fa آسیب شناسی ورزشی و حرکات اصلاحی Sport injury and corrective exercises پژوهشي Research <p style="text-align: justify;"><strong>Purpose:</strong> Osteoarthritis is one the most common chronic diseases of the joints that mostly affects knee joint. Low power laser is one the minimally invasive treatments of osteoarthritis. However, the efficacy of this therapy is still controversial. This study aimed to evaluate the efficacy of low power laser therapy in reducing pain and increasing knee range of motion compared to placebo laser.<br> <strong>Methods:</strong> This study was a double-blind clinical trial performed at a day clinic in Tehran, Iran, from March 2016 to February 2017. Treatment for both groups comprised 10 sessions, 5 times a week. In this study, low power laser with a wavelength of 810 nm and a power of 100 mW at 8 points was applied around the knee during two weeks of treatment. At the same time, laser was applied in the placebo group, but the power output was zero. Data analyses were performed by repeated measures ANOVA test using SPSS version 20.<br> <strong>Results:</strong> The mean pain score at rest (P=0.691), as well as at activity (P=0.751) were not significantly different between the two groups. The range of motion of knee flexion (P=0.435), and the range of motion of knee extension (P=0.885) showed no significant difference between two groups. Both interventions over time lead to a significant change in pain at rest and at activity and also increase in the range of motion in knee flexion and extension (P<0.05).<br> <strong>Conclusion:</strong> Both placebo treatment with routine physiotherapy and active laser therapy with routine physiotherapy decreased knee pain and increased knee range of motion in patients affected with osteoarthritis and there was no significant difference between two therapies.</p> <p></p> Osteoarthritis, Knee, Low power laser, Physical therapy 13 18 http://ptj.uswr.ac.ir/browse.php?a_code=A-10-42-14&slc_lang=fa&sid=1 Forouzan Rastgar Koutenaei فروزان رستگار کوتنائی No Department of Physiotherapy, University of Social Welfare and Rehabilitation Sciences, Tehran, Iran. گروه فیزیوتراپی دانشگاه علوم بهزیستی و توانبخشی Zahra Mosallanezhad زهرا مصلی نژاد zmosallanezhad@yahoo.com Yes Department of Physiotherapy, University of Social Welfare and Rehabilitation Sciences, Tehran, Iran. گروه فیزیوتراپی دانشگاه علوم بهزیستی و توانبخشی Mehrdad Naghikhani مهرداد نقی خانی No Department of Physiotherapy, University of Social Welfare and Rehabilitation Sciences, Tehran, Iran. دانشگاه علوم بهزیستی و توانبخشی kamran Ezati کامران عزتی No Department of Physical Therapy, School of Medicine, Guilan University of Medical Sciences, Guilan, Iran. استادیارگروه فیزیوتراپی دانشگاه علوم پزشکی گیلان، گیلان، ایران Akbar Biglarian اکبر بیگلریان No Department of Biostatistics,University of Social Welfare and Rehabilitation Sciences, Tehran, Iran. دانشگاه علوم بهزیستی و توانبخشی Mahdi Nouroozi مهدی نوروزی No Social Determinants of Health Research Center, University of Social Welfare and Rehabilitation Sciences, Tehran, Iran. دانشگاه علوم بهزیستی و توانبخشی Maryam Ghodrati مریم قدرتی No Department of Physiotherapy, University of Social Welfare and Rehabilitation Sciences, Tehran, Iran. دانشگاه علوم بهزیستی و توانبخشی