Two metabolic bone diseases decrease bone mass: osteoporosis and osteomalacia.

In osteoporosis, bone mass decreases, but the ratio of bone mineral to bone matrix is normal.

In osteomalacia, the ratio of bone mineral to bone matrix is low.

Osteoporosis results from a combination of low peak bone mass, increased bone resorption, and impaired bone formation. Osteomalacia is due to impaired mineralization, usually because of severe vitamin D deficiency or abnormal vitamin D metabolism (see ). Osteomalacia can be caused by disorders that interfere with vitamin D absorption (eg, celiac disease) and by certain drugs (eg, antiseizure drugs). Osteoporosis is much more common than osteomalacia in the United States. The two disorders may coexist, and their clinical expression is similar; moreover, patients with osteoporosis may have mild to moderate vitamin D deficiency.

Osteomalacia should be suspected if the patient has bone pain, recurrent rib or other unusual fractures, and the vitamin D