Transformation of bone mineral morphology: From discrete marquise-shaped motifs to a continuous interwoven mesh

Revisiting the physical and chemical nature of the mineral component of bone

The physico-chemical characteristics of bone mineral remain heavily debated. On the nanoscale, bone mineral resides both inside and outside the collagen fibril as distinct compartments fused together into a cohesive continuum. On the micrometre level, larger aggregates are arranged in a staggered pattern described as crossfibrillar tessellation. Unlike geological and synthetic hydroxy(l)apatite, bone mineral is a unique form of apatite deficient in calcium and hydroxyl ions with distinctive carbonate and acid phosphate substitutions (CHAp), together with a minor contribution of amorphous calcium phosphate as a surface layer around a crystalline core of CHAp. In mammalian bone, an amorphous solid phase has not been observed, though an age-dependent shift in the amorphous-to-crystalline character is observed. Although octacalcium phosphate has been postulated as a bone mineral precursor, there is inconsistent evidence of calcium phosphate phases other than CHAp in the extracellular matrix. In association with micropetrosis, magnesium whitlockite is occasionally detected, indicating pathological calcification rather than a true extracellular matrix component. Therefore, the terms 'biomimetic' or 'bone-like' should be used cautiously in descriptions of synthetic biomaterials. The practice of reporting the calcium-to-phosphorus ratio (Ca/P) as proxy for bone mineral maturity oversimplifies the chemistry since both Ca2+ and PO43- ions are partially substituted. Moreover, non-mineral sources of phosphorus are ignored. Alternative compositional metrics should be considered. In the context of bone tissue and bone mineral, the term 'mature' must be used carefully, with clear criteria that consider both compositional and structural parameters and the potential impact on mechanical properties. STATEMENT OF SIGNIFICANCE: Bone mineral exhibits a unique hierarchical structure and is classified into intrafibrillar and extrafibrillar mineral compartments with distinct physico-chemical characteristics. The dynamic nature of bone mineral, i.e., evolving chemical composition and physical form, is poorly understood. For instance, bone mineral is frequently described as "hydroxy(l)apatite", even though the OH- content of mature bone mineral is negligible. Moreover, the calcium-to-phosphorus ratio is often taken as an indicator of bone mineral maturity without acknowledging substitutions at calcium and phosphate sites. This review takes a comprehensive look at the structure and composition of bone mineral, highlighting how experimental data are misinterpreted and unresolved concerns that warrant further investigation, which have implications for characterisation of bone material properties and development of bone repair biomaterials.

Mineralized collagen plywood contributes to bone autograft performance

Autologous bone (AB) is the gold standard for bone-replacement surgeries1, despite its limited availability and the need for an extra surgical site. Traditionally, competitive biomaterials for bone repair have focused on mimicking the mineral aspect of bone, as evidenced by the widespread clinical use of bioactive ceramics2. However, AB also exhibits hierarchical organic structures that might substantially affect bone regeneration. Here, using a range of cell-free biomimetic-collagen-based materials in murine and ovine bone-defect models, we demonstrate that a hierarchical hybrid microstructure-specifically, the twisted plywood pattern of collagen and its association with poorly crystallized bioapatite-favourably influences bone regeneration. Our study shows that the most structurally biomimetic material has the potential to stimulate bone growth, highlighting the pivotal role of physicochemical properties in supporting bone formation and offering promising prospects as a competitive bone-graft material.

Curved mineral platelets in bone

Bone is a composite material principally made up of a mineral phase (apatite) and collagen fibrils. The mineral component of bone occurs in the form of polycrystalline platelets 2-6 nm in thickness. These platelets are packed and probably glued together in stacks of two or more, ranging up to >30 platelets. Here we show that most of these stacks are curved flat sheets whose cylindrical axes are oriented parallel to the long axes of collagen fibrils. Consequently, the curvature of the platelets is not detectable in TEM sections cut parallel to the collagen fibril axes. The radius of curvature around these axes ranges from about 25 nm (the average radius of the collagen fibrils) to 100's of nm. The shapes of these curved forms contribute to the compressive strength of bone. STATEMENT OF SIGNIFICANCE: Bone, the material of which bones are made, is mainly composed of a protein, collagen, and the mineral apatite (calcium phosphate). The crystals have long been known to be flat plates about 5 nanometers (nm) thick. Here we show that the crystals are bound together in curved platelets with a radius of curvature between 25 and several hundred nm, which weave between fibrils of collagen. Some platelets wrap tightly around fibrils. The platelets form stacks of from two to up to 30. The crystals in the platelets are all oriented parallel to the cylindrical fibrils even though most crystals are not in contact with collagen. These curved structures provide greater strength to bone.

