High-throughput quantification of the mechanical competence of murine femora--a highly automated approach for large-scale genetic studies.

Animal models are widely used to gain insight into the role of genetics on bone structure and function. One of the main strategies to map the genes regulating specific traits is called quantitative trait loci (QTL) analysis, which generally requires a very large number of animals (often more than 1000) to reach statistical significance. QTL analysis for mechanical traits has been mainly based on experimental mechanical testing, which, in view of the large number of animals, is time consuming. Hence, the goal of the present work was to introduce an automated method for large-scale high-throughput quantification of the mechanical properties of murine femora. Specifically, our aims were, first, to develop and validate an automated method to quantify murine femoral bone stiffness. Second, to test its high-throughput capabilities on murine femora from a large genetic study, more specifically, femora from two growth hormone (GH) deficient inbred strains of mice (B6-lit/lit and C3.B6-lit/lit) and their first (F1) and second (F2) filial offsprings. Automated routines were developed to convert micro-computed tomography (micro-CT) images of femora into micro-finite element (micro-FE) models. The method was experimentally validated on femora from C57BL/6J and C3H/HeJ mice: for both inbred strains the micro-FE models closely matched the experimentally measured bone stiffness when using a single tissue modulus of 13.06 GPa. The mechanical analysis of the entire dataset (n=1990) took approximately 44 CPU hours on a supercomputer. In conclusion, our approach, in combination with QTL analysis could help to locate genes directly involved in controlling bone mechanical competence.

Screening for bipolar disorder among outpatients with substance use disorders.

BACKGROUND: Comorbidity of bipolar disorder and alcohol or substance abuse/dependence is frequent and has marked negative consequences on the course of the illness and treatment compliance. The objective of this study was to compare the validity of two short instruments aimed at screening bipolar disorders among patients treated for substance use disorders. METHODS: The Mood Disorder Questionnaire (MDQ) and the Hypomania Checklist-32 (HCL-32) were tested with reference to the mood section of the Structured Clinical Interview for DSM-IV axis I disorders (SCID) in 152 patients, recruited in two outpatient clinics providing specialized treatment for alcohol and opiate dependence. RESULTS: According to the SCID, 33 patients (21.7%) had a diagnosis within the bipolar spectrum (two bipolar I, 21 bipolar II and 10 bipolar not otherwise specified). The HCL-32 was more sensitive (90.9% vs. 66.7%) and the MDQ more specific (38.7% vs. 77.3%) for the whole sample. The MDQ displayed higher sensitivity and specificity in patients treated for alcohol than for opiate dependence, whereas the HCL-32 was highly sensitive but poorly specific in both samples. Both instruments had a positive predictive value under 50%. CONCLUSIONS: Caution is needed when using the MDQ and HCL-32 in patients treated for substance use disorders.

A non-linear homogeneous model for bone-like materials under compressive load.

Finite element (FE) models accurately compute the mechanical response of bone and bone-like materials when the models include their detailed microstructure. In order to simulate non-linear behavior, which currently is only feasible at the expense of extremely high computational costs, coarser models can be used if the local morphology has been linked to the apparent mechanical behavior. The aim of this paper is to implement and validate such a constitutive law. This law is able to capture the non-linear structural behavior of bone-like materials through the use of fabric tensors. It also allows for irreversible strains using an elastoplastic material model incorporating hardening. These features are expressed in a constitutive law based on the anisotropic continuum damage theory coupled with isotropic elastoplasticity in a finite strain framework. This material model was implemented into metafor (LTAS-MNNL, University of Liège, Belgium), a non-linear FE software. The implementation was validated against experimental data of cylindrical samples subjected to compression. Three materials with bone-like microstructure were tested: aluminum foams of variable density (ERG, Oakland, CA, USA), polylactic acid foam (CERM, University of Liège, Liège, Belgium), and cancellous bone tissue of a deer antler (Faculty of Veterinary Medicine, University of Liège, Liège, Belgium).

The importance of murine cortical bone microstructure for microcrack initiation and propagation.

