This study is the first to use the Chinese version of the JFLS scale to describe limitations in jaw function in Chinese patients with TMD and to comprehensively explore the risk factors associated with jaw functional limitation based on a biopsychosocial model. The main limitations identified were in mastication and mobility, with females showing more significant functional restriction than males. Diagnosis subgroup, pain intensity, oral behaviors, and anxiety were significant predictors of jaw functional limitation. Anxiety could further increase the degree of jaw function limitation by inducing oral behaviors and exacerbating pain. These findings fill a gap in the data on jaw function among Chinese patients with TMD, providing a reference for subsequent studies comparing findings between different populations as well as providing a theoretical basis for developing targeted strategies to improve mandibular function in clinical practice.
Basic jaw functional limitation characteristics in Chinese patients with TMD
Although no standard JFLS scores have been established, the OPPERA study, which included 185 patients with chronic TMD, reported the following median scores [17]: mastication, 2.22; mobility, 2.22; verbal and emotional communication, 0.72; and global limitations, 1.74. By comparison, the scores for our study sample group were higher: mastication, 3.19; mobility, 5.25; verbal and emotional communication, 0.52; and global limitations, 2.27. These differences might reflect differences in the inclusion criteria between our study and the OPPERA study. Specifically, both acute and chronic TMDs were included in our study, as long as the DC/TMD criteria were met. Moreover, all patients in our study were of Chinese ethnicity, mostly Han Chinese. Therefore, ethnic and cultural factors may have biased our results.
The most prominent restriction among our patients was the inability to open the mouth wide enough to bite an apple or a sandwich, possibly due to articular problems and some jaw movement limitations. This markedly affected patients’ eating functions, prompting them to seek medical help. Patients also often had difficulty in chewing tough food or hard bread, exhibiting masticatory dysfunction. This may be related to altered muscle control and dental occlusion patterns. Patients with TMD have impaired orofacial motor functions as compared to healthy controls, with changes in masseter and temporal muscle recruitment during mastication altering motor control of the jaw–facial muscles and eventually hampering chewing [18]. Additionally, Nickel et al [19]. found that healthy individuals had higher incisor biting symmetry in muscle organization (P < 0.03) than did patients with TMD. We found fewer verbal and emotional communication limitations than Kuć et al [20]. who studied 50 Polish patients with TMD who self-administered the JFLS, and reported difficulty in smiling and in producing facial expressions and emotions. This may be due to cultural differences: East Asians express feelings more subtly, requiring fewer muscle movements than Eastern Europeans. This view is supported by Zhi et al [21]. who argue that there are many inconsistencies between Asians and Westerners in the intensity of facial reactions due to cultural differences. This also suggests that the sensitivity of JFLS as a tool for assessing jaw function may be inadequate for detecting verbal and emotional communication, which limits the detection of limited social function in Chinese patients with TMD.
Women had significantly higher jaw function limitation scores in mastication and jaw mobility, as well as higher global limitation scores, than men did. We suggest that these sex differences in jaw function are most likely related to psychological effects. Liu et al [22]. noted that men and women have different coping strategies and performance when experiencing pain and stress; women are alert and sensitive to adverse emotions, which can make their disease experience worse. Similarly, Kim et al [23]. pointed out that women are more affected by general mental health issues including stress, depressive symptoms, and suicidal thoughts, than men. Bagis et al [24]. likewise found that women showed TMD signs and symptoms more frequently than men did. Hence, sex differences should be considered in clinical practice, and health education and psychological guidance targeted toward female patients are necessary.
Risk factors for jaw functional limitations
The diagnosis subgroup was a significant predictor of limitations in jaw function, with patients diagnosed with intra-articular TMD and combined TMD having higher JFLS scores, consistent with the progression and underlying characteristics of the disease. Intra-articular TMD is defined as joint dysfunction due to disc displacement, degenerative joint disease, and joint subluxation, often manifested as restricted mouth opening, pain in the area of the TMJ, or active restriction of jaw movement due to fear of TMJ dislocation. The above manifestations affect the vertical jaw movement, as well as jaw functions, such as chewing hard objects. In contrast, pain-related TMD mainly includes muscle pain, due to muscle tension and spasm, and joint pain, due to joint effusion, synovitis, or osteoarthritis, where the pain is the primary manifestation and actual mouth opening is not significantly restricted. Therefore, patients with pain-related TMD experience less jaw restriction compared to patients with intra-articular TMD. Patients with combined and pain-related TMD have a more complex condition, with more severe symptoms and more jaw restrictions as they have both restricted mouth opening and painful symptoms.
