Structure and Function of the Skeleton in Dogs

Below is information about the structure and function of the canine skeleton. We will tell you about the general structure of skeleton, how bones work in dogs, common diseases that affect the skeleton and common diagnostic tests performed in dogs to evaluate the skeleton.

What Is the Skeleton?

The skeleton is the bony framework of the body that is present in all vertebrate animals, including dogs. It consists of bones, ligaments, and cartilage. The skeleton is composed of the hard tissues of the body, and its primary functions are to support the body, to provide a system of levers used in locomotion, to protect the soft organs of the body, and to produce red blood cells (hematopoiesis).

A dog’s skeleton is formed so the dog can run fast, hunt and chase. For example, a dog’s shoulder blades are not tightly connected to its skeleton, so the dog has potential for greater motion and flexibility. The dog skeleton has an average of 319 bones.

Where Is the Skeleton Located in Dogs?

The skeleton is located throughout the entire head and body.

What Is the General Structure of the Canine Skeleton?

The skeleton is composed of three skeletal subunits:

• Appendicular skeleton – the bones of the limbs
• Axial skeleton – the bones of the skull, spine, ribs and sternum
• Visceral skeleton – bone that forms part of an organ (such as the middle ear ossicles)

Bones are organs composed of hard, mineralized tissue that provide structural support to the body. Not all dogs have the exact same size and shape to their bones. Since people have been breeding dogs for hundreds of years, bones vary greatly in their length and thickness depending on the specific breed. Even though the tiny Chihuahua has the same number and type of bones as the Great Dane, the size and shape of their bones are very different. Dogs have the greatest variety in the size and configuration of their skeletons of any species of animal.

The skeleton consists of bones that may be classified according to shape:

• Long bones are found in the limbs.
• Short bones are confined to the wrist (carpus, metacarpus) and ankle (tarsus, metatarsus) regions.
• Sesamoid bones are present near freely moving joints, such as the wrist and the knee (stifle).
• Flat bones are found in the pelvis where they provide for the attachment of muscles and long bones, and in the head where they surround and protect the eye, ear, sinuses, and brain.
• Irregular bones include the vertebral column, all bones of the skull that are not of the flat type, and three parts of the hip bone.

Bones contain several layers of tissue. The periosteum, a fibrous membrane, covers the outside of bone. This membrane is rich in small blood vessels called capillaries, which are responsible for nourishing bone.

The firm, dense, outer layer of bone is called cortical bone. Eighty percent of skeletal bone mass is cortical bone. Cortical bone assumes much of the weight bearing of the body. Cancellous bone (also called trabecular bone) is an inner spongy structure that resembles honeycomb. Cancellous bone accounts for 20 percent of bone mass. This spongy mesh-like bone is specially designed for strength, with the meshwork behaving similar to the steel rebar rods that are buried within concrete.

Bones also contain bone marrow within the hollow center shaft of bone (medullary cavity). Marrow is yellow when it is made up of mostly fat, and it is red in areas where red and white blood cells are produced. Red marrow is present in certain bones, like the leg (femur), upper arm (humerus), pelvis (ilium), and ribs.

What Are the Functions of the Skeleton in Dogs?

The skeleton serves four functions:

• Bones support and protect the body.
• Bones serve as levers for muscular action.
• Bones serve as a storehouse for calcium and phosphorus, and many other elements.
• Bones serve as a factory for red blood cells and for several kinds of white blood cells. In the normal adult dog, it also stores fat.

What Are Some Diseases of the Skeleton in Dogs?

Congenital diseases

Certain congenital and developmental bone diseases occur in the dog. Examples include the following:

