Effectiveness of low-intensity laser on pain in patients with supraspinatus tendinopathy: A triple-blind randomized controlled trial

Abstract

BACKGROUND

Tendinopathy is an inflammatory process that occurs in and around the tendon when both are affected by a specific injury. The supraspinatus muscle is one of the most common causes of shoulder pain.

AIM

To test the effectiveness of laser treatment in reducing pain, improving mobility, quality of life and pressure pain thresholds (PPT) in patients with supraspinatus tendinopathy.

METHODS

A randomised controlled clinical trial was conducted in which a physiotherapeutic intervention with a therapeutic laser was performed for four weeks to observe the influence of supraspinatus tendinopathy in the shoulder on pain. In addition, a triple-blind study was conducted, in which the physiotherapist performing the intervention was blinded, as were the patients receiving the treatment and the person interpreting the study results. A sample of 80 patients was recruited and randomly divided into two groups: An experimental group and a control group. Laser therapy was applied to the first group, and a placebo was applied to the second group.

RESULTS

Eighty participants were randomized to a therapeutic laser group (n = 40) or a placebo group (n = 40), with no baseline differences in demographic or clinical variables. Post-intervention analyses revealed significantly greater improvements in the laser group compared with placebo for pain intensity, muscle strength, and active shoulder range of motion in flexion, extension, abduction, and external rotation (P < 0.05). No between-group differences were observed for PPT, adduction, or internal rotation. Within-group analyses showed significant improvements in all clinical variables only in the laser group. Repeated-measures the Analysis of Variance demonstrated a significant time × group interaction for Disability of Arm, Shoulder, and Hand scores (P < 0.001), indicating superior and sustained functional improvement in the laser group up to three months post-intervention.

CONCLUSION

The therapeutic laser therapy produced superior and sustained improvements in pain, strength, shoulder mobility, and functional outcomes compared with placebo, supporting its effectiveness as an adjunct treatment.

Key Words: Laser; Tendinopathy; Supraspinatus; Pain; Physiotherapy; Rehabilitation

Core Tip: This triple-blind randomized controlled trial demonstrates that therapeutic laser therapy produces meaningful and lasting benefits in patients with supraspinatus tendinopathy by reducing shoulder pain and enhancing joint mobility. The intervention also leads to relevant gains in rotator cuff muscle strength and overall shoulder function, facilitating better performance in daily and overhead activities. These improvements are not explained by placebo responses alone, supporting therapeutic laser as an effective, evidence-based option within a multimodal rehabilitation approach for this common shoulder disorder.

INTRODUCTION

Shoulder pathology is one of the main causes of musculoskeletal pain in the adult population, with a significant impact on physical function, work capacity and quality of life[1]. Within this broad spectrum, rotator cuff tendinopathies represent one of the most frequent clinical entities, with the supraspinatus tendon being the most commonly affected because of its biomechanical role in glenohumeral stability and its vulnerability to repetitive loads, subacromial compression, and age-related degenerative processes[2,3]. Supraspinatus tendinopathy is clinically characterised by localised or referred pain in the shoulder and proximal arm, limited active and passive range of motion (ROM), and alterations in upper limb function, which directly affect activities of daily living and the patient's overall perception of health[4].

There is evidence that exposure to various physical stresses in the workplace, such as working above head height, lifting heavy objects, and working in an uncomfortable position, increases the risk of shoulder disorders. Psychosocial risk factors are also associated with this condition[4].

Shoulder pathology is very diverse and can be caused by various factors. Shoulder pain is a significant health problem affecting approximately 25% of the general population[5]. The prognosis for people with musculoskeletal shoulder pain varies widely among individuals. Fifty percent of these individuals still report symptoms six months after visiting their primary care physician[6]. A recent study reported that the incidence of shoulder pain ranges from 7.7-62 per 1000 people per year (median 37.8 per 1000 people per year) in high-, middle, and low-income countries. These data demonstrate that a significant proportion of the population worldwide will experience shoulder pain on a daily, annual, and lifetime basis[7].

From a pathophysiological point of view, supraspinatus tendinopathy is currently understood as a multifactorial process involving structural and biochemical changes in tendon tissue, with alterations in collagen organisation, increased neovascularization, and peripheral and central sensitisation[2]. These mechanisms partly explain the persistence of pain and the variable response to traditional therapeutic interventions. In this context, a conservative approach is the first line of treatment, and physiotherapy plays a central role through the combined use of therapeutic exercise, manual techniques, and physical modalities aimed at pain control and functional improvement[4,5].

