Although few studies have showed beneficial effect of platelet rich plasma (PRP) for osteoarthritis of knee (OA knee), only one study applied randomized, control method and none of them has the objective measurements such as balance test or muscle strength which were reported as decrease in patients with OA knees. We assess the analgesic effect and the extent of improved muscle strength and balance after PRP in patients with OA knee.

Patients with bilateral OA knee will be enrolled and randomized into intervention and control knee. One dose of PRP is applied in the intervention knee and normal saline in control side. Outcome measurements included visual analog scale (VAS)、Lequesne's severity index、Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)、balance test (Biodex) and isokinetic strengthening test at different follow-up frame (2nd week, 1st month, 3rd month and 6th month after treatment).

Inclusion Criteria:

1. Age between 35 to 75 y/o.
2. Alert consciousness
3. Symptom of knee osteoarthritis persist at least 6 months and stage I or II scored by Ahlbäck grading system
4. The pain score measured by VAS at least 4 points

Exclusion Criteria:

1. Has received hyaluronic acid or steroid injection within 6 monts
2. Has received NSAIDs or steroid within one week
3. Tumor or metastasis surrounding the knee joint
4. Has received total knee replacement, major surgery in knee, rheumatoid arthritis
5. Patient who cant tolerance the balance or muscle strength test.