The purpose of this study is to determine the effect of a chondroitin sulphate conventional treatment on the degree of severity of synovitis, as measured by magnetic resonance in patients with knee OA with clinical synovitis.

Inclusion Criteria:

• Primary OA of the knee according to ACR with synovitis criteria (warmth, swelling or effusion);
• OA of radiological stages 2 and 3 according to Kellgren-Lawrence;
• Minimum joint space width ≥2 mm in the medial femorotibial compartment on standing knee X-ray;
• VAS of pain while walking ≥40 mm.

Exclusion Criteria:

• Known allergy to chondroitin sulphate;
• Progressive or serious pathologies (cancer, AIDS,...);
• Other bone and articular diseases (antecedents and/or current signs) such as chondrocalcinosis, Paget's disease, rheumatoid arthritis, aseptic osteonecrosis, gout, septic arthritis, ochronosis, acromegaly, hemochromatosis, Wilson's disease, osteochondromatosis;
• Corticosteroids (oral, injectable), indomethacin, therapeutic dose of glucosamine or chondroitin sulphate during the 12 weeks preceding inclusion;
• Hyaluronic Acid (intra-articular pathway) during the 26 weeks preceding inclusion;
• Radioactive synovectomy during the 12 weeks preceding inclusion;
• If the patient is receiving NSAID at baseline (D0), it should be maintained at a stable dosage as much as possible from D0 to D180. In the open phase of the study (D180 to D360), NSAID will also be allowed as rescue medication along with acetaminophen.