*3.1.1 Bending*

The definition of bending comprehends the axial compressive load that is applied eccentrically (off-center) to the bone column. Studies have demonstrated that 85–89% of the predominant normal physiological stresses that most bones experience during weight bearing are from bending loads due to the curvilinear shape of the femur, humerus, and, in some breeds, tibia e radius [1]. In these cases, the axial compressive load is applied eccentrically during locomotion [1]. The convex face of the bones experiences the maximal load of tension forces, whereas the concave side experiences the maximal load of compression forces. The bone column experiences a gradient distribution of this opposite force perpendicular to the bone, the axis being the center

**Figure 7.** *Different force vectors acting on long bone fractures.*

of the medullary cavity, an imaginary point where there are no tensile or compressive stresses and strains. This plane is called the neutral axis or neutral plane of the bone and experiences no axial stress (**Figure 7**) [1]. The magnitude of the compressive and tension forces acting on the bone increases as the distance from the neutral axis increases (**Figure 7**). When a supraphysiological bending load exceeds the yield point and the load-bearing capacity of bone, for example, in extremely soft bone such as immature or diseased bone, when it is subjected to a bending load. A fracture line starts at the tension side (because the cortical bone is weaker in tension than in compression) and propagates to the compression surface, producing, in most cases, a transverse fracture.

If an internally generated shear stress is added to these forces, it results in a short oblique fracture line toward the compression band surface of the bone. Two oblique fracture lines can occur near the compression surface, if the magnitude of shearing forces increases, forming a loose wedge. This fracture pattern is referred to as a butterfly fracture and is a result of two divergent planes of shear stresses near the compression surface.
