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Bone density, or bone mineral density, is the amount of bone mineral in bone tissue. The
concept Concepts are defined as abstract ideas. They are understood to be the fundamental building blocks of the concept behind principles, thoughts and beliefs. They play an important role in all aspects of cognition. As such, concepts are studied by s ...
is of mass of mineral per
volume Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). Th ...
of bone (relating to density in the physics sense), although clinically it is measured by proxy according to
optical density Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative lo ...
per square centimetre of
bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...
surface upon imaging. Bone density measurement is used in clinical medicine as an indirect indicator of
osteoporosis Osteoporosis is a systemic skeletal disorder characterized by low bone mass, micro-architectural deterioration of bone tissue leading to bone fragility, and consequent increase in Bone fracture, fracture risk. It is the most common reason for a ...
and
fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displ ...
risk. It is measured by a procedure called densitometry, often performed in the
radiology Radiology ( ) is the medical discipline that uses medical imaging to diagnose diseases and guide their treatment, within the bodies of humans and other animals. It began with radiography (which is why its name has a root referring to radiati ...
or nuclear medicine departments of
hospital A hospital is a health care institution providing patient treatment with specialized health science and auxiliary healthcare staff and medical equipment. The best-known type of hospital is the general hospital, which typically has an emergenc ...
s or clinics. The measurement is painless and non-invasive and involves low
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
exposure. Measurements are most commonly made over the lumbar spine and over the upper part of the
hip In vertebrate anatomy, hip (or "coxa"Latin ''coxa'' was used by Celsus in the sense "hip", but by Pliny the Elder in the sense "hip bone" (Diab, p 77) in medical terminology) refers to either an anatomical region or a joint. The hip region is ...
. The forearm may be scanned if the hip and lumbar spine are not accessible. There is a
statistical Statistics (from German: ''Statistik'', "description of a state, a country") is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics to a scientific, industria ...
association between poor bone density and higher probability of fracture. Fractures of the legs and
pelvis The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The ...
due to falls are a significant public health problem, especially in elderly women, leading to much medical cost, inability to live independently and even risk of death. Bone density measurements are used to screen people for osteoporosis risk and to identify those who might benefit from measures to improve bone strength.


Testing

Bone density tests are not recommended for people without risk factors for weak bones, which is more likely to result in unnecessary treatment rather than discovery of a true problem.


Indications for testing

The risk factors for low bone density and primary considerations for a bone density test include: *females age 65 or older. *males age 70 or older. *people over age 50 with: **previous bone fracture from minor trauma. **rheumatoid arthritis. **low body weight. **a parent with a hip fracture. * individuals with vertebral abnormalities. * individuals receiving, or planning to receive, long-term glucocorticoid (
steroid A steroid is a biologically active organic compound with four rings arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes that alter membrane fluidity; and a ...
) therapy. * individuals with primary hyperparathyroidism. * individuals being monitored to assess the response or efficacy of an approved osteoporosis drug therapy. * when androgen deprivation therapy is being planned for
prostate cancer Prostate cancer is cancer of the prostate. Prostate cancer is the second most common cancerous tumor worldwide and is the fifth leading cause of cancer-related mortality among men. The prostate is a gland in the male reproductive system that sur ...
. * individuals with a history of eating disorders. Other considerations that are related to risk of low bone density and the need for a test include smoking habits, drinking habits, the long-term use of corticosteroid drugs, and a vitamin D deficiency.


Conditions found

A bone density test may detect
osteoporosis Osteoporosis is a systemic skeletal disorder characterized by low bone mass, micro-architectural deterioration of bone tissue leading to bone fragility, and consequent increase in Bone fracture, fracture risk. It is the most common reason for a ...
or osteopenia. The usual response to either of these indications is consultation with a physician.


Test result terms

Results of the test are often reported in three terms: * Measured areal density in g cm−2. * Z-score: the number of standard deviations above or below the mean for the patient's age, sex and ethnicity. * T-score: the number of standard deviations above or below the mean for a healthy 30-year-old adult of the same sex and ethnicity as the patient.


Types of tests

While there are many different types of bone mineral density tests, all are non-invasive. Most tests differ according to which bones are measured to determine the test result. These tests include: * Dual-energy X-ray absorptiometry (DXA or DEXA) * Trabecular bone score * Dual X-ray Absorptiometry and Laser (DXL) * Quantitative computed tomography (QCT) * Quantitative ultrasound (QUS) *
Single photon absorptiometry Single photon absorptiometry is a measuring method for bone density invented by John R. Cameron and James A. Sorenson in 1963. History Single photon absorptiometry (SPA) was developed in 1963 by Steichen et al. In 1976, it was an important tool ...
(SPA) *
Dual photon absorptiometry Dual or Duals may refer to: Paired/two things * Dual (mathematics), a notion of paired concepts that mirror one another ** Dual (category theory), a formalization of mathematical duality *** see more cases in :Duality theories * Dual (grammatical ...
(DPA) * Digital X-ray radiogrammetry (DXR) * Single energy X-ray absorptiometry (SEXA) DXA is currently the most widely used, but quantitative ultrasound (QUS) has been described as a more cost-effective approach to measure bone density. The DXA test works by measuring a specific
bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...
or bones, usually the spine, hip, and wrist. The density of these bones is then compared with an average index based on age, sex, and size. The resulting comparison is used to determine the risk for fractures and the stage of osteoporosis (if any) in an individual. Average bone mineral density = BMC / W /cm2*BMC = bone mineral content = g/cm *W = width at the scanned line


Interpretation

Results are generally scored by two measures, the T-score and the Z-score. Scores indicate the amount one's bone mineral density varies from the mean. Negative scores indicate lower bone density, and positive scores indicate higher. Less than 0.5% of patients who underwent DXA-scanning were found to have a T- or Z-score of more than +4.0, often the cause of an unusually ''high bone mass (HBM)'' and associated with mild skeletal dysplasia and the inability to float in water.


