Figure 3.8 A shear force is an external force acting on an object or surface parallel to the slope or plane in which the surface lies. Cyclic shear loading of an interface between regions of different subchondral bone densities in the distal condyles of the third metacarpal bone predisposes to condylar fracture in Thoroughbred racehorses.

      Source: Dr Ryan Carpenter.

      Shear

A shear force is one that is applied parallel to a surface or section through a bone, causing a tendency for surfaces or sections to slide past one another (Figure 3.8). Shear loading is distinct from shear stresses and strains induced by other loading conditions discussed earlier [44]. Bone is disproportionately weaker in shear than in tension or compression [45–47].

      Combined Loading

The in vivo environment is more complex than uniaxial loading. Bone geometry, gait, muscle forces, ground surface conditions and disease processes all influence bone stresses generated by both external and internal loads. Ideally, experimental studies should recreate physiological loading conditions. Combined loading circumstances can be achieved by multiaxial loading (e.g. compression and torsion) and/or applying loads at soft tissue attachment sites.

Relationships Between Location and Morphology

      Fracture configurations in a clinical setting are often not easily categorized into the classic patterns because clinical fractures occur under complex, multidirectional loading conditions at high strain rates and are influenced by local bone quality and surrounding soft tissues. However, an understanding of the predominant biomechanical forces involved in fracture generation in different parts of the skeleton is crucial to executing a successful repair and formulating strategies to reduce the risks of repair complications.

      Mid‐body fractures of the proximal sesamoid bones provide a good (albeit simplified) example of the relationship between transverse fracture configuration, location and morphology. The proximal sesamoid bones are composed of dense trabecular bone [48], and are subjected to high tensile loads exerted proximally and distally by the suspensory and distal sesamoidean ligaments, respectively [49, 50]. Complete, mid‐body fractures of the proximal sesamoid bones typically have a transverse orientation, attributable to longitudinally directed tensile forces [51].

      Short oblique and butterfly fractures generally result from bending forces, which cause tensile loading on the convex side and compressive loading on the concave side of the bone. The radius is particularly susceptible to side‐impact loads like kick injuries, typically resulting in comminuted fractures in adult horses [52, 53]. Simple fractures are less common, but when they do occur the configuration is usually either short oblique or butterfly, with the base of the butterfly fragment on the same side of the bone as the impact [53]. In one ex vivo study mimicking kicking injuries in intact radii and tibiae, most oblique fractures also had a second divergent fissure which may have extended into a butterfly fracture if the impact velocity had been higher [53].

      Long oblique and spiral fracture configurations occur in the diaphyses of the femur, [54–56], tibia [37, 57] and humerus [58]. Traumatic diaphyseal fractures of the proximal long bones in adult horses are often severely comminuted due to substantial energy release at the time of the fracture [54–56]. However, in foals, diaphyseal fractures of the femur, tibia and humerus commonly occur in spiral or long oblique configurations due to a combination of compressive and torsional forces placed on the limb during axial loading [54, 55, 57, 59].

      Cyclic shear loading plays an important role in the formation of third metacarpal (MCIII) or metatarsal (MTIII) condylar fractures [60, 61]. During high‐speed locomotion, load is concentrated on the palmar aspect of the distal condyles of MCIII and adaptive modelling leads to increased density of the subchondral bone [62]. Bone forming the sagittal ridge, which is not directly loaded during locomotion, remains of relatively lower density [63]. The resulting variation in bone density between the two condyles and the sagittal ridge creates a stiffness gradient, leading to concentration of shear force at the interface of the regions of different densities at the parasagittal groove, where increased shear strain will result in fatigue damage [62]. Continued cyclic shear loading of the condyle leads to the propagation of a single dominant crack until structural failure occurs [60].

      Pure compression fractures are uncommon in horses [44]. Fractures that involve significant compressive forces can occur in the cervical vertebrae as a result of trauma, such as falls or impact into a fixed object [64–66]. Dorsal cortical stress fractures of the third metacarpal bone can also be related to compression [43]. During fast‐gaited exercise, the dorsal cortex of the third metacarpal bone is subjected to greater compressive forces than the rest of the cortex [67]. Consequently, dorsal cortical stress fractures have a short oblique configuration, typically propagating in a palmaroproximal to dorsodistal direction [68].

The Mechanical Behaviour of Bone

      When externally applied forces (loads) act on an object, it will undergo deformation (i.e. changes in dimensions). Forces expressed relative to the areas of application are termed stresses, and deformations caused by these stresses and expressed as a proportion of the original dimensions are called strains.

The mechanical behaviour of a bone organ (e.g. a humerus) is related to its structural properties (dependent on size/shape/geometry, etc.) and material properties (the bone tissue that it is made of, which is independent of shape/geometry, etc.). When testing the structural properties of bone, the relationship between load and displacement is represented