Understanding bone strength: size isn’t everything

In vivo models, particularly mouse mutations, are increasingly being used to investigate the impact of the absence or overexpression of a gene product on musculoskeletal load-bearing capacity. Skeletal functional integrity can be assessed by structural strength tests that measure how well the whole bone can bear loads. Although the importance of performing these tests is well-recognized, care must be taken in designing the experiments and interpreting the data. The aim of this report is to clarify the relationship of whole bone structural strength to material and geometric properties and the interpretation of these data in the context of in vivo models, especially mice. In particular, we emphasize that there is no alternative to testing whole bone strength and that conclusions regarding bone mechanical function based solely on geometry or bone mineral content are inappropriate and likely misleading. What is a whole bone structural test and what does it measure? Different types of loads, such as bending or torsion, can be applied to whole bones in vitro to determine the structure’s stiffness and failure load (structural strength). The structural stiffness is a measure of the resistance to deformation under the applied load, and the structural strength is the load required to fail the whole bone. These two whole bone measurements are structural properties and are influenced by both the material from which the structure is composed (the tissue material properties) as well as how and where that material is distributed (the geometric form of the tissue) (Figure 1). Therefore, both material and geometric properties are required to assess the structural integrity of a long bone, and neither material nor geometry alone is sufficient to predict the structural failure load. Currently, there is no substitute for a mechanical test to measure whole bone structural behavior; no alternative parameter has been identified that is fully indicative of strength and can serve as a surrogate measure. Bone material properties are the tissue level mechanical properties that describe the constituent material and are independent of the size and shape of the bone. Material properties include the tissue ultimate stress and modulus of elasticity. These tissue properties are determined by machining precise samples from the bone of interest and testing them in a particular loading mode. The material properties are influenced by compositional measures such as mineral density, collagen content, and ash fraction. In addition to composition, factors such as collagen cross-linking, collagen fiber orientation, mineral crystal size, and the microstructural organization (e.g., lamellae, osteons) also influence material behavior. From a mechanical perspective, the composition and organization of the material clearly influence the tissue’s ability to bear loads, but most measures, except for mineral density, have not yet been related directly to the tissue properties derived from mechanical tests. When designing a structural test, the relevant material and geometric measures are determined by the loading mode applied to the whole bone to measure strength (e.g., torsion, bending, or compression) as well as the outcome parameter of interest (e.g., stiffness or failure load) (Figure 1). For example, if we test a bone to failure in torsion, then we will measure the torsional load to failure (a structural parameter). The appropriate geometric and material properties are the torsional section modulus (a geometric parameter) and the ultimate shear stress of the bone (a material parameter). The section modulus represents the geometric resistance to torsion and increases as the material lies further from the axis of rotation (Figure 2). The ultimate shear stress is the strength of the bone tissue when loaded in torsion. A biomechanics tutorial by Turner and Burr has provided a more complete presentation of mechanical assessment of whole bone and bone tissue. The contribution of structural, geometric and material analyses can be illustrated with a hypothetical example (Figure 3). Consider the case of a mutant and wild-type comparison in which animal age, gender, and weight are matched, but the bone material and geometry may be affected by the mutation. A whole bone torsion test to failure showed that both the control and mutant failed at the same torque of 8.7 Nzmm. Based on this analysis alone, we would conclude that the mutation had no effect. Additional analyses, either geometric or material, would be necessary to reveal the true effect of the mutation. On the other hand, if we only measure the geometry, we find the mutant bone to have a 21% lower section modulus than wild-type. Therefore, based on geometry alone, we might conclude that the mutant is structurally weaker than the control. However, combined with the structural information, we would know that the smaller mutant bones must have increased material properties to achieve the same structural failure load. Conversely, a tissue level material test would determine that the ultimate shear stress is 26% higher in the mutant than the wild-type. Therefore, based on material differences alone, we might conclude that the mutant is structurally stronger than the control. In each case, global conclusions based on a single analysis (structural, geometric, or material) are different, contradictory, and potentially incorrect. Ideally, all three tests would be required, but, at a minimum, combining two analyses is sufficient to understand the effect of the mutation on the structural properties of the whole bone. Address for correspondence and reprints: Marjolein C. H. van der Meulen, Ph.D., Sibley School of Mechanical and Aerospace Engineering, Cornell University, 219A Upson Hall, Ithaca, NY 14853. E-mail: mcv3@cornell.edu Bone Vol. 29, No. 2 August 2001:101–104