T-score

The T-score is the relevant measure when screening for osteoporosis. It is the bone mineral density at the site when compared to the "young normal reference mean". It is a comparison of a patient's bone mineral density to that of a healthy 30-year-old. The US standard is to use data for a 30-year-old of the same sex and ethnicity, but the WHO recommends using data for a 30-year-old white female for everyone. Values for 30-year-olds are used in post-menopausal women and men over age 50 because they better predict risk of future fracture. The criteria of the World Health Organization are: * Normal is a T-score of −1.0 or higher * Osteopenia is defined as between −1.0 and −2.5 *
Osteoporosis Osteoporosis is a systemic skeletal disorder characterized by low bone mass, micro-architectural deterioration of bone tissue leading to bone fragility, and consequent increase in Bone fracture, fracture risk. It is the most common reason for a ...
is defined as −2.5 or lower, meaning a bone density that is two and a half standard deviations below the mean of a 30-year-old man/woman.


Z-score

The Z-score for bone density is the comparison to the "age-matched normal" and is usually used in cases of severe osteoporosis. This is the
standard score In statistics, the standard score is the number of standard deviations by which the value of a raw score (i.e., an observed value or data point) is above or below the mean value of what is being observed or measured. Raw scores above the mean ...
or number of
standard deviation In statistics, the standard deviation is a measure of the amount of variation or dispersion of a set of values. A low standard deviation indicates that the values tend to be close to the mean (also called the expected value) of the set, while ...
s a patient's bone mineral density differs from the average for their age, sex, and ethnicity. This value is used in premenopausal women, men under the age of 50, and in children and adolescents. It is most useful when the score is less than 2 standard deviations below this normal. In this setting, it is helpful to scrutinize for coexisting illnesses or treatments that may contribute to osteoporosis such as glucocorticoid therapy, hyperparathyroidism, or
alcoholism Alcoholism is, broadly, any drinking of alcohol that results in significant mental or physical health problems. Because there is disagreement on the definition of the word ''alcoholism'', it is not a recognized diagnostic entity. Predomina ...
.


Prevention

To prevent low bone density it is recommended to have sufficient
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...
and vitamin D. Sufficient calcium is defined as 1,000 mg per day, increasing to 1,200 mg for women above 50 and men above 70. Sufficient vitamin D is defined as 600
IUs __NOTOC__ ''Ius'' or ''Jus'' (Latin, plural ''iura'') in ancient Rome was a right to which a citizen (''civis'') was entitled by virtue of his citizenship ('' civitas''). The ''iura'' were specified by laws, so ''ius'' sometimes meant law. As on ...
per day for adults 19 to 70, increasing to 800 IUs per day for those over 71. Exercise, especially weight-bearing and resistance exercises are most effective for building bone. Weight-bearing exercise includes walking, jogging, dancing, and hiking. Resistance exercise is often accomplished through lifting weights. Other therapies, such as estrogens (e.g., estradiol, conjugated estrogens),
selective estrogen receptor modulator Selective estrogen receptor modulators (SERMs), also known as estrogen receptor agonist/antagonists (ERAAs), are a class of drugs that act on the estrogen receptor (ER). A characteristic that distinguishes these substances from pure ER agonists a ...
s (e.g., raloxifene,
bazedoxifene Bazedoxifene, used as bazedoxifene acetate, is a medication for bone problems and possibly (pending more study) for cancer. It is a third-generation selective estrogen receptor modulator (SERM). Since late 2013 it has had U.S. FDA approval for ba ...
), and bisphosphonates (e.g., alendronic acid, risedronic acid), can also be used to improve or maintain bone density. Tobacco use and excessive alcohol consumption have detrimental effects on bone density. Excessive alcohol consumption is defined as more than one standard-sized alcoholic beverage per day for women, and drinking two or more alcoholic beverages per day for men.


Genetics

Bone mineral density is highly variable between individuals. While there are many environmental factors that affect bone mineral density, genetic factors play a large role as well. Bone Mineral Density variation has been estimated to have 0.6-0.8
heritability Heritability is a statistic used in the fields of Animal husbandry, breeding and genetics that estimates the degree of ''variation'' in a phenotypic trait in a population that is due to genetic variation between individuals in that population. T ...
factor, meaning that 60-80% of its variation is inherited from parents. Because of the heritability of Bone Mineral Density, family history of fractures is considered as a risk factor for osteoporosis. Bone Mineral Density is polygenic and many of the genetic mechanisms still remain poorly understood.


Genetic Diseases Associated with Bone Mineral Density

There are several rare genetic diseases that have been associated with pathologic changes in bone mineral density. The table below summarizes these diseases:


References

{{Bone/cartilage physiology Density Skeletal system