The carpus of the dog can be equated with the wrist in man. It is made of many small bones that form three major joints. Normally, the top joint is very flexible, whereas the remaining joints have limited range of motion. The backside of the carpus is supported by a very strong ligamentous structure called the palmar fibrocartilage, which prevents the joints from overextending (hyperextension).
Cause of carpal hyperextension injury
The cause of a hyperextension injury is due to tearing of the palmar fibrocartilage. This type of injury can be due to a single isolated traumatic event, such as jumping off a roof or out of a window. In other cases, it is due to repeated injury to the palmar fibrocartilage due to jumping off elevated surfaces or out of vehicles. Once the palmar fibrocartilage tears, conservative measures such as splinting or casting the limb are consistently ineffective in nearly all cases.
Signs and Diagnosis
Carpal hyperextension injury is most commonly seen in large active dogs; however, small breeds certainly can sustain a similar injury. Warning signs of this condition include lameness that worsens with exercise. Swelling and hyperextension of the carpus also are common clinical signs.
The diagnosis of a carpal hyperextension injury is based on physical examination findings and X-rays of the affected forelimb. Stress X-rays of the affected limb, with your pet under anesthesia, may be required to further demonstrate the problem. This involves applying a stress to the carpus to see which joints have torn ligaments (white arrow points to the unstable joints). If only the lower two rows of joints are broken down, a partial fusion of the joint will be recommended. If all of the joints are broken down or the top joint is broken down, a fusion of all of the joints will be required.
In order to correct this problem, it is essential to perform an arthrodesis, or fusion of the carpal joints. This involves removing the cartilage from the surfaces of the bones within the joints, packing the joints with a bone graft that is collected from the humerus bone (near the shoulder), and stabilizing the bones with a plate and screws. The surgeon will make a determination prior to surgery if a partial arthrodesis (photos right) or a full arthrodesis (photos below) is necessary. The full arthrodesis will remove all movement within the carpus, whereas the partial arthrodesis retains about 50 to 75% of normal range of motion of the carpus. After surgery, a cast or splint is applied to the limb to support the repair for about four to six weeks.
Surgical arthrodesis will relieve your companion’s pain once the bones have fused together. A partial arthrodesis commonly will allow your pet to have normal function of the limb with good range of motion of the carpus. A full arthrodesis of the carpus typically results in good function of the limb; however, a gait abnormality is expected, as the carpus will not have any movement. Uncommon complications following surgery include infection, failure of the bones to fuse together, breakage of the implants (metal plate and screws) and cold sensitivity. In general, about 90 to 95% of the patients will respond well to the surgery.
For more information on this subject, speak to the veterinarian who is treating your pet.