Implant failure

image of Implant failure
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The aim of fracture stabilization is to restore a bone’s structure and function temporarily whilst fracture healing restores it permanently. This chapter reviews implant material failure.

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29.1 Fractured cancellous screw head from a clinical case. The surgeon had mistakenly used this screw in conjunction with a plate where a cortical screw would have been more appropriate. Note the plastic deformation in the thread prior to core failure. The radius (r) of the core is significantly smaller than in a cortical screw and core resistance to torque stress is proportional to r (see Chapter 4). Relative over-tightening led to acute material failure. (Courtesy of D Strong)
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29.2 Curve for stress number of cycles, determined for 1045 carbon steel. The fatigue limit is a level of stress that will never cause material failure regardless of how often the stress is applied. The curve shows that even quite modest increases in stress amplitude can greatly reduce the number of cycles to failure: a 50% increase in stress might reduce the life of the material by a factor of 10 or more. Much larger stresses (higher than those recorded on this curve) will lead to plastic deformation or even acute fracture of the material. (Redrawn from )
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29.3 When a metal bar is placed under tension, the stress (shown here by lines of force) is spread evenly across the bar. Drawn by Vicki Martin Design, Cambridge, UK and reproduced with her permission.
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29.4 With the bar under tension, the number of lines of force at any cross-section must remain constant. Cutting a hole or notch into the bar will lead to stress concentration around the hole or at the extremity of the notch. The similarities between these hypothetical models and bone plates and screws are obvious. Drawn by Vicki Martin Design, Cambridge, UK and reproduced with her permission.
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29.5 A load-sharing bone plate placed on the lateral (tension) surface of a dog’s femur experiences cyclic tensile (bending) stresses of greater magnitude at the abaxial surface of the implant compared to those applied to the underside surface. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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29.6 (a) An anatomically reduced mid-femoral fracture fixed with a plate and screws. The dashed line represents the estimated location of the neutral axis. This bone and plate composite is inherently stable and has a high AMI as a result of the location of the mass of material (bone plate laterally and cortex medially) at some distance from the neutral axis. (b) A mid-femoral fracture, without the benefit of a mechanically competent medial cortex. Here the neutral axis lies within the plate itself and as such the AMI of the bone and plate composite is very much lower. Load-bearing will cause cyclic stress to be concentrated in the small area of plate overlying the fracture, leading to fatigue failure. Drawn by Vicki Martin Design, Cambridge, UK and reproduced with her permission.
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29.7 (a) Medial application of a 3.5 mm dynamic compression plate to manage this mildly comminuted, distal tibial fracture in an active Labrador Retriever. Here the surgeon has underestimated the effect of the gap in the opposite cortex, resulting in (b) fatigue failure of the implant and collapse of the lateral fracture gap 4 weeks postoperatively.
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29.8 (a) This mildly comminuted fracture of the distal radius and ulna in a Golden Retriever (b) was managed initially using only a 3.5 mm locking compression plate and screws applied to the cranial aspect of the distal radius. (c) Fatigue failure occurred 3 weeks postoperatively. (d) The application of an orthogonal radial plate, as undertaken at the review, or separate fixation of the fractured ulna would almost certainly have prevented this complication.
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29.9 Crevice corrosion occurring at a typical location within the screw hole of a 316L stainless steel bone plate. Note the resulting focal, irregular loss of material substance and the production of superficial orange–brown deposits of hydrated metal oxides and hydroxides.
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