Synovitis occurs in osteoarthritis, it is termed as inflammation of the synovial membrane. It has been studied that administration of chondroitin sulfate (CS) on synovitis significantly improves patients outcomes in patients with knee osteoarthritis (KOA).

To evaluate by ultrasonography the effect of chondroitin sulfate (CS) on synovitis in patients with knee osteoarthritis (KOA). To collaborate in the understanding of the biochemical mechanisms involved in the synovial inflammation process.

Randomized, single-blind, controlled trial involving 70 patients with primary KOA treated for 6 months with CS or acetaminophen (ACT). Evaluation of KOA status at baseline, 6 weeks, 3 and 6 months included: ultrasonography to assess synovitis (following the OMERACT expertise group definition), visual analogue scale and Lequesne index to measure pain and function, and ELISA to quantify inflammatory mediators in serum and synovial fluid.

Synovitis presence was reduced by 50% in the CS group while a 123% increase was observed in ACT group. Conversely, patients without initial synovitis and treated with ACT reached 85.71% synovitis onset, but only 25% in CS group. Both therapies improved articular function, but only CS resulted in significant pain improvement at the end of the treatment. Changes in RANTES and UCN synovial fluid concentration were associated with CS treatment.

Treatment with CS had a sustained beneficial effect, preventing synovitis onset or reducing its presence as well as reducing KOA symptoms. ACT ameliorated clinical symptoms but had no effect on inflammation. The CS anti-inflammatory effect could be related to the observed changes in RANTES and UCN concentration.