Causes of Angular Limb Deformities
Angular limb deformities can be caused by the premature closure of a growth plate (called a “physis”), by retained cartilaginous cores or by bony bridging (synostosis) between the radius and ulna. In dogs, these deformities typically occur in one or both front legs, although occasionally a hind leg is affected. To understand this condition, it is worthwhile to have a general understanding of canine forelimb anatomy, and of how long bones grow.
The lower part (forearm) of a dog’s front leg (forelimb) has two long bones: the radius and the ulna. These are the largest paired bones in a dog’s body. The radius is the main weight-bearing bone of the forearm. Above the radius and ulna is the elbow joint, which connects the lower long bones to the humerus (upper arm), which in turn articulates with the scapula (shoulder blade) to form the shoulder joint. Below the radius and ulna are a number of small bones, tendons and ligaments that form the carpus (in people, this is the wrist joint). Below the carpus is the paw, which is comprised of various joints and bones that are collectively referred to as the metacarpus, paralleling the body of the hand in people. At the very end of the forelimb are the toes – called digits or phalanges – which in people are the fingers of the hands.
Angular limb deformities typically come from abnormalities in the radius and/or ulna. In puppies, immature long bones are made of a shaft called the “diaphysis”, which is the middle or center part of the bone. At both ends of the diaphysis is another area called the “metaphysis,” and beyond that is the terminal end of the long bone, called the “epiphysis.” The area between the metaphysis and epiphysis is called the “physis.” It is also commonly referred to as a “growth plate.” Longitudinal growth of the radius and ulna (and other long bones) comes almost entirely from the growth plates (physes), which have their own extensive blood supply that is separate from the blood supply that nourishes the central shaft (diaphysis) and the very ends (epiphyses) of the bones. The growth plates are cartilage made up of five separate developmental zones. Longitudinal growth, which causes the puppy to grow taller, comes from active division and growth of cartilage cells in the growth plates. Ultimately, those cartilage cells (called chondrocytes) degenerate, causing calcium to be deposited into the bony matrix of the area, which mineralizes and forms mature hard bone.
This growth process normally continues until the growth plates close. In dogs, the growth plates of the radius and ulna typically close between 6 and 9 months of age. The physis at the lower (distal) end of the ulna bone is responsible for most of the longitudinal growth of that bone. It also is particularly vulnerable to becoming crushed during trauma because of its unique shape.
Because of their intimate association and contribution to the same joints at either end, the paired radius and ulna bones need to grow in a synchronous manner for the foreleg to develop normally. Premature closure of a growth plate at the lower end of the radius or ulna (or rarely, the tibia in the hind leg) will cause the affected bone to stop growing earlier than it should. Because of their close connection, abnormally stunted growth of one of the long bones of the forearm will affect development of the other long bone, as well as the elbow and/or the carpus (wrist) joints. Put simply, if the ulna stops growing while the radius continues to grow, the radius will begin bowing at the top, rotating outward and suffering from what is called “valgus deformity,” which is a twisting of the leg away from the midline of the dog’s body. The dog’s elbow and carpal (wrist) joints are commonly affected, as well.
Premature growth plate closure most commonly is caused by some traumatic injury to the affected limb of a young, growing puppy less than 9 months of age. Trauma can damage the vessels that normally provide the blood supply necessary for bone growth. Far less common causes of angular limb deformities are retained cartilage cores and bony bridging of the radius and ulna, which are beyond the scope of this article.
Prevention of Angular Limb Deformities
The best way to prevent angular limb deformities is to prevent any type of traumatic injury to the lower legs of growing dogs. Early recognition and treatment are essential to preventing (or at least minimizing) severe leg deformities, pain and discomfort in affected animals.
Angular limb deformities have become desired genetic characteristics of certain breeds, such as the Bassett Hound, Corgi, Dachshund and others. In those breeds, the limbs are “naturally” shortened and deformed, without traumatic injury causing premature closure of the growth plates.