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An electrochemical investigation of TMJ implant metal alloys in an artificial joint fluid environment: The influence of pH variation
Journal of Cranio-Maxillo-Facial Surgery xxx (2014) 1e10
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An electrochemical investigation of TMJ implant metal alloys in an artificial joint fluid environment: The influence of pH variation Dmitry Royhman a, b, Rashmi Radhakrishnan b, Judy Chia-Chun Yuan a, Mathew T. Mathew a, b, Louis G. Mercuri b, c, Cortino Sukotjo a, * a University of Illinois at Chicago College of Dentistry, Department of Restorative Dentistry (Head: Dr. Stephen Campbell, DDS, MMSc), 801 S. Paulina Ave, Chicago, IL 60612, USA b Rush University Medical Center, Department of Orthopedics (Head: Dr. Joshua Jacobs, MD), 1611 W. Harrison St., Chicago, IL 60612, USA c TMJ Concepts, 2233 Knoll Drive, Ventura, CA 93003, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Paper received 18 July 2013 Accepted 6 January 2014
Objective: To investigate the corrosion behaviour of commonly used TMJ implants alloys (CoCrMo and Ti6Al4V) under simulated physiological conditions. Methods: Corrosion behaviour was evaluated using standard electrochemical corrosion techniques and galvanic corrosion techniques as per ASTM standards. Standard electrochemical tests (Ecorr, Icorr, Rp and Cf) were conducted in bovine calf serum (BCS), as a function of alloys type and different pHs. Galvanic corrosion tests were conducted in BCS at a pH of 7.6. Alloy surfaces were characterized using white-light interferometry (WLI) and scanning electron microscopy (SEM). Results: The potentiodynamic test results exhibited the enhanced passive layer growth and a better corrosion resistance of Ti6Al4V compared to CoCrMo. Electrochemical impedance spectroscopy measurements demonstrated the influence of protein as a function of pH on corrosion mechanisms/kinetics. Galvanic coupling was not a major contributor to corrosion. SEM and WLI images demonstrated a significantly higher in surface roughness in CoCrMo after corrosion. Conclusions: The results of this study suggest that Ti6Al4V shows superior corrosion behaviour to CoCrMo due to its strong passive layer, simulated joint fluid components can affect the electrochemical nature of the metal/electrolyte interface as a function of pH, and the galvanic effect of coupling CoCrMo and Ti6Al4V in a single joint is weak. Published by Elsevier Ltd on behalf of European Association for Cranio-Maxillo-Facial Surgery.
Keywords: Electrochemistry TMJ Titanium Alloy CoCrMo alloy Corrosion Artificial joint fluid
1. Introduction Temporomandibular dysfunction (TMD) is a generic term used to describe the extra-articular and intra-articular disorders associated with this joint. It may be caused by injury to the mandible, temporomandibular joint (TMJ), muscles of mastication, or by disc displacement, arthritic disease, developmental TMJ abnormalities, neoplasia or ankylosis (Ingawale and Goswami, 2009; Stegenga et al., 1991). TMD is most common between the ages 20 and 40, having a higher occurrence in women (Warren and Fried, 2001). Twenty to twenty-five percent of the population have symptoms of TMD (Detamore and Athanasiou, 2003; Van Loon et al., 2002; Warren
* Corresponding author. University of Illinois at Chicago College of Dentistry, Department of Restorative Dentistry, 801 S. Paulina Ave. Room #365B, Chicago, IL 60612, USA. Tel.: þ1 312 355 0360; fax: þ1 312 996 3535. E-mail address: [email protected] (C. Sukotjo).
and Fried, 2001). In cases where intra-articular TMD demonstrates end-stage arthrosis, TMJ total joint replacement (TMJ TJR) may provide the best management option (Guarda-Nardini et al., 2008a, b; Ingawale and Goswami, 2009; Landes et al., 2013; Mercuri, 2000; Schuurhuis et al., 2012; Wolford et al., 2003). In order to achieve the best outcome, any material used to make a joint prosthesis should have biocompatibility, functional compatibility, low wear coefficients when loaded under functional conditions, adaptability to anatomical structures, rigidity to stabilized components, corrosion resistance, and non-toxic effects (Wolford et al., 2003). Currently, there are 2 types of total TMJ replacement devices approved by the Food and Drug Administration (FDA) for use in the United States: (1) stock or “off-the-shelf devices”: the cast CoCr-onall polyethylene device (Biomet Microfixation, Jacksonville, FL), and the cast all CoCr-on-cast CoCr device (Nexus CMF, Salt Lake City, UT); and (2) or “custom” devices: the wrought Ti6Al4V/CoCrMo-onwrought CP Ti backed polyethylene patient-fitted device (TMJ
1010-5182/$ e see front matter Published by Elsevier Ltd on behalf of European Association for Cranio-Maxillo-Facial Surgery. http://dx.doi.org/10.1016/j.jcms.2014.01.029
Please cite this article in press as: Royhman D, et al., An electrochemical investigation of TMJ implant metal alloys in an artificial joint fluid environment: The influence of pH variation, Journal of Cranio-Maxillo-Facial Surgery (2014), http://dx.doi.org/10.1016/j.jcms.2014.01.029
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D. Royhman et al. / Journal of Cranio-Maxillo-Facial Surgery xxx (2014) 1e10
Nomenclature Ecorr Icorr Ipass Rs Cdl Rp: CPE SEM WLI EIS OCP SCE
