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:

• Provide informed consent, documented in a signed and dated form.
• Will comply with all study procedures, including daily ratings filling before and after the in-person study visit, and be available for the study duration.
• All participants of both genders between ages 18 to 74 years.
• Cases and controls will be matched for age within ±5 years, and all participants must understand English commands to follow study procedures (e.g., during CPM testing).

Pain-free controls:

• Age matching (within ±5 years)
• No previous diagnosis for the most common pain-related TMD as defined in the DC/TMD criteria (myalgia, arthralgia, headache attributed to TMD)
• 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 past month)
• 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 past month), e.g., low back pain, fibromyalgia, migraine headaches
• Not meeting any of the most common pain-related TMD diagnoses as described in the DC/TMD criteria (myalgia, arthralgia, headache attributed to TMD) upon clinical exam following the DC/TMD protocol

Chronic painful TMD cases:

• Primary TMD case criteria: Myalgia (masticatory muscle pain) based on clinical exam following the DC/TMD protocol
• Concurrent arthralgia (TMJ pain) and/or headache attributed to TMD allowed
• Myofascial pain must meet the following criteria:

• Onset >3 months, occurring >15 days/month on average for >3 months
• Minimum of 10 jaw pain episodes since onset, each lasting at least 30 minutes and 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:

• Traumatic facial injury or surgery on the face/jaw, arms, or hands
• Presence of pain related to dental and periodontal pathology
• Pregnant
• Has any of the following medical conditions by self-report:

• Renal failure or dialysis
• Heart disease (examples: uncontrolled arrhythmia or hypertension, cardiomyopathy) or heart failure
• Non-allergic bronchospasm (chronic obstructive pulmonary disease and emphysema)
• Diabetes (type I or II) that is not controlled with medication or diet
• Hyperthyroidism
• Uncontrolled seizures
• Used any injection therapy (e.g., tender or trigger point injections, steroid injections), acupuncture, biofeedback, or TENS for managing facial/jaw pain within 2 weeks before the screening assessment.
• If undergoing botulinum toxin injections in the head and neck area, must be 3 months since the last set of injections, and refrain from this treatment until study participation has ended
• History of major depression or other major psychiatric disorder requiring inpatient hospitalization within the last 6 months before the screening assessment
• History of treatment for drug or alcohol abuse within the last 12 months
• Current pain medication use (e.g., opioids, ibuprofen, acetaminophen) that cannot be stopped <24 hours before each study visit
• Other conditions/diseases associated with altered pain perception: neurological or development disorders (dementia, autism spectrum disorder), neoplasm, multiple sclerosis, trigeminal neuralgia
• Adults lacking the capacity to provide informed consent for themselves
• Unable to understand instructions for sensory testing in English.
• Thermal threshold for Pain-50 (pre-determined at the beginning of visit 1) is outside the temperature range of 40ºC to 49ºC
• Inability to complete at least four daily ratings between the Informed consent, Pre-visit 1 procedures, and in-person visit 1
• Lack of access to electronic devices with internet connection during the study participation
• Anything that would place the individual at increased risk or preclude the individual's full compliance with study procedures or completion of the study. "