Stifle injuries are common career ending injuries for equine athletes. In particular, meniscal injuries even when surgically managed have a relatively poor prognosis for return to prior level of performance. A retrospective study by Ferris et al. concluded that compared to historical controls, horses treated with mesenchymal stem cells (MSCs) showed an improvement in the ability to return to work compared to surgery alone. In a recent study of equine specialists, MSCs were reported to be the most commonly used biologic following surgery for meniscal injury. However, recent reports are controversial with a new retrospective study of meniscal injuries showing no effect of biologic therapies (including MSCs) on outcome. There is a clear gap in knowledge as to whether intra-articular stem cell administration improves outcomes for meniscal injury. Part of the challenge of answering this question has been the lack of a consistent, experimental model of meniscal disease in the horse. Therefore, we will conduct a randomized, controlled, blinded prospective study evaluating the use of intra-articular mesenchymal stem cells for treatment of meniscal injury using a novel, experimental model of meniscal disease. This study will achieve two important objectives. Firstly, it will establish the novel experimental model of meniscal disease which can be used now and in the future for evaluating the effectiveness of intra-articular treatments for meniscal disease. Secondly, the study will evaluate the effectiveness of intra-articular MSCs for meniscal disease using multiple outcome measures including clinical parameters, imaging, synovial fluid characteristics, gross anatomical evaluation and histological outcomes. The study will have an immediate impact on the treatment of meniscal disease in the horse by determining the efficacy of intra-articular MSC use, and a longer-lasting impact by validating a meniscal model for future investigation of therapeutic interventions.