The growth plates are much softer than other regions of the bones, therefore are more prone to injury
The wrist or carpus is prone to developing an injury of the growth plate of the ulna because it is “V” or cone-shaped
If the dog is beyond the rapid growth phase then both radius and ulna bones must be cut to straighten the limb
Growth plates, which are responsible for longitudinal bone growth are located near the ends of the bones and are found in animals less one year of age. The majority of a dog’s growth occurs during four to eight months of age. Usually by one year of age, when the pet is mature, the growth plates are closed and are not visible on x-rays. The growth plates are much softer than other regions of the bones, therefore are more prone to injury.
Growth plate injury
Since most of the longitudinal growth of bones occurs up to eight months of age, growth plate injuries that occur after this time are not as devastating. A growth plate is the weakest link in the bone construct, therefore is more susceptible to injury. A Salter-Harris classification of growth plate fracture has been described in humans and may to relate to prognosis in dogs and cats. A type 5 injury results in a crushing injury the growth plate resulting in damage to the cells responsible for growth of the bone. The result is a scar at the level of the growth plate and no additional growth of the bone occurs. If the growth plate injury occurs in one of two paired bones (one bone grows and the other does not) the final outcome is usually devastating (see photo right of a Collie with twisted right forelimb due to premature closure of the ulnar growth plate).
Mechanism of limb twisting
- The wrist or carpus is prone to developing an injury of the growth plate of the ulna because it is is V or cone-shaped (see illustration below). An impact such as taking a fall and landing on the front limb with full force can drive this cone-shaped growth plate together and result in severe damage to the cells. This type of injury is common in short-legged dogs. Another injury can be due to damage to the blood supply to a section of the growth plate and a cartilaginous core will develop and the bone growth slows down or stops. Cartilaginous core formation is more common in giant breed dogs.
- The growth plate at the lower part of the ulna bone (red line in illustration) is responsible for 90% of the growth of the entire bone. Therefore, the ulna essentially stops growing. The radius bone, which is the parallel bone to the ulna, continues to grow. This causes a bowstring effect, with the radius being bent like a bow due to its continued growth and the ulna staying the same length like the string of the bow.
- The wrist therefore twists and the elbow joint also gets pulled out of alignment (see illustration). The lameness in such a case comes primarily from the elbow incongruency (dashed circle in illustration), but also may be due to stress put on the collateral ligaments of the carpus.
- Another type of injury is closure of one side of the growth plate of the lower radius bone. In this situation the bone could twist toward or away from the midline of the pet (depending on which side of the growth plate is affected). This injury occurs when the limb is struck on one side of the foot.
- If the dog is still growing and the carpus is not twisted more than 25 degrees then removing a small section of the ulna bone and its associated covering (periosteum) will usually allow the limb to become straight again as the dog grows (see photo right).
- If the dog is beyond the rapid growth phase then both radius and ulna bones must be cut to straighten the limb (see fig 4 before and fig 5 after surgery; a circular fixator was placed to keep the bones aligned). Sometimes the elbow congruency needs to be improved by cutting the ulna bone just below the elbow joint.
- Anesthetic death
- Limb continues to twist
- Poor bone healing or nonhealing of the bone
- Breakage of the surgical implants (screws, pins, plates)
- Limb is shorter than the normal thus resulting in a gait abnormality