Depression and Anxiety as Predictors of Pain and Sensory Thresholds in Adults with and Without Temporomandibular Disorder: A Case–Control Study

ABSTRACT

Aim:

Temporomandibular disorders (TMDs) are often associated with psychological distress and altered pain perception. However, the relationship between depression, anxiety, and sensory/pain thresholds in TMD remains underexplored. The present study aimed to assess the association of depression and anxiety with sensory and pain thresholds in adults with and without TMD using standardized electrical stimulation.

Materials and Methods:

This observational case–control study included 56 adults with TMD and 56 matched healthy controls. Psychological distress was evaluated using the SCL-90-R questionnaire. Sensory and pain thresholds were assessed via electrical stimulation of the dental pulp. The sensory threshold was the minimum intensity of a stimulus, and pain threshold is the minimum intensity of a stimulus that triggers the perception of pain. Statistical analyses included Mann–Whitney U, Kruskal–Wallis, Spearman correlation, and multivariate logistic regression, adjusted for age and gender.

Results:

TMD subjects demonstrated significantly higher depression and anxiety scores than controls (P < 0.01). Pain thresholds were negatively correlated with depression and anxiety in both groups (r ≈ –0.63 to –0.66, P < 0.001), while anxiety correlated with lower sensory thresholds in TMD subjects only (r = –0.37, P < 0.01). Multivariate analysis revealed that a low pain threshold was independently associated with moderate/severe depression (OR = 4.38; 95% CI: 1.13–17.04), while a low sensory threshold was linked with moderate/severe anxiety, TMD status, female gender, and older age.

Conclusion:

Depression is independently associated with reduced pain thresholds in both TMD and healthy subjects. Anxiety is specifically linked to sensory threshold reduction in TMD patients. These findings underscore the need to integrate psychological assessment in chronic orofacial pain management.

INTRODUCTION

It is estimated that more than 50 million Americans suffer from chronic pain.[1] Chronic pain is related to psychological distress including anxiety and depressive disorders.[2,3] Studies examined the role of psychological distress in headache,[4] fibromyalgic pain,[2] neck pain,[5] and low back pain.[6] Objective evaluations of the pain threshold have been performed commonly under stimulation by dipping the forearm in coldwater,[7] electrical stimulus,[8] pressure,[2] or other means such as acupuncture.[9] The literature supports that individuals reporting chronic pain anxiety and depression present a reduced pain threshold. Indeed, reactivity to pain can vary significantly among subjects. However, while responsiveness is influenced by individual characteristics such as gender and age,[2] it may also depend on the psychological status.[10] Temporomandibular disorder (TMD) is highly prevalent in the general population,[11,12] which is associated with headache,[13] neck pain,[14] and psychological distress, especially depression.[15] Despite the potential clinical and practical relevance of the possible link between pain sensitivity and psychological factors in TMD,[16] there is a lack of experimental studies examining the relationship between TMD and evoked pain. It was found that subjects suffering from TMD exhibited high responsiveness to pain. Sherman[17] tested patients with TMD and/or myofascial pain according to the criteria of the Research Diagnostic Criteria for TMD[18] and concluded that patients reporting anxiety or depression overestimated the cognitive and behavioral component of pain reaction. Slade[19] found in a longitudinal study conducted on healthy women that psychological characteristics such as depression or perceived stress are associated with pain sensitivity and influence the risk of developing TMD. In addition to pain sensitivity, assessing the sensory responsiveness to stimulation might be valuable as altered sensory reactivity could be an early indicator of individual discomfort and possibly reflect a psychological distress. However, there is currently a lack of studies assessing this issue in subjects suffering from TMD. The aim of this study was to examine the relationship between psychological distress (depression and anxiety) and sensory and pain thresholds under electrical stimulation in adults with TMD and to evaluate whether any difference may exist with subjects without TMD.

