**5. Applied fracture biomechanics to common clinical presentations in small animal osteosynthesis**

When the concepts of fracture biomechanics are applied to clinical situations, in simple terms, it is possible to define strain as movement and stress as force or a magnitude of load that is applied to the bone and/or the implant.

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

### **5.1 Strain**

The plate strain is the strain (movement) experienced by a plate when a load of the force vector is applied to it. More specifically, it is the amount of movement that the plate experienced with a certain force (proportional to the original length). Areas of high strain on the plate are areas of high stress. Areas of high plate strain should be avoided because small increases in stress on a plate decrease the fatigue life of an implant. The majority of implant failures after small animal orthopedic surgery are fatigue failures.

It is generally accepted that strain/movement at the fracture gap needs to be within the tolerable levels for tissues. The fracture stability dictates the type of healing that will occur. With strain below 2%, primary bone healing can occur, whereas at 100% strain, the only tissue that can form is granulation tissue. In secondary bone healing, the initial tissue at the fracture site in the inflammatory phase of bone regeneration is granulation tissue. The tissue then progressively stiffens until cartilage can form. Cartilage has a strain tolerance of around 10%. *In vitro* data suggests that callus is stimulated at strains of around 5–10% and bone is stimulated at strains between 1 and 5%. The bone formation starts in lower strain zones at the periphery near the periosteum before spreading inward across the entire fracture gap. .

### **5.2 Stiffness**

The concept of stiffness can be thought of as the magnitude of movement when a force is applied (it is the slope of the stress/strain curve). If the implant is stiff, it does not move when force is applied. One of the determinants of stiffness is working length. If the working length is increased, the stiffness decreases. This means more movement of the plate and higher stress (and higher strain). However, if the working length of the plate decreases, the stress and the strain will be concentrated in a smaller area, which can also predispose to plate failure.

#### **5.3 The strain paradox**

Stoffel et al. found that in an *in vitro* situation of a 1 mm simple fracture gap, the strain experienced on the plate in tension bending was lower with a long working length [18]. However, if you have a more flexible plate, the fracture ends touch and suddenly load sharing is produced and therefore less movement and lowered strain, however, only in tension bending. Although with a stiffer plate, the plate does not bend in tension bending. Basically, in the situation of a 1 mm gap, the strain was paradoxically decreased with a longer working length. However, this phenomenon can be explained by the fact that fracture ends touched when the bone is loaded, preventing further movement of the plate in the *'in vitro*'situation. If strain and stress on implants are increased, the fatigue life of a plate decreases. If the fatigue life of a plate from 100,000 cycles is reduced to 10,000 cycles, this could be the difference between the fracture healing before implant failure and catastrophic failure requiring surgical revision. .

#### **5.4 The concept of micromotion**

It is widely recognized that micromotion contributes to fracture healing by stimulating the formation of bridging calli. Osteosynthesis methods that are based on relative stability allows micromotion creating a biomechanically optimal construct for secondary bone healing by promoting bone callus formation and has already been associated with early bone healing in several high-risk cases [19]. On the other hand, delayed unions resulting from insufficient mechanical stability, or hypertrophic nonunion, may also be associated with large callus formation.

The concept of micromotion is also applied to joint prostheses at the bone-implant interface Excessive micromotion of an implant in bone renders bone ingrowth impossible and reduces osteointegration of prosthesis. The tolerated minimal movement within an interface has been reported to be 28–150 μm, and repetitive higher displacements values allow only the ingrowth of fibrous tissue to avoid osteointegration [20]. Micromotion magnitude is primarily a function of implant stability, although is influenced by the differences in the elastic modulus of bone and implants.

Axial micromotion can be created with circular external skeletal fixators (because the wires allow motion at the fracture site that is axial in direction), with some configurations of interlocking nail and with special plate designs. However, when using a locking plate with a long working length, the micromotion observed at the fracture site is characterized by not only an axial vector; it is also multidirectional.

Besides the influence of the magnitude of micromotion, the characteristics of interfragmentary micromotion are also influential in bone healing. Applying cyclic interfragmentary micromotion for short periods has been shown to influence the repair process significantly [21]. In a study by Goodship et al., it was reported that interfragmentary cyclic micromovement applied for the short term at a high strain rate produced a greater amount of periosteal callus when compared to the same stimulus applied at a low strain rate. It was also shown if a high-strain-rate stimulus is applied later in the regeneration period, this physiological process was significantly inhibited [21]. The beneficial effect of this particular biophysical stimulus early in the healing period may be related to the viscoelastic nature of the differentiating connective tissues in the early endochondral callus. In the early endochondral callus, high rates of movement induce a greater deformation of the fracture fragments because of the stiffening of the callus [19, 21].

An experimental study proved that stimulation of new bone formation by dynamization with micromovement was effective mainly in the early healing phase (4 weeks postoperatively), while dynamization had no significant influence in the late healing phase (8 weeks postoperatively). The beneficial effects of micromotion are hampered by the influence of the gap size in the healing process [22]. From that evidence, with dynamization, the negative effects related to a large gap size overcome the positive effects of dynamization [22]. If a flexible fixation of a simple diaphyseal fracture is performed in clinical practice, the fracture gap should therefore be reduced to as small as possible. But if for some reason a large fracture gap cannot be avoided, dynamization (i.e., enabling micromovement) of the fracture should be performed very carefully and only in the first weeks postoperatively [23]. A large callus formation does not necessarily lead to greater mechanical stability [23, 24]. From that conclusion, was not the size of the radiological evident callus, but the amount of newly formed bone of the peripheral callus that was important for gaining mechanical stability. After the early healing phase, a large amount of new bone is formed, which is mainly responsible for the biomechanical stability of the fracture line.

The amount of callus, more specifically the periosteal callus, is, to some extent, related to the flexural rigidity of the fracture. Research that has found a consistently positive effect of interfragmentary movement on the mechanical stability of regenerating bone has applied only small and controlled interfragmentary movements in the early healing phase [25] or allowed larger movement and loads in a later phase [26].
