The hock joint of a dog is analogous to the ankle joint of humans. The shin bone, called the tibia, is connected to the hock joint which then joins the talus bone (one of the bones of the paw). The fibula (splint bone) runs along the full length of the tibia. Ligaments on the inner and outer part of the hock joint hold the bones together, and side of the hock joint has two important ligaments, making a total of four main ligaments that hold the hock together. Instability can be due to tearing of ligaments that hold the bones of the hock in place, a fracture of the fibula, or a fracture of the bottom of the tibia bone.
Any dog or cat can suffer from hock instability, as it is usually due to trauma.
Hock instability results in a sudden onset of lameness. There may be pain, swelling, and heat associated with the affected joint as well.
Hock instability can be diagnosed relatively easily on physical exam. The veterinarian must test the hock by trying to open the joint from the outside and inside (medial and lateral) sides of the joint with the hock in both extension and flexion. In addition, the hock must be stressed by twisting it from side to side (internal and external torsion) to see if the ligaments are stable. It is important to determine what structures have been compromised in the injury leading to instability of the joint. This can be done by taking stress radiographs, in which forces are applied to the hock in different direction and radiographs are taken. Different radiographical positions and views are needed to identify if and where there is a bone fracture.
Hock instability is created when ligaments that hold the bones of the hock in place are torn, when the fibula is fractured, or when the bottom of the tibia is fractured. All of these occur due to trauma, which can include a fall, accident, or similar event.
Treatment requires surgery, but the surgery technique employed depends on the type of injury that is present, ligament damage versus a fracture. If a facture is present surgery using pins, wires, and screws to repair the fracture is performed. In order to minimize the development of arthritis in the joint, the fracture must be perfectly aligned and the pin must not penetrate the joint. If a ligament is torn, screws are placed in the bones above and below the joint, and heavy permanent suture is tied around screws to simulate the ligament. With time, scar tissue develops to add support to the surgically repaired joint.
The best outcome for injuries that cause instability of the hock is with surgery. If instability is due to a broken bone, the damage is more easily and successfully repaired than if caused by torn ligaments.