Bone Abstracts

Skeletal development is characterized by sex-, race- and maturation-specific increases in bone strength. Studies using conventional QCT in the spine and femur, and peripheral QCT (pQCT) in the extremities provided insight into differences in compartment volumetric BMD (vBMD) and cortical dimensions but were limited by inadequate resolution to assess microarchitecture. For example, pQCT studies demonstrated that cortical vBMD was greater in females, while cortical section modulus was greater in males and these differences in structure were more pronounced in later Tanner stages (Leonard JCEM 2010). In contrast, high resolution (HR-pQCT) scanners have a voxel size of 82 μm (conventional pQCT scanners have a voxel size of 400 μm and a slice thickness of 2.3 mm) and provide estimates of trabecular microarchitecture and cortical porosity. HR-pQCT images can be used as input for micro-finite element (μFE) analysis to estimate bone strength. These methods have been used to identify changes in cortical porosity and the proportion of load borne by cortical bone during mid- to late puberty that mirror the timing and sex differences in distal forearm fractures in epidemiologic studies (Kirmani JBMR 2009). Similarly, a study comparing the tibia and radius suggested that more rapid modeling at the distal radial metaphysis results in a greater dissociation between growth and mineral accrual than observed at the distal tibia with transitory low cortical thickness and vBMD in boys but not in girls (Wang JBMR 2010). To our knowledge, no prospective studies have examined associations between HR-pQCT results and subsequent fractures. However, Chevallley et al. reported that fractures in healthy females during childhood were associated with lower trabecular thickness and μFE measures of bone stiffness and failure load at age 20 years (JCEM 2012). Micro-MRI also has sufficient resolution for in vivo assessment of bone microarchitecture; however, relatively long scan times (e.g. 20 min) and the lack of automated methods for the quantitative analysis of microstructural parameters have limited use in children. Of note, MR spectroscopy measures of marrow adipose tissue has been used in children and may provide an index of adipocyte vs osteoblast differentiation in the mesenchymal stem cell pool.