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

smaller and when a force is applied to a fracture site if the variation of the gap is bigger, the strain also increases.

The illustrative example of two different interfragmentary displacement scenarios is given in **Figure 4**. In the first two images, a simple line fracture with a 10 mm gap between bone fragments is shown. When both fragments are displaced by 5 mm, a displacement (strain) of 100% occurs. In the lower images, a multiline or communitive fracture is depicted, and there is a gap of 10 mm between each fragment, totaling a gap of 30 mm. When the same displacement of 5 mm in each major fragment takes place, a total of 10 mm is also added to the fracture gap; however, in this case, the final gap undergoes deformation (strain) of approximately 30%, because the final displacement was distributed over all fragments.

In the biological context, the strain concept is used to explain the relative deformation and its effects on bone tissue regeneration. In bone callus formation, the tissue can resist a different amplitude of elongation (distancing of fragments). If the movement exceeds the critical value of elongation, there will be dysfunction at a cellular level and, consequently, no delay in the tissue formation. In bone regeneration, the predominant cells in each phase show different tolerance to different magnitudes of elongation movements. As bone regeneration progresses, the tissue is less tolerant, demanding a more rigid mechanical environment (with less micromotion). During the inflammatory phase, the granulation tissue is the most tolerant to movement when compared to cartilaginous or bone tissue in subsequent phases (**Table 1**).

To illustrate the difference in instability tolerance between a simple fracture and a multi-fragmentary fracture, consider the following scenario:

Assuming both fractures have the same initial gap width (5 mm) and overall displacement (5 mm) in (A) and (B), the full displacement (5 mm) is active within a single gap in a simple transverse fracture (A), resulting in a strain of 100%, which is the limit of tolerated strain for granulation tissue. In contrast, in a multi-fragmentary fracture with five gaps (B), the overall displacement is shared among the gaps, resulting in each gap displacing from 5 mm to 6 mm, and the resulting strain is only 20% [3].

Additionally, and from a mechanical point of view, different fracture patterns presented different strain behaviors when subjected to the same stress load. Generally, the larger is the lever arm, the more movement at the interfragmentary interface will be observed; this occurs in single-line fractures (transverse and oblique) in which the fragments are long relative to the fractured section. In the previous scenario, the gap is small and the variation is large, determining a high-strain fracture environment. In simple words, these fracture lines are more sensitive to load/movement forces. In the opposite scenario, multiline fractures, the lever arm is smaller (multiple fragments smaller in length) and the total interfragmentary space (gap) is inevitably larger; with greater gap and equal length variation, with the same stress/load forces applied, the strain value will be smaller (Eq. (4)). For this reason, these fractures are considered low strain or less sensitive to movement or load forces [3].


**Table 1.**

*Tolerance of tissues of the osteogenic pathway to elongation and shortening.*

#### **Figure 5.**

*Illustration of fracture lines with high and low strain. High-strain fracture pattern, A and B—fracture line without load and with a little gap (x); when the bone is loaded, the fracture gap increase by 50% (1.5 x); lowstrain fracture, C and D—multiple line fracture with a gap equivalent to fracture line in A; however, when the bone is loaded, the displacement is more subtle among fragments (1.1 x) because the displacement is distributed between all fracture fragments.*

Clinically, for the surgical decision-making process, the surgeon should consider two groups of fractures based on the strain theory:


chosen over osteosynthesis methods. This type of fracture is caused by highenergy trauma with more need to preserve the soft tissue envelope of the fracture. Considering this scenario, bone osteosynthesis methods rely on bridging plates (biological osteosynthesis plates) and external fixation.

It is important to remind that several biological and clinical factors influence bone regeneration such as age, time elapsed since trauma, trauma intensity, and soft tissue disruption (open or closed fracture). For this reason, the decision process in bone osteosynthesis is multifactorial and should not be based exclusively on mechanical factors like the strain theory.
