Bone density is a medical term normally referring to the amount of mineral matter per square centimeter of bone

Bone

Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

s. Bone density (or BMD) is used in clinical medicine as an indirect indicator of osteoporosis

Osteoporosis

Osteoporosis is a disease of bones that leads to an increased risk of fracture. In osteoporosis the bone mineral density is reduced, bone microarchitecture is deteriorating, and the amount and variety of proteins in bone is altered...

 and fracture

Fracture

A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...

 risk.

This medical bone density is not the true physical

Physics

Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

 "density" of the bone, which would be computed as mass per volume. It is measured by a procedure called densitometry

Densitometry

Densitometry is the quantitative measurement of optical density in light-sensitive materials, such as photographic paper or film, due to exposure to light...

, often performed in the radiology

Radiology

Radiology is a medical specialty that employs the use of imaging to both diagnose and treat disease visualized within the human body. Radiologists use an array of imaging technologies to diagnose or treat diseases...

 or nuclear medicine departments of hospital

Hospital

A hospital is a health care institution providing patient treatment by specialized staff and equipment. Hospitals often, but not always, provide for inpatient care or longer-term patient stays....

s or clinic

Clinic

A clinic is a health care facility that is primarily devoted to the care of outpatients...

s. The measurement is painless and non-invasive and involves low radiation

Radiation

In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

 exposure. Measurements are most commonly made over the lumbar spine and over the upper part of the hip. The forearm may be scanned if the hip and lumbar spine are not accessible. Average density is around 1500 kg m⁻³

There is a statistical

Statistics

Statistics is the study of the collection, organization, analysis, and interpretation of data. It deals with all aspects of this, including the planning of data collection in terms of the design of surveys and experiments....

 association between poor bone density and higher probability of fracture. Fractures of the legs and pelvis

Pelvis

In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...

 due to falls are a significant public health

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" . It is concerned with threats to health based on population 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 women for osteoporosis risk and to identify those who might benefit from measures to improve bone strength.

Terms

Results are often reported in 3 terms:

1. Measured areal density in g cm⁻²
2. z-score, the number of standard deviations above or below the mean for the patient's age, sex and ethnicity
3. 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

Limitations

Use of BMD has several limitations.

1. Measurement can be affected by the size of the patient, the thickness of tissue overlying the bone, and other factors extraneous to the bones.
2. Bone density is a proxy measurement for bone strength, which is the resistance to fracture and the truly significant characteristic. Although the two are usually related, there are some circumstances in which bone density is a poorer indicator of bone strength.
3. Reference standards for some populations (e.g., children) are unavailable for many of the methods used.
4. Crushed vertebrae can result in falsely high bone density so must be excluded from analysis.

Candidates

The National Osteoporosis Foundation recommends BMD testing for the following individuals:

• All women aged 65 and older regardless of risk factors
• Younger postmenopausal
  Menopause
  Menopause is a term used to describe the permanent cessation of the primary functions of the human ovaries: the ripening and release of ova and the release of hormones that cause both the creation of the uterine lining and the subsequent shedding of the uterine lining...
  
   women with one or more risk factors.
• Postmenopausal women who present with fractures (to confirm the diagnosis and determine disease severity).
• Estrogen
  Estrogen
  Estrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...
  
   deficient women at clinical risk for osteoporosis.
• Individuals with vertebral abnormalities.
• Individuals receiving, or planning to receive, long-term glucocorticoid (steroid
  Steroid
  A steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone.The core...
  
  ) therapy.
• Individuals with primary hyperparathyroidism
  Hyperparathyroidism
  Hyperparathyroidism is overactivity of the parathyroid glands resulting in excess production of parathyroid hormone . The parathyroid hormone regulates calcium and phosphate levels and helps to maintain these levels...
  
  .
• Individuals being monitored to assess the response or efficacy of an approved osteoporosis drug therapy.
• Individuals with a history of eating disorders

Types of tests

While there are many different types of BMD tests, all are non-invasive. Most tests differ in which bones are measured to determine the BMD result.

These tests include:

• Dual-energy X-ray absorptiometry (DXA or DEXA)
• Quantitative computed tomography (QCT)
• Qualitative ultrasound
  Qus
  Qus is a city in the modern Qena Governorate, Egypt, located on the east bank of the Nile. Its modern name is one of many borrowings in Egyptian Arabic from Coptic, the last living phase of Ancient Egyptian...
  
   (QUS)
• Single photon absorptiometry  (SPA)
• Dual photon absorptiometry  (DPA)
• Digital X-ray radiogrammetry
  Digital X-ray Radiogrammetry
  Digital X-ray Radiogrammetry is a means of measuring bone mineral density . Digital X-ray radiogrammetry is based on the old technique of radiogrammetry. In DXR the cortical thickness of the three middle metacarpal bones in the hand is measured in a digital X-ray image by a computer and is through...
  
   (DXR)
• Single energy X-ray absorptiometry (SEXA)

DEXA is currently the most widely used, but ultrasound

Ultrasound

Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...

 has been described as a more cost-effective approach to measure bone density.

The test works by measuring a specific bone

Bone

Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

 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 risk for fractures and the stage of osteoporosis in an individual.

Average bone mineral density = BMC / W [g/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.

T-score

The T-score is a comparison of a patient's BMD to that of a healthy thirty-year-old of the same sex and ethnicity. This value is used in post-menopausal women and men over aged 50 because it better predicts risk of future fracture. The criteria of the World Health Organization

World Health Organization

The World Health Organization is a specialized agency of the United Nations that acts as a coordinating authority on international public health. Established on 7 April 1948, with headquarters in Geneva, Switzerland, the agency inherited the mandate and resources of its predecessor, the Health...

 are:

• Normal is a T-score of -1.0 or higher
• Osteopenia
  Osteopenia
  Osteopenia is a condition where bone mineral density is lower than normal. It is considered by many doctors to be a precursor to osteoporosis. However, not every person diagnosed with osteopenia will develop osteoporosis...
  
   is defined as between -1.0 and -2.5
• Osteoporosis
  Osteoporosis
  Osteoporosis is a disease of bones that leads to an increased risk of fracture. In osteoporosis the bone mineral density is reduced, bone microarchitecture is deteriorating, and the amount and variety of proteins in bone is altered...
  
   is defined as -2.5 or lower, meaning a bone density that is two and a half standard deviations below the mean of a thirty year old man/woman.

Hip fractures per 1000 patient-years

WHO category	Age 50-64	Age > 64	Overall
Normal	5.3	9.4	6.6
Osteopenia Osteopenia Osteopenia is a condition where bone mineral density is lower than normal. It is considered by many doctors to be a precursor to osteoporosis. However, not every person diagnosed with osteopenia will develop osteoporosis...	11.4	19.6	15.7
Osteoporosis Osteoporosis Osteoporosis is a disease of bones that leads to an increased risk of fracture. In osteoporosis the bone mineral density is reduced, bone microarchitecture is deteriorating, and the amount and variety of proteins in bone is altered...	22.4	46.6	40.6

Z-score

The Z-score is the number of standard deviations a patient's BMD differs from the average BMD of their age, sex, and ethnicity. This value is used in premenopausal women, men under the age of 50, and in children.