Torn cruciate ligaments are one of the most common causes of rear leg lameness in dogs. Dogs and people have 2 cruciate ligaments in their knee. Cruciate means "cross" in latin, and these cruciate ligaments cross the knee stabilizing the knee joint. Bigger dogs are more prone to tearing their cruciate ligaments, but we can also see it happending in small dogs as well. The injury usually occurs when a dog has their foot planted, and then rotates around the foot. The dog often goes from running normally to holding up one of the rear legs and using just 3 legs. Cruciate ligament injury in dogs is diagnosed by feeling instability in the knee, and by taking radiographs (x-rays) of the knee joint. Often, dogs will need surgery if their cruciate ligament is completely torn and the knee joint is no longer stable.