Collagen Fiber Orientation Is Coupled with Specific Nano-Compositional Patterns in Dark and Bright Osteons Modulating Their Biomechanical Properties

Abstract

Bone continuously adapts to its mechanical environment by structural reorganization to maintain mechanical strength. As the adaptive capabilities of bone are portrayed in its nano-and microstructure, the existence of dark and bright osteons with contrasting preferential collagen fiber orientation (longitudinal and oblique-angled, respectively) points at a required tissue heterogeneity that contributes to the excellent fracture resistance mechanisms in bone. Dark and bright osteons provide an exceptional opportunity to deepen our understanding of how nanoscale tissue properties influence and guide fracture mechanisms at larger length scales. To this end, a comprehensive structural, compositional, and mechanical assessment is performed using circularly polarized light microscopy, synchrotron nanocomputed tomography, focused ion beam/scanning electron microscopy, quantitative backscattered electron imaging, Fourier transform infrared spectroscopy, and nanoindentation testing. To predict how the mechanical behavior of osteons is affected by shifts in collagen fiber orientation, finite element models are generated. Fundamental disparities between both osteon types are observed: dark osteons are characterized by a higher degree of mineralization along with a higher ratio of inorganic to organic matrix components that lead to higher stiffness and the ability to resist plastic deformation under compression. On the contrary, bright osteons contain a higher fraction of collagen and provide enhanced ductility and energy dissipation due to lower stiffness and hardness.

Influence of X-rays and gamma-rays on the mechanical performance of human bone factoring out intraindividual bone structure and composition indices

“…Thus, the elastic region with respect to the bending stiffness described by the Young's modulus, which varies by the mineral, is maintained at very high irradiation doses, as shown in previous studies [ 8 ]. However, the yield point was lower in the 31.2 ​kGy group than in the control group, reflecting the interaction of the two components of bone, namely, the collagen and mineral in a composite material, to transfer load [ 3 , 12 ]. The decreased yield point and drastic decrease in the plastic region reflects impaired load transfer between minerals and collagen [ 45 ] when the plastic behavior of the bone is reached.…”

“…Further radiological methods include clinical radiation therapy (in the range of Gy [ [5] , [6] , [7] ]), and irradiation sterilization of bone grafts (in the range of kGy [ 1 , [8] , [9] , [10] , [11] ]), which can affect the mechanical quality of bone depending on the irradiation dose. Therefore, the question arises, how ex vivo research methods utilizing X-rays (Gy to kGy) such as ex vivo HR-pQCT, μCT imaging, synchrotron studies for high resolution 3D-imaging [ 12 ] and crystal quantification [ 3 ]) would affect the biomechanical tissue properties. Sterilization methods for bone grafts such as gamma irradiation in the range of kGy have been known for a long time to impair mechanical competence [ 11 , 13 ].…”

Machine learning denoising of high-resolution X-ray nanotomography data

“…Femoral bone was prepared in line with previously published protocols (Stockhausen et al, 2021). Briefly, femoral cross sections from the mid-diaphysis were cut using a diamond saw and fixed in 3.7% formaldehyde.…”

“…Thereafter, the sample was embedded undecalcified in glycol methacrylate (Technovit 7200, Heraeus Kulzer GmbH, Wehrheim, Germany) and ground to a thickness of 100 mm using an automatic grinding machine. Images from other high-resolution imaging techniques can be found in Stockhausen et al (2021). Cylinders of 25 mm diameter were extracted by focused ion beam (FIB) milling and measured at an FOV of 40 mm  40 mm, with a 130 mm diameter FZP and an effective pixel size of 18.7 nm.…”

“…The nanotomographic scans thereby helped in revealing nanometre-thick tunnels (canaliculi, yellow arrows) that connect mechanosensitive cells within the bone matrix and play a key role in maintaining bone health (Milovanovic et al, 2013). Moreover, lamellar structures with alternating collagen fiber orientation (red arrows) between neighbouring lamellae are visualized (Stockhausen et al, 2021). These structures can also be identified in SEM images (Stockhausen et al, 2021), which can be used in a complementary manner to the shown 3D volumes, opening doors for correlative imaging.…”