This study evaluated the effectiveness of two different LLLT devices on patients with MPS using both objective and subjective methods. In order to achieve homogeneity in the study, the patients included in the study were selected from the same age group, social status and profession. Studies show that 55% of patients with maxillofacial pain complain of MPS [18]. Traditionally, temporary treatment with massage, heat, analgesics and myorelaxants has been used to reduce this pain. Treatment options such as intraoral appliances (occlusal splints) or Botox injections are also available [8, 9]. However, these treatments may have some disadvantages. Generally, patients cannot tolerate the use of intraoral appliances, and since Botox application is temporary, it must be repeated every 4–6 months. Since long-term application of Botox paralyses the muscle, the stimulus from the brain is not transmitted to the muscle, muscle tone is affected, and atrophy of the tendons and bone to which the muscle is attached may occur in the future. Therefore, although Botox treatment is effective in the short term, long-term use is not recommended [19]. In recent years, LLLT have also been used to treat TMD [6,7,8]. Laser equipment has been produced in a variety of models and structures to enable regional application. As the masseter muscle is a large and thick muscle covering the cheek area, there may be a difference in the effectiveness of treatment between regional and point application. Factors such as the laser type, type of application, wavelength and depth of penetration of the laser have a significant effect on the success of the treatment [7]. Therefore, in this study, the efficacy of LLLT was compared using different methods of application, different penetration depths and different wavelengths.

The most common symptom of MPS is pain, but there are other effects as well: muscle hypertrophy, muscle fatigue, ischaemia of muscle tissue, decreased blood flow and damage to muscle fibres [20]. Studies have reported that continuous clenching changes the blood oxygen supply to the masseter muscle [17]. One of the causes of TMD is clenching and grinding of the teeth, if the problem is not solved, the severity of the clenching and grinding will increase [21]. Therefore, the severity of the clenching is expected to decrease after the treatment we apply, and the measurement of the bite force (BF) is a factor that determines the effectiveness of the treatment [21]. Considering all these factors, in the present study, pain level, which is a qualitative method, and bite forces, masseter muscle oxygenation, which allow us to obtain quantitative data, were measured to evaluate the effectiveness of LLLT.

Subjective assessment was performed with a pain scale (VAS) measured by palpation of the masseter muscle. VAS is a proven, reliable measurement method recommended by the American Dental Association (ADA). It has been used in many other studies to measure the pain in the affected area by palpation in joint disorders [22, 23]. In our study, the VAS results showed a decrease in all three groups. However, the reduction in pain level was less in the placebo group than in the treatment groups. As stress is known to be a major factor in the etiology of temporomandibular disorders, we assume that the reduction in pain in the placebo group is due to stress reduction. Because when the other two objective data were analysed, there was no change in the placebo group. The thought of receiving treatment may have caused a reduction in pain levels. In a study by Olavi et al., it was reported that LLLT applied to trigger points significantly reduced the pain level [24]. Similarly, other studies have reported a reduction in pain with LLLT applied to trigger points in MPS, even claiming that the patient can live comfortably for 2–3 years after LLLT [16, 25,26,27,28]. However, in the study by Thorsen et al., it was reported that there was no difference in pain level in the treatment group compared to the placebo group [29]. There are many reasons for the differences in these results. There is no definite protocol regarding the applied laser parameters. In the studies, the wavelength, energy density and application time of LLLT are different.

Various lasers with different wavelengths such as 780–904 nm GaAs, 830–904 nm GaAlAs, 1064 nm Nd: YAG have been used in low-level laser studies for TMD [30,31,32,33,34]. The energy density of the laser is also an important parameter. There is no definitive information in the literature on the effective dose for MPS, but 6–10 J/cm2 per session have been recommended for myogenic disorders and 4–6 J/cm2 per session for arthritis/arthrosis [34]. Studies have generally used an energy density in the range of 3–8 J/cm2 at each trigger point [31, 32, 35]. With reference to these studies, we used an energy density of 8 J/cm2 for the Nd: YAG laser in our study and successful results were obtained.

In our study, the effectiveness of LLLT was also evaluated with objective data such as bite force and masseter muscle oxygenation. In the literature, the number of studies measuring oxygenation in the masseter muscle is limited and the difference in oxygen level that occurs when the jaws are at rest, clenching or chewing is usually measured [17, 20, 36]. In a study by Puel et al., when healthy individuals and individuals with TMD were compared, it was reported that masseter oxyhaemoglobin values decreased at rest and during contraction, and it is thought that oxygenation decreases with increased contraction of the masseter muscle as a result of stress [37]. In the study by İspirgil et al., haemodynamic changes were observed in the masseter muscle as a result of occlusal splint treatment. It was reported that splint treatment caused a decrease in blood flow and a quantitative decrease in HbO2 [38]. In our study, there was no statistical difference in the level of HbO2 in both laser treatment groups, but there was a small quantitative decrease, while there was no change in the control group. According to the results of our study, there was a significant decrease in bite force values, which is another quantitative data, in the treatment group. There are no studies in the literature measuring bite force after LLLT, but studies on occlusal splint treatment support the findings of a decrease in EMG amplitude and a decrease in maximum bite force [38].

One of the limitations of the study is the lack of long-term follow-up of the results. Especially in the placebo group, it would have been good to see whether the reduction in pain that occurred immediately after the end of treatment returned to its previous level in the long-term.