BACKGROUND: Interference is an important feature of the waves. When two or more in phase light waves meet, a new and reinforced wave is generated. Shoulder pain is a common clinical problem and laser is one of the treatments frequently used to relieve it. OBJECTIVE: To test the safety of interferential laser therapy generated by two independent low level lasers and compare its effectiveness with conventional single laser therapy in the reduction of shoulder musculoskeletal pain and associated disability. DESIGN: Randomised and single-blind controlled clinical trial. SETTING: Physiotherapy Unit and Rehabilitation Department of Ramon y Cajal University Hospital (Madrid). PARTICIPANTS: 200 patients with shoulder musculoskeletal pain were randomly assigned in two groups, 100 people each. INTERVENTIONS: Group I, experimental (n = 100) received interferential laser, placing two probes opposite each other over the shoulder joint. Group II, control (n = 100) received conventional laser therapy, using a single probe along with a second inactive dummy probe. Lasers used were GaAlAs diode (810 nm, 100 mW), in continuous emission. Laser was applied in contact mode through ten sessions, on 5 shoulder points (7 Joules/point) per session. MAIN OUTCOME MEASURES: Visual analogue scale (VAS) score and Shoulder Pain Disability index (SPADI), recorded before and after laser treatment. RESULTS: There were no differences between both groups in the reduction of pain, either assessed by VAS scale (median difference 0, 95% CI of the difference -0.6 to 0.5, p = 0.81) or SPADI index (median difference 0.4, 95% CI of the difference -2.9 to 3.8, p = 0.80), using the Mann-Whitney U-test. Comparison between the scores recorded before and after the treatment, within each group, showed significant differences for VAS during movement (median difference 3, 95% CI of the difference 2.07 to 4, p < 0.001) and SPADI index (median difference 3.5, 95% CI of the difference 2.67 to 3.85, Wilcoxon test, p < 0.001), for both groups. CONCLUSIONS: In this study, the application of two low level lasers in order to generate interference inside the irradiated tissue showed to be a safe therapy. Both interferential and conventional laser therapy reduced shoulder pain and disability. Nevertheless, differences between them were not detected. Future research in this field could include applying this technique with other laser parameters or application forms.

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