Excessive or repetitive activities can lead to tendinopathy and degeneration or tears of the rotator cuff, which inevitably compromise the ability of the tendon to stabilises and depress the humeral head. Around the fifth decade of life, degenerative changes are detected in the rotator cuff tendons, particularly thinning and fibrillation in the “critical zone” (hypovascular area) of the cuff. These changes are believed to be part of physiological aging, but it appears that as degeneration increases, repair mechanisms fail, and microtears develop that can progress to macrotears. Pain typically worsens when sleeping on the affected shoulder and when moving the shoulder in certain directions. The overlying bone can exert pressure on the tendons when the arm is raised, a phenomenon described as “impingement”[8].

Age also becomes an intrinsic risk factor, as blood flow to the tendon decreases with increasing age[9]. The deep and anterior fibres of the rotator cuff (inserting at the greater humeral tuberosity) are the most susceptible to tearing because they are the least vascularized[10]. Occasionally, this condition is associated with excessive deposits of hydroxyapatite crystals in the tendons, thus forming calcifications within the structure[11]. In some cases, this pathology is asymptomatic, but in others it manifests with pain and joint restriction in the area where these calcium deposits are located[12,13].

Most techniques used to treat this condition (nonsteroidal anti-inflammatory drugs), local steroid injections, physical therapeutic agents such as exercise, cold or heat, ultrasound, and laser) alone or in combination can relieve symptoms; however, these are not curative procedures, but rather analgesic therapies[14].

Low-level laser therapy (LLLT) has been commonly used in clinical practice for years. The effect of this therapy depends on the intensity and duration of exposure, and its short-term effects include an increase in the adenosine triphosphate content in the area. The long-term effects cause increased blood flow or hypervascularization in the area of application, which accelerates fibroblast proliferation and promotes type I and III collagen synthesis[15]. Research has shown that LLLT alters cellular processes, resulting in anti-inflammatory effects, such as the modulation of the inflammatory response and myonecrosis, have been described in the treatment of muscle injuries[16]. Several studies have described the positive effects of this therapy on tendinopathy, osteoarthritis, arthritis, wound healing, back pain, neck pain, and peripheral nerve injuries. In addition, positive effects have been described in the treatment of muscle injuries, such as modulation of the inflammatory response and myonecrosis[15,16].

Therefore, the objective of this clinical trial was to analyse the efficacy of LLLT compared with that of a placebo intervention in patients with supraspinatus tendinopathy, evaluating its effect on pain relief, improvement in shoulder ROM, quality of life, and the pressure pain thresholds (PPT). This study aims to contribute to the existing evidence through a controlled and rigorous design, providing relevant information for clinical decision-making in the physiotherapeutic management of this prevalent pathology.

MATERIALS AND METHODS

Study design

A randomized, triple-blind controlled clinical trial was conducted from 2023-2024 on patients with rotator cuff tendinopathy who attended a specific physiotherapy clinic in Ávila, Spain. This study was carried out in accordance with the recommendations of the CONSORT statement for randomized clinical trials[17].

Participants

Patients attended physiotherapy consultations with a prior diagnosis from an orthopedic surgeon that they were suffering from supraspinatus tendinopathy after being assessed through orthopedic tests and complementary radiodiagnostic tests. The patients were subsequently randomly assigned to the treatment group or the control group via Epidat 3.1 software. Assignment within the intervention and placebo groups was performed completely randomly in groups A (treatment) and B (placebo). A total of 95 patients were recruited and, after they met the inclusion and exclusion criteria and passed the various phases of the study and randomization, 80 patients were assigned as follows: The intervention group (n = 40) received LLLT treatment on the supraspinatus tendon and the control group (n = 40) received placebo LLLT therapy on the same tendon. The sample size was calculated via the software proposed by the Cleveland Clinic Quantitative Health Sciences calculator for parallel clinical trials of superiority. The Disability of Arm, Shoulder, and Hand (DASH) scale score was taken as the main variable, and a standard deviation of 15 points was assumed[18,19]. With these parameters and a significant level of 5%, and a power of 80%, and assuming a loss rate of 15%, it was estimated that 35 subjects per group would need to be included.

The DASH questionnaire was selected as the primary outcome measure because it is a validated, region-specific, patient-reported instrument that comprehensively captures upper extremity symptoms and functional disability that are clinically meaningful for patients and clinicians. Prioritizing a functional, patient-centered measure as the primary endpoint is consistent with current recommendations for outcome selection in clinical research and allows a more holistic assessment of treatment benefit beyond symptom relief alone. In addition, pain intensity was assessed using a Visual Analog Scale (VAS), which was prespecified as a key secondary outcome to quantify patients’ subjective perception of pain. Considering both functional status and pain is important to uphold methodological rigor and ethical standards, ensuring that the intervention’s impact on daily activities and symptom burden is adequately evaluated for informed clinical decision-making.

Subjects were selected consecutively from January 2024 to March 2024, as they arrived for consultation and after verifying compliance with a series of selection criteria.

