Knee Osteoarthritis and Thermal Spa Treatment

The goal of this interventional study is to evaluate the effectiveness at 6 months of the Enghien-les-Bains thermal spa treatment on functional disability and pain, assessed by the Minimal Clinically Important Improvement (MCII), in patients with knee osteoarthritis. The study is conducted in patients aged 50 to 80 years with knee osteoarthritis (Kellgren-Lawrence grade 2 to 4) and a baseline functional impairment defined by a normalized WOMAC function score ≥ 30. The main questions it aims to answer are: * Does the Enghien-les-Bains thermal spa treatment result in a clinically meaningful improvement in pain and functional disability at 6 months in patients with knee osteoarthritis? * Is the effectiveness of the Enghien-les-Bains thermal spa treatment superior to standard care, as assessed through an indirect comparison with an external control group from the ANGELLO randomized controlled trial (NCT0581943)? Patients receiving the Enghien-les-Bains thermal spa treatment are compared to the external control group of the ANGELLO randomized controlled trial (Saint-Jean-d\'Angély), composed of non-spa-treated patients receiving standard care, to assess differences in pain and functional disability outcomes at 6 months. Patients of the study: * Undergo a standardized thermal spa treatment at Enghien-les-Bains. * Continue usual medical care for knee osteoarthritis. * Complete questionnaires and clinical assessments at baseline and at 6 months.