MATERIALS AND METHODS

DESIGN AND STUDY SAMPLE

This observational case–control study included 56 subjects affected by TMD (13 males and 43 females) consecutively admitted to the Department of TMDs and Orofacial Pain, San Paolo Hospital, Milan, between June 2023 and December 2023, and 56 gender and age (+1 year) matched healthy controls randomly selected from the population included in the Reference Local Health Authorities register of individuals resident within the hospital district. Inclusion criteria were as follows: age ≥ 18 years, presence of at least both central natural superior incisors, and being of White origin. Exclusion criteria were as follows: patients with type I diabetes mellitus, pregnancy, oral contraceptive use, hormone replacement therapy use, those undergoing anticonvulsant therapy, treatment with immunosuppressive drugs (including systemic corticosteroids), those with immunodeficiency (i.e., leukemia, cancer, and seropositivity for HIV), acute or serious or chronic medical conditions (i.e., myocardial infarction, stroke, and neurological diseases), and with participation in another study in the past 6 months or being under follow-up. The sample size estimation was 70 subjects, but we excluded 14 subjects from the research due to the exclusion criteria, especially because we realized they had previously participated in another similar study. Controls were initially contacted telephonically by our department equipe and invited to participate in the study. If they agreed, they were screened for eligibility and subsequently scheduled for an eligibility verification visit at our department. This visit included a dental evaluation to rule out the presence of TMD. All eligible subjects received explanation in detail about the study’s objective and provided written informed consent. The study design comprised two main phases. In the first, conducted on the day of recruitment, subjects were evaluated for TMD, depression, and anxiety. In the second phase, subjects underwent clinical assessment of sensory and pain thresholds.

CLINICAL EXAMINATION

On the day of enrollment, subjects underwent a clinical examination to assess general health and dental status and presence and severity of TMD. Additionally, they completed a self-administered questionnaire evaluating depression and anxiety status. Baseline socio-demographic data were also collected.

Temporomandibular disorder

The presence and severity of TMD were assessed using clinical variables recognized to be valuable clinical practice.[20] While the Diagnostic Criteria for Temporomandibular Disorders (DC-TMD) were available, this study opted for the Helkimo scale, a basic instrument with fewer parameters that incorporates both painful and not painful TMD signs and symptoms, thereby including patients beyond those with solely orofacial pain. In detail, TMD was evaluated and classified by the Helkimo criteria,[21] which include the revelation of temporomandibular joint sounds, feeling of stiffness of the jaws or fatigue on the masticatory musculature, difficulty in opening the mouth at all or in closing the mouth, pain on jaws movement, and facial and jaw pain. Each item is scored from 0 to 5, and the overall score ranges from 0 to 25. According to Helkimo, the severity of TMD was classified as follows: absence (overall score 0), mild (score 1–4), moderate (score 5–9), and severe (score ≥10).

SCL-90 questionnaire[22,23]

Depression and anxiety levels were evaluated using the SCL-90 questionnaire of the Research Diagnostic Criteria for TMDs (RDC/TMD), as developed by Dworkin and LeResche.[18,19,20,21,22,23] The depression subscale comprises 20 non-pain related items, while the anxiety includes 12 items, five pain-related, and seven non-pain-related. For every specific item, subjects rated the degree to which they had been distressed in the past month, each being scored on a 0 to 4 scale. Depression and anxiety subscale scores range from 0 to 80 and from 0 to 48, respectively, and the mean individual score was used as an indicator of severity. The mean individual score have been validated and used in TMD patients.[23] For the analysis, the mean scores were additionally categorized as normal, moderate, and severe, in accordance with the RDC/TMD cut offs derived from population-based studies,[23] namely, depression: normal, 0 to 0.534; moderate, 0.535 to 1.105; severe >1.105; anxiety: normal, 0 to 0.427; moderate, 0.428 to 0.857; severe >0.857.

Sensitivity evaluation

Following the first phase, all participants returned within 4 ± 2 days after enrollment at our department for evaluation of sensory and pain thresholds. The session was performed in the morning at 9 h ± 30 m under the same conditions for all subjects.

