If more bone calcium is absorbed than is replaced, the density or the mass of the bone is reduced. The bone becomes progressively weaker, increasing the risk that it may break.

Osteoporosis means "porous bone." This condition develops when bone is no longer replaced as quickly as it is removed. More than 1.5 million fractures occur related to osteoporosis each year. Most people are unaware that they have osteoporosis until a fracture occurs.

The exact medical cause for osteoporosis is not known, but a number of factors are known to cause osteoporosis: aging, physical inactivity, reduced levels of estrogen, heredity, excessive Cortisone or thyroid hormone, smoking, and excessive alcohol intake.

Although osteoporosis will occur in all persons as they age, the rate of progression and the effects can be modified with proper early diagnosis and treatment.

During growth and young adulthood, adequate calcium nutrition and vitamin D and regular weight-bearing exercises, such as walking, jogging, and dancing three to four hours a week, build strong bones and are investments in future bone health. Smoking and consuming excessive amounts of alcohol should be avoided because they increase bone loss. As people age, appropriate intake of calcium and vitamin D and regular exercise, as well as avoidance of smoking and excessive alcohol use, are necessary to reduce loss of bone mass.

Family doctors working with your orthopaedic surgeon can evaluate whether your bone density has been reduced, and can evaluate the cause for the reduction. Early treatment for osteoporosis is the most effective way to reduce bone loss and prevent fractures. However, treatment programs after a fracture also are of value and may help to prevent future fractures.

Current treatment methods can reduce bone loss, but there are no proven methods of restoring lost bone. Building bones through adequate calcium intake and exercise when you are young is an investment that will pay off years later with a reduced risk of hip and other fractures.

The loss of bone tends to occur most in the spine, lower forearm above the wrist, and upper femur or thigh-the site of hip fractures. Spine fractures, wrist fractures, and hip fractures are common injuries in older persons.

A gradual loss of bone mass, generally beginning about age 35, is a fact of life for everyone. After growth is complete, women ultimately lose 30 to 50 percent of their bone density, and men lose 20 to 30 percent.

Women lose bone calcium at an accelerated pace once they go through menopause. Menstrual periods cease because a woman's body produces less estrogen hormone, which is important for the maintenance of bone mass or bone strength. Your family doctor or gynecologist may evaluate and recommend a treatment program of estrogen replacement therapy, calcitonin or other medications. To be most effective, the treatment program should begin at menopause.

Osteoporosis is a major health threat for more than 28 million Americans. In the U.S., eight million women and two million men already have osteoporosis. More than 18 million Americans have low bone mass placing them at increased risk for osteoporosis.

Here are some warning signs that your bones are in trouble making you susceptible to osteoporosis later in life:

• You break a bone. Breaking a bone as an adult does not always mean you have osteoporosisbut it could be a warning sign that your bones are weak.

• You have a family history of osteoporosis or hip fractures. If someone in your family has osteoporosis, you have a 60 to 80 percent chance of getting the condition, too. And if your mother has broken a hip, your chances double for having a hip fracturea strong indicator of bone weakness.

• You do not get enough calcium. People need to incorporate calcium throughout their lives to prevent osteoporosis. Milk and other dairy products can provide a major boost of bone-building calcium to most diets. Leafy green vegetables are another good source of calcium.

• You smoke or drink excessive amounts of alcohol. If you smoke, you double your risk of suffering an osteoporotic fracture. Researchers have found that smoking reduces your ability to absorb calcium. Thats because cigarette smoke is thought to damage bone cells and prevent new bone growth. People who drink too much alcohol on a regular basis are prone to bone loss and fractures due to poor nutrition as well as increased risk of falling.

• You are thinner than normal. Low body weight is an important risk factor for osteoporosis and increased risk of fractures. Being too thin means you have less padding of muscle and fat to protect your bones which are more vulnerable to injury. Frequent dieting can be risky because when you lose weight, you also lose fat, muscle and bone density. Fat and muscle may eventually come back, but bone could be gone forever.

• You have chronic medical problems. If you suffer from rheumatoid arthritis, hyperthyroidism (excessive production of thyroid hormones), hyperparathyroidism (a condition caused by excessive amounts of parathyroid hormone), diabetes or liver disease, your chances of having osteoporosis increase.

• You have persistent back pain. Back pain that will not quit could be a sign that you have a spinal fracture. Spinal fractures occur when bones in your back become so weak that they fracture and collapse. This can happen without warning even during ordinary activities such as bending over to pick up a newspaper.

• You are shorter than you used to be. If you fracture a bone in your spine, it could collapse onto itself causing you to shrink. Multiple fractures also can cause the spine to form a curve causing the disfigurement known as a dowagers hump. However, not all height loss is caused by osteoporosis. As you age, the disks in your back lose their elasticity causing your frame to shift downward. It is not uncommon to lose anywhere from 1/2 - to 1-inch in height between the ages 60 and 80.

• Osteoporosis causes bones to thin and weaken. With osteoporosis, bone mineralsmainly calciumare lost, causing bones to become so brittle that they could result in a wrist, hip or spine fracture.

