Introduction
Adhesive capsulitis or frozen shoulder is among the most common causes of pain and disability in people whose prevalence is estimated at 2-5% [1, 2, 3]. An important feature of this disease is pain and limited movement in the shoulder [4]. Numerous studies have examined the pathophysiology of adhesive capsulitis and the best modality and treatment method for it [5, 6, 7]. There are numerous interventions for adhesive capsulitis, including drug therapies [8], arthroscopic release [9], intra-articular injections [10, 11], suprascapular nerve block [12], manipulation under anesthesia [13], and physical therapy and exercise therapy [14, 15, 16, 17].
Physical therapy interventions have received more attention due to their non-invasive nature [16, 17]. Common interventions in the physical therapy of patients with adhesive capsulitis include manual techniques, exercise therapy, and physical modalities generally used to reduce pain and increase range of motion [16]. Among the physical modalities, laser therapy known as Photobiomodulation (PBM) may increase metabolic capacity, ATP production, and cellular energy level and lead to cell function and repair [18, 19]. PBM uses non-ionizing light and is a nonthermal process that changes molecular functions due to dose-response effects [19].
The shoulder’s rotator cuff muscles play the most crucial role in regulating dynamic stability and arthrokinematics [20]. The supraspinatus muscle has a more influential role and is the most biomechanically active shoulder muscle [21]. On the other hand, poor blood supply is more prone to degeneration than other muscles. Studies indicated that supraspinatus muscle injury directly alters the arthrokinematics of the shoulder joint and, depending on the severity of the injury, may completely impair joint movement [22, 23]. Therefore, the evaluation and treatment of structural and functional defects of this muscle are critical in completing the treatment process of adhesive capsulitis [24].
Various studies use different tools to measure the thickness of soft tissues, including magnetic resonance imaging, computed tomography scans, and ultrasonography [25]. In the present study, measuring muscle function using ultrasonography after treatment methods is a new perspective that previous studies have not developed [26]. 
There is no study examining the thickness of the supraspinatus muscle, as the most effective shoulder muscle in the occurrence of adhesive capsulitis has been measured by ultrasound [27]. Therefore, this study aimed to evaluate the effect of PBM on pain, Range of Motion (ROM), and thickness of supraspinatus muscle in patients with adhesive capsulitis.

Materials and Methods
This randomized controlled trial was performed from February 2019 to June 2020. Overall, 52 Subjects with shoulder adhesive capsulitis voluntarily participated in the present study. The inclusion criteria for the study included the age range of 18-50 years and diagnosing adhesive capsulitis as unilateral shoulder pain duration 3-9 months by an orthopedic surgeon [24]. The exclusion criteria were shoulder pain of less than 3-month, humeral fractures, cervical and shoulder surgeries, and bursitis. Moreover, the subjects with a history of physical therapy or any local injection within the last 3 months, psychological problems, and athletics were excluded. The study variables were pain, ROM, the supraspinatus muscle thickness in rest and contraction, the rest-thickness-contraction percentage.
The Visual Analogue Scale (VAS) assessed the intensity of pain. The patient’s shoulder pain was recorded between 0 and 10 with the necessary explanation to the person, and they expressed their average pain over the past 24 hours [28]. Then, the height and weight of the subject were measured and recorded using a tape measure and scales. Besides, the Body Mass Index (BMI) was calculated by measuring Height (H) in centimeters and Weight (W) in kilograms. The range of abduction and external rotation was also assessed by a goniometer [29]. Honda ultrasound machine (Honda electronics, HS 2000, Japan) and 5-10 MHz linear probe measured muscle thickness. 

