This study tests Wolff's law of trabecular bone adaptation by examining if induced changes in joint loading orientation cause corresponding adjustments in trabecular orientation. Two groups of sheep were exercised at a trot, 15 min/day for 34 days on an inclined (7°) or level (0°) treadmills. Incline trotting caused the sheep to extend their tarsal joints by 3-4.5° during peak loading (P<0.01) but has no effect on carpal joint angle (P=0.984). Additionally, tarsal joint angle in the incline group sheep were maintained more extended throughout the day using elevated platform shoes on their forelimbs. A third "sedentary group" group did not run but wore platform shoes throughout the day. As predicted by Wolff's law, trabecular orientation in the distal tibia (tarsal joint) were more obtuse by 2.7 to 4.3° in the incline group compared to the level group; trabecular orientation was not significantly different in the sedentary and level groups. In addition, trabecular orientations in the distal radius (carpal joint) of the sedentary, level and incline groups did not differ between groups, and were aligned almost parallel to the radius long axis, corresponding to the almost straight carpal joint angle at peak loading. Measurements of other trabecular bone parameters revealed additional responses to loading, including significantly higher bone volume fraction (BV/TV), Trabecular number (Tb.N) and trabecular thickness (Tb.Th), lower trabecular spacing (Tb.Sp), and less rod-shaped trabeculae (higher structure model index, SMI) in the exercised than sedentary sheep. Overall, these results demonstrate that trabecular bone dynamically adjusts and realigns itself in very precise relation to changes in peak loading direction, indicating that Wolff's law is not only accurate but also highly sensitive.
STUDY DESIGNIn vitro compressive load-displacement experiments on intact rat lumbar vertebrae and on the same vertebrae after part of their trabecular bone was removed.
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.
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.
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.
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.
This study examines the question of whether the stiffness (Young's 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 Young's modulus is slightly (6%), but significantly greater than that of the compressive Young's modulus. It is also shown that within a relatively small bone specimen there are considerable variations in the modulus, presumably caused by structural inhomogeneities.
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.
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.
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.
The mechanical behaviour of material bone can be completely described by a group of material properties. The mechanical behaviour of the entire bone organ, however, is much more difficult to predict; it is the result both of the properties of the material of which it is made, and of the geometric spatial architecture in which this is arranged. This review first describes material bone in terms of its complex, graded and hierarchical structure. Basic concepts used in the field of mechanics of materials are defined and explained and then used to describe the mechanical properties of whole bone. Some clinical implications of these properties are provided. Commonly used mechanical testing methods for the study of the mechanical behaviour of whole bone are reviewed and the technical difficulties associated with them are discussed.
Young's modulus and Poisson's ratios of 6mm-sized cubes of equine cortical bone were measured in compression using a micro-mechanical loading device. Surface displacements were determined by electronic speckle pattern-correlation interferometry. This method allows for non-destructive testing of very small samples in water. Analyses of standard materials showed that the method is accurate and precise for determining both Young's modulus and Poisson's ratio. Material properties were determined concurrently in three orthogonal anatomic directions (axial, radial and transverse). Young's modulus values were found to be anisotropic and consistent with values of equine cortical bone reported in the literature. Poisson's ratios were also found to be anisotropic, but lower than those previously reported. Poisson's ratios for the radial-transverse and transverse-radial directions were 0.15+/-0.02, for the axial-transverse and axial-radial directions 0.19+/-0.04, and for the transverse-axial and radial-axial direction 0.09+/-0.02 (mean+/-SD). Cubes located only millimetres apart had significantly different elastic properties, showing that significant spatial variation occurs in equine cortical bone.
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, Young's 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 Poisson's 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.