Inclusion criteria

Age between 35 years and 65 years, was used to reduce the influence of age-related degenerative changes and ensure a more homogeneous sample. Patients were previously diagnosed by an orthopedic surgeon with supraspinatus tendinopathy and chronic unilateral shoulder pain (≥ 3 months) located in the proximal lateral region of the arm and exacerbated by active abduction and external rotation of the shoulder. A pain intensity of ≥ 5 points on the VAS at rest or during shoulder movement at baseline was given. Ultrasound-confirmed supraspinatus tendinopathy, defined by at least one of the following structural findings: Tendon thickening, hypoechoic areas, fibrillar disorganisation, or neovascularization, was assessed by an experienced musculoskeletal radiologist at the study center. Preserved cognitive ability sufficient to understand the study procedures and provide informed consent. Ultrasound imaging was used exclusively as a diagnostic tool to confirm eligibility and was not considered an outcome measure in this study.

Exclusion criteria

A full-thickness rotator cuff tear, calcifications, adhesive capsulitis, glenohumeral instability, or severe glenohumeral or acromioclavicular osteoarthritis can be identified clinically or by imaging. Previous surgery involving the affected shoulder. History of shoulder fracture, inflammatory rheumatic disease (e.g., rheumatoid arthritis), polymyalgia rheumatica, or neurological disorders affecting the upper limb. Systemic conditions potentially influence pain perception or tissue healing, including diabetes mellitus, significant cardiac dysfunction, or active malignancy. Contraindications to laser therapy, such as pregnancy or breastfeeding, the presence of pacemakers or implantable defibrillators, or known photosensitivity. Current use of anticoagulant or antiplatelet therapy. Receipt of infiltrative treatments (e.g., corticosteroid injections) or structured physiotherapy for the affected shoulder within the two months prior to recruitment. The presence of other musculoskeletal conditions causing pain in the upper extremity that could confound the outcome assessment. The diagnosis of major depressive disorder or other psychiatric conditions that could interfere with pain reporting or adherence to the intervention. Inability to comply with the treatment protocol or follow-up assessments. Participation in any other therapeutic intervention targeting shoulder pain during the study period.

Ethical considerations

The study subjects were enrolled voluntarily and received no financial compensation for their participation. The informed consent was obtained from all participants before data collection began. The research was conducted between January 2024 and March 2024 and was previously approved by the Ethics Committee of the University Hospital of Móstoles with registration code CEI 2024/021 and clinical trial registration on the clinicaltrial.gov website with number NCT07261449. Furthermore, compliance with the Declaration of Helsinki, the Biomedicine Act, the Patient Autonomy in Data Processing Act and the Organic Data Protection Act is noted.

Outcome assessment

The patients recruited previously were diagnosed by an orthopedic surgeon with supraspinatus tendinopathy via orthopaedic tests and complementary tests, but the physiotherapist performed a preliminary ultrasound scan to confirm this previous diagnosis and observe the presence and involvement of the tendon in B-mode, to determine its precise location and assess the severity of the tendinopathy and distinguish between the different stages of tendon involvement. A LOGIQ P9 ultrasound scanner (Prym, Madrid, Spain, 2024) was used for this purpose. Laser therapy was performed via BTL brand equipment, model 4000, with a BTL-485-05IC probe and an output power of 50 mW and a wavelength of 830 nm (BTL, Madrid, Spain, 2024). The measurements were taken by a physiotherapist with more than 20 years of experience. In addition, this examination revealed signs of tendon inflammation, hypoechoic areas, calcifications, fibrillar disorganisation and/or muscle neovascularization[16]. The images were taken under the supervision of a physiotherapist trained in this discipline. The variables analyzed in this study were pain, pain threshold, quality of life, shoulder ROM, and muscle strength.

For pain, the VAS represents pain intensity on a 10 cm line. A value between 4 and 6 indicates moderate to severe pain, and a value above 6 indicates very intense pain. An algometer was used to measure the pain threshold. In this case, pressure was applied to the second marked point, as this corresponds to the central area of the tendon. This area was chosen because it was the most affected area in most patients according to ultrasound findings. The DASH scale was used to assess quality of life, the only variable tested in three phases of the intervention[18,19]. To measure pressure pain sensitivity, pressure algometry was used to assess the PPT at these anatomical locations[20]. The ROM of the shoulder was assessed via a goniometer. The ROM was measured via a universal goniometer[21]. Active flexion, extension, abduction, adduction, internal rotation, and external rotation of the affected shoulder were measured. Muscle strength was measured via a handheld dynamometer (KINVENT Muscle Controller), which is a portable, battery-powered device that measures the maximum force in Newtons when performing a specific action[22,23]. The device is connected via Bluetooth to the Kforce APP data center to store test information and patient data. Each patient underwent three measurements of the force exerted at 45° abduction of the affected shoulder. The highest value obtained was recorded for the study, corresponding to the maximum force exerted.

