Meir Max Barak

Trabecular evidence for a human-like gait in Australopithecus africanus

Although the earliest known hominins were apparently upright bipeds, there has been mixed evidence whether particular species of hominins including those in the genus Australopithecus walked with relatively extended hips, knees and ankles like modern humans, or with more flexed lower limb joints like apes when bipedal. Here we demonstrate in chimpanzees and humans a highly predictable and sensitive relationship between the orientation of the ankle joint during loading and the principal orientation of trabecular bone struts in the distal tibia that function to withstand compressive forces within the joint. Analyses of the orientation of these struts using microCT scans in a sample of fossil tibiae from the site of Sterkfontein, of which two are assigned to Australopithecus africanus, indicate that these hominins primarily loaded their ankles in a relatively extended posture like modern humans and unlike chimpanzees. In other respects, however, trabecular properties in Au africanus are distinctive, with values that mostly fall between those of chimpanzees and humans. These results indicate that Au. africanus, like Homo, walked with an efficient, extended lower limb.

Are tensile and compressive Young’s moduli of compact bone different?

This study examines the question of whether the stiffness (Youngs modulus) of secondary osteonal cortical bone is different in compression and tension. Electronic speckle pattern interferometry (ESPI) is used to measure concurrently the compressive and tensile strains in cortical bone beams tested in bending. ESPI is a non-contact method of measuring surface deformations over the entire region of interest of a specimen, tested wet. The measured strain distributions across the beam, and the determination of the location of the neutral axis, demonstrate in a statistically-robust way that the tensile Youngs modulus is slightly (6%), but significantly greater than that of the compressive Youngs modulus. It is also shown that within a relatively small bone specimen there are considerable variations in the modulus, presumably caused by structural inhomogeneities.

Enamel dictates whole tooth deformation: a finite element model study validated by a metrology method.

In order to understand whole tooth behavior under load the biomechanical role of enamel and dentin has to be determined. We approach this question by comparing the deformation pattern and stiffness of intact teeth under load with the deformation pattern and stiffness of the same teeth after the enamel has been mechanically compromised by introducing a defect. FE models of intact human premolars, based on high resolution micro-CT scans, were generated and validated by in vitro electronic speckle pattern interferometry (ESPI) experiments. Once a valid FE model was established, we exploit the flexibility of the FE model to gain more insight into whole tooth function. Results show that the enamel cap is an intrinsically stiff biological structure and its morphology dictates the way a whole tooth will mechanically behave under load. The mechanical properties of the enamel cap were sufficient to mechanically maintain almost its entire stiffness function under load even when a small defect (cavity simulating caries) was introduced into its structure and breached the crown integrity. We conclude that for the most part, that enamel and not dentin dictates the mechanical behavior of the whole tooth.

Optical metrology methods for mechanical testing of whole bones.

Classical mechanical methods for testing whole bone have been critically assessed in a previous review where their limitations in terms of precision, accuracy and the amount of data yielded were described. This article describes the use of optical metrology methods and their novel adaptation to the study of whole bone response to mechanical load. Such methods overcome many of the limitations of mechanical testing: they do not require contact with the tested sample, are non-destructive, can be conducted on wet samples, and results comprise deformation maps of entire surfaces. The concepts upon which each method is based are reviewed, and examples of their use in biomechanical studies of bone are presented. Potential future applications that are expected to make significant contributions to the understanding of whole bone mechanics are outlined.

The contribution of trabecular bone to the stiffness and strength of rat lumbar vertebrae.

Study Design. In vitro compressive load-displacement experiments on intact rat lumbar vertebrae and on the same vertebrae after part of their trabecular bone was removed. Objective. To determine the contribution of the trabecular bone component to the stiffness and strength of rat lumbar vertebrae. Summary of Background Data. Vertebral fractures are common in the aging population, possibly resulting from the deterioration of the mechanical properties of vertebral bone. Studies of the contribution of trabecular bone to the mechanical behavior of whole vertebra were published, but yielded mixed results. Here, we propose a novel optical metrology approach to address this important question. Methods. The bodies of intact rat lumbar vertebrae and the bodies of the same vertebrae after part of their trabecular bone was removed were loaded within their elastic region in a wet environment. The amount of trabecular bone removed was determined by micro-computer tomography scanning. Deformation maps of the dorsal vertebral surface of the intact and manipulated vertebrae were obtained using an optical metrology method, and compared. Intact and manipulated vertebrae were also loaded to failure in compression and their strengths and stiffness were compared. Results. The preferred trabecular orientation was found to be along the anterior-posterior axis, which is similar to humans. Removal of up to 42% of the trabecular tissue in the intact vertebrae did not significantly affect lumbar vertebral stiffness. However, removal of even smaller amounts of the intact trabecular tissue significantly reduced vertebral strength. Conclusion. Trabeculae in rat lumbar vertebrae fulfill an important role in failure resistance (strength), but have little or no effect on the deformational behavior (stiffness) of the bone. These results differ from previous results we reported for rat femora, where removal of trabecular bone surprisingly increased the stiffness of the whole bone, and suggest that trabecular tissue may have different functions depending on anatomic location, bone function and morphology, and mode of loading.

Importance of the integrity of trabecular bone to the relationship between load and deformation of rat femora: an optical metrology study

Bone is a composite hierarchical structure composed of a cortical shell and inner trabecular tissue. One of the most basic questions in whole-bone function is the relative contributions of cortical and trabecular bone tissues to the loaded whole bone. In this study, the manner in which the cortical surfaces of an intact proximal rat femur deform under load is compared to the same femur after some of the trabecular bone in the distal femoral neck was removed. The surface displacements were measured by electronic speckle pattern interferometry (ESPI) and the extent of trabecular bone removed was determined by high resolution micro-CT scanning. The results show that after damaging the trabecular bone tissue in the distal femoral neck, the manner in which compressive loads are transformed to other regions of the femoral neck changed. The whole bone behaved in a ‘stiffer’ manner. This demonstrates the importance of connectivity of the trabeculae and that beyond a certain threshold of damage the normal load-transferring mechanism is impaired. Since these experiments were carried out in a non-contact non-destructive manner in a wet environment and the rat femur was loaded in a close-to physiological manner, we postulate that our results have a direct relevance to the in vivo biomechanical behavior of the femoral neck.