Shoulder ROM evaluations 
The abduction was recorded using a goniometer. The subject was supine with the upper limbs next to the body and anatomical position. The patient moved his shoulder to abduct in the frontal plane. The goniometer was placed on bone landmarks; the fixed arm of the goniometer was placed parallel to the sternum. The axis of the goniometer was the acromion process, and its movable arm on the humerus was in the direction of the medial epicondyle [30, 31].
Furthermore, the range of external shoulder rotation was assessed by a goniometer. Thus, the study subject was in a supine position with the shoulder in 90-degree abduction, 90-degree elbow flexion, and forearm pronation. The fixed arm of the goniometer was perpendicular to the ground. The axis of the goniometer was on the ulceranon process, and its moving arm was on the forearm in the direction of the ulnar styloid process [31].
Ultrasound measurements were obtained while the study subject was sitting using the Honda ultrasound device and linear probe (5-10 MHz, HS, Japan) on the supraspinatus muscle. The shoulder was neutral (hanging next to the body), and the elbow was fully extended. Initially, the scapula was palpated, and a line was drawn along the spine of the scapula. The acromion and coracoid processes were also palpated, and a line was drawn between them. Another line is drawn along the length of the acromion. The probe was placed parallel to the spine of the scapula, and the scapular notch was identified. The probe was then rotated parallel to the line between the acromion and the coracoid process at the scapular notch, with the scapular notch centered. When the cross-section of the supraspinatus muscle was detectable, the closest distance between the center of the acromion-coracoid line and the outer edge of the probe was measured [30]. The upper and lower margins of the supraspinatus muscle were recorded as muscle thickness in the resting position of the upper limb on the side of the body and 60 degrees of scaption. The reliability of these measurements was reported previously [30]. All evaluation methods were recorded before, after 5 and 10 sessions, and a 1-month follow-up.

Intervention 
The treatments of the control group consisted of the following: 
1) Infrared (IR) lamp for 20 minutes: The IR lamp was placed at a distance of 50 cm from the person’s shoulder and turned on. The patient also felt a mild heat. 
2) Transcutaneous Electrical Nerve Stimulation (TENS) brief type for 20 minutes: In this method, 4 electrodes of the TENS were used on both sides of the affected shoulder. The frequency used varied between 3 and 120 Hz, and the patient felt an electric current to the point of tolerance with muscle tremor. 
3) Ultrasound: The 1 MHz probe was used for 5 minutes on the anterior shoulder. The intensity was 0.8 w/cm2, and the duty cycle was 20%. 
4) Shoulder strengthening exercises: including 5 exercises, were performed twice daily from the third to the tenth session [17, 24, 32]. 
The treatment in the control group is a routine physical therapy treatment for patients with frozen shoulders. Since the same treatment is performed for the PBM group, this group can be considered a control group [33].
In the present study, a laser therapy device (CARSI, Brazil) with a wavelength of 905 nm, energy density 8J /cm2, power 50 mW was radiated at 10 points of the shoulder, including 3 points in the anterior part of the capsule, 3 points in the posterior part of the capsule, two points on the root of the C5 nerve and two on the axilla was employed for the PBM group. The duration of irradiation for each point was 91s. The area was thoroughly cleaned with alcohol to minimize the skin’s resistance. The therapist and the subject wore special glasses during treatment to prevent eye damage [34].
The Shapiro-Wilks test was used to evaluate the distribution of numerical variables. We used the Independent Samples t-test to compare variables, such as age, height, and weight among the patients before interventions. The Analysis of Variance (ANOVA) was used to evaluate the differences among groups, and the Bonferroni post hoc test was used to determine the mean differences between the study groups. The data analyses were performed in SPSS.

Results
In total, 52 subjects (26 per group) participated in this study. The demographic information of the patients with adhesive capsulitis is presented in Table 1.

There were no significant differences between the two groups concerning demographic characteristics. Furthermore, other comparative variables didn’t show significant differences between the research groups before treatment (P>0.05).
The control and PBM groups revealed a significant reduction in shoulder pain after the intervention. In the control group, the initial mean±SD value of pain (VAS) decreased from 7.23±1.36 to 3.85±3.25 at the final follow-up (P<0.001). Likewise, pain in the PBM group declined from 7.38±1.29 to 2.35±2.74 (P<0.001). The interaction of group and time (2×4) was significant for pain (P=0.02) (Figure 1). 

Finally, the Bonferroni post hoc test results indicated the pain reduction in the PBM group was higher than the control group at follow-up (P=0.02) (Table 2).

The mixed two-factor analysis of variance showed that the interaction of the group and time of shoulder abduction (P=0.6), shoulder external rotation (P=0.46), supraspinatous muscle thickness, rest (P=0.31), supraspinatus muscle thickness, contraction (P=0.11), and % rest-contraction (P=0.66) were not significant (Table 2). 

