Читать книгу Fractures in the Horse - Группа авторов - Страница 44

During normal daily activity, forces and moments are applied to whole bone structures in various directions simultaneously [35]. Loading conditions are often simplified to single axis loading in tension, compression, shear, bending, torsion or combined loading of a homogeneous structure with isotropic material properties (Figure 3.2). Such analyses provide insight into the circumstances that led to bone fracture, but may not accurately reflect the in vivo loading conditions.

Bone is an anisotropic material, meaning its mechanical properties depend on the direction of the applied forces. In general, because of the structure and orientation of osteons, compact bone is strongest in axial compression, weaker in tension and weakest in shear. As a result, fractures usually propagate along tension and shear planes. Shear planes run at approximately 45° angles from compressive and tensile stresses. Fractures will also follow the path of least resistance. Therefore, fracture lines or cracks will often be diverted around heavily buttressed areas. Likewise, fractures may terminate at suture lines or pre‐existing cracks as these dissipate fracture energy more efficiently.

Classic loading modes and fracture configurations are readily applicable to long bones, and diaphyseal fractures are mostly used to illustrate these concepts. They are also recognized in some types of epiphyseal [37], proximal sesamoid [38] and carpal cuboidal bone fractures [39, 40] in horses. However, these loading modes and associated fracture configurations are currently not well understood in relation to a number of other common equine sites, such as distal phalangeal, distal sesamoidean and tarsal cuboidal bone fractures.