Structure and Function of the Skeleton in Cats

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

What Is the Skeleton?

The skeleton is the bony framework of the body that is present in all vertebrate cats, dogs, and animals. 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). The cat skeleton has an average of 250 bones.

The cat’s skeletal design is very similar to ours, although there are two significant differences. First, a cat’s spine or backbone contains more bones that ours, mainly because of the tail. Their vertebrae are not as tightly connected as ours, making the cat’s spine extremely flexible. This feature enables the cat to arch his back and to twist or turn his body so that he can squeeze through the tiniest gaps. Second, the cat lacks a clavicle or collarbone. A collarbone would broaden the chest, thus reducing the cat’s ability to get through narrow spaces, and limiting the length of his stride.

Where Is the Skeleton Located?

The skeleton is located throughout the entire head and body of cats.

What Is the General Structure of the Feline 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 cats have the exact same size and shape to their bones. Since people have been breeding cats for thousands of years, bones may vary in their length and thickness depending on the specific breed.

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 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 Feline Skeleton?

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 cat, it also stores fat.

What Are Some Diseases of a Cat’s Skeleton?

Congenital diseases. Certain congenital and developmental bone diseases occur in the cat, but are usually uncommon. Examples include the following:

• Mucopolysaccharidosis VI is a genetic defect in the metabolism of connective tissue that causes secondary changes in bone. It usually affects Siamese cats and causes them to have a broad, flat face. They are often lame and may have trouble walking.
• Osteodystrophy is a rare inherited disorder of Scottish fold cats. Bony deformities develop in the toes, tail and bones of the wrist and ankle.
• Multiple cartilaginous exostoses are abnormal proliferations of bone in certain areas, such as the long bones, ribs and vertebrae. They occur in young adult cats and may cause lameness and discomfort.

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. 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 cats fed an all meat diet. In kittens 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 seriously affect the development of bone. Lameness, bony deformities and fractures may occur.
• Excessive vitamin A in the diet of cats causes deformities to develop in bones. Excessive levels of this vitamin are most likely to occur when the cat is fed a diet that contains predominantly liver and milk. Lameness, weakness, and reluctance to move are common signs.

Trauma. Trauma to bones is perhaps the most common skeletal disorder encountered in the cat, especially outdoor cats. Cats that are injured through falls, automobile accidents and fights can experience a variety of bony fractures and dislocations.

Cancer. Neoplasia or cancer of bone is uncommon in the cat. Tumors may arise within the tissues of the bone or may invade bones from the surrounding soft tissues. Cats are most prone to cancers of the bone marrow, such as lymphosarcoma. Cancers of cortical and cancellous bone are more rare in the cat.

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, vitamin A 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 cat 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.