How big of a problem is osteoporosis?
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Osteoporosis is a reduction in the density of bone. The remaining bone is then compromised in strength and resistance against fracture. The National Osteoporosis Foundation estimates that 44 million Americans or about 55% of the people (including all races) age 50 and older will be affected by osteoporosis and occurs six to eight times more frequently in women than in men. It is estimated that 10 million adults already have osteoporosis, 8 million of which are women, and as many as 34 million are estimated to have low bone mass called (osteopenia). In fact as many as 1.5 million new fractures are attributable to osteoporosis in the United States each year. Sadly, many of these fractures result in permanent immobility. Osteoporosis is a reduction in the density of bone, which is then compromised in strength and resistance against fracture. |
How is osteoporosis diagnosed?
The World Health Organization (WHO) has set guidelines for characterizing the degree of bone loss. In order to do so, bone density must be compared to what is typically seen in younger people. Osteopenia is a level of bone density reduction that places a person at greater risk of fracture. It is said to be a measured bone density that is 1 to 2.5 standard deviations (SD) below an average (or statistical mean) for a younger person of the same gender. Osteoporosis is more severe, whereby the reduction of bone density is greater than 2.5 SD below the average. Individuals should talk to their physicians about where they are relative to others and X-ray measurements such as DXA (dual energy x-ray absorptiometry) are used in the diagnosis. Generally, osteoporosis develops without symptoms. It is usually not until a person fractures a bone or complains of severe back pain that an X-ray diagnosis is made.
What is the composition of bone?
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There are 206 bones in the human body and the entire skeleton represents about 12 percent and 15 percent of young adult women’s and men’s body weight, respectively. That is roughly 5.5 kg (15 lb) for a woman and 11 kg (23 lb) for a man. Typically, when we think of bone, we think of minerals such as calcium and phosphorus, but minerals make up only 30 to 40 percent of bone weight. Beyond minerals, bone is also comprised of bone cells, nerves, blood vessels, collagen, and other proteins. Bone protein, primarily collagen is stalk-like. Bone mineral compleses, called hydroxyapatite lay on top of of the collagen fibers. (See Collagen and Hydroxyapatite Figure) |
Is bone actively modified?
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Bone is often considered inactive tissue. Maybe this comes from images of skeletons at Halloween or the bone fossils of animals that lived long ago. Whatever the case, bone is actually fairly active. It is constantly engaged in remodeling processes by bone cells called osteoblasts and osteoclasts. The osteoblasts are responsible for making and laying down new collagen protein and other substances. The collagen provides the network for the deposition of calcium and phosphorus mineral complexes such as hydroxyapatite to cling to. This is an important and often overlooked point, because without collagen you cannot mineralize bone. For this reason the osteoblasts are said to be active in making new bone tissue and are often called “bone makers.” |
Osteoclasts, on the other hand, are primarily responsible for initiating the events leading to the breakdown of bone substances. For this reason they are often called “bone destroyers.” Osteoclasts ooze acids that will dissolve the mineral complexes as well as enzymes (collagenase) that will dissolve collagen. The actions of osteoclasts may seem destructive, but their role in bone remodeling is pivotal. Also, when osteoclasts break down bone mineral complexes, the minerals can become available to the blood. This could be important in maintaining blood calcium levels.
How is bone remodeled?
The activities of osteoblasts and osteoclasts are indeed antagonistic and occur simultaneously. Therefore the body is building new bone at the same time as it is breaking down older bone. This is referred to as “bone turnover” and is similar to tearing up and pouring new concrete for a street or tearing down and constructing a new wall in a building. This reconstruction allows for that street or wall to be most appropriate in its functions.
Throughout life there are periods when the activities of these cells are out of balance. This can be purposeful or pathological. The imbalance in turnover results in either a net gain or loss of bone. For example, in childhood, as bones are lengthening and growing thicker, the activities of osteoblasts will exceed those of the osteoclasts, and new bone is built. On the contrary, in later adulthood, osteoclast cell activity seems to be greater than osteoblast activity. This results in a slow loss of bone matrix. During periods when there is neither a net loss nor gain of bone, the activities of osteoblasts and osteoclasts are in balance and coordinated to properly remodel bone.
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Even though a finalized bone length and therefore adult height is realized in the late teens to early twenties, bone is constantly being remodeled. The turnover process is governed by factors such as hormones (growth hormone, PTH, estrogen, testosterone, and calcitonin) and vitamin D. Mechanical forces, such as pressure exerted upon bone during resistance exercise, also play a big role in bone turnover. These factors affect bone remodeling primarily by increasing or decreasing the activity of osteoclasts and osteoblasts. |
