Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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SECTION ONE • General Principles

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Figure 1-22.  Importance of working length. A: The working length of an implant is defined by the max- imal spread of its fixation points in a bone segment. A plate with a long working length has a better mechanical advantage, or “leverage” to counter-balance the bending force imparted by the proximal femur than a short plate. A plate with a short working length imparts a higher stress riser at the end screw to counter-balance bending forces. B: Stabilizing a mid-diaphyseal fracture with an intramedullary nail will yield a comparable working length of the proximal and distal nail segment. However, a distal fracture will yield a short distal working length, whereby the proximal segment will have a long lever arm, capable of inducing large stress risers at the distal nail segment.

train or bus. The increase in stability is gained by increasing the lever arm that can resist destabilizing forces. In the case of a distal femur plating construct, the long lever arm of the proximal femur generates considerable bending forces that must be counterbalanced by the lateral plate (Fig. 1-22A). A long plate with a long working length has a greater mechanical advantage than a short plate, and will induce a smaller stress riser at the end screw for a given load. Therefore, a long plate with diaphyseal screws spaced far apart to achieve maximal working length will provide the highest construct strength. 100 In contrast, a long bridge span, achieved by omitting screws in proximity of the fracture, will decrease the working length, weaken the construct, 203 and increase shear motion at the frac- ture. 82,111 The number of diaphyseal screws is less important than the working length of the plate. 184 Beyond three screws, adding a fourth screw in a diaphyseal segment has little effect on axial stability, but slightly improves stability in torsion. 203 Using more than four bicortical diaphyseal screws will not sig- nificantly improve fixation strength, regardless of plate length. 64 Additional screws should only be considered in presence of deficient screw purchase in osteoporotic bone.

For intramedullary nailing constructs, stabilizing a mid-di- aphyseal fracture with a canal-filling long nail will result in com- parable working lengths of the proximal and distal nail segments (Fig. 1-22B). However, stabilizing a distal fracture will yield a short working length of the distal nail segment, while the proxi- mal segment will have a long lever arm, capable of inducing large stress risers at the distal interlocking screws and nail–bone inter- face. This unbalanced construct can exhibit a drastically reduced strength, especially in case of comminuted fractures that do not support load transfer. Therefore, despite advances in nail designs, caution is advised in the nailing of unstable distal fractures. For external fixators, the working length is determined by the spacing of fixation pins or pin clusters, or the spacing of circular frames within the same bone segment. A longer working length achieved by placing one pin close to the fracture and one far away will lead to a more stable and stronger construct. The working length concept applies equally to the sub- components of fixation constructs. For example, the working length of a unicortical screw to resist torsion corresponds to the cortical thickness (Fig. 1-23A). 100 Even a small torsional force can induce stress risers at the screw–bone interface that exceed

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