In order to better understand bone postyield behavior and consequently bone failure behavior, this study aimed first to investigate cortical bone microstructure and second, to relate cortical bone microstructure to microdamage initiation and propagation in C57BL/6 (B6) and C3H/He (C3H) mice; two murine inbred strains known for their differences in bone phenotype. Murine femora of B6 and C3H were loaded axially under compression in a stepwise manner. For each loading step, 3D data sets at a nominal resolution of 700 nm were acquired by means of synchrotron radiation-based computed tomography. Cortical bone microstructure was divided into three phases: the canal network, the osteocyte lacunar system, and microdamage. Canal volume density and canal unit volume both correlated highly to crack number density (canal volume density: R(2)=0.64, p<0.005 and canal unit volume: R(2)=0.75, p<0.001). Moreover, the large canal units in C3H bone were responsible for more microdamage accumulation compared to B6 bones. This more pronounced microdamage accumulation due to large intracortical bone voids, which eventually leads to a fatal macrocrack (fracture), represents a potential contributing factor to the higher incidence of bone fractures in the elderly.

Time-lapsed assessment of microcrack initiation and propagation in murine cortical bone at submicrometer resolution.

The strength of bone tissue is not only determined by its mass, but also by other properties usually referred to as bone quality, such as microarchitecture, distribution of bone cells, or microcracks and damage. It has been hypothesized that the bone ultrastructure affects microcrack initiation and propagation. Due to its high resolution, bone assessment by means of synchrotron radiation (SR)-based computed tomography (CT) allows unprecedented three-dimensional (3D) and non-invasive insights into ultrastructural bone phenotypes, such as the canal network and the osteocyte lacunar system. The aims of this study were to describe the initiation and propagation of microcracks and their relation with these ultrastructural phenotypes. To this end, femora from the two genetically distinct inbred mouse strains C3H/He (C3H) and C57BL/6 (B6) were loaded axially under compression, from 0% strain to failure, with 1% strain steps. Between each step, a high-resolution 3D image (700 nm nominal resolution) was acquired at the mid-diaphysis using SR CT for characterization and quantitative analysis of the intracortical porosity, namely the bone canal network, the osteocyte lacunar system and the emerging microcracks. For C3H mice, the canal, lacunar, and microcrack volume densities accounted typically for 1.91%, 2.11%, and 0.27% of the cortical total volume at 2% apparent strain, respectively. Due to its 3D nature, SR CT allowed to visualize and quantify also the volumetric extent of microcracks. At 2% apparent strain, the average microcrack thickness for both mouse strains was 2.0 microm for example. Microcracks initiated at canal and at bone surfaces, whereas osteocyte lacunae provided guidance to the microcracks. Moreover, we observed that microcracks could appear as linear cracks in one plane, but as diffuse cracks in a perpendicular plane. Finally, SR CT images permitted visualization of uncracked ligament bridging, which is thought to be of importance in bone toughening mechanisms. In conclusion, this study showed the power of SR CT for 3D visualization and quantification of the different ultrastructural phases of the intracortical bone porosity. We particularly postulate the necessity of 3D imaging techniques to unravel microcrack initiation and propagation and their effects on bone mechanics. We believe that this new investigation tool will be very useful to further enhance our understanding of bone failure mechanisms.

Time-lapsed investigation of three-dimensional failure and damage accumulation in trabecular bone using synchrotron light.

Synchrotron radiation micro-computed tomography (SRmicroCT) is a very useful technique when it comes to three-dimensional (3D) imaging of complex internal and external geometries. Being a fully non-destructive technique, SRmicroCT can be combined with other experiments in situ for functional imaging. We are especially interested in the combination of SRmicroCT with mechanical testing in order to gain new insights in the failure mechanism of trabecular bone. This interest is motivated by the immense costs in health care due to patients suffering from osteoporosis, a systemic skeletal disease resulting in decreased bone stability and increased fracture risk. To better investigate the different failure mechanisms on the microlevel, we have developed a novel in situ mechanical compression device, capable of exerting both static and dynamic displacements on experimental samples. The device was calibrated for mechanical testing using solid aluminum and bovine trabecular bone samples. To study different failure mechanisms in trabecular bone, we compared a fatigued and a non-fatigued bovine bone sample with respect to failure initiation and propagation. The fatigued sample failed in a burst-like fashion in contrast to the non-fatigued sample, which exhibited a distinct localized failure band. Moreover, microscopic cracks - microcracks and microfractures - were uncovered in a 3D fashion illustrating the failure process in great detail. The majority of these cracks were connected to a bone surface. The data also showed that the classification of microcracks and -fractures from 2D section can sometimes be ambiguous, which is also true for the distinction of diffuse and distinct microdamage. Detailed investigation of the failure mechanism in these samples illustrated that trabecular bone often fails in delamination, providing a mechanism for energy dissipation while conserving trabecular bone architecture. In the future, this will allow an even better understanding of bone mechanics related to its hierarchical structural organization.