Tumorigenic potential and disease manifestations of malignant B-cell variants differing in their fibronectin adhesiveness

OBJECTIVE Microenvironmental interactions of malignant B cells can modulate various in vitro physiological responses, including proliferation, migration, apoptosis, and drug resistance. Disease manifestations of human malignant B-cell variants, isolated based on their differential interactions with fibronectin, were examined in nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. MATERIALS AND METHODS Disease manifestations were assessed by pathological examinations and skeletal imaging of NOD/SCID mice injected with malignant B-cell variants. Dissemination patterns were analyzed by whole-body real-time imaging of mice injected with fluorescence-labeled malignant cells. RESULTS Initial dissemination patterns and dynamics of both high (type A) and low (type F)-adherent variants, following intravenous inoculation, were similar. Both cell types reached the spleen and liver within 30 minutes after injection, then increasingly accumulated within the bone marrow. Mice injected with type-A cells developed multiple myeloma-like disease within the bone marrow, with multiple lytic bone lesions. In contrast, type-F cells displayed low tumorigenic capacity in spite of their efficient homing to the bone marrow niche. In addition, type-A cells grew as extramedullary tumors in some of the intravenous-inoculated mice, and formed solid tumors following subcutaneous injection. Both cell variants retained their characteristics surface markers following in vivo outgrowth as tumors, indicating that at least some of their properties are relatively stable. CONCLUSION Data suggest that the differential tumorigenicity of B-cell adhesive variants is attributable to the capacity of type-A cells to survive and proliferate within the bone marrow, rather than to different initial dissemination of the two cell populations.

Tooth and bone deformation: structure and material properties by ESPI

In order to understand complex-hierarchical biomaterials such as bones and teeth, it is necessary to relate their structure and mechanical-properties. We have adapted electronic speckle pattern-correlation interferometry (ESPI) to make measurements of deformation of small water-immersed specimens of teeth and bones. By combining full-field ESPI with precision mechanical loading we mapped sub-micron displacements and determined material-properties of the samples. By gradually and elastically compressing the samples, we compensate for poor S/N-ratios and displacement differences of about 100nm were reliably determined along samples just 2~3mm long. We produced stress-strain curves well within the elastic performance range of these materials under biologically relevant conditions. For human tooth-dentin, Youngs modulus in inter-dental areas of the root is 40% higher than on the outer sides. For cubic equine bone samples the compression modulus of axial orientations is about double the modulus of radial and tangential orientations (20 GPa versus 10 GPa respectively). Furthermore, we measured and reproduced a surprisingly low Poissons ratio, which averaged about 0.1. Thus the non-contact and non-destructive measurements by ESPI produce high sensitivity analyses of mechanical properties of mineralized tissues. This paves the way for mapping deformation-differences of various regions of bones, teeth and other biomaterials.

The orthotropic elastic properties of fibrolamellar bone tissue in juvenile white-tailed deer femora.

Fibrolamellar bone is a transient primary bone tissue found in fast‐growing juvenile mammals, several species of birds and large dinosaurs. Despite the fact that this bone tissue is prevalent in many species, the vast majority of bone structural and mechanical studies are focused on human osteonal bone tissue. Previous research revealed the orthotropic structure of fibrolamellar bone, but only a handful of experiments investigated its elastic properties, mostly in the axial direction. Here we have performed for the first time an extensive biomechanical study to determine the elastic properties of fibrolamellar bone in all three orthogonal directions. We have tested 30 fibrolamellar bone cubes (2 × 2 × 2 mm) from the femora of five juvenile white‐tailed deer (Odocoileus virginianus) in compression. Each bone cube was compressed iteratively, within its elastic region, in the axial, transverse and radial directions, and bone stiffness (Youngs modulus) was recorded. Next, the cubes were kept for 7 days at 4 °C and then compressed again to test whether bone stiffness had significantly deteriorated. Our results demonstrated that bone tissue in the deer femora has an orthotropic elastic behavior where the highest stiffness was in the axial direction followed by the transverse and the radial directions (21.6 ± 3.3, 17.6 ± 3.0 and 14.9 ± 1.9 Gpa, respectively). Our results also revealed a slight non‐significant decrease in bone stiffness after 7 days. Finally, our sample size allowed us to establish that population variance was much bigger in the axial direction than the radial direction, potentially reflecting bone adaptation to the large diversity in loading activity between individuals in the loading direction (axial) compared with the normal (radial) direction. This study confirms that the mechanically well‐studied human transverse‐isotropic osteonal bone is just one possible functional adaptation of bone tissue and that other vertebrate species use an orthotropic bone tissue structure which is more suitable for their mechanical requirements.

THE CONTRIBUTION OF TRABECULAR BONE TISSUE TO THE

Introduction Bone is a hierarchical material composed at its highest level of compact and trabecular bone tissues [Weiner, 1999]. The mechanical behavior of whole bone is a result of its material properties, (which are determined primarily by mineral content and porosity [Currey, 2003]), and its geometric architecture [Weiner, 1998]. Although each of these two bone tissue types has been studied extensively, much less is known about their relative contributions to whole bone function. This fundamental question is addressed using novel tools for investigating whole bone response to physiological load.