Luna, a 9-year-old Thoroughbred mare, was presented with a gradually worsening lameness issue. Luna displayed lameness, which was initially mild and subtle but had progressed to become pronounced, even at a walk. Luna exhibited increased lameness and discomfort when her affected joint, in this case, her hock, was flexed. There was noticeable swelling and heat in the hock joint, along with a mild effusion. what is the likely diagnosis?