The purpose of this study is to assess the efficacy and safety of Hyalgan after proximal tibial osteotomy in treatment of knee osteoarthritis patients. Normally, OA patients who were treated with osteotomy will not be treated with any SYSADOA after operation, even though their cartilage's not completely loss, so osteotomy plus HA injection should provide more benefit to patients than osteotomy alone.

Inclusion Criteria:

1. Patients between 35 and 65 years with primary knee OA with malalignment
2. Mild to moderate OA of grade II or III severity on the Kellgren-Lawrence scale with require treatment by osteotomy
3. Malalignment is not exceed 15 degree (+,-)
4. Pain on walking (15 m) ≥ 40 mm.
5. Range of motion > 90 degree
6. Evidence of adequate contraceptive methods in women of childbearing age

Exclusion Criteria:

1. Previous surgery on affected knee
2. Previous intraarticular intervention within the last 3 months (eg. Steroid, anaesthetic, Sodium hyaluronate)
3. Oral SYSADOA treatment (chondroitin, glucosamine, diacerein) within 2 months prior to study start
4. Known or suspected infection of the affected joint
5. Painful knee condition due to another cause than chondral lesions, such as Sudeck's atrophy, intraarticular neoplasm, pigmented villonodular synovitis, chondromatosis
6. Poor general health or other conditions which would make regular hospital attendance difficult
7. Ascertained hypersensitivity to any component used in the study (eg. Hyaluronic acid, diclofenac, morphine, omeprazole and paracetamol)
8. Hypersensitivity to avian protein
9. Ongoing or previous participation in a clinical study within the last 3 months