View source for Bone density

{{Short description|Amount of bone mineral in bone tissue}}
[[Image:Bone density scanner.jpg|right|thumb|A scanner used to measure bone density using [[dual energy X-ray absorptiometry]]]]
'''Bone density''', or '''bone mineral density''', is the amount of [[bone mineral]] in [[bone tissue]]. The [[concept]] is of [[mass]] of mineral per [[volume]] of bone (relating to [[density]] in the [[physics]] sense), although [[medicine#Clinical practice|clinically]] it is measured by proxy according to [[optical density]] per square centimetre of [[bone]] surface upon [[medical imaging|imaging]].<ref>{{MeshName|Bone+Density}}</ref> Bone density measurement is used in clinical medicine as an indirect indicator of [[osteoporosis]] and [[fracture]] risk. It is measured by a procedure called [[densitometry]], often performed in the [[radiology]] or [[nuclear medicine]] departments of [[hospital]]s or [[clinic]]s. The measurement is painless and non-invasive and involves low [[radiation]] exposure. Measurements are most commonly made over the [[lumbar spine]] and over the upper part of the [[hip]].<ref name="pmid18587077">{{cite journal |author=Cole RE |title=Improving clinical decisions for women at risk of osteoporosis: dual-femur bone mineral density testing |journal=J Am Osteopath Assoc |volume=108 |issue=6 |pages=289–95 |date=June 2008 |pmid=18587077 |url=http://www.jaoa.org/cgi/pmidlookup?view=long&pmid=18587077}}</ref> The forearm may be scanned if the hip and lumbar spine are not accessible.

There is a [[statistics|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.<ref>{{Cite web|title=Preventing Falls and Related Fractures |work=NIH Osteoporosis and Related Bone Diseases National Resource Center|url=https://www.bones.nih.gov/health-info/bone/osteoporosis/fracture/preventing-falls-and-related-fractures|access-date=2021-03-12 |publisher=National Institute for Health}}</ref> 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]].<ref name="whenyouneed" /> The usual response to either of these indications is consultation with a physician.<ref name="whenyouneed" /> Bone density tests are not recommended for people without risk factors for weak bones,<ref>{{Citation |author1 = American Academy of Family Physicians |author1-link = American Academy of Family Physicians |others = presented by [[ABIM Foundation]] |title = Five Things Physicians and Patients Should Question |publisher = [[American Academy of Family Physicians]] |work = Choosing Wisely: an initiative of the [[ABIM Foundation]] |url = http://choosingwisely.org/wp-content/uploads/2012/04/5things_12_factsheet_Amer_Acad_Fam_Phys.pdf |access-date = August 14, 2012 |url-status = dead |archive-url = https://web.archive.org/web/20120624075453/http://choosingwisely.org/wp-content/uploads/2012/04/5things_12_factsheet_Amer_Acad_Fam_Phys.pdf |archive-date = June 24, 2012 }}</ref><ref name="whenyouneed">{{Citation | author1 = Consumer Reports | author-link = Consumer Reports | author2 = American Academy of Family Physicians | author2-link = American Academy of Family Physicians | date = May 2012| title = Bone-density tests: When you need them – and when you don't| publisher = [[Consumer Reports]]| work = Choosing Wisely: an initiative of the [[ABIM Foundation]]| url = http://consumerhealthchoices.org/wp-content/uploads/2012/05/ChoosingWiselyBoneDensityAAFP.pdf| access-date = August 14, 2012| archive-date = March 4, 2016| archive-url = https://web.archive.org/web/20160304075232/http://consumerhealthchoices.org/wp-content/uploads/2012/05/ChoosingWiselyBoneDensityAAFP.pdf| url-status = dead}}</ref> which is more likely to result in unnecessary treatment rather than discovery of a weakness.<ref name="whenyouneed"/>

===Indications for testing===
The risk factors for low bone density and primary considerations for a bone density test include:

