Dimorphism in axial and appendicular dimensions, cortical and trabecular microstructure and matrix mineral density in Chinese and Caucasian women

Highlights

• Chinese women build a more robust skeleton than Caucasians during growth.

• The more robust skeleton is not observed in advanced age.

• There may be greater bone loss or race-specific secular trends in bone morphology.

Abstract

Introduction

Appendicular fractures are less common in Chinese than Caucasian women. Bone mineral density (BMD) is lower, not higher than in Caucasians because Chinese have smaller appendicular dimensions than Caucasians. However, smaller bones may offset the liability to fracture by being assembled with a more robust microstructure. We hypothesized that Chinese assemble an appendicular skeleton with a thicker, less porous and more mineralized cortex that is less deteriorated in advanced age than in Caucasians.

Methods

We compared anthropometry in 477 Chinese and 278 Caucasian women and compared bone microstructure using high-resolution peripheral quantitative computed tomography in another cohort of 186 Chinese and 381 Caucasian women aged 18 to 86 years, all living in Melbourne, Australia. Trabecular plate (p) and rod (r) bone volume/total volume (BV/TV) were quantified using individual trabecula segmentation (ITS). Bone strength was estimated using micro-finite element analysis (μFEA).

Results

Premenopausal Chinese were shorter than Caucasian women, mainly due to shorter leg length. Distal radial total cross sectional area (CSA) was 14.8% smaller (p < 0.001). After adjusting for age and total CSA, Chinese had similar cortical and medullary areas but 0.30 SD lower cortical porosity and 0.27 SD higher matrix mineral density (both p < 0.05). Trabecular plate-to-rod ratio was 0.55 SD higher due to a 0.41 SD higher pBV/TV and 0.36 SD lower rBV/TV (p ranging 0.001 to 0.023). Chinese also had 0.36 SD greater whole bone stiffness and 0.36 SD greater failure load than Caucasians (both p < 0.05). After adjusting for age and total CSA, postmenopausal Chinese had 3.3% smaller cortical area, medullary area was 2.1% larger, cortical porosity was no lower, matrix mineral density and pBV/TV were no higher compared with Caucasians at the distal radius. Whole bone stiffness was 0.39 SD lower and failure load was 0.40 SD lower in Chinese (both p < 0.05).

Conclusion

Chinese build a more robust skeleton than Caucasians during growth, an advantage not observed in advanced age due to greater bone loss or race-specific secular trends in bone morphology.

Introduction

The incidence of appendicular fractures is lower in Chinese than Caucasians [[1], [2], [3]]. This is not explained by the racial differences in bone mineral density (BMD). Indeed, BMD is lower, not higher than in Caucasians because Chinese have smaller appendicular dimensions than Caucasians [4]. However, resistance to fracturing of a smaller skeleton may be achieved by assembling it more robustly during growth; forming thicker cortices relative to the total cross-sectional area with lower porosity and thicker more connected trabeculae of the metaphyseal region in Chinese [5]. Shorter stature or leg length in Chinese may also be associated with better balance and a lower impact following a fall [6].

Quantification of racial differences in bone microstructure is challenging because meticulous attention is needed in choosing the region of interest (ROI) [7]. In an individual, adjacent cross sections of a long bone are assembled using similar amounts of mineralized bone matrix [7,8]. What differs is how that mineralized matrix is fashioned as cortical and trabecular bone along the length of the bone. Distally the constant amount of material is mainly trabecular with a thin cortical shell whereas proximally in the metaphyseal-diaphyseal region and especially the midshaft more of the constant amount of mineralized matrix is assembled as cortical bone.

For racial comparisons to be valid, the same position of a ROI relative to the length of the bone must be compared. Chinese have a shorter appendicular skeleton than Caucasians. When a ROI of the distal radius or distal tibia is chosen using a fixed distance from the midpoint of the radio-carpal joint space, the ROI is positioned more proximally in Chinese [7]. The more proximal positioning exaggerates the racial difference described above; Chinese will have thicker less porous cortices, the matrix mineral density will be higher and trabecular density will be lower [7]. Several approaches can be taken to correct for this positioning error. The recommended approach is to use a percentage of the radial or tibial bone length (~4–6%). If this is not available, correcting for racial differences in bone length by racial differences in the total bone cross-sectional area (CSA) achieves similar accuracy as using the 4–6% of the bone length [7].

In addition, secular drifts in sitting and standing height are race specific. Height generally increases in more recently born individuals forming the young cohort in a cross sectional sample, but this may differ by race, body segment and sex [9,10]. If the secular increase in height is associated with secular trends towards an earlier puberty then the taller stature, irrespective of race, is likely to be due to greater trunk length (because leg length should be shorter if puberty occurs earlier in life in more recently born women).

We hypothesized that, (i) relative to Caucasians, Chinese are shorter mainly due to their shorter leg length, (ii) more recently born women of each race are taller than earlier born women, (iii) Chinese have an earlier menarche and so will have shorter leg length but comparable trunk length, (iv) across age, the taller stature in younger women is due to trunk length not leg length, irrespective of race, and (v) Chinese have lower cortical porosity, higher matrix mineral density, and higher trabecular plate to rod ratio than Caucasians after adjusted for total bone CSA.