Both biomedical devices and oil pipelines have corrosion over time. While seemingly completely different environments, the mechanisms of corrosion may be surprisingly similar. Both environments have the capacity to sustain certain sulfur-reducing bacteria that have been associated with metallic corrosion.
Current explanations of biomedical alloy degradation are focused on the physicochemical mechanisms of galvanic, pitting, crevice, and fretting corrosion; however, recent studies have shown that galvanic corrosion of biomedical alloys, specifically Ti6Al4V (ASTM F136 ELI) and CoCrMoC (ASTM F75 and ASTM F1537), is not a concern in modular constructs where these two alloys are in contact.1,2 In galvanic corrosion, there are no apparent reactive pathways between Ti6Al4V and CoCrMoC. This has been well studied in spine instrumentation, where the mixing of alloys is common (eg, a Ti6Al4V pedicle screw with a CoCrMoC tulip and an interlocking Ti6Al4V or CoCrMoC spine rod).3 Other forms of corrosion, common with titanium and its alloys, include crevice and pitting corrosion. Each has been described in numerous in vitro and retrieval analysis studies.4–9 Ultimately, these studies dismiss the corrosion mechanism as a function of the local microbiome.10
There is ample evidence that bacteria and macrophage influences can be identified with orthopedic biomedical alloy degradation.4–7 The corrosion of metal alloys (A316L surgical stainless steel; ASTM F136 ELI Ti6Al4V; ASTM F75/F1537/F799 CoCrMoC) in vivo has been well documented.8,9 Further, significant corrosion and tissue retention of metal ions in both single alloy spine constructs and combination alloy constructs has now been documented.11 As early as 1999, Propionibacterium acnes and Staphylococcus species were identified in orthopedic infections, detected in 63% of sonicated samples obtained from 120 patients after total hip revision.6 As recently as 2016, P acnes was highlighted as a possible contributor to postoperative infections in orthopedic procedures.7 These commensal skin-dwelling microbes are anaerobic, sulfur-reducing bacteria that are also associated with corrosion of metals in other industries.12–15
The presence of sulfur-reducing bacteria and their biofilm formation on metals has been extensively studied in the petroleum industry, dating as far back as 1926 when sulfur-reducing bacteria were analyzed and found to be associated with petroleum deposits.16 In 2003, Zhu et al12 identified Propionibacterium species strain V07/12348, Propionibacterium species strain WJ6, and Escherichia coli in natural gas pipelines. In 2005, Yoshida et al13 identified P acnes in crude oil samples in Japanese stockpiles as well as Staphylococcus species. The presence of Propionibacterium species and Staphylococcus species is not unexpected, given that the sulfur content of these supplies is 1 to 2 wt%.13 Additionally, sulfur-reducing bacteria have been shown to be capable of residing on titanium,17,18 as well as carbon steels.19 The organisms cause damage through metabolite acids, such as H2S and propionic acid, as they use sulfate, nitrate, nitrite, carbon dioxide, Fe3+, Mn4+, Cr6+, and other metal ions and bacterial waste products as electron acceptors for metabolism.19,20
Current interest in orthopedic biofilm-forming microbes has focused on P acnes.5,21–23P acnes, S aureus, and S epidermidis biofilms have been isolated from oil line pipe in fields and refineries,24–28 as well as in water treatment facilities.29,30 Using the knowledge of other industries, this suggests that the presence of an implant provides a suitable substrate on which these normally symbiotic bacteria can colonize and lead to infection, corrosion, and potentially hardware failure. Regardless of whether a patient is diagnosed with a specific infection, several sulfur-reducing, biofilm-forming bacteria are common in human tissues. Given that line pipe corrosion is indicative of microbial-influenced corrosion (MIC) in other fields, it is appropriate to consider that such observations may also be valid in orthopedics.
Materials and Methods
After university institutional review board approvals were obtained, 60 spine hardware constructs were examined. Constructs in this study were considered as hardware sets consisting of all rods and pedicle screws removed from a single patient at the time of revision surgery for any reason other than catastrophic (mechanical) failure. Immediately after explantation, all hardware was examined for biofilm formation in the operating theater. Operating room imaging was obtained using an iPhone 8 (Apple, Cupertino, California), as larger camera and lighting systems used for metallurgical imaging are intrusive to the environment and more difficult to sterilize based on hospital requirements for equipment entering operating theaters. Images were obtained within 5 minutes of explantation to allow differentiation between blood film and biofilm (Figure 1). Areas where only biofilm was present were noted for subsequent sectioning.