*females age 65 or older.<ref name="whenyouneed"/>
*males age 70 or older.<ref name="whenyouneed"/>
*people over age 50 with:
**previous bone fracture from minor trauma.<ref name="whenyouneed"/>
**rheumatoid arthritis.<ref name="whenyouneed"/>
**low body weight.<ref name="whenyouneed"/>
**a parent with a hip fracture.<ref name="whenyouneed"/>
* individuals with [[vertebrae|vertebral]] abnormalities.<ref name="titleNOF - Bone Mass Measurement">{{cite web|url=http://www.nof.org/osteoporosis/bonemass.htm |title=Bone Mass Measurement |access-date=2008-03-20 |work=NOF |archive-url=https://web.archive.org/web/20080307014020/http://www.nof.org/osteoporosis/bonemass.htm |archive-date=2008-03-07 |url-status=dead }}</ref>
* individuals receiving, or planning to receive, long-term glucocorticoid ([[steroid]]) therapy.<ref name="titleNOF - Bone Mass Measurement"/>
* individuals with primary [[hyperparathyroidism]].<ref name="titleNOF - Bone Mass Measurement"/>
* individuals being monitored to assess the response or efficacy of an approved osteoporosis drug therapy.<ref name="titleNOF - Bone Mass Measurement"/>
* when [[androgen deprivation therapy]] is being planned for [[prostate cancer]].
* individuals with a history of eating disorders.<ref name="titleNOF - Bone Mass Measurement"/>

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.<ref name="whenyouneed"/>

===Test result terms===
Results of the test are reported in three forms:
* Measured areal density in g cm<sup>−2</sup>.
* [[#Z-score|Z-score]]: the number of standard deviations above or below the mean for the patient's age, sex and ethnicity.
* [[#T-score|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===
[[File:Blausen 0095 BoneDensitometryScan.png|thumb|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:
*[[Dual-energy X-ray absorptiometry]] (DXA or DEXA)
*[[Trabecular bone score]]
*[[Dual X-ray absorptiometry and laser|Dual X-ray Absorptiometry and Laser (DXL)]]
*[[Quantitative computed tomography]] (QCT)
*[[Quantitative ultrasound]] (QUS)
*[[Single photon absorptiometry]]  (SPA)
*[[Dual photon absorptiometry]]  (DPA)
*[[Digital X-ray radiogrammetry]] (DXR)
*[[Single energy X-ray absorptiometry]] (SEXA)

DXA is the most commonly used testing method {{asof|2016|lc=y}}.<ref name=":5" /> 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.<ref name="urlBone densitometry">{{cite web |title=Bone densitometry |url=http://courses.washington.edu/bonephys/opbmd.html |access-date=2008-09-02}}</ref>

Average bone mineral density = BMC / W [g/cm<sup>2</sup>]
*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 [[buoyancy|float in water]].<ref>{{cite journal |vauthors=Gregson CL, Steel SA, O'Rourke KP, Allan K, Ayuk J, Bhalla A, Clunie G, Crabtree N, Fogelman I, Goodby A, Langman CM, Linton S, Marriott E, McCloskey E, Moss KE, Palferman T, Panthakalam S, Poole KE, Stone MD, Turton J, Wallis D, Warburton S, Wass J, Duncan EL, Brown MA, Davey-Smith G, Tobias JH |title='Sink or swim': an evaluation of the clinical characteristics of individuals with high bone mass |journal=Osteoporos Int |volume=23 |issue=2 |pages=643–54 |date=February 2012 |pmid=21455762 |pmc=3261396 |doi=10.1007/s00198-011-1603-4 }}</ref>

