target="_blank" rel="nofollow" href="#ulink_89ec345f-e162-56c4-ad20-e99739e33cbf">Figure 3.5 Shear stresses arising from torsional loading result in tensile and compressive forces at ~45° to the plane of shear. The fracture propagates perpendicular to the principal tensile stress in a spiral configuration relative to the longitudinal axis (A). The fracture becomes complete when the proximal and distal ends of the spiral are connected by a longitudinal fissure. This humeral fracture in a four‐month‐old Arabian foal provides an example of a spiral/long oblique fracture occurring predominantly due to torsional forces.
Source: Dr Scott Katzman.
Figure 3.6 Bending creates tensile and compressive loads on different sides of the bone. Failure occurs first on the side under tension resulting in a transverse distraction fracture. The fracture then propagates on the side under compression in an oblique configuration, with or without a butterfly fragment, illustrated by a Salter–Harris type II fracture of the proximal tibial physis in a 10‐day‐old foal and a mid‐diaphyseal butterfly fracture of the third metatarsal bone in a foal.
Source: Drs. Susan Stover and Larry Galuppo).
Figure 3.7 Three‐point bending configurations have a central load point at the location of highest bending moment (stress) on a bone supported near its ends. Four‐point bending configurations have two inner load points between two outer support points to produce a constant bending moment between the two inner supports.
Source: Lopez [43] Reproduce with permission of Elsevier.
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
