Bone density, orbone mineral density, is the amount ofbone mineral inbone tissue. Theconcept is ofmass of mineral pervolume of bone (relating todensity in thephysics sense), althoughclinically it is measured by proxy according tooptical density per square centimetre ofbone surface uponimaging.[1] Bone density measurement is used in clinical medicine as an indirect indicator ofosteoporosis andfracture risk. It is measured by a procedure calleddensitometry, often performed in theradiology ornuclear medicine departments ofhospitals orclinics. The measurement is painless and non-invasive and involves lowradiation exposure. Measurements are most commonly made over thelumbar spine and over the upper part of thehip.[2] The forearm may be scanned if the hip and lumbar spine are not accessible.
There is astatistical association between poor bone density and higher probability of fracture. Fractures of the legs andpelvis due to falls are a significantpublic health problem, especially in elderly women, leading to substantial medical costs, inability to live independently and even risk of death.[3] Bone density measurements are used to screen people for osteoporosis risk and to identify those who might benefit from measures to improve bone strength.
A bone density test may detectosteoporosis orosteopenia.[4] The usual response to either of these indications is consultation with a physician.[4] Bone density tests are not recommended for people without risk factors for weak bones,[5][4] which is more likely to result in unnecessary treatment rather than discovery of a weakness.[4]
The risk factors for low bone density and primary considerations for a bone density test include:
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.[4]
Results of the test are reported in three forms:
While there are many types of bone mineral density tests, all are non-invasive. The tests differ according to which bones are measured to determine the test result.
These tests include:
DXA is the most commonly used testing method as of 2016[update].[7] The DXA test works by measuring a specificbone 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.
Quantitative ultrasound (QUS) has been described as a more cost-effective approach for measuring bone density, as compared to DXA.[8]
Average bone mineral density = BMC / W [g/cm2]
Results are generally scored by two measures, theT-score and theZ-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 underwentDXA-scanning were found to have a T- or Z-score of more than +4.0, often the cause of an unusuallyhigh bone mass (HBM) and associated with mild skeletaldysplasia and the inability tofloat in water.[9]
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.[10] 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.[11] Values for 30-year-olds are used in post-menopausal women and men over age 50 because they better predict risk of future fracture.[12] The criteria of theWorld Health Organization are:[13]
| WHO category | Age 50–64 | Age > 64 | Overall |
|---|---|---|---|
| Normal | 5.3 | 9.4 | 6.6 |
| Osteopenia | 11.4 | 19.6 | 15.7 |
| Osteoporosis | 22.4 | 46.6 | 40.6 |
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 thestandard score or number ofstandard 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.[12][16] 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 asglucocorticoid therapy,hyperparathyroidism, oralcoholism.
To prevent low bone density it is recommended to have sufficientcalcium andvitamin D.[17][18] Sufficient calcium is defined as 1,000 mg per day, increasing to 1,200 mg for women above 50 and men above 70.[18] Sufficient vitamin D is defined as 600IUs per day for adults 19 to 70, increasing to 800 IUs per day for those over 71.[18] 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.[19] Other therapies, such asestrogens (e.g.,estradiol,conjugated estrogens),selective estrogen receptor modulators (e.g.,raloxifene,bazedoxifene), andbisphosphonates (e.g.,alendronic acid,risedronic acid), can also be used to improve or maintain bone density.Tobacco use and excessivealcohol consumption have detrimental effects on bone density.[20][18] Excessive alcohol consumption is defined as more than onestandard-sized alcoholic beverage per day for women, and drinking two or more alcoholic beverages per day for men.[18]
Bone mineral density is highly variable between individuals. While there are many environmental factors that affect bone mineral density, genetic factors play the largest role.[7][21] Bone mineral density variation has been estimated to have 0.6–0.8heritability factor, meaning that 60–80% of its variation is inherited from parents.[22] Because of the heritability of bone mineral density, family history of fractures is considered as a risk factor for osteoporosis.[23] Bone mineral density ispolygenic and many of the genetic mechanisms remain poorly understood.[21]
There are several rare genetic diseases that have been associated with pathologic changes in bone mineral density. The table summarizes these diseases:[24][23]
| Disease | Gene affected | Inheritance | Source |
|---|---|---|---|
| Osteogenesis imperfecta | COLIA1 | Autosomal recessive | [24][23] |
| Osteogenesis imperfecta | COLIA2 | Autosomal recessive | [24][23] |
| Osteoporosis pseudoglioma syndrome | LRP5 | Autosomal recessive | [23] |
| Osteopetrosis | TCIRGI | Autosomal recessive | [23] |
| Camurati-Engelmann disease | TGFβ-1 | Autosomal recessive | [23] |
| Van Buchem disease | SOST | Autosomal recessive | [23] |
| Severe infantile osteopetrosis | CLCN7 | Autosomal recessive | [23] |