Effect of Compressive Straining on Nanoindentation Elastic Modulus of Trabecular Bone

Abstract

Trabecular bone with its porous structure is an important compressive load bearing member. Different structural factors such as porosity, non-homogeneous deformation, varying trabeculae thickness, connectivity, and nanoscale (10 nm to 1 μm) to macroscale (~0.1 mm to 10 mm) composition hierarchy determine the failure properties of trabecular bone. While the above factors have important bearing on bone properties, an understanding of how the local nanoscale properties change at different macroscale compressive strain levels can be important to develop an understanding of how bone fails. In the present work, such analyses are performed on bovine femoral trabecular bone samples derived from a single animal. Analyses focus on measuring nanoindentation elastic moduli at three distinct levels of compressive strains in the bone samples: (1) when the samples are not loaded; (2) after the samples have been loaded to a strain level just before apparent yielding and the macroscale compression test is stopped; and (3) after the samples have been compressed to a strain level after apparent yielding and the macroscale compression test is stopped. Nanoindentation elastic modulus values are two orders of magnitude higher than the macroscale compressive elastic modulus values of all samples. A high variability in macroscale compressive elastic modulus values is observed because of porous architecture and small sample size. Nanoindentation elastic modulus values show a progressive reduction with increase in the extent of macroscale compressive deformation. Apparent yielding has a significant effect on this trend. The decrease in nanoindentation modulus value for all samples accelerates from approximately 20% before yielding to approximately 60% after yielding in comparison to the nanoindentation modulus values at 0% strain level. The level of variation in the predicted nanoindentation modulus values is the lowest for uncompressed samples (~16-18%). However, with increase in the extent of compression, the level of variation increases. It varied between 50% and 90% for the samples tested after yielding showing a widespread heterogeneity in local nanoscale structural order after apparent yielding. Scanning electron microscope (SEM) observations suggest that apparent yielding significantly destroys local nanoscale structural order. However, quantitative results suggest that a significant residual nanoscale stiffness varying from 5 GPa to 8 GPa among different samples still remains for possible repair facilitation.

Solvent specific persistence length of molecular type I collagen

“…For example, a persistence length of ∼18 μm (midway between the lowest and highest projected 3D‐WLC persistence length values) would imply an elastic modulus of 19 GPa for a cylindrical rod with a 1.5 nm diameter, appropriate for collagen. For comparison, recent reports for the elastic modulus of bovine femoral trabecular bone, obtained through nanoindentation experiments, under no macroscopic compressive strain, is ∼20 GPa . The mineralized bone would be expected to be significantly more rigid than the unmineralized collagen molecules.…”

Cyclic cryopreservation affects the nanoscale material properties of trabecular bone

“…In a standard preparation procedure (Dall'Ara et al, 2012;Gan et al, 2010;Giambini et al, 2012;Rho et al, 1997;Zysset et al, 1999), specimens were cleaned in an ultrasonic bath (Branson 5210, Branson Ultrasonics; Danbury CT), dehydrated in a series of alcohol baths followed by air dying, and embedded in low-viscosity epoxy (EpoxySet, Allied High Tech Inc., Rancho Dominguez, CA). The embedded specimens were polished with five grades of silicon carbide paper (320-1200 grit) and two grades of polishing slurry (1 mm diamond and 0.05 mm alumina-silica) (Allied High Tech Inc., Rancho Dominguez, CA).…”

The Evolution of the Field of Biomechanics Through the Lens of Experimental Mechanics

“…Other recent works have conducted in-depth investigations into the biomechanics of the femur in fracture fixation and into the phenomena of strain shielding [22,23]. The mechanics of trabecular bone, which is of tremendous interest due to its role in osteoporosis, has been investigated using a novel non-destructive method based on micro-computed tomography technology [24] and was also the subject of a nanoscale biomechanical study by Gao et al [25].…”