Figure 9 - available via license: Creative Commons Attribution 4.0 International
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Individual slice images before and after indentation showing plastic deformation, collapse and cracking as a result of the indenter at a distance of 66 µm from the indenter tip (see also animated sequences in supplementary video online).
Source publication
Bone is a complex material comprising high stiffness, but brittle, crystalline bio-apatite combined with compliant, but tough, collagen fibres. It can accommodate significant deformation, and the bone microstructure inhibits crack propagation such that micro-cracks can be quickly repaired. Catastrophic failure (bone fracture) is a major cause of mo...
Context in source publication
Context 1
... Figure 9, we can see how a crack interacts with an existing pore within the bone structure, and the growth of the crack appears to be terminated at this point. However, the 2D view of cracks and their interactions with the pores illustrated in these figures can be highly misleading. ...
Citations
... Moreover, most indentation tests have so far restricted the examination of cracks to the material surface upon unloading using scanning electron microscopy [22,24], thus, hindering the deformation analysis of the material below the indenter tip during indentation. Recent in situ micro-computed tomography (μCT) indentation tests have proved to be useful to visualise the three-dimensional (3D) cracking and fracture behaviour not only in bone [25,26], but also in dentin [27], and non-biological brittle materials [28,29]. Most importantly, the deformation beneath the indenters could be measured by digital volume correlation (DVC) of the acquired tomographs [25,27,29]. ...
The development of treatment strategies for skeletal diseases relies on the understanding of bone mechanical properties in relation to its structure at different length scales. At the microscale, indention techniques can be used to evaluate the elastic, plastic, and fracture behaviour of bone tissue. Here, we combined in situ high-resolution SRμCT indentation testing and digital volume correlation to elucidate the anisotropic crack propagation, deformation, and fracture of ovine cortical bone under Berkovich and spherical tips. Independently of the indenter type we observed significant dependence of the crack development due to the anisotropy ahead of the tip, with lower strains and smaller crack systems developing in samples indented in the transverse material direction, where the fibrillar bone ultrastructure is largely aligned perpendicular to the indentation direction. Such alignment allows to accommodate the strain energy, inhibiting crack propagation. Higher tensile hoop strains generally correlated with regions that display significant cracking radial to the indenter, indicating a predominant Mode I fracture. This was confirmed by the three-dimensional analysis of crack opening displacements and stress intensity factors along the crack front obtained for the first time from full displacement fields in bone tissue. The X-ray beam significantly influenced the relaxation behaviour independent of the tip. Raman analyses did not show significant changes in specimen composition after irradiation compared to non-irradiated tissue, suggesting an embrittlement process that may be linked to damage of the non-fibrillar organic matrix. This study highlights the importance of three-dimensional investigation of bone deformation and fracture behaviour to explore the mechanisms of bone failure in relation to structural changes due to aging or disease. STATEMENT OF SIGNIFICANCE: : Characterising the three-dimensional deformation and fracture behaviour of bone remains essential to decipher the interplay between structure, function, and composition with the aim to improve fracture prevention strategies. The experimental methodology presented here, combining high-resolution imaging, indentation testing and digital volume correlation, allows us to quantify the local deformation, crack propagation, and fracture modes of cortical bone tissue. Our results highlight the anisotropic behaviour of osteonal bone and the complex crack propagation patterns and fracture modes initiating by the intricate stress states beneath the indenter tip. This is of wide interest not only for the understanding of bone fracture but also to understand other architectured (bio)structures providing an effective way to quantify their toughening mechanisms in relation to their main mechanical function.
... Similar to Hengsberger et al., they used different parts of the same specimen for the imaging and indentation. More recently, Lowe et al., performed an in situ high-resolution XCT indentation in dried mouse femoral head, using a lab-XCT system, to visualise and track plastic deformation and crack propagation; an experiment that lasted 52 h (Lowe et al., 2018). Conventional lab-based XCTs use polychromatic x-ray cone beam, whereas Synchrotron CTs use monochromatic parallel beam. ...
