Mineralization and collagen orientation throughout aging at the vertebral endplate in the human lumbar spine.

The human vertebral body and intervertebral disc interface forms the region where the cartilaginous endplate, annulus fibrosis and bone of the vertebral body are connected through an intermediate calcified cartilage layer. While properties of both the vertebral body and components of the disc have been extensively studied, limited quantitative data exists describing the microstructure of the vertebral body-intervertebral disc interface in the spine throughout development and degeneration. Quantitative backscattered scanning electron and second harmonic generation confocal imaging were used to collect quantitative data describing the mineral content and collagen fiber orientation across the interface, respectively. Specimens spanned ages 56 days to 84 years and measurements were taken across the vertebral endplate at the outer annulus, inner annulus and nucleus pulposis. In mature and healthy endplates, collagen fibers span the calcified cartilage layer in all regions, including the endplate adjacent to the central nucleus pulposis. We also observed an abrupt transition from high mineral volume fractions (35-50%) to 0% over short distances measuring 3-15 microns in width across the transition from calcified cartilage to unmineralized cartilage. The alignment of collagen fibers at the outer annulus and thickness of the CC layer indicated that collagen fiber mineralization adjacent to the bone may serve to anchor the soft tissue without a gradual change in material properties. Combining backscattered scanning electron microscopy and second harmonic generation imaging on the same sections thus enable a novel assessment of morphology and properties in both mineralized and soft tissues at the vertebral body-intervertebral disc throughout development and aging.

Influences of spherical tip radius, contact depth, and contact area on nanoindentation properties of bone.

Nanoindentation has been widely used as a means to measure the micro-mechanical properties of bone and to predict the macroscopic properties. The role of indent depth and indenter tip geometry in measuring the hierarchical properties of bone tissue was explored experimentally using a range of spherical indenter tips of R=5, 25, 65, and 200 µm. Nanoindentation arrays, not targeted to fall on specific structures or locations, enabled statistical sampling of osteons within PMMA-embedded, bovine, cortical bone on a single sample to a range of maximum displacements (minimum of 100 nm and maximum of 2000 nm). Elastic finite element models were then utilized to isolate the contributions of indenter tip radius, contact area, and position within the lamellar structure in comparison to the experimental results. For a small, R=5 µm indenter tip, indentation modulus consistently increased with contact depth and increased plastic deformation, resulting in an artificial increase in elastic properties. While larger radius tips (R=25, 65, and 200 µm) did not enable evaluation of a high spatial resolution on the surface, they produced data that was representative of the lower load and contact depth measurements with the smaller tip. However the sensitivity to mechanical property variations across the 2-D surface of the material was lost with increase in indenter tip size. Correspondingly, measurement variance was also decreased as the volume contributing to the indent response represented an average of surface roughness, varying mineral content, defects, and underlying tissue type and structure.