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(1) To report the clinical and synovial effects of a platelet-rich product (PRPr) in normal equine joints, (2) to assess the persistence of platelets within synovial fluid after intra-articular injection, (3) to compare responses to different preparations of that product, and (4) to evaluate a gravity filtration system for PRPr preparation in horses.Experimental.A platelet-rich saline product (PRPr) was prepared from 7 normal horses using a proprietary preparation device and was divided into 3 treatments: resting, CaCl2-activated (23 mM, final), and bovine thrombin-activated (10 U/mL, final). Each horse had 3 concurrent randomly assigned intra-articular PRPr treatments administered in their metacarpophalangeal/metatarsophalangeal joints; the fourth limb was injected with saline (0.9% NaCl) solution as a control. Clinical assessments, cytologic analysis of synovial fluid and hemograms were performed at 6, 24, 48, and 96 hours after injection. PRPr composition and growth factor content were analyzed.The gravity filtration system produced a moderately concentrated PRPr. At 6 and 24 hours, when compared to control values, all PRPr treatments caused a significant increase in synovial WBC concentration (P < .0059) and neutrophil percentage (P < .0005). Bovine thrombin-activated PRPr injection consistently caused increased effusion scores and periarticular signs. At all time points, the synovial WBC concentration after thrombin-activated PRPr was significantly greater (P < .001) than for the control, CaCl2-activated or resting PRPr. Intact platelets could be observed in synovial fluid for up to 5 days after intra-articular PRPr injection.Resting and CaCl2-activated PRPr may be safely used to treat equine joints, but bovine thrombin activation is not recommended at 10 U/mL. A PRPr can be prepared using a gravity filtration system, eliminating the need for centrifugation.