With respect to the intervention procedure, 86 patients were initially entered into an application to generate random groups. This resulted in a group of 43 participants who made up the placebo group and another group of 43 participants who made up the experimental group. Before proceeding with the intervention, all patients were provided with an informed consent document so that they could give their written consent for the physiotherapy procedure and the possible publication of their data. After the participants signed the document, the initial data for the study variables were recorded. These were noted in the assessment notebook corresponding to each patient. At the end of the study, all variables were reexamined. Three months after the intervention, the participants completed the quality of life test[24].

Blinding

To ensure blinding, two physiotherapists participated: One was responsible for applying the intervention, without knowledge of the assigned group, and the other was responsible for randomization and laser configuration. The latter activated or did not activate the device according to the assigned group, also playing a recorded sound from the equipment to simulate the activation of the light beam in the placebo group. Both physiotherapists performed 10 preliminary tests each to confirm that the recorded sound was indistinguishable from the real sound, and neither of them could identify whether the sound corresponded to the laser in operation or to the recording. This ensured the blinding of the physiotherapist performing the intervention, as they did not know whether the device was emitting laser therapy or not. Furthermore, neither the physiotherapist administering the treatment, nor the statistician responsible for the data analysis, nor the patients were aware of group allocation during the study. Group identities were disclosed only after completion of the statistical analyses; therefore, the trial was considered triple-blinded.

Procedure

Patients assigned to group 1 underwent LLLT therapy. A BTL model 4000 device was used. A BTL-485-05IC probe was used. The output power is 50 mW. The wavelength was 830 nm. Laser class 3B. For safety reasons, both the patient and the therapist wore approved protective eyewear, which prevented any glimpse of light that might come from the laser.

Three applications of 1 minute and 40 seconds were performed. Area of 1 cm², power of 50 m, wavelength of 830 nm, frequency of 5 Hz, resulting in a total energy delivery of 5 J, equivalent to an energy density of 4.0 J/cm². Therefore each session lasted 4 minutes and 20 seconds. Three applications were performed per week for one month, meaning that each patient received a total of 12 sessions. The interventions were performed on alternate days to avoid overtreating the tendon. The aim was to target the areas closest to the tendon structure: The supraspinatus insertion tendon. The probe was positioned at a 90° angle to the skin and the area to be treated. This area was selected beforehand via ultrasound and marked with three dots painted on the skin with indelible ink. The points were the same throughout the procedure and correlated with the proximal, distal, and central parts of the tendon. During the sessions, the patient was in the supine position with his hand behind his back to induce internal rotation, extension, and adduction so that the tendon was more exposed[25].

Patients were assigned to group 2, namely, LLLT placebo therapy. The placebo group was treated with the same equipment and methodology as the experimental group. However, in this case, the specialist did not press the power button on the laser controller.

At the same time, as the patient was wearing other protective glasses that did not allow vision, he recorded the sound emitted by the laser equipment, which prevented the patient from seeing the laser beam. Moreover, as the patient was wearing other protective glasses that prevented vision, the patient recorded the sound emitted by the laser equipment, which prevented both the physiotherapist performing the intervention and the patient from distinguishing whether the real intervention or the placebo was being applied. In both groups, the three treatment points were marked again with a permanent marker, session after session. The points were always confirmed on the ultrasound image. During the month of treatment, patients were informed that they should avoid the use of anti-inflammatories (medications, creams, ice, etc.) so that they would not influence the physiological processes of the laser.

The statistician responsible for the data analysis remained blinded to treatment allocation throughout the entire analytical process. The second physiotherapist provided a fully coded dataset in which treatment groups were labelled as Group A and Group B, without indicating whether they corresponded to active laser therapy or placebo. The randomization key linking group codes to treatment allocation was held exclusively by the physiotherapist in charge of randomization. All statistical analyses and interpretations were therefore conducted using the coded dataset, with no knowledge of the actual treatment allocation. The identity of the treatment groups was revealed only after the statistician had submitted the results and their corresponding interpretations.

Statistical analysis

A statistical study was conducted via the Statistical Package for the Social Sciences (SPSS) statistical program (version 24.0) (SPSS Inc, Chicago, IL, United States). The analyses were performed using complete cases, including only participants who completed the follow-up outcome assessments. First, a descriptive analysis of the variables was performed. Quantitative variables were summarised using the mean and standard deviation, and qualitative variables were summarised using absolute and relative frequencies.

To check the normality assumption of the variables the Kolmogorov-Smirnov test (P > 0.05) was used to assess whether the samples followed a normal distribution in the quantitative variables, given the size of the samples, which exceeded 30 individuals per group.

To compare the scores between the experimental group (laser) and the control group (placebo) at each time point, Student’s t-test for independent samples or, in the nonparametric case, the Mann-Whitney U test was used.

