Cells that remove old bone are called osteoclasts and cells that replace and repair the tissue are called osteoblasts. Osteoclasts stick to the bone and destroy minute sections by releasing acid, then ingesting the remnants. Osteoblasts then move in to cover the excavated area and start the process of building new bone. Bone mass is maintained at a constant level due to the tight balance of bone removal and rebuilding. Both osteoblasts and osteoclasts are created from precursors in the bone marrow, and after performing their respective functions, they are programmed to die by a process called apoptosis.
Osteoporosis can lead to fractures after little or no trauma.
Common sites for fractures are the wrist, rib, vertebra, and the hip. Such fractures may occur in women and men of any age and may leave a patient confined to a wheel chair. The main risk factors for the development of this disease are sex hormone deficiency in females or males, old age, and the use of anti-inflammatory steroids. The form of osteoporosis that is most prevalent in women is caused by a deficiency in the female hormone, estrogen, occurring around the time of menopause. Estrogen keeps the rate of bone remodeling low and shortens the working life of osteoclasts while it increases the working life of osteoblasts.