In teeth that remain decayed for a long time, the muscles in that area atrophy because the patient cannot use the decayed area much. As a result, muscle length and mass decrease. To compare the amount of this reduction before and after treatment with ultrasound and to treat accordingly

Material and method Clinical examination Apical periodontitis is inflammation of the periodontium at the entry portals of the root canal system. Typically, the lesion is found at the root apex, but it can occur at various levels along the root surface, and lesions can also develop in the lateral and furcal regions Acute apical periodontitis (AAP) is characterized by acute clinical symptoms (pain with the teeth in occlusion, tenderness to percussion or palpation in the periapical region, intrasulcular/intracanal pus drainage and swelling) but no evidence of periapical radiolucency except for mild widening of the periodontal ligament.

Patients will be selected from patients who apply to the Department of Endodontics, Faculty of Dentistry, Harran University, as a result of clinical and radiological examinations for acute apical periodontitis cases. After the patients are informed about the treatments to be performed, ultrasound measurements will be performed on the patients. Ultrasound(USG) imaging procedures will be performed by a single operator. Data will be recorded. Then, under local anesthesia and rubber dam isolation, the relevant tooth will be opened with round (Intensiv212) access cavities. Working length (WL) was determined using an electronic apex locator (Root ZX II; J. Morita Corp., Kyoto, Japan) in combination with a size 10 K-file. ProTaper Next X1 (#17, 0.04 taper) and X2 (#25, 0.06 taper) files (Dentsply Sirona, Ballaigues, Switzerland) were used sequentially with an endodontic motor (VDW Silver Reciproc; VDW, Munich, Germany) using gentle in-and-out movements in accordance with the manufacturer's instructions. The canals will be irrigated with 2 ml sodium hypochlorite at each file change. final irrigation will be done with 5ml 17% EDTA (Ethylenediaminetetraacetic acid), 2.5ml DW (distilled water) and 5.25% NaOCl (sodium hypochlorite). Then, after the canals are dried, they will be filled with calcium hydroxide and kept for 14 days with glass ionomer filling. At the end of this period, the patients will be called back, the necessary checks will be done, the canals will be washed again and kept for another 14 days with calcium hydroxide. At the end of the total of 28 days, the measurements will be repeated with a second ultrasound from the patients whose symptoms have completely disappeared and before and after comparisons will be made.

USG imaging In USG evaluation, LOGIQ P9 XDclear (GE HealthCare, USA) Masseter muscle thickness measurements will assume that the part with the highest repeatability is the middle part of the mediolateral line along the mandibular ramus. Therefore, when making measurements, the "L 3-12 RS (3-12 MHz)" linear probe will be placed approximately from the region corresponding to the middle of the mediolateral line along the mandibular ramus, perpendicular to the fibers of the masseter muscle, and parallel to the long axis of the corpus mandibula, in the middle between the zygomatic arch and the gonial angle, close to the occlusal plane, which is the thickest part of the masseter muscle.

Muscle thickness was defined as the maximum distance between the inner and outer fascia. Shear wave elastography (SWE) method will be used in ultrasonographic elastography examinations of muscles. In this study, the value recorded as Young's modulus will be defined as hardness and kPa units will be used.

Masseter Muscle SWE Examination For each participant, the stiffness values of the muscles at rest and at maximum contraction will be calculated for the right and left masseter muscles. The probe will be placed at the widest part in the middle of the muscle belly, perpendicular to the long axis of the masseter muscle. In the second method, the probe will be placed at the widest part in the middle of the muscle, parallel to the long axis of the masseter muscle.

In both methods, the middle part of the masseter muscle will be determined as the most prominent part when the participant bites in the B-Mode images. Then, the width of the region to be examined will be adjusted and the "Elasto" mode will be selected and the SWE system will be activated.

With the activation of the SWE system, the screen will be divided into right and left sections, where the routine grayscale image will be displayed on the left and the elastography image will be displayed on the right. Then, three consecutive elastography images will be obtained and a 10 cm2 circular region of interest (ROI) will be placed at the center of the examined muscle tissue in each image. After three measurements are taken in this way, the average of the measurements will be taken as the stiffness value of the muscle.

Inclusion Criteria:

Systemically healthy adults aged 18-65 years (American Society of Anesthesiologists Class I) Diagnosis of acute apical periodontitis Presence of at least four posterior occlusal units (one premolar pair counted as one unit; one molar pair counted as two units) Age ≥18 years Absence of maxillofacial pathology or systemic disease No history of temporomandibular joint or masticatory muscle disorders No complaints of dysphoni No diagnosed muscular disorders

Exclusion Criteria:

Presence of systemic disease Current use of any medication Fewer than 20 natural teeth Presence of prosthetic restorations (single crowns, fixed dental prostheses, or removable prostheses) Dental implant restorations History of periodontal treatment within the previous year Diagnosis of symptomatic pulpitis Pregnancy Smoking Unilateral edentulism