Finally, to analyse the temporal evolution of the scores obtained on the DASH scale, a repeated measures analysis of variance was performed. The intrasubject factor was time, i.e., the time of measurement of the DASH scale score in patients, and consisted of four levels: Preintervention, postintervention, one-month follow-up, and three-month follow-up. The intersubject factor corresponds to the intervention variable and consists of two levels: The intervention group with laser therapy and the placebo group.

Mauchly’s test was used to verify that the variables met the sphericity assumption. For those that did not (P < 0.05), the Greenhouse-Geisser correction was applied. To analyse the causes of significance within the interaction factor, a post hoc analysis was performed via the Bonferroni method. A significance level of 5% was considered for all analyses performed.

RESULTS

The total number of initial participants was 95. Nine participants were excluded from the study: n = 6 for not meeting the selection criteria and n = 3 for declining to participate. The 86 participants finally included in the study were randomized, resulting in 22 men and 21 women in the experimental group and 24 men and 19 women in the control group. During the study, one man and female in the experimental group and another in the control group two men, reducing the sample size to 40 patients in the experimental and control groups, respectively. The information can be found in Figure 1.

Figure 1  Flow diagram.

A total of 80 participants were included and randomly assigned to the placebo group (n = 40) or the therapeutic laser intervention group (n = 40). The participants’ ages ranged from 19-65 years. The mean age was 41.8 ± 13.2 years in the placebo group and 44.2 ± 14.4 years in the laser group, with no statistically significant difference between the groups (P = 0.478). The distribution by sex was comparable between the two groups (P = 0.823).

Similarly, no significant differences were observed between the groups in any of the baseline clinical variables evaluated, including pain (VAS), strength, PPT, and active shoulder ROM (all P > 0.05), indicating adequate initial homogeneity of the sample.

Table 1 shows the descriptive statistics for the clinical variables of interest that were the main focus of the study, collected according to the intervention group. Measurements were taken before the intervention.

Table 1 Preintervention clinical variables by intervention group, n (%)/mean ± SD.

	Placebo group (n = 40)	Intervention group (n = 40)	P value
VAS (cm)	6.8 (1.3)	6.9 (1.5)	0.746
Strength (N)	8.8 (3.4)	9.1 (3.5)	0.761
PPT (kg/cm²)	4.7 (1.3)	4.4 (1.1)	0.279
Flexion (º)	155. 6 (16.1)	154.5 (13.4)	0.751
Extension (º)	68.9 (11.3)	70.3 (13.6)	0.631
Aduction (º)	24.7 (6.6)	22.9 (5.1)	0.183
Abduction (º)	118.9 (32.9)	116.9 (36.2)	0.794
Internal rotation (º)	66.5 (18.4)	60.9 (16.7)	0.162
External rotation (º)	70.1 (16.7)	69.6 (16.4)	0.791

Summary measures of clinical variables of interest prior to intervention. The P value resulting from the application of Student’s t-test for independent samples or the Mann-Whitney test, as appropriate. N: Newtons of force; PPT: Pressure pain threshold.

There were no significant differences between the scores for the clinical variables of interest between the laser and placebo groups, measured before the intervention

Following the intervention, statistically significant differences in several secondary clinical variables were observed in favour of the therapeutic laser treatment, as shown in Table 2.

Table 2 Between-group comparison of post-intervention clinical variables, n (%)/mean ± SD.

	Placebo group (n = 40)	Intervention group (n = 40)	P value
VAS (cm)	6.1 (1.3)	4.1 (1.6)	< 0.001a
Strength (N)	8.2 (2.9)	13.04 (3.5)	< 0.001a
PPT (kg/cm²)	5.6 (1.1)	5.9 (1.1)	0.125
Flexion (º)	158.9 (13.6)	168.4 (12.6)	0.002a
Extension (º)	69.5 (10.0)	81.4 (9.3)	< 0.001a
Aduction (º)	26.3 (4.9)	27.9 (3.4)	0.338
Abducción (º)	127.8 (32.4)	163.3 (19.6)	< 0.001a
Internal rotation (º)	66.9 (18.4)	74.3 (13.1)	0.104
External rotation (º)	70.8 (18.0)	80.5 (12.7)	0.025a

^aP < 0.05.

Summary measures of clinical variables of interest after the intervention. The P value (P) resulting from the application of Student’s t-test for independent samples or the Mann-Whitney test, as appropriate. VAS: Visual analogue scale; PPT: Pressure pain threshold.

Compared with the placebo group, the laser group presented a significantly greater reduction in pain as measured by the VAS (P < 0.001), as well as superior improvements in muscle strength (P < 0.001) and in the active ROM for flexion (P = 0.002), extension (P < 0.001), abduction (P < 0.001) and external rotation (P = 0.025).

No statistically significant differences were observed between the groups in the PPT, adduction or internal rotation at the post-intervention assessment (P > 0.05).

The analysis of pre-post comparisons within each group clearly revealed different patterns, as can be seen in Table 3.

