Microarchitecture and bone quality in the human calcaneus: Local variations of fabric anisotropy

Abstract

The local variability of microarchitecture of human trabecular calcaneus bone is investigated using high resolution microCT scanning. The fabric tensor is employed as the measure of the microarchitecture of the pore structure of a porous medium. It is hypothesized that a fabric tensor-dependent poroelastic ultrasound approach will more effectively predict the data variance than will porosity alone. The specific aims of the present study are i) to quantify the morphology and local anisotropy of the calcaneus microarchitecture with respect to anatomical directions, ii) to determine the interdependence, or lack thereof, of microarchitecture parameters, fabric, and volumetric bone mineral density (vBMD), and iii) to determine the relative ability of vBMD and fabric measurements in evaluating the variance in ultrasound wave velocity measurements along orthogonal directions in the human calcaneus. Our results show that the microarchitecture in the analyzed regions of human calcanei is anisotropic, with a preferred alignment along the posterior-anterior direction. Strong correlation was found between most scalar architectural parameters and vBMD. However, no statistical correlation was found between vBMD and the fabric components, the measures of the pore microstructure orientation. Therefore, among the parameters usually considered for cancellous bone (i.e., classic histomorphometric parameters such as porosity, trabecular thickness, number and separation), only fabric components explain the data variance that cannot be explained by vBMD, a global mass measurement, which lacks the sensitivity and selectivity to distinguish osteoporotic from healthy subjects because it is insensitive to directional changes in bone architecture. This study demonstrates that a multi-directional, fabric-dependent poroelastic ultrasound approach has the capability of characterizing anisotropic bone properties (bone quality) beyond bone mass, and could help to better understand anisotropic changes in bone architecture using ultrasound.

Changes of Elastic Constants and Anisotropy Patterns in Trabecular Bone During Disuse-Induced Bone Loss Assessed by Poroelastic Ultrasound

“…These observations are in agreement with previous studies of trabecular bone anisotropy in the human calcaneus [30]. These results suggest that vBMD and fabric could be considered as the main independent descriptors of trabecular bone microarchitecture.…”

“…To produce high-contrast, low-noise images, the projections were averaged five times, and a median filter was used to prevent speckle noise formation. Hydroxyapatite rods (HA, 2 mm diameter, 0.25 and 0.75 gHA/cm3) were scanned using the same protocol to calibrate images for TMD [30,33].…”

“…The threshold value was determined by analyzing the images using an edge detection algorithm (imagej v 1.37, National Institutes o f Health, Bethesda, MD). The TMD threshold obtained through this edge detection procedure was 0.45 gHA/cm3 [30,33]. An anatomical Cartesian coordinate system with axes defined along the ML, AP, and SI directions was intro duced to investigate the relationship between directional measure ments of bone microarchitecture and ultrasound measurements.…”

“…For the present study, the longitudinal fast wave velocity was computed using equations in Cardoso and Cowin [23] along three anatomical directions ML, SI, and AP, using the measurements of porosity and fabric from each trabecu lar bone sample. TMD from pCT was used to calculate the tissue modulus for each sample [30,33] and the apparent density of the solid constituent. The material properties of the fluid repre senting the mixture of blood and marrow were defined as follows: mass density = 1055 kg/m3; Lame constants A = 2634 MPa and H = 0 MPa; and dynamic viscosity r; = 0.1 Pa-s.…”

“…This approach provides the potential to use ultrasound mea surements in bone to examine tissue architecture in addition to bone mass. Recent studies show that this approach significantly improves prediction o f mechanical properties o f bovine and human cancellous bone when compared to density-based approaches [28][29][30],…”