1. Research project objectives/Research hypothesis Masticatory muscles disorders constitute a serious and frequent diagnostic and therapeutic problem in dental practice. Its symptoms appear in 40-90% of the population. The most characteristic symptoms include acute and chronic pain in masticatory muscles and/or temporomandibular joints, sounds in temporomandibular joints (clicking, popping), and abnormal jaw movements. Those main symptoms may be associated with a headache and otolaryngological disorders, as well as increased tension and pain in the muscles of the neck and shoulder girdle. Physical examination shows that masseter muscles disorders are often associated with their hypertrophy and pain during palpation. The pathogenesis of this disorder is unclear and multifactorial. Diagnostic criteria are based on recommendations of INfORM (International Network for Orofacial Pain and Related Disorders Methodology). The main goal of treatment is the elimination of pain in the masticatory muscles.

   The aim of the project is to evaluate the usefulness of shear wave sonoelastography in the diagnosis and monitoring of the progress of treatment of masticatory muscles disorders. To date, few reports attempting to establish normal values have been published, but the available reports have significant limitations.
2. Research project methodology The study is divided into two stages. The aim of the first stage is the development of the sonoelastography methodology with precise determination of the positioning of the sonography head and the patient's condyles in temporomandibular joints as well as the determination of the normal range of masticatory muscle structure in the population of healthy subjects. This stage includes people without the history of TMD and any deviations from normal values according to DC-TMD protocol. Patients treated with drugs affecting masticatory muscles function as well as those with systemic diseases which affect muscle function (e.g., neurological and hormonal disorders, neoplastic disease), above 60 years of age, patients with mental disorders and not agreeing to participate in the study will be excluded.

   Measurement of the masticatory muscle structure will be performed based on the shear wave sonoelastography (SWE) used in the noninvasive assessment of soft tissue hardness. In this method, share waves generated by pressure are detected by longitudinal ultrasonography waves which propagate in the tissues much faster than shear waves. The measurement reflects the quantitative measurement of tissue hardness expressed in kilopascals. Among its advantages are low dependence on the operator, high repeatability and the possibility of quantitative evaluation of tested parameters. The main disadvantage of this method is the fact that every organ has its unique characteristics and hence, normative values must be determined separately for each of them. Examination of masticatory muscles will be conducted by one experienced operator in the same conditions using the same head and the same device. During the examination, the patients will lie on the bed in a supine position. Both masseter muscles will be examined in jaw bite and rest positions.

   The second stage concerning the evaluation of the sonoelastography in the monitoring of the efficacy of treatment. Patients with temporomandibular disorders classified as group II - Masticatory Muscle Disorders (DC-TMD). Exclusion criteria will include previous treatment for masticatory muscles disorders and clinical criteria used in the first stage. Control group will include patients without temporomandibular disorders. The treated group will undergo treatment for masticatory muscles disorders with manual therapy and stabilization occlusal splint. The assessment of severity of pain with Numerical Rating Scale and quality of life associated with oral health using standardized international questionnaire - the Oral Health Impact Profile-14 (OHIP-14) will be conducted at baseline and after 12 weeks of treatment. Masticatory muscle structure as measured with sonoelastography and the results of DC-TMD protocols will be compared in order to evaluate treatment efficacy.
3. Expected impact of the research project on the development of science The investigators expect that the investigator's study will have a large and multidirectional impact on the development of science and the health of society. Gaining reliable method of masticatory muscle structure measurement in the course of masticatory muscles disorders will facilitate the conduct of clinical research by delivering an objective method for comparison of results of treatment conducted in various centers with various methods. The project is also important for public health. Better diagnosis and monitoring of the treatment of this chronic disorder gives a chance for improving oral health care for the entire population. Among the additional values of the project is the interdisciplinarity which is based on the use of cutting-edge non-invasive diagnostic imaging method in oral medicine.