Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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CHAPTER 1 • Biomechanics of Fractures and Fracture Fixation

Figure 1-23.  A: The working length of a uni- cortical screw corresponds to the cortical thick- ness. Torsion can induce high stress risers at the screw–bone interface, leading to progressive “toggle” and eventual screw pull-out failure. B: The working length of a bicortical screw cor- responds to the outer diameter of the diaphysis and can effectively resist torsion without induc- ing excessive stress risers at the screw–bone interface.

A

B

the yield strength of cortical bone, leading to progressive “tog- gle” and eventual fixation failure of unicortical screws. Because bicortical screws contact both cortices, the working length is much greater and stress risers are not expected to approach the yield strength of bone (Fig. 1-23B). Adding a single bicortical screw to a unicortical locked plating construct can increase its torsional strength by 73%. 88 With the ever-increasing choice of implants and fixation options, selecting and configuring the appropriate fixation method has become ever more complex. The intention of this section was not to provide a comprehensive overview of the many and often- conflicting guidelines and technical tricks of how best to configure a fixation construct. Instead, it should provide a clear and logical decision strategy, and illustrate key concepts on practical exam- ples. The surgeon must first decide on the targeted healing mode, and then select a fixation method and configuration to achieve a construct stiffness suitable to promote the targeted healing mode. Finally, the fixation construct must be applied in a manner to maximize construct strength and durability without impeding the desired stiffness. The strength of a given construct can be improved by maximizing its working length and by reducing stress concen- trations. The latter can be achieved by load distribution, reduc- tion of stiffness gradients and by avoiding introduction of prestress during application of the fixation construct. Three general types of fracture fixation constructs exist for the operative treatment of fractures: plates, intramedullary nails, and external fixators. 153 From a biomechanical point of view, these constructs either splint the fracture providing an environ- ment of relative stability or compress the fragments resulting in an environment of absolute stability. Fracture stabilization by interfragmentary compression, once the gold standard, is becoming less common with the introduction of new implants and techniques. Absolute stability is typically obtained using compression plates and lag screws. Provided sufficient compression between the fragments is maintained throughout the entire duration of fracture healing, direct bone healing without callus formation is expected. The demanding technique of compression plating does not allow FIXATION CONSTRUCTS

the surgeon options to modify or control the biomechanical environment at the fracture site, especially since introduction of any motion will disrupt the primary bone healing process. When using fixation constructs in a splinting method, the construct stability and stiffness can be controlled by the sur- geon’s choice of implant type and dimensions, the operative preparation of the bone, and the construct configuration. A sur- geon’s implant choices can lead to large variances in construct stability and stiffness, resulting in considerable differences in healing progression. Depending on the fracture fixation stabil- ity, bone healing may proceed with optimal callus formation. Alternatively a delay in healing may occur because of overly stiff or too flexible fixation, or a nonunion may manifest in the presence of excessive instability. 57 To achieve optimal clinical outcomes, it is important for the surgeon to understand how the stability and stiffness of fracture fixation influence the bone healing process, the characteristic stiffness profiles provided by the commonly used constructs, and how the stability of these constructs be best controlled by the surgeon. The stiffness of a fracture fixation construct can generally be determined in three loading directions and around three rota- tion axes. Stiffness depends on the loading direction, construct configuration, and method of attachment to the bone. A com- plete six-degree-of-freedom assessment of construct stiffness is available only for a small number of fixation constructs. 76,215 Construct stiffness cannot be measured directly, rather it is cal- culated from measurements of the applied load and the resulting construct deformation (Fig. 1-24). Most studies focus on tibial fracture fixation constructs, since the tibia exhibits complication rates in fracture healing of up to 22%. 27 Because of the inherent challenges in determining load and deformation in patients, 60 fix- ation stiffness studies are often performed in vitro under labora- tory conditions. Bench-top studies have several limitations that can affect stiffness results. For example, simplified loading direc- tions that may not properly reflect the range of physiologically feasible loading conditions. Studies typically do not account for the soft-tissue contribution to the stiffness of the extremity. 195 In addition, stiffness measurements frequently isolate the tibia and do not consider the stabilizing effect of the fibula to lower-leg BIOMECHANICAL CHARACTERIZATION OF FIXATION CONSTRUCTS

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