Estimation of mechanical stiffness by finite element analysis of ultrasound computed tomography (UCT-FEA); a comparison with X-ray µCT based FEA in cancellous bone replica models

The mechanical integrity of a bone is determined by its quantity and quality. Conventional mechanical testing is the ‘gold standard’ for assessing bone strength, although not applicable in vivo since it is inherently invasive and destructive. A mechanical test measurement of stiffness (N mm−1) provides an accurate estimate of strength, although again inappropriate in vivo. Several non-destructive, non-invasive, in vivo techniques have been developed and clinically implemented to serve as surrogates for bone strength assessment including dual-energy X-ray absorptiometry along with axial and peripheral quantitative computed tomography, and quantitative ultrasound. Finite element analysis (FEA) is a computer simulation method that predicts the behaviour of a structure such as a bone under mechanical loading, being previously combined with in vivo bone imaging, reporting higher predictions of mechanical integrity than imaging alone. We hypothesised that ultrasound computed tomography (UCT) may be combined with FEA, thereby predicting the stiffness of bone. The objective of this study was to apply finite element analysis to UCT derived attenuation images of trabecular bone replica samples, thereby providing an estimate of mechanical stiffness that could be compared with both a gold standard mechanical test and a surrogate X-ray µCT-FEA. Replica bone samples were 3D-printed from four anatomical sites (femoral head, lumbar spine, calcaneus and iliac crest), with two cylindrical volumes of interest extracted from each sample. Each replica sample was scanned by X-ray µCT and a bespoke UCT system, from which finite element analysis was performed to estimate mechanical stiffness. The samples were then mechanically tested, yielding the gold standard stiffness value. The coefficient of determination (R2) to estimate mechanical test derived stiffness was 99% for µCT-FEA and 84% for UCT-FEA. In conclusion, UCT-FEA is a promising tool for estimating the mechanical integrity of a bone. This study demonstrated that UCT-FEA, based upon quantitative attenuation images, provided a comparable estimation of gold standard mechanical-test stiffness and therefore has significant potential clinical utility for osteoporotic fracture risk assessment and quantitative assessment of musculoskeletal tissues