• Panosteitis is an inflammation in the marrow cavity of the long bones. It affects primarily young, large- or giant breed dogs. Affected animals show signs of pain in the bone, lameness, fever and lethargy.
• Hip dysplasia is abnormal development of the hip joint that causes the ball and socket of the joint to be looser than normal. Hip dysplasia occurs primarily in large breed dogs. Clinical signs may occur in young dogs or arise later in life as degenerative arthritis develops. Affected dogs may develop lameness, a bunny-hopping gait in their rear legs, a reluctance to walk, and stiffness in the rear legs.
• Osteochondrosis dissecans (OCD) is a defect in the smooth cartilage surface within one or more joints, especially in the shoulder, elbow, hock and knee (stifle). It occurs primarily in young, growing, large breed dogs.
• Hypertrophic osteodystrophy (HOD) is an inflammation in the growth plates of the long bones. It usually causes swelling and pain of the bone just above the joints, and primarily affects young, growing, large- and giant breeds of dogs.
• Multiple cartilaginous exostoses are abnormal proliferations of bone in certain areas, such as the long bones, ribs, and vertebrae. They occur in young dogs and may cause lameness and discomfort.

Numerous other developmental abnormalities of joints may affect young dogs, such as aseptic necrosis of the head of the femur, dislocation of the knee cap (patella), and elbow dysplasia.

Osteomyelitis

Osteomyelitis is an inflammation of bone that is usually caused by a bacterial infection. Infections of the bone may also arise with certain fungal infections and in the presence of bone implants, such as bone plates and pins.

Nutritional disorders

• Some disorders that cause abnormalities in the circulating levels of calcium, phosphorous and certain vitamins can adversely affect bones. Examples include:
• Rickets is severe weakening of the bone due to calcium deficiency or imbalances in the diet. It arises most often in young dogs fed an all meat diet. In puppies it causes lameness, deformities and fractures of the bone.
• Chronic kidney failure affects bone by altering the amount of phosphorus and vitamin D in the body. The bones become soft, thin, and weak.
• A deficiency in vitamin D in the diet, or low conversion of vitamin D in the body from a lack of exposure to sunlight, can affect the development of bone. Lameness, bony deformities and fractures may occur.

Trauma. Trauma to bones is perhaps the most common skeletal disorder encountered in the dogs, especially dogs allowed to roam free. Dogs that are injured through falls, automobile accidents or fights can experience a variety of bony fractures and dislocations.

Cancer. Neoplasia or cancer of bone occurs in the dog. Tumors may arise within the tissues of the bone or may invade bones from the surrounding soft tissues. Large breed dogs develop more bone cancer than do small breed dogs. Dogs may also develop cancers of the bone marrow, such as lymphosarcoma.

What Types of Diagnostic Tests Are Used to Evaluate the Skeleton?

• Physical examination and palpation of bones. The initial evaluation of the skeleton involves a thorough examination, with palpation of the bones and joints. The animal may be observed in the examination room for signs of lameness and abnormalities in gait.
• Radiography. Cortical and cancellous bone and some forms of cartilage show up very well on plain x-rays. For this reason, plain x-rays are an important tool in evaluating the skeleton. Some x-rays may be taken with the animal awake and sedated, while other x-rays may require that the animal be completely anesthetized.
• Routine laboratory tests. A complete blood count and biochemistry profile may be taken to look for signs of infection and abnormalities in circulating levels of calcium and phosphorus. These tests are also helpful to detect other disorders that may affect the bones, such as kidney disease, anemia and leukemia.
• Special laboratory tests. If an infection of bone is suspected, then samples may be collected for bacterial and fungal cultures. Serologic tests for fungal diseases may also be submitted. Occasionally the levels of vitamin D and parathyroid hormones are measured in the blood.
• Bone biopsy. Identification of the type of bone disease present may require a bone biopsy. This is especially true in cases of congenital and developmental bone diseases, osteomyelitis and tumors of bone.
• Bone marrow biopsy. Biopsy of the bone marrow cavity is done via passage of a needle into the red bone marrow of one or more bones and aspiration of a sample of the bone marrow. The bone marrow sample is then sent to a veterinary pathologist for microscopic examination.
• Advanced imaging techniques. CT and MRI are very useful in examining bones and their adjacent soft tissues, such as cartilage, ligaments, and tendons. Radioisotope bone scans are also helpful in some cases. To arrange these tests often requires that your dog be referred to a veterinary orthopedic specialist or a veterinary radiologist.
• Chest x-rays. Chest x-rays may be taken to look for evidence of infection or tumors that have spread to the lungs, and to identify abnormalities in the ribs or vertebrae of the chest.