ASSESSMENT OF SENSORY AND PAIN THRESHOLDS

Electrical stimulation of tooth pulp was performed in order to measure sensory and pain thresholds, as suggested by several authors.[24] This stimulation would stress only nociceptive terminals A delta and C fibers and is not harmful. The following procedure was followed. The subject was seated on a dental chair resting his/her forearms on the chair arms. The right central incisive was analyzed. The tooth was isolated with the use of a dam, and the dental surface was cleaned with alcohol, cotton pellets, and air-drying. A reduction of the resistance was obtained with an electro-conductive gel applied on the dental surface. Consecutive single-square wave impulses with a duration of 5 msec, ranging from 0 to 64 µA, generated by a direct current oscilloscope (Model 2001 Pulp Tester, Analytic Technology, Redmond, WA, USA) were used. On appliance of the electrical stimulator to the dental pulp, the intensity was increased at a rate of 1 arbitrary unit (AU) on a scale ranging from 0 to 80 (1 AU corresponds to 4/5 µA). Sensory and pain thresholds were defined as the minimum current that elicited a positive sensory or pain response, respectively. Before examination, subjects were instructed to indicate when they first felt any sensory sensation by tapping the left hand on the chair arm and then as they felt pain by tapping once more the left hand on the chair. The same experienced examiner, blinded to the clinical and psychological status of the subject, performed all measurements. The examiner underwent prior on a training and calibration process during an iterative process performed on 20 voluntary subjects. Three measurements were performed for each subject at a 10-minute interval, and the average value was used for the analyses. The intra-subject variability evaluated by the coefficient of variation (ratio of standard deviation to mean) ranged from 3.2% to 6.1% in the TMD group and from 3.0% to 5.7% in the control groups. This comparable inter-group variation and the degree of intra-subject measurement variation were considered acceptable and deemed to yield reliable data.

STATISTICAL ANALYSIS

A statistician blinded to the subjects’ clinical status performed the statistical analyses. In order to identify if continuous data were normally distributed, we used the Kolmogorov–Smirnov test (P > 0.05). Depression (P 0.118) and anxiety (P 0.104) levels as well as sensitivity (P 0.146) and pain (P 0.158) thresholds were not normally distributed. Descriptive results are expressed as mean (SD) or median (interquartile range [IQR]) and percentage. Pearson’s x² test and Mann-Whitney U test or the Kruskal–Wallis were performed to assess differences between or among unpaired groups. Significance of multiple comparisons was adjusted on the Bonferroni correction. Additionally, sensory and pain thresholds were dichotomized at the lowest tertile, and multiple logistic regression models were fitted to evaluate their independent association with depression anxiety, and TMD, further adjusting for gender and age. A significance level of 0.05 was used, and the statistical tests are two-tailed. Statistical analyses were conducted using the SPSS software, version 15.0 (SPSS Inc., Chicago, IL, USA).

RESULTS

All sensory and pain measurements were successful obtained. Among the 56 TMD subjects, 21 (37.5%), 18 (32.1%), and 17 (30.4%) had mild, moderate, or severe TMD, respectively. Table 1 compares the characteristics between TMD and control subjects. Higher depression and anxiety scores were observed in TMD subjects. No inter-group difference was found for sensory or pain thresholds. Higher depression and anxiety scores were correlated positively with increasing age both in TMD (Spearman correlation coefficient, r = 0.362, P < 0.0001 and r = 0.315, P < 0.001) and control (r = 0.316, P = 0.018 and r = 0.259, P = 0.047) subjects. Depression and anxiety levels did not differ between women and men in both groups, except for anxiety in TMD, which was higher in women (median [IQR], 0.46 [0.33–0.92] vs. 0.25 [0.21–0.54], P = 0.0037). A lower sensory threshold was associated with increasing age in both groups (TMD, r = −0.387, P < 0.001; controls, r = −0.451, P < 0.0001), while a lower pain threshold was related with increasing age in the TMD group only (r = −0.502, P < 0.0001). Women exhibited a lower sensory threshold than men both in TMD (median [IQR], 15 [11–21] vs. 26 [17–31], P < 0.01) and control (median [IQR], 13 [12–16] vs. 17 [14.5–24], P = 0.024) groups and a lower pain threshold in the TMD group only (median [IQR], 29 [22–40] vs. 39 [26.5–62], P = 0.025). A multiple logistic regression analysis showed that the only variable independently differentiating between TMD subjects and controls was anxiety, with TMD subjects exhibiting higher (moderate/severe) anxiety scores (odds ratio, OR, 3.15, 95% confidence interval [95% CI], 1.43 to 6.96). Depression and anxiety scores and sensory and pain thresholds were not significantly associated with the severity of TMD [Table 2]. However, there was a broad inter-subject variability in each of three groups of TMD severity for both psychological variables and sensory and pain thresholds, with a coefficient of variation ranging from 52.2% to 59.4% in mild TMD, from 44.5% to 90.0% in moderate TMD, and from 45.3% to 54.0% in severe TMD. Depression [Figure 1] and anxiety [Figure 2] scores were negatively associated with the pain threshold both in TMD (depression, Spearman correlation coefficient, r, −0.650; anxiety, r, −0.633, P < 0.0001) and controls (r, −0.629 and −0.665, P < 0.0001) [Figure 3]. The sensory threshold was negatively associated with anxiety in TMD subjects only (r, −0.369, P < 0.010). Table 3 reports the relationship between sensory and pain thresholds according to the severity of depression or anxiety. Among subjects with normal depression or anxiety scores, TMD subjects showed a significantly higher sensory threshold compared to controls. In subjects having moderate/severe depression or anxiety scores, no significant difference occurred between TMD and controls for both sensory and pain thresholds. In the TMD group, subjects with moderate/severe depression or anxiety exhibited lower sensory and pain thresholds than subjects with normal psychological scores; the sensory threshold did not significantly differ between subjects with moderate or severe psychological scores, while a lower pain threshold was observed in subjects with severe than moderate scores. In controls, no difference in the sensory threshold was found with respect to the severity of depression or anxiety, while the pain threshold was lower in subjects with moderate/severe than normal psychological scores and in subjects having severe versus moderate depression score. Lastly, a multivariate logistic regression analysis showed that a low (being in the lowest tertile of) pain threshold was independently associated with a higher depression score (moderate/severe vs. normal, OR=4.38, 95%Cl, 1.13 to 17.04 P = 0.033). A low (being in the lowest tertile of) sensory threshold was independently associated with a higher anxiety score (moderate/severe vs. normal odds ratio OR =6.06, 95%CI, 1.49 to 24.6 P = 0.012), being a TMD patient (OR=2.47, 95%CI 1.03 to 5.95, P = 0.043), or woman (OR=5.59, 95%CI 1.38 to 21.94, P = 0.016), and increasing age (OR=1.04, 95%CI 1.01 to 1.07, P = 0.012).

