The objective of this study is to quantify salivary oxidative stress biomarkers in patients with temporomandibular disorders and to quantify recently isolated endogenous peptide opiorphin in saliva of these patients. As chronic exposure to stress may cause hyperalgesia as a result of the stress response in the hypothalamic-pituitary-adrenal axis, aim is to test this as an underlying mechanism by correlating opiorphin and oxidative stress markers to salivary cortisol levels. The aim is to assess the association of oxidative stress salivary biomarkers with muscle and joint pain and to measure opiorphin, a potential biomarker of different pathological states.

Temporomandibular disorders (TMD) are most common chronic orofacial pain conditions of non-dental origin, with prevalence in the general population of 3.6% to 7%. Despite signs and symptoms being well described in the literature, there is still an absence of underlying pathophysiological mechanisms. Evidence based strategies for diagnosis and management of temporomandibular pain still aren't available. Psychological and mechanical stress factors could contribute to oxidative stress (OS) and lead to TMD. Therefore, identification of oxidative stress biomarkers would objectively indicate implication of OS in TMD pain onset mechanisms, and provide a basis for early detection, and a potential target for therapeutic agents to prevent progression to more severe dysfunction.

Aim is to quantify salivary OS markers and total antioxidant capacity (TAC), as well as recently isolated endogenous peptide opiorphin (OP) (2006 Inst. Pasteur), in TMD patients and compare them to controls. As chronic exposure to stress may cause hyperalgesia as a result of the stress response in the hypothalamic-pituitary-adrenal (HPA) axis, aim is to test this as an underlying mechanism by correlating OP and OS markers to salivary cortisol (SC) levels.

Hypotheses: OS has a role in TMD onset and maintenance, thus salivary markers of OS will increase and/or TAC will decrease; OP influences orofacial pain syndromes, such as TMD, and its salivary level will differ between TMD patients and controls. If decreased OP levels in TMD patients were encountered, we hypothesize that OP downregulation contributes to TMD onset as its analgesic effect is absent. Conversely, increased OP levels would suggest that OP is upregulated merely as a reaction to painful stimuli. Disbalanced SC levels in TMD patients would corroborate involvement of HPA axis in TMD mechanism, which is known to affect the intensity of OS.

Saliva of 50 TMD patients (diagnosed by validated diagnostic criteria and MRI) and 50 controls will be collected. OS markers (8-hydroxydeoxyguanosine, malondialdehyde, etc.) will be assessed by ELISA with spectrophotometric detection and by spectrophotometric reagent kits. OP levels will be measured by HPLC-MS/MS method, originally developed and validated by team members. The electrochemiluminescence immunoassay ECLIA will be used for measuring SC.

Pain and stress will be subjectively assessed using questionnaires: all subjects will fill in Perceived Stress Scale 10 (PSS-10); in TMD patients the worst experienced pain will be recorded using Visual Analogue Scale at the initial and at subsequent visits, as well as Graded Chronic Pain Scale (GCPS), Visual Analogue Scale (VAS), Patient Health Questionnaire (PHQ), Jaw function limitation scale (JFLS), Oral Behaviours Checklist and Oral Health Impact Profile (OHIP)-14.

TMD patients will be randomized in 2 treatment groups (1: stabilization splint + placebo pills; 2: placebo splint + 1g of vitamin C daily). Measurements will be repeated after 3 and 6 months of treatment. Monitoring of OP, OS markers and SC during that period will, depending on observed changes in TMD symptoms, further elucidate underlying proposed mechanism by performing multivariate analyses including treatment outcomes.

This translational research aims to make basic science findings useful for clinical applications. Findings of higher concentrations of OS biomarkers would, besides their significance as "a piece of puzzle" of TMD mechanism, could also be important in establishment of TMD diagnosis and as prospective therapeutic targets. Salivary opiorphin, due to its proven analgesic effect might additionally serve as a possible drug for orofacial pain syndromes.

Novel approach to understanding neuroendocrine mechanisms in TMD and their links to OS, as well and use of saliva as non-invasively available diagnostic biofluid represent significant scientific advances.

Inclusion Criteria:

• for TMD patients: diagnosis of TMD using Croatian version of research diagnostic criteria for temporomandibular disorders (RDC/TMD)
• for control group: gender- and age-matched healthy volunteers

Exclusion Criteria:

• for TMD patients: patients younger than 18 years; smoking; other local diseases and/or systemic disorders; the use of anti-inflammatory drugs, analgesics and/or muscle relaxants; individuals who had already been under treatment for TMD
• for control group: younger than 18 years; cardiac/circulatory/metabolic/muscle abnormalities; smoking; the use of medications, supplements or dietary aids that might affect the outcome results

It will be a single-blind interventional study (as placebo splint cannot be mistaken for a stabilization splint by a clinician).