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Quadrupedal Mechanics - Anatomy & Physiology (view source)
Revision as of 07:43, 30 April 2013
, 07:43, 30 April 2013→3 The design of the passive supporting tissues
The study of bones, osteology, is often the only opportunity to deduce the natural history of an animal long since dead. The oldest quadruped bone fossils are 350 million years old (Fig. 3.1). The distribution and density of mineralised tissue indicate the magnitude and direction of the forces acting on a bone in life, no matter how long ago these forces were applied.
The study of bones, osteology, is often the only opportunity to deduce the natural history of an animal long since dead. The oldest quadruped bone fossils are 350 million years old (Fig. 3.1). The distribution and density of mineralised tissue indicate the magnitude and direction of the forces acting on a bone in life, no matter how long ago these forces were applied.
:::::'''Fig.3.1 Ichthyostega'''
:::::'''Fig.3.1 Ichthyostega'''
:::::The oldest known tetrapod is a late Devonian amphibian that measured about 1 m in length.
:::::The oldest known tetrapod is a late Devonian amphibian that measured about 1 m in length.
'''Sculpture within bones'''
'''Sculpture within bones'''
The combination of lightness and compression strength is obtained by internal sculpturing. Trabeculae (Latin, little beams) are oriented parallel to the compression forces. These forces act, for instance, between opposing ends of the bodies of vertebrae, & therefore the trabeculae are parallel to the axis of each vertebral body (Fig. 3.2). At the end of a long bone, the compress-ion force transmitted across a flexed joint is not parallel to the shaft, and will vary in direction. Here, the design most economical of material is a network of interconnecting trabeculae following the compression stress lines (Fig. 2.8). Study the pattern in a long bone that has been sectioned longitudinally.
The combination of lightness and compression strength is obtained by internal sculpturing. Trabeculae (Latin, little beams) are oriented parallel to the compression forces. These forces act, for instance, between opposing ends of the bodies of vertebrae, & therefore the trabeculae are parallel to the axis of each vertebral body (Fig. 3.2). At the end of a long bone, the compress-ion force transmitted across a flexed joint is not parallel to the shaft, and will vary in direction. Here, the design most economical of material is a network of interconnecting trabeculae following the compression stress lines (Fig. 2.8). Study the pattern in a long bone that has been sectioned longitudinally.
:::::'''Fig.3.2 Trabecular compression lines in bone'''
:::::'''Fig.3.2 Trabecular compression lines in bone'''
:::::A median section of the 10th thoracic vertebra of the horse, showing trabeculae within the vertebral body directed along the long axis of the body.
:::::A median section of the 10th thoracic vertebra of the horse, showing trabeculae within the vertebral body directed along the long axis of the body.
'''Design to resist bending in one plane'''
'''Design to resist bending in one plane'''
Couples acting in planes parallel to the length of a bone result in bending, which involves a combination of compression (on the inside of the bending curve) and tension (on the outside of the bending curve). If the bending is predominantly in one plane, the shape of the bone is such that the longer dimension is oriented in the same plane as the bending couple. Thus the zygomatic arch is a bony beam turned on edge to the force acting on it, as are the body and the ramus of the mandible, the spinous and the transverse processes of the vertebrae, the scapula, and the wing of the ilium.
Couples acting in planes parallel to the length of a bone result in bending, which involves a combination of compression (on the inside of the bending curve) and tension (on the outside of the bending curve). If the bending is predominantly in one plane, the shape of the bone is such that the longer dimension is oriented in the same plane as the bending couple. Thus the zygomatic arch is a bony beam turned on edge to the force acting on it, as are the body and the ramus of the mandible, the spinous and the transverse processes of the vertebrae, the scapula, and the wing of the ilium.
'''Design to resist bending in several planes'''
'''Design to resist bending in several planes'''
The long bones of the limbs, however, must resist bending forces in many directions. Compression and tension due to bending are greatest towards the outside of a bone; hence a hollow cylinder achieves the most strength for the least material. The same principle applies in the design of bamboo and scaffolding. Maximum bending stress occurs half way along the length of the shaft, and here the cortical bone is thicker and denser (Fig. 2.8). An analogous manmade structure is the leafed spring of a car. In this case, one leaf is sufficient at the ends, but all the leaves overlap in the middle of the spring.
The long bones of the limbs, however, must resist bending forces in many directions. Compression and tension due to bending are greatest towards the outside of a bone; hence a hollow cylinder achieves the most strength for the least material. The same principle applies in the design of bamboo and scaffolding. Maximum bending stress occurs half way along the length of the shaft, and here the cortical bone is thicker and denser (Fig. 2.8). An analogous manmade structure is the leafed spring of a car. In this case, one leaf is sufficient at the ends, but all the leaves overlap in the middle of the spring.
