Meniscal injuries of the stifle joint are common injuries in horses that are challenging to diagnose, have limited treatment options, and have a relatively poor prognosis for return to prior performance. With continuing advancements and expansion in available imaging modalities, the most effective modality for diagnosing and monitoring stifle injuries has not been clearly established. Previous publications have compared the ability of multiple imaging modalities to detect meniscal injuries in clinical patients, but none have utilized a controlled injury model, nor have they monitored meniscal healing following a well-characterized, acute injury. Our group has completed a preliminary study establishing a model of arthroscopically induced meniscal injury resulting in consistent lameness, detectable imaging abnormalities, and gross and histologic evidence of disease. The proposed project is an in vivo randomized, controlled, blinded study utilizing our new model of acute meniscal injury. The Study Aim is to compare the accuracy of radiographs, ultrasound, magnetic resonance imagining (MRI) and computed tomography arthrography (CTA) for diagnosing meniscal pathology/ joint disease and monitoring healing over 120 days. Twelve horses (between 2-6 years old) with no history of lameness and no stifle joint pathology will have a meniscal defect created arthroscopically in one randomly chosen limb. The contralateral limb will undergo sham arthroscopy. Horses will be turned out 10 days following surgery with repeated lameness and imaging evaluations over a 120-day period following surgery. This grant application will compliment a co-study funded by the Grayson Jockey Club Research Foundation that utilizes this meniscal disease model to determine the effect of autologous bone marrow-derived mesenchymal stem cells on meniscal healing. At two weeks post-surgery, six horses will receive a single treatment with intra-articular autologous mesenchymal stem cells; the remaining six horses will receive a saline placebo. Lameness examinations will be performed every 15 days; ultrasound and radiographs every 30 days by a blinded radiologist, and a CTA (with and without intra-articular contrast) on day 30. At the time of euthanasia (day 120), limbs will undergo an additional CTA and an MRI. This study will be the first to characterize imaging findings following an acute meniscal injury over a 120-day period, with comparison of imaging results to gross and histopathological findings. The study will have an immediate impact on equine health by filling a critical gap in stifle disease diagnosis and the monitoring of meniscal healing and development of osteoarthritis.