Rods immediately after explantation (A). The longer rod has been wiped down to expose surface defects (B). These rods were removed from a patient undergoing revision due to sagittal balance imbalance.
After imaging, 1 rod from each construct was cleaned 4 times using sterile wipes (Sani-Cloth wipes; PDI Healthcare, Woodcliff, New Jersey) to remove all non-adherent biologic material. This allowed exposure of areas of corrosion and mechanical damage (Figure 2). Sections where biofilm was noted were marked. The second rod remained untouched and packaged in a sterile specimen transport bag for biofilm DNA/RNA extraction in a laboratory environment.
Sectioning of the left rod in Figure 1 such that they only included areas where biofilm was present, and surface mechanical damage was not visually evident.
Cleaned rods were imaged in the laboratory to construct a complete measure of surface damage. Marked areas were cut using rod cutters to approximately 35-mm lengths to fit the scanning electron microscopy (SEM) chamber (Figure 3). Each section was cleaned again 2 times using sterile wipes. Marked rod sections were then imaged using SEM with energy dispersive x-ray spectroscopy (EDS) (JSM-6010LA; JEOL USA, Inc, Peabody, Massachusetts). Backscatter mode (backscatter electron shadow [BES]) was employed to better image the topology of the surface. Additionally, BES can reveal elemental differences indicating possible areas where low electron energy organic elements are present relative to the higher energy metal elements.
Rod sections marked and placed on carbon tape on the scanning electron microscopy stage. Marking on the sections denotes areas where only biofilm and corrosion were present as noted in the operating room. The silver-colored sections are from Figure 1. The gold-colored sections are from a patient who underwent revision due to a diagnosed implant infection.
Corrosion in areas of biofilm appears as surface discoloration indicative of repassivation of the titanium oxide that spontaneously forms on this metal and its alloys. Figure 4 shows a clear example of how biofilm corrosion appears, specifically on an anodized Ti6Al4V spine rod. There is dull discoloration of the surfaces, rather than a bright red coloration, as the result of blood (Figure 4). The original anodized surface has been removed and subsequently replaced with a thinner repassivated surface. The bottom rod in Figure 4 was wiped in the operating room, showing damage from both mechanical components, fretting wear, and discoloration due to corrosion. It is also clear that wiping the hardware enables a more accurate assessment of corrosion and mechanical damage on surfaces. There is clear differentiation between discoloration due to corrosion vs mechanical damage (Figure 4). Although the most extreme example of what was noted in the operating room, this is typical of all hardware removed from the included patients.
Rods explanted from a patient with diagnosed lumbar spinal stenosis and pseudarthrosis after joint fusion. The upper rod retained biologic material in the “as removed” condition. The bottom rod had been cleaned, showing the deep purple original anodization as well as areas of repassivation and mechanical damage. Mechanical damage had clear material deformation such as the dents and scrapes seen. Corrosion as the result of crevices and fretting surround the mechanical damage.
Under SEM BES examination, the authors noted that not all biofilm was removed using the surgical wipes. This is indicative of the tenacity of bacterial bio-films as described in other fields. There are regions where the grain structure of the titanium is clear, while other areas appear amorphous with small nodules present (Figure 5). The film was noted predominantly in the presence of pitting, such as the black areas in Figure 5, which are corrosion pits. These pits were noticeably larger and numerous in areas of biofilm. In areas not associated with bio-films, there were few pits even if mechanical wear was evident, as well as crevice cracking (Figure 6). There is no evidence that the surface corrosion is modified between clinically diagnosed infected and noninfected patients.
Scanning electron microscopy imaging of rod surfaces from Figure 1. Corrosion pits (white arrows) surrounded by biofilm. The center pits showed additional crevice corrosion (black arrows) where the biofilm had been removed, thus exposing the substrate surface and titanium crystal grains and grain boundaries.
A rod surface region where biofilm is present. The black dots are corrosion pits. The dark line to the right is an ink mark denoting the boundary of biofilm as observed in the operating room (A). The rod surface outside of the biofilm. Surface cracks are readily apparent (arrows) (B).
Closer examination showed pit 3-dimensionality (Figures 7–8), including the presence of corrosion products such as aluminum oxides. No other metal oxide appeared to be present within the pits; however, that may have been the result of limitations in the measurement technique. The presence of the barium signal in Figure 8 was most likely the result of the characteristic radiographs of barium Lα energy (4.47 keV) being very close to the titanium Mα energy of 4.51 keV. The presence of calcium may have been the result of precipitation of calcium from body fluids during air drying, as no bone adherence was seen on any rods.