'''Design to resist shearing'''
'''Design to resist shearing'''
The junction of bones with ligaments and tendons
'''The junction of bones with ligaments and tendons'''
Where tendons and ligaments are attached to a bone, tension, as well as shearing, occurs. At the junction, the collagen fibres of each tissue pass uninterrupted from a mineralised tissue to a non-mineralised tissue. At these junctions, the angle at which tension and shearing occurs must not change with different positions of the limb (Fig. 3.3). This is a special requirement in the design of joints and the location of tendon and ligament attachments.
Where tendons and ligaments are attached to a bone, tension, as well as shearing, occurs. At the junction, the collagen fibres of each tissue pass uninterrupted from a mineralised tissue to a non-mineralised tissue. At these junctions, the angle at which tension and shearing occurs must not change with different positions of the limb (Fig. 3.3). This is a special requirement in the design of joints and the location of tendon and ligament attachments
.
'''Elastic energy and bone fractures'''
'''Elastic energy and bone fractures'''
Fractures occur when the elastic energy (the area under the stress-strain curve in Fig. 2.2) builds up to a point beyond which any recovery is possible. Highly mineralised bone breaks more easily, for the same percentage of strain.
Fractures occur when the elastic energy (the area under the stress-strain curve in Fig. 2.2) builds up to a point beyond which any recovery is possible. Highly mineralised bone breaks more easily, for the same percentage of strain.
Elastic energy can also be increased by a higher loading rate. Bones fracture most under sudden, violent forces. The same forces applied in a slow, uniform manner would not cause as much injury. A fatigue fracture occurs when muscle fatigue results in, for instance, stumbling, and hence abnormally high loading rates. Damage to bone from external forces depends on the speed, from slight damage due to low speeds of impact, to more damage when an animal is hit by a car, is kicked, or runs into a fence, to the very high energy fractures caused by a bullet.
Elastic energy can also be increased by a higher loading rate. Bones fracture most under sudden, violent forces. The same forces applied in a slow, uniform manner would not cause as much injury. A fatigue fracture occurs when muscle fatigue results in, for instance, stumbling, and hence abnormally high loading rates. Damage to bone from external forces depends on the speed, from slight damage due to low speeds of impact, to more damage when an animal is hit by a car, is kicked, or runs into a fence, to the very high energy fractures caused by a bullet.
'''Kinds of fractures'''
Bone, we have noted, is stronger under compression than under tension, while shear is intermediate. Appropriate combinations of forces cause tensile fractures (Fig. 3.4 A) in bone mainly at the sites of attachment of tendons and ligaments. Common sites of fracture are, in the horse, the proximal ulna, the patella, the proximal sesamoids, the calcaneus and the accessory carpal bone. Such fractures are usually transverse.
Compression fractures, (Fig. 3.4 B) by contrast, are usually oblique, at an angle of 45° that corresponds to the plane of maximal shear stress due to the compressive load.
Because bone is damaged more by tension than compression, bending fractures (Fig. 3.4 C) begin on the tension side, are transverse where there is tension, and oblique on the compression side.
In torsion fractures, (Fig. 3.4 D) shear stress occurs over the entire shaft of a long bone, and is greatest at the periosteal surface. The result is a spiralling fracture.
:::::'''3.4 Ways in which a long bone may fracture'''
:::::The type of external load applied to a bone will determine the pattern of the fracture:
::::::A Tension
::::::B Compression
::::::C Bending (tension and compression)
::::::D Torsion (shear).
'''The role of tendons'''
Tendons transmit to bones the force produced by muscles. Muscular force is thereby concentrated on to a small area of the skeleton, contributing to precision of movement and allowing several muscles to act on the same structure in different ways. By the use of tendons, the mass of a muscle may be at a considerable distance from the site of skeletal movement, thereby controlling weight distribution and contours of the body. Consider the insertions of the extensor muscles of the hip joint.
'''Antifriction devices associated with tendons'''
Limbs are, of course, not straight, and the angles of joints can change markedly. Tendons allow the transmission of a force round these angles. The simplest way to smooth their passage is to provide a cushion. This is a bursa (Latin, wine sac made of goatskin), a pouch lined by synovial membrane and filled with synovial fluid. This lies between the tendon and the bone (Fig. 3.5 a), or it may wrap around the tendon to form a synovial sheath (Fig. 3.5 b). Sheaths are often held in place by transversely arranged collagenous structures, retinacula. A further adaptation occurs when the surface of the bone becomes changed to articular cartilage and forms one boundary of the bursal cavity. A pulley-like arrangement is formed (Fig 3.5 c).
