participants. We tested the appropriateness of the scale using the KMO measure and the Bartlett spherical test. The KMO coe cient for this questionnaire was 0.927 and the p-value of Bartlett's test was < 0.001. We used exploratory factor analysis (EFA) to examine the degree of association between questions. A project analysis was carried out to test the degree of differentiation of the questions on a 27% scale. The associations that existed between self-rated oral health, subjective oral conditions, oral health behaviours, and OHRQoL were analyzed by constructing a SEM. Before constructing the SEM, we rst performed a con rmatory factor analysis (CFA) to assess whether the factor structure chosen based on our study was also acceptable in these data or whether it should be modi ed [36]. When constructing the SEM, each observed variable is imported and calculated as a mean value. We use the Bootstrap method [35,37] to verify whether there are mediating effects between the variables of interest in the model. We bootstrapped 5000 samples from the raw data (n = 1,708) by putting back a random sample. The con dence interval (CI) was 95% for the direct and indirect effects of each of the suggested mediating variables beyond zero. Using the skewness-kurtosis test to check the normality of the observed variables [38].

Demographic data
We analyzed a total of 1,708 data from college students, aged 22.0 ± 4.3 years. The majority of them were female (57.4%), medical non-oral majors (54.3%), junior (27.5%), and with an annual household income less than ¥50,000 (42.7%). Table 1 shows the characteristics of the respondents.   Correlation analysis between variables Figure 2 shows the standardized estimated parameters of the nal structural model. Table 3 shows the results of the correlations between the variables. All paths are signi cant (p < 0.001). The model showed that (1)  And (7) oral health behaviours had a negative effect on subjective oral conditions (β = 0.143) as well as tooth condition perception (β = 0.595). We used the Bootstrap method to test the mediating effects of the model with a bias correction of 95% CI. The direct and indirect effect results are shown in Table 4. The results showed that (1)

Discussion
This study analyzed the potential associations between self-rated oral health, subjective oral conditions, oral health behaviours, and OHRQoL among college students by constructing SEM. The results showed that there were in uential associations between the variables. To our knowledge, there are few such studies for college students in Anhui Province. Our study aimed to explore in advance the oral health behavioural problems of college students as well as their self-perceptions of oral health to improve OHRQoL and further improve their health status. It is instructive for targeted improvement and prevention of oral health problems among college students, as well as for systematic oral health education.
We found a direct negative relationship between college students' oral health behaviours and their selfrated oral health (β = -0.326, p < 0.001). According to the questionnaire score design, correct oral health behaviours lead to better self-rated oral health, and good self-rated oral health also promotes the development of correct oral health behaviours. Ueno M et al. found that when people become aware of a problem with their oral conditions, it may lead to corresponding health behaviours change, such as visiting the dentist and improving brushing patterns [10]. Meanwhile, when poor oral health behaviours are identi ed, they can be improved in order to enhance oral health [42]. There are similar to our ndings. Oral health behaviours are important determinants of oral health, including regular brushing with uoride toothpaste, checkups, and preventive treatments (scaling, sealants, and uoride treatments) [43]. Because previous study has generally recognized health as a subjective state, self-rated health has been used more in dental epidemiology and health services research [44]. Health education and the development of lifelong health behaviours by making the most of the golden age of college [31]. It is possible to fundamentally improve students' self-rated oral health.
The results of the study showed that the better the oral health behaviours, the more oral health interventions were implemented by the students. This is similar to the results of the previous study [45,46]. Oral interventions included seeing a dentist and receiving education about oral health. Study has shown that, oral health behaviours such as annual dental checkups and brushing after every meal are essential to reduce plaque and tartar [47]. Nakre PD et al. found that oral health education had a signi cant effect on improving people's oral health behaviours [48]. Therefore, regular diagnosis of dental disease and dental checkups are more likely to help improve the condition of mouth as well as overall health [49,50]. College students are mainly 17 to 25 years old and are in the transitional stage of life, which is an important period for the formation of health concepts and behaviours [51]. Hence, during this period, it is important to provide the necessary oral health interventions and dental education to make them aware of the importance of oral health in order to maintain their oral health in the future.
In addition, the results indicated that when the subjective oral conditions are worse, the poorer the OHRQoL of college students. This nding is also supported by similar studies. A study of Japanese college students by Yamane et al. found signi cant effects of oral diseases such as oral pain, malocclusion, dental caries, and TMD on OHRQoL [12]. When people suffer from toothache, they inevitably experience unbearable pain, which has a negative impact on their daily work and quality of life [52]. It has been found that subjective health condition assessments re ect more accurately on a patient's quality of life than clinical assessments [53]. Thus, when oral diseases are reduced and oral health is improved, it may bene t mental health and quality of life will improve accordingly [54]. We encourage college students to have regular oral health examinations and to treat oral diseases as soon as possible when they occur so that their quality of life is not compromised.
Our study demonstrated that the better the self-rated oral health of college students, the better their subjective oral conditions as well as OHRQoL. Rui Arantes et al. found that symptoms of oral diseases such as toothache play an important role in oral health self-assessment [13]. Our nding was similar to the result of previous study in Japan, which showed that self-rated oral health had positive correlations with subjective chewing ability as well as OHRQoL [12]. The perception of oral health was strongly related to their utilization of oral health services and whether they were aware that they had oral diseases. In other words, subjective oral disease conditions also in uences college students' perceptions of their oral health [55]. On the other hand, perceived oral health re ects an individual's subjective impressions, including not only health status, but also satisfaction with the aesthetic condition and of oral health [56].
When college students have good oral and dental health, they also have clean breath and beautiful smiles, and they make a better impression when interacting with their peers, which is important for today's adolescents [31]. This can be interpreted to mean that when the oral and tooth conditions are good, college students also develop enough self-con dence in themselves to perform better in all aspects of lives, thus contributing to their well-being and quality of life.
The path from oral health behaviours to OHRQoL did not t our expected model (β = 0.119, p < 0.001).
The results indicated that oral health behaviours have a negative impact on OHRQoL. However, Vigu AL et al. 's study of Romanian college students found a association between oral behaviours and OHRQoL [57]. A study by Broadbent et al. found that oral health behaviours in adults have a signi cant impact on symptoms such as dental caries or tooth loss, thus further in uencing OHRQoL [29]. The reason for this result, we speculate, may be related to the way the survey was a questionnaire distribution, the recall bias of college students, or to the fact that there are few young college students with severe oral diseases.
Our results suggest that when college students perform better oral health behaviours, they may have worse subjective oral conditions and poorer tooth condition perception. This differs from previous nding [58]. Lalani A et al. 's oral health survey of dental students found that oral health behaviours were signi cantly and negatively associated with plaque and gingivitis [59]. The reason for this phenomenon can be explained by the fact that a small percentage (14.8%) of the population in our survey were dental students, and the rest of the college students may not have really learned the correct brushing or oral cleaning methods, which did not play a signi cant role in improving their oral health. They felt overly good about their oral health behaviours, but the actual subjective oral and tooth conditions were not satisfactory. This results in college students' oral health behaviours negatively impacting subjective oral and tooth status. It has been shown that people who do not have expertise have a poorer grasp of oral health behaviours than professional dentists, and their periodontal condition is worse as a result [60].
Notably in our results, subjective oral conditions play a partial mediating role in both the effect of oral health behaviours on OHRQoL and the effect of self-rated oral health on OHRQoL. This is similar to previous studies [12]. It could indicate that the impact of subjective oral conditions on the quality of life of college students cannot be ignored. Therefore, it can be assumed that the subjective oral conditions are a prerequisite for OHRQoL. In addition, oral health behaviours had the greatest impact on tooth condition perception. Therefore, improving oral health behaviours may be the most effective in changing the tooth condition of college students.

