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High molecular weight (MW) hyaluronan (HA) preparation is considered to be more biologically active than HAs of lower MWs. However, many of the HA preparations currently used to treat osteoarthritis (OA) have lower MWs by the enhanced penetration of HA molecules into the synovial lining cells. In this study, we determined the effectiveness of sonophoresis on the delivery of high MW HA into synovial membrane using an animal model of OA. A total of 1000 kDa (HA1000) and 3000 kDa (HA3000) HA were labeled with fluorescein and injected into the knees of rabbits. Low-intensity continuous ultrasound at 1 MHz, 400 mW/cm² was applied to the knees for 10 min treatment bid. Synovial fluid analysis revealed increased absorption and fluorescence microscopy showed deeper penetration of both HA1000 and HA3000, more so with the latter. Histologic examination indicated that ultrasound treatment resulted in …