  No matter what your age, you can take steps to prevent bone loss. Here are some preventative tips developed by the American Academy of Orthopaedic Surgeons that you should follow during the stages of your life.

  10 and 20 years old:

  Start preventing osteoporosis now by putting calcium deposits in your "bone bank." To make your bones as strong and healthy as possible, you need 1,300 mg of calcium each day. Instead of soft drinks, choose milk-at least three 8-oz. glasses each day. You also can choose other foods-dairy products like cheese or green leafy vegetables are other great sources of calcium.

  For females, remember that having regular menstrual periods is important to prevent osteoporosis. See a physician if you are older than 16 and have never menstruated or if you have begun to menstruate but have noticed changes in your cycle.

  20 and 35 years old:

  Though you are not forming new bone as readily as before, your bones will reach their peak strength during these years. It is important to get adequate calcium and exercise to help achieve peak bone density. You will need at least 1,000 mg of calcium each day. Engaging in weight-bearing exercises like jogging or walking also will help make your bones stronger.

  35 and 50 years old:

  You may have begun to gradually lose bone. At this stage in your life, getting enough calcium (1,000 mg each day) and exercise are crucial to keep bone loss to a minimum. Most women enter menopause between the ages of 42 and 55. If your period becomes irregular or if you develop signs of menopause, talk with your physician. You also might want to ask about bone density screening examinations.

  Over 50 years old:

  For women who have gone through menopause, they may be losing bone at a rate of 1 to 6 percent per year. Ask your physician about the various therapies on the market, and if it is appropriate for you.

  Adequate calcium intake and exercise still are important. You should be getting 1,000 mg of calcium each day. If you rarely get out in the sun, vitamin D also may be recommended. Try to walk, jog or perform a resistance workout for at least 20 minutes, three times a week.

• The history of bone density measurement goes back to the 1940s. Attempts to measure bone density at that time utilized plain radiographs. Since demineralization of bone is not apparent on a plain X-ray until about 40 percent of the bone has been lost, different methods of bone density determination were developed by measuring size or shape of different anatomical structures. Grading systems were developed based on the appearance of trabecular patterns. The Singh Index was based on the trabecular pattern in the proximal femur.

  Radiographs were graded 1 through 6 based on the disappearance of the normal trabecular pattern usually seen in the femoral neck. Studies showed a correlation between a Singh Index of less than 3 and fractures of the hip, wrist and spine. Radiographic absorptiometry was developed during the late 1980s as an easy way to determine BMD with plain X-rays. An X-ray of the hand is taken incorporating an aluminum reference wedge. This film is then analyzed and the density of the bone compared to the reference wedge. The correlation between the RA values and the actual density is excellent.

  Single photon absorptiometry

  In the early 1960s, a new method of determining bone density using a radioactive isotope was developed-single photon absorptiometry (SPA). A single energy photon beam was passed through bone and soft tissue to a detector. The amount of mineral in the path could then be quantified. The amount of soft tissue the beam had to penetrate needed to be small so the distal radius was usually utilized. SPA measurements are accurate and the test usually takes about 10 minutes. The radioactive source gradually decays, however, and must be replaced after a time.

  Dual photon absorptiometry

  The principle of dual photon absorptiometry (DPA) is the use of a photon beam that has two distinct energy peaks. One energy peak will be more absorbed by soft tissue and the other by bone. The soft tissue component then can be mathematically subtracted and the BMD thus determined. For the first time, BMD of the spine and proximal femur could be determined. Although accurate for predicting fracture risk, precision is poor due to decay of the isotope, and the machine has limited usefulness in monitoring BMD changes over time.

  Dual-energy X-ray absorptiometry

  Dual-energy X-ray absorptiometry (DXA) works in a similar fashion to DPA, but uses an X-ray source instead of an isotope. This is superior because the radiation source does not decay and the energy stays constant over time. Scan times are much shorter than with DPA and radiation dose is very low. The skin dose for an AP spine scan is in the range of 3 mrem. DXA scans are extremely precise. Precision in the range of 1 percent to 2 percent has been reported. DXA can be used as an accurate and precise method to monitor changes in bone density in patients undergoing treatments. The first generation DXAs used a pencil beam type scanner. The X-ray source moves with a single detector. Second generation machines use a fan beam scanner that incorporates a group of detectors instead of a single one. These machines are considerably faster and produce a higher resolution image. DXA has become the "gold standard" for BMD measurement today.

  Quantitative computed tomography

  Measurement of BMD by quantitative computed tomography (QCT) uses most standard CT scanners with software packages that allow them to determine bone density in the hip or spine. This technique is unique in that it provides for true three-dimensional imaging and reports BMD as true volume density measurements. The advantage of QCT is its ability to isolate the area of interest from surrounding tissues. It can, therefore, localize an area in a vertebral body of only trabecular bone leaving out the elements most affected by degenerative change and sclerosis. The QCT radiation dose is about 10 times that of DXA and QCT tests may be more expensive than DXA.

