Ultrasonographic Assessment of Muscle Echogenicity in Myofascial Pain

Overview

Myofascial pain is one of the most important symptoms in patients with temporomandibular disorders (TMD). Although the etiological factors leading to TMD have not been definitively established, parafunctional habits such as bruxism and clenching, as well as conditions such as stress and anxiety, are thought to contribute to inflammation of the joint capsule and to muscle spasm and pain in the masticatory muscles. This condition is defined as a clinical entity affecting a significant proportion of patients and may result in pain and hypertrophy of the masticatory muscles. One of the key clinical findings of myofascial pain is the presence of trigger points, which induce pain perception and involve the sensory component of sensitized nociceptors. Muscle tenderness on palpation is an important clinical finding in the diagnosis of masticatory system dysfunctions and can be observed in approximately 90% of patients. In ultrasonography, echogenicity refers to the ability of tissues to reflect sound waves and varies depending on differences in acoustic impedance between tissues; as impedance differences increase, echogenicity also increases. In muscle tissue, internal structure can be evaluated based on the visibility and density of intramuscular echogenic bands. In the presence of inflammation, edema, or intramuscular structural changes, a decrease in the number of these echogenic bands, loss of their definition, and alterations in internal echogenicity may be observed. Therefore, ultrasonography is a valuable imaging method not only for evaluating muscle thickness but also for assessing internal structural characteristics and treatment-related changes. Posterior cervical muscles include the trapezius, longissimus, sternocleidomastoid (SCM), splenius, and levator scapulae muscles. Although these muscles do not directly affect mandibular movement, they may become symptomatic in the presence of TMD and can refer pain to the facial region. For this reason, their palpation is routinely performed during clinical examination. The trapezius muscle is a large and easily palpable muscle located in the back, shoulder, and neck region, which does not directly affect jaw function but may contribute to widespread headache. The primary purpose of its palpation during examination is not to assess shoulder function, but to identify active trigger points that may cause referred pain. The sternocleidomastoid (SCM) muscle plays an important role in head posture and movement. It can be palpated along its entire length, including its lower portion near the clavicle. Trigger points in this muscle may cause referred pain to the temple, temporomandibular joint, and ear region. The treatment of myofascial pain is complex and generally varies according to the severity of symptoms and clinical presentation. Primary treatment options include occlusal splint therapy, supportive exercises, stress and anxiety reduction approaches, muscle exercises, postural modifications, pharmacological treatments (nonsteroidal anti-inflammatory drugs, muscle relaxants, benzodiazepines, selective serotonin reuptake inhibitors), and symptom-oriented interventions such as acupuncture. In addition, local anesthetic injections without vasoconstrictors may be applied to trigger points. Lidocaine injections into the muscles have been reported to contribute to pain control by reducing muscle activity and pain mediators. The present study aims to evaluate the effects of occlusal splint therapy and lidocaine injection on the masseter, trapezius, and sternocleidomastoid muscles in patients diagnosed with myofascial pain, by analyzing internal echogenicity measurements obtained from archived ultrasonographic images.