**3. Biomechanics of bone fracture**

Bones from the appendicular skeleton are continuously subject to physiologic and non-physiologic mechanical forces. Physiologic forces are generated through weightbearing and muscular contraction during physical activity or even at rest. Didactically, it is established that force vectors act isolated in long bones such as flexion or bending, axial compression, tension, and shear and torsion forces; however, clinically, one force vector is predominant. In healthy animals, the physiologic loads applied to bones rarely exceed the yield point, or more practically, physiological forces do not cause plastic (permanent) deformation of bone (**Figure 6**). Nonetheless, when nonphysiologic forces are the result of externally applied loads (vehicular trauma, horse kick, fall from height, and gunshot), it is easily exceeding the yield point and loadbearing capacity of the bone is easily exceeding, and as consequence, fracture will occur (**Figure 6**). The resistance of the bone will vary according to the direction of the load, and depending on the intensity and type of forces applied to the bones, different fracture lines will form. In general, oblique fractures originate from supraphysiological axial compression forces; transverse fractures are related to tension (avulsion) and flexion forces applied on opposite sides of the long bone; spiral fracture lines are expected from torsional forces, which create an angular line running around the circumference of the bone and a longitudinal one joining the two ends of the spiral. The combination of forces will give rise to other patterns of fracture lines, such as segmental fractures (butterfly fragment) caused by shear failure on the compression side before the tension fault line; it then propagates throughout the bone, creating the compression side plus a fracture line, usually a single line fragment. The combination of flexion and compression forces potentially generates early failure on the compression side, and a larger fragment will break loose (major butterfly fracture).

#### **Figure 6.**

*Load/deformation curve of long bones (A—starting load, B—yield point of deformation, C—fracture or failure point).*

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

The minority of clinically encountered fractures are a result of pure isolated loading; the majority of fractures presented to a veterinary surgeon are caused by more complex loading situations. Most clinical fractures are produced by a combination of three or more loading modes, resulting in a fracture line initiation and progressing in numerous planes. For example, a fracture caused by vehicular trauma is often the result of a combination of bending, shear, and torsional loads. Additionally, the values of the different loads would most likely be different, causing further variations in the fracture patterns observed.

Fractures caused by high-energy trauma (e.g., road traffic accidents) involve the greater accumulation of energy associated with the combination of forces, often resulting in greater fragmentation (comminuted lines). These fractures will also cause greater muscle damage and vascular compromise. The direction in which the energy is applied to the bone is as important as the intensity with which it propagates. Energy is absorbed by the bone and then released with the fracture. Damage applied to soft tissue and bone is proportional to the amount of energy released, and it is concluded that complex fractures are associated with greater soft tissue enveloping lesions.
