Temporomandibular disorder (TMD) is a musculoskeletal disorder, and it majorly presents symptoms such as pain in the temporomandibular joint and masticatory muscles, joint noise, and mouth opening limitation. Occlusion, trauma, emotional stress, deep pain input, and parafunctional activity are known as the primary causes [1]. In recent years, the prevalence of TMDs has been steadily increasing, and the prevalence is especially high among young adults [1,2]. It is thought that stress due to the complexity of modern social life is the major cause of such a situation, but it has not been proven.

Stress impacts disease via physiological responses to psychological stimuli. In general, an integrated biological response appears after exposure to stress, with changes in the neuroendocrine system helping to overcome the stress response and also influencing the onset or course of the disease. That is, diverse hormones are secreted to increase the body’s resistance to stress factors, and when these reactions are altered, it is reported that many diseases are caused [3].

What is being used to assess hormonal activity against stress include catecholamine, cortisol, a sexual hormone, thyroid hormone, growth hormone, and the like. Among them, cortisol is frequently used in studies as an important indicator that best represents the changed physiological state in response to stress stimuli [4].

Stress-related hormone levels can be measured in blood, urine, and saliva, for example. Although hormone measurements in the blood are widely used, there are some that stress, such as venipuncture to obtain a blood sample. Therefore, much saliva research has recently been conducted [3].

In a recent study on salivary cortisol in TMDs patients, Kobayashi et al. [5] found no difference in salivary cortisol concentrations between TMDs patients and controls, but Alresayes et al. [6] discovered that adolescents with TMDs had high salivary cortisol concentrations. The connection between TMDs and stress has yet to be determined.

Therefore, in this study, the causative factors associated with TMDs stress were investigated using salivary cortisol analysis in young adults with TMDs.

1. Study Subjects

Between June 1, 2021, and August 31, 2021, 32 TMDs patients (16 males, 16 females) and 34 patients without a history of TMDs (12 males, 22 females) who visited Chosun University Dental Hospital were the study subjects. Both the TMD patient group and the control group were assessed with young adults in their late teens, 20s, and 30s. This study was conducted with the approval of the Institutional Review Board (IRB) of Chosun University Dental Hospital (CUDHIRB-02103-010).

2. Study Method

More than 2 mL of nonstimulated saliva from the TMD patient and control groups was collected in an experimental tube (Fig. 1). Saliva was collected between 3 and 5 pm to mitigate the difference in salivary cortisol concentration throughout the day, and the collected saliva in the tube was instantly frozen at −20°C. The age and gender, perceived stress level, and history of bruxism and clenching were evaluated in the experimental and control groups. The experimental group’s persistent symptoms of TMDs were divided into less than 3 months and more than 3 months to differentiate acute and chronic.

Recently, the degree of perceived stress was classified into three stages: + (mild), ++ (moderate), and +++ (severe), and the self-reported history of bruxism and clenching was investigated. Saliva was kept frozen for about 3 months until all samples were collected for salivary cortisol analysis. Salivary cortisol concentration was determined using an enzyme-linked immunosorbent assay, and for the experiment, it was requested by the menopause health center (Leaders Union Co. Ltd., Gwangmyeong, Korea).

3. Statistical Analysis

Statistical analysis was conducted using IBM SPSS Statistics for Windows, Version 26.0 (IBM Co., Armonk, NY, USA). Mann–Whitney U test and independent t-test were used to contrast and analyze the salivary cortisol concentration of the TMD patient group and the control group, as well as the salivary cortisol concentration between the two groups based on the presence or absence of bruxism and clenching. The Mann–Whitney U test was used to contrast and analyze the salivary cortisol concentration based on the duration of symptoms in the TMD patient group and the salivary cortisol concentration based on the believed stress level in both groups.

1. Distribution of Patients

The average age of the TMD patient group was 28.25 years, while the control group was 25.97 years old. The sex ratio was 50.0% male and 50.0% female in the TMD patient group, and 35.3% male and 64.7% female in the control group (Table 1).

2. Comparison of Concentration of Salivary Cortisol in the TMD Patients and the Control Group

In the TMD patients and the control groups, there was a statistically significant difference (p<0.05) (Table 2).

3. Comparison of Concentration of Salivary Cortisol According to the Symptom Duration in the TMD Patients

There was a statistically significant difference (p<0.05) in salivary cortisol concentrations solely in males, but there was no significant difference according to the symptom duration (p>0.05) (Table 3).

4. Comparison of Concentration of Salivary Cortisol According to the Perceived Stress Level in the TMD Patients and the Control Group

In the group with mild and moderate perceived stress levels, there was a statistically significant difference (p<0.05) (Table 4).

5. Comparison of Concentration of Salivary Cortisol According to the Existence of Bruxism/Clenching in the TMD Patients and the Control Group

There was no statistically significant difference between the two groups in the TMD patients and the control group (p>0.05) (Table 5).

TMDs cause temporomandibular joint and masticatory muscle pain and dysfunction. It primarily affects teens and 20s, and the prevalence rises with age, with women being more affected than men [1,2]. A recent domestic study also found a rapid increase in the number of patients in their 20s, but the cause is unknown [2]. Therefore, in this study, while there are numerous causes of TMDs, salivary cortisol was measured for the group in their 20s to assess the relationship with stress (Table 1).

Many diseases are caused or aggravated by stress. Stress-related diseases include tension-type headaches, migraine, coronary artery disease, gastric and duodenal ulcers, irritable bowel syndrome, and various psychiatric disorders. In the orofacial area, recurrent aphthous ulcers, oral lichen planus, and TMDs are linked to stress [3].

