Osteoporosis is a disease of the bone where the balance between new bone deposition and old bone resportion is disordered causing a loss of bone mass. Although the disease mechanism is well understood, the causes are not always as clear. Osteoporosis can broadly divided into primary and secondary, where there is no causative disease (primary) or it occurs as a consequence of some underlying disease (secondary).
Primary osteoporosis occurs in the absence of any disease that affects the bone either directly or indirectly. In most instances the cause is not well understood and may even be due to unknown reasons. The medical term for a condition with no known cause is idiopathic. It can be classified further into :
Menopause is a known risk factor for osteoporosis. It is due to declining female hormones like estrogen. Certain bone cells that are responsible for constantly remodeling the bone have estrogen receptors. This means that they are sensitive to estrogen and once estrogen levels decline, the activity of these bone cells are also affected. It appears that the drop in estrogen levels increases osteoclast activity – the cells that are responsible for bone resorption.
Post-menopausal osteoporosis is not just related to age. Bone density has a direct relationship with estrogen, and even in premature menopause which may occur before the age of 45 years, osteoporosis can develop. The pattern of bone loss makes certain bones more at risk of fractures. This includes the wrist, forearm and vertebrae.
As the name indicates, this type of osteoporosis is associated with advancing age. Although it is more common after the age of 70 years, it may sometimes be seen earlier in the senior years. Aging appears to reduce the number of osteoblasts – the bone cells that lay down new bone. The exact reason why this occurs has not been conclusively identified but there are numerous changes in the body with aging that may be responsible.
Calcium and vitamin D are two substances that are important for bone health. While calcium is largely acquired through food, vitamin D is linked to sunlight exposure. There appears to be a decreased intake of calcium or impairment in absorbing calcium from the gut with advancing age. These factors can further contribute to age-associated osteoporosis. Fractures of the wrist, vertebrae and hip are more likely with age-associated osteoporosis.
Bone health is maintained by a number of different factors that are not isolated to the bone alone. Hormones, nutrients and force on the bones all determine bone density. In secondary osteoporosis, there is some underlying disorder that affects bone health. It includes a wide array of causes such as :