Efficacy Study of Intra-articular Hyaluronic Acid in the Knee Osteoarthritis

Inclusion Criteria: * Age between 40 and 80 years * Primary knee OA of the medial or lateral femoro-tibial compartment * Symptoms for at least 3 months * Diagnosis according to ACR criteria * Kellgren \& Lawrence radiological grade 2-3 * Patients who failed to respond sufficiently (not responders) to analgesics and/or NSAIDs taken regularly or responders intolerant to the regular use of analgesics and/or NSAIDs. * Mean WOMAC pain subscore at the target knee \>= 40 mm and \< 80 mm on a VAS at baseline after wash-out from analgesics/NSAIDs· If bilateral knee OA: definition of the most symptomatic joint at screening based on patient's evaluation and Investigator's clinical judgment and mean WOMAC pain subscore at baseline in the contra-lateral knee \< 30 mm * Written informed consent * Subject able to understand, co-operative and reliable Exclusion Criteria: * BMI \>= 32 kg/m2 * Secondary (post-traumatic) knee OA * Predominantly patello-femoral pain/syndrome * No remaining joint space width * Symptomatic hip OA or other interfering health condition * Severe varus/valgus deformity (\>15°) * History/present evidence of (target knee): inflammatory, infective or metabolic joint diseases; recurrent clinical chondrocalcinosis; crystal arthropaties; osteo-articular pathologies differing from arthrosis; articular fracture; ochronosis; acromegaly; haemochromatosis; Wilson's disease; primary osteochondromatosis; heritable disorders; collagen gene mutations. * Concomitant rheumatic disease * Significant injury to the target knee in the last 6 months * Previous joint replacement/arthroplasty (target knee) * Arthroscopy/osteotomy/surgery in the past 1 year (target knee) * Any surgery scheduled in the next 6 months * Venous or lymphatic stasis in the relevant limb * Skin infection/disease/trauma at the injection site * Systemic or i.a. (target knee) corticosteroids in the past 3 months * I.a. corticosteroids (non-target joint) in the past 4 weeks * Viscosupplementation (target knee) in the past year * Recently started (in the last 3 months) physical therapy (target knee) * Recently (in the last 3 months) started therapy or change in dosage of SYSADOAs * Ongoing anticoagulant therapy * Chronic or recurrent use of NSAIDs/analgesics/narcotics because of diseases different from OA of the target knee * History of allergy or hypersensitivity to hyaluronic acid or paracetamol or avian proteins * Participation in a clinical study in the last 3 months * Pregnant or lactating women, and women of childbearing potential not willing to use adequate contraception * Patients unable to stay in the study for 6 months, non-cooperating, not able to understand