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 Vitamin D Vitamin D Deficiency and Dependency Inadequate exposure to sunlight predisposes to vitamin D deficiency. Deficiency impairs bone mineralization, causing rickets in children and osteomalacia in adults and possibly contributing... read more ). Osteomalacia can be caused by disorders that interfere with vitamin D absorption (eg, celiac disease Celiac Disease Celiac disease is an immunologically mediated disease in genetically susceptible people caused by intolerance to gluten, resulting in mucosal inflammation and villous atrophy, which causes malabsorption... read more ) and by certain drugs (eg, antiseizure drugs Drug Treatment of Seizures No single drug controls all types of seizures, and different patients require different drugs. Some patients require multiple drugs. (See also the practice guideline for the treatment of refractory... read more ). Osteoporosis is much more common than osteomalacia in the US. 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 level is consistently very low. To definitively differentiate between the two disorders, clinicians can do a tetracycline-labeled bone biopsy, but this is rarely warranted.