Andres Laib

Direct three-dimensional morphometric analysis of human cancellous bone : microstructural data from spine, femur, iliac crest and calcaneus

The appearance of cancellous bone architecture is different for various skeletal sites and various disease states. During aging and disease, plates are perforated and connecting rods are dissolved. There is a continuous shift from one structural type to the other. So traditional histomorphometric procedures, which are based on a fixed model type, will lead to questionable results. The introduction of three‐dimensional (3D) measuring techniques in bone research makes it possible to capture the actual architecture of cancellous bone without assumptions of the structure type. This requires, however, new methods that make direct use of the 3D information. Within the framework of a BIOMED I project of the European Union, we analyzed a total of 260 human bone biopsies taken from five different skeletal sites (femoral head, vertebral bodies L2 and L4, iliac crest, and calcaneus) from 52 donors. The samples were measured three‐dimensionally with a microcomputed tomography scanner and subsequently evaluated with both traditional indirect histomorphometric methods and newly developed direct ones. The results show significant differences between the methods and in their relation to the bone volume fraction. Based on the direct 3D analysis of human bone biopsies, it appears that samples with a lower bone mass are primarily characterized by a smaller plate‐to‐rod ratio, and to a lesser extent by thinner trabecular elements.

Reproducibility of direct quantitative measures of cortical bone microarchitecture of the distal radius and tibia by HR-pQCT☆

Quantitative cortical microarchitectural end points are important for understanding structure-function relations in the context of fracture risk and therapeutic efficacy. This technique study details new image-processing methods to automatically segment and directly quantify cortical density, geometry, and microarchitecture from HR-pQCT images of the distal radius and tibia. An automated segmentation technique was developed to identify the periosteal and endosteal margins of the distal radius and tibia and detect intracortical pore space morphologically consistent with Haversian canals. The reproducibility of direct quantitative cortical bone indices based on this method was assessed in a pooled data set of 56 subjects with two repeat acquisitions for each site. The in vivo precision error was characterized using root mean square coefficient of variation (RMSCV%) from which the least significant change (LSC) was calculated. Bland-Altman plots were used to characterize bias in the precision estimates. The reproducibility of cortical density and cross-sectional area measures was high (RMSCV <1% and <1.5%, respectively) with good agreement between young and elder medians. The LSC for cortical porosity (Ct.Po) was somewhat smaller in the radius (0.58%) compared with the distal tibia (0.84%) and significantly different between young and elder medians in the distal tibia (LSC: 0.75% vs. 0.92%, p<0.001). The LSC for pore diameter and distribution (Po.Dm and Po.Dm.SD) ranged between 15 and 23 microm. Bland-Altman analysis revealed moderate bias for integral measures of area and volume but not for density or microarchitecture. This study indicates that HR-pQCT measures of cortical bone density and architecture can be measured in vivo with high reproducibility and limited bias across a biologically relevant range of values. The results of this study provide informative data for the design of future clinical studies of bone quality.

Calibration of trabecular bone structure measurements of in vivo three-dimensional peripheral quantitative computed tomography with 28-μm-resolution microcomputed tomography

It has recently been shown that high-resolution computed tomography and magnetic resonance imaging have the potential to assess information about the microarchitecture of bone in a noninvasive way. However, due to the limited spatial resolution of the in vivo measurements, the individual trabeculae are not depicted with their true thickness. Nevertheless, the spacing of the structural elements allows the assessment of trabecular number. In a previous publication, the ridge number density (RND) was introduced as a measure for this structural index. It can be extracted from high-resolution three-dimensional (3D) images of patients and shows a reproducibility of 1.6%. In this work the Ridge extraction procedure is compared to and calibrated with microcomputed tomography (microCT) measurements. Three-dimensional measurements of 15 bone biopsies are made with a 28-microm-resolution microCT scanner as well as with a 165-microm-resolution peripheral quantitative computed tomography (pQCT) scanner. For the latter, the same settings are used as for patient examinations. The 15 pairs of measurements are analyzed and the resulting structural indices are compared. The results show that structural indices such as trabecular number, mean thickness, and mean separation can be determined from the 3D pQCT data with an r2 of between 0.81 and 0.96 if the microCT data are taken as the gold standard. The calibration equation found for the bone volume fraction has an intercept of 0.04 and a slope of 0.86 (r2 = 0.98), and trabecular number as the main additional structural index shows a nonsignificant intercept and a calibration slope of 0.91 with the microCT. The calibration procedure can be used directly for patient examinations. Applied to time-series measurements it may be of value for monitoring and quantifying microarchitectural changes due to therapy or aging.

3D micro-computed tomography of trabecular and cortical bone architecture with application to a rat model of immobilisation osteoporosis.

