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A Dog's Musculoskeletal System

The musculoskeletal system is the framework that holds your dog's body upright and protects the delicate internal organs of the chest and abdomen. It is made up of between 319 and 321 bones (depending on tail length) and the hundreds of muscles required to move and control the bones and control other bodily functions.

Bone Structure and Function

The skeletal anatomy of a dog is remarkably similar to a human's. Starting at the head, the major bones of the appendicular skeleton include the skull, and the vertebrae which run down the back and to the tip of the tail. Scapulae form the shoulder blades you can generally feel above your dog's front legs. You can also usually feel the rib cage, if your dog is not overweight. The front legs are made up of the humerus at the top, the ulna and radius at the bottom, and the front feet contain carpals, metacarpals, and phalanges. The back legs arise from the pelvic bones. The femur makes up the top part of the legs, with the fibula and tibia making up the bottom portion. The back feet comprise the tarsals, metatarsals, and phalanges. The dog's kneecap is called the patella, or sometimes the rotula. These bones in the limbs make up the axial skeleton. There are also small bones located within body organs like the middle ear. These are known as the visceral skeleton.

In contrast to the human skeleton however, the canine skeleton is built primarily to run. Motion and flexibility are key. For example, the shoulder blades aren't tightly connected to the rest of the skeleton to allow a huge range of motion as the dog runs. The rib cage is extraordinarily large to allow for maximum lung expansion

Although it is easy to visualize bones as "dead" because we often see them in skeletons, bones are actually living organs, constantly developing and changing, with new bone replacing old bone cells, even when the dog has reached full size in adulthood.

The bone is made up of four layers. The periosteum is a fibrous membrane that covers the outside of the bone proper. This membrane is full of blood vessels which bring nourishment to the bone tissue. The majority of bone is called cortical bone, responsible for weight-bearing. About 20% of the body's bone mass is made up of cancellous bone, which is a spongy layer that looks like a honeycomb. Cancellous bone acts like rebar to provide strength to the cortical bone. Inside the hard bony structures is a core of marrow which produces blood cells.

X-ray of severe hip dysplasia in a dog.

Bone Injury and Disease

Bones can be damaged by birth defects, injury, infections, and tumors. One of the most common congenital defects in dogs is hip dysplasia, where the head of the femur is shaped incorrectly and will not stay in place inside the pelvic bone socket. There is no known treatment for hip dysplasia, and the dog ends up lame and in pain from arthritis as a result of the bones rubbing together and wearing away the cartilage meant to protect the hip joints. The best way to avoid hip dysplasia is to have breeding stock carefully evaluated to rule out hip problems before producing puppies.

The primary injury that can damage a dog's bones is a fracture. Just as with humans, extreme trauma can break through a dog's bones, sometimes even causing the bone to break through the dog's skin in an injury known as a compound fracture. Your vet will set the dog's bone back into place and may apply a cast to hold it in place while it heals. Once healed, a bone is generally stronger in the broken place than at any other location.

Bone infections are relatively rare in dogs. Osteomyelitis is generally caused by a staph infection, although sometimes by a streptococcus bacteria or fungus. Most commonly, bone infections occur as a result of another injury. For example, when a dog sustains a puncture wound or a compound fracture, bacteria may be carried into the dog's bones from the outside environment. Treatment with antibiotics will usually cure a bone infection without any subsequent problems.

Osteosarcoma is the most common type of cancer to affect a dog's bones. It is most common in giant breeds, particularly as they get older. The most common location for a tumor to develop is in the growth plates of the long bones in the legs. By the time the tumor causes enough pain to make your dog limp, chances are it will have metastasized to other locations throughout the body, most commonly the lungs. Treatment generally includes amputation of the affected limb and chemotherapy to control the metastases. However, life expectancy after diagnosis is frequently one year or less.

Arthritis occurs when the cartilage that normally coats and protects a joint wears away, causing the two bone ends to run on each other. It commonly worsens with age, but injuries around a joint can hasten the onset of the pain and inflammation associated with arthritis.

Muscle Structure and Function

Striated muscles, the ones which are most obvious when you look at a dog, serve the dual functions of keeping the bones attached to each other and allowing the bones to move. Muscles are attached to bone by means of tendons, while ligaments stretch across joints to connect two bones while allowing movement of the joint.

Smooth muscles, on the other hand, are muscles embedded in the walls of the dog's internal organs like the stomach, intestines, and bladder. These muscles are not under conscious control of the dog's brain, but operate to allow the dog to clear waste.

Muscle Injury and Disease

Canine muscles can be injured by trauma, just as they can be in humans. A muscle may be stretched beyond its normal capacity or it may even tear under stress. A stretched muscle or tendon is known as a strain, while a stretched or torn ligament is known as a sprain.

A strain usually results in some pain, weakness, and muscle spasms, but complete healing can usually be achieved in a very short time with rest and anti-inflammatory medications. Sprains can range from a very minor stretched ligament that can be treated similarly to a strain with equally good results, to a serious tear completely through the ligament which must be treated with surgery. Full recovery from a sprain is often not achieved if the damage is severe. A sprain may cause the joint to become unstable, causing long-term arthritis and pain.

A dog's muscles are typically not affected by infection, cancer or birth defect. Diseases that affect the canine muscle tissue normally start in other body systems and then impact the muscles. For example, Cushing's disease can cause muscle wasting.