Leptin receptor gene deficiency minimally affects osseointegration in rats

Metabolic syndrome represents a cluster of conditions such as obesity, hyperglycaemia, dyslipidaemia, and hypertension that can lead to type 2 diabetes mellitus and/or cardiovascular disease. Here, we investigated the influence of obesity and hyperglycaemia on osseointegration using a novel, leptin receptor-deficient animal model, the Lund MetS rat. Machined titanium implants were installed in the tibias of animals with normal leptin receptor (LepR+/+) and those harbouring congenic leptin receptor deficiency (LepR-/-) and were left to heal for 28 days. Extensive evaluation of osseointegration was performed using removal torque measurements, X-ray micro-computed tomography, quantitative backscattered electron imaging, Raman spectroscopy, gene expression analysis, qualitative histology, and histomorphometry. Here, we found comparable osseointegration potential at 28 days following implant placement in LepR-/- and LepR+/+ rats. However, the low bone volume within the implant threads, higher bone-to-implant contact, and comparable biomechanical stability of the implants point towards changed bone formation and/or remodelling in LepR-/- rats. These findings are corroborated by differences in the carbonate-to-phosphate ratio of native bone measured using Raman spectroscopy. Observations of hypermineralised cartilage islands and increased mineralisation heterogeneity in native bone confirm the delayed skeletal development of LepR-/- rats. Gene expression analyses reveal comparable patterns between LepR-/- and LepR+/+ animals, suggesting that peri-implant bone has reached equilibrium in healing and/or remodelling between the animal groups.

Bone without borders - Monetite-based calcium phosphate guides bone formation beyond the skeletal envelope

Calcium phosphates (CaP) represent an important class of osteoconductive and osteoinductive biomaterials. As proof-of-concept, we show how a multi-component CaP formulation (monetite, beta-tricalcium phosphate, and calcium pyrophosphate) guides osteogenesis beyond the physiological envelope. In a sheep model, hollow dome-shaped constructs were placed directly over the occipital bone. At 12 months, large amounts of bone (∼75%) occupy the hollow space with strong evidence of ongoing remodelling. Features of both compact bone (osteonal/osteon-like arrangements) and spongy bone (trabeculae separated by marrow cavities) reveal insights into function/need-driven microstructural adaptation. Pores within the CaP also contain both woven bone and vascularised lamellar bone. Osteoclasts actively contribute to CaP degradation/removal. Of the constituent phases, only calcium pyrophosphate persists within osseous (cutting cones) and non-osseous (macrophages) sites. From a translational perspective, this multi-component CaP opens up exciting new avenues for osteotomy-free and minimally-invasive repair of large bone defects and augmentation of the dental alveolar ridge.

Bone mineral organization at the mesoscale: A review of mineral ellipsoids in bone and at bone interfaces

Much debate still revolves around bone architecture, especially at the nano- and microscale. Bone is a remarkable material where high strength and toughness coexist thanks to an optimized composition of mineral and protein and their hierarchical organization across several distinct length scales. At the nanoscale, mineralized collagen fibrils act as building block units. Despite their key role in biological and mechanical functions, the mechanisms of collagen mineralization and the precise arrangement of the organic and inorganic constituents in the fibrils remains not fully elucidated. Advances in three-dimensional (3D) characterization of mineralized bone tissue by focused ion beam-scanning electron microscopy (FIB-SEM) revealed mineral-rich regions geometrically approximated as prolate ellipsoids, much larger than single collagen fibrils. These structures have yet to become prominently recognized, studied, or adopted into biomechanical models of bone. However, they closely resemble the circular to elliptical features previously identified by scanning transmission electron microscopy (STEM) in two-dimensions (2D). Herein, we review the presence of mineral ellipsoids in bone as observed with electron-based imaging techniques in both 2D and 3D with particular focus on different species, anatomical locations, and in proximity to natural and synthetic biomaterial interfaces. This review reveals that mineral ellipsoids are a ubiquitous structure in all the bones and bone-implant interfaces analyzed. This largely overlooked hierarchical level is expected to bring different perspectives to our understanding of bone mineralization and mechanical properties, in turn shedding light on structure-function relationships in bone. STATEMENT OF SIGNIFICANCE: In bone, the hierarchical organization of organic (mainly collagen type I) and inorganic (calcium-phosphate mineral) components across several length scales contributes to a unique combination of strength and toughness. However, aspects related to the collagen-mineral organization and to mineralization mechanisms remain unclear. Here, we review the presence of mineral prolate ellipsoids across a variety of species, anatomical locations, and interfaces, both natural and with synthetic biomaterials. These mineral ellipsoids represent a largely unstudied feature in the organization of bone at the mesoscale, i.e., at a level connecting nano- and microscale. Thorough understanding of their origin, development, and structure can provide valuable insights into bone architecture and mineralization, assisting the treatment of bone diseases and the design of bio-inspired materials.