Table 1.

Characteristic of participants

	TMD subjects (n = 56)	Controls (n = 56)	P a
Age
Mean (SD)	43.1 (17.1)	42.2 (16.6)	0.793
Gender
Females n (%)	43 (76.8%)	43 (76.8%)	1.00
SCL-90 scale score: median (IQR)
Depression	0.93 (0.50–130)	0.53 (0.25–1.01)	0.005
Anxiety	0.79 (0.42–1.40)	0.42 (0.25–0.91)	0.003
Sensory and pain threshold (AU): median (IQR)
Sensory	16.0 (12.0–24.7)	14.5 (12.0–18.0)	0.098
Pain	29.5 (222–42.7)	31.5 (25.0–48.7)	0.340

‘Statistical significance of difference between groups. All P values were by Mann–Whitney U test for any variable except gender (Chi-square). 1 AU corresponds to 4/5 µA.

Table 2.

Median (IQR) of depression and anxiety levels and sensory and pain thresholds in TMD subjects, according to the severity of TMD

	TMD severity’			pb
	Mild (n = 21)	Moderate (n = 18)	Severe (n = 17)
SCL-90 scale score
Depression	0.90 (0.53–1.30)	0.90 (0.30–1.30)	1.05 (0.55–1.50)	0.640
Anxiety	1.0 (0.75–1.54)	0.64 (0.31–1.21)	0.83 (0.54–1.29)	0.157
Sensory and pain threshold (AO
Sensory	14 (11–24)	19.5 (14.5–30)	18 (12.5–24.5)	0.121
Pain	28 (16.5–36.5)	29.5 (22.5–58.5)	33 (23–41.5)	0.454

‘According to Helkimo index: mild, 1 to 4; moderate, 5 to 9; severe, 10 to 25. ^bStatistical significance of difference among TMD groups (Kruskal–Wallis test). 1 AU corresponds to 4/5 µA

Figure 1.

Depression vs. Pain Threshold (TMD group) – shows a strong negative correlation: as depression scores increase, the pain threshold decreases

Figure 2.

Anxiety vs. Sensory Threshold (TMD group only) – illustrates a moderate negative correlation, unique to TMD subjects

Figure 3.

Depression vs. Pain Threshold (Control group) – also shows a strong inverse relationship

Table 3.

