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When a bending load is applied, compressive stress is induced on the concave side and tensile stress is induced on the convex side of the deforming bone. Bending creates a longitudinally oriented plane, called the neutral axis, where neither compressive nor tensile stresses are present. The greater the distance from the neutral axis, the larger the tensile or compressive stress. This has implications for internal fracture fixation, as implants (such as intramedullary nails) that are positioned at the neutral axis are exposed to lower levels of bending (and torsional) strain compared to implants placed away from the neutral axis (such as bone plates and external fixators) [42].

Failure is initiated on the tensile (convex) side of the bone because bone material is weaker in tension than compression. Tensile failure causes transverse crack propagation until compressive stresses on the concave side of the bone induce failure in shear at 45° to the longitudinal axis of the bone. Failure along the plane of highest shear stress drives the fracture line in oblique directions, producing an oblique fracture face or a butterfly fragment (failure in two shear planes at right angles to one another) on the compressive side of the bone (Figure 3.6). When the contribution of the compressive loading component is substantial, a larger butterfly fragment will result.

There are several modes by which bending deformation is induced. Axial compression of a curved bone (e.g. radius) and external forces applied to the side of a long bone both induce bending. in vitro, bending can be induced through three‐point and four‐point bending (Figure 3.7), and cantilever bending (where one end of a beam is fixed and a force is applied to the free end). Bending can also occur during axial compression secondary to specimen buckling. Bone fracture from four‐point bending is uncommon in clinical settings, but is useful experimentally as it creates a uniform bending moment between two central load points. This avoids concentrating stress under the central load fixture of three‐point bending and thus potential failure due to artefactual stress concentration.

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.