What is Paget's disease of the bone?
Paget's disease of the bone is a chronic bone disorder in which bones become enlarged and deformed. Bone may become dense, but fragile, because of excessive breakdown and deformation of bone. Named for Sir James Paget (1814-1899), this disease affects 1-3% of people over 50 years of age, but affects over 10% of people over 80 years of age. Paget's disease is the second most common bone disorder in people over 50, after osteoporosis. It is rarely diagnosed in young adults.
Paget's disease can affect one or more bones in the body. Most often, the pelvis, bones in the skull, the long bones (the large bones that make up the arms and legs), and the collarbones are affected by Paget's disease. In addition, the joints between bones (the knees or elbows, for example) can develop arthritis because of this condition.
Normally, cells that break down old bone (osteoclasts) and cells that form new bone (osteoblasts) work in balance to maintain bone structure and integrity. In Paget's disease, both osteoclasts and osteoblasts become overactive in some areas of bone, and the rate at which bone is broken down and rebuilt in these areas increases tremendously. The overactive areas enlarge but are structurally abnormal and therefore weaker than normal areas. In Paget's disease, more bone breaks down than usual and more new bone forms than usual. These changes in the bone can lead to bone enlargement and deformity. The new bone growth tends to be softer and more fragile than normal bone, and can develop in a haphazard pattern. Because of this, the bone can fracture. The long bones, especially the legs, tend to bow, and the skull may enlarge, particularly over the forehead.
Paget's disease can cause bones to weaken and may result in bone pain, arthritis, bone deformities and fractures. Paget's disease is characterized by changes in the normal mechanism of bone formation. Bone is a living material made by the body through the continual processes of formation and breakdown (resorption). The combination of these two actions is called remodeling and is used by the body to build bone tissue that is strong and healthy. Strong bones are formed when bone tissue is made up of plate-shaped crystals of minerals called hydroxyapatite. Normal wear and tear on the skeletal system is repaired throughout life by the ongoing process of remodeling. In fact, the entire human skeleton is remodeled every five years.
Paget's disease causes abnormal bone growth. Normally bone is constantly being broken down and rebuilt. In the early stages of Paget's disease, bone is broken down more quickly than it is rebuilt. To try to keep up with this breakdown of bone, the body speeds up the bone rebuilding process. However, this new bone is often weak and brittle, causing it to break more easily. Paget's disease most often affects the bones in the pelvis, spine, skull, chest, and legs. Most cases of Paget's disease do not cause symptoms. However, the most common symptoms, when they occur, are bone pain and deformed bones (bowed legs, enlarged skull or hips, or a curved backbone).