Bone mass and microarchitecture are the main determinants of bone strength. Three-dimensional micro-computed tomogrpahy has the potential to examine complete bones of small laboratory animals with very high resolution in a non-invasive way. In the presented work, the proximal part of the tibiae of hindlimb unloaded and control rats were measured with 3D MicroCT, and the secondary spongiosa of the scanned region was evaluated using direct evaluation techniques that do not require model assumptions. For determination of the complete bone status, the cortex of the tibiae was evaluated and characterised by its thickness. It is shown that with the proposed anatomically conforming volume of interest (VOI), up to an eight-fold volume increase can be evaluated compared to cubic or spherical VOIs. A pronounced trabecular bone loss of −50% is seen after 23 days of tail suspension. With the new evaluation techniques, it is shown that most of this bone loss is caused by the thinning of trabeculae, and to a lesser extent by a decrease in their number. What changes most radically is the structure type: the remaining bone is more rod-like than the control groups bone. Cortical bone decreases less than trabecular bone, with only −18% after 23 days.

The skeletal structure of insulin-like growth factor I-deficient mice

The importance of insulin‐like growth factor I (IGF‐I) for growth is well established. However, the lack of IGF‐I on the skeleton has not been examined thoroughly. Therefore, we analyzed the structural properties of bone from mice rendered IGF‐I deficient by homologous recombination (knockout [k/o]) using histomorphometry, peripheral quantitative computerized tomography (pQCT), and microcomputerized tomography (μCT). The k/o mice were 24% the size of their wild‐type littermates at the time of study (4 months). The k/o tibias were 28% and L1 vertebrae were 26% the size of wild‐type bones. Bone formation rates (BFR) of k/o tibias were 27% that of the wild‐type littermates. The k/o bones responded normally to growth hormone (GH; 1.7‐fold increase) and supranormally to IGF‐I (5.2‐fold increase) with respect to BFR. Cortical thickness of the proximal tibia was reduced 17% in the k/o mouse. However, trabecular bone volume (bone volume/total volume [BV/TV]) was increased 23% (male mice) and 88% (female mice) in the k/o mice compared with wild‐type controls as a result of increased connectivity, increased number, and decreased spacing of the trabeculae. These changes were either less or not found in L1. Thus, lack of IGF‐I leads to the development of a bone structure, which, although smaller, appears more compact.

High-Resolution pQCT Analysis at the Distal Radius and Tibia Discriminates Patients With Recent Wrist and Femoral Neck Fractures

We depict a fragility bone state in two primitive osteoporosis populations using 3D high‐resolution peripheral in vivo QCT (HR‐pQCT). Postmenopausal women (C, controls, n = 54; WF, wrist, n = 50; HF, hip, n = 62 recent fractured patients) were analyzed for lumbar and hip DXA areal BMD (aBMD), cancellous and cortical volumetric BMD (vBMD), and microstructural and geometric parameters on tibia and radius by HR‐pQCT. Principal component analysis (PCA) allowed extracting factors that best represent bone variables. Comparison between groups was made by analysis of covariance (ANCOVA). Two factors (>80% of the entire variability) are extracted by PCA: at the radius, the first is a combination of trabecular parameters and the second of cortical parameters. At the tibia, we found the reverse. Femoral neck aBMD is decreased in WF (8.6%) and in HF (18%) groups (no lumbar difference). WF showed a ∼20% reduction in radius trabecular vBMD and number. Radius cortical vBMD and thickness decrease by 6% and 14%, respectively. At the tibia, only the cortical compartment is affected, with ∼20% reduction in bone area, thickness, and section modulus and 6% reduction in vBMD. HF showed same radius trabecular alterations than WF, but radius cortical parameters are more severely affected than WF with reduced bone area (25%), thickness (28.5%), and vBMD (11%). At the tibia, trabecular vBMD and number decrease by 26% and 17.5%, respectively. Tibia cortical bone area, thickness, and section modulus showed a >30% decrease, whereas vBMD reduction reached 13%. Geometry parameters at the tibia displayed the greatest differences between healthy and fractured patients and between wrist and hip fractures.