In the stress response, activation of the sympathetic nervous system and the hypothalamic-pituitary-adrenal axis (HPA axis) plays important roles. The body secretes multiple hormones to increase resistance due to stress factors, and hormones like catecholamine and glucocorticoid are best used in stress studies [7]. Acute stress stimulates the adrenal medulla through the activation of the pituitary gland, which secretes catecholamines like epinephrine and norepinephrine to induce physiological responses. When stress progresses, the pituitary gland secretes adrenocorticotropic hormone, which in turn secretes the glucocorticoid hormones (cortisol and aldosterone) from the adrenal cortex [3,4].

Cortisol is an important biomarker that can best indicate the physiological state altered in response to stress stimuli [8]. Cortisol secretion peaks around 8 am and is lowest at midnight and is secreted either spontaneously or by various biochemicals and psychological stimuli [7].

The study of neuroendocrine function is based on hormone measurement in the blood and urine. When blood is used for sampling, stress hormone levels can be raised only by the stress of blood collection itself. Therefore, salivary stress hormone analysis is an excellent tool for studying stress-related diseases [9-11]. Because salivary hormone levels closely mirror blood hormone levels, saliva has been widely used in recent studies. Saliva sample storage is simple. Kahn et al. [12] discovered that storing samples at room temperature for 2 weeks did not affect the salivary cortisol levels. Garde and Hansen [13] reported that samples can be stored for up to 3 months at 5°C or at least 1 year at −20°C or −80°C when used for cortisol analysis after storage. In this study, to lower changes in salivary cortisol concentration during the day, more than 2 mL of saliva was collected from a plastic tube between 3 and 5 pm and stored frozen at −20°C until analysis (Fig. 1).

As the cause of TMDs, one factor that can influence masticatory function is the increase in emotional stress experienced by the patient. Stress-induced activation of the HPA axis increases the activity of gamma-efferent fibers via a complex neural pathway, causing the fibers in the muscle spindles to contract. The net result increases muscle tension [14]. In a study by Da Silva Andrade et al. [15], salivary cortisol was higher in women with TMDs with elevated depression and somatization scores, and a study by Salameh et al. [16] also reported that adult patients with TMDs had higher salivary cortisol concentrations than controls, with females showing higher concentrations than males. This study also found a statistical difference in the salivary cortisol concentration between the male and female TMD patient group and the control group, verifying that TMDs are linked to psychological stress (p<0.05) (Table 2).

Cortisol levels in saliva reflect the physiological response to mental stress [17]. The HPA axis is activated when the recipient feels threatened by an impending event with negative consequences. Bassett et al. [18] found that an increased salivary cortisol level was sustained for 2 hours before speaking in front of an audience in a study, and Lehnen et al. [19] also found that salivary cortisol level was highest 10 minutes before public speaking.

Salivary cortisol is also raised by various physical stress or exercise. Stahl and Dörner [20] discovered an increase in salivary cortisol after painful medical procedures such as prostatic biopsy and sternal puncture, and in a study by O’Connor and Corrigan [21], 30 minutes of ergometer exercise at 75% VO_{2 max} indicated a significant increase in salivary cortisol. Remarkably, the cortisol response to repeated physical stress is not habituated.

In research on salivary cortisol concentrations in TMDs patients, Alresayes et al. [6], Da Silva Andrade et al. [15], and Salameh et al. [16] reported high salivary cortisol concentrations in patients with TMDs. However, Lambert et al. [22] found that the Chronic HPA axis response to stress was weak in TMDs patients, and Jasim et al. [23] reported that salivary cortisol levels in women with chronic and acute orofacial pain were not distinct from those in the control group.

In this study, the statistical significance of salivary cortisol concentration was demonstrated in male and mild and moderate groups in comparison according to the duration of TMDs symptoms and perceived stress level (p<0.05) (Tables 3, 4). A comparison based on the duration of the TMDs symptoms revealed a stressed relationship in the male group, but it is believed that a more detailed period setting and a study involving a large number of subjects are required in the future. Furthermore, it was difficult to clearly understand the link between perceived stress level and changes in salivary cortisol concentration because there was no statistical significance in the group conveyed as severe and it is thought that studies with various psychological tests are required in the future.

Causes of TMDs include parafunctional activities like bruxism or teeth clenching. Increased emotional stress aggravates not only muscle tension in the head and neck but also nonfunctional muscle activity such as bruxism and clenching [24]. Bruxism and clenching may be linked to the onset of TMDs or torus mandibularis because they cause muscle hyperactivity beyond physiological tolerance in the orofacial area [25]. Therefore, in this study, tooth clenching and bruxism were investigated to determine whether the cause of tooth clenching and bruxism is stress-related and induces TMDs. In a study of salivary cortisol related to bruxism, Khayamzadeh et al. [26] found that salivary levels were significant in patients with parafunction habits. However, in this study, there was no statistical relevance of salivary cortisol concentration in the group with and without bruxism or clenching (p>0.05) (Table 5). This result is considered to be due to the multiple causes of bruxism or clenching.

In this study, salivary cortisol was measured in the young adult TMD patient group to examine the causative factors related to stress in TMD. In the TMD patient group and control group, the salivary cortisol concentration indicated statistical significance, denoting that stress was the causative factor.

Although salivary cortisol concentration linked to the clinical symptom duration of TMD, perceived stress level, and bruxism or clenching were not statistically significant between the two groups, it was discovered that salivary cortisol is a reliable marker for stress-related studies.

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

Conceptualization: JMA. Data curation: HJP. Formal analysis: JWR. Funding acquisition: JMA. Methodology: CYC, JMA. Project administration: JWR. Visualization: HJP. Writing original draft: CYC, JMA. Writing review & editing: HJP, JMA.