Knee osteoarthritis (KOA) is a common condition that causes long-lasting knee pain and difficulty with daily activities. Many patients have pain that is stronger than expected from joint changes because the nervous system becomes more sensitive to pain. Transcranial direct current stimulation (tDCS) is a non-invasive technique that uses a small electrical current applied to the scalp to help reduce pain sensitivity.

This study will test whether active tDCS over the primary motor cortex can reduce pain and improve function in people with knee osteoarthritis. A total of 102 participants will be randomly assigned to receive either active tDCS or sham (placebo) stimulation. All participants will receive 15 sessions over three weeks. We will measure pain intensity, pain sensitivity, physical function, depression, cognition, and quality of life before the treatment, after the 3-week treatment program, and again at the 1-month follow-up.

Knee osteoarthritis (KOA) is a major cause of chronic pain, disability, and reduced quality of life. In many patients, the severity of pain exceeds structural joint damage due to peripheral and central sensitization, including impaired conditioned pain modulation (CPM). Such dysfunction in descending inhibitory pathways contributes to pain amplification and poor response to standard treatments. Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulatory technique capable of modulating cortical excitability, enhancing descending inhibition, and potentially restoring altered pain processing mechanisms.

This randomized, assessor- and participant-blinded, sham-controlled clinical trial will investigate the neuromodulatory effects of anodal primary motor cortex (M1) tDCS in patients with KOA exhibiting impaired CPM. A total of 102 participants who meet eligibility criteria will be randomly assigned (1:1) to receive either active anodal M1-tDCS (2 mA, 20 minutes per session) or sham stimulation, using identical electrode placement and brief initial stimulation to maintain blinding. Both groups will receive 15 sessions administered over three consecutive weeks.

Assessments will be conducted at baseline, immediately after the intervention, and at 1-month follow-up. Primary outcomes include changes in pain intensity measured by the Visual Analogue Scale (VAS) and Brief Pain Inventory (BPI). Secondary outcomes include peripheral and central sensitization measures-Pressure Pain Threshold (PPT), Conditioned Pain Modulation (CPM), Central Sensitization Inventory (CSI), and PainDETECT-as well as functional outcomes assessed using the WOMAC index, Timed Up and Go (TUG) test, and One-Leg Stance. Additional outcomes include depressive symptoms (Beck Depression Inventory, BDI), cognitive function (Mini-Mental State Examination, MMSE), and health-related quality of life (SF-12).

This study aims to provide a comprehensive evaluation of the analgesic and neuromodulatory effects of tDCS in KOA, and to clarify its impact on pain modulation, pain sensitization, physical function, depression, cognition, and overall quality of life.

Inclusion Criteria:

• Diagnosis of knee osteoarthritis (KOA) according to the American College of Rheumatology (ACR) clinical criteria.
• Adults of both sexes aged 35 years or older.
• Clinical knee pain persisting for at least 3 months.
• Average knee pain intensity ≥ 4/10 on the Numerical Rating Scale (NRS; 0-10) during the previous 24 hours, with one dominant affected knee.
• Central sensitization phenotype: impaired conditioned pain modulation (CPM), defined as no change or a reduction in pressure pain threshold (PPT) after the conditioning stimulus, corresponding to a PPT ratio ≥ 1 (pre-to-post stimulus).
• Prior pharmacological pain management with NSAIDs and/or SNRIs discontinued due to adverse effects, intolerance, or contraindications, and able to complete the required washout period (2 weeks for NSAIDs and 4 weeks for SNRIs) before baseline assessment.
• Able and willing to provide written informed consent and comply with study procedures.

Exclusion Criteria:

Participants will be excluded if they have any condition that could affect cortical excitability, confound outcome measures, or interfere with tDCS safety, including:

• Other chronic pain conditions associated with central sensitization (e.g., fibromyalgia).
• Inflammatory arthropathies (e.g., rheumatoid arthritis, psoriatic arthritis, systemic lupus erythematosus).
• Neuropathic pain syndromes (e.g., lumbar or cervical radiculopathy).
• Any clinically significant or unstable systemic disease (cardiovascular, hepatic, renal, or metabolic disorders).
• Pregnancy or active malignancy.
• Neurological or psychiatric disorders including epilepsy, history of syncope, traumatic brain injury with residual deficit, or major depressive disorder.
• Current participation in physiotherapy, electrotherapy, or exercise rehabilitation programs.
• Metallic implants or implanted electrical devices (e.g., pacemakers, cochlear implants, deep brain stimulators).
• Cognitive impairment interfering with understanding instructions or providing informed consent.
• Dermatological contraindications at stimulation or testing sites (e.g., open wounds, infection, irritation).
• History of alcohol or substance abuse, or current use of centrally acting medications that alter cortical excitability (e.g., benzodiazepines).

Individual participant data (IPD) will not be shared because this is a single-center academic doctoral research project, and the ethical approval obtained does not permit public sharing of patient-level data. All collected data are confidential and will be used solely for the purposes of this study in accordance with institutional and national data protection regulations.

This is a participant- and assessor-blinded, sham-controlled clinical trial. All tDCS sessions will be delivered by a trained physiotherapist who is aware of group allocation, while both participants and the outcome assessor remain blinded. To maintain allocation concealment, the physiotherapist recorded each participant's assignment using coded geometric symbols, the meaning of which remained confidential until completion of data collection.

Active and sham tDCS will be applied using identical electrode placements and device settings. In the sham condition, the current will be delivered for approximately 30 seconds and then gradually ramped down to zero to reproduce the initial tingling sensation of active stimulation, ensuring effective participant blinding. After all data are collected, group identities will be unmasked for statistical analysis.