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Bone den­si­ty (or bone min­er­al den­si­ty) is a med­ical term nor­mal­ly refer­ring to the amount of min­er­al mat­ter per square cen­time­ter of bones. Bone den­si­ty (or BMD) is used in clin­i­cal med­i­cine as an indi­rect indi­ca­tor of osteo­poro­sis and frac­ture risk. This med­ical bone den­si­ty is not the true phys­i­cal “den­si­ty” of the bone, which would be com­put­ed as mass per vol­ume. It is mea­sured by a pro­ce­dure called den­sit­o­m­e­try, often per­formed in the radi­ol­o­gy or nuclear med­i­cine depart­ments of hos­pi­tals or clin­ics. 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. 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 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.