An angular limb deformity is an abnormally shaped, curved, or crooked limb. Angular limb deformities occur in young, growing animals after a traumatic event, such as being hit by a car, getting stepped on or dropped.
Bones grow lengthwise by elongation and multiplication of the cells at the growth plates or physes. There is a growth plate on each end of the bone.
Angular limb deformities most often occur in the forearm (radius/ulna), but can also occur in the lower hind limb (tibia/fibula). An injury to the growth plate causes crushing of the growing cells and stops their growth (premature closure of the physis). This type of crushing injury to the cells is not apparent on X-rays.
The crushed cells stop growing, while unaffected cells continue to grow. Often the damaged portion is on one side of the bone, causing one side to stop growing while the other side continues to grow. This makes the bone grow in a curved shape, similar to a plant that curves toward sunlight.
The forearm consists of a two-bone system: the radius and the ulna. If the growth plate in one of these bones is damaged, then that bone will stop growing. The other bone will continue to grow and is tethered by the damaged bone, causing a curvature or bowstring effect. Most often, the ulna is damaged after an injury to the forelimb because the shape of the physis is cone-shaped and can be injured in many different directions.
Unequal lengthening of the bones can also cause abnormal joint contact, or joint incongruity. This can result in pain, arthritis or abnormal mobility.
It can take a few weeks after an injury for an angular limb deformity to become apparent, depending on the growth rate of the pet. Some breeds are predisposed to premature closure of the growth plates, causing abnormally short, crooked legs. This is normal in breeds such as the basset hound, shih tzu
, but sometimes this "normal" abnormality is more pronounced than expected and causes lameness.