Trabecular bone is a complex material with substantial heterogeneity Its elastic and strength properties vary widely across anatomic sites, and with aging and disease Although these properties depend very much on density, the role of architecture and tissue material properties remain uncertain It is interesting that the strains at which the bone fails are almost independent of density Current work addresses the underlying structure-function relations for such behavior, as well as more complex mechanical behavior, such as multiaxial loading, time-dependent failure, and damage accumulation A unique tool for studying such behavior is the microstructural class of finite element models, particularly the "high-resolution" models It is expected that with continued progress in this field, substantial insight will be gained into such important problems as osteoporosis, bone fracture, bone remodeling, and design/analysis of bone-implant systems This article reviews the state of the art in trabecular bone biomechanics, focusing on the mechanical aspects, and attempts to identify important areas of current and future research

Biomechanics of trabecular bone.

Trabecular bone is a highly porous, heterogeneous, and anisotropic material which can be found at the epiphyses of long bones and in the vertebral bodies. Studying the mechanical properties of trabecular bone is important, since trabecular bone is the main load bearing bone in vertebral bodies and also transfers the load from joints to the compact bone of the cortex of long bones. This review article highlights the high dependency of the mechanical properties of trabecular bone on species, age, anatomic site, loading direction, and size of the sample under consideration. In recent years, high resolution micro finite element methods have been extensively used to specifically address the mechanical properties of the trabecular bone and provide unique tools to interpret and model the mechanical testing experiments. The aims of the current work are to first review the mechanobiology of trabecular bone and then present classical and new approaches for modeling and analyzing the trabecular bone microstructure and macrostructure and corresponding mechanical properties such as elastic properties and strength.

Biomechanics and Mechanobiology of Trabecular Bone: A Review

Publisher Summary This chapter focuses on the biomechanical properties of bones. Bone is a physiologically dynamic tissue whose primary functions are to provide a mechanical support system for muscular activity, provide for the physical protection of organs and soft tissues, and act as a storage facility for systemic mineral homeostasis. The resulting structure of the skeleton is influenced heavily by mechanical principles, acting both as constraints and as driving forces in its architecture. The mechanical behavior of bone may be studied at two levels, material and structural. The material, or tissue, level properties of bone are evaluated by performing standardized mechanical tests on uniform bone tissue samples. They may also be estimated in vivo by using densitometric projections, but these are less accurate than actual mechanical testing. The assessment of bone mechanical properties can be made using techniques ranging from noninvasive imaging to in vitro mechanical tests of excised specimens or whole bones. Microcomputed tomography (CT), magnetic resonance imaging (MRI), and peripheral qualitative CT (pQCT) methods of imaging, especially in use with finite element models, continue to improve in their accuracy of mechanical property assessment. The strength and rigidity of a bone are typically greater in the direction of customary loading, particularly in cortical bone, where osteons are oriented in a longitudinal direction as indicated by the bone's loading history. Cortical bone is stronger in compression than in tension, and after maturation the tensile strength and the modulus of elasticity of femoral cortical bone decline by approximately 2% per decade. Another important mechanical property exhibited by bone is known as viscoelasticity, which is largely due to its water content, whereas material properties, such as strength and toughness, come from its solid-phase components.

