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Quadrupedal Mechanics - Anatomy & Physiology (view source)
Revision as of 10:22, 29 April 2013
, 10:22, 29 April 2013no edit summary
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.
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.
====3 The design of the passive supporting tissues====
'''Osteology: bones provide long-lasting clues to ancient forces'''
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.
:::::3.1 Ichthyostega
:::::The oldest known tetrapod is a late Devonian amphibian that measured about 1 m in length.
'''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.
:::::'''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.
'''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.
'''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.
'''Design to resist shearing'''
Shearing forces are applied to bones at the sites of insertion of tendons and ligaments. Resistance to these forces is enhanced by localised thickenings of the bone. Tubercles such as the ischial tuberosity and the greater trochanter of the femur (Fig. 2.8), which serve to increase the torque of muscles acting about a joint, are large because the shearing stress is great. Couples acting in planes at right angles to the length of a bone result in twisting, which also shears the material. The shearing stress is, as for bending, greatest at the outside, and zero along the central axis of the bone. Hollow shafts give strength with lightness in twisting as well as in bending, and again, such stress is greatest midway along the length of the bone.
:::::'''Fig.3.3 Attachments of tendons to bone'''
:::::The direction of the tensile force in the tendon of origin of the biceps brachii muscle of the horse does not change with changing angulation of the shoulder joint, because the tendon bends around the intertuberal groove of the humerus (A). Were this not so, the stress would be concentrated on one part of the junction of bone and tendon (B, arrowed).
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.
'''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.
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.
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[[Category:Musculoskeletal System - Anatomy & Physiology]]
[[Category:Musculoskeletal System - Anatomy & Physiology]]