Tornetta Rockwood Adults 9781975137298 FINAL VERSION

21

CHAPTER 1 • Biomechanics of Fractures and Fracture Fixation

A, B

C Figure 1-21.  Strategies to minimize stress concentrations. A: In the metaphysis, using more and longer screws will improve load distribution and fixation strength. B: In the diaphysis, the stiffness gradient between the plated and native cortex can be reduced by using thinner and more elastic titanium plates, applied with a nonlocking endscrew. C: When using hybrid fixation with locked and nonlocked screws, nonlocked screws are used before locking screws to prevent preloading.

Locking screw fixation in the metaphysis is an example of the importance of load distribution. A metaphyseal locking screw rests on individual trabeculae that are in direct contact with the screw shaft (Fig. 1-21A). A longer screw will distribute load transfer over more trabeculae. Each additional screw will increase the number of trabeculae that participate in load transfer from the plate to the metaphyseal bone. Sharing the load between multiple screws decreases the load, or stress riser, imposed on the individual trabeculae. Hence, the maximal number of locking screws of maximal length will provide the strongest fixation in the metaphysis. 85 For intramedullary nails, using two multi-planar interlocking screws rather than one not only reduces the axial load on each screw, but also distributes the load and reduces the stress riser at the screw–bone interface. 102 Reduction of stiffness gradients can be understood by evalu- ating the effect of a diaphyseal plate on the stress distribution of the surrounding bone. A plate increases the bending stiffness of the diaphysis. This will lead to a stiffness gradient at the transition from the plated to the nonplated diaphysis section (Fig. 1-21B). Bending forces induce a stress riser at the end screw, which can cause a transverse fracture through the end screw hole. This complication has an incident rate of 1% to 3%. 29,47 Because tita- nium is approximately half as stiff as stainless steel, the stress riser at the end screw can be reduced by using plates made of titanium. 156 The stress riser can further be reduced by using a nonlocking end screw, since the rigid, angle-stable fixation of locking screws can further exacerbate the stiffness gradient and resulting stress riser at the plate end. Replacing a locked end screw with a nonlocked end screw increases the bending strength of a construct by 40%. 31 For the same reason, modern intramedullary nails are typically made of titanium alloy, and

have a tapered design with a rounded tip to reduce the stiffness gradient and resulting stress riser at the nail tip. Prevention of preloading during fixation is particularly important for intramedullary nails. If the entry portal is in the incorrect location or orientation, nail insertion can not only cause malalignment of the fracture, but can also induce stress risers, leading to acute cortical fracture or splitting at the insertion site or nail tip. 86 Even if a sub-optimal nail insertion does not cause an acute fracture, it is likely to induce permanent stress risers that predispose the fixation construct to early fatigue failure. As such, even today’s advanced nail designs do not alleviate the need for accurate nail insertion to prevent malreduction and minimize stress risers. In the case of hybrid locked plate constructs, it is important to place all nonlocking screws before inserting lock- ing screws to prevent preloading the screw–bone interface (Fig. 1-21C). If fixed angle locked screws are placed first, the plate might be held elevated from the bone. Nonlocking screws placed after the locked screws cannot effectively compress the elevated plate onto the bone surface and will instead induce additional axial preload to the screw–bone interfaces. Maximizing Working Length The working length of fixation refers to the maximum distance between fixation points on one side of a fracture (Fig. 1-22). It is not to be confused with the bridge span, which is the length of the plate segment that spans the fracture zone. A greater work- ing length will improve the strength of the fixation construct, and will reduce stress risers at the fixation points. The stabiliz- ing effect of an increased working length effect can be felt by separating one’s feet to gain a firm stance while standing on a

Copyright © 2020 Wolters Kluwer Health, Inc. Unauthorized reproduction of this content is prohibited.

LWBK1698-C01_p001-042.indd 21

05/12/18 8:38 PM