**2.8 The helicopter effect**

The tightening of the first screw (head locking) in one extreme of the plate without stabilization of the other end of the plate will cause the "helicopter effect". This effect also occurs if only two screws were applied in auxiliary plating used in the orthogonal plating technique. In orthogonal plating, the auxiliary plate must have a minimum of two screws applied for the segment to prevent the helicopter effect (**Figure 3**).

## **2.9 The strain theory**

The control of interfragmentary micromotion is the key point for correct fracture stabilization. The knowledge of the factors that dynamically influence the distances between fracture fragments is fundamental to controlling micromotion. The strain theory is the most important concept used in the decision-making process, regarding fracture osteosynthesis from a mechanical perspective.

Essentially, strain or relative deformation is the amount of movement (distancing/ approaching) between fracture fragments relative to the original distance (gap).

It is expressed as a percentage of movement, that is, movement of gap/original fracture gap when the fragments were subject to mechanical stimulus (weight support, muscular contraction, and passive movement, among others). The calculus of interfragmentary movement in a laboratory environment is obviously more precise than in clinical settings. In clinical scenarios, several factors can influence the magnitude and direction of the fragments in the process of distancing/approaching movement; among these factors are the great variability of fracture patterns and the correspondent mix pattern of strain simultaneously present at the fracture lines [3].

Mathematically, the strain is determined by the formula (Eq. (4)):

Formula for strain determination:

$$E = \frac{\Delta L}{L}$$

where the *E* is the strain expressed by % value, Δ*L* is the variation of interfragmentary space (gap variation), and *L* is the initial gap. By the equation, is possible to infer that if the initial gap is bigger, the final strain value inversely will be

#### **Figure 4.**

*Illustration of interfragmentary displacement. A and B—simple line fracture with a gap of 10 mm; when the displacement of 5 mm between both fragments occurs, a strain of 100% is produced; C and D—multiple line fracture with a gap between fragments of 10 mm, totalizing a fracture 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.*