Discussion 
The aim of this study was to evaluate the effect of PBM on pain, shoulder ROM, and thickness and function of supraspinatus muscle in patients with frozen shoulders. The present study data highlighted that using PBM compared to routine physical therapy suggested a similar effect on the above variables. Both increased shoulder ROM and altered thickness and muscle function in patients with shoulder pain; however, compared between the two methods, neither was superior to the other. Nevertheless, the effects of PBM were superior in pain reduction. 
Some studies presented consent, and others indicated contrary findings with the present study [35, 36, 37, 38, 39, 40]. Recently, Alfredo documented that combining laser therapy with exercises reduces pain intensity and improves shoulder function over 3 months in patients with subacromial impingement syndrome [41]. In the present study, the analgesic effects of laser therapy were more outstanding than the control group. The results of previous studies revealed that laser was also irradiated to the roots of the 5 cervical nerves; therefore, it revealed a more significant effect on reducing patients’ pain than the control group. High-dose laser radiation to the nerve roots blocks the conduction of pain-receptors to higher centers and reduces patients’ pain [19]. Therefore, choosing the correct dosage and appropriate points for laser radiation are essential factors in reducing patients’ pain perception [42].
Additionally, improved shoulder ROM and supraspinatus muscle function were significant in both groups after 1 month. These patients were in the freezing or painful phase of the disease, which pain being the most predominant symptom [24]. Therefore, reducing pain in these patients may have more prolonged effects, which were studied in the present study for up to one month [35, 36]. However, more prolonged analgesic effects of up to 6 months of laser therapy were indicated in previous studies [24]. Another noteworthy point is that reducing pain following laser radiation starts from the initial sessions and is more [34, 39]. However, improving ROM and muscle function takes more time. A longer follow-up time probably shows different results [23, 35].
The present study was the first to investigate the effects of PBM on muscle thickness and function using ultrasonography in patients with adhesive capsulitis. Improvement in muscle function and thickness did not differ significantly between the two groups after one month. One reason was the practice of the same exercise in both groups [36]. Furthermore, the location of the laser radiation was not on the target muscle; thus, it could not have more effects on its performance [43]. In future studies, laser radiation is suggested to be applied to the muscle to increase its thickness and function. However, other studies have supported these findings [44, 45].
The present study indicated that laser does not have more anti-inflammatory effects than other modalities, like ultrasound. Failure to consider the exact depth of the lesion and the use of low power laser and radiation on small and limited points (three points in front and three points behind the joint capsule with an area of 0.19 cm2) in the present study may be the reasons [42, 46]. The results of previous studies revealed that using a laser with higher power or radiation over a wider area and selecting more points may have better effects [36, 39, 42].
The main limitations of the present study were as follows: challenges in diagnosing adhesive capsulitis due to lack of objective instruments, the relatively small sample size for each group, the lack of long-term follow-up, and the lack of sham group.

Conclusion
The PBM indicated greater analgesic effects than routine physical therapy in the short term and one-month follow-up. However, the effects on ROM and muscle thickness and function were insignificant in the short term and one-month follow-up. 

Ethical Considerations
Compliance with ethical guidelines
All study procedures complied with the ethical guidelines of the Declaration of Helsinki 2013. All subjects filled in the consent forms (Ethics code: IR.GUMS.REC.1396.434) and were registered in the Iranian Clinical Trials Register (Code: IRCT20170516034003N2).

Funding
The present study was supported by the Deputy of Research and technology of Guilan University of Medical Sciences, Iran.

Authors contributions
Conceptualization: Kamran Ezzati, Saemeh Khani, Amir Salari, Arash Aris; Methodology: Kamran Ezzati, Saemeh Khani, Amir Salari, Arash Aris; Software: Saemeh Khani, Kamran Ezzati; Validation: Kamran Ezzati, Amir Salari, Arash Aris; Formal analysis: Kamran Ezzati, Saemeh Khani; Investigation: Kamran Ezzati, Saemeh Khani, Amir Salari, Arash Aris; Data curation: Kamran Ezzati, Saemeh Khani, Amir Salari; Writing – original draft preparation: Saemeh Khani, Kamran Ezzati, Amir Salari, Arash Aris; Writing – review & editing: Kamran Ezzati, Amir Salari, Saemeh Khani, Arash Aris; Visualization: Amir Salari, Arash Aris; Supervision: Kamran Ezzati; Project administration: Kamran Ezzati, Amir Salari, Saemeh Khani, Arash Aris; Funding Acquisition: Kamran Ezzati, Amir Salari.

Conflict of interest
The authors reported no conflict of interest.

Acknowledgements
The authors would like to thank the Deputy of Research and technology of Guilan University of Medical Sciences for supporting this research.

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