Steve Teta
Department of Exercise Science
University of Massachusetts Amherst
Physiology of Bone | About Osteoporosis | Causes | Who's at Risk | Bone Health | Equipment Manufacturers | References
Bones as Organs
The human body contains over 200 bones all joined together at articulations known as joints. Joints aid in movement by providing junctions for bones whereby they move when muscles exert force on them. The skeleton is divided up into two categories: the axial skeleton which includes the bones of the skull, vertebral column, ribs, and sternum; and the appendicular skeleton which includes the bones of the hips, shoulders, and extremities. There are several different shapes of bones- long, short, flat, and irregular. Each shape is specific to its function.
Bone Tissue
There are two types of bone tissue: cortical and trabecular bone. Cortical bone, also called compact bone is densely packed and makes up about 80% of the skeleton. Trabecular bone, also called spongy bone, is more porous and is surrounded by cortical bone.
Composition of Various Bones
MEASUREMENT SITE | CORTICAL PERCENTAGE | TRABECULAR PERCENTAGE |
Culcaneus | 5 | 95 |
Lumbar spine: Anterior-posterior view | 50 | 50 |
Lumbar spine: Lateral view | 10 | 90 |
Proximal femur | 60 | 40 |
Total body | 80 | 20 |
Cortical Bone is composed of osteons, which are the functional units of bone. Osteons are organized into concentric layers of matrix called lamellae, which are surrounded by widely dispersed cells. The matrix is the intracellular space, and it is made up of organic and inorganic substances. Trabecular bone is composed of branching projections called trabeculae, which form a latticelike network of interconnecting spaces. Its appearance is responsible for its other name, spongy bone. Trabecular bone has the same cell and matrix elements as cortical bone, but it has a greater degree of porosity.
About 80-90% of the volume of cortical bone is calcified; only 15-20% of trabecular bone is calcified, the remaining volume is occupied by bone marrow, blood vessels, and connective tissue. Therefore cortical bone is best suited for structural support and protection, and trabecular bone is best suited for bone's physiological functions. It is in the trabecular bone that the age-related loss in bone mineral density occurs. Therefore the most common sites of osteoporotic fractures occur in areas composed predominately of trabecular bone (wrist, hip, and spine).
Bone Development
Bone development can be divided into three processes: bone growth, bone modeling, and bone remodeling. Each process occurs at different times throughout an individual's life.
Growth
Bone growth refers to the increase in size of the bone due to an increase in the number of bone cells. There are two types of bone growth: appositional growth and longitudinal growth. Appositional growth indicates that a bones increases in thickness and mass. Longitudinal growth occurs at the epiphyseal plate prior to a person's reaching adult height. This is an area of concern about stunting of a child's growth as a result of excessive exercise.
Modeling
Bone modeling is the process of altering the shape of bone by bone resorption and bone formation. Micromodeling is the microscopic level of cell organization that occurs during formation; it determines what kind of tissue will be formed. Macromodeling controls if, when, and where new tissue will form or old tissue will be removed. this process ensures that the shape of the bone is matched to the role that bone will serve.
Remodeling
Bone remodeling refers to the continual process of bone turnover, maintenance, replacement, and repair. It reflects the balance between the coupled processes of bone resorption and bone formation. This ongoing process occurs because of the coupled action of bone cells.
There are three types of bone cells: osteoclasts, osteoblasts, and osteocytes.The cells are the living part of the bone. Osteoclasts are large, multinucleated bone cells that cause the resorption of bone tissue (bone-destroying cells). Osteoblasts are bone cells that cause the deposition of bone tissue (bone-forming cells). Osteoblasts are cells that produce an organic bone matrix that will become calcified and harden when minerals are deposited in it. The hardening of bone matrix is known as ossification. Osteocytes are mature osteoblasts surrounded by calcified bone that help regulate the process of bone remodeling. Osteocytes appear to initiate the process of calcification.
Osteoporosis is a thinning and weakening of the bones that happens as we get older. If this deterioration in our bones goes untreated, our skeleton eventually will become extremely fragile, and some bones will be likely to break - or fracture - with very little trauma.
Although the process of bone loss begins gradually when we are in our mid to late thirties, it is so slow that it may take many years before we become aware of it. Women, generally, are at greater risk of developing osteoporosis than men. This is because, following menopause, women experience a rapid loss of bone from the skeleton due to the decrease in estrogen production.
Bone loss is usually a painless process until a fracture occurs. Thus, women often are unaware that they have osteoporosis until it is brought to their attention, suddenly, with an unexpected and painful fracture, when they are in their fifties, sixties, or seventies. The most common way that a woman may discover that she has osteoporosis is when she breaks her wrist or hip following a minor fall. Other women, as they grow older, may find themselves losing height or developing a hunched back ("dowager's hump") or find that their clothes no longer fit properly. This occurs when the vertebrae - the bones that make up the spine - become so weak that a simple daily movement such as coughing or lifting may cause them to collapse. This, too, can be extremely painful. Osteoporotic fractures, particularly those of the hip and spine, often lead to significant pain and disability. Although a fracture of the wrist often will heal with little deformity, a patient may not make a complete recovery from a fractured hip or spine. The resulting disability may affect the individual's ability to work and care for her family and may be so severe that she becomes dependent on family members or community caregivers.
