*3.1.2 Compression (axial)*

Compression loading, also called axial compression, is produced when equal and opposite loads are applied toward the center and parallel to the longitudinal axis of the bone, causing compressive stress and strain within the bone (**Figure 8**). In long bones, compressive loads cause a decrease in height and an increase in width. Maximum compressive loads occur on a plane perpendicular to the applied load and can be defined by a series of small forces directed toward the center of the bone that

#### **Figure 8.**

*Illustration of shear and bending forces acting on a long bone, A—shear loading causes angular deformations, B bending loading induces tensile loading along the convex surface and compressive loading along the concave surface causing a transverse fracture pattern.*

### *Biomechanical Basis of Bone Fracture and Fracture Osteosynthesis in Small Animals DOI: http://dx.doi.org/10.5772/intechopen.112777*

potentially can compact or crush the bone. Rationally, we would expect that compression fractures developed perpendicularly to the applied compressive load will crush the bone. Nonetheless, the perpendicular tensile strain is usually not too important, because the expansion of cortical bone is highly unlikely and internally generated tensile strain also develops outward from the center of the bone, perpendicular to its longitudinal axis.

Nonetheless, compression loading also produces internal shear loading that develops oblique to the longitudinal axis and is maximal on a plane of 45° from the axis of compressive loading (**Figure 9**) [9]. Macroscopically, the fracture line of bone loaded under pure compression is typically a short oblique fracture and is created by these internal shear stresses, generated partly because of the bone's anisotropy and the fact that bone is weaker in shear forces and more tolerant in compression loads. These oblique fracture configurations produced by compressive loading are commonly seen clinically with jump or fall injuries of the distal tibia and radius (bones that are loaded along their central axis) [9].

A transverse fracture pattern also can appear as a result of compressive loading and is occasionally seen in vertebral bodies or the growth plates of long bones in young animals, also called impaction or impacted fracture (type V or VI Salter-Harris fracture) [1].

### *3.1.3 Shearing*

Shear loads occur when a force is applied parallel to the bone's surface, causing it to have a tendency to slide past another surface and causing an angular deformation (**Figure 9**). With the shear forces acting in opposite directions on opposing surfaces, shear loads within the bone lead to deforming it in an angular manner (right angles within the bone are deformed to acute or obtuse angles). In general, the bone offers the weakest strength when subjected to shear forces. Therefore, bone fractures along the plane of maximal shear stress. Clinically, fractures developing from shear loading often occur in the metaphyseal region of long bones with high cancellous bone content [9].

A classic example of fracture that occurs in small animals as a result of pure shear loading is the fracture of the lateral aspect of the distal humeral condyle seen in immature animals (Salter-Harris type IV). This fracture occurs as axial compressive forces are transmitted from the foot through the head of the radius to the lateral and/ or intercondylar component of the condyle of the distal humerus, resulting in a concentration of shearing forces at these regions of the distal humerus, producing a classical type IV Salter-Harris fracture [1]. Other common fractures created by shear loading would include "T" or "Y" intercondylar fractures of the distal humerus, fractures of the tibial plateau, isolated condylar or intercondylar femoral fractures, fractures of the glenoid cavity of the scapula, vertebral body fractures, and carpal or tarsal bone fractures. As previously described, shear loads also occur in most long bones subjected to pure axial compression, resulting in short oblique fractures along the plane of maximal shear stress [1, 9].

#### *3.1.4 Stress concentration or stress risers*

Osteopenia and bone defects on bone structure caused by iatrogenic conditions such as drilled holes (biopsy tract, bone graft collection, or screw removal) or acquired conditions like neoplasia, bone cysts, and bone infection (bacterial or fungal

#### **Figure 9.**

*Illustration of stress and strain produced by compressive and tensile loading. A—Compressive loading induces compressive and shear stresses and strains that, if excessive, may induce a short oblique fracture. B—Tensile loading induces tensile stresses and strains, which, if excessive, induce a transverse fracture.*

osteomyelitis) cause stress concentrations in bone that can initiate failures [10–12]. These areas of stress concentration can lead to local stress risers in the bone near the defect, which is many times higher than the stress physiologically applied to the bone. The concept of the stress concentration effect is based on the mechanical phenomenon that physiological loads must flow through the bone and, in a healthy bone tissue, without defects or heterogeneity. The applied forces flow equally through all regions, creating equal stress throughout. However, in bone with defects (e.g., holes from removed screws), the load cannot flow through the areas with defects and thus must flow around the holes. This leads to a concentration of stress in points adjacent to defects and osteopenia areas [9]. The clinical consequence is that bone stress-rising points break at lower loads than homogenous bones. The weakening effect of a stress riser is particularly noted for torsional loading where the decrease in strength may approach 90% and is proportional to the defect size [10]. However, defects smaller than 10% of the bone diameter may be of negligible significance in torsional resistance and may resist under physiological loadings [13].

Another form of stress concentration is illustrated by the difference in the elastic moduli (stiffness) of two materials (e.g., stainless steel and bone) under load. The
