ACL tears occur most commonly in young and middle-aged large breed dogs. The most commonly affected breeds are Labrador retrievers, golden retrievers, Rottweilers, pit bulls, and mastiffs.
The knee is a complex joint that is held together by a series of strong ligaments. The cranial cruciate ligament, also known as anterior cruciate ligament or ACL, is the strongest and most important of these ligaments. Approximately 90% of the knee problems that occur in dogs are due to damage to the ACL. In people, it is usually an acute athletic injury that causes this ligament to rupture. In dogs, however, it is a cumulative stress problem due to the unique configuration of the dog's knee joint. The ACL provides the most stability and is under constant stress during weight bearing. In susceptible dogs, this constant stress eventually leads to complete or partial tearing of the ligament.
Symptoms of ACL tears may include sudden lameness when your dog completely holds his leg off the ground, intermittent limping, or very mild limping or stiffness that progressively becomes worse.