Central Sensitization and Somatization in Adults with Temporomandibular Disorders: A Prospective Study

ABSTRACT

Introduction:

Temporomandibular disorders (TMD) are chronic conditions affecting the masticatory muscles and joints. This study aimed to evaluate central sensitization (CS) and somatization in adults with TMD.

Materials and Methods:

Eighty adult TMD patients were evaluated using the Central Sensitization Inventory (CSI) and Somatic Symptom Scale-8 (SSS-8). Clinical exams and pain ratings were conducted, and statistical analyses applied.

Results:

Most participants fell into the subclinical CSI category. Women reported higher somatization scores. A significant correlation existed between somatic symptom severity and chronic muscle pain.

Conclusion:

CS is less prevalent than expected in TMD patients, while somatization is more pronounced in women and those with chronic muscle pain. Integrated biopsychosocial approaches are recommended.

INTRODUCTION

Temporomandibular disorders (TMD) are a group of conditions affecting the masticatory muscles, temporomandibular joints, and associated tissues.[1] Symptoms include orofacial pain and joint dysfunction,[1,2] with a prevalence of 5%–12%.[3] In a Polish cohort, 55% had at least one symptom.[4] TMD is the second leading cause of musculoskeletal pain after low-back pain.[5,6] and occurs twice as often in women.[7] Chronic TMD can impair quality of life.[5,6,7] The biopsychosocial model best explains its multifactorial origin. However, its pathophysiology remains unclear.[4,8] Central sensitization (CS), defined as enhanced central nervous system (CNS) responsiveness to stimuli, is a key mechanism in chronic pain syndromes. It is linked to reduced pressure pain thresholds and pain without tissue injury in TMD patients. TMD often overlaps with other CS syndromes (CSS),[8] making CS essential to evaluate. Somatization, also common in TMD, reflects the broader symptom burden.[7] The Somatic Symptom Scale-8 (SSS-8) questionnaire captures this and, along with the CSI, helps clarify the role of CS in TMD symptomatology.

MATERIALS AND METHODS

Participants and study design

Eighty adult patients from a TMD outpatient clinic participated after giving written informed consent. The study was approved by the ethics committee (Ref: MAIDS/Ethical Committee/2016/3273).

Inclusion and exclusion criteria

Inclusion criteria: age ≥18, TMD diagnosis per diagnostic criteria/TMD, and chronic primary/secondary masticatory muscle pain (MMP) per I nternational Classification of Orofacial Pain. Exclusion criteria: long-term CNS-impairing drugs, severe psychiatric/systemic/autoimmune/cognitive disorders, substance abuse, cancer, pregnancy/lactation, other severe pain/headache conditions, or abnormal brain imaging (computed tomography/magnetic resonance imaging).

Data collection and clinical examination

Data were collected from June 2020 to January 2021. Participants completed the CS inventory (CSI) and SSS-8, followed by clinical examination by calibrated dentists, supervised by a TMD expert (>10 years’ experience). Pain in masseter and temporalis muscles was rated using a 0–10 Numeric Rating Scale.

Questionnaires

CSI (α = 0.92): 25 items, total score 0–100; cutoff = 40. Part B assessed comorbid CS conditions.

SSS-8 (α = 0.72): 8 items, score 0–32; severity categorized from minimal to very high.

Statistical analysis

Mann–Whitney U, Kruskal–Wallis with Dunn’s post hoc, Chi-square, and Spearman correlation were used. Power analysis required ~155–200 participants (α = 0.05, power = 0.8). Software: TIBCO Statistica v13 and R (fifer 1.0, MAPI Research Trust, Lyon, France).

RESULTS

Sample characteristics

The study included 80 Caucasian adults with TMD, of whom 75% were women (n = 60) and 25% men (n = 20). The average age was 37.92 years (standard deviation = 14.14), with no statistically significant age difference between genders. Patients were categorized as having joint-only disorders (n = 10), muscle-only disorders (n = 27), or combined disorders (n = 43). No significant age differences were found among these groups [Table 1]. SSS-8 scores varied significantly across CSI categories as shown in Figure 1.

Table 1.

