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In vivo examinations of bone microarchitecture have become available recently through high resolution computed tomography (3D-QCT) and magnetic resonance imaging. The spatial resolution of the resulting images, however, is not sufficient to depict individual trabeculae in their true shape. Nevertheless, structural indices such as relative bone volume, trabecular number, mean thickness and mean separation can be extracted with the help of a ridge detection algorithm. Precision of the procedure is of the order of 1%, accuracy is ascertained using a micro-CT based calibration. In this work we report first results of time serial examinations. Eighteen healthy postmenopausal women (no HRT) were measured at months 0, 6, and 12, and the temporal changes were analyzed. Examination site was the distal radius. The above mentioned structural indices, the average densities and the thickness of the cortical shell were determined. Of the 18 women 6 showed no significant bone loss of any kind, 5 lost primarily cancellous bone, 4 lost primarily cortical bone, and 3 had a substantial loss of cortical as well as cancellous bone. We conclude that even in a homogenous group such as postmenopausal women, there are considerable differences in the reason why bone is weakened and that high resolution 3D-QCT allows to differentiate between various types of bone loss.

The structural properties of trabecular bone have been shown to vary significantly with age, anatomic location, and metabolic condition. Micro-computed tomography (microCT) is an emerging technique for the nondestructive assessment and analysis of the three-dimensional trabecular bone architecture. Within the framework of the European Union BIOMED I project "Assessment of Bone Quality in Osteoporosis," a total of 350 bone biopsies from five different anatomical locations were harvested post mortem from 70 donors (aged 23 to 92 years). These biopsies were measured using a newly devised compact micro-tomographic system, also referred to as desk-top microCT. Samples with a diameter from a few millimeters to a maximum of 18 mm and a length of up to 55 mm can be measured. For this study fresh, untreated bone biopsies with a diameter of 8 mm were measured micro-tomographically with a nominal isotropic resolution of 14 microns. For all samples, the volumes of interest (4 x 4 x 4 mm3) were binarized using a uniform threshold. Subsequently, standard structural indices such as bone volume density (BV/TV), bone surface density (BS/BV), trabecular plate number (Tb.N), trabecular thickness (Tb.Th), trabecular separation (Tb.Sp) and the degree of anisotropy (DA) were computed for all specimens incorporating mean intercept length (MIL) measurements. Regression analysis was used to estimate the correlations of single structural parameters with age or with a second parameter and also among different measurement sites. The nondestructive microCT measurements allowed not only to perform quantitative bone morphometry but also to assess other important microstructural features in the determination of the mechanical integrity of trabecular bone such as the incidence and prevalence of microcallus formations. The findings from the BIOMED I study are expected to improve our understanding of the relative importance of bone architecture, damage accumulation, and bone mineralization in the characterization of bone quality in the progress of age- and disease-related bone loss.

Most standard methods to predict bone quality are merely based on apparent density measurements. However, apparent density alone does neither explain all variation of the mechanical properties nor does it account for the structural anisotropy of trabecular bone. Thus, apparent density alone might not be sufficient to accurately predict the quality of bone. This study investigates if a new approach based on microstructural computer models can provide additional and relevant information on bone quality. 58 human trabecular bone samples from the femoral head were measured with a 3-D micro-Computed Tomography (micro-CT) system providing a voxel representation of the bone microarchitecture with a resolution of 28 microns. Based on such representations, the orthotropic stiffness matrices and the principal directions were computed for 5 mm cubes with microstructural Finite Element Analysis (FEA). For a subset of six samples the moduli were then validated with tri-axial mechanical compression tests. The results show that on average 15% of the variation of the elastic properties are not explained by bone volume fraction. Differences of elastic properties between samples with the same bone volume fraction range up to 53%. The variation of the degree of anisotropy is unrelated to that of the bone volume fraction. Finally, the direction-dependent stiffness of the trabecular bone differs by a factor of four, indicating that one single (isotropic) modulus as predicted from apparent density measurements might not be adequat. It is concluded that micro-CT-based FEA provides new and additional information about anisotropy and mechanical properties in a direct and non-destructive way, and thus will be important in the future for advanced failure risk prediction. An extension to patient examinations using high-resolution CT or MRI techniques is envisaged.

Micro-computed tomography (microCT) is an emerging technique for the non-destructive assessment and analysis of the three-dimensional cancellous bone architecture. However, the procedures the procedures and applications used to quantify bone structures are not yet standardized. The aim of this study was to provide more insight in the resolution-dependency of microstructural properties of three-dimensional trabecular bone. Ten iliac crest bone biopsies were measured using a newly devised microCT system providing a nominal isotropic resolution of 14 microns. To study the resolution dependency the measured data were reconstructed on reduced image arrays with reduction factors ranging from 2 to 20. To assess the structural properties, morphometric parameters were computed based on a truly three-dimensional approach. The results showed a strong resolution dependency of the structural properties and that, if very precise results are needed, only the highest resolution will predict the correct values. Nevertheless, since the properties either decrease or increase monotonously up to a nominal resolution of about 175 microns, the values appear to be restorable using a suitable calibration procedure.

26 European centres participated in a concerted research action Biomedical Engineering: Quantitative Assessment of Osteoporosis. With a newly designed European spine and forearm phantom, the stability, accuracy, precision of dual energy absorption (DXA) and quantitative computer tomography (QCT) densitometry machines have been evaluated. Marked and clinically significant differences were found between brands and between techniques. Cross-calibration formulae have been made and normative data evaluated for different regions (spine, femoral neck, femoral trochanteric and forearm). A general fit for all data obtained from different machines was established.The cross-calibration formulae will allow a sensitivity analysis to assist the choice of equipment for clinical management of different categories of patients with bone disease. The present results obtained with an internationally accepted European spine and forearm phantom can now serve to stimulate the manufacturers to improve the comparability of bone measurements between machines.