Table 3 Within-group pre-post comparison of clinical variables in each treatment group.

Comparation	Placebo group	Intervention group
	Pre; post; P value	Pre; post; P value
VAS pre - VAS post	6.8 (1.3); 6.1 (1.3); < 0.001a	6.9 (1.5); 4.1 (1.6); < 0.001a
Strength pre - strength post	8.8 (3.4); 8.2 (2.9); 0.920	9.1 (3.5); 13.0 (3.4); < 0.001a
PPT pre - PPT post	4.7 (1.3); 5.6 (1.1); < 0.001a	4.4 (1.1); 5.9 (1.1); < 0.001a
Flexion pre - flexión post	155.6 (16.1); 158.9 (13.6); 0.169	154.5 (13.4); 168.4 (12.6); < 0.001a
Extension pre - Extension post	68.9 (11.3); 69.5 (10.0); 0.620	70.3 (13.6); 81.4 (9.4); < 0.001a
Aduction pre - aduction post	24.7 (6.6); 26.3 (4.9); 0.003a	22.9(5.1); 27.9(3.4); < 0.001a
Abduction pre - abduction post	118.9 (32.9); 127.8 (32.4); < 0.001a	116.9 (36.2); 163.3 (19.6); < 0.001a
Internal rotation pre - internal rotation post	66.5 (18.4); 66.9 (18.4); 0.763	60.9 (16.7); 74.4 (13.1); < 0.001a
External rotation pre - external rotation post	70.1 (16.7); 70.8 (18.0); 0.421	69.6 (16.4); 80.5 (12.7); < 0.001a

^aP < 0.05.

Comparison of the scores on variables measured in patients before and after the intervention according to the group to which they belong. P value (P), result of applying Student’s t-test for related samples or Wilcoxon’s signed-rank test for related samples, as appropriate. VAS: Visual analogue scale; PPT: Pressure pain threshold.

In the placebo group, significant improvements were observed only in pain (VAS score), PPT, adduction and abduction (P < 0.05), whereas the remaining variables did not show clinically relevant changes.

In contrast, in the laser-treated group, all the clinical variables evaluated including pain, strength, PPT, and all the joint movements analysed significantly improved after the intervention (all P < 0.001).

Primary outcome

Functionality measured via the DASH scale. Upper limb functionality, assessed via the DASH scale, was defined as the primary outcome variable of the study.

Analysis using a repeated measures the Analysis of Variance model revealed highly significant differences in DASH scores over time (P < 0.001; η² = 0.473), treatment group (P = 0.006; η² = 0.176) and time × group interaction (P < 0.001; η² = 0.216), indicating that functional evolution differed significantly between the two groups throughout the follow-up period. This information can be found in Table 4.

Table 4 Results of the repeated-measures Analysis of Variance for the Disability of Arm, Shoulder, and Hand scale, n (%).

Results	Pre	Post	Month	3 months	Time	Group	Time × group
					F (gL); P value; η2	F (gL); P value; η2	F (gL); P value; η2
Disability of Arm, Shoulder, and Hand scale					F (2.3; 90.8) = 35.36, P < 0.001, 2 = 0.473	F (1; 39) = 8.32, P = 0.006, 2 = 0.176	F (3; 117) = 10.74, P < 0.001, 2 = 0.216
Placebo	36.2 (12.9)	24.7 (12.8)	42.9 (14.0)	24.8 (11.9)
Laser	36.6 (13.3)	27.4 (13.8)	26.6 (11.0)	22.7 (13.1)

The values are expressed as the means (SD). F, P value, and η² correspond to the repeated-measures Analysis of Variance. Greenhouse-Geisser correction was applied when need.

In the therapeutic laser treatment group, a progressive and sustained decrease in the DASH score was observed from baseline to three months of follow-up, reflecting a clinically relevant functional improvement. In contrast, DASH scores showed a transient increase at the one-month follow-up before returning toward baseline values at three months. The information can be verified in the Figure 2.

Figure 2 Evolution of mean the Disability of Arm, Shoulder, and Hand scores in the laser and placebo groups over time. The error bars represent 95% confidence intervals. DASH: Disability of Arm, Shoulder, and Hand.

Post hoc comparisons of the DASH scale with Bonferroni correction revealed statistically significant differences between groups in the DASH score at the one-month follow-up, with significantly greater values (worse function) in the placebo group than in the laser group (mean difference = 16.34; 95%CI: 11.65-21.03; P < 0.001).

Within the laser group, significant improvements in DASH scores were observed in all time comparisons relative to baseline (postintervention, 1 month, and 3 months), confirming the persistence of the therapeutic effect over time. In contrast, the placebo group showed a less consistent evolution, with significant differences only at certain time intervals. Tables 5 and 6 show the post hoc comparisons for the DASH scale scores.

