A “Toolbox” for Forensic Engineers

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Failure of Medical Implements 299 Longitudinal Scratches Main Ductile Fracture Tip Shear Lips Ductile Shear Lips Brittle Fracture Zone Inner Lip Hole Tear Zone Possible Origin (Mirror Zone) Ductile Fiber Ductile Fiber Brittle Fracture Zone Figure 9.7 Fracture surface map of failed distal end of catheter, describing main surface features and possible origin (sketch magnification: 50¥). Figure 9.8 SEM fractograph of a cut surface on a new TPE catheter, the cut being deliberately made with a Tuohy needle (a hollow steel needle with sharp tip for insertion into the back). Defects in the edge of the needle leave striations in the catheter surface. causing great distress to the patient. 6 The device in question was designed to stretch muscle and other chest tissues prior to implantation of a permanent prosthesis. The operation is commonly performed after mastectomy for
300 Forensic Materials Engineering: Case Studies Figure 9.9 General view of breast tissue expander, with dome at the left for filling via hypodermic needle. removal of cancerous bodies and, following implantation, the tissue expander bag is inflated in a series of steps with saline solution over a time period of some 40 days. The chest tissues are thus strained progressively, as the bag becomes larger. In simple terms a tissue expander can be likened to a balloon (bag), with an attached tube (catheter) through which saline solution is passed to expand the balloon (Figure 9.9). Failure of the device under consideration occurred during the fill sequence, with total loss of bag volume occurring some 40 days into the procedure (Table 9.1). Since the surgeon appeared to have overfilled the bag, the sequence of events thus raised the issues of possible medical negligence as well as product liability. Closer inspection of the fractured tissue expander showed a simple fracture in the catheter tube where it connected to the bag (Figure 9.10). There were no outstanding features on the fracture surface, the tube having broken by growth of a single brittle crack (Figure 9.11). There were no traces of fatigue striations to show crack arrest typical of cyclic failure, the fracture surface appearing smooth and clean. The origin lay at the edge of the catheter wall where it joined the shoulder of the bag, a zone of maximum stress concentration. ESEM examination of that zone revealed much finer cracks at the interface of the tube and the shoulder (Figure 9.12). The cracks extended around the periphery of the joint and had grown catastrophically in a single event that broke the tube entirely, fracture morphology indicative of failure in the adhesive used to join bag to catheter. The bag was also examined and showed large salt crystals contaminated with traces of blood, indicating cross leakage while still implanted, as well as the presence of some residual saline after removal from the patient’s body. As the catheter was found to have completely broken away from the
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Library of Congress Cataloging-in-P
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Preface Case studies of product fai
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could not be simpler in shape, they
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Acknowledgments We thank The Open U
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The Authors Peter R. Lewis, Ph.D.,
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Laboratories of Imperial College of
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3 Establishing the Load Transfer Pa
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6.5.2 A Problem in Eire 197 6.5.3 M
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9.4.2 Failure in Femoral Stem of To
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13.3.7 Design Development Sequence
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Materials in Distress 2.1 Introduct
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Materials in Distress 27 It is usef
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Materials in Distress 29 (A) Unload
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Materials in Distress 31 1. Flowing
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Materials in Distress 33 to compens
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Materials in Distress 35 Fracture o
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Materials in Distress 37 Force A Fr
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Materials in Distress 39 Such fract
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Materials in Distress 41 somewhat b
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Materials in Distress 43 Figure 2.8
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Materials in Distress 49 Stress Amp
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Materials in Distress 53 solutions.
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Materials in Distress 59 short time
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Fraudulent Insurance Claims 349 11
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Intellectual Property Cases 13.1 In
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A “Toolbox” for Forensic Engineers

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