Mineral tessellation in bone and the stenciling principle for extracellular matrix mineralization

We review here the Stenciling Principle for extracellular matrix mineralization that describes a double-negative process (inhibition of inhibitors) that promotes mineralization in bone and other mineralized tissues, whereas the default condition of inhibition alone prevents mineralization elsewhere in soft connective tissues. The stenciling principle acts across multiple levels from the macroscale (skeleton/dentition vs soft connective tissues), to the microscale (for example, entheses, and the tooth attachment complex where the soft periodontal ligament is situated between mineralized tooth cementum and mineralized alveolar bone), and to the mesoscale (mineral tessellation). It relates to both small-molecule (e.g. pyrophosphate) and protein (e.g. osteopontin) inhibitors of mineralization, and promoters (enzymes, e.g. TNAP, PHEX) that degrade the inhibitors to permit and regulate mineralization. In this process, an organizational motif for bone mineral arises that we call crossfibrillar mineral tessellation where mineral formations - called tesselles - geometrically approximate prolate ellipsoids and traverse multiple collagen fibrils (laterally). Tesselle growth is directed by the structural anisotropy of collagen, being spatially restrained in the shorter transverse tesselle dimensions (averaging 1.6 × 0.8 × 0.8 μm, aspect ratio 2, length range 1.5-2.5 μm). Temporo-spatially, the tesselles abut in 3D (close ellipsoid packing) to fill the volume of lamellar bone extracellular matrix. Poorly mineralized interfacial gaps between adjacent tesselles remain discernable even in mature lamellar bone. Tessellation of a same, small basic unit to form larger structural assemblies results in numerous 3D interfaces, allows dissipation of critical stresses, and enables fail-safe cyclic deformations. Incomplete tessellation in osteomalacia/odontomalacia may explain why soft osteomalacic bones buckle and deform under loading.

Magnesium whitlockite - omnipresent in pathological mineralisation of soft tissues but not a significant inorganic constituent of bone

Whitlockite is a calcium phosphate that was first identified in minerals collected from the Palermo Quarry, New Hampshire. The terms magnesium whitlockite [Mg-whitlockite; Ca₁₈Mg₂(HPO₄)₂(PO₄)₁₂] and beta-tricalcium phosphate [β-TCP; β-Ca₃(PO₄)₂] are often used interchangeably since Mg-whitlockite is not easily distinguished from β-Ca₃(PO₄)₂ by powder X-ray diffraction although their crystalline structures differ significantly. Being both osteoconductive and bioresorbable, Mg-whitlockite is pursued as a synthetic bone graft substitute. In recent years, advances in development of synthetic Mg-whitlockite have been accompanied by claims that Mg-whitlockite is the second most abundant inorganic constituent of bone, occupying as much as 20-35 wt% of the inorganic fraction. To find evidence in support of this notion, this review presents an exhaustive summary of Mg-whitlockite identification in biological tissues. Mg-whitlockite is mainly found in association with pathological mineralisation of various soft tissues and dental calculus, and occasionally with enamel and dentine. With the exception of high-temperature treated tumoural calcified deposits around interphalangeal and metacarpal joints and rhomboidal Mg-whitlockite crystals in post-apoptotic osteocyte lacunae in human alveolar bone, this unusual mineral has never been detected in the extracellular matrix of mammalian bone. Characterisation techniques capable of unequivocally distinguishing between different calcium phosphate phases, such as high-resolution imaging, crystallography, and/or spectroscopy have exclusively identified bone mineral as poorly crystalline, ion-substituted, carbonated apatite. The idea that Mg-whitlockite is a significant constituent of bone mineral remains unsubstantiated. Contrary to claims that such biomaterials represent a bioinspired/biomimetic approach to bone repair, Mg-whitlockite remains, exclusively, a pathological biomineral. STATEMENT OF SIGNIFICANCE: Magnesium whitlockite (Mg-whitlockite) is a unique calcium phosphate that typically features in pathological calcification of soft tissues; however, an alarming trend emerging in the synthetic bioceramics community claims that Mg-whitlockite occupies 20-35 wt% of bone mineral and therefore synthetic Mg-whitlockite represents a biomimetic approach towards bone regeneration. By providing an overview of Mg-whitlockite detection in biological tissues and scrutinising a diverse cross-section of literature relevant to bone composition analysis, this review concludes that Mg-whitlockite is exclusively a pathological biomineral, and having never been reported in bone extracellular matrix, Mg-whitlockite does not constitute a biomimetic strategy for bone repair.