Table 5 Post hoc comparisons (Bonferroni adjustment) for Disability of Arm, Shoulder, and Hand scale scores.

Time	Comparison group	Average DASH score differences	SD	P value	95%CI
Pre-intervention	Placebo-laser	-0.40	2.72	0.884	(-5.90, 5.10)
Post-intervention	Placebo-laser	-2.72	3.00	0.371	(-8.79, 3.36)
One-month	Placebo-laser	16.34	2.32	< 0.001	(11.65, 21.03)
Three-months	Placebo-laser	2.06	3.39	0.396	(-2.79, 6.90)

CI: Confidence interval; DASH: Disability of Arm, Shoulder, and Hand.

Table 6 Post hoc comparisons (Bonferroni adjustment) for Disability of Arm, Shoulder, and Hand scale scores.

	Temporal comparison	Average DASH score differences	DE	P value	95%CI
Placebo	Pre - post	11.49	2.43	< 0.001	(4.74, 18.24)
	Pre - one month	-6.76	2.94	0.161	(-14.93, 1.41)
	Pre - three months	11.41	2.12	< 0.001	(5.52, 17.30)
	Post - one month	-18.25	2.13	< 0.001	(24.16, -12.33)
	Post - three months	-0.08	1.19	1.000	(-3.39, 3.23)
	One - three months	18.17	2.43	< 0.001	(11.41, 24.93)
Laser	Pre - post	9.17	2.40	0.003	(2.51, 15.83)
	Pre - one month	9.98	2.66	0.003	(2.59, 17.38)
	Pre - three months	13.87	2.30	< 0.001	(7.47, 20.26)
	Post - one month	0.81	2.94	1.000	(-7.36, 8.98)
	Post - three months	4.69	2.24	0.257	(-1.54, 10.92)
	One - three months	3.88	2.31	0.604	(-2.53, 10.30)

CI: Confidence interval; DASH: Disability of Arm, Shoulder, and Hand.

DISCUSSION

This clinical trial evaluated the efficacy of a therapeutic laser in patients with supraspinatus tendinopathy, compared it with that of a placebo intervention, used upper limb functionality measured by the DASH scale as the primary outcome. The main findings indicate that laser treatment produced significant and sustained functional improvements over time, accompanied by reduced pain and improved physical performance of the shoulder, clearly surpassing the effects observed in the placebo group.

The magnitude of the effect observed on the DASH scale (η² = 0.176 for the group factor and η² = 0.216 for the time × group interaction) can be considered clinically relevant and comparable, or even superior, to that reported in previous studies that have evaluated conservative interventions in rotator cuff pathologies, such as isolated therapeutic exercise or conventional electrophysical modalities. Unlike previous studies that have shown inconsistent results regarding the use of therapeutic lasers, the present study benefits from a placebo-controlled design and a temporal follow-up that allows for analysis of the persistence of the effect. From a methodological point of view, the use of the DASH scale as the primary variable provides a more comprehensive view of the functional impact of treatment, overcoming the limitation of focusing exclusively on self-reported pain. This approach is consistent with current recommendations, which emphasise the need to include measures of functionality and quality of life in the evaluation of interventions for shoulder tendinopathies.

Compared with previous studies that have evaluated the use of therapeutic lasers in rotator cuff tendinopathies, the results of the present study are consistent with those described by Saunders, who reported significant improvements in pain and function after the application of low-intensity lasers in patients with supraspinatus tendinopathy in a double-blind study, but with a smaller sample size (n = 24)[25]. However, many of the previous studies have methodological limitations, such as the absence of a placebo group or the lack of longitudinal follow-up, which makes it difficult to attribute the observed changes to the specific effect of the intervention. In this context, the placebo-controlled design of the present study strengthens the internal validity of the results.

