Читать книгу Fractures in the Horse - Группа авторов - Страница 47

Torsional loading occurs when opposite moments (rotational forces) are applied to the ends of a bone, such that the bone twists around the longitudinal axis. Fracture surfaces produced by torsional loads are helical, creating a fracture that circles or spirals around the shaft. Torsional loading induces shear stresses in planes parallel and perpendicular to the longitudinal axis. The magnitude of shear stresses increases proportionally with increasing distance from the central axis of rotation (typically the longitudinal axis of the bone). Tensile and compressive stresses are also induced, orientated approximately 45° from the shear direction. The fracture originates where shear stress is greatest on the periphery of the bone and then propagates due to tensile stresses distracting bone fragments along a spiral configuration until the fracture ends are approximately parallel or above one another (Figure 3.5). The fracture becomes complete when a longitudinal fissure occurs, connecting the proximal and distal ends of the spiral crack.

It must be recognized that this classic spiral fracture pattern occurs when an isotropic and homogeneous prismatic cylinder is loaded in pure torsion. As such, it is rarely seen in vivo, because of the asymmetric geometry of equine long bones, the forces exerted on those bones by soft tissues, regional variations in predominant collagen fibre orientation and other material characteristics within the bone.

Figure 3.3 Tensile loads cause the bone to elongate and narrow. Failure occurs due to tensile forces perpendicular to a transverse plane. Tension from the suspensory ligament and distal sesamoidean ligaments influenced the transverse configuration of the mid‐body proximal sesamoid bone fracture shown.

Source: Dr Ryan Carpenter.

Figure 3.4 Compressive loads cause the bone to shorten and widen. Failure occurs along the plane of maximum shear stress, oriented approximately 45° from the axis of compressive loading. An incomplete dorsal cortical stress fracture of the third metacarpal bone illustrates a fracture due predominantly to compressive loading and shear failure.

Source (inset): Based on O'Brien et al. [41].