====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.<ref>{{Cite web|title=Bone Density Scan: MedlinePlus Medical Test|url=https://medlineplus.gov/lab-tests/bone-density-scan/|access-date=2020-10-29|website=medlineplus.gov|language=en}}</ref> 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.<ref name="Univ Washington Bone Physics">{{cite web|url=http://courses.washington.edu/bonephys/opbmdtz.html |title=T and Z scores. |access-date=2013-06-22  |date=2011-07-29 |publisher=University of Washington Bone Physics }}</ref> Values for 30-year-olds are used in post-menopausal women and men over age 50 because they better predict risk of future fracture.<ref name="Richmond">{{cite web |url=http://www.guideline.gov/summary/summary.aspx?ss=15&doc_id=11559&nbr=5990 |title=Osteoporosis and bone mineral density. |access-date=2008-05-11 |last=Richmond |first=Bradford |date=2007-11-13 |publisher=American College of Radiology |archive-url=https://web.archive.org/web/20080917114051/http://guideline.gov/summary/summary.aspx?ss=15&doc_id=11559&nbr=5990 |archive-date=2008-09-17 |url-status=dead }}</ref> The criteria of the [[World Health Organization]] are:<ref name="WHOcriteria">{{cite web | author=((WHO Scientific Group on the Prevention and Management of Osteoporosis)) |url=http://whqlibdoc.who.int/trs/WHO_TRS_921.pdf |title=Prevention and management of osteoporosis : report of a WHO scientific group| year=2003 |access-date=2007-05-31 }}</ref>
* Normal is a T-score of −1.0 or higher<ref>{{Cite web|title=Bone Mass Measurement: What the Numbers Mean |work=NIH Osteoporosis and Related Bone Diseases National Resource Center|url=https://www.bones.nih.gov/health-info/bone/bone-health/bone-mass-measure#b|access-date=2021-03-12 |publisher=National Institute for Health}}</ref>
* [[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.

{| class="wikitable" align="center"
|+ Hip fractures per 1000 patient-years<ref name="pmid17846439">{{cite journal |vauthors=Cranney A, Jamal SA, Tsang JF, Josse RG, Leslie WD |title=Low bone mineral density and fracture burden in postmenopausal women |journal=Canadian Medical Association Journal |volume=177 |issue=6 |pages=575–80 |year=2007 |pmid=17846439 |doi=10.1503/cmaj.070234 |pmc=1963365}}</ref>
! 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 deviation]]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.<ref name="Richmond" /><ref name="pmid16456501">{{cite journal | vauthors = Bachrach LK | title = Assessing bone health in children: who to test and what does it mean? | journal = Pediatr Endocrinol Rev | volume = 2 | issue = Suppl 3| pages = 332–6 | date = February 2005 | pmid = 16456501 | doi = | url = }}</ref> 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 in biology#Dietary recommendations|calcium]] and [[Vitamin D deficiency|vitamin D]].<ref>{{Cite web|url=https://www.webmd.com/osteoporosis/features/building-stronger-bones|title=Building Stronger Bones|last=Lawrence|first=Jean|website=WebMD|language=en|access-date=2020-04-20}}</ref><ref name=":4">{{Cite web|url=https://www.mayoclinic.org/healthy-lifestyle/adult-health/in-depth/bone-health/art-20045060|title=How to keep your bones healthy|website=Mayo Clinic|language=en|access-date=2020-04-20}}</ref>  Sufficient calcium is defined as 1,000 mg per day, increasing to 1,200 mg for women above 50 and men above 70.<ref name=":4" /> Sufficient vitamin D is defined as 600 [[International unit|IUs]] per day for adults 19 to 70, increasing to 800 IUs per day for those over 71.<ref name=":4" /> 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.<ref>{{Cite web|title=Exercise for Your Bone Health |work=NIH Osteoporosis and Related Bone Diseases National Resource Center|url=https://www.bones.nih.gov/health-info/bone/bone-health/exercise/exercise-your-bone-health|access-date=2020-11-09 |publisher=National Institute for Health }}</ref> Other therapies, such as [[estrogen (medication)|estrogen]]s (e.g., [[estradiol (medication)|estradiol]], [[conjugated estrogens]]), [[selective estrogen receptor modulator]]s (e.g., [[raloxifene]], [[bazedoxifene]]), and [[bisphosphonate]]s (e.g., [[alendronic acid]], [[risedronic acid]]), can also be used to improve or maintain bone density. [[Health effects of tobacco|Tobacco use]] and excessive [[Health effects of alcohol|alcohol consumption]] have detrimental effects on bone density.<ref>{{Cite web |title=Smoking and Bone Health |work=NIH Osteoporosis and Related Bone Diseases National Resource Center |url=https://www.bones.nih.gov/health-info/bone/osteoporosis/conditions-behaviors/bone-smoking |access-date=2022-09-19 |publisher=National Institute for Health }}</ref><ref name=":4" /> Excessive alcohol consumption is defined as more than one [[Standard drink|standard-sized alcoholic beverage]] per day for women, and drinking two or more alcoholic beverages per day for men.<ref name=":4" />

