High-resolution three-dimensional-pQCT images can be an adequate basis for in-vivo microFE analysis of bone

Micro-finite element (microFE) models based on high-resolution images have enabled the calculation of elastic properties of trabecular bone in vitro. Recently, techniques have been developed to image trabecular bone structure in vivo, albeit at a lesser resolution. The present work studies the usefulness of such in-vivo images for microFE analyses, by comparing their microFE results to those of models based on high-resolution micro-CT (microCT) images. Fifteen specimens obtained from human femoral heads were imaged first with a 3D-pQCT scanner at 165 microns resolution and a second time with a microCT scanner at 56 microns resolution. A third set of images with a resolution of 165 microns was created by downscaling the microCT measurements. The microFE models were created directly from these images. Orthotropic elastic properties and the average tissue von Mises stress of the specimens were calculated from six FE-analyses per specimen. The results of the 165 microns models were compared to those of the 56 microns model, which was taken as the reference model. The results calculated from the pQCT-based models, correlated excellent with those calculated from the reference model for both moduli (R2 > 0.95) and for the average tissue von Mises stress (R2 > 0.83). Results calculated from the downscaled micro-CT models correlated even better with those of the reference models (R2 > 0.99 for the moduli and R2 > 0.96 for the average von Mises stress). In the case of the 3D-pQCT based models, however, the slopes of the regression lines were less than one and had to be corrected. The prediction of the Poisson's ratios was less accurate (R2 > 0.45 and R2 > 0.67) for the models based on 3D-pQCT and downscaled microCT images respectively). The fact that the results from the downscaled and original microCT images were nearly identical indicates that the need for a correction in the case of the 3D-pQCT measurements was not due to the voxel size of the images but due to a higher noise level and a lower contrast in these images, in combination with the application of a filtering procedure at 165 micron images. In summary: the results of microFE models based on in-vivo images of the 3D-pQCT can closely resemble those obtained from microFE models based on higher resolution microCT system.