Median (IQR) sensory and pain thresholds in TMD subjects and controls, according to different depression or anxiety levels, intragroup comparisons across psychological categories

Threshold	TMD (n = 56)				Controls (n = 56)
	Depression (SCL-90 scale score
	Normal	Moderate	Severe	PQ	Normal	Moderate	Severe	PQ
(AU)	(n = 17)	(n = 18)	(n = 21)		(n = 28)	(n = 16)	(n = 12)
Sensory	26 (14–39)b	15 (11.5–21)°	16 (12–20)°	0.034	16 (12–23.5)	14 (12–18.5)	12 (11–15.5)	0.113
Pain	58 (33–62)b	30.5 (24.5–40)°	23 (16–26.5)d	<0.0001	44.5 (31.5–58.5)b	30.5 (27.5–33)°	18.5 (13.5–22)d	<0.001
Anxiety (SCL-90 scale score)
Threshold	Normal	Moderate	Severe	PQ	Normal	Moderate	Severe	PQ
(AU)	(n = 15)	(n = 16)	(n = 25)		(n = 30)	(n = 11)	(n = 15)
Sensory	24 (15–39)b	16 (12–24.5)°	14 (10.5–20)°	0.012	16 (12–22)	12 (11–15)	14 (11–17)	0.061
Pain	56 (38–62)b	32 (26–39.5)°	24 (17.5–28.5)d	<0.0001	44.5 (30.5–59)b	30 (25–39)°	22 (15–29)°	<0.0011

^a Within-group statistical significance of difference among depression and anxiety groups (Kruskal–Wallis test). Different superscripts indicate significant difference between groups (Bonferroni correction).

Significant difference in the sensory threshold between TMD and controls in subjects having normal depression or anxiety levels (P < 0.05, Mann-Whitney U test). 1 AU corresponds to 4/5 µA.

DISCUSSION

MAIN FINDINGS

This observational case–control study suggested that, independently of age and gender, an association was observed between increased depression and anxiety levels with a reduced pain threshold in both TMD and healthy adults, while increased anxiety may additionally be correlated with a reduced sensory threshold in TMD subjects. To our knowledge, this is the first study to experimentally examine the relationship between psychological factors (depression and anxiety) and sensory threshold under electrical stimulation in TMD subjects. Psychological status was evaluated using a previously validated procedure.[22,23] Examinations of sensory and pain thresholds induced under electrical stimuli of dental pulp[24] were performed according to a standardized procedure that emphasizes nociceptive response. Intra-individual variability was minimal (around 6%), ensuring reliability. Notably, electrical stimulation was additionally applied by Ayesh[8] to delineate the nociceptive properties from the temporomandibular joint. When analyzing the pooled data, the present study proved that the levels of depression or anxiety were associated with age, in both TMD and healthy subjects. This is consistent with findings by other authors and may be not unexpected due to the high prevalence of psychosis and vulnerability to depression disorders in older people.[25] In accordance with Chiu,[2] no significant association was found of age with pain threshold in healthy subjects, whereas in the TMD group, both sensory and pain thresholds were associated with age. This may suggest that in subjects suffering from TMD, a detrimental sensitivity to pain may progress over time. The observation that TMD subjects exhibited lower depression and anxiety scores than healthy sex- and age-matched controls further supports this possibility, implying that a persistent clinical discomfort may influence the individual psychological status. Studies are needed to clarify this hypothesis. No significant difference between TMD and healthy subjects was found with respect to sensory and pain thresholds. However, while the sensory threshold was lower in women than men in both groups, a lower pain threshold was significantly associated with being woman in TMD only. The association of gender with pain threshold has been found by other authors[17] in TMD subjects, and by Chiu[2] in a population that included both subjects free from chronic pain and subjects with widespread pain. In healthy subjects, Ayesh[8] found no significant gender-related differences in mechanical or electrical sensory or pain thresholds when stimuli were applied to the skin overlying the temporomandibular joint, although women demonstrated a lower temporomandibular joint pain tolerance before stimulation.