Mapping Bone Surface Composition Using Real-Time Surface Tracked Micro-Raman Spectroscopy

The surface of bone tells a story - one that is worth a thousand words - of how it is built and how it is repaired. Chemical (i.e., composition) and physical (i.e., morphology) characteristics of the bone surface are analogous to a historical record of osteogenesis and provide key insights into bone quality. Analysis of bone chemistry is of particular relevance to the advancement of human health, cell biology, anthropology/archaeology, and biomedical engineering. Although scanning electron microscopy remains a popular and versatile technique to image bone across multiple length scales, limited chemical information can be obtained. Micro-Raman spectroscopy is a valuable tool for nondestructive chemical/compositional analysis of bone. However, signal integrity losses occur frequently during wide-field mapping of non-planar surfaces. Samples for conventional Raman imaging are, therefore, rendered planar through polishing or sectioning to ensure uniform signal quality. Here, we demonstrate ν1 PO43- and ν1 CO32- peak intensity losses where the sample surface and the plane of focus are offset by over 1-2 μm when underfocused and 2-3 μm when overfocused at 0.5-1 s integration time (15 mW, 633 nm laser). A technique is described for mapping the composition of the inherently irregular/non-planar surface of bone. The challenge posed by the native topology characteristic of this unique biological system is circumvented via real-time focus-tracking based on laser focus optimization by continuous closed-loop feedback. At the surface of deproteinized and decellularized/defatted sheep tibial cortical bone, regions of interest up to 1 mm2 were scanned at micrometer and submicrometer resolution. Despite surface height deviations exceeding 100 μm, it is possible to seamlessly probe local gradients in organic and inorganic constituents of the extracellular matrix as markers of bone metabolism and bone turnover, blood vessels and osteocyte lacunae, and the rope-like mineralized bundles that comprise the mineral phase at the bone surface.

Ellipsoidal mesoscale mineralization pattern in human cortical bone revealed in 3D by plasma focused ion beam serial sectioning

Visualizing bone mineralization and collagen fibril organization at intermediate scales between the nanometer and the hundreds of microns range, is still an important challenge. Similarly, visualizing cellular components which locally affect the tissue structure requires a precision of a few tens of nanometers at maximum while spanning several tens of micrometers. In the last decade, gallium focused ion beam (FIB) equipped with a scanning electron microscope (SEM) proved to be an extremely valuable structural tool to meet those ends. In this study, we assess the capability of a recent plasma FIB-SEM technology which provides a potential increase in measurement speed over gallium FIB-SEM, thus paving the way to larger volume analysis. Nanometer-scale layers of demineralized and mineralized unstained human femoral lamellar bone were sequentially sectioned over volumes of 6-16,000 μm³. Analysis of mineralized tissue revealed prolate ellipsoidal mineral clusters measuring approximately 1.1 µm in length by 700 nm at their maximum diameter. Those features, suggested by others in high resolution studies, appear here as a ubiquitous motif in mineralized lamellar bone over thousands of microns cubed, suggesting a heterogeneous and yet regular pattern of mineral deposition past the single collagen fibril level. This large scale view retained sufficient resolution to visualize the collagen fibrils while also partly visualizing the lacuno-canalicular network in three-dimensions. These findings are strong evidence for suitability of PFIB as a bone analysis tool and the need to revisit bone mineralization over multi-length scales with mineralized tissue.