The cruciate ligament is a band of tough fibrous tissue that attaches the femur (thigh bone) to the tibia (shin bone), preventing the tibia from shifting forward relative to the femur. It also helps to prevent the stifle (knee) joint from over-extending or rotating.
Trauma to the equivalent ligament in humans is common, and damage most frequently occurs during some form of sporting activity such as skiing or playing football. However; cruciate ligament disease is very different in dogs as rather than the ligament suddenly breaking due to excessive trauma, it usually degenerates slowly over time due to the fibres within the ligament weakening.
The exact cause of this is unknown, but genetic factors may be important with certain breeds being more susceptible - including Labradors, Rottweilers, Boxers, West Highland White Terriers and Newfoundlands. Other factors such as obesity, hormonal imbalances and certain inflammatory conditions of the joint may also play a role. It’s common for dogs that have ruptured the ligament in one leg to develop the condition in the other leg.