Electric zaps to the brain may soothe aching knees

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

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).