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OBJECTIVE Polyacrylamide hydrogel (4% PAHG) is an inert viscoelastic supplement used to manage osteoarthritis in horses. Even with a prolonged clinical effect, horses may be administered multiple doses during their performance career. The effect of the serial 4% PAHG treatments is not known. The objectives of this study were to evaluate the clinical, histologic, and synovial fluid biomarker effects following serial administration of 4% PAHG in normal equine fetlock joints. ANIMALS 8 healthy horses. METHODS In a blinded, controlled in vivo study, horses received serial intra-articular injections of 4% PAHG (Noltrex Vet; Nucleus ProVets LLC) and contralateral 0.9% saline control on days 0, 45, 90, and 135. Treatment and control joints were randomly assigned. Synovial fluid was collected before administration of 4% PAHG or 0.9% saline on day 0 and at study completion for cellular and biomarker evaluation. Serial physical and lameness examinations were performed throughout the study. On day 240, gross examination and harvest of cartilage and synovial membrane for histology were completed. RESULTS There were no histologic changes in articular cartilage or synovial fluid biomarkers. The 4% PAHG was seen on the surface of the synovium in 5 of 8 treated joints 105 days after the last treatment. There are minimal effects following serial injections of 4% PAHG on normal joints in horses following administration at 0, 45, 90, and 135 days, with final evaluation on day 240. CLINICAL RELEVANCE Serial administration of intra-articular 4% PAHG in horses may provide long-term joint lubrication with no detrimental effects. [ABSTRACT FROM AUTHOR]