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Bone density, or bone mineral density, is the amount of
bone mineral Bone mineral (also called inorganic bone phase, bone salt, or bone apatite) is the inorganic component of bone tissue. It gives bones their compressive strength. Bone mineral is formed predominantly from carbonated hydroxyapatite with lower cry ...
in
bone tissue 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, a ...
. 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 Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...
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 Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
in the
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
sense), although clinically it is measured by proxy according to optical density per square centimetre of
bone A bone is a Stiffness, rigid Organ (biology), organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red blood cell, red and white blood cells, store minerals, provid ...
surface upon
imaging Imaging is the representation or reproduction of an object's form; especially a visual representation (i.e., the formation of an image). Imaging technology is the application of materials and methods to create, preserve, or duplicate images. ...
. 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 fracture risk. It is the most common reason for a broken bone ...
and fracture risk. It is measured by a procedure called
densitometry Densitometry is the quantitative measurement of optical density in light-sensitive materials, such as photographic paper or photographic film, due to exposure to light. Overview Optical density is a result of the darkness of a developed picture ...
, 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 radiat ...
or
nuclear medicine Nuclear medicine or nucleology is a medical specialty involving the application of radioactive substances in the diagnosis and treatment of disease. Nuclear imaging, in a sense, is " radiology done inside out" because it records radiation emi ...
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 emerge ...
s or
clinic A clinic (or outpatient clinic or ambulatory care clinic) is a health facility that is primarily focused on the care of outpatients. Clinics can be privately operated or publicly managed and funded. They typically cover the primary care needs ...
s. 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 The lumbar vertebrae are, in human anatomy, the five vertebrae between the rib cage and the pelvis. They are the largest segments of the vertebral column and are characterized by the absence of the foramen transversarium within the transverse p ...
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 association between poor bone density and higher probability of fracture. Fractures of the legs and pelvis due to falls are a significant
public health Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals". Analyzing the det ...
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 The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordates ...
abnormalities. * individuals receiving, or planning to receive, long-term glucocorticoid ( steroid) therapy. * individuals with primary
hyperparathyroidism Hyperparathyroidism is an increase in parathyroid hormone (PTH) levels in the blood. This occurs from a disorder either within the parathyroid glands (primary hyperparathyroidism) or as response to external stimuli (secondary hyperparathyroidism). ...
. * individuals being monitored to assess the response or efficacy of an approved osteoporosis drug therapy. * when
androgen deprivation therapy Androgen deprivation therapy (ADT), also called androgen suppression therapy, is an antihormone therapy whose main use is in treating prostate cancer. Prostate cancer cells usually require androgen hormones, such as testosterone, to grow. ADT red ...
is being planned for prostate cancer. * 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 fracture risk. It is the most common reason for a broken bone ...
or
osteopenia Osteopenia, known as "low bone mass" or "low bone density", is a condition in which bone mineral density is low. Because their bones are weaker, people with osteopenia may have a higher risk of fractures, and some people may go on to develop osteop ...
. 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 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 ...
: the number of standard deviations above or below the mean for the patient's age, sex and ethnicity. *
T-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 mea ...
: 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 Dual-energy X-ray absorptiometry (DXA, or DEXA) is a means of measuring bone mineral density (BMD) using spectral imaging. Two X-ray beams, with different energy levels, are aimed at the patient's bones. When soft tissue absorption is subtracted ...
(DXA or DEXA) *
Trabecular bone score The trabecular bone score is a measure of bone texture correlated with bone microarchitecture and a marker for the risk of osteoporosis. Introduced in 2008, its main projected use is alongside measures of bone density in better predicting fracture r ...
* Dual X-ray Absorptiometry and Laser (DXL) *
Quantitative computed tomography Quantitative computed tomography (QCT) is a medical technique that measures bone mineral density (BMD) using a standard X-ray Computed Tomography (CT) scanner with a calibration standard to convert Hounsfield Units (HU) of the CT image to bone min ...
(QCT) * Quantitative ultrasound (QUS) * Single photon absorptiometry (SPA) * Dual photon absorptiometry (DPA) *
Digital X-ray radiogrammetry Digital X-ray radiogrammetry is a method for measuring bone mineral density (BMD). Digital X-ray radiogrammetry is based on the old technique of radiogrammetry. In DXR, the cortical thickness of the three middle metacarpal bones of the hand is ...