== 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.<ref name=":5">{{Cite journal |last1=Goolsby |first1=Marci A. |last2=Boniquit |first2=Nicole |date=2016 |title=Bone Health in Athletes |journal=Sports Health |volume=9 |issue=2 |pages=108–117 |doi=10.1177/1941738116677732 |pmc=5349390 |pmid=27821574}}</ref><ref name=":0">{{Cite journal |last1=Yang |first1=Tie-Lin |last2=Shen |first2=Hui |last3=Liu |first3=Anqi |last4=Dong |first4=Shan-Shan |last5=Zhang |first5=Lei |last6=Deng |first6=Fei-Yan |last7=Zhao |first7=Qi |last8=Deng |first8=Hong-Wen |date=February 2020 |title=A road map for understanding molecular and genetic determinants of osteoporosis |journal=Nature Reviews Endocrinology |language=en |volume=16 |issue=2 |pages=91–103 |doi=10.1038/s41574-019-0282-7 |pmc=6980376 |pmid=31792439}}</ref> Bone mineral density variation has been estimated to have 0.6–0.8 [[heritability]] factor, meaning that 60–80% of its variation is inherited from parents.<ref name=":1">{{Cite journal |last1=Peacock |first1=Munro |last2=Turner |first2=Charles H. |last3=Econs |first3=Michael J. |last4=Foroud |first4=Tatiana |date=2002-06-01 |title=Genetics of Osteoporosis |journal=Endocrine Reviews |volume=23 |issue=3 |pages=303–326 |doi=10.1210/edrv.23.3.0464 |pmid=12050122|doi-access=free }}</ref> Because of the heritability of bone mineral density, family history of fractures is considered as a risk factor for osteoporosis.<ref name=":3" /> Bone mineral density is [[polygenic]] and many of the genetic mechanisms remain poorly understood.<ref name=":0" />

=== 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:<ref name=":2">{{Cite journal |last1=Marom |first1=Ronit |last2=Rabenhorst |first2=Brien M. |last3=Morello |first3=Roy |date=2020-10-01 |title=Management of Endocrine Disease: Osteogenesis imperfecta: an update on clinical features and therapies |url=https://eje.bioscientifica.com/view/journals/eje/183/4/EJE-20-0299.xml |journal=European Journal of Endocrinology |volume=183 |issue=4 |pages=R95–R106 |doi=10.1530/EJE-20-0299 |pmc=7694877 |pmid=32621590}}</ref><ref name=":3">{{Cite journal |last1=Huang |first1=Qing-Yang |last2=Kung |first2=Annie Wai Chee |date=2006-08-01 |title=Genetics of osteoporosis |url=https://www.sciencedirect.com/science/article/pii/S1096719206001648 |journal=Molecular Genetics and Metabolism |volume=88 |issue=4 |pages=295–306 |doi=10.1016/j.ymgme.2006.04.009 |pmid=16762578 |url-access=subscription }}</ref>
{| class="wikitable"
|+
!Disease
!Gene affected
!Inheritance
!Source
|-
|[[Osteogenesis imperfecta]]
|COLIA1
|Autosomal recessive
|<ref name=":2" /><ref name=":3" />
|-
|[[Osteogenesis imperfecta]]
|COLIA2
|Autosomal recessive
|<ref name=":2" /><ref name=":3" />
|-
|[[Osteoporosis-pseudoglioma syndrome|Osteoporosis pseudoglioma syndrome]]
|LRP5
|Autosomal recessive
|<ref name=":3" />
|-
|[[Osteopetrosis]]
|TCIRGI
|Autosomal recessive
|<ref name=":3" />
|-
|[[Camurati–Engelmann disease|Camurati-Engelmann disease]]
|TGFβ-1
|Autosomal recessive
|<ref name=":3" />
|-
|[[Van Buchem disease]]
|SOST
|Autosomal recessive
|<ref name=":3" />
|-
|Severe infantile osteopetrosis
|CLCN7
|Autosomal recessive
|<ref name=":3" />
|}

==References==
{{Reflist}}

{{Bone/cartilage physiology}}

[[Category:Mass density]]
[[Category:Bones]]