MicroCT evaluation of normal and osteoarthritic bone structure in human knee specimens

Although trabecular bone structure has been evaluated, variation with knee compartment and depth from joint surface is not completely understood. Cadaver knees were evaluated with microcomputed tomography analysis for these variations. Objective differences were compared between: medial vs. lateral compartments; femoral vs. tibial bone; and normal vs. arthritic knees. Depth dependent changes in the parameters were observed for the first 6 mm of the cores in normal knees: BV/TV, Tb.N and Conn.D gradually decrease, while Tb.Sp and SMI increase. In the first 6 mm of the normal tibia BV/TV, Tb.N, and Tb.Th are greater than in the femur on both the medial and lateral compartments while Tb.Sp, SMI, and Conn.D are lower. The medial compartment values for BV/TV, Tb.N, Tb.Th and Conn.D are generally greater than for the lateral in both the femur and tibia while Tb.Sp and SMI are lower. In comparison of normal vs. arthritic knees significant differences are observed in the first 6 mm of the medial tibia. With arthritis BV/TV and Tb.Th are lower, while SMI and Tb.Sp are higher. Tb.N and Conn.D show no statistically significant difference. The bone structure variations are, thus, most prominent in the first 6 mm of depth and medial compartment bone is generally more structurally sound than lateral. Severely arthritic bone changes are most prominent in the medial compartment of the tibia and bone structure is less sound in severe arthritis.

New model-independent measures of trabecular bone structure applied to in vivo high-resolution MR images.

Abstract: Complementing measurements of bone mass with measurements of the architectural status of trabecular bone is expected to improve predictions of fracture risk in osteoporotic patients and improve the assessment of response to drug therapy. With high-resolution MRI the trabecular network can be imaged with 156×156×500 mm3 voxels, sufficient to depict individual trabeculae, albeit with inaccurate thickness. In this work, distance transformation techniques were applied to the three-dimensional image of the distal radius of postmenopausal patients. Structural indices such as trabecular number (app.Tb.N), thickness (app.Tb.Th) and separation (app.Tb.Sp) were determined without model assumptions. A new metric index, the apparent intra-individual distribution of separations (app.Tb.Sp.SD), is introduced. The reproducibility of the MR procedure and structure assessment was determined on volunteers, and the coefficient of variation was found to be 2.7–4.6% for the mean values of structural indices and 7.7% for app.Tb.Sp.SD. The distance transformation methods were then applied to two groups of patients: one of postmenopausal women without vertebral fracture and one of postmenopausal women with at least one vertebral fracture. It was found that app.Tb.Sp.SD discriminates fracture subjects from non-fracture patients as well as dual-energy X-ray absorptiometry (DXA) measurements of the radius and the spine, but not as well as DXA of the hip. Using receiver operating characteristic analysis, the area under the curve (AUC) values were 0.67 for app.Tb.Sp.SD, 0.72 for DXA radius, 0.67 for DXA spine and 0.81 for DXA of the hip. A combination of MR indices reached an AUC of 0.75. Age-adjusted odds ratio ranged from 1.85 to 2.03 for app.Tb.N, app.Tb.Sp and app.Tb.Sp.SD (p<0.003). We conclude that in vivo high-resolution MRI not only has the potential of imaging trabecular bone, but in combination with novel metrics may offer new insight into the structural changes occurring in postmenopausal women.

Load transfer analysis of the distal radius from in-vivo high-resolution CT-imaging

Prevention of osteoporotic bone fractures requires accurate diagnostic methods to detect the increase in bone fragility at an early stage of osteoporosis. However, todays bone fracture risk prediction, primarily based on bone density measurement, is not sufficiently precise. There is increasing evidence that, in addition to bone density, also the bone microarchitecture and its mechanical loading conditions are important factors determining the fracture risk. Recently, it has been shown that new high-resolution imaging techniques in combination with new computer modeling techniques based on the finite-element (FE) method can account for these additional factors. These techniques might provide information that is more relevant for the prediction of bone fracture risk. So far, however, these new imaged-based FE techniques have not been feasible in-vivo. The objectives of this study were to quantify the load transfer through the trabecular network in a distal radius using a computer model based on in-vivo high-resolution images and to determine if common regions of fractures can be explained as a result of high tissue loading in these regions. The left distal radius and the two adjacent carpal bones of a healthy volunteer were imaged using a high-resolution three-dimensional CT system providing an isotropic resolution of 165 microm. The bone representation was converted into a FE-model that was used to calculate stresses and strains in the trabecular network. The two carpal bones were loaded using different load ratios (for each load case 1000 N in total) representing impact forces on the hand either in near-neutral position or ulnar/radial deviation. The load transfer through the trabecular network of the radius was characterized by the tissue strain energy density (SED) distribution for all load cases. It was found that the distribution of the tissue loading depends on the ratio of the forces acting on the carpal bones. For all load cases the higher SED values (on average: 0.02 +/- 0.08 (S.D.) N mm(-2)) are found in a 10 mm region adjacent to the articular surface which corresponds well with the region where Colles- or Chauffeur-fractures occur. We expect that, eventually, this new approach can lead to a better prediction of the fracture risk than methods based on bone density alone since it accounts for the bone microstructure as well as its loading conditions.

High-Resolution Three-Dimensional-pQCT Images Can Be an Adequate Basis for In-Vivo μFE 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 Poissons 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.