The cranial cruciate ligament stabilizes the knee by attaching the femur and tibia. If the ligament ruptures, the shin “gives out” in a forward direction and inwardly rotates. The ligament can rupture all of a sudden or gradually depending on the use of the leg.
Large breed dogs such as Rottweilers and Labrador Retrievers rupture acutely at <3 years of age, but the average for all dogs is 5 years and only 20% are acute. The dog will limp all of a sudden after an activity such as catching a frisbee and not bear weight on the affected leg. A gradual rupture may appear during a routine activity such as stepping off a curb. A gradual rupture occurs due to aging (>5 years) or poor conformation such as straight knees or bow legs. Diseases of the immune system and infection also weaken the ligament. A partial tear may cause limping over a long period of time. The muscles of the hind leg shrink when dogs use the legless. When the ligament ruptures, you can feel the laxity in the knee called “Cranial Drawer”. Sometimes a click can be heard because of a torn cartilage shock absorber called the meniscus. The meniscus is torn >50% of the time when the ligament is no longer stabilizing the knee.
