Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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CHAPTER 1 • Biomechanics of Fractures and Fracture Fixation

the stabilization element and the bone, by increasing the dis- tances between stabilization elements and the fixator pins, or by using larger diameter fixator pins (Fig. 1-26B). The stiffness of monolateral fixators has only been investi- gated in a small number of clinical studies, reporting axial stiff- ness values of 174 N/mm, 58 240 N/mm, 65 and 510 N/mm. 98 In vitro animal studies of the AO/ASIF-double tube system yielded axial stiffness reports of 148 N/mm, 99 250 N/mm, 114 and 364 N/mm. 17 Similar axial stiffness results were reported for bilat- eral AO/ASIF external fixators. 17 In vitro studies of monolat- eral external fixators resulted in stiffness values as low as 40 N/ mm. 17,97 The magnitude of shear motion perpendicular to the diaphysis is usually similar to that of axial motion under phys- iologic loading. 76,215 The stiffness of ring-fixator constructs depends mainly on the number of rings, ring diameter, wire diameter, and the number of fixator pins. Axial-stiffness values of 33 to 117 N/ mm 99,190 and torsional stiffness values of 0.7 to 1.5 Nm/deg have been reported. 190 Ring fixators have a lower axial stiffness than monolateral fixators but a far greater torsional stiffness than monolateral external fixators and intramedullary nail con- structs. Most of these studies were performed on isolated tibiae. With an intact fibula that is thought to improve the healing pro- cess, 196 the stiffness in vivo can be assumed to be approximately one-third higher. 114 STIFFNESS OF PLATE CONSTRUCTS Fracture fixation with conventional compression plates relies on very high stiffness of the implant–bone construct, which can result in direct bone healing. 167 However, a loss of compres- sion between the fragments can cause interfragmentary motion, inducing large tissue strains and bone resorption. 167 In recent years, the use of locking plates has become more frequent. These locked plating constructs do not rely on compression. Rather, they achieve stability with fixed angle locking screws and typi- cally result in a residual fracture gap. 167,203 Plate bending leads to the stimulation of callus formation mainly at the opposite side of the plate. 36,147 However, directly beneath the plate, the interfragmentary motion and tissue strain are extremely low, leading to the suppression of the bone formation stimulus. 39,180 For large fragment locking plates, an axial stiffness of up to 3,000 N/mm has been measured. 39 Reducing the bending stiff- ness by increasing the bridge span 203 increases interfragmentary motion only opposite the plate, but not appreciably beneath the plate (Fig. 1-27A,B). Elevation of the plate from the bone surface could increase interfragmentary motion both near and opposite the plate, but also decreases the load to failure under cyclic loading. 2 For a long bridge span, interfragmentary shear motion can dominate axial motion. 39 The asymmetric axial motion at the fracture gap makes it dif- ficult to define axial and shear stiffness. It is therefore more prac- tical to evaluate interfragmentary motion individually at specific locations of the fracture. In vitro studies of locking plate con- structs showed 1-mm axial interfragmentary motion at the cortex opposite the plate in response to 350 N loading, correspond- ing to 0.5 × body weight. However, motion beneath the plate remained minimal. 54 Assuming a patient with partial load bearing

making shear motion the principal concern for intramedul- lary fixation. 12 This translational and rotational shear motion can furthermore be exacerbated by motion between inter- locking screws and interlocking holes in the nail (Fig. 1-26A). To reduce shear motion due to toggle of interlocking screws inside the nail, angle stable interlocking systems have been developed. 116,123 Removal of interlocking screws can be used to dynamize a nail. While this will facilitate telescopic motion of the bone fragments and closer approximation of the fracture surface, it will also decrease shear stiffness and may further increase shear motion. STIFFNESS OF EXTERNAL FIXATOR CONSTRUCTS External fixation is primarily used for temporary stabilization of open or infected fractures. Despite its temporary applica- tion, sufficient fixation stiffness is important to initiate the bone healing process, which may under favorable conditions proceed to successful fracture healing without the need for revision to an alternate fixation type. Most external fixator systems can be applied in a variety of configurations that can greatly differ in stiffness. 17 Unilateral external fixator systems are used most frequently for long bone fractures. The stiffness of unilateral external fixa- tors mainly depends on the size, configuration, and material of the fixator elements and fixator pins. Fixation stiffness can be increased by several means: by decreasing the distance between B Figure 1-26.  A: The stiffness of unreamed intramedullary nails depends mainly on the transverse shear motion (SM) in mediolateral and anterior–posterior directions, the difference between the nail diam- eter (N) and diameter of the marrow cavity (MC), and the rotational motion (RM). The bending stiffness can be increased with angle-sta- ble interlocking screws (IS). 59 B: Axial motion (AM) and shear motion (SM) of external fixators under load depend mainly on the distance between the fixator bars and the bone (A), the distance between bars (B), the pin diameter (C), and the distance between pins (D). 59 Fixator stability is increased by decreasing A and by increasing B, C, and D. F , force. (Reprinted by permission from Springer: Claes L. Mechanobiol- ogy of fracture healing, part 2: relevance for internal fixation of frac- tures. Unfallchirurg . 2017;120(1):23–31. Copyright © 2016 Springer Medizin Verlag Berlin.) A

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