Clinical and Histologic Evaluation of Polyacrylamide Gel in Normal Equine Metacarpal/Metatarsal-Phalangeal Joints

Polyacrylamide hydrogel (PAHG) is being developed to treat osteoarthritis in horses. Prior to application in osteoarthritic joints, evaluation of PAHG in normal joints is indicated. The objectives of this study are to evaluate the clinical, cytologic, histologic, and metabolic effects of PAHG in normal fetlock joints. This study is an in vivo controlled study utilizing six horses that had each of the four fetlocks assigned to either a 7, 28, 56 days or control group subjected to arthrocentesis only. Synovial fluid was collected prior to administration of 2.5 mL of 4% PAHG and again at the completion of the study for macroscopic, cytological, and cartilage metabolism evaluation. The completion of the study included gross and histologic examination of the cartilage and synovial membrane. There was a small but significant ( P = .0242) increase in cell count in the synovial fluid at 7 days. There were significant changes in the synovial membrane histology score ( P = .0277) as a result of hypertrophic synoviocytes. Biomarkers indicated a small increase in cartilage turnover 7 days after PAHG administration. The PAHG was visible on the surface of the synovium at 7 days, and PAHG appeared in the interstitial spaces of the synovium and intracellular at days 28 and 56. Data from this study provide information as to the tolerance and disposition on an intraarticular injection of PAHG in a normal fetlock joint during a 56-day study. There were no major or permanent detrimental effects seen with the administration of PAHG in normal joints.