Читать книгу Fractures in the Horse - Группа авторов - Страница 35
Structural (Whole Bone) Properties
ОглавлениеThe degree of deformation that a structure, such as a whole bone, undergoes when loaded will be determined by the magnitude and nature of the load, the geometric properties of the structure (its mass and the distribution of that mass around the axis of loading) and the mechanical properties of the material from which it is made.
There is relatively little diversity in the material properties of cortical bone from similar bones between different individuals of the same species or even between species. Numerous studies have shown that most variations in the mechanical properties of whole bones are largely accounted for by differences in their geometric properties. These can vary greatly, particularly in animals subject to different exercise. In galloping Thoroughbreds, strain gauges bonded to the dorsal cortices of third metacarpal bones demonstrated significant variation in the maximum extent of deformation (peak strain magnitude) between different animals. Peak strains measured in young, exercise naive horses were 1.5–2 times higher than those recorded in older animals, which had been in training for a prolonged period [36]. The difference was accounted for by a significant variation in the geometric properties of the bones between the two groups [37].
Figure 2.16 Eccentric compressive loading on bones of the appendicular skeleton results in bending forces, with one cortex in tension and the opposite in compression. Stress magnitude increases linearly with distance from the axis about which the object bends (the neutral axis).
Most bones of the appendicular skeleton of quadrupeds are loaded in axial compression due to gravity. However, the principal load is frequently applied eccentrically, resulting in a bending moment, which is often exaggerated by the pull of musculature. Loads due to bending result in a stress gradient across the bone, with one side loaded in compression and the other tension (Figure 2.16). The further the mass of the structure is from its neutral axis (the axis about which it bends), the greater ‘leverage’ the material has to resist the loads and the stronger and stiffer the bone is in that plane. In a situation where loads are unpredictable and an object may be subject to bending forces in any plane, a hollow cylinder provides the most mechanically effective distribution of mass. If one plane is loaded more heavily and more frequently than another, then eccentric distribution of mass around the circumference of the cylinder will offer optimum resistance to the predominant loads while also providing support in other planes (Figure 2.17).
Load is transmitted between bones at joints. Typically, bones flare at their ends, providing a wider surface area at the articulation and reducing stress on the load‐bearing structures: hyaline articular cartilage, mineralized articular cartilage, subchondral bone and deeper metaphyseal bone. A framework of cancellous bone supports the entire joint surface and transmits the load to the diaphyseal cortex. The relatively fine trabeculae forming the cancellous network provide a compliant structure more capable of absorbing impact loads than dense cortical bone. Fat and other soft tissues in the spaces between trabeculae may also play an important role in damping these loads. In moderation, localized fracture of individual trabeculae may be another physiological mechanism of mitigating loads that could be more damaging to other tissues.
Figure 2.17 Line drawing illustrating the stress intensity (colour density) either side of the neutral axis at the mid‐diaphysis of a bone which is loaded in bending. Bone modelling can alter the geometric properties of the bone to place the tissue at the optimal location to resist bending stresses.