Osteoporosis is the loss of bone density and strength Figure 01. A normal skeleton is made up of two major types of bone. One type, called cortical bone, is the dense, stiff bone that is the major component of the long bones of the legs and arms and other sites. Another type of bone, called trabecular bone (or “spongy” bone), occurs in areas such as the spine, hips, heels, and wrists, where flexibility and shock absorption are required. Trabecular bone contains many small holes (pores) through which blood circulates. Within these small pores, special cells called osteoclasts break down bone, and other special cells called osteoblasts build bone back up.
The constant process of breakdown and re-formation is called remodeling. One cycle of remodeling takes up to six months. In fact, your entire skeleton is slowly replaced by remodeling every four to five years. Osteoporosis happens when osteoclasts break down bone more quickly than the osteoblasts form new bone. When this happens, your bones will lose strength and density. Trabecular bone loses density more readily than cortical bone.
Early bone loss is just thinning of the bone around the already porous areas (osteopenia) Figure 02. At this early stage, bone loss may be fully reversible. If the process of bone loss is not reversed, however, it may eventually lead to severe osteoporosis. Osteoporotic bone contains much larger holes connected only by thin strips of weakened bone. This weakened bone is less dense, and is much more prone to fractures. Sites containing more spongy (trabecular) bone tend to fracture first, which is why hip, spine, and wrist fractures are more common than fractures at other sites. People with more severe osteoporosis have a greater risk of fracturing the ribs and the long bones of the arms and legs.
Figure 01. Scanning electron micrograph of A) normal bone density and B) decreased bone density characteristic of osteoporosis
Figure 02. Fracture sites in osteoporosis
As you age, your bones break down faster than they can build back up. The primary cause of osteoporosis is increasing age. Most women reach their maximum bone density by the age of 35—most men reach their maximum bone density by age 40. After you reach your peak bone density, your bone mass is maintained by a process whereby cells break down and re-form bone (remodeling). As you get older, however, your bones break down more quickly than they re-form. Therefore, your bones become thinner and more likely to break. Age-related losses of balance and muscle strength also increase your risk of falling and breaking a bone.
Poor nutrition and lack of physical activity contribute to decreased bone density. It is important to consume calcium and vitamin D during adolescence and young adulthood. During this time your bones are still building mass, and calcium and vitamin D help to make them as strong as possible. It is also important to get regular physical exercise during this time; especially weight-bearing exercise.
A poor diet and a sedentary lifestyle can hinder bone-building and contribute to brittle bones in later age. Drinking caffeine and soda on a regular basis, in particular, is thought to increase the rate of calcium loss from bones. This is particularly disturbing since many young people now drink soda and coffee instead of milk.
It remains important to get exercise and consume calcium and vitamin D as you age to slow bone loss.
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