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Quadrupedal Mechanics - Anatomy & Physiology (view source)
Revision as of 09:52, 28 April 2013
, 09:52, 28 April 2013→2 Elasticity: external forces and stored energy
:::::Phase III: Breakdown of the crystalline components of the collagen fibres. Deformation takes place with progressively less stress.
:::::Phase III: Breakdown of the crystalline components of the collagen fibres. Deformation takes place with progressively less stress.
'''Putting the stress on to collagen'''
When a collagenous structure, such as tendon, progressively undergoes tensile stress in a laboratory preparation, three stages o stress strain relationships are recognised (Fig. 2.11).
During the '''first stage''', the wavy pattern or crimp seen in bundles of fibres is removed. This crimp is probably due to an in–series elastic component, accounting for the high elasticity over this stage. This first stage is less apparent when the strain rate is high. Such a high strain rate is the likely situation in tendon in vivo.
The '''second stage''' is the true elastic stage for collagen fibres. With stretch, the axial periodicity of the fibres is increased. The elasticity of collagen is about one three hundredth that of elastin. This makes it a suitable material for structures that must withstand large forces without stretching too much.
In '''stage three''', there is a loss of elasticity. The crosslinks between the fibrils have been broken by the stress and the material exhibits viscous flow. The strain remains after removal of the stress, defined as residual deformation in Fig 2.3.
'''Organisation of collagen in bone and fibrocartilage'''
In both bone and fibrocartilage, collagen fibrils are embedded in a matrix. For bone, the matrix is made of mineral crystals and for cartilage the matrix is mucopolysaccaride. Bone and fibrocartilage are therefore both heterogeneous. Each is a composite material consisting of several different components.
In bone, collagen forms one third of the weight and one half of the volume of the tissue. The arrangement of the collagen determines the quality of the bone. In the most structured bone, the collagen fibres are arranged parallel to one another in sheets, or lamellae, 5µm thick (Fig. 2.12). The alignment of the collagen alternates between the lamellae. In some bones, the lamellae form concentric rings around blood vessels. Each set of such rings is called an osteone. Not all bone is, however, lamellar bone; such highly organised bone is only common in larger reptiles and mammals. The predominant component of bone, by weight, is the organic part. Crystals of hydroxyapatite that lie between the collagen fibrils within each lamella form this. The resulting mineralised tissue is one twentieth as elastic as pure collagen in tension.
Fibrocartilage contains collagen in a less organised pattern than lamellar bone. The more elastic type of cartilage contains elastin also. The matrix of cartilage is a polymer made up of proteins and polysaccharides in hydrated complexes, known as mucopolysaccharide.
:::::'''Fig. 2.12 Bone structure'''
:::::A magnified, schematic view of a section from the shaft of a bovine femur.
::::::a periosteum
::::::b lamellar bone
::::::c osteone
::::::d vascular canal
'''The advantage and organisation of composite materials'''
As examples of composite materials, the physical properties of bone and fibrocartilage are unlike those of either component taken alone, and are not the sum of the two taken together. Collagen fibres resist tension forces and hydroxyapatite resists compression forces. The composites, bone and fibrocartilage may therefore be likened to manmade materials such as ferroconcrete, fibreglass or filled rubber. The discovery of these revolutionised construction of bridges, boats and tyres, as was animal construction revolutionised when bony skeletons were invented much longer ago.
Structures made up of bone or fibro-cartilage are seldom homogeneously constituted. Their composite materials have specific orientations and densities, resulting in compact or spongy bone, or hyaline cartilage with little collagen or dense fibrocartilage. The elasticities and strengths of these depend on both the orientation of the force to the direction of the fibrous component, or "grain", and the proportions of each component. For instance, bone lamellae are densest around the surface and along the shaft of a bone (Fig. 2.13). The reasons for this are apparent once the forces within the structure are considered.
:::::'''Fig. 2.13 Structure of a long bone'''
:::::The architectural requirements specify that only sufficient material should be used to allow for stresses the bone is expected to tolerate. Thus, the extremities are composed of spongy bone internally, the shaft is osseous only on the outside, and the dense laminar bone of the shaft is thickest halfway along its length. During growth, an increase in the length of the bone occurs by ossification of cartilage, produced from the epiphyseal line, in the metaphyseal region. This growth is minimally affected by stresses in tendons and ligaments, because these structures are attached to the epiphyseal regions beyond the epiphyseal lines.
'''Compression and tensile strength of bone and cartilage'''
The strength of a tissue is determined by measuring the minimum stress needed to produce permanent deformity in the material. Deformation comes about by pairs of opposing forces that cause compression, tension or shear (Fig 2.14).
The compressive strength of fresh compact bone loaded parallel to the grain is about 170 MN.m-2. This is impressive, considering that the compressive strength of marble and granite is little more, at 200MN.m-2. Cartilage has a compressive strength lower than bone.
• Fresh, compact bone loaded parallel to the grain has a tensile strength of 80MN.m-2. For steel it is 500MN.m-2. Again, cartilage has a compressive strength lower than bone.
