Objective: This study is designed to evaluate the short-term efficacy of photobiomodulation for improving pain and function in patients that will undergo total knee replacement and decrease in morphine consumption and adverse effects of opioids. Methods: thirty four patients of both genders that will undergo total knee replacement will participate in this randomized controlled double-blind clinical trial. They will be randomly allocated into two groups: Photobiomodulation group 1 with 18 patients and placebo group with 18 patients and group 3 control without device . Photobiomodulation will be performed twice during 48h, using adivice Light-Aid da BrightPhotomedicine- Brasil, com LED de Arsenieto, Gálio and Alumínio (AsGaAl) of 850 nm device 3B class, measured and calibrated previously .Five points were irradiated on the knee. The placebo group will be treated with the same laser device, but will not be turned on. All patients will receive treatment sessions of photobiomodulation after 24h and 48 h after peripheral nerve blockade (femoral nerve block and nerve supply), applied in 5 points for 5 minutes (300 seconds) close to the incision (that will be protected by transparent film) and repeated every 24h after totaling 2 applications. Patients will be assessed for duration of surgery and anesthesia, assessment of the pain, time to first rescue analgesia request, physical examination, dosimetry of photobiomodulation and amount of morphine consumption using PCA 24h and 48 hours after nerve blockade after surgery. In addition, adverse effects , sedation, paresthesias, motor blockade, nausea and vomiting will be assessed ever 6h during 48h.

thirty-four patients treated at the Institute of Orthopedics and Traumatology, University of São Paulo School of Medicine Hospital das Clínicas (IOT-HC-FMUSP), already identified with knee OA awaiting TKA; the thirty-four selected patients will be divided into two groups with 17 patients in each group:

• Photobiomodulation + PCA Group (G1): patients in the PO of knee arthroplasty surgery treated with the photobiomodulation device turned on, 24h and 48h after peripheral nerve block (femoral nerve and obturator nerve) .With conventional analgesia and with the device of PCA.
• Placebo + PCA group (G2): patients in the postoperative period of knee arthroplasty surgery treated with the photobiomodulation device switched off at 24h and 48h after peripheral nerve block (femoral nerve and obturator nerve). With conventional analgesia and with the PCA device.

All patients will undergo photobiomodulation treatment sessions after 24h and 48h of peripheral nerve block (femoral and adductor nerve block), applied in two regions: in the femoral nerve region for 4 minutes (240 seconds) and near the surgical incision (protected with clear film) for 2 minutes (120 seconds) and repeated every 24 hours after, totaling 2 applications in 2 regions (see table 1). Patients will be systematically evaluated with identification, duration of surgery and duration of anesthesia, pain assessment, time of first analgesic rescue request, physical examination, photobiomodulation dosimetry and amount of PCA bolus requested every 6h within within 48 hours after nerve block after knee arthroplasty surgery. In addition to assessing and quantifying adverse effects (motor block, sedation, nausea and vomiting). All pain assessments (VAS) and rescue number requests will be performed by "blinded examiners", ie professionals who will not be aware of the group to which the patient will belong.

Inclusion Criteria:

• Men and women diagnosed with OAJ with comorbidities (metabolic syndrome, ie, OAJ + at least two overweight / central obesity, diabetes, dyslipidemia, high blood pressure)
• Age between 60 and 75 years awaiting primary TKA at IOT-HC-FMUSP
• Patients not submitted to previous lower limb arthroplasty.
• Patients not submitted to knee infiltration up to 6 months before study.
• Patients without personal history of cognitive, psychiatric and / or neurological disorders, whose symptoms presented at the moment of the evaluation are related or significantly interfere with the functions of attention, memory, logical reasoning, understanding, in order to impair the assimilation of the given guidelines.
• Patients with no personal history of inflammatory arthritis (rheumatoid arthritis, for example)

Exclusion Criteria:

• Contraindication for spinal anesthesia and / or peripheral nerve block
• Not having consent signed
• Prior use of strong opioids
• Unable to use PCA method
• Patients undergoing knee infiltration during the study.
• Patients diagnosed with chronic inflammatory arthritis (rheumatoid arthritis, for example) during the study.