Reference point indentation is not indicative of whole mouse bone measures of stress intensity fracture toughness

Abstract

Bone fragility is a concern for aged and diseased bone. Measuring bone toughness and understanding fracture properties of the bone are critical for predicting fracture risk associated with age and disease and for preclinical testing of therapies. A reference point indentation technique (BioDent) has recently been developed to determine bone's resistance to fracture in a minimally invasive way by measuring the indentation distance increase (IDI) between the first and last indentations over cyclic indentations in the same position. In this study, we investigate the relationship between fracture toughness KC and reference point indentation parameters (i.e. IDI, total indentation distance (TID) and creep indentation distance (CID)) in bones from 38 mice from six types (C57Bl/6, Balb, oim/oim, oim/+, Phospho1−/− and Phospho1 wild type counterpart). These mice bone are models of healthy and diseased bone spanning a range of fracture toughness from very brittle (oim/oim) to ductile (Phospho1−/−). Left femora were dissected, notched and tested in 3-point bending until complete failure. Contralateral femora were dissected and indented in 10 sites of their anterior and posterior shaft surface over 10 indentation cycles. IDI, TID and CID were measured. Results from this study suggest that reference point indentation parameters are not indicative of stress intensity fracture toughness in mouse bone. In particular, the IDI values at the anterior mid-diaphysis across mouse types overlapped, making it difficult to discern differences between mouse types, despite having extreme differences in stress intensity based toughness measures. When more locations of indentation were considered, the normalised IDIs could distinguish between mouse types. Future studies should investigate the relationship of the reference point indentation parameters for mouse bone in other material properties of the bone tissue in order to determine their use for measuring bone quality.

Tissue-Level Mechanical Properties of Bone Contributing to Fracture Risk

“…resistance to crack initiation and propagation) for a set of single-edge notched beams from 62 human femurs (64±22 [21–99] yo) [29]. On the other hand, Carriero et al [73] argued that cRPI is not indicative of fracture toughness as indentation depths were higher in mouse bones with very brittle ( oim/oim ) and with very ductile ( Phospho1 −/− ) behavior, compared to their controls. However, since cRPI outcomes do not only reflect differences in fracture toughness, it is conceivable that the higher indentation depth observed in the oim model occurred because the brittle bone was susceptible to local damage generated by the probe, while the higher indentation depth observed in the Phospho1 −/− model occurred because poorly mineralized bone with a reduced hardness lowered resistance to indentation.…”

An Investigation of the Mineral in Ductile and Brittle Cortical Mouse Bone

“…These mice lack a phosphatase, which is required for the generation of inorganic phosphate for bone mineralization (19–21). Phospho1 −/− mice have more ductile bones at the macroscopic scale (13,19), and reduced mineral to matrix ratio as shown by Raman experiments (19). …”

“…Two mouse phenotypes were chosen, based on known fracture toughness values (Fig. 1) to represent brittle and ductile bone (13,14). We used osteogenesis imperfecta murine ( oim −/− ) mice, which replicates the moderate to severe condition of osteogenesis imperfecta in humans.…”

“…Osteogenesis imperfecta, also called brittle bone disease, is primarily caused by mutations in type 1 collagen genes, which lead to bone fragility (11,15,16). Oim −/− bone has decreased ultimate stress (11) and toughness (13,14) at the whole bone level, higher mineral density measured by backscattered techniques (11,17) and smaller and less arranged apatite crystals (8,11,17,18). For ductile bones we used PHOSPHO1 ( Phospho1 −/− ).…”

Understanding the structure–property relationship in cortical bone to design a biomimetic composite

“…Moreover, novel less invasive methodologies for in vivo measuring fracture toughness on small animals are of great clinical relevance, since they can be helpful in understanding the material properties of bone during preclinical testing for reducing fracture risks [42]; At nano-to subnanoscale, numerical simulations -from atomistic to coarse grain -allow researchers to examine the small scale chemo-mechanical behavior, studying characteristic phenomena, which are difficult to be reached by experiments [43].…”