The associations between work-related factors and temporomandibular disorders among female full-time employees: findings from the Fourth Korea National Health and Nutrition Examination Survey IV (2007–2009)

Companies are introducing innovative technologies in an effort to enhance production and efficiency in today’s competitive global market. The accompanying changes in the occupational environment such as longer working hours, more temporary workers, and greater job instability have resulted in a greater workload, faster pace of work, and greater complexity of work for many workers; accordingly, their physical and psychological burden has increased [1].

Long working hours are reported to be associated with all-cause mortality and have a negative effect on circulatory system diseases, diabetes, depression, anxiety, and other types of mental disorders as well as sleep patterns, cognitive function, and health-related behaviors [2–4]. Furthermore, temporary workers are more likely to experience psychological distress and depression than regular workers [5, 6].

Temporomandibular disorders (TMD) are one of the main contributors to pain of the face and are the second most common musculoskeletal condition after chronic low back pain [7]. TMD is a term that refers to all types of pain and functional problems in the masticatory muscles and jaw joints [8]. Some of the most common features of TMD include pain in the face and in front of the ear, limited jaw movement, and clicking or popping noises in the joint when the jaw moves [9].

Women show a higher level of TMD prevalence (2 to 4 times) than men [9]. Sex hormones, especially estrogens, play an important role in the pathogenesis of masticatory muscles and increase sensitivity to pain in TMD [10, 11]. Furthermore, many research articles have reported that women are considered to be more typically associated with TMD and pain control for TMD, because women are more sensitive to the development of pain [11, 12]. In addition to physiological factors, psychosocial factors such as depression, stress, and anxiety are correlated with TMD, and such factors occur more frequently among women than men [13, 14]. However, studies among women on the association between occupational environment and TMD are quite limited in Korea as well as in other countries.

Therefore, this research aimed to identify the association between TMD and work-related factors, such as long working hours and type of employment among female full-time workers in the Fourth Korea National Health and Nutrition Examination Survey (KNHANES IV).

This cross-sectional study investigated the association between work-related factors and TMD. Among work-related factors, the association between non-manual occupation and TMD and the association between working hours and TMD was significant. This study found that the risk of TMD was higher among women who worked more than 60 h per week than among those who worked less than 40 h per week, after adjusting for the general characteristics and work-related factors of this study population. However, the association between TMD and other work-related factors such as shift work and temporary work was not significant.

Younger women were more likely to have a higher TMD prevalence rate (Table 1). This result is consistent with the findings of other study [9]. Young women have a lower threshold for pain and they are more likely to perceive TMD symptoms than men [11]. Women that has a higher educational level showed a higher TMD prevalence rate (Table 1). A study showed jaw dysfunction symptoms were associated with higher education level [20]. Significant associations of pain in the orofacial region have been found with both higher and lower educational levels [21]. The association between educational level and TMD is not conclusive. About marital status in this study, single female workers showed a higher TMD prevalence rate (Table 1). A study reported that unmarried status correlated with poorer health [22]. Another study, however, suggested no correlation between marital status and TMD and whether a correlation exists between them is controversial [23]. For drinking, problem drinkers’ TMD prevalence rate was higher (Table 1). Miettinen O et al. reported that drinking at least once a week correlated with TMD symptoms [24]. The hypothalamic–pituitary–adrenal (HPA) axis’s dysregulation observed in chronic active alcoholism is closely related to psychiatric stress-related disorders, and TMD patients had higher prevalence rates for these disorders [25–27].

According to a 2015–2016 OECD report, the annual number of working hours in Korea was 2,464 in 2002 and this has declined each year. Nonetheless, the annual number of working hours in 2014 stood at 2,124, the second-highest among the OECD member states next to Mexico [28]. The number of working hours per week is limited to no more than 48, including overtime hours, according to the EU [16]. In Korea, death by overwork is considered for those working 60 h or more per week [17].

There are few researches to directly show the association between long working hours and TMD. However, many studies revealed a correlation between long working hours and psychiatric and musculoskeletal problems [29, 30]. Meanwhile, biologic (joint trauma), behavioral (psychiatric problems), environmental (head and cervical posture), and cognitive factors (pain threshold) all play a role in the development of TMD symptoms [31]. Since TMD is a disorder that presents with symptoms of the temporomandibular joint and masticatory muscles, it can also be considered a musculoskeletal disorder. Accordingly, we assume that mental and physical stress arising from long working hours affected TMD prevalence.

Chen Y et al. reported that the job stress of female workers who worked for 48 h or more per week was 1.79 times as that of women who worked for fewer than 48 h [32]. In the prospective Whitehall II cohort study, showed an excess risk of depression (hazard ratios [HR] 2.67, 95% CI 1.07–6.68) and anxiety (HR 2.84, 95% CI 1.27–6.34) associated with long working hours among women [33].