After the menopause, almost all women are at increased risk of osteoporosis, although certain lifestyle, hereditary, or medical factors may increase this risk. This web page will help you identify the factors that may increase your risk of developing osteoporosis and the lifestyle changes that you can make to reduce this risk.
The cause of osteoporosis is not known. However, many factors are known to contribute to the development of this condition. When you think about the cause of osteoporosis it might help you to think of a jigsaw puzzle. There are many "pieces", or factors, to the puzzle. Some of these factors include:
-diet (especially calcium intake)
-exercise
-heredity
-hormones (particularly the female sex hormone estrogen)
-intestinal and kidney function
-overall health
-medical conditions and their treatments
*Each of these factors is a "piece of the puzzle" and is important to consider.
Certain people are more likely to develop osteoporosis than others. Factors that increase your
likelihood of developing osteoporosis are called "risk factors." If you have several risk factors,
that does not mean you will definitely develop osteoporosis or get a fracture, but rather your
chances of this happening are increased. On the other hand, some individuals who do not
appear to be at risk might end up with a fracture caused by osteoporosis. Several risk factors
have been identified. Women are about four times more likely to develop osteoporosis than
men. One reason is that women generally have thinner, lighter bones throughout life than men;
another reason is the rapid loss of bone that occurs at menopause and thereafter. Also, women
have a longer life span than men. The longer you live, the higher the likelihood of developing
osteoporosis. Age has been associated with osteoporosis because after peak bone mass is
reached, it gradually begins to decline. The more years that pass, the greater the loss of bone.
As a rough estimate, we lose 10% of our bone mass per decade of life after the age of thirty.
Menopause Women are at greater risk of developing osteoporosis than men. Of course, this does not mean that every woman will get osteoporosis, it simply means that osteoporosis occurs with greater frequency in women.
After the menopause, the ovaries stop producing the female hormone, estrogen. Estrogen is important for maintaining bonestrength. Without estrogen, bone loses calcium, one of its most important components. If you have had a particularly early menopause (before the age of 45, for example), your bones will have lost the important benefits of estrogen much earlier than usual, and it is likely that you are at an increased risk of developing osteoporosis.
The amount of estrogen in your body may also decrease due to reasons other than the menopause. If, for example, you have had a hysterectomy, and if your ovaries were removed, you are likely to have a very low estrogen level, and your risk of osteoporosis may be higher than normal.
Women whose periods have stopped for a long time, for any reason other than pregnancy, may have weaker bones as a result of having a deficiency of estrogen during that time. If you are still having regular periods, your ovaries probably are producing enough estrogen.
Smoking/Alcohol We all know about the health risks associated with smoking and a high alcohol intake. In addition to all the other problems that they can cause, smoking and a regular high alcohol intake can also interfere with the body's ability to maintain normal, healthy bones. Smoking speeds up the rate at which you lose bone, which makes you much more likely to suffer from osteoporosis.
Lack of Exercise The strength of our bones is determined partly by the physical demands placed on the skeleton. Similar to the way in which our muscles weaken if they are not used, bones need a certain amount of exercise to stay strong and healthy. People who are confined to bed or a wheelchair, or who lead a particularly inactive lifestyle, have a higher risk of developing osteoporosis.
Diet We are what we eat! To a certain extent this is true, particularly with regard to the maintenance of healthy bones. An inadequate intake of calcium in the diet deprives the body of the raw materials it needs to maintain bone mass and strength. Anyone who is nutritionally deficient or who has a low dietary intake of calcium-rich foods, such as dairy products and fresh vegetables, may be at increased risk of developing osteoporosis. In addition, as we grow older, the amount of calcium that our body needs changes. For example, a higher calcium intake is recommended for children, adolescents, women who are breast-feeding, and postmenopausal women.
As we grow older, we should be more conscious about what we eat. This is because, as we age, the ability of our digestive system to absorb the important vitamins and minerals contained in the food that we eat is reduced. Therefore, it is often easy for older persons to become malnourished, even if they think that they are eating properly.
Family History Susceptibility to fracture may be, in part, hereditary. Young women whose mothers have a history of vertebral fractures also seem to have reduced bone mass.
Medical History Some medical problems, for example, an overactive thyroid gland, liver disease, or anorexia nervosa, can cause osteoporosis. In addition, certain medications, such as steroids, when used for a long time can have a detrimental effect on bones. Steroids often are used in the long-term treatment of asthma and conditions like rheumatoid arthritis. If you have been taking steroids, you should discuss this with your physician.
Previous Fracture A previous broken bone, particularly of the hip, wrist, or spine, that has resulted from minor trauma may be an indication that your bones are already weak. If this is the case, the likelihood of breaking additional bones, or possibly the same bone, could be increased. Your physician will advise you on how to try to lower your chances of having more fractures.
Low Body Weight Petite women are at greater risk because they have less bone to lose than larger, big-boned women. Women who are thin have a greater risk of fractures than obese women. (This is not to suggest that being overweight is a good idea. Obese women may have other equally serious medical problems, such as diabetes or high blood pressure. Both overweight and underweight individuals should try to attain their desirable weight).
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