Ethnic differences in bone health

There are differences in fracture risk between ethnic groups around the globe, in both men and women.1 To determine strategies for prevention of osteoporosis and bone fragility in an ever-changing environment, we must understand the phenotype underlying these differences.

Most studies report ethnic differences in areal bone mineral density (aBMD) measured by dual energy X-ray absorptiometry (DXA), which do not consistently parallel ethnic patterns in fracture rates. Variations in body size and composition are likely to contribute to reported differences. More recently, three-dimensional bone-imaging modalities, including quantitative computed tomography (QCT), peripheral QCT (pQCT) and high resolution pQCT (HRpQCT), have enabled the measurement of volumetric bone mineral density (vBMD), structural dimensions and internal organisation of cortical and trabecular bone. As a result, more and more studies are now focusing on bone microarchitecture and how this influences fracture risk in different ethnic groups.

The literature has shown that the lowest fracture rates are in populations with African ancestry,2 with a greater than 10-fold variation in age-standardised hip fracture risk among 63 countries,2 demonstrating the variation in fracture risk between ethnic groups. Large population studies have used aBMD measured by DXA as a marker for bone health and fracture risk to study ethnic differences in bone health. The third National Health and Nutrition Examination Survey (NHANES III) has shown that African Americans have the highest mean femoral neck and total hip aBMD levels compared with Caucasian men, who had the lowest levels.3

One of the largest multi-ethnic studies, the National Osteoporosis Risk Assessment (NORA), showed that African American women had the highest aBMD and that Asians had the lowest.4 Even after adjusting for body weight and other risk factors, the greater aBMD in African American women persisted. However, Asian women had similar values to Caucasian women.4 This study led to the suggestion that there may be other components of bone health that need to be considered: namely bone geometry, size and microarchitecture.

The Osteoporotic Fractures in Men Study (MrOS) used QCT and showed that African American and Asian men have increased bone strength compared with White men, due to greater vBMD and thicker cortices of the femoral neck.5 In the UK, data from the European Male Ageing Study (EMAS) were compared with a group of Afro-Caribbean and South Asian men.6 Afro-Caribbean men had higher aBMD compared with White and South Asian men, and these differences were independent of weight and height. In contrast, differences in aBMD between White and South Asian men were attenuated by correcting for body size.6 With the exception of cortical vBMD, which was lower in White men compared with both Afro-Caribbean and South Asian men, the differences in vBMD were far fewer than in DXA outcomes where Afro-Caribbean men did not differ from White or South Asian men. Rather, the geometry of bone differed between the groups and mostly at the diaphyseal sites, and hip axis length was longer in White men.6 At the radius and tibia diaphysis, Afro-Caribbean men had more cortical bone within a slightly larger periosteal envelope, and consequently greater bending strength than the other two groups.6

Fracture and bone mineral density data in Aboriginal and Torres Strait Islander people of Australia are sparse.7 Aboriginal and Torres Strait Islander men and women were 50% and 26% more likely to have a hip fracture compared with non-Indigenous men and women respectively. Hip fractures occur at a much younger age in Aboriginal and Torres Strait Islander people (in men, 65 compared with 81 years; in women, 74 compared with 83 years).8 Between 1999 and 2009, a study reported a significantly disproportionate increase in minimal trauma hip fracture rates among Aboriginal and Torres Strait Islander people aged ≥40 years,9 in contrast to declining agerelated rates of hip fracture in non-Indigenous Australians.10 Use of 3D-imaging modalities in future studies will help shed light on why fracture rates are higher among Aboriginal and Torres Strait Islander people.11

Studies are now beginning to demonstrate that bone geometry and microarchitecture are different in ethnic groups, and these variations are likely to contribute to the ethnic differences in fracture rates. Chinese American women have smaller bones; however, they have higher cortical vBMD12 and better trabecular microarchitecture, with more plate-like structures, when compared with Caucasian American women.13 African American women have a high aBMD which contributes greatly to a reduced fracture risk; however, thicker cortices and better trabecular microarchitecture also contribute to decreasing fracture risk when compared with Caucasian women,14 suggesting that cortical and trabecular bone together contribute to bone strength.

The Study of Women’s Health Across the Nation (SWAN) reported no differences in aBMD between Chinese, Japanese and Caucasian women, but that bone geometry varied greatly.15 Femoral neck cross-sectional area and section modulus of the hip, measured by DXA, were higher in Japanese compared with Caucasian women.15 Thus, Japanese women have better resistance to axial compressive and bending stresses due to greater bone area, which conferred a larger section modulus. HRpQCT has been used to explain the structural basis of bone fragility, but is somewhat limited today. A study in premenopausal Chinese American women showed that the trabecular microarchitecture of Eastern Asian women appears to have a structural advantage when compared with White Caucasian women, with more plate-like and larger trabeculae, and greater plate−rod junction density – a parameter indicating trabecular network connections.13

There is a need to go beyond aBMD and assess skeletal parameters contributing to a healthy bone phenotype, i.e. bone shape, mineralisation and distribution, to ultimately decrease the burden of osteoporosis worldwide and to highlight that a ‘one-size fits all approach’ is not appropriate.

Ayse ZenginAustralia

REFERENCES

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3. Looker et al. 1998 OsteoporosisInternational https://doi.org/10.1007/s001980050093

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5. Marshall et al. 2008 JournalofBone&MineralResearch https://doi.org/10.1359/jbmr.070908

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©Pixabay/stokpic https://doi.org/10.1016/j.bone.2016.07.018

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8. Australian Institute of Health & Welfare 2010

TheProblemofOsteoporoticHipFractureinAustralia https://www.aihw.gov.au/reports/chronic-musculoskeletalconditions/osteoporotic-hip-fracture-in-australia/summary

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JournalofBone&MineralResearch https://doi.org/10.1002/jbmr.352

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JournalofBone&MineralResearch https://doi.org/10.1002/jbmr.378

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JournalofBone&MineralResearch https://doi.org/10.1002/jbmr.1953

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JournalofBone&MineralResearch https://doi.org/10.1002/jbmr.1781