... High-resolution XCT has been extensively used to assess the microstructure of trabecular bone (Gillard et al., 2014;Judex et al., 2003;Levrero-Florencio et al., 2016). To investigate features in the micro/nano-scale, such as the residual deformation of the bone caused by the indentation and additionally extract important and more localised mechanical properties of bone tissue (i.e. via DVC), high-resolution and long exposure scans are required (Lowe et al., 2018;Peña Fernández et al., 2019). Long exposure to radiation is known to affect the mechanical properties of cortical bone and cartilage (Akkus et In this study, indentation was performed to assess the bone tissue mechanical properties before and after exposure in the X-CT. ...
Exposure to X-ray radiation for an extended amount of time can cause damage to the bone tissue and therefore affect its mechanical properties. Specifically, high-resolution X-ray Computed Tomography (XCT), in both synchrotron and lab-based systems, has been employed extensively for evaluating bone micro-to-nano architecture. However, to date, it is still unclear how long exposures to X-ray radiation affect the mechanical properties of trabecular bone, particularly in relation to lab-XCT systems. Indentation has been widely used to identify local mechanical properties such as hardness and elastic modulus of bone and other biological tissues. The purpose of this study is therefore, to use indentation and XCT-based investigative tools such as digital volume correlation (DVC) to assess the microdamage induced by long exposure of trabecular bone tissue to X-ray radiation and how this affects its local mechanical properties. Trabecular bone specimens were indented before and after X-ray exposures of 33 and 66 h, where variation of elastic modulus was evaluated at every stage. The resulting elastic modulus was decreased, and micro-cracks appeared in the specimens after the first long X-ray exposure and crack formation increased after the second exposure. High strain concentration around the damaged tissue exceeding 1% was also observed from DVC analysis. The outcomes of this study show the importance of designing appropriate XCT-based experiments in lab systems to avoid degradation of the bone tissue mechanical properties due to radiation and these results will help to inform future studies that require long X-ray exposure for in situ experiments or generation of reliable subject-specific computational models.
... In physiological conditions, especially regarding microcracks [10], this linear damage occurs in compression as it appears as ellipsoidal planes of separation, particularly in interstitial, extra-osteonal, highly mineralized areas within the cortex [11]. Besides the microcracks, other types of bone microdamage (e.g., microfractures and diffuse damage) are expected to occur and are important in the events leading to remodeling, both in cortical and cancellous bone [12,13]. ...
Objective
The aim of this study was to evaluate bone microdamage in sites prepared for implant placement by using an ex vivo model with three drilling rotation speeds.
Methods
Bovine bone ribs were used for the creation of 18 osteotomy sites at different rotation speeds: 1200 rpm, 800 rpm, and 400 rpm. Specimens were stained with xylenol orange and prepared for histological analysis by using fluorescence and polarized light microscopies. Bone microdamage was evaluated by number and based on total bone area, as follows: microfracture density (Fr.D = n/mm²), microcrack morphology (diffuse or linear), and density (Cr.D = n/mm²), and presence of bone chips. To complement the analysis, linear microcracks were assessed by using confocal microscopy for three-dimensional visualization.
Results
Bone microdamage on the osteotomy sites included microcracks, diffuse damages, microfracture, and bone chip formation. There was an association between bone microdamage and cancellous bone (p 0.0016), as well as a positive correlation between Fr.D and Cr.D (p 0.05, r 0.54). BM occurrence was not different between the three rotation speeds. In 3D, the height of the microcrack depth was 60.81 µm.
Conclusion
Bone microdamage occurs during osteotomy, and the ex vivo model used was effective for the assessment of these biomechanical parameters. In addition, microdamage was not influenced by the drilling rotation speed in this experimental condition.
Indexing terms
Bone and bones; Histology; Osteotomy