(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 Stiffness, rigid Organ (biology), organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red blood cell, red and white blood cells, store minerals, provid ...
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 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 mea ...
and the
Z-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 ...
. 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 Dual-energy X-ray absorptiometry (DXA, or DEXA) is a means of measuring bone mineral density (BMD) using spectral imaging. Two X-ray beams, with different energy levels, are aimed at the patient's bones. When soft tissue absorption is subtracted ...
-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 Dysplasia is any of various types of abnormal growth or development of cells (microscopic scale) or organs (macroscopic scale), and the abnormal histology or anatomical structure(s) resulting from such growth. Dysplasias on a mainly microscopic ...
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 The World Health Organization (WHO) is a specialized agency of the United Nations responsible for international public health. The WHO Constitution states its main objective as "the attainment by all peoples of the highest possible level of h ...
are: * Normal is a T-score of −1.0 or higher *
Osteopenia Osteopenia, known as "low bone mass" or "low bone density", is a condition in which bone mineral density is low. Because their bones are weaker, people with osteopenia may have a higher risk of fractures, and some people may go on to develop osteop ...
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 fracture risk. It is the most common reason for a broken bone ...
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 deviations 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 Glucocorticoids (or, less commonly, glucocorticosteroids) are a class of corticosteroids, which are a class of steroid hormones. Glucocorticoids are corticosteroids that bind to the glucocorticoid receptor that is present in almost every vertebr ...
therapy,
hyperparathyroidism Hyperparathyroidism is an increase in parathyroid hormone (PTH) levels in the blood. This occurs from a disorder either within the parathyroid glands (primary hyperparathyroidism) or as response to external stimuli (secondary hyperparathyroidism). ...
, or
alcoholism Alcoholism is, broadly, any drinking of alcohol (drug), alcohol that results in significant Mental health, mental or physical health problems. Because there is disagreement on the definition of the word ''alcoholism'', it is not a recognize ...
.


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 to ...
and
vitamin D Vitamin D is a group of fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and many other biological effects. In humans, the most important compounds in this group are vitamin D3 (c ...
. 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 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
estrogen Estrogen or oestrogen is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal acti ...
s (e.g.,
estradiol Estradiol (E2), also spelled oestradiol, is an estrogen steroid hormone and the major female sex hormone. It is involved in the regulation of the estrous and menstrual female reproductive cycles. Estradiol is responsible for the development of f ...
,
conjugated estrogens Conjugated estrogens (CEs), or conjugated equine estrogens (CEEs), sold under the brand name Premarin among others, is an estrogen medication which is used in menopausal hormone therapy and for various other indications. It is a mixture of the ...
), selective estrogen receptor modulators (e.g.,
raloxifene Raloxifene, sold under the brand name Evista among others, is a medication used to prevent and treat osteoporosis in postmenopausal women and those on glucocorticoids. For osteoporosis it is less preferred than bisphosphonates. It is also used to ...
, bazedoxifene), and
bisphosphonate Bisphosphonates are a class of drugs that prevent the loss of bone density, used to treat osteoporosis and similar diseases. They are the most commonly prescribed drugs used to treat osteoporosis. They are called bisphosphonates because they ...
s (e.g.,
alendronic acid Alendronic acid, sold under the brand name Fosamax among others, is a bisphosphonate medication used to treat osteoporosis and Paget's disease of bone. It is taken by mouth. Use is often recommended together with vitamin D, calcium supplementat ...
,
risedronic acid Risedronic acid, often used as its sodium salt risedronate sodium, is a bisphosphonate. It slows down the cells which break down bone. It's used to treat or prevent osteoporosis, and treat Paget's disease of bone. It is taken by mouth. It was ...
), can also be used to improve or maintain bone density.
Tobacco use Tobacco smoking is the practice of burning tobacco and ingesting the resulting smoke. The smoke may be inhaled, as is done with cigarettes, or simply released from the mouth, as is generally done with pipes and cigars. The practice is believed ...
and excessive
alcohol consumption An alcoholic beverage (also called an alcoholic drink, adult beverage, or a drink) is a drink that contains ethanol, a type of alcohol that acts as a drug and is produced by fermentation of grains, fruits, or other sources of sugar. The cons ...
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 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. The concept of h ...
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