Fractures in the Horse. Группа авторов

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Fractures in the Horse - Группа авторов


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and its location relative to the collagen fibrils are poorly understood. Whether mineral forms within fibrils, outside them or a combination of the two remains contentious. There is evidence that mineral is initially deposited in the gaps within fibrils (between collinear collagen molecules) by a process of heterogeneous nucleation – a surface‐catalyzed or assisted nucleation process. However, there are those who argue that the data and the structural restraints imposed by collagen within the fibrils do not support or permit such an arrangement. Similarly, the morphology of the crystals is not universally accepted. There is evidence that mineral is deposited as needle‐like crystals, whereas others argue that it is really in the form of flakes or plates, which appear as needles when viewed from side on. There is general agreement though that the crystals are anisotropic: they are elongated along their crystallographic c‐axis, which is aligned parallel with the collagen fibrils. Schwarcz et al. [20] have recently proposed a model whereby mineral that is not in the form of apatite initially forms in the gap zones of fibrils. It then extends out into the extra‐fibrillar space where apatite crystals form sheets or lamellae that partially wrap around the fibrils (Figure 2.10). Several mineral lamellae may form around a single fibril, and lamellae surrounding one fibril and those of adjacent fibrils bind firmly together through strong bonds.

Schematic illustration of schematic diagram showing progressive steps in the mineralization of collagen molecules in a single fibril, assuming that most mineral in bone is intrafibrillar. (a) Early mineralization in gap zones; (b) further mineralization extends into adjacent overlap zones.

      Source: Landis et al. [23]. Reproduced with permission of Elsevier.

      Mineralization

      Mineralization of osteoid involves an interaction of processes that either promote or inhibit deposition. Initial nucleation of mineral may be enhanced by the formation or exposure of nucleators and by the removal or modification of inhibitors. However, details of the mechanisms involved and the location in, on or around the fibrils remain subjects of controversy. Many believe that specific atomic groups located in the gap zones of collagen fibrils are arranged in such a way as to induce heterogeneous nucleation of hydroxyapatite [24]. These nuclei subsequently expand by addition of further inorganic ions, so giving rise to crystals. Certain factors, principally non‐collagenous proteins, have been shown to promote or inhibit mineralization. For example, phosphoproteins, such as bone sialoprotein, bind calcium and thereby act as mineral nucleators. Conversely, proteoglycans may inhibit the process by masking critical zones or occupying essential spaces within fibrils, thereby reducing diffusion, chemical interaction and sequestration of calcium ions.

      In most healthy adult bones, the mineral fraction (proportion of dry weight accounted for by mineral) is between 60 and 70%. Fractions in this range engender material properties that provide an optimal compromise between strength, stiffness and toughness. Osteoblasts and osteocytes limit the ultimate extent of matrix mineralization through the adjustment of extracellular ion concentrations [25, 26]. Loss of these cells, for instance in osteonecrosis, is associated with hypermineralization, which can have profound effects on material properties causing bone to become brittle.

      Mineralization of bone matrix makes it appropriately stiff and strong to fulfil its primary roles. The physical nature of its primary functions means that the mechanical properties of bone as a material (tissue) and structure (whole bone) are critical. A vast body of literature documents the mechanical properties of bone from many different species. The degree of matrix mineralization, variation in matrix organization (microstructure), porosity and orientation of collagen fibres within the matrix all significantly influence the strength, stiffness and toughness of bone. A brief review of mechanical terminology follows to assist readers less familiar with these terms to understand the concepts that follow.

Schematic illustration of graphical and schematic illustrations of the relationship between stress imposed on an object by a tensile load and deformation of the object.

      Tissue (Material) Properties


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