Metabolic bone disease: an update.

Metabolic bone disease and some of the disorders of the skeleton are associated with alterations in calcium and phosphorus homeostasis. For example, calcium salts are not freely soluble in body fluids and increases in either calcium or phosphate or both can lead to deposition of calcium salts in the soft tissues. In addition, because the crystalline material in bone is very tiny, there is a rapid and very active exchange system with the extracellular space so that disorders of calcium or phosphorus are reflected by a change in bone structure and, in fact, contribute to the ease with which fractures occur. Also, in many disorders, bone formation and destruction are equal in activity but opposite in sign; in some conditions such as osteoporosis, destruction exceeds production and lead to weakness of bone structure. Finally, the transfer of calcium across lumens such as the gut, kidney, or bone requires a system, which consists of parathyroid hormone and 1,25-dihydroxyvitamin D and a low phosphate level.