Pain intensity also significantly predicts limitation in jaw function. First, patients may exhibit painful mouth opening and mastication, which affects jaw function. It is worth noting that this may also be related to pain-induced fear of movement, which is supported by the study by Marshall et al [25]. In that study, fear of movement was found to be a mediating factor between pain intensity and disability.
To the best of our knowledge, this study is the first to identify the OBC score as a significant predictor of limitation in jaw function. Oral behaviors refer to activities other than normal oral physiological functions, such as clenching teeth, chewing gum, grinding teeth, biting nails, and holding the jaw, which may adversely affect the teeth, TMJ, and masticatory muscles. Excessive movement, tension, and spasm of masticatory muscles cause chronic damage to the masticatory system, resulting in jaw-facial pain and affecting jaw function [26]. Tuncer et al [27]. found that the probability of having high JFLS scores was 3.551-fold greater in patients with sleep bruxism (SB) than in those without SB (P = 0.002). Our study confirms that oral behaviors are associated with jaw functional limitation, with a higher the OBC score being associated to a higher the JFLS score.
Anxiety was also significant predictors of jaw functional limitation. Anxiety has previously been documented to influence muscular activities. It can disturb the harmonization among closely matched muscles around the joint and reduce mechanical properties, eventually leading to masticatory dysfunction [28, 29]. Furthermore, anxiety may dysregulate various interacting neurobiological systems, resulting in central sensitization and elevating the patient’ s self-perceived degree of jaw functional limitation [30, 31].
Moreover, anxiety can indirectly increase the degree of jaw function limitation by inducing oral behaviors and exacerbating pain. Numerous previous studies have shown that mental stress can be relieved by actions such as grinding, clenching, or finger sucking. Owczarek et al [32]. found a significant tendency for submandibular muscle tone to increase as indicators of stress, anxiety, and depression increased. It is suggested that anxiety can promote the development of certain oral parafunctions and that these oral behaviors will further lead to increased tension in the masticatory muscles, triggering dysfunction of the masticatory muscles and the TMJ, which in turn affects jaw function.
The relationship between anxiety and pain may be related to its effects on the hypothalamic endocrine system and the autonomic nervous system. Abnormalities in the secretion of body fluids, enzymes, and hormones increase the secretion of endogenous pain substances, decrease pain-suppressing substances, and lower pain thresholds [33]. Asquini et al [34]. explored the effects of coronavirus disease 2019 on the psychological state, central sensitization, and intensity of oral and maxillofacial pain in patients with TMD and found that patients with chronic TMD were more vulnerable to the psychological/social aspects of the pandemic, pointing to the amplifying effect of anxiety on pain.
Although not all included psychological risk factors were found to be significant predictors of jaw functional limitation in multiple regression analysis, we cannot ignore the potential impact of these factors on jaw function. Kuć et al. [20] constructed a multiple regression model with the GAD-7 score as the outcome variable and identified the PHQ-15 and PHQ-9 scores as significant predictors. These studies showed that psychological risk factors, such as depression, anxiety, somatization, sleep quality, and oral behaviors, are interrelated and have a combined influence on jaw function. These risk factors may play a critical role in reducing the jaw function of patients with TMD. It is, therefore, essential to assess patients’ psychological status.
Strengths and limitations
The strength of our study is that it is the first to characterize jaw function among Chinese patients with TMD and to explore the relationship between limitations in jaw function and a full range of biopsychosocial factors. Furthermore, this study clarified, for the first time, the effect of oral behaviors on jaw function limitation and found that anxiety can increase jaw function limitation by inducing oral behaviors and exacerbating pain. The results provide theoretical guidance for future research relevant to the diagnosis and treatment of TMD.
This study also had some limitations. First, this cross-sectional study could not explore the causal association between biopsychosocial risk factors and jaw functional limitation. Second, there was no healthy control group, and we could not compare the jaw functional status of healthy subjects with those of patients with TMD. Third, psychological variables were collected using self-reported questionnaires, and the influence of subjective bias cannot be excluded. Lastly, we did not include pain catastrophizing or fear of movement among the psychological risk factors. Future studies could include healthy controls and explore the association of more comprehensive biopsychosocial risk factors with jaw functional limitation through longitudinal studies.