Biomechanics of Bone

Publisher Summary The cells of bone influence its structure by means of four processes: growth, repair, modeling, and remodeling, the last being the basis of bone tissue turnover in the adult skeleton. The purposes of growth and repair are obvious. Modeling serves to adapt bones to changes in mechanical loading, a process that is most effective during growth. In the analysis of bone remodeling, it seems reasonable to make two assumptions. First, periodic replacement of bone serves to maintain its ability to carry out its functions. Second, because the most obvious difference between the old bone removed and the new bone put in its place is in their ages, excessive age of bone in some way compromises its functional capacity; in general, there is a reciprocal relationship between the rate of turnover and the mean age of the bone. Because the primary function of bone is mechanical, the primary purpose of remodeling of bone is to maintain its load-bearing capacity. This is accomplished partly by preventing structural damage at the microscopic and submicroscopic levels and partly by repairing such damage after it occurs. Further, the relative importance of the mechanical and metabolic aspects of remodeling, debated inconclusively for many years, is evidently different in different regions of the skeleton, although both are essential to the organism as a whole. Moreover, the most important non-mechanical function of bone remodeling concerns the regulation of calcium homeostasis. Bone is involved in both determining the steady-state target value for plasma-free calcium and correcting deviations from the target value. Both of these processes depend on a relatively high rate of bone remodeling, but in quite different ways. Bone mineral also functions as a reservoir for sodium and as a buffer for hydrogen ion regulation. Bone remodeling may also provide biochemical support for hematopoiesis as well as the mechanical support provided by the bone itself.

Skeletal Heterogeneity and the Purposes of Bone Remodeling: Implications for the Understanding of Osteoporosis

Additive manufacturing technique is a promising approach for fabricating cellular bone substitutes such as trabecular and cortical bones because of the ability to adjust process parameters to fabricate different shapes and inner structures. Considering the long term safe application in human body, the metallic cellular implants are expected to exhibit superior fatigue property. The objective of the study was to study the influence of cell shape on the compressive fatigue behavior of Ti-6Al-4V mesh arrays fabricated by electron beam melting. The results indicated that the underlying fatigue mechanism for the three kinds of meshes (cubic, G7 and rhombic dodecahedron) is the interaction of cyclic ratcheting and fatigue crack growth on the struts, which is closely related to cumulative effect of buckling and bending deformation of the strut. By increasing the buckling deformation on the struts through cell shape design, the cyclic ratcheting rate of the meshes during cyclic deformation was decreased and accordingly, the compressive fatigue strength was increased. With increasing bending deformation of struts, fatigue crack growth in struts contributed more to the fatigue damage of meshes. Rough surface and pores contained in the struts significantly deteriorated the compressive fatigue strength of the struts. By optimizing the buckling and bending deformation through cell shape design, Ti-6Al-4V alloy cellular solids with high fatigue strength and low modulus can be fabricated by the EBM technique.

The influence of cell morphology on the compressive fatigue behavior of Ti-6Al-4V meshes fabricated by electron beam melting.

Repetitive, low-intensity loading from normal daily activities can generate fatigue damage in trabecular bone, a potential cause of spontaneous fractures of the hip and spine. Finite element models of trabecular bone (Guo et al., 1994) suggest that both creep and slow crack growth contribute to fatigue failure. In an effort to characterize these damage mechanisms experimentally, we conducted fatigue and creep tests on 85 waisted specimens of trabecular bone obtained from 76 bovine proximal tibiae. All applied stresses were normalized by the previously measured specimen modulus. Fatigue tests were conducted at room temperature; creep tests were conducted at 4, 15, 25, 37, 45, and 53 degrees C in a custom-designed apparatus. The fatigue behavior was characterized by decreasing modulus and increasing hysteresis prior to failure. The hysteresis loops progressively displaced along the strain axis, indicating that creep was also involved in the fatigue process. The creep behavior was characterized by the three classical stages of decreasing, constant, and increasing creep rates. Strong and highly significant power-law relationships were found between cycles-to-failure, time-to-failure, steady-state creep rate, and the applied loads. Creep analyses of the fatigue hysteresis loops also generated strong and highly significant power law relationships for time-to-failure and steady-state creep rate. Lastly, the products of creep rate and time-to-failure were constant for both the fatigue and creep tests and were equal to the measured failure strains, suggesting that creep plays a fundamental role in the fatigue behavior of trabecular bone. Additional analysis of the fatigue strain data suggests that creep and slow crack growth are not separate processes that dominate at high and low loads, respectively, but are present throughout all stages of fatigue.

D. W. Cheng

Papers

Creep contributes to the fatigue behavior of bovine trabecular bone.