In humans and dogs the lower leg or tibia is pulled forward at the stifle or knee joint. If movement occurs then a diagnosis of ACL tear is made. If there is very minimal movement a partial tear is diagnosed. If more than a minimal amount of movement occurs than a full tear is diagnosed. X-rays are important only to rule out other conditions such as fracture and or cancer. Soft tissue structures such as the Anterior Cruciate ligament are not visible on x-ray. Clicking when a dog walks can be a sign of a torn meniscus. In humans however we have the benefit of an MRI to follow up on the initial exam. This wonderful diagnostic device is not generally available to pet owners. If it were the cost would be very high.