Patients will have been asked to answer symptom questionnaire (WOMAC) before the spa therapy (Day 0), at the end of the spa therapy (Week 3) and 6 months later (M6). It is a self-administered questionnaire consisting of 24 items divided into 3 subscales: Pain (5 items), Stiffness (2 items) and Physical Function (17 items). The test questions are scored on a scale of 0-4, which correspond to: None (0), Mild (1), Moderate (2), Severe (3), and Extreme (4). The scores for each subscale are summed up, with a possible score range of 0-20 for Pain, 0-8 for Stiffness, and 0-68 for Physical Function.

Patients will have been asked to answer pain questionnaire (EVA) before the spa therapy (Day 0), at the end of the spa therapy (Week 3) and 6 months later (M6). The EVA is in the form of a 10 cm plastic strip graduated in mm. On the face presented to the patient, there is a cursor which he mobilizes along a straight line, one end of which corresponds to "No pain", and the other to "Maximum pain imaginable". The patient must, along this line, position the cursor at the place that best locates his pain. On the other side, there are millimeter graduations seen only by the caregiver. The position of the cursor mobilized by the patient makes it possible to read the intensity of the pain, which is measured in mm.

Patients will have been asked to answer and quality of life questionnaire (SF-36) before the spa therapy (Day 0), at the end of the spa therapy (Week 3) and 6 months later (M6). There are 11 questions in SF-36, with 36 items in total.

The SF-36 items are divided into 8 different areas: Physical component (4 areas: Physical activity, Limitations due to physical condition, Physical pain, Perceived health) and Mental component (4 areas: Life and relationships with others , Psychic health, Limitations due to mental state, Vitality).

It is requested to respond according to the last 4 weeks. The rating system recommended for the SF-36 is a weighted Likert system for each item. The items in the subscales are totaled in order to obtain a summary score for each subscale or dimension. Each of the 8 summary scores is linearly transformed on a scale of 0 (negative for health) to 100 (favorable for health) to obtain a score for each subscale.

A global clinical evaluation done with evaluation of different exams: Knee flexion (in degrees), joint swelling (measuring the perimeter of the joint (in cm)) before the spa therapy (Day 0), at the end of the spa therapy (Week 3) and 6 months later (M6).