Endogenous Pain Inhibition Deficiency in Chronic TMD Pain

Inclusion Criteria:

To be eligible to participate in this study, an individual must meet all the following criteria:

1. Provide informed consent, documented in a signed and dated form.

2. Will comply with all study procedures, including daily ratings filling before andafter the in-person study visit, and be available for the study duration.

3. All participants of both genders between ages 18 to 74 years.

4. Cases and controls will be matched for age within ±5 years, and all participantsmust understand English commands to follow study procedures (e.g., during CPMtesting).

Pain-free controls:

1. Age matching (within ±5 years)

2. No previous diagnosis for the most common pain-related TMD as defined in the DC/TMDcriteria (myalgia, arthralgia, headache attributed to TMD)

3. No significant orofacial pain (jaw pain, TMJ pain) in the past 3 months (""significant"" meaning 5 or more days in any month or any pain during the pastmonth)

4. No report of significant pain in the last 3 months elsewhere in the body (""significant"" meaning 5 or more days in any month or any pain during the pastmonth), e.g., low back pain, fibromyalgia, migraine headaches

5. Not meeting any of the most common pain-related TMD diagnoses as described in theDC/TMD criteria (myalgia, arthralgia, headache attributed to TMD) upon clinical examfollowing the DC/TMD protocol

Chronic painful TMD cases:

1. Primary TMD case criteria: Myalgia (masticatory muscle pain) based on clinical examfollowing the DC/TMD protocol

2. Concurrent arthralgia (TMJ pain) and/or headache attributed to TMD allowed

3. Myofascial pain must meet the following criteria:

4. Onset >3 months, occurring >15 days/month on average for >3 months

5. Minimum of 10 jaw pain episodes since onset, each lasting at least 30 minutesand no less than 2 hours within the day, OR unremitting."

Exclusion Criteria:

An individual who meets any of the following criteria will be excluded from participation in this study:

1. Traumatic facial injury or surgery on the face/jaw, arms, or hands

2. Presence of pain related to dental and periodontal pathology

3. Pregnant

4. Has any of the following medical conditions by self-report:

5. Renal failure or dialysis

6. Heart disease (examples: uncontrolled arrhythmia or hypertension,cardiomyopathy) or heart failure

7. Non-allergic bronchospasm (chronic obstructive pulmonary disease and emphysema)

8. Diabetes (type I or II) that is not controlled with medication or diet

9. Hyperthyroidism

10. Uncontrolled seizures

11. Used any injection therapy (e.g., tender or trigger point injections, steroidinjections), acupuncture, biofeedback, or TENS for managing facial/jaw pain within 2weeks before the screening assessment.

12. If undergoing botulinum toxin injections in the head and neck area, must be 3 monthssince the last set of injections, and refrain from this treatment until studyparticipation has ended

13. History of major depression or other major psychiatric disorder requiring inpatienthospitalization within the last 6 months before the screening assessment

14. History of treatment for drug or alcohol abuse within the last 12 months

15. Current pain medication use (e.g., opioids, ibuprofen, acetaminophen) that cannot bestopped <24 hours before each study visit

16. Other conditions/diseases associated with altered pain perception: neurological ordevelopment disorders (dementia, autism spectrum disorder), neoplasm, multiplesclerosis, trigeminal neuralgia

17. Adults lacking the capacity to provide informed consent for themselves

18. Unable to understand instructions for sensory testing in English.

19. Thermal threshold for Pain-50 (pre-determined at the beginning of visit 1) isoutside the temperature range of 40ºC to 49ºC

20. Inability to complete at least four daily ratings between the Informed consent,Pre-visit 1 procedures, and in-person visit 1

21. Lack of access to electronic devices with internet connection during the studyparticipation

22. Anything that would place the individual at increased risk or preclude theindividual's full compliance with study procedures or completion of the study. "