corrosion potential (Tafel extrapolation) corrosion current (Tafel extrapolation) passivation current solution resistance double layer capacitance polarization resistance constant phase element scanning electron microscopy white-light interferometry electrochemical impedance spectroscopy open circuit potential saturated calomel electrode
Concepts, Ventura, CA), and the cast all CoCr-on-cast CoCr custom device (Nexus CMF, Salt Lake City, UT). A custom made, patient fitted device is available for markets outside US (Biomet, Jacksonville, FL, US), which is designed by a virtual planning company (Medical Modeling, Golden, CO, US). Ti6Al4V and CoCrMo are the metal materials of choice for implants due to their biocompatibility, strength, ductility, corrosion resistance, and rigidity (Chaturvedi, 2009; Guarda-Nardini et al., 2008a; Langford and Frame, 2002; Songür et al., 2009; Virtanen et al., 2008). In the body, the TMJ implant is exposed to extremely complex and variable conditions from the electrolytic environment. Such factors include: temperature, biofilm, protein, joint fluid, and pH changes due to the severe pathological changes in the surrounding tissues from the surgical procedure or the presence of implant itself (Chaturvedi, 2009). Furthermore, there is often an inflammatory response, which can lead to the generation of hydrogen peroxide and increased levels of protein and cytokines (Katou et al., 1996; Virtanen et al., 2008). All of these factors may accelerate the electrochemical nature of the metal/electrolyte interface and lead to pitting corrosion (Virtanen et al., 2008). Furthermore, galvanic coupling (from mixed metal implants) may play a significant role as well (Reclaru and Meyer, 1994). Corrosion can lead to roughening of the surface, weakening of the restoration, liberation of elements from the metal or alloy, and toxic reactions (Cohen, 1998; Schiff et al., 2002; Songür et al., 2009; Virtanen et al., 2008). Corrosion can severely limit the strength and lifespan of the material leading to mechanical failure of the implant (Cohen, 1998; Schiff et al., 2002; Songür et al., 2009; Virtanen et al., 2008). Additionally, it can cause the release of metal ions into the surrounding tissues, which can cause several cytotoxic responses including, a decrease in enzyme activity, interference with biochemical pathways, carcinogenicity, mutagenicity, localized tissue irritation, and toxicity leading to skeletal and nervous system disorders (Westesson et al., 1987). Corrosion products have been implicated in causing local pain or swelling in the region of the implant in the absence of infection (Chaturvedi, 2009). Fig. 1a and b shows the fossa component of a TMJ total joint replacement device (Christensen (TMJ, Inc. Golden, CO) Metal on Metal prosthesis), which was retrieved due to implant failure. There is evident damage at the articulating surface, most likely attributed to wear and corrosion. There is a limited amount of information regarding TMJ implant material degradation and the associated mechanisms that may cause it. The objective of this study was to investigate the corrosion behaviour of the two most commonly used TMJ total joint replacement device metals (Ti6Al4V and CoCrMo) using simulated
Fig. 1. (a) Image of the fossa component of a failed TMJ total joint replacement device shows degradation at the articulating surface and (b) SEM image shows a 1000 magnification of the region inside the white box from (a).
in vivo environmental conditions. An artificial joint fluid environment (Bovine Calf Serum; BCS) was chosen, with a varying pH range to simulate the pH values at physiological tissues that interface surgical implants (pHs from approximately 4.0 to 7.7) (Konttinen et al., 2001). In addition, since corrosion is a function of the nobility of the exposed surfaces, especially when considering the use of implants with more than one metal type or alloy (Chaturvedi, 2009; Ciszewski et al., 2007; Virtanen et al., 2008), possible effects of galvanic corrosion were also examined. 2. Materials and methods 2.1. Sample preparation Ti6Al4V and CoCrMo were chosen for this experiment due to their implementation in current TMJ joint replacement devices. The chemical composition of the Ti6Al4V and CoCrMo alloy discs are shown in Table 1. Fifteen Ti6Al4V discs (Mac-Master Carr, Elmhurst, IL, USA) and fifteen wrought high-carbon CoCrMo discs (ATI Allvac, Pittsburgh, PA, USA) were milled from rods. The discs were then brought to a mirror finish (Ra 25 nm) in order to obtain standardized surfaces for which the measurements could be compared. In order to obtain the mirror finished surfaces, the Ti6Al4V discs were wet-ground using a series of abrasive discs (#320, #400, #600, and #800) (Carbimet 2, Buehler, Lake Bluff, IL, USA), followed by polishing with diamond paste (MetaDi 9-micron, Buehler, Lake Bluff, IL, USA) and lubricant (MetaDi Fluid, Buehler, Lake Bluff, IL,
Please cite this article in press as: Royhman D, et al., An electrochemical investigation of TMJ implant metal alloys in an artificial joint fluid environment: The influence of pH variation, Journal of Cranio-Maxillo-Facial Surgery (2014), http://dx.doi.org/10.1016/j.jcms.2014.01.029
D. Royhman et al. / Journal of Cranio-Maxillo-Facial Surgery xxx (2014) 1e10 Table 1 Chemical composition of Ti6Al4V and CoCrMo. Composition (in wt%) Ti alloy CoCrMo alloy
