Bone density

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Bone mineral density
)
dual energy X-ray absorptiometry

Bone density, or bone mineral density, is the amount of

lumbar spine and over the upper part of the hip.[2]
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 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.

Testing

A bone density test may detect osteoporosis or osteopenia.[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]

Indications for testing

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]

Test result terms

Results of the test are reported in three forms:

  • 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

Illustration of Bone Densitometry Scan

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.[7] The DXA test works by measuring a specific bone 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]

  • BMC = bone mineral content = g/cm
  • W = width at the scanned line

Interpretation

Results are generally scored by two measures, 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.[9]

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.[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 the World Health Organization are:[13]

  • Normal is a T-score of −1.0 or higher[14]
  • Osteopenia is defined as between −1.0 and −2.5
  • Osteoporosis 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.
Hip fractures per 1000 patient-years[15]
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

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 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.[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 as glucocorticoid therapy, hyperparathyroidism, or alcoholism.

Prevention

To prevent low bone density it is recommended to have sufficient calcium and vitamin 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 600 IUs 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 as estrogens (e.g., estradiol, conjugated estrogens), selective estrogen receptor modulators (e.g., raloxifene, bazedoxifene), 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.[20][18] 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.[18]

Genetics

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.

polygenic and many of the genetic mechanisms remain poorly understood.[21]

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 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]

References

  1. ^ Bone+Density at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
  2. PMID 18587077
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  3. ^ "Preventing Falls and Related Fractures | NIH Osteoporosis and Related Bone Diseases National Resource Center". www.bones.nih.gov. Retrieved 2021-03-12.
  4. ^
    ABIM Foundation, Consumer Reports, archived from the original
    (PDF) on March 4, 2016, retrieved August 14, 2012
  5. (PDF) on June 24, 2012, retrieved August 14, 2012
  6. ^ a b c d e "Bone Mass Measurement". NOF. Archived from the original on 2008-03-07. Retrieved 2008-03-20.
  7. ^
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  8. ^ "Bone densitometry". Retrieved 2008-09-02.
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  10. ^ "Bone Density Scan: MedlinePlus Medical Test". medlineplus.gov. Retrieved 2020-10-29.
  11. ^ Unknown, Unknown (2011-07-29). "T and Z scores". University of Washington Bone Physics. Retrieved 2013-06-22.
  12. ^ a b Richmond, Bradford (2007-11-13). "Osteoporosis and bone mineral density". American College of Radiology. Archived from the original on 2008-09-17. Retrieved 2008-05-11.
  13. ^ WHO Scientific Group on the Prevention and Management of Osteoporosis (2000 : Geneva, Switzerland) (2003). "Prevention and management of osteoporosis : report of a WHO scientific group" (PDF). Retrieved 2007-05-31.{{cite web}}: CS1 maint: numeric names: authors list (link)
  14. ^ "Bone Mass Measurement: What the Numbers Mean | NIH Osteoporosis and Related Bone Diseases National Resource Center". www.bones.nih.gov. Retrieved 2021-03-12.
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  17. ^ Lawrence, Jean. "Building Stronger Bones". WebMD. Retrieved 2020-04-20.
  18. ^ a b c d e "How to keep your bones healthy". Mayo Clinic. Retrieved 2020-04-20.
  19. ^ "Exercise for Your Bone Health | NIH Osteoporosis and Related Bone Diseases National Resource Center". www.bones.nih.gov. Retrieved 2020-11-09.
  20. ^ "Smoking and Bone Health | NIH Osteoporosis and Related Bone Diseases National Resource Center". www.bones.nih.gov. Retrieved 2022-09-19.
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