Corrosion pits associated with the presence of biofilm. Image from the rods shown in Figure 1 (A). The true surface of the rod that was obscured by the film. Small nodules are cellular remnants and as determined by their dark color indicating light elements. Surface cracking can be seen where biofilm has been removed (B).
Representative energy dispersive x-ray spectroscopy analysis of the surfaces of the rods from Figure 1. Calcium, oxygen, and aluminum were present in a corrosion pit. Similar spectra were observed regardless of diagnosis for revision.
The existence of biofilms on retrieved hardware is not unexpected, as their presence and characterization has been described in many previous studies.4–7,9,17,21–23 In the current study, it was clear that biofilms were visually present on all of the rod hardware removed. This was in a series of patients in whom no clinical sign of infection was noted. At the same time, the corrosion of biomedical alloys has also been very well studied.4,11,18 This suggests that the most likely mechanism for corrosion was MIC as the result of bacteria endemic to the tissues. Although the patients have not been found to have a clinically diagnosed infection (as currently defined), the interaction of the bacteria and the metal alloys was apparent, which has been previously proposed.31 Gilbert et al4 described biofilm and infection, but suggested that macrophage-influenced corrosion is possible. However, the concept of orthopedic MIC as the result of an endemic microbiome has not been proposed until now. From continued observation of the corrosion surfaces of removed hardware, this seems to be the simplest explanation for ongoing hardware corrosion when wear or other sources of surface damage are not present.
Some of the retrieved hardware in this study was sonicated to determine if the biofilms were bacterial. Following established methods,32 using matrix-assisted laser desorption ionization-time of flight mass spectrometry, the authors measured the presence of up to 10 different species of bacteria residing on retrieved spine hardware (rods and pedicle screws). Organisms identified included Klebsiella variicola, Citrobacter freundii, Citrobacter braakii, and Citrobacter youngae. None of these species were cultured from peri-hardware tissue at the time. In one case, a patient was identified as having E coli and S hominis based on culturing of spine tissue; however, the hardware removed at the same time indicated Klebsiella variicola and Citrobacter freundii. Other species consistently cultured from peri-implant tissues obtained during revisions included P acnes as well as S aureus and S epidermidis. All of these are sulfur-reducing bacteria that have been identified in MIC in oil and water.23,25,27
The disruption of the passive oxide surface occurs due to metabolic products from the bacteria subsequently creating a localized corrosion in the form of pitting.18 After calcium and phosphorous, sulfur is the most common mineral in the human body and is available for microbial metabolism and the formation of H2S metabolites from the local amino acids cysteine and methio-nine.14,15,33 Thus, sulfur-reducing bacteria are well suited to function in this environment, with any metal providing a scaffold and energy source.
Other corrosion studies suggest other possible mechanisms, such as increasing tensile stress up to 125% yield, increasing corrosion.34 However, observations of the metal surface from this finding suggested a highly organized pitting along lines of stress unlike those observed in the current study. In 2015, Gilbert et al4 examined modular taper regions in hip devices. These regions are also subjected to fretting corrosion and are unlikely able to support microbial biofilms due to the cyclic micromotion between components—similar to what was observed in the current study, with biofilms not being substantially present under modular connectors.4 In 2017, Kubacki et al35 also suggested that damage and corrosion on metal surfaces in vivo could be the result of electrocautery. Although this is possible, the damage described in that study showed evidence of melting as well as pitting.
The observed corrosion pitting was similar to what has been described and presented in other fields.18,26 In each study, the predominant bacteria were different from what populated the patient population in the current study. In a study by AlAbbas et al,26 a mixed consortium of bacteria as determined by 16S gene sequencing was found, with 3 major phylotypes: Proteo-bacteria (Desulfomicrobium species), Firmicutes (Clostridium species), and Bacteroidetes (Anaerophaga species). The pitting shown matches what was seen in the current study—randomly distributed pits along the surfaces. Rao et al18 provided a detailed mechanism as to how titaniums can be corroded by sulfur-reducing bacterias, as well as displayed pits from in vitro studies that appeared to have profiles similar to what was observed in the current study.
The surface damage present on explanted Ti6Al4V spine rods is uniquely similar to damage found in other industries where MIC is prevalent. Given that similar anaerobic, sulfur-reducing bacteria reside in and on human tissues, it is most likely that corrosion observed on explanted hardware is the result of MIC and not from inflammatory or other processes. Using analysis methods from other industries to characterize the microbiome present on explanted hardware is necessary. In so doing, a new definition of hardware-induced infection will be forthcoming.
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