CLINICAL IMPLICATIONS

Clinically, these findings suggest that while the higher prevalence of TMD in women than men may be partially linked to a higher pain sensitivity of the temporomandibular joint in women,[26] this factor alone does not fully explain the clinical disparity. The finding that TMD subjects exhibited higher depression and anxiety scores than healthy controls is aligned with the findings of the existing literature[27] and is unsurprising given the chronic nature of TMD.[28] It has been moreover suggested that an association may exist between severity of chronic pain and psychological disorders.[29] However, in this study, anxiety and depression were not associated with the severity of TMD. While no comparative data currently exist in literature, this issue merits further investigation. Carlson[30] described a physiological mechanism that modulates pain sensitivity (sensitization of peripheral nociceptors, central sensitization, autonomic deregulation, and the inhibitory processes of pain), and Sessle[31] proposed that peripheral and central mechanisms (e.g., the individual psychological status) can modify the perception of pain; our belief was that all subjects in this study exhibited a central sensitization to pain. Several studies investigated the relationship of psychological distress with pain sensitivity.[2,3,4,5,6,7,8,9,10,11,12,13,14,15,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34] While there is a consensus that subjects suffering from chronic pain exhibit a reduced pain threshold, findings in healthy subjects remain conflicting.[2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34] Dickens[34] suggested that depressed individuals may be less sensible to perceive a stimulus as painful, whereas Chiu[2] evidenced that an association between depression and a reduced pain threshold may occur also in healthy subjects. The results obtained in the present study support that a low (being in the lowest tertile) pain threshold may be independently associated with increased depression level, both in TMD and healthy subjects. Notably, depression and anxiety are associated with a reduced sensory threshold in TMD subjects only. Of particular interest, while no difference in sensory or pain thresholds was observed between TMD and control subjects with moderate and severe depression or anxiety, in subjects with normal depression score, the median sensory threshold was around 62.5% higher in TMD than control subjects, and in subjects with normal anxiety score, this percentage was around 50%. These findings may suggest that psychological distress in TMD may modulate not only the individual pain responsiveness but also sensory function. Furthermore, in TMD subjects with normal depression and anxiety, the disorder itself may reduce the sensitivity to stimuli applied in the temporomandibular joint region. These hypotheses required further investigation, including studies of potential underlying neurological and physiological mechanisms.

LIMITATIONS

Some consideration should be taken in interpreting and generalizing the present findings due some methodological limitations. The major drawback was the cross-sectional study design. Therefore, no etiological conclusion can be inferred about the interrelationships of TMD with depression and anxiety or sensory and pain thresholds. Indeed, results from the study by Slade[19] suggest that psychological status linked to pain sensitivity could influence the first onset of TMD in adult women. No other study exists currently that assesses the etiological relationship between depression or anxiety, neither in healthy or TMD subjects. Chiu[2] hypothesized that depression is more likely a precursor to reduced pain thresholds rather than a consequence. Prospective studies should be hopefully conducted to clarify these dynamics. Second, wide variability in sensory and pain thresholds across both groups highlight the role of individual differences, consistent with the literature findings.[32] Indeed, recognition that pain responsiveness may be associated with gender and age[2] and psychological status,[15] and that heterogeneity as far as genetic variation could possibly account for some different individual responses to pain stimuli[33] support plausibility of the findings found in this study.

CONCLUSIONS

In conclusion, within the limitations of this study, we infer that depression is independently associated with a reduced pain threshold, as induced by electrical stimulus applied in the dental pulp in both TMD and healthy subjects, while anxiety may be independently associated with a reduced sensory threshold in TMD subjects only. Large longitudinal studies are needed to clarify the association of psychological disorders with sensory and pain thresholds and to investigate the etiology of this relationship in both healthy individuals and those with chronic pain.

ACKNOWLEDGMENTS

Not applicable.

CONFLICTS OF INTEREST

The authors certify that there are no conflicts of interest with any financial organization with reference to the material discussed in the manuscript.

AUTHORS CONTRIBUTIONS

Alessandro Marchesi have contributed to the conception of the manuscript, to the acquisition of the data, the analysis and interpretation of the statistical data, and to the processing of the manuscript.

ETHICAL POLICY AND INSTITUTIONAL REVIEW BOARD STATEMENT

The study was carried out following the Helsinki Declaration on Human Clinical Research. The authors applied for the written approval from the Institutional Ethics Committee, however it was not considered necessary and therefore not requested given the fact that the procedures applied in this case study were observational only and did not include treatment of the patient.

PATIENT DECLARATION OF CONSENT

All participant patients of the research have provided a written consent and a signed disclaimer to use and publish data from their medical records.

DATA AVAILABILITY STATEMENT

Data is available by corresponding e-mail: gnato.sanpaolo@gmail.com.

LIST OF ABBREVIATIONS

• TMD: Temporomandibular disorders

• TMJ: Temporomandibular joint

• SD: Standard deviation

• RDC: Research diagnostic criteria

• AU: Arbitrary unit

• IQR: Interquartile range

• OR: Odds ratio

• CI: Confidence interval

Funding Statement

This research is not subject to external funding.