• The resistance of compact bone to shear may be as low as 50MN.m-2 if stressed parallel to the grain, and as high as 120 MN.m-2 if stressed transversely to the grain. Cartilage, tendon and ligament have less resistance to shear.
If bone is mechanically superior to other materials, why is it not used more often for preference?
:::::'''Fig. 2.14 Types of stress'''
:::::Pairs of forces can exert three types of stress on solids
'''Weight economy and the choice of building materials'''
In nature, the most successful animal would be expected to economise on the use of building materials. Living in a highly competitive athletic environment, neither the hunter of the hunted can afford the luxury of excessive inertia-producing mass. Wild animals are lean and mean. The successful animal will employ in any given situation the least dense material for the task, in relation to the maximal mechanical demands that are made. With regard to weight economy, therefore, the specifications of the animal machine will use bones where high compression and shear forces are to be resisted, cartilage where these forces are lower and where lightness and elasticity are required, and pure collagen fibres where tensile forces alone are to be resisted.
Because animals economise on materials, the size and shape of their various parts indicate to us much about their function. Since its establishment as the first modern science by Vesalius in 1543, anatomy has depended very much on this principle.
'''Grace and coordination: the significance of strain rate'''
In locomotion, and when the animal must absorb accidental impact, dynamic, rather than the static physical properties described above are much more relevant. A small stress suddenly applied may do more damage than a large stress gradually applied. To break a piece of string, it is useful to apply a high-tension strain rate (give it a sudden jerk) as well as shear (pull the string over the edge of a table). Bone fractures are more likely when movement becomes less coordinated. The actual strain rate is difficult to measure in the living animal under the conditions in which the supporting tissues are used. Nevertheless, we can appreciate that an animal should run in such a way that strain rates are as low as possible by eliminating hard impact with the ground. Animals generally move smoothly and gracefully.
'''The advantages and disadvantages of stiffness'''
A major dynamic physical property of the supporting tissues is their energy absorption ability (Fig. 2.2) or stiffness. The stiffer the material, the less kinetic energy it is able to absorb. Since bones function as levers, stiffness is a desirable property; the higher the proportion of mineral, the stiffer the bone. Bone mineral first appears in bones of the developing fetus at sites where stiffness is needed most. This is either for protection, such as in the cranium to protect the brain from external pressure, or to aid correct development, as in the long bones of the limbs, the shape of which is necessary for the correct form of the muscular system (Fig. 2.15). The most highly mineralised bones in the body are the aptly named petrous temporal bone and the auditory ossicles, both involved in the detection and interpretation of sound waves. These bones assist the process of hearing best if they manage to avoid the absorption of the energy of sound waves. But because of this, they are brittle, and must be protected from other forms of mechanical stress by being enclosed by softer bone.
:::::'''Fig. 2.15 Ossification of limb bones'''
:::::These views of a sheep fetus at 38 and 44 days after conception have the mineralised bone stained with alizarin. They show which bones and parts of bones need the stiffening properties of hydroxyapatite crystals to ensure normal development, even though limb bones are not needed to support the weight of the animal at this stage.
:::::Why are the bones of the skull and thorax relatively well developed?
'''Energy economy – getting the most out of legs'''
The unmineralised collagenous structures, tendons and ligaments, absorb energy during locomotion as elastic energy, which reappears when the stress is removed, mainly as kinetic energy and partly as heat (Fig. 2.5). The collagen in the limb of a horse behaves like the spring inside a pogo stick (Fig. 2.16). It is the elasticity of legs that offsets for them the inherent disadvantages they have when compared with that unique human invention, the wheel.
:::::'''Fig. 2.16 Elasticity of collagen"'
:::::Action of the suspensory ligament a.,
:::::the proximal sesamoid bones b,.
:::::and the sesamoidean ligaments c.,
:::::of the fetlock joint d. of the horse at three stages of the stride.
:::::During a fast gallop and when landing from a jump, the ergot on the palmar surface of the fetlock joint contacts the ground
'''Tissue proportions and the absorption of energy'''
The greater the mass of a particular material, the more energy it can absorb. The weight of a bone, or for that matter that of the entire skeleton within the body, is understandably dependent more on how much energy need be absorbed during exercise, rather than by the ability to resist a static load. Body design is not well explained by considering only static forces. This will be discussed later in relation to the weight of the skeleton in animals of different body size, in Chapter 7.
'''The economical design of bones'''
A bone must contain the minimum material necessary to allow for the combination of compression, tension and shear forces to which it is subjected. Economic gain can be achieved by organising the distribution of ossified and nonossified tissues within a bone, or by designing the shape of the bone to take account of the predominant tension, compression and shear force couples that are applied to it. The next chapter shows how whole scientific disciplines, the osteological aspects of anatomy, archeology and paleontology, are based on this premise.
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[[Category:Musculoskeletal System - Anatomy & Physiology]]
[[Category:Musculoskeletal System - Anatomy & Physiology]]