:::::'''3.5 Structures reducing friction between tendon and bone'''
:::::(a) to (d) represent increasing complexity
:::::Left side: longitudinal sections of the tendon
:::::Right side: transverse sections of the tendon
:::::Note that that the opposing surfaces of a synovial space are normally in contact.
::::::a. bursa
::::::b. synovial sheath and retinaculum
::::::c. articular cartilage, bursa and retinaculum
::::::d. sesamoid bone, articular cartilage, synovial joint and collateral sesamoid ligaments
'''Shearing within tendons'''
Loaded tendons that are straight sustain only tensile forces, but where a tendon bends shearing forces also occur. Accordingly, tendons compensate by becoming thicker at such places. If forces other than tension are particularly severe, small bones, which are better able to withstand compression and shear, interrupt the tendons. These are called sesamoid bones (Latin, resembling a sesame seed) (Fig. 3.5d, & 3.6). A bone inserted into a tendon also creates the possibility of attaching ligaments to hold the tendon precisely in place. These are collateral sesamoidean ligaments (Fig. 3.6). Both the sesamoid bone, and the bone with which it is in contact, have articular cartilage on their opposed surfaces. This arrangement of a synovial sac interposed between two bony surfaces covered by articular cartilage constitutes a synovial joint. Examples of sesamoid bones in the dog are those interposed in the tendon of insertion of the quadriceps muscle at the stifle joint (the patella) (Fig 3.6), and the bones interposed in the tendon of insertion of the Interosseous muscles at each of the metacarpo- and the metatarsophalangeal joints (the proximal sesamoid bones (Fig. 2.16)). In both these examples, the synovial pouch of the seamoid bone is continuous with the pouch of the adjacent joint.
'''The lubrication of joints: multigrade specification'''
The lubrication fluid of bursae, synovial sheaths and joints is secreted from protrusions of synovial membrane, synovial villi, located in loose parts of the joint capsule away from any compressed surfaces. This is synovial fluid. In a normal synovial sac, only a small amount of fluid is present. The viscosity of this clear or pale yellow liquid is more during slow movement, which is when the rate of shear is low. But when the shear rates are highest, the viscosity becomes much lower. The fluid drag and hence the supporting ability of the joint is therefore greatest when the limb is quietly bearing weight; the impedance is conveniently reduced for fast movements.
Lubrication is effected in two ways, so that the pressure in the fluid film supports the load. Not only does a film of synovial fluid separate opposing articular surfaces (fluid film lubrication), but also a single layer of glycoprotein molecules of synovial fluid is absorbed into the surface of the articular cartilage and protects the surfaces against wear (boundary lubrication).
The resulting lubrication is termed elastohydrodynamic. The cartilage surfaces are elastic; the deformation at contact increases the surface area and the lubricant escapes less readily; and the load-bearing capacity of the joint is greatly increased.
:::::'''3.6 The largest sesamoid bone'''
:::::The patella (b) is interposed in the tendon of insertion of the quadriceps muscles (a), that insert on the tibial tuberosity through the patellar ligament (c). The patella is held in place by collateral femoropatellar ligaments (d), and the hinge movement of the joint is maintained by the collateral femorocrural ligaments (e).
'''The resistance of joint surfaces to wear'''
Large repetitive compression and shear forces between opposing bones are catered for by an elastic surface 1 to 7mm thick composed of hyaline cartilage. This cartilage is unusual in that it has no perichondrium. The collagen within this articular cartilage differs from that of fibrocartilage in that the fibrils no not aggregate to form fibres. They do, however, align parallel to the surface for a shallow depth over the articular surface, and in a deep, thick zone are perpendicular to the surface and are continuous with the collagen of the underlying bone. Since articular cartilage is 70 to 80% water, its elasticity probably incorporates hydrostatic properties as well.
While joints usually cope well with a lifetime of wear, the demands of the breeders and trainers of athletic and working animals are often overoptimistic. Thus injuries to joints arise not only from accidental trauma but also because of genetic selection for high performance, and from overuse.
'''Orthopedics: assisting natural wear'''
In various ways, not only joints, but also all other components of the passive musculoskeletal system of these animals can fail. While natural repair is often possible, with rest, diverse orthopedic techniques have been developed to save valuable animals. The application of each of these techniques depends on an understanding of the mechanical principles of the structures needing repair, and the materials used to assist. An orthopedic surgeon constantly depends on good anatomical knowledge.
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[[Category:Musculoskeletal System - Anatomy & Physiology]]
[[Category:Musculoskeletal System - Anatomy & Physiology]]