Limitations
Our study has some limitations. First, our study was only conducted among colleges in Anhui Province, which had a limited representative population and cannot be generalized to college students nationwide.
Second, our data were obtained by distributing questionnaires, which were subjectively reported by college students, and the lack of clinical examination made it impossible to avoid recall bias and over-or under-evaluation by college students. Third, our survey was a cross-sectional study, and it was not possible to infer causal associations between variables. Later we will extend the study to longitudinal sections. In addition, the in uence of other factors (e.g. gender, age) on each variable may have been overlooked.

Conclusion
The self-rated oral health was well in this group of Chinese college students, and some oral health behaviours were also well done, but the subjective oral conditions were not very good. In addition, we found a positive correlation between oral health behaviours and self-rated oral health. Self-rated oral health had a positive effect on subjective oral conditions and OHRQoL. Subjective oral conditions had a direct positive effect on OHRQoL. Oral health behaviours had a negative effect on subjective oral conditions, tooth condition perception and OHRQoL. Oral health behaviours had a positive effect on oral health interventions. Furthermore, subjective oral conditions play a partial mediating role in both the effect of oral health behaviours on OHRQoL and the effect of self-rated oral health on OHRQoL. Further in-depth studies are needed to con rm these results.

List Of Abbreviations
OHRQoL: oral health-related quality of life; OHIP: Oral Health Impact Pro le; SEM: structural equation modelling; MLE: maximum likelihood estimate; EFA: exploratory factor analysis; CFA: con rmatory factor analysis; CI: con dence interval; DF: degree of freedom; GFI: goodness of t index; CFI: comparative t index; NFI: normed t index; IFI: incremental t index; AGFI: adjusted goodness of t index; RMSEA: root mean square error of approximation Declarations Ethics approval and consent to participate The protocol of this study was approved by the Medical Ethics Committee of Bengbu Medical College, China (project code 2019-062). All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. Before data collection, all participants read the consent information, and provided verbal informed consents by clicking "Continue" to take this online survey. The informed consent protocol was approved by the Medical Ethics Committee of Bengbu Medical College, China.

Consent for publication
Not applicable.
Availability of data and materials The datasets used and/or analysed during the current study were available from the corresponding author on reasonable request.