Stress Corrosion Cracking of an Aluminum Alloy Used in External Fixation Devices Jacob L. Cartner, Warren O. Haggard, Joo L. Ong, Joel D. Bumgardner Biomedical Engineering Department, Herff College of Engineering, The University of Memphis and the Joint Program in Biomedical Engineering, The University of Memphis-University of Tennessee Health Science Center-Memphis, Memphis, TN, 38152 Received 23 May 2007; revised 24 October 2007; accepted 5 November 2007 Published online 6 February 2008 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.31038 Abstract: Treatment for compound and/or comminuted fractures is frequently accomplished via external fixation. To achieve stability, the compositions of external fixators generally include aluminum alloy components due to their high strength-to-weight ratios. These alloys are particularly susceptible to corrosion in chloride environments. There have been several clinical cases of fixator failure in which corrosion was cited as a potential mechanism. The aim of this study was to evaluate the effects of physiological environments on the corrosion susceptibility of aluminum 7075-T6, since it is used in orthopedic external fixation devices. Electrochemical corrosion curves and alternate immersion stress corrosion cracking tests indicated aluminum 7075-T6 is susceptible to corrosive attack when placed in physiological environments. Pit initiated stress corrosion cracking was the primary form of alloy corrosion, and subsequent fracture, in this study. Anodization of the alloy provided a protective layer, but also caused a decrease in passivity ranges. These data suggest that once the anodization layer is disrupted, accelerated corrosion processes occur. ' 2008 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 86B: 430–437, 2008 Keywords: external fixation; corrosion; aluminum 7075; stress corrosion cracking; anodization INTRODUCTION Treatment for compound and/or comminuted fractures is frequently accomplished via external fixation, especially in displaced tibial diaphyses, soft tissue injuries, or when dis- eases like diabetes or peripheral vascular disease are preva- lent. Successful treatment is accomplished in part by providing maximal intraoperative access to injury and clini- cal construct stability for healing. 1 To achieve this stability, external fixation devices are frequently used. The composi- tion of external fixators often include aluminum alloy com- ponents due to their high strength-to-weight ratios. The Stryker Hoffman I and Hoffman II series fixators, Smith & Nephew Jet-X Central Body and Taylor Spatial Frame sys- tems, and Orthofix Limb Reconstruction and Sheffield Cir- cular Systems, among other examples, are composed of aluminum alloys. While external fixators are a proven method of fracture stabilization, there are many possible complications which can lead to neurovascular, ligamentous, or tendon injuries. Inadequate pin purchase can cause loosening and draining postoperatively, making wound site infection or nonunion likely. 2 Improper, mismatched fixator placement and patient misuse can both place higher than normal mechanical loads on components, sometimes causing slippage, deformation, or failure. 2 Additionally, external fixator components may experience significant corrosion due to bodily fluids, clean- ing reagents, extreme pH levels and moisture. 3 Corrosion of fixator components can lead to loss of material and the release of metal corrosion products into host tissues, which may result in device failure or impairment of healing. 4 Most aluminum external fixators contain components composed of 6XXX- and 7XXX-series alloys. These alloys can be susceptible to corrosion in chloride environments. 5–16 External fixation also, by design, places tensile forces on fixator components which can exacerbate corrosion in a process known as stress corrosion cracking (SCC). SCC causes materials and devices to fail at stress levels much lower than their design stress levels. To the authors’ knowl- edge, there have been 11 clinical cases of fixator failure due to stress corrosion reported to the Food and Drug Administration (FDA) in the last 6 years for which surgery time was delayed or patient injury was sustained. 4 SCC typically initiates at specific localized sites and can propagate either intergranularly, transgranularly, or both, depending on the alloy composition. As a general rule, cracking usually proceeds orthogonal to the applied stress. 7 Five main variables that have been found to greatly affect Correspondence to: J. D. Bumgardner (e-mail: jbmgrdnr@memphis.edu) Contract grant sponsor: Smith & Nephew, Inc. (Memphis, TN) ' 2008 Wiley Periodicals, Inc. 430