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No studies comparing the effects of platelet-rich plasma (PRP) injection and placebo injection in bilateral knee osteoarthritis in the same patient, or discussing muscle strength after PRP injection, have been published.Twenty patients with bilateral knee osteoarthritis were eligible, and 40 knees were randomized into two groups: PRP (knees [right or left by a coin toss] receiving a single intra-articular PRP injection) and saline group (the contralateral knee of the same patient, into which single 4-mL intra-articular injection of normal saline was administered). The primary outcome measure was Western Ontario and McMaster's Universities Osteoarthritis Index and the secondary included isokinetic test results. The evaluation was at baseline and at 2 wks, 1, 3, and 6 mos after injection.The PRP group showed a significant reduction in the Western Ontario and McMaster's Universities Osteoarthritis Index pain and total scores compared with normal saline group (P < 0.05). Although a significantly greater percentage of knee strength (extensor > flexor) was found in the PRP group during a longer follow-up period, PRP treatment resulted in insignificant differences in muscle strength compared with normal saline.Platelet-rich plasma treatment significantly improves pain, stiffness, and disability in patients with knee osteoarthritis compared with normal saline treatment. Additional strength training is recommended to enhance muscle strength recovery.