Tornetta Rockwood Adults 9781975137298 V2

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SECTION ONE • General Principles

Figure 1-3.  Most biologic tissues are composed of multiple components, organized in a structurally opti- mized microstructure. They exhibit distinct mechanical properties, depending on the direction of loading (anisotropy; A ), as exemplified by the longitudinally oriented osteons of cortical bone, and the rate or speed of loading (viscoelasticity; B ), as shown for articular cartilage. A B

Arthroplasty implants are designed with a fatigue limit in excess of physiologic loading, and are not expected to fail in fatigue. Fracture fixation implants are designed to only carry load until the fracture consolidates. Therefore, if a bone fracture fails to unite, prolonged loading of the osteosynthesis construct will eventually lead to fatigue failure of fixation hardware. Analo- gous to material characterization under compressive loading described here, the stiffness, yield strength, ultimate strength, and fatigue limit of materials can also be determined under ten- sion, bending, torsion, and shear loading. Such comprehensive assessment of material properties specific for each principal loading mode is beyond the scope of this chapter but is well described in the literature. 93,152,197 Because biologic tissues are typically composed of multiple components to support unique functional properties, the mate- rial property characterization of biologic tissues is more complex than that of metals or polymers. For example, many tissues have fibrous components, whereby the fiber orientation delivers spe- cific material properties along distinct loading directions. Such direction-dependent material property is termed anisotropy . Bone is an anisotropic material, meaning it has different material prop- erties depending on the loading direction. The ultimate strength

of cortical bone in compression is 50% greater than in tension. Bone is also transversely anisotropic in that its stiffness is about 50% higher when loaded in a longitudinal direction parallel to its osteon orientation ( E = 17 GPa) than in the transverse direction ( E = 12 GPa) (Fig. 1-3A). This transversely anisotropic behav- ior of cortical bone is also evident in greater ultimate strength in a longitudinal direction (193 MPa) than in a transverse direc- tion (133 MPa). Materials such as titanium and stainless steel are isotropic , meaning they have the same properties regardless of the direction of loading, and their stiffness can be sufficiently described by a single E-modulus value. Tissues can also exhibit time-dependent viscoelastic prop- erties, whereby the stiffness of the tissue is not constant, but increases in response to faster loading. Conversely, if a static, constant load is applied to a viscoelastic tissue, such as articular cartilage, the resulting strain is not constant, but will gradually increase over time as interstitial fluid is being depleted from the loaded area (Fig. 1-3B). This gradual increase in deformation under constant loading of a viscoelastic material is called creep . Similarly, the stiffness and strength of bone vary depending on how fast it is loaded. For high loading rates, such as a fall from a height, the modulus of bone increases up to twofold, making it

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