Your dog has just finished playing in the park with all her friends and comes back to you with an on again, off again hind-limb lameness. Chances are she has ruptured the cruciate ligament in her stifle.
This joint is held together by four ligaments and the joint capsule. Two ligaments pass through the joint to keep it stable during flexion and extension (forward and backwards movement), another ligament can be found down either side to prevent medial and lateral (sideways) movement. The usual cause of ligament rupture is a sudden jolt to the joint, like when the leg is caught up while jumping a fence or the sudden twisting of the stifle when spinning around. Sometimes the angle between the femur and tibia is too great, placing pressure on the ligaments and causing them to eventually tear or rupture with time. Occasionally the menisci or fibrous pads which cushion the bones in the joint may get small tears which are also painful and cause lameness.
There are many different ways to treat this problem, depending on the size of the dog, its lifestyle and what caused the injury. Occasional rest will help (especially with very small dogs or partial ruptures) but usually one of the many variations of surgery for this injury will be required. Surgery is needed to stabilise the joint and to help slow down the development of arthritis, which will eventually form in the joint as a result of the initial injury and any ongoing instability. The joint is usually x-rayed first to rule out any other possible causes of lameness such as fractures or tumours. You and your Vet will then decide which surgery is best suited for your dog, including the cost and recovery time.