Distribution of pain severity of masticatory muscles divided by gender

Muscle Site	Gender	Mean	95% CI Mean (Lower)	95% CI Mean (Upper)	Std. Deviation
Left masseter muscle	Female	4.944	4.460	5.428	3.098
	Male	3.389	2.646	4.132	2.722
Right masseter muscle	Female	5.356	4.881	5.831	3.043
	Male	3.778	3.057	4.498	2.640
Left temporal muscle	Female	3.656	3.145	4.167	3.274
	Male	2.500	1.789	3.211	2.604
Right temporal muscle	Female	3.781	3.279	4.284	3.219
	Male	2.741	2.010	3.472	2.679

Figure 1.

Somatic Symptom Scale-8 scores across central sensitization inventory categories

Temporomandibular disorder distribution

Diagnoses included disc displacement (with/without reduction), arthralgia, degenerative joint disease, subluxation, myalgia, myofascial pain, and myofascial pain with referral. Myalgia was most prevalent (75%), followed by disc displacement with reduction (36%) and arthralgia (19%). A Chi-square test found no significant association between gender and diagnosis (P = 0.557), but women had significantly higher pain scores in all four examined muscle sites:

• Left masseter (P = 0.001)

• Right masseter (P < 0.001)

• Left temporal (P = 0.019)

• Right temporal (P = 0.034).

Temporomandibular disorder and central sensitization inventory

CSI scores placed 65% of participants in the “subclinical” group, with fewer in mild (16%), moderate (10%), severe (7%), and extreme (2%) ranges. No age or diagnosis-based differences were found, but men were more likely to fall into the “subclinical” group. Higher CSI levels correlated with greater muscle pain, especially in the right masseter and both temporal muscles.

Temporomandibular disorder and Somatic Symptom Scale-8

Participants were distributed across five symptom levels, from “no to minimal” (25%) to “very high” (14%). A weak positive correlation was found between age and SSS-8 (rs = 0.173, P = 0.008), and women had significantly higher scores (P = 0.018).

Central sensitization inventory and Somatic Symptom Scale-8

Kruskal–Wallis analysis showed significantly lower SSS-8 scores in the CSI “subclinical” group (P < 0.001), though interpretation was limited due to group size variation.

DISCUSSION

This investigation assessed the prevalence of CS using the CSI and somatization using the SSS-8 among individuals with TMD. Most participants were female, consistent with previous findings linking this trend to hormonal, metabolic, or pain sensitivity differences.[6,7] Myalgia remained the most common diagnosis.[4,6] Interestingly, most participants scored below the subclinical threshold for CS on the CSI, implying that CS, as measured by the CSI, may be less prevalent among TMD patients than previously believed. This contrasts with findings by La Touche et al., who reported high CS symptom levels in TMD, and Meng et al., who found more CS-related symptoms like hyperalgesia in chronic TMD.[8]

The absence of chronicity assessment and exclusion of comorbidities in our study may partly explain these discrepancies. CS has been strongly linked to chronic conditions, and other studies included patients with multiple comorbidities. Methodological variations, including tools like QST and CPM, may further account for conflicting results.

We found no significant association between TMD diagnosis categories or MMP intensity and CS severity, diverging from past findings. However, Baroni et al. and Ferillo et al. reported links between orofacial pain and CS through decreased pain thresholds and CNS dysfunction. Our findings support the biopsychosocial pain model discussed by Nugent et al.[6]

SSS-8 results showed higher somatization among women and those with greater MMP intensity, affirming gender-based symptom reporting differences. Strong associations between CS and somatization were evident, reinforcing the link between heightened CNS sensitivity and somatic burden. These findings underscore the importance of comprehensive evaluations and integrated treatment strategies addressing both CS and somatization.

CONCLUSION

This study assessed the frequency of CS and somatization among patients with TMD, indicating that severe CS is less widespread in this population than previously assumed, based on data received from study participants. Notably, women reported larger somatic symptom loads than males, underscoring gender disparities in pain experience. A strong association was established between the SSS-8 questionnaire and chronic MMP. Furthermore, the subclinical category of the CSI varied considerably from other groups, demonstrating lower SSS-8 scores.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.