Effect of water on nanomechanics of bone is different between tension and compression

Abstract

Water, an important constituent in bone, resides in different compartments in bone matrix and may impose significant effects on its bulk mechanical properties. However, a clear understanding of the mechanistic role of water in toughening bone is yet to emerge. To address this issue, this study used a progressive loading protocol, coupled with measurements of in situ mineral and collagen fibril deformations using synchrotron X-ray diffraction techniques. Using this unique approach, the contribution of water to the ultrastructural behavior of bone was examined by testing bone specimens in different loading modes (tension and compression) and hydration states (wet and dehydrated). The results indicated that the effect of water on the mechanical behavior of mineral and collagen phases at the ultrastructural level was loading-mode dependent and correlated with the bulk behavior of bone. Tensile loading elicited a transitional drop followed by an increase in load bearing by the mineral phase at the ultrastructural level, which was correlated with a strain hardening behavior of bone at the bulk level. Compression loading caused a continuous loss of load bearing by the mineral phase, which was reflected at the bulk level as a strain softening behavior. In addition, viscous strain relaxation and pre-strain reduction were observed in the mineral phase in the presence of water. Taken together, the results of this study suggest that water dictates the bulk behavior of bone by altering the interaction between mineral crystals and their surrounding matrix.

Keywords: Bone; Collagen; Mineral; Synchrotron X-ray scattering; Toughness.

Fig. 1

Schematic of the X-ray diffraction experimental setup: The diffraction angle (θ) was measured based on the radius (r) of the diffraction (Debye) ring and the distance of the sensor panel to the specimen (L), which was employed to determine the lattice spacing using the Bragg’s equation.

Fig. 2

Bulk and ultrastructural behavior of bone under different conditions: stress-strain curve of bone under wet and dry conditions in tension (A) and in compression (B); bulk vs. internal strain of the mineral phase of bone in tension (C) and in compression (D); and bulk vs. internal strain of the collagen phase in bone, in tension (E) and in compression (F), respectively. Not all load cycles have been shown for compression wet specimens as the measured properties saturate beyond ~5% applied strain.

Fig. 3

Mineral and collagen fibril stresses vs. the macroscopic stress under wet and dry conditions in tension (A & C) and compression (B & D). The contribution of the collagen phase to load bearing of bone is minimal compared to the mineral phase. Both mineral and collagen stresses showed a linear relationship with the bulk stress under dry condition for both loading modes. Under the hydrated condition, mineral stress increased prior to yield, dropped at the yield point and gradually increased thereafter in tension, but had almost a linear relationship with the bulk strain in compression. Mineral strain values were much higher than those of bulk stress, indicating a remarkable stress concentration in the mineral phase. The mineral stress was estimated using the Kröner-Eshelby model.

Fig. 4

Strain in the mineral phase plotted against the collagen fibril strain in tension (A) and compression (B). Slope obtained by a linear fit quantifies the degree of cooperation between the mineral and collagen fibril deformation. In compression, no considerable changes in cooperation between the mineral and collagen deformation was observed between dry (0.65) and wet (0.57) specimens in compression. However, in tension the degree of deformation cooperation remarkably decreased (0.32) under wet condition, whereas the degree of cooperation between the mineral and collagen deformation was comparable with that in compression (0.64) if bone is dehydrated.

Fig. 5

Initial d-spacing of specimens in the unloaded state: The d-spacing value of bone powder represented the d-spacing of bone crystals without any pre-straining. The contraction of d-spacing in longitudinal and the extension in transverse direction of wet specimens indicated compressive pre-straining in the mineral phase along the longitudinal direction of bone. Dry specimens exhibited contraction in the mineral phase in both transverse and longitudinal directions, indicating a biaxial shrinkage in the matrix surrounding the mineral crystals. Collagen fibrils also exhibited shrinkage in the longitudinal direction by dehydration.

Fig. 6

Pre-strain reduction in wet vs. dry bone in tension (A) and in compression (B): Pre-strain reduction index (ζ) varies from one to zero, one indicating the original pre-strained state of the mineral phase and zero indicating a complete removal of pre-strain. Dry specimens exhibited no reduction in pre-strain, whereas wet specimens showed significant reduction in pre-strain. In compression, the pre-strain reduction index saturated at bulk strains higher than 2%.

Fig. 7

Viscous strain relaxation in the mineral phase (Δε_m) vs. bulk strain (ε_i) during the dwelling period of each loading cycle for wet and dry specimens in tension (A) and compression (B): The mineral strain relaxation is the magnitude of strain difference between the mineral strain at the onset of dwelling for bulk stress relaxation and the mineral strain at the end of dwelling. Dry specimens did not show considerable viscous strain relaxation, whereas wet specimens exhibited significant viscous strain relaxation after yielding in both loading modes.