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tensile stress occurs on a plane perpendicular to the direction of applied load. The structure lengthens and narrows, and subsequently fails as a result of debonding at cement lines and osteonal pull out to result in a transverse fracture configuration (Figure 3.3). Long bones are not well adapted to resist uniaxial tensile loads, which are not common during normal physiological activities.
Figure 3.2 Common fracture configurations and simplified causative forces illustrated on diagrams of the dorsal surface of the third metacarpal bone.
Source: Modified from Morgan and Bouxsein [36].
Compression
Compressive loading occurs when equal and opposite loads are applied to push the ends of the bone closer together. Axial compression causes the bone to shorten and widen. Because bone material is weakest in shear and the plane of maximum shear stress is offset 45° from the axis of loading, diaphyseal fractures due to longitudinal compression have an oblique configuration (Figure 3.4) [41].
in vivo, oblique fractures often result from a combination of compression, bending and/or torsion forces that cause the bone to break diagonally to the long axis. The fracture morphology reflects the predominate type of load. If compression forces are predominant, a short oblique configuration will occur. If bending forces are predominant, the fracture will have a transverse component, with or without a butterfly fragment. Long oblique fractures, which are often difficult to differentiate from spiral fractures, are common when torsion is the predominant force.
Torsion
Torsional loading occurs when opposite moments (rotational forces) are applied to the ends of a bone, such that the bone twists around the longitudinal axis. Fracture surfaces produced by torsional loads are helical, creating a fracture that circles or spirals around the shaft. Torsional loading induces shear stresses in planes parallel and perpendicular to the longitudinal axis. The magnitude of shear stresses increases proportionally with increasing distance from the central axis of rotation (typically the longitudinal axis of the bone). Tensile and compressive stresses are also induced, orientated approximately 45° from the shear direction. The fracture originates where shear stress is greatest on the periphery of the bone and then propagates due to tensile stresses distracting bone fragments along a spiral configuration until the fracture ends are approximately parallel or above one another (Figure 3.5). The fracture becomes complete when a longitudinal fissure occurs, connecting the proximal and distal ends of the spiral crack.
It must be recognized that this classic spiral fracture pattern occurs when an isotropic and homogeneous prismatic cylinder is loaded in pure torsion. As such, it is rarely seen in vivo, because of the asymmetric geometry of equine long bones, the forces exerted on those bones by soft tissues, regional variations in predominant collagen fibre orientation and other material characteristics within the bone.
Figure 3.3 Tensile loads cause the bone to elongate and narrow. Failure occurs due to tensile forces perpendicular to a transverse plane. Tension from the suspensory ligament and distal sesamoidean ligaments influenced the transverse configuration of the mid‐body proximal sesamoid bone fracture shown.
Source: Dr Ryan Carpenter.
Figure 3.4 Compressive loads cause the bone to shorten and widen. Failure occurs along the plane of maximum shear stress, oriented approximately 45° from the axis of compressive loading. An incomplete dorsal cortical stress fracture of the third metacarpal bone illustrates a fracture due predominantly to compressive loading and shear failure.
Source (inset): Based on O'Brien et al. [41].
Bending
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.