And many studies have supported that psychosocial factors related to long working hours have a correlation with TMD [13, 14]. In a case-control study, patients with TMD pain showed a higher level of anxiety (OR 5.1), somatization (OR 2.7), and depression (OR 3.5) than the control group [34]. Depression (incidence density ratio [IDR] 3.2), perceived stress (IDR 2.6), and mood (IDR 7.3) increased the risk of TMD [35]. A prospective cohort study on orofacial pain confirmed several psychological variables (stress, previous stressful life events, and negative affect) that could predict the onset of TMD pain [36].

In this study results, the TMD prevalence rate was significantly high for workers who perceived stress a lot (Table 1). Although we might think of the role of stress in the relationship between long working hours and TMD, the stress used in this study was not evaluated using a quantitative stress assessment tool but using only a questionnaire survey on stress perception in general. Therefore, it is not enough to regard the stress of our research as stress associated with long working hours. A more detailed analysis would be needed in the future, using an assessment tool that can accurately show a correlation between stress and long working hours.

Although the participant group with depressive symptoms showed a higher prevalence of TMD than the group with no depressive symptoms, the difference was not significant (Table 1). One possible reason of inconsistent findings with other previous studies could be that we used a single question to evaluate whether someone had depression; therefore this could affect the validity of our results. This should be addressed in future follow-up research.

In this study, we found no significant differences between day work and shift work when it came to prevalence of TMD. One of the reasons could be that there was possible underestimation owing to the healthy worker effect. Another reason is too small number of study subjects in this study.

In addition, we assume that satisfaction with shift work, not shift work itself, is associated with TMD. Symptoms related to TMD were correlated with alexithymia and depressive mood [37]. Dissatisfaction with shift work, not shift work itself, has been reported to have a positive correlation with depressive mood [37]. Kim et al. reported that whereas shift work increased metabolism risk factors for cardiovascular diseases or had a negative effect on mental health, shift work offers less responsibility outside of normal working hours and greater economic reward than day work [38]. Accordingly, it is necessary to more accurately evaluate the level of job satisfaction arising from shift work.

When the association between type of employment and TMD was examined, the TMD risk among temporary workers (OR 1.14, 95% CI 0.75–1.72) was higher but it was not significant (Table 3). According to 2016 data of Statistics Korea [39], 40.3% of female wage earners in Korea are temporary workers; the percentage of temporary female workers (27.8%) was lower in our research. It is believed that different definitions and assessment methods of temporary workers are used, which would result in different percentages of temporary workers and ultimately different TMD prevalence rates among published studies. Accordingly, future research should be conducted using an agreed definition, if possible, rather than a questionnaire as in our study, to identify whether a participant is a temporary worker.

Only few people have difficulty in daily life owing to TMD. However, such disorders worsen while unnoticed, and patients miss the right treatment timing and the disorders develop into chronic ones. Greene et al. reported that chronic TMD pain can result in the absence or loss of work or social interaction, and ultimately reduces the overall quality of life [40]. In the United States, it is estimated that 17.8 million working days are lost annually for 100 million full-time adult workers due to serious TMD [41].

The following limitations should be noted in this study. First, this research was a cross-sectional study to identify the association between work-related factors and TMD, therefore, we were unable to find any causal relationships. Second, this study was unable to reflect the double burden of female full-time workers. Third, information error may have been introduced during data collection using the questionnaire. We only checked whether participants worked shifts and did not evaluate their levels of satisfaction or stress regarding shift work, which would have resulted in a more compelling conclusion. In addition, in this study, participants were evaluated using only the questionnaire, without establishing a clear definition of regular and temporary workers. Lastly, various confounding variables, such as oral and maxillofacial habits that could affect TMD (clenching and bruxism), were not considered in this research. Tooth contacting habits (TCH) such as clenching and bruxism are considered to have an effect on the incidence, continuation, and deteriorating condition of TMD [42]. However, we did not include evaluation items for clenching and bruxism owing to the nature of the data used, which could not accurately reflect these factors and therefore could not produce significant outcomes. Follow-up research should incorporate evaluation items for clenching and bruxism to delve further into these factors.

Despite these limitations, our study is the first to address the associations between work-related factors and TMD using representative national data in Korea. There are various factors that contribute to TMD. If TMD is treated at an appropriate time and by considering various factors, the quality of life for people with TMD would be greatly improved. In addition, we believe it can reduce socioeconomic loss resulting from TMD.

In this study, we found the risk of TMD was higher among women with long working hours. We believe that this study holds significance, as it provides basic data on the negative effects of long working hours on health.