Tornetta Rockwood Adults 9781975137298 V2

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CHAPTER 1 • Biomechanics of Fractures and Fracture Fixation

the lower segment, the resulting fracture will be “right handed” in direction, similar to a right hand threaded screw. 4,62 There is often a soft tissue hinge associated with the vertical compo- nent of the fracture. The corresponding reduction maneuver is a counterclockwise force that utilizes the soft tissue hinge to afford the reduction. Relative to other fracture patterns, spiral fractures are thought to be relatively low energy. 62 Comminuted Fractures Comminuted fractures are high-energy fractures with multi- ple fragments and are known to have worse clinical outcomes than the simple fractures previously discussed. 121 The degree of comminution is directly related to the fracture energy. Under- standing the energy required to create these severe fractures is an important component in developing a treatment strategy. At present, the energy required to produce a given injury is largely described qualitatively, making assessment of the severity of the injury inexact. Recently, a quantitative technique relat- ing the degree of bony comminution to the amount of energy delivered at the time of injury was introduced. 15 The basic idea, grounded in principles of engineering fracture mechanics, is that the mechanical energy absorbed in producing a fracture directly correlates to the amount of interfragmentary surface area created during impact loading. Computed tomography (CT) scans provide the opportunity to directly measure inter- fragmentary surface area, from which the fracture energy can be quantified (Fig. 1-8). 6 For intra-articular fractures of the tibia, this technique found that comminuted proximal tibia and dis- tal tibia fractures resulted from similar fracture energy, but the degree of articular surface involvement was greater in the distal tibia. 73 Because the articular surface area of the proximal tibia is roughly twice that of the distal tibia, the energy absorbed per unit area is most likely higher in the distal tibia, resulting in greater local damage to the joint surface. This potentially explains the worse clinical outcomes for distal tibia fractures relative to proximal tibia fractures. PHYSIOLOGIC LOADS DURING NORMAL ACTIVITIES Different from the loads required to cause a fracture, the loads generated by activities of daily living define the forces a fixation construct will experience during the healing process. In gen- eral, fracture fixation constructs must provide enough stability to resist prolonged loading in the range of these activities. In the lower extremity, loads are generated during ambulation, while in the upper extremity these loads are related to utilizing the hand for activities such as eating and personal care. In addi- tion, the influence of ambulation aids upon postoperative load- ing affects the loads a fracture fixation construct must resist. In the lower extremity, ambulation aids decrease the postoperative loads; while in the upper extremity, loads are increased while using crutches. Upper Extremity Upper extremity forces are generated by muscle contraction and the weight of the arm as it is positioned in space. Different from the lower extremity, upper extremity activities of daily living

combination of bending and torsion. 62,121 Similar to pure axial loading, torsional loading also produces dominant forces at an angle of 45 degrees to the long axis of the bone, but the bending component results in a fracture line that is more vertical. When the torsional force is dominant, long oblique rather than short oblique fractures occur. 62 Butterfly Fractures The classically described mechanism for butterfly fractures is a fracture resulting from combined bending and compression forces on the bone. 4 The bending force creates tension at the far side of the neutral axis and compression at the near side of the neutral axis (Fig. 1-7). The fracture begins with a trans- verse tension fracture on the far cortex. Compression at the near cortex results in failure in shear with typical 45-degree oblique fracture lines. The combination of oblique compressive fracture lines joining with the transverse tension fracture line generates the butterfly fragment. 136 The energy required to form a but- terfly fracture is higher than for transverse or simple oblique fractures. Butterfly fractures may also occur after progressive loading of short or long oblique fractures wherein the short or long oblique fragment is sheared by the adjacent bone segment, resulting in a butterfly fragment. 4 In this case, all fracture lines are oblique, without a transverse component. Spiral Fractures Spiral fractures occur as the result of torsional forces. 178 The fracture has long, sharp ends with a vertical component. The resulting 45-degree fracture has a characteristic orientation depending on the direction of the torsional load. If the upper segment is fixed and a clockwise torsional load is applied to Figure 1-7.  Combined axial loading and bending results in the classic butterfly fracture. The bending component of the load results in ten- sion on the cortex on the far side of the neutral axis and compression on the near side from the applied load. Tension causes a transverse fracture line, and compression results in two oblique fracture lines, generating a butterfly fracture.

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