Bone fractures.
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Biomechanical Phenomena↗
Biomedical subjects
H D Wagner
Publications and source records attributed to H D Wagner.
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.
Animals↗
Anisotropic mechanical properties of lamellar bone using miniature cantilever bending specimens.
The flexural modulus and work-to-fracture properties of circumferential lamellar bone from baboon tibia are presented, based on experiments with miniature cantilever bending specimens. Data is provided for specimens in three orthogonal directions that show marked anisotropy. The advantages of such miniature specimens (about 150 microm in diameter and 2 mm long) include the possibility of sampling very small volumes within a heterogeneous structure such as osteonal bone, or studying biological materials that are not available in large enough volumes for conventional mechanical analysis.
Animals↗
Microstructure-microhardness relations in parallel-fibered and lamellar bone.
Understanding the mechanical function of bone material in relation to its structure is a fascinating but very complicated problem to resolve. Part of the complexity arises from the hierarchical structural organization of bone. Microhardness measurements, initially on relatively simply structured parallel-fibered bone, show a marked anisotropy in three orthogonal directions. This may, in part, be due to the highly anisotropic structure of the basic building block of bone, the mineralized collagen fibril. Microhardness measurements made face-on to the layers of crystals and collagen triple helical molecules, show much lower values than those made edge-on to these layers. Microhardness measurements of the much more complex "rotated-plywood" structure of lamellar bone, reveal the well-known general tendency toward anisotropy in relation to the long axis of the bone. A detailed examination of microhardness-microstructure relations of lamellar bone, however, shows that only in certain orientations can microhardness values be related directly to a specific attribute of the lamellar structure. Clearly, the gradual tilting and rotating of the mineralized collagen fibrils that form this structure produce a material that tends toward having isotropic microhardness properties, even though its basic building block is highly anisotropic. This may be an important structural attribute that allows lamellar bone to withstand a variety of mechanical challenges.
Animals↗
On the relationship between the microstructure of bone and its mechanical stiffness.
A recent study of bone structure shows that the plate-shaped carbonate apatite crystals in individual lamellae are arranged in layers across the lamellae, and that the orientation of these layers are different in alternate lamellae. Based on these findings, a new micromechanical model for the Young's modulus of bone is proposed, which accounts for the anisotropy and geometrical characteristics of the material. The model incorporates the platelet-like geometry of the basic reinforcing unit, the presence of alternating thin and thick lamellae, and the orientations of the crystal platelets in the lamellae. The thin and thick lamellae are modeled as orthotropic composite layers made up of thin rectangular apatite platelets within a collagen matrix, and classical orthotropic elasticity theory is used to calculate the Young's modulus of the lamellae. Bone is viewed as an assembly of such orthotropic lamellae bent into cylindrical structures, and having a constant, alternating angle between successive lamellae. The micromechanical model employs a modified rule-of-mixtures to account for the two types of lamellae. The model provides a curve similar to the published experimental data on the angular dependence of Young's modulus, including a local maximum at an angle between 0 and 90 degrees. A rigorous testing of the model awaits additional experimental data.
Animals↗
Elastic and ultimate properties of acrylic bone cement reinforced with ultra-high-molecular-weight polyethylene fibers.
A study of the fracture behavior of poly-(methyl methacrylate) (PMMA) bone cement reinforced with short ultra-high-molecular-weight polyethylene (Spectra 900) fibers is presented. Linear elastic and nonlinear elastic fracture mechanics results indicate that a significant reinforcing effect is obtained at fiber contents as low as 1% by weight, but beyond that concentration a plateau value is reached and the fracture toughness becomes insensitive to fiber content. The flexural strength and modulus are apparently not improved by the incorporation of polyethylene fibers in the acrylic cement, probably because of the presence of voids, the poor mixing practice and the weakness of the fiber/matrix interfacial bond. The present polyethylene/PMMA composite presents several advantages as compared to other composite cements, but overall the mechanical performance of this system resembles that of Kevlar 29/PMMA cement, with a few differences. Scanning electron microscopy reveals characteristic micromechanisms of energy absorption in Spectra 900/PMMA bone cement. A scheme for the strength of random fiber-reinforced composites, which is a simple extension of the Kelly and Tyson model for the strength of unidirectional composites, is presented and discussed. Young's modulus and the fracture toughness results are discussed in the framework of existing theories. More fundamental modeling treatments are needed in terms of fracture micromechanisms to understand and optimize the various mechanical properties with respect to structural parameters and cement preparation technique.
Bone Cements↗
Use of high-performance polyethylene fibres as a reinforcing phase in poly(methylmethacrylate) bone cement.
First results are presented concerning the elastic and ultimate mechanical behaviour of p(MMA) bone cement reinforced with as-received and surface-modified Spectra 900 polyethylene fibres. Even though the surface chemistry and reactivity of the fibres was modified, the surface oxidation and surface grafting treatments of the polyethylene fibres apparently did not significantly affect the mechanical properties of the polyethylene-reinforced p(MMA) bone cement or improve the interfacial bonding. This may be attributed to the rather unfavourable area-to-volume ratio of PE fibres for such treatments, as well as to the necessarily low content of PE fibres in the bone cement which does not allow a clear differentiation between the various samples.
Bone Cements↗
Fracture toughness of Kevlar 29/poly(methyl methacrylate) composite materials for surgical implantations.
A study of the fracture behaviour of Kevlar 29 reinforced dental cement is undertaken using both linear elastic and nonlinear elastic fracture mechanics techniques. Results from both approaches--of which the nonlinear elastic is believed to be more appropriate--indicate that a reinforcing effect is obtained for the fracture toughness even at very low fibre content. The flexural strength and modulus are apparently not improved, however, by the incorporation of Kevlar 29 fibres in the PMMA cement, probably because of the presence of voids, the poor fibre/matrix interfacial bonding and unsatisfying cement mixing practice. When compared to other PMMA composite cements, the present system appears to be probably more effective than carbon/PMMA, for example, in terms of fracture toughness. More experimental and analytical work is needed so as to optimize the mechanical properties with respect to structural parameters and cement preparation technique.
Biomedical Engineering↗
[Experience in the clinical use of an imidazoline derivative with varying application].
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Administration, Oral↗
[Influencing the intra- and extracellular water-electrolyte balance in acute renal failure using various therapeutic measures (experimental studies in animals].
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Acute Kidney Injury↗