Water-filtered Infrared-A Radiation in Patients With Hand Osteoarthritis

Inclusion Criteria: * Diagnosis of finger polyarthritis (confirmed by a specialist, radiological findings available) * Local pain for more than 50% of the day in the past 3 months. * Subjective pain intensity of local pain in the last 24 hours on a VAS scale of 0 to 10 being \>4. * No planned local treatment in the next 6 months. * No planned intra-articular injections within the next 6 months. * The patient must be able to understand the study explanations and appropriately follow the instructions of the investigating physician. Exclusion Criteria: * Chronic inflammatory joint diseases. * Poor general health. * Febrile diseases. * Fibromyalgia syndrome. * Psychotic disorders. * Severe comorbidities. * Acute unclear skin diseases. * Heat urticaria. * Painful diseases/diagnoses of the shoulder and/or arm. * Lymphatic vessel diseases (lymphedema, lymphangitis). * Porphyria. * Pain-associated diseases/diagnoses of the shoulder and/or arm. * Intra-articular injections or arthroscopies within the last 3 months. * Previous or planned surgical procedure on the affected joints in the last 8 weeks or in the upcoming 6 months. * Intake or application of photosensitizing drugs or extracts (e.g., porphyrins, tetracyclines, sulfonamides, psoralenes, St. John's wort). * Systemic medication with glucocorticoids or immunosuppressants. * Pain medication with opioid analgesics. * Known pregnancy. * Concurrent participation in another clinical trial.