Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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CHAPTER 1 • Biomechanics of Fractures and Fracture Fixation

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Figure 1-39.  For fixation constructs, a high stiffness does not imply superior strength, but may in fact be detrimental to construct strength. The difference between the low bending stiffness of ribs ( A ) and the high stiffness of early rib plates induced stress risers at the screw–bone interface ( B ) and caused fixation failure by screw pull-out ( C ). Similarly, a rigid locking screw at the end of a plate induces a stress riser than can weaken the fixation constructs ( D ).

For fracture healing, deficient stiffness of external fixator or intramedullary nail constructs can lead to hypertrophic nonunions, 155,216 while excessive stiffness of locked plating constructs can suppress biologic bone healing. 147 Construct stiffness is frequently calculated by dividing the applied load by the actuator motion of the test system. However, if the effect of construct stiffness on fracture healing is of interest, stiffness should be calculated based on the motion measured at the fracture site. If motion across the fracture site is highly nonuniform, gap motion should be assessed individually at multiple locations within the fracture gap. In addition to the amount of motion, the direction of motion between adjacent

fracture surfaces can be assessed to quantify the type of inter- fragmentary displacement in terms of shear and axial motion. Since stiffness can be assessed under static loading conditions, the amount and direction of motion at the fracture site can be measured manually by photographic tracking of markers, or automatically with optical or electromagnetic motion tracking systems (Fig. 1-40). For example, a recent study tracked the motion of interfragmentary markers and demonstrated that increasing the bridge span of locked plating constructs has only a small effect on axial motion, but can greatly increase shear motion, leading to shear-dominant interfragmentary motion. 111

Figure 1-40.  Interfragmentary motion assessment in terms of mediolateral shear and axial motion ( A ) can be accomplished by photographic tracking of LED markers attached to the medial and lateral aspects of adjacent fracture surfaces ( B ). C: Relative changes in the distance between markers reveal shear motion d S and axial motion at the near cortex ( d A, NC) and far cortex ( d A, FC) of the fracture. (Adapted fromHenschel J, Tsai S, Fitzpatrick DC, et al. Comparison of 4 methods for dynamization of lock- ing plates: differences in the amount and type of fracture motion. J Orthop Trauma . 2017;31:531–537.)

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