  Peripheral bone density testing

  Lower cost portable devices that determine bone mineral density at peripheral sites, such as the radius, the phalanges or the calcaneus are being utilized more and more for osteoporosis screening. The advantage of these devices is the ability to bring bone density assessment to a large portion of the population who otherwise would not be able to have the test. These machines cost considerably less than those that measure the hip and spine.

  The Norland pDXA is a true dual energy X-ray device that is dedicated to measure BMD in the distal radius. It is portable and weighs about 59 pounds.

  Hologic makes a quantitative ultrasound (QUS) unit, called the Sahara. This device measures the speed of sound and the attenuation of the sound signal through the calcaneus. It gives an "estimated" BMD. In two recent prospective studies of postmenopausal women over age 65, QUS predicted hip fractures as well as BMD measurements of the hip by DXA.

  One of the problems with peripheral testing is that only one site is tested and low bone density in the hip or spine may be missed. This results because of discordance of bone density between different skeletal sites. Although these peripheral machines are considered accurate, there have been doubts raised about their precision. The reproducibility of peripheral machines may not be good enough to monitor patients undergoing treatment for osteoporosis.

  Discordance in BMD among various skeletal sites is more common in the years just following menopause. BMD may be normal at one site and low at another site. In these early postmenopausal years, bone density in the spine decreases first because the turnover in this highly trabecular bone is higher than other skeletal sites. Bone density at various skeletal sites begins to coincide at about age 70.

  In early post menopausal women, therefore, up to the age of about 65, the most accurate site to measure is probably the spine. In older women over the age of 65, the concordance of skeletal sites is much closer and it may not make much difference which site is measured. Caution must be used in interpreting spine scans in elderly patients because of degenerative changes falsely elevating the BMD values. Measurements are, however, mostly site specific and the most accurate predictor of fracture risk at any site is a bone density measurement at that site.

  The peripheral devices are, at present, good screening devices because of their portability, availability and lower cost, but patients may still need central testing, even in light of a normal peripheral test. The following can be used as a guide to which patients with a normal peripheral test should be tested centrally:

  Bone density report interpretation

  The main purpose of obtaining a bone density test is determining fracture risk. The bone mineral density correlates very well with risk of fracture. It is more powerful in predicting fractures than cholesterol is in predicting myocardial infarction or blood pressure in predicting stroke.

  1. Postmenopausal patients not on hormone replacement therapy (HRT), concerned about osteoporosis and concerned about prevention, who would consider HRT, bisphosphonates or SERMs, if a low bone mass is discovered  2. Maternal history of hip fracture, smoking, tallness (more than 5'7") or thinness (less than 125 pounds)  3. Patients on medications associated with bone loss  4. Patients with secondary conditions associated with low bone mass (hyperthyroidism, posttransplantation, malabsorption, hyperparathyroidism, alcoholism, etc.  5. Patients found to have high urinary collagen crosslinks (NTx, etc.) 6. History of previous fragility fracture

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• The T-score is the number of standard deviations (SD) above or below the young adult mean. The young adult mean is the expected normal value for the patient compared to others of the same sex and ethnicity in a reference population the manufacturer builds into the DXA software. It is approximately what the patient should have been at their peak bone density at about age 20.

  As a general rule, for every SD below normal the fracture risk doubles. Thus, a patient with a BMD of 1 SD below normal (a T-score of - 1) has 2 times the risk of fracture as a person with a normal BMD. If the T-score is -2 the risk of fracture is 4 times normal. A T-score of -3 is 8 times the normal fracture risk. Patients with a high risk can then be treated and future fractures prevented.

  Different manufactures of DXA equipment report the hip BMD differently. In LunarTM machines, the average value for the femoral neck (designated as "Neck" in the report) is usually the most accurate for the hip. In HologicTM machines, the "Total" value should be used. In the spine, the most accurate T-score is the average of the first four lumbar vertebrae (L1-L4) in all manufactures of DXA machines.

  There are other factors that determine fracture risk such as a person's eyesight, balance, leg strength, and conditions that might cause them to fall more easily. Age itself is an independent risk factor for fracture independent of bone density. Anyone with osteoporosis that has had a previous fragility fracture is considered to have severe osteoporosis and has a very high risk for future fractures.

  Z-score

  The Z-score is the number of standard deviations the patient's bone density is above or below the values expected for the patient's age. By comparing the patient to the expected BMD for his or her own age, the Z score can help classify the type of osteoporosis. Primary osteoporosis is age-related where no secondary causes are found. Secondary osteoporosis occurs when underlying agents or conditions induce bone loss. Some common causes of secondary osteoporosis are thyroid or parathyroid abnormalities, malabsorption, alcoholism, smoking and the use of certain medications especially corticosteriods.

  A Z-score lower then -1.5 should make you suspicious of secondary osteoporosis. If secondary causes are suspected, the patient should usually undergo further work up including laboratory testing to find out if there is an underlying reason for the osteoporosis. This is important because treating the underlying condition may be necessary to correct the low bone density.