The purpose of this study is to prove the non-inferiority of the intra-articular injection of hyaluronic acid (Sinovial®) in the symptomatic treatment of knee OA in comparison to Synvisc®.

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