Mapping dynamical mechanical properties of osteonal bone by scanning acoustic microscopy in time-of-flight mode

Mechanical properties of stingray tesserae: High-resolution correlative analysis of mineral density and indentation moduli in tessellated cartilage

Skeletal tissues are built and shaped through complex, interacting active and passive processes. These spatial and temporal variabilities make interpreting growth mechanisms from morphology difficult, particularly in bone, where the remodeling process erases and rewrites local structural records of growth throughout life. In contrast to the majority of bony vertebrates, the elasmobranch fishes (sharks, rays, and their relatives) have skeletons made of cartilage, reinforced by an outer layer of mineralized tiles (tesserae), which are believed to grow only by deposition, without remodeling. We exploit this structural permanence, performing the first fine-scale correlation of structure and material properties in an elasmobranch skeleton. Our characterization across an age series of stingray tesserae allows unique insight into the growth processes and mechanical influences shaping the skeleton. Correlated quantitative backscattered electron imaging (qBEI) and nanoindentation measurements show a positive relationship between mineral density and tissue stiffness/hardness. Although tessellated cartilage as a whole (tesserae plus unmineralized cartilage) is considerably less dense than bone, we demonstrate that tesserae have exceptional local material properties, exceeding those of (mammal) bone and calcified cartilage. We show that the finescale ultrastructures recently described in tesserae have characteristic material properties suggesting distinct mechanical roles and that regions of high mineral density/stiffness in tesserae are confined predominantly to regions expected to bear high loads. In particular, tesseral spokes (laminated structures flanking joints) exhibit particularly high mineral densities and tissue material properties, more akin to teeth than bone or calcified cartilage. We conclude that these spokes toughen tesserae and reinforce points of contact between them. These toughening and reinforcing functions are supported by finite element simulations incorporating our material data. The high stresses predicted for spokes, and evidence we provide that new spoke laminae are deposited according to their local mechanical environment, suggest tessellated cartilage is both mutable and responsive, despite lacking remodeling capability. STATEMENT OF SIGNIFICANCE: The study of vertebrate skeletal materials is heavily biased toward mammal bone, despite evidence that bone and cartilage are extremely diverse. We broaden the perspective on vertebrate skeleton materials and evolution in an investigation of stingray tessellated cartilage, a curious type of unmineralized cartilage with a shell of mineralized tiles (tesserae). Combining high-resolution imaging and material testing, we demonstrate that tesserae have impressive local material properties for a vertebrate skeletal tissue, arguing for unique tissue organization relative to mammalian calcified cartilage and bone. Incorporating our materials data into mechanical models, we show that finescale material arrangements allow this cartilage to act as a functional and responsive alternative to bone, despite lacking bone's ability to remodel. These results are relevant to a diversity of researchers, from skeletal, developmental, and evolutionary biologists, to materials scientists interested in high-performance, low-density composites.

Cortical bone properties in the Brtl/+ mouse model of Osteogenesis imperfecta as evidenced by acoustic transmission microscopy

Higher skeletal fragility has been established for the Brtl/+ mouse model of osteogenesis imperfecta at the whole bone level, but previous investigations of mechanical properties at the bone material level were inconclusive. Bone material was analyzed separately at endosteal (ER) and periosteal regions (PR) on transverse femoral midshaft sections for 2-month old mice (wild-type n = 6; Brtl/+ n = 6). Quantitative backscattered electron imaging revealed that the mass density computed from mineral density maps was higher in PR than in ER for both wild-type (+2.1%, p < 0.05) and Brtl/+ mice (+1.8%, p < 0.05). Electron induced X-ray fluorescence analysis indicated significantly lower atomic Ca/P ratios and higher Na/Ca, Mg/Ca and K/Ca ratios in PR bone compared to ER independently of genotype. Second harmonic generation microscopy indicated that the occurrence of periodically alternating collagen orientation in ER of Brtl/+ mice was strongly reduced compared to wild-type mice. Scanning acoustic microscopy in time of flight mode revealed that the sound velocity and Young's modulus (estimated based on sound velocity and mass density maps) were significantly greater in PR (respectively +6% and +15%) compared to ER in wild-type mice but not in Brtl/+ mice. ER sound velocity and Young's modulus were significantly increased in Brtl/+ mice (+9.4% and +22%, respectively) compared to wild-type mice. These data demonstrate that the Col1a1 G349C mutation in Brtl/+ mice affects the mechanical behavior of bone material predominantly in the endosteal region by altering the collagen orientation.

Confocal laser scanning microscopy-a powerful tool in bone research

The confocal laser scanning microscope (CLSM) enables the collection of images picturing selected planes in depth of thick samples, thus giving 3D information while keeping the sample intact. In this article we give an overview of our CLSM applications in bone research: (i) the characterization of osteoblasts and osteoclasts properties in cell biology, (ii) the visualization of the three dimensional (3D) osteocyte lacunar canalicular network in undemineralized plastic-embedded bone samples, (iii) the observation of tetracycline labels in bone biopsy samples from patients in combination with information on the mineralization density from quantitative backscatter electron imaging, which enables the time course of mineral accumulation in newly formed bone to be followed, (iv) the precise measurement of the thickness of thin ground bone sections, a prerequisite for the mapping of local mechanical properties by scanning acoustic microscopy.

Bone quality assessment techniques: geometric, compositional, and mechanical characterization from macroscale to nanoscale

This review presents an overview of the characterization techniques available to experimentally evaluate bone quality, defined as the geometric and material factors that contribute to fracture resistance independently of areal bone mineral density (aBMD) assessed by dual energy x-ray absorptiometry. The methods available for characterization of the geometric, compositional, and mechanical properties of bone across multiple length scales are summarized, along with their outcomes and their advantages and disadvantages. Examples of how each technique is used are discussed, as well as practical concerns such as sample preparation and whether or not each testing method is destructive. Techniques that can be used in vivo and those that have been recently improved or developed are emphasized, including high resolution peripheral quantitative computed tomography to evaluate geometric properties and reference point indentation to evaluate material properties. Because no single method can completely characterize bone quality, we provide a framework for how multiple characterization methods can be used together to generate a more comprehensive analysis of bone quality to complement aBMD in fracture risk assessment.