Indentation of bone tissue: a short review

Abstract

IntroductionBone is a highly hierarchical natural composite material,which mechanical properties are investigated from thephysiological elastic behavior up to impact or fatiguedamage accumulation responsible for traumatic or stressfractures. Each hierarchical level of organization contrib-utes to the global mechanical response of a bone structure:– The mineralized collagen fibril (MCF, 200 nm)– The lamella (2–7 μm)– The bone structural unit (BSU, 60 μm)– Bone tissue, cortical shell, or trabeculae (100–3,000 μm)– Trabecular bone (TB, mm)– Organ (cm)Bone matrix quality becomes a central focus in under-standing the etiology of fractures in diseases such asosteoporosis. Macroscopic indentation was adopted in thefirst half of the 20th century to investigate hardness of boneat the tissue level (compact and trabecular) and wasextended to micro- and nano-indentation in the past 10 yearsto evaluate specific mechanical properties of bone structuralunits and single lamellae, respectively. Over this period oftime, more than 375 papers or reviews were published(Scopus, “*indentation and bone,” June 23rd 2008).Consequently, the objective of this paper is to review therecent progress of indentation techniques to evaluate themechanical properties of human bone at these intermediatelevels of organization.Materials and methodsIndentationIndentation consists in pressing a hard tip with a knownforce into a semi-infinite half-space and measuring directlyor indirectly the contact area. In the classical hardness testperformed at the macroscopic or microscopic level, thecontact area is estimated optically from the imprint createdby the tip on the material and leads to the definition ofhardness as the force divided by this area. The mostcommon hardness measuring indenters are sphere (Brinell,Rockwell), four-sided pyramid (Vickers), and asymmetricpyramid (Knoop).Recent depth-sensing technologies allow for measure-ment of the tip displacement during the indentation processof a semi-infinite half-space with micro- and even nano-precision. Indirect estimation of the contact area is obtainedby preliminary calibration of the tip shape and systemcompliance. Shallow indentation depths down to 100 nmallow for spatial resolution of the measurements in themicron range, and the sample position is typically con-trolled by high-precision motorized tables or piezoelectricscanners. Micro- and nano-indentation tips are often madeof diamond and can be found in spherical, conical, andmost commonly, three-sided pyramidal (Berkovich) shapes.The blunted extremity of the tip has a radius of approxi-mately 100 nm. Spherical tips minimize plastic deformationand damage but are difficult to manufacture, flat puncheslead to high stress concentrations, conical tips have an axialsymmetry that remains in the assumption of most theoret-