Abrisham et al[26] also investigated the effectiveness of LLLT, but in this case for subacromial syndrome. They had one group that received laser therapy in combination with therapeutic exercise and another group that performed the same exercise protocol, but in this case the laser administered was a placebo. This project once again shows that laser therapy combined with other therapies is effective, since combining its analgesic effect with the increased ROM provided by exercise allows for much more comprehensive and effective interventions. Notably, 10 sessions were carried out per week for only 2 weeks, which leads to the conclusion that there was no overtreatment that could be counterproductive for the patient. The main difference in this study is that both groups were given a progressive exercise programme and superficial cold therapy both in the clinic and at home[27]. The results of this study were very similar to those of the previous study, as the dose administered was similar and each group individually improved significantly in terms of pain, ROM and quality of life; however, the comparison between the two groups revealed that there is no significant improvement in these variables. With regard to pain, the significant reduction observed in the VAS score in the laser-treated group is consistent with the findings of previous systematic reviews suggesting an analgesic effect of therapeutic lasers on tendon pathologies. Bjordal et al[28] noted that the clinical effects of laser therapy depend largely on the dosage parameters used, which could explain the heterogeneity of results described in the literature. The results of the present study support the hypothesis that proper application of laser therapy can generate clinically relevant benefits, not only in terms of pain, but also in terms of function. Yeldan et al[29] conducted a similar trial related to subacromial syndrome, which also causes shoulder pain in many and is related to inflammation of the supraspinatus tendon, among other things. There were also two study groups, one placebo (laser) group and one intervention group (only exercises), which received sessions five days a week for three weeks. The doses administered as follows: 90 seconds at each point (from 3 to 5) with a frequency of 2000 Hz using a GaAs diode laser instrument (Roland Serie Elettronica Pagani, wavelength 904 nm, frequency range 5-7000 Hz and maximum power of 27 W, 50 W or 2764 W). Strength did not improve significantly in either group. In contrast, the parameters in our study (4.0 J/cm²) enable deeper tissue interaction and more effective photobiomodulation in the supraspinatus tendon region. These differences likely account for the divergent clinical outcomes. Perhaps there were no improvements in either study due to the few weeks of treatment compared with some of the other studies[30-34], even though this project also implemented exercises. Another hypothesis to consider is the number of points at which the treatment is applied, with 3/5 points being insufficient for structural improvement. Hernández Díaz et al[35] focused on acute shoulder pain caused by calcifications. In their study, they applied conventional treatment for this condition, i.e. iontophoresis, to one group of patients and LLLT to the other group. The final results showed that laser therapy was statistically more effective than iontophoresis in improving the ROM. In terms of pain and strength, both groups improved significantly. These patients also underwent mechanotherapy, digital ladder and ceiling pulley exercises in the rehabilitation room, as well as Codman pendulum exercises at home[32]. Eslamian et al[34] investigated the effectiveness of low-intensity lasers combined with other physiotherapy techniques such as ultrasound, transcutaneous electrical nerve stimulation and heat packs... for the treatment of rotator cuff tendinopathy. Laser treatment was applied to a maximum of 10 painful points on the shoulder over a period of 3 weeks[34]. In this case, the combined therapy was effective in reducing shoulder pain, but as in the present study, there were no significant improvements in ROM. The improvements in pain may have been the result of a combination of several physiotherapy treatments. ROM may not have increased because these therapies only have analgesic and not curative effects on the affected structure. The improvement produced by laser therapy in the case of calcifications may have been due to the large amount of time invested, as it was carried out over three years despite only 10 sessions being applied. In the other studies, as they involved only tendon inflammation, fewer weeks of treatment were needed, but a greater number of sessions were needed. This research suggests that it may be more appropriate to take longer breaks between laser sessions for the treatment to be more effective. This article was taken into account because a large proportion of the participants in the project had one or more calcifications in the supraspinatus tendon. Hernández Díaz et al[35] cite the limitations of this study, particulary the sample size, patient variability, loss to follow-up, duration of the study, and attitudes of the participants. A larger number of participants would have increased the statistical quality, thus providing more meaningful and generalizable data for the population. Each patient is unique and may react differently to treatment. These differences can be a problem when generalising the results to other individuals, so the ideal situation would be to find people with characteristics that are as similar as possible. Furthermore, as this was a several-week intervention, some participants were unable to continue due to work or personal circumstances. This reduced the sample size and, consequently, the number of results collected in the final statistics. Had it been possible to devote more weeks to the treatment, we would have been able to carry out a more specific follow-up and assessment. From a pathophysiological point of view, the observed effects could be explained by ability of the therapeutic laser to modulate inflammatory processes, improve microcirculation, and promote metabolic activity in tendon tissue, influencing both peripheral mechanisms and nervous system sensitisation[28]. These actions could explain the improvements observed in the PPT and in the active ROM of the shoulder.

Finally, as laser therapy is a treatment that requires little time and is painless, some patients are skeptical about its effectiveness, which means that psychological factors may have been included that could have affected the results of the questionnaires on both quality of life and pain perception[36,37].

Limitations

The study assessed participants over a three-month period and evaluated the results in the short and medium term, thus, a longer-term evaluation, at least six months, would have been useful to compare the results. Therefore, for future research, it would be interesting to add more patients to similar studies, with more homogeneous samples and extended over time to verify the medium- and long-term effects.

Future lines

Future research should consider, for example, a longer treatment time to obtain better results long-term follow-up, and even further investigations of the influence of psychosocial factors and the patient environment and how these factors affect pathology.

CONCLUSION

Low-intensity laser treatment is effective in reducing pain in patients with supraspinatus tendinopathy. Compared with placebo therapy, laser therapy improved quality of life, pain tolerance, greater active ROM across multiple shoulder movements and strength in patients who participated in the study. These benefits were not only immediate but persisted through three months of follow-up, contrasting with the placebo group's limited and inconsistent gains, which primarily involved transient pain relief and partial ROM improvements.