The main objective is the comparison of three different dosage regimens of hyaluronic acid in the treatment of osteoarthritis of the knee.

Inclusion Criteria:

• Outpatient>45 years of age diagnosed with primary osteoarthritis according to ACR and Kellgren criteria
• Knee pain ≥ 55 mm and ≤90 mm measured by Visual Analogue Scale (VAS) 100 mm
• Ability to understand and follow study procedures
• Written informed consent

Exclusion Criteria:

• Patients with secondary osteoarthritis of the knee according to ACR criteria.
• Severe inflammation of the knee diagnosed by physical examination or a sedimentation speed rate < 40 mm/h and serum rheumatoid factor titre < 1:40
• Patients having previously received surgery, including arthroscopy
• Patients with joint inflammatory disease (spondilitis, systemic rheumatoid arthritis), microcrystalline arthropathies (chondrocalcinosis, gout, fibromyalgia) and significant osteoarthritis symptoms in other joints apart from the knee, and which require pharmacological treatment
• Previous administration of any of the following treatments: hyaluronic acid ia (last year); steroids i.a. or joint lavage (last 3 months), glucosamine sulfate, chondroitin sulfate or diacerein (last 3 months), NSAIDs (last 14 days), any investigational drug (last month) or its administration during this study