Ti 89.62 C 0.241
Al 6.1 Co 64.60
V 4.0 Cr 27.63
C 0.004 Mo 6.04
Fe 0.16 Si 0.66
O2 0.106 Mn 0.70
N2 0.008 Al 0.02
H2 0.0022
USA) on polishing cloth (TexMet Polishing Cloth, Buehler, Lake Bluff, IL, USA), and finally brought to a mirror finish using colloidal silica polishing suspension (MasterMet, Buehler, Lake Bluff, IL, USA) on chemomet polishing cloth (Chemomet I, Buehler, Lake Bluff, IL, USA). CoCrMo alloy discs were wet-ground using a slightly different series of abrasive discs (#240, #320, #400, and #600) (Carbimet 2, Buehler, Lake Bluff, IL, USA), followed by polishing with 6 mm Polycrystalline Diamond Suspension (MetaDi, Buehler, Lake Bluff, IL, USA) on polishing cloth (TexMet Polishing Cloth, Beuhler, Lake Bluff, IL, USA), and finally brought to a mirror finish using 1 mm Polycrystalline Diamond Suspension (MetaDi, Buehler, Lake Bluff, IL, USA) on nylon cloth (Premium Cloths, Buehler, Lake Bluff, IL, USA). Before testing, the samples were ultrasonically cleaned with 70% isopropanol and deionized water, and finally dried with nitrogen gas stream. 2.2. Corrosion tests For corrosion tests, the samples were divided into 8 groups (n ¼ 3) as a function of different disc types (Ti6Al4V or CoCrMo) and pH (3.0, 4.5, 6.0, or 7.6) as shown in Fig. 2a. Electrochemical behaviour of Ti6Al4V and CoCrMo was studied by analysing open circuit potential (OCP), corrosion current density (Icorr), passivation current (Ipass), and corrosion potential (Ecorr). All electrochemical tests were conducted using a custom-made electrochemical cell (made of acrylic) using a standard 3-electrode configuration in which the potentiostat (G300, Gamry Inc., Warminster, PA, USA) was used to perform the corrosion measurements. A saturated calomel electrode (SCE) was used as the reference electrode, a graphite rod as a counter-electrode, and the metal alloy as a working electrode (exposed surface (0.38 cm2)). All tests followed the American Society for Testing for Metals (ASTM) guidelines (G61 and G31-72). The corrosion behaviour of Ti6Al4V and CoCrMo alloy
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in various artificial joint fluid pHs was examined by modifying the pH of a bovine calf serum (BCS) solution (electrolyte volume ¼ 10 mL). The BCS solution contained: BCS (30 g/L protein content), NaCl (18 g/L), Propylene Phenoxetol (30 mL/L), Tris (54 g/ L), and HCl (approximately 55 mL/L), or enough to bring the pH of the solution down to 7.6. Once the initial pH of 7.6 was reached, lactic acid was added to attain the rest of the pH levels (3.0, 4.5, and 6.0). The samples were mounted into the electrochemical cell and the temperature of the test solution was maintained at 37 C. The sequence of tests included open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and finally a cyclic polarization test. Open circuit potential was monitored for a period 3600 s. The EIS measurements were performed in the frequency range from 100 kHz to 5 MHz, with AC sine wave amplitude of 10 mV applied to the electrode at its corrosion potential. EIS results were used to propose an equivalent circuit to represent the electrochemical process in relation to the observed electrochemical phenomena. A simple model of this circuit is presented in Fig. 2b, which consists of solution resistance (Rs), polarization resistance (Rp), and capacitance of the electrochemical double layer (Cdl). Under the exposure to complex solution like BCS and to account the heterogeneity of surface, Cdl was replaced by constant phase element (CPE). The modelling was done with Echem Analyst (Gamry Inc., Warminster, PA, USA). The cyclic polarization scan was carried out from 0.8 V to 1.8 V (2 mV/s scan rate). Corrosion measurements Ecorr, Icorr, and Ipass were obtained from the polarization curves. Ecorr and Icorr values were estimated using Tafel extrapolation. 2.3. Galvanic corrosion tests The galvanic corrosion of coupled Ti6Al4V/CoCrMo (n ¼ 3) was measured at pH ¼ 7.6 as is shown in Fig. 2a. Measurements were taken using a 3-electrode configuration with Ti6Al4V as the working electrode, CoCrMo as a counter-electrode, and a saturated calomel electrode (Gamry Inc., Warminster, PA, USA) as the reference electrode for 24 h. For each trial mean values were taken of the total current density, potential, and integration throughout the experiment. 2.4. Surface characterization In order to obtain surface roughness measurements (Ra and rms), three-dimensional images of the corroded surfaces were taken using a white-light interferometry microscope (Zygo New View 6300, Zygo Corporation, Middlefield, CT, USA). Six measurements were taken per disc. Further surface characterization was performed using a scanning electron microscope (SEM) (Jeol JSM6490 LV, Oxford Instruments, Oxford, UK) in order to examine surface changes on the metal surfaces due to the corrosion process (passive films and pitting behaviour). 2.5. Statistical analysis
Fig. 2. (a) Schematic diagram of the design of experiment and sample groups and (b) schematic diagram for the three-element Randle’s equivalent circuit used to determine the polarization resistance (Rp). Rs represents the solution resistance and CPE represents the constant phase element used to replace the double layer capacitance (Cdl).
Corrosion analysis was performed by evaluating 5 parameters from standard corrosion tests (Icorr, Ecorr, Ipass, Cdl and Rp), 3 parameters from galvanic corrosion tests (current density, potential, and integration), and surface roughness measurements (Ra and rms), taken from white-light interferometry. One-way analysis of variance was used to determine the effects of pH (factor 1, four levels) and its effect on Icorr, Ecorr, Ipass, Cdl and Rp in each alloy type (factor 2, two levels). Tukey’s post hoc analysis was used for pairwise comparisons within these groups. A two-sample independent t-test was performed to compare the differences between Ti6Al4V and CoCrMo for Icorr, Ecorr, Ipass, Cdl, Rp, as well as surface
Please cite this article in press as: Royhman D, et al., An electrochemical investigation of TMJ implant metal alloys in an artificial joint fluid environment: The influence of pH variation, Journal of Cranio-Maxillo-Facial Surgery (2014), http://dx.doi.org/10.1016/j.jcms.2014.01.029
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D. Royhman et al. / Journal of Cranio-Maxillo-Facial Surgery xxx (2014) 1e10
Fig. 3. Electrochemical results (a) cyclic polarization curves, (b) Icorr, (c) Ecorr, and (d) Ipass.
roughness parameters (Ra and rms) with a critical value of p ¼ 0.05 (Statistical Package for the Social Sciences, version 17.0; SPSS Inc., Chicago, IL, USA). 3. Results 3.1. Cyclic polarization scans The potentiodynamic curves for CoCrMo and Ti6Al4V in BCS are presented in Fig. 3a. The values for OCP, Icorr, Ipass, and Ecorr are presented in Table 2, Fig. 3bed respectively. In general, all four parameters decreased with increasing pH in both alloys. Significant differences among pHs (p < 0.05; ANOVA) were noted within both metals for Ecorr, Icorr and Ipass values. In the CoCrMo group, there was a significant difference among all pHs for Ecorr, Icorr, and Ipass (p < 0.05; Tukey HSD test), except for Icorr pH 4.5 vs. pH 6.0 (p ¼ 0.071) and Ipass pH 6.0 vs. 7.6 (p ¼ 0.151). In the Ti6Al4V group, significant differences were seen at all pHs for all 3 parameters (p < 0.05; Tukey HSD test), except for Icorr pH 6.0 vs. 7.6 (p ¼ 0.113) and Ipass pH 4.5 vs. 3.0 (p ¼ 0.104), pH 4.5 vs. 6.0 (p ¼ 0.658), and pH 4.5 vs. 7.6 (p ¼ 0.527). Ti6Al4V shows significantly lower Icorr values and higher Ipass values than CoCrMo in all pHs (p < 0.0001; t-test), the Ecorr value for Ti6Al4V was significantly higher in pH 3.0 (p ¼ 0.004; t-test) and lower in 6.0 (p ¼ 0.035; t-test) compared to CoCrMo.
3.2. EIS measurements The EIS circuit parameters, polarization resistance (Rp), and capacitance (Cdl) are presented in Fig. 4c and d for both alloys. Rp is the polarization resistance at the metal/electrolyte interface. Cdl is the specific double-layer capacitance at the working electrode/ electrolyte interface. In general, Cdl was significantly different among pHs for both metals (p < 0.05, ANOVA); however, Rp only showed significant values for Ti6Al4V (p ¼ 0.14, ANOVA). In CoCrMo, Cdl pH 3.0 was significantly lower compared to other pHs (p < 0.01, Tukey HSD test). In Ti6Al4V, pH 3.0, Cdl was significantly higher only when compared to pH 7.6 (p ¼ 0.013, Tukey HSD test) and Rp was significantly lower only when compared to pH 7.6 (p ¼ 0.01, Tukey HSD test). When comparing between the metal alloys, Rp for Ti6Al4V was significantly higher in all pHs compared to CoCrMo (p < 0.05, ttest). Ti6Al4V only showed significantly higher value of Cdl in pHs 3.0 (p ¼ 0.000, t-test) and 6.0 (p ¼ 0.002, t-test) compared to CoCrMo. 3.3. Galvanic corrosion The mean values for potential coupling Ti6Al4V and CoCrMo were 1.02 to 0.46 (V vs. SCE) and the deviations of the mixed potentials are within the range of 0.01e0.04 (V vs. SCE). The mean
Table 2 Evolution of open circuit potential as a function of pH with standard deviation for tested conditions. Sample type
Ti alloy CoCrMo alloy
Parameter
OCP (Eq¼0) (V vs. SCE) OCP (Eq¼0) (V vs. SCE)
pH level pH 3.0
pH 4.5
pH 6.0
pH 7.6
0.42 (0.00) 0.28 (0.02)
0.50 (0.01) 0.31 (0.08)
0.66 (0.01) 0.52 (0.02)
0.76 (0.00) 0.79 (0.00)
Please cite this article in press as: Royhman D, et al., An electrochemical investigation of TMJ implant metal alloys in an artificial joint fluid environment: The influence of pH variation, Journal of Cranio-Maxillo-Facial Surgery (2014), http://dx.doi.org/10.1016/j.jcms.2014.01.029
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Fig. 4. EIS results: (a) Nyquist plot, (b) Bode plot, (c) polarization resistance, and (d) capacitance.
values for current were 5.29 to 121.42 (nA/cm2) and the range of deviations of the mixed current was between the range of 10.72e 0.99 (nA/cm2). The galvanic current gradually increased towards zero as can be seen in Fig. 5. The galvanic current was integrated during the last 6 h. The mean integrated value change was 1.98 C. 3.4. Surface characterization The surface parameters (Ra and rms values), before and after corrosion, are displayed in Table 3. In the CoCrMo group, Ra and rms values were significantly different among the 5 groups tested (p < 0.0001, ANOVA) and all corroded groups yielded significantly higher Ra and rms values compared with the control (p < 0.05, Tukey HSD test). When comparing metals, CoCrMo yielded significantly higher Ra (p < 0.05, t-test) and rms (p < 0.001, t-test) compared to Ti6Al4V within each pH. WLI images showed a higher surface roughness after corrosion in both alloys, but CoCrMo alloy surfaces demonstrated higher surface roughness compared to the Ti6Al4V counterpart at the same pHs (Fig. 6a). SEM inspection demonstrated an increase in surface roughness and overall pitting
Fig. 5. Galvanic coupling of Ti6Al4V and CoCrMo with potential (Left Y-axis) and current (Right Y-axis) over time.
behaviour after corrosion in both alloys, especially in CoCrMo (Fig. 6b). 4. Discussion The findings in this study were consistent with previous findings obtained from this laboratory (Barão et al., 2012; Mathew et al., 2011), which show that lower pH levels increased the rate of transfer of electrons at the metal/electrolyte interface, BCS can be used as a suitable electrolyte to simulate joint fluid in an electrochemical cell, and that dissolved biomolecules can influence corrosion kinetics in the test system. 4.1. EIS analysis The EIS technique was utilized for characterizing electrochemical reactions at the metal/electrolyte interface and assessing the interfacial changes from the effect of protein at the metal surface. EIS data is commonly analyzed through two stages. The first stage (stage a) is analysis via data plots, namely Nyquist plot (imaginary component of impedance vs. real component) and Bode plots (impedance and phase angle vs. frequency). The second stage (stage b) is analysis by using the EIS data to develop an equivalent electrical circuit or model for the electrochemical process. This is used to quantify electrochemical kinetic parameters such as polarization resistance (Rp) and capacitance (Cdl). The EIS data is presented as Nyquist plot and Bode plot in Fig. 4a and b respectively. These plots show the variation of impedance as a function of frequency of the double layer formed at the interface of the electrolyte and the metal during the corrosion process. The Nyquist plot (imaginary component of impedance vs. real component of impedance) demonstrates the corrosion resistance of the metal surface. A decreased semicircular diameter of capacitance loop indicates a decreased corrosion resistance. In the second stage
Please cite this article in press as: Royhman D, et al., An electrochemical investigation of TMJ implant metal alloys in an artificial joint fluid environment: The influence of pH variation, Journal of Cranio-Maxillo-Facial Surgery (2014), http://dx.doi.org/10.1016/j.jcms.2014.01.029
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Table 3 Representative surface roughness parameters (Ra and rms), mean and standard deviation, of Ti6Al4V and CoCrMo as a function of corrosion in different BCS pH levels. Sample type
Roughness parameter
Corrosion process Control (non-corroded)
Corroded (pH 3.0)
Corroded (pH 4.5)
Corroded (pH 6.0)
Corroded (pH 7.6)
CoCrMo
Ra (mm) rms (mm)
0.007 (0.001) 0.023a (0.020)
1.035 * (0.817) 1.270b* (0.956)
0.626 * (0.237) 0.782b* (0.252)
0.929 * (0.138) 1.116b* (0.157)
1.175b* (0.007) 1.394b* (0.090)
Ti6Al4V
Ra (mm) rms (mm)
0.009 (0.003) 0.013 (0.003)
0.009* (0.003) 0.014* (0.007)
0.014* (0.002) 0.018* (0.002)
0.011* (0.003) 0.014* (0.004)
0.009* (0.002) 0.012* (0.003)
a
b
b
b
For each variable, small letters compare different pH values at the same metal alloy type. Different letters indicate significant differences among the groups (a ¼ 0.05). Asterisk (*) indicates a statistical difference between Ti6Al4V with CoCrMo in each corresponding pH level (a ¼ 0.05).
Fig. 6. (a) White-light interferometry microscopy 3D images of Ti6Al4V and CoCrMo before and after corrosion. X and Y scales are in mm and Z scale is in mm. The Z scale covers 0.2 mm in amplitude. Colours represent the peaks (from w0 to þ0.1 mm of amplitude in the Z axis) (red and yellow) and valleys (from w0 to 0.1 mm of amplitude in the Z axis) (green and blue) on the titanium and CoCrMo surfaces topographies. (b) Surface characterization SEM surface profile images for Ti6Al4V and CoCrMo before and after corrosion as a function of pH.
of EIS analysis, the data was employed to develop an equivalent electrical circuit of the corrosion process. The electrical circuit of this particular electrochemical process was similar to Randle’s circuit (Fig. 2b); however, Nyquist results appear to show more complicated behaviour than that which could be modelled with the simple Randle’s circuit. The semicircles appeared to be depressed (vertical radius is shorter than horizontal radius). To compensate for the peculiar nature of this Nyquist plot, the simple capacitance component in the Randle’s circuit was replaced by constant phase element (CPE). CPE behaviour is related to the heterogeneous properties of the surface and models the behaviour of the double layer as an imperfect capacitor. 4.2. The influence of pH on Ti6Al4V Under air-exposure, an oxide layer is spontaneously formed on the Ti surface, which protects the metal against corrosion (LavosValereto et al., 2004; Pan et al., 1996; Tamura et al., 2002). As this layer is subjected to chemical attack, it can break down and cause the Ti surface to be exposed, which will lead to an increased
corrosion rate on the Ti surface. This layer is represented by the Ipass value. Even though the protectiveness of this oxide layer is reduced as pH is decreased, the Ipass values in Ti exhibited relatively slight changes as acidity increased (Fig. 3c); which demonstrates the stability of the oxide layer in Ti. The cyclic polarization scan (Fig. 3a) also showed that the passive region remained constant with increasing current. The Icorr values exhibited a relatively small change in current as a function of pH (Fig. 3b), which also demonstrated a stable oxide layer in Ti. This is confirmed by the SEM images (Fig. 6b), which do not show any pitting behaviour. The EIS measurements showed the increased rate of corrosion in an acidic environment due to the fact that Rp values decreased with increasing acidity and Cdl values increased (Fig. 4c and d respectively). This observation was further verified by examining the classical Pourbaix diagram, which explains the corrosion mechanisms as a function of electrode potential and pH level of the environment. The diagram for Ti is shown in Fig. 7a, which indicates the efficiency of the Ti to form an oxide layer, as it has a large passivation zone.
Please cite this article in press as: Royhman D, et al., An electrochemical investigation of TMJ implant metal alloys in an artificial joint fluid environment: The influence of pH variation, Journal of Cranio-Maxillo-Facial Surgery (2014), http://dx.doi.org/10.1016/j.jcms.2014.01.029
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Fig. 7. Classical Pourbaix diagrams for (a) Ti and (b) Cr, and schematic diagrams of the electrochemical process and ion exchange in presence of protein content solution for, (c) Ti6Al4V, (d) CoCrMo.
4.3. The Influence of pH on the CoCrMo alloy The polarization resistance values from the Rp graph (Fig. 4c) for CoCrMo seem to remained relatively steady despite the change in pH. This result is also supported by Nyquist and Bode plots (Fig. 4a and b respectively), which show only slight changes in resistance to polarization. The Ipass values in CoCrMo exhibited larger changes than Ti6Al4V as acidity increased (Fig. 3d); which demonstrates a less stable oxide layer than Ti6Al4V. Additionally, the higher Icorr values in CoCrMo alloy also indicated a less stable oxide layer (Fig. 3b). The cyclic polarization scan also showed that the passive region did not remain constant with increasing current (Fig. 3a), which further demonstrates a less stable passive layer. From these results, it can be concluded that the formation and growth of the passive film in CoCrMo was decreased with decreasing pH, but this decrease in the passive layer did not have a significant role in the corrosion kinetics of the material. In the corrosion process of CoCrMo, Cr is the electrochemically active metal and dominates the oxide film formation and passivation process (Ciszewski et al., 2007; Cohen, 1998; Virtanen et al., 2008). This process mainly forms Cr2O3, which acts as a protective barrier between the metal surface and the corrosive environment. Fig. 7b shows the classical Pourbaix diagram for the Cr. This diagram demonstrates how the passive region is shifted to higher pH, which implies corrosion behaviour at low pH environments. Hence, the reason for the decreased affinity of CoCrMo could be coupled with the low tendency of passive film formation of Cr and its relatively low presence in the alloy (w27%) as shown in Table 1. Additionally, even though Co is generally considered less active (The oxidation of Cr is preferential to that of Co.), it still dominates in terms of its presence in the alloy (w64%) (Lewis et al., 2005). 4.4. The possible influence of protein on corrosion kinetics Synovial fluid contains mucin, fat, albumin, leukocytes and cytokines (Kim et al., 2012), all of which may affect the corrosion
kinetics of the implant material (Yang and Black, 1994). Additionally, some of these biomolecules may attach to the implant surface and affect the surface reactivity of that implant. For example, it was previously reported that the formation of a protein-containing biofilm on a metal surface increased corrosion of the base alloy in CoCrMo (Yan et al., 2007). Another study determined that albumin significantly altered the surface properties of the Ti6Al4V by affecting the repassivation process (Khan et al., 1999). Furthermore, it was also discovered that metal alloys exhibit higher polarization resistance in the presence of synovial fluid than NaSO4 solution on several implant metal alloys, and that this may be due to the adsorbed layer of organic molecules on the metal surface (Contu et al., 2003). Still, it is unclear whether biomolecules inhibit or accelerate electrochemical reactions. The presence of protein can induce deviation from classical Pourbaix observations on the corrosion behaviour. It is likely that different molecules, metals, and combinations therein may have different ranges of effects. 4.5. The comparison between Ti6Al4V and CoCrMo 4.5.1. Difference and similarity in basic corrosion behaviour The polarization curves explicitly indicate the superior corrosion behaviour of Ti6Al4V compared to CoCrMo (Fig. 3a). The passivation behaviour and corrosion kinetics of Ti6Al4V was very distinct from CoCrMo. In the case of Ti6Al4V, the vertical passivation plateau after a potential of 0.5 V vs. SCE showed no evidence of pitting behaviour; however, CoCrMo demonstrated an initial passivation zone, ranging from 0.5 to þ0.5 V vs. SCE, and after that, the surface was subject to pitting behaviour. With both alloys, the Icorr value decreased as pH increased (Fig. 3b). This indicates that as the solution shifts from an acidic environment to a more basic nature, the corrosion rate declines. A similar trend was observed in the Ti6Al4V and CoCrMo. Fig. 3d shows that Ecorr trends and values were very similar in both metal types; however, there were significant differences in electrochemical parameters (Icorr and Ipass) between Ti6Al4V and CoCrMo (Fig. 3b and c respectively).
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Ti6Al4V showed superior corrosion properties compared to CoCrMo. CoCrMo had much larger Icorr values, and much smaller Ipass values. These differences may be caused by presence of a more stable oxide layer on the Ti surface, which offers superior corrosion resistance (Chaturvedi, 2009; Cohen, 1998; Khan et al., 1999; LavosValereto et al., 2004; Pan et al., 1996; Tamura et al., 2002). The impedance responses in phase angles were similar for both alloys; however, the Nyquist plot (Fig. 4a), demonstrated the higher corrosion resistance of Ti6Al4V compared to CoCrMo. Similarly, the Bode impedance plot (Frequency vs. Impedance) and phase plot (Frequency vs. Phase angle) (Fig. 4b) show that in the high frequency region, CoCrMo exhibited a lower impedance value compared to Ti6Al4V. A close observation to the phase angle plot indicates that both alloys exhibited similar maximum relative angle value. Generally, the highest delay of current with respect to potential (i.e. maximum value of angle) specifies better corrosion behaviour. In contrast, the greatest width of the capacitive behaviour at the intermediate frequency zone implies the better corrosion behaviour by a decrease of susceptibility to corrosion. The Ti6Al4V shows higher width compared to CoCrMo (Fig. 4b). This behaviour is possibly caused by the protection of the metal surface due to the presence of passive film, which retards the relaxation time. This affects the electrochemical characteristics of the surface and creates an overlapping effect of such relaxation periods. In doing so, this influences the charge transfer process. Polarization resistance showed a steady trend for CoCrMo and a linear increment in Ti6Al4V. Furthermore, in Ti6Al4V, the values exhibited much more dispersion compared to CoCrMo. From this, it can be concluded that acidity in BCS accelerated the ion exchange between the metale electrolyte interface in both metal types, but to a much higher extent in CoCrMo. Despite having a higher polarization resistance, Ti also showed a higher capacitance compared to CoCrMo. This could be due to the fact that Ti is considered to be a more electrochemically active metal and has a high tendency to form the passive oxide film (Chaturvedi, 2009; Cohen, 1998; Khan et al., 1999; Lavos-Valereto et al., 2004; Pan et al., 1996; Tamura et al., 2002). Although Ti6Al4V exhibited better corrosion behaviour, its strength-to-weight ratio is significantly lower than CoCrMo (Cohen, 1998; Virtanen et al., 2008). For this reason, CoCrMo is employed for heavy bearing applications, such as with hip implants, more often than Ti6Al4V (Cohen, 1998; Kawalec et al., 1995). The TMJ joint is considered to be low load-bearing and highly active (Van Loon et al., 1999, 2002). The choice between Ti6Al4V and CoCrMo alloy might be critical. The spontaneous ability of Ti6Al4V to form the oxide layer is an advantage if the joint is active. The sliding action under low contact pressure might support this selection; however, under high impact pressure, the selection of CoCrMo alloy might be favourable (Medley et al., 1996; Virtanen et al., 2008). 4.5.2. Surface conditions and schematic diagram WLI and SEM images (Fig. 5a and b respectively) showed that there is a drastic increase in roughness of CoCrMo compared with the relatively modest increase of roughness of Ti6Al4V. Acidity levels in BCS seemed to play a much larger role in CoCrMo corrosion than it did in Ti6Al4V and this may be due to a more stable oxide layer which forms at the Ti surface. As reported earlier in many studies (Goldberg and Gilbert, 2004; Songür et al., 2009) SEM images from this study showed evident pitting behaviour on CoCrMo (Fig. 6b). Ti6Al4V exhibited good immunity to pitting behaviour in all pH environments compared to CoCrMo (Fig. 6b). A schematic diagram of the surface of Ti6Al4V and CoCrMo after or during the corrosion exposure is provided in Fig. 7(c and d). The
Ti6Al4V surface exhibited a more stable passive film and the surrounding protein played a larger role on the corrosion kinetics. Metal ions were released to the surrounding electrolyte and their role in corrosion kinetics is very difficult to predict. It should be noted that during the corrosion exposure, the surface is exposed to protein and may have an adhered protein layer. Protein adhesion to the surface under electrochemical exposure could be explained through two main mechanisms, chemical adsorption and physical adsorption (Ivarsson et al., 1985; Kasemo, 1998; Lausmaa et al., 1990; Rosengren et al., 1996; Vojvodic et al., 2006; Wälivaara et al., 1994). CoCrMo exhibits a higher protein affinity than Ti6Al4V. This could be attributed to the stable passive film generated on the surface that inhibits both chemical adsorption and physical adsorption process; which prevents further adhesion of protein on the surface (Hodgson et al., 2004; MetikoHukovi c and Babi c, 2007; Valero Vidal et al., 2010). 4.6. Galvanic corrosion influence Galvanic corrosion results from coupling dissimilar metals in direct electrical contact in corrosive solutions; such as the TMJ implant joint, which often utilize Ti6Al4V and CoCrMo alloy together to make a single joint. In this study, the potential pathway that leads to galvanic corrosion was not a strong contributor to corrosion. This is most likely attributed to the passivation behaviour of Ti6Al4V and to a lesser extent, the passive layer in CoCrMo as well. This passivation acts as a protective barrier to prevent galvanic interaction (Kummer and Rose, 1983; Mears, 1975; Virtanen et al., 2008). 4.7. Clinical relevance of the finding with respect to TMJ implant In February 2011, the FDA issued a press release ordering postmarket surveillance studies on the three manufacturers of TMJ implants in order to determine the length of time before the implants are removed or replaced due to pain or failure (Press Announcements e FDA orders postmarket surveillance of certain TMJ implants, 2011). The FDA stated that a substantial number of patients required implant revision surgery within three years (Press Announcements e FDA orders postmarket surveillance of certain TMJ implants, 2011). Some studies that examined the tissue surrounding retrieved TMJ implants found wear debris from the breakdown of the implant due to wear and corrosion processes (Guarda-Nardini et al., 2008a; Mercuri, 2000; Wolford et al., 2003). This paper presents the results of an initial investigation into the corrosive behaviour of commonly used TMJ implants alloys (CoCrMo and Ti6Al4V) under simulated physiological conditions as a possible cause of early TMJ total joint replacement devices. This study demonstrates that increased acidity in artificial joint fluid has a negative effect on TMJ implant corrosion resistance. Increased acidity during inflammation process may increase the release of corrosion products into the TMJ joint, which over time, can cause adverse biological reactions (Westesson et al., 1987). Given these implications, it is important to find the best biomaterial for implantation with optimal corrosion properties. In general, this study demonstrates that of the currently used TMJ total joint replacement device alloys, Ti6Al4V shows superior corrosion behaviour to CoCrMo. 4.8. Future work and limitations In this study, only electrochemical tests were performed. Other factors, which may cause the degradation of the metals and alloys, such as wear, were not considered. Additionally, only one type of artificial joint fluid was chosen as the electrolyte and it contained
Please cite this article in press as: Royhman D, et al., An electrochemical investigation of TMJ implant metal alloys in an artificial joint fluid environment: The influence of pH variation, Journal of Cranio-Maxillo-Facial Surgery (2014), http://dx.doi.org/10.1016/j.jcms.2014.01.029
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only one protein concentration (30 g/L). Other artificial joint fluids and concentrations may yield different results. Finally, the testing conditions were meant to mimic the physiological environment in a TMJ, but the exact environment cannot be fully duplicated. There are no methods to duplicate the exact complex environment of the human TMJ in vitro. Still, the methods used in this experiment are standard methods designed to simulate the physiological environment in order to test for specific effects. In the future, attempts will be made to examine the effects of corrosion of TMJ TJR implants in a simulated TMJ environment in conjunction with the effect of wear debris on the system. Selection of the implant metal should be based on the mechanical conditions as well as the electrochemical nature for which it is designed. Further investigations are required in this area, addressing the current limitations. 5. Conclusions Within the limitation of this study, currently used TMJ TJR device alloys, Ti6Al4V shows superior corrosion resistance to CoCrMo. Increased acidity in artificial joint fluid has a negative effect on alloy corrosion resistance and the coupling of CoCrMo and Ti6Al4V in a single joint prosthesis causes a weak galvanic reaction. Ethical approval Not required. Funding This study was partially supported by Oral and Maxillofacial Surgery Foundation to C.S., M.T.M. and L.G.M. Conflict of interest None. Acknowledgements The authors would like to acknowledge Dr. M. A. Wimmer (Director, Wear Testing Lab, Rush University Medical Centre), Prof. R. Urban for the SEM facilities (Rush University Medical Centre), the College of Dentistry at University of Illinois at Chicago (UIC) for the financial support, and to ATI Allvac for the CoCrMo alloy specimens. References Barão VA, Mathew MT, Assunção WG, Yuan JC, Wimmer MA, Sukotjo C: Stability of cp-Ti and Ti-6Al-4V alloy for dental implants as a function of saliva pH e an electrochemical study. Clin Oral Implants Res 23(9): 1055e1062, 2012 Chaturvedi TP: An overview of the corrosion aspect of dental implants (titanium and its alloys). Indian J Dent Res 20(1): 91e98, 2009 Ciszewski A, Baraniak M, Urbanek-Brychczynska M: Corrosion by galvanic coupling between amalgam and different chromium-based alloys. Dent Mater 23(10): 1256e1261, 2007 Cohen J: Current concepts review. Corrosion of metal orthopaedic implants. J Bone Joint Surg Am 80(10): 1554, 1998 Contu F, Elsener B, Bohni H: Characterization of implant materials in fetal bovine serum and sodium sulfate by electrochemical impedance spectroscopy. II. Coarsely sandblasted samples. J Biomed Mater Res A 67(1): 246e254, 2003 Detamore MS, Athanasiou KA: Structure and function of the temporomandibular joint disc: implications for tissue engineering. J Oral Maxillofac Surg 61(4): 494e506, 2003 Goldberg JR, Gilbert JL: The electrochemical