The primary objective of our study is to compare the thickness of the masseter muscle, measured by ultrasound, in individuals with chronic neck pain to that of healthy individuals. The secondary objective of the study is to examine the relationships between masseter muscle thickness and pain intensity, neck disability level, pain catastrophizing level, and clinical diagnostic criteria in individuals with chronic neck pain.

The aim of this study is to evaluate masseter muscle thickness in individuals with chronic neck pain using ultrasonography and to compare it with healthy individuals. Thus, it is intended to reveal possible morphological changes related to the craniomandibular system in chronic neck pain and to determine whether these changes contribute to clinical evaluations. The literature reports that there are biomechanical and neuromuscular connections between the temporomandibular system and the cervical spine, and that changes in masseter muscle activity and morphology may be related to neck pain. The literature emphasizes that there are biomechanical, neuromuscular, and sensory connections between the temporomandibular system and the cervical spine, and that dysfunctions in these two regions may influence each other reciprocally. The masseter muscle, in particular, being one of the primary muscles of the temporomandibular system, may exhibit increased muscle activity, parafunctional habits, and morphological adaptations due to central sensitization mechanisms in chronic pain conditions. Chronic neck pain is reported to be more common in individuals with temporomandibular disorders, along with increased muscle sensitivity and functional limitations, and this condition is associated with cervical pain severity and disability level. Ultrasonography is a reliable, valid, and non-invasive method for assessing muscle thickness and is widely used to evaluate the morphological characteristics of the masseter muscle. Measuring masseter muscle thickness by ultrasonography allows for the evaluation of the muscle's structural response to functional loading and enables comparisons between different clinical groups. However, the literature shows that studies comparing possible changes in masseter muscle thickness in individuals with chronic neck pain with healthy individuals are limited, and this relationship has not been sufficiently elucidated. Accordingly, the primary objective of this study is to compare the masseter muscle thickness measured by ultrasonography in individuals with chronic neck pain with that in healthy individuals. The secondary objective of the study is to examine the relationships between masseter muscle thickness and pain intensity, neck disability level, pain catastrophizing level, and clinical diagnostic criteria in individuals with chronic neck pain.

Inclusion Criteria:

• Be between the ages of 18 and 45
• Have neck pain that has persisted for at least 3 months
• Agree to participate in the study voluntarily and provide written informed consent

Exclusion Criteria:

• History of cervical spine surgery or severe trauma
• Diagnosis of cervical radiculopathy, myelopathy, or neurological disease
• Rheumatic, inflammatory, or systemic musculoskeletal diseases
• Diagnosed temporomandibular joint disorder (TMD)
• Diagnosis of bruxism or a pronounced habit of teeth clenching/grinding
• History of manual therapy, injection, or surgical intervention in the face, jaw, or neck region within the last 6 months
• Currently undergoing orthodontic treatment or history of orthodontic treatment within the last 6 months
• Cognitive impairment or conditions that prevent understanding and responding to questionnaires

The study will include individuals aged 18-45 who have experienced neck pain for at least 3 months but do not have a diagnosed cervical pathology. The control group will include individuals aged 18-45 who do not have any health problems.