ReviewWhole bone mechanics and mechanical testing
Introduction
Understanding the relationship between the mechanical responses of whole entities, their material properties and their shape, is a challenge. This is greatly enhanced when the material itself is complex, and when the entity it forms has a convoluted shape. This is almost always the case in biology. Despite major technical advances made in recent years in experimental techniques, we are still unable to predict and fully understand the mechanical functions of whole biological entities such as bones.
Various attempts have been made to address these objectives. These have been based mainly on mechanically measuring deformations of whole bones at a few individual locations (Burr et al., 1996, Milgrom et al., 2004), or developing constitutive theoretical models that can predict the behaviour of the whole bone based on the mechanical properties of its individual constituent materials (Silva et al., 2005). However due to limitations inherent in these methods, both approaches have, for the most part, only been partially successful; single-point strain measurements (the concept of ‘strain’ is defined in detail below) miss local variations that are extremely significant in a complex structure such as whole bone, while the accuracy of numerical models relies on knowledge of the material properties that is not available. Recent advances in optical metrology have opened up new opportunities as they enable the precise and accurate mapping of the manner in which the entire surface of a whole bone deforms.
Detailed data describing the displacement maps of the entire surface of bones create the exciting possibility of relating the complex distribution of mechanical properties of loaded bone and its microstructures to deformations and strains. Such studies could improve our understanding of normal physiological processes, such as skeletal aging, as well as disease processes such as osteoporosis. They also provide opportunities for engineers designing bio-inspired materials to study the principles, advantages and characteristics of the behaviour of hierarchical and multifunctional materials (Gao, 2006, Mayer, 2005). In this review we shall describe the material bone, introduce the basic terms used when describing mechanics of bones, and describe the methods commonly used to test the mechanical performance of whole bone.
Section snippets
The material bone
‘Bone’ is a term used to describe both the organ bone and the material it is made of. The material of all bones has the same constituents. These include mineral, carbonated hydroxyapatite (also known as dahllite), the framework protein type I collagen, many other so-called non-collagenous proteins and water (Delmas et al., 1984).
The material of bones has a hierarchical structure that changes at different length scales (Fig. 1). It is also a graded material, as its composition, structure, and
Basic concepts of mechanics of materials
The mechanical behaviour of a structure is determined by its geometry and the properties of the material (or materials) of which it is made. The material properties are independent of the geometry and are inherent to the material itself. The determination of these properties requires definition of the concepts ‘stress’ and ‘strain’. The following is a short, and by no means rigorous or complete, description of these concepts. Further information can be found in texts on the theory of elasticity
Mechanics of whole bones
Study of the mechanical performance of whole bones is clearly of much interest to researchers and clinicians alike. However, due to its complexity as described above, this is a most challenging and technically difficult endeavour. The stiffness of a structure (the extent to which it deforms under a particular load) depends on the type of loading, its geometry and on the mechanical properties of the materials of which it is made. This concept will be demonstrated using a structure which bears
Mechanical testing methods
Most investigations of the mechanical performance of whole bones rely on in vivo or in vitro experiments in which bones were tested in compression or by 3- or 4-point bending. Torsional loading and impact loading are also used, though less frequently. Measurements in these experiments consist of single values like load-to-yield and load-to-failure. Strain measurements are made at a very limited number of points (1–20) by placing strain gauges on the surface of bones. Alternatively, theoretical
Bending
Bending tests are by far the most common methods used to test whole bones. They are used particularly to characterise the mechanical behaviour of bones of small experimental animals such as mice and rats belonging to different strains or treated by various drugs, which may affect the skeleton (Rubin and Lanyon, 1985, Robling et al., 2001, Ammann et al., 2003, Warden et al., 2005, Lane et al., 2006, Judex et al., 2007). Whole-bone bending tests are mostly 3-point or 4-point bending experiments.
Torsion
Torsional testing of whole bone is accomplished by firmly embedding the epiphyseal ends of the tested bone in rectangular or cylindrical blocks of plastic material which are fitted into the grips of the torsion testing machine. Using one of several available testing devices, a torque (twisting moment) is applied to one of these grips while the other is kept firm, and load and angular deformation are recorded. This setup allows the approximate determination of the shear modulus of bone.
Impact loading
The simulation of trauma-associated loading conditions requires high strain rates and is very important in the prediction of bone behaviour under sudden loads in non-physiological directions (i.e. falling). This fact stimulated the development of a class of impact-type loading devices, such as pendulum loading, which uses a hammer of precisely known weight which is dropped from a known height (thus its potential energy is known) and hits the sample in impact; such experiments can yield
Conclusions
Bone is a composite hierarchical material, therefore investigation of the relationship between the materials properties and the geometry and mechanical behaviour of whole bone is challenging and very complicated. A thorough understanding of this relationship is of importance to clinicians and researchers alike; it helps to understand the normal behaviour of whole bones during physiological loading, identifies areas of peak stresses which are more likely to fracture during intense activity, and
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