The relationship of weathering cracks to split-line orientation in bone

The essential identity of weathering cracks and split-lines is demonstrated experimentally. Innominate bones, left in the open to weather, developed cracks similar to the split-line orientation typically observed in the same region. These bones were decalcified and intermittent split-lines prepared along their course. The orientation of the split-lines always corresponds to the weathering splits. The same results were obtained with bird and mammal bones collected in the field and showing weathering cracks. The same also applies to cracks in bones of a gorilla which had been partly defleshed and salted down for preservation. Apparently any process which shrinks bone will produce cracks with the same orientation as split-lines. Possibilities for split-line analysis of fossil and archeological bone are opened up by these experiments.     

Advanced weathering cracks as an improvement on split-line preparations for analysis of structural orientation in compact bone.

Weathering cracks have been demonstrated to be equivalent to split-lines in compact bone. The amount of cracking becomes more extensive with time. Advanced weathering cracks have the potential for giving more complete information on orientation patterns than split-lines. They can also be superior in accuracy and objectivity of results obtained, in preserving the original condition of the bone, and in elimination of much of the work involved in making split-line preparations.     

The relationship between split-line orientation and in vivo bone strain in galago (G. crassicaudatus) and macaque (Macaca mulatta and M. fascicularis) Mandibles

There is still disagreement concerning the functional significance of split-line patterns in bone. This study was undertaken to reexamine the mechanical forces hypothesis for split-line formation by comparing split-line patterns with in vivo mandibular bone strain patterns. The relationship between split-line orientation and in vivo stress and strain patterns on macaque and galago mandibles was examined during jaw opening and the power stroke of mastication and incision. An attempt was made to relate split-line orientation to the direction of tensile stress and strain along the primate mandible. In addition, we also investigated the alternative possibility that split-line orientation is related to the direction of low stresses (and strains) on the primate mandible. The results of this study showed that there was no consistent relationship between split-line orientation and the principal strains or stresses. Thus, split-lines did not run consistently in the direction of high or low stress and strain. Therefore, we have concluded that split-line orientation provides little useful information for inferring patterns of stress and strain in bone.     

Two orinetational features of compact bone as predictors of split-line patterns

Ways of determining orientation patterns in compact bone are needed for specimens which are not available for split-line analysis. Weathering cracks have been shown to be essentially identical in orientation to split-lines, but they are more often than not absent or very few in number. Two other features of compact bone which promise to extend the range of orientational analysis are illustrated and described. These are (1) red or brown lines near the surface, and (2) numerous light-colored filaments, at or near the surface, laid parallel to each other. The latter manifestation has been observed more frequently and usually gives a more complete picture of orientational patterns. Specimens showing the two indicators were decalcified and processed for split-line patterns, which were essentially identical to the indicators in orientation. The first type of indicator is shown microscopically to be a manifestation of resorption spaces and immature osteons containing coagulated blood. The second represents oriented mature osteons in monkey pelves, but the microscopic basis in a cat pelvic bone is not clear.     

Effect of superficial collagen patterns and fibrillation of femoral articular cartilage on knee joint mechanics-a 3D finite element analysis

Collagen fibrils of articular cartilage have specific depth-dependent orientations and the fibrils bend in the cartilage surface to exhibit split-lines. Fibrillation of superficial collagen takes place in osteoarthritis. We aimed to investigate the effect of superficial collagen fibril patterns and collagen fibrillation of cartilage on stresses and strains within a knee joint. A 3D finite element model of a knee joint with cartilage and menisci was constructed based on magnetic resonance imaging. The fibril-reinforced poroviscoelastic material properties with depth-dependent collagen orientations and split-line patterns were included in the model. The effects of joint loading on stresses and strains in cartilage with various split-line patterns and medial collagen fibrillation were simulated under axial impact loading of 1000 N. In the model, the collagen fibrils resisted strains along the split-line directions. This increased also stresses along the split-lines. On the contrary, contact and pore pressures were not affected by split-line patterns. Simulated medial osteoarthritis increased tissue strains in both medial and lateral femoral condyles, and contact and pore pressures in the lateral femoral condyle. This study highlights the importance of the collagen fibril organization, especially that indicated by split-line patterns, for the weight-bearing properties of articular cartilage. Osteoarthritic changes of cartilage in the medial femoral condyle created a possible failure point in the lateral femoral condyle. This study provides further evidence on the importance of the collagen fibril organization for the optimal function of articular cartilage.     

Lamellar bone: structure-function relations.

The term "bone" refers to a family of materials that have complex hierarchically organized structures. These structures are primarily adapted to the variety of mechanical functions that bone fulfills. Here we review the structure-mechanical relations of one bone structural type, lamellar bone. This is the most abundant type in many mammals, including humans. A lamellar unit is composed of five sublayers. Each sublayer is an array of aligned mineralized collagen fibrils. The orientations of these arrays differ in each sublayer with respect to both collagen fibril axes and crystal layers, such that a complex rotated plywood-like structure is formed. Specific functions for lamellar bone, as opposed to the other bone types, could not be identified. It is therefore proposed that the lamellar structure is multifunctional-the "concrete" of the bone family of materials. Experimentally measured mechanical properties of lamellar bone demonstrate a clear-cut anisotropy with respect to the axis direction of long bones. A comparison of the elastic and ultimate properties of parallel arrays of lamellar units formed in primary bone with cylindrically shaped osteonal structures in secondary formed bone shows that most of the intrinsic mechanical properties are built into the lamellar structure. The major advantages of osteonal bone are its fracture properties. Mathematical modeling of the elastic properties based on the lamellar structure and using a rule-of-mixtures approach can closely simulate the measured mechanical properties, providing greater insight into the structure-mechanical relations of lamellar bone.     

Study on the Microstructure of Human Articular Cartilage/Bone Interface

For improving the theory of gradient microstructure of cartilage/bone interface, human distal femurs were studied. Scanning Electron Microscope (SEM), histological sections and MicroCT were used to observe, measure and model the microstructure of cartilage/bone interface. The results showed that the cartilage/bone interface is in a hierarchical structure which is composed of four different tissue layers. The interlocking of hyaline cartilage and calcified cartilage and that of calcified cartilage and subchondral bone are in the manner of “protrusion-pore” with average diameter of 17.0 μm and 34.1 μm respectively. In addition, the cancellous bone under the cartilage is also formed by four layer hierarchical structure, and the adjacent layers are connected by bone trabecula in the shape of H, I and Y, forming a complex interwoven network structure. Finally, the simplified structure model of the cartilage/bone interface was proposed according to the natural articular cartilage/bone interface. The simplified model is a 4-layer gradient biomimetic structure, which corresponds to four different tissues of natural cartilage/bone interface. The results of this work would be beneficial to the design of bionic scaffold for the tissue engineering of articular cartilage/bone.     

Multiscale modeling of bone tissue with surface and permeability control

Natural biological materials usually present a hierarchical arrangement with various structural levels. The biomechanical behavior of the complex hierarchical structure of bone is investigated with models that address the various levels corresponding to different scales. Models that simulate the bone remodeling process concurrently at different scales are in development. We present a multiscale model for bone tissue adaptation that considers the two top levels, whole bone and trabecular architecture. The bone density distribution is calculated at the macroscale (whole bone) level, and the trabecular structure at the microscale level takes into account its mechanical properties as well as surface density and permeability. The bone remodeling process is thus formulated as a material distribution problem at both scales. At the local level, the biologically driven information of surface density and permeability characterizes the trabecular structure. The model is tested by a three-dimensional simulation of bone tissue adaptation for the human femur. The density distribution of the model shows good agreement with the actual bone density distribution. Permeability at the microstructural level assures interconnectivity of pores, which mimics the interconnectivity of trabecular bone essential for vascularization and transport of nutrients. The importance of this multiscale model relays on the flexibility to control the morphometric parameters that characterize the trabecular structure. Therefore, the presented model can be a valuable tool to define bone quality, to assist with diagnosis of osteoporosis, and to support the development of bone substitutes.     

Lamellar Bone: Structure–Function Relations

The term “bone” refers to a family of materials that have complex hierarchically organized structures. These structures are primarily adapted to the variety of mechanical functions that bone fulfills. Here we review the structure–mechanical relations of one bone structural type, lamellar bone. This is the most abundant type in many mammals, including humans. A lamellar unit is composed of five sublayers. Each sublayer is an array of aligned mineralized collagen fibrils. The orientations of these arrays differ in each sublayer with respect to both collagen fibril axes and crystal layers, such that a complex rotated plywood-like structure is formed. Specific functions for lamellar bone, as opposed to the other bone types, could not be identified. It is therefore proposed that the lamellar structure is multifunctional—the “concrete” of the bone family of materials. Experimentally measured mechanical properties of lamellar bone demonstrate a clear-cut anisotropy with respect to the axis direction of long bones. A comparison of the elastic and ultimate properties of parallel arrays of lamellar units formed in primary bone with cylindrically shaped osteonal structures in secondary formed bone shows that most of the intrinsic mechanical properties are built into the lamellar structure. The major advantages of osteonal bone are its fracture properties. Mathematical modeling of the elastic properties based on the lamellar structure and using a rule-of-mixtures approach can closely simulate the measured mechanical properties, providing greater insight into the structure–mechanical relations of lamellar bone.     

Prediction of bone adaptation in the ulnar-osteotomized sheep's forelimb using an anatomical finite element model

A method for the prediction of the time-course of bone adaptation based on an alternative hypothesis of strength optimization has been previously investigated and developed by Prendergast and Taylor¹. This paper extends our work in the study of the effectiveness of this bone adaptation model in predicting similar bone remodelling to that observed in animal experiments. In particular the experimental work which has been modelled is that of Lanyon, Goodship, Pye and McFie². An anatomical finite element model of the sheep's forelimb has been generated for this purpose and is used to estimate stresses in the bone structure for the normal and osteotomized condition. The propensity for remodelling of the altered bone structure is predicted using the proposed remodelling law for the new stress field in the bone structure. The preliminary results indicate an initial bone adaptation pattern similar to that observed experimentally without the necessity to use arbitrarily different constants for the endosteal and periosteal surfaces. We therefore suggest that the remodelling law based on damage and repair gives a better predictive model of bone adaptation than previous models.     

Modelling external bone adaptation using evolutionary structural optimisation

External remodelling is significant in the bone healing process, and it is essential to predict the bone external shape in the design of artificial bone grafts. This paper demonstrates the effectiveness of the evolutionary structural optimisation (ESO) method for the simulation of bone morphology. A two-dimensional ESO strategy is developed which is capable of finding the modified bone topology beginning with any geometry under any loading conditions. The morphology of bone structure is described by the quantitative bone adaptation theory, which is integrated with the finite element method. The evolutionary topology optimisation process is introduced to find the bone shape. A rectangle, which occupies a larger space than the external shape of the bone structure, is specified as a design domain; the evolutionary process iteratively eliminates and redistributes material throughout the domain to obtain an optimum arrangement of bone materials. The technique has been tested on a wide range of examples. In this paper, the formation of trabecular bone architecture around an implant is studied; as another example, the growth of the coronal section of a vertebral body is predicted. The examples support the assertion that the external shape of bone structure can be successfully predicted by the proposed ESO procedure.     

FORMATION OF BONE TISSUE IN CULTURE FROM ISOLATED BONE CELLS

A system is described for the formation of bone tissue in culture from isolated rat bone cells. The isolated bone cells were obtained from embryonic rat calvarium and periosteum or from traumatized, lifted periosteum of young rats. The cells were cultured for a period of up to 8 wk, during which time the morphological, biochemical, and functional properties of the cultures were studied. Formation of bone tissue by these isolated bone cells was shown, in that the cells demonstrated osteoblastic morphology in light and electron microscopy, the collagen formed was similar to bone collagen, there was mineralization specific for bone, and the cells reacted to the hormone calcitonin by increased calcium ion uptake. Calcification of the fine structure of the cells and the matrix is described. Three stages in the calcification process were observed by electron microscopy. It is concluded that these bone cells growing in vitro are able to function in a way similar to such cells in vivo. This tissue culture system starting from isolated bone cells is therefore suitable for studies on the structure and function of bone.     

Organ printing: the future of bone regeneration?

In engineered bone grafts, the combined actions of bone-forming cells, matrix and bioactive stimuli determine the eventual performance of the implant. The current notion is that well-built 3D constructs include the biological elements that recapitulate native bone tissue structure to achieve bone formation once implanted. The relatively new technology of organ/tissue printing now enables the accurate 3D organization of the components that are important for bone formation and also addresses issues, such as graft porosity and vascularization. Bone printing is seen as a great promise, because it combines rapid prototyping technology to produce a scaffold of the desired shape and internal structure with incorporation of multiple living cell types that can form the bone tissue once implanted.     

Taking cues from the extracellular matrix to design bone-mimetic regenerative scaffolds

There is an ongoing need for effective materials that can replace autologous bone grafts in the clinical treatment of bone injuries and deficiencies. In recent years, research efforts have shifted away from a focus on inert biomaterials to favor scaffolds that mimic the biochemistry and structure of the native bone extracellular matrix (ECM). The expectation is that such scaffolds will integrate with host tissue and actively promote osseous healing. To further enhance the osteoinductivity of bone graft substitutes, ECM-mimetic scaffolds are being engineered with a range of growth factors (GFs). The technologies used to generate GF-modified scaffolds are often inspired by natural processes that regulate the association between endogenous ECMs and GFs. The purpose of this review is to summarize research centered on the development of regenerative scaffolds that replicate the fundamental collagen-hydroxyapatite structure of native bone ECM, and the functionalization of these scaffolds with GFs that stimulate critical events in osteogenesis.     

Les relations isopedine - tissu osseux dans le post-temporal et les ecailJes de la ligne laterale de Latimeria chalumnae (Smith)

The scaly part of the posttemporal bone and the lateral-line scales have the same structure as other scales. However, the lateral line with its bony canal induces some local transformations in both structures. The posttemporal bone can be interpreted as a lateral-line scale whose "dermo"-component is extensively developed. With its composite structure, both osseous and scaly, the posttemporal bone affords an instance peculiarly well suited for studying the relationships between isopedine and bone. The observations demonstrate that isopedine whose superposed collagen layers make a double-twisted plywood, is a specialized structure which evolved from a bony tissue. The fibrous basal plate of a Latimeria scale is homologous to the osseous basal plate of a cosmoid scale.     

Finite element analysis of ramming in Ovis canadensis

The energy produced during the ramming of bighorn sheep (Ovis canadensis) would be expected to result in undesirable stresses in their frontal skull, which in turn would cause brain injury; yet, this animal seems to suffer no ill effects. In general, horn is made of an α-keratin sheath covering a bone. Despite volumes of data on the ramming behavior of Ovis canadensis, the extent to which structural components of horn and horn-associated structure or tissue absorb the impact energy generated by the ramming event is still unknown. This study investigates the hypothesis that there is a mechanical relationship present among the ramming event, the structural constituents of the horn, and the horn-associated structure. The three-dimensional complex structure of the bighorn sheep horn was successfully constructed and modeled using a computed tomography (CT) scan and finite element (FE) method, respectively. Three different three-dimensional quasi-static models, including a horn model with trabecular bone, a horn model with compact bone that instead of trabecular bone, and a horn model with trabecular bone as well as frontal sinuses, were studied. FE simulations were used to compare distributions of principal stress in the horn and the frontal sinuses and the strain energy under quasi-static loading conditions. It was noticed that strain energy due to elastic deformation of the complex structure of horn modeled with trabecular bone and with trabecular bone and frontal sinus was different. In addition, trabecular bone in the horn distributes the stresses over a larger volume, suggesting a mechanical link between the structural constituents and the ramming event. This phenomenon was elucidated through the principal stress distribution in the structure. This study will help designers in choosing appropriate material combinations for the successful design of protective structures against a similar impact.     

Adaptive diffuse domain approach for calculating mechanically induced deformation of trabecular bone

Remodelling of trabecular bone is essentially affected by the mechanical load of the trabeculae. Mathematical modelling and simulation of the remodelling process have to include time-consuming calculations of the displacement field within the complex trabecular structure under loading. We present an adaptive diffuse domain approach for calculating the elastic bone deformation based on micro computer tomogram data of real trabecular bone structures and compared it with a conventional voxel-based finite element method. In addition to allowing for higher computational efficiency, the adaptive approach is characterised by a very smooth representation of the bone surface, which suggests that this approach would be suitable as a basis for future simulations of bone resorption and formation processes within the trabecular structure.     

Magnetic resonance microscopy for the quantitative analysis of trabecular bone architecture

A major concern for long-term spaceflight is the effect of microgravity on bone structure and mass as a loss of cortical and trabecular bone volume and density, both of which can lead to decreased bone strength and an increased risk of bone fracture. Detailed analysis of the three-dimensional structure of trabecular bone, and its relation to bone strength has become feasible only recently using high-resolution 3D imaging techniques. In particular, magnetic resonance microscopy (MRM) has proved to be particularly useful for the ex vivo evaluation of the complex architecture of trabecular bone. In this study, we describe the use of two different MRM-based methods for the quantitative evaluation of the three-dimensional structure of trabecular bone explants and for the prediction of their biomechanical properties. The in vivo application of such methods is also discussed.     

Examination of bone chemical composition in osteoporosis using fluorescence-assisted synchrotron infrared microspectroscopy.

Although it is clear that osteoporosis is associated with a reduction in bone mass and a fragile skeleton, it is not understood whether the chemical composition of osteoporotic bone is different from normal bone. In this study, cynomolgus monkeys (Macaca fascicularis) were administered fluorochrome labels at one and two years after ovariectomy (Ovx) or Sham ovariectomy (intact), that were taken up into newly remodeled bone. Using fluorescence-assisted synchrotron infrared microspectroscopy, the chemical composition of bone from intact versus Ovx monkeys has been compared. Results from overall composition distributions (labeled + non-labeled bone) reveal similar carbonate/protein and phosphate/protein ratios, but increased acid phosphate content and different collagen structure in the Ovx animals. Analysis of the fluorochrome-labeled bone indicates similar degrees of mineralization in bone remodeled after one year, but decreased mineralization in Ovx bone remodeled two years after surgery. Thus, bone from monkeys with osteoporosis can be characterized as having abnormal collagen structure and reduced rates of mineralization. Coupled with factors such as trabecular architecture and bone shape and size, these ultrastructural factors may play a contributing role in the increased bone fragility in osteoporosis.