Introduction
The temporomandibular joint (TMJ) is a bilateral synovial joint that is located on both sides of the jaw and is involved in chewing, swallowing, speech and other automatic movements, such as yawning and grinding. About 2000-2500 jaw movements occur daily. Temporomandibular disorders (TMDs) include a group of diseases of the TMJ, jaw muscles, masticatory muscles and the nerves that control them, which cause pain and dysfunction of the jaw joint and muscles controlling jaw movement [2]. In most cases, the cause of TMD is not precisely known, but in general, parafunctional habits, trauma, stress, anxiety and depression, heredity, grinding teeth, occlusal disharmony, and orthodontic treatment can be mentioned as the causes of TMDs [3]. The diagnostic criteria for TMD (DC/TMD) is a framework that consists of four distinct components: History and clinical examination, imaging studies, psychological testing, and laboratory tests. By combining the information obtained from DC/TMD with a complete clinical examination and additional diagnostic tests, an accurate diagnosis can be achieved [4]. In general, TMDs can be divided into three main categories: Disorders of the joints, including disc disorders, disorders of the muscles used for chewing (masticatory muscles), and headaches associated with a TMD. Each of these categories has different branches. The symptoms of TMDs include: Pain in the chewing muscles and/or jaw joint (most common symptom), pain that spreads to the face or neck, jaw stiffness, limited movement or locking of the jaw, painful clicking, popping, or grating in the jaw joint when opening or closing the mouth, ringing in the ears, hearing loss, or dizziness, and a change in the way the upper and lower teeth fit together. TMD is prevalent in 31% of adults and 11% of children and adolescents, and its symptoms increase with aging and stress. TMD in women has a higher prevalence than men (5:1) due to psychosocial and hormonal factors [3, 6]. 
Several randomized clinical trials have been conducted to evaluate the effectiveness of low-level laser therapy (LLLT) in the treatment of TMD. However, the clinical effectiveness of this method is controversial. Some studies observed that LLLT had the best treatment results compared to other methods, while some others did not find significant differences. The aim of this study is to investigate the effects of LLLT on myofascial pain and mouth opening reported in different studies.

Methods
In this study, randomized clinical trials, case-control studies, and systematic review studies that investigated the treatment of TMD with LLLT were reviewed. To identify the articles, a search was conducted in PubMed, Scopus and Google Scholar databases. The search strategy in PubMed using the MeSH terms was as below: 
(Treating temporomandibular myofascial pain[MeSH terms]) AND (low level laser[title/abstract])
(“therapy”[MeSH subheading] OR “therapy”[all fields] OR “treat”[all fields] OR “treating”[all fields] OR “treated”[all fields] OR “treats”[all fields]) AND “temporomandibular”[all fields] AND “myofascial”[all fields]) AND “pain”[MeSH terms] AND “low level laser”[Title/Abstract] 

Translations
Treating: “Therapy”[subheading] OR “therapy”[all fields] OR “treat”[all fields] OR “treating”[all fields] OR “treated”[all fields] OR “treats”[all fields]
Pain[MeSH terms]: “Pain”[MeSH terms]
The search strategy in Scopus was as following: TITLE-ABS-KEY (myofascial) AND (pain) AND (low-level laser)
The studies published from 1976 to the present were included. The studies were also selected based on the DC/TMD criteria. Then, the studies that did not meet the criteria were excluded and the remaining studies were reviewed. The studies with unavailable full texts, studies written in a language other than English, animal studies, studies that did not directly mention TMD and LLLT in their titles, duplicates, studies that evaluated the prevalence of TMD using the criteria different from the DC/TMD criteria, and case reports were excluded.

Results
In the initial search, 271 articles were found. After removing non-English articles, animal studies, case reports, and those with no available full text, 141 articles remained. After screening, 130 articles remained, 95 of which were not in line with the objectives of our study. Of the remaining 35 articles, 6 were duplicates and 21 did not have the words TMD and LLLT in their titles. Finally, 8 articles were included in the review.
Out of 8 reviewed articles, 6 were in favor of the effectiveness of LLLT in the treatment of TMD and related disorders, such as masticatory muscle pain and TMJ pain and difficulty in jaw opening. One study stated that LLLT is not effective for the treatment of TMD and related disorders. One study believed that another method, i.e. superluminous device, is more effective than LLLT.

Conclusion
The TMDs include pain or tenderness in the jaw, neck, or face, clicking in the jaw joint, difficulty opening or closing the mouth, and headache [2]. One of the methods to treat this disorder is LLLT. This method can reduce pain, inflammation and edema, and heal wounds and deeper tissues and nerves. The LLLT increases the production of adenosine triphosphate, which leads to the improvement of local microcirculation, reduction of edema by increasing lymph flow, and reduction of prostaglandin E2 and cyclooxygenase-2 levels [11, 13]. In this study, out of 8 reviewed articles, 6 studies reported the effectiveness of LLLT in the treatment of TMD and the reduction of associated pain. Therefore, it can be said that LLLT is a suitable method for the treatment of TMD and can lead to acceptable results. Considering the limited studies in this field, it is not possible to draw a definite conclusion and more studies are needed.

Ethical Considerations

Compliance with ethical guidelines
There were no ethical considerations to be considered in this research.

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors.

Authors' contributions
Conceptualization and study design: Amirreza Hendi, Data collection and analysis: Mahsa Koochaki; The initial draft preparation: Bahare Mahmoudpour and Sina Sarlak; Critical revision: Mehsa Khoki.

Conflicts of interest
The authors declared no conflict of interest.




References