Fibrous dysplasia is a chronic condition of the skeleton where a portion of a bone develops abnormally.

The condition begins before birth. It is caused by a gene mutation that affects the cells that produce bone. Although the abnormal bone forms before birth, its presence is often not discovered until childhood, adolescence, or even adulthood.

The gene mutation causes the cells to form an abnormal type of fibrous bone. This fibrous bone gradually grows and expands over a period of years, causing a weakened area of the bone. The area of weak bone can cause pain. It can crack (fracture) the bone, and may lead to deformity.

Fibrous dysplasia accounts for approximately 7% of all benign bone tumors. Any bone may be affected. The most common bones involved are the thighbone, shinbone, ribs, skull, upper arm bone, and pelvis.

Usually, only one bone is involved (monostotic fibrous dysplasia). Less often, multiple bones are involved (polyostotic fibrous dysplasia). The polyostotic form is generally more severe and is discovered earlier. This form can involve as few as two bones in the same limb or multiple bones throughout the skeleton.

The same abnormality that occurs in the bone cells of fibrous dysplasia may also occur in the cells of some of the body's glands. This can lead to hormonal abnormalities. This is rare and generally only happens with severe forms of polyostotic fibrous dysplasia. McCune-Albright syndrome is a condition where polyostotic fibrous dysplasia occurs with pigmented skin lesions ("cafe au lait" spots) and hormonal abnormalities.

The cause of the gene mutation is not known. It is not inherited or passed on to the children of affected patients. No dietary or environmental cause is known. It occurs equally among males and females of all races.

As the abnormally formed fibrous tissue grows and expands, the involved area of bone becomes weaker. The weakened area of bone can become painful. Pain is more likely to occur in the weightbearing leg and pelvis bones. This type of pain generally begins as a dull ache that is made worse with activity and lessened with rest. It can progressively increase with time.

Sometimes, the bone breaks (fractures) through the weak area, causing sudden severe pain. This can happen after there has already been less severe pain for a time, or it may happen suddenly with no prior pain. The weakened area of bone can cause deformity of the bone over time. Deformity of the facial bones and bowing of the leg bones can be noticeable.

Severe deformity can lead to loss of vision or hearing when facial bones are involved. When the leg and pelvis bones are severely deformed, arthritis may develop in nearby joints.

Young patients with hormonal abnormalities may develop early puberty. This problem is more common in girls than boys. This is usually caused by overactivity of the ovaries. Overactivity may also occur in other glands of the body, including:

• the thyroid gland (causing anxiety, loss of weight, and abnormal sweating)
• the adrenal glands (causing weight gain, diabetes)
• the pituitary gland (causing milk production in women, gigantism, acromegaly)
• the parathyroid glands (causing high levels of calcium in the blood)

Pigmented skin lesions are often seen in patients with fibrous dysplasia and hormonal abnormalities.

The elevated hormone levels normally associated with pregnancy may speed up the growth of fibrous dysplasia lesions, causing increased pain.

Warning signs that an area of fibrous dysplasia may have become cancerous include increasing pain, particularly pain that wakes you up at night or does not go away with rest. The presence of a mass should always be investigated.

The explosion of genetic research may lead to a better understanding of the exact mutation involved in fibrous dysplasia. That may lead to more effective nonsurgical treatments.

Newer generations of medications, like the bisphosphonates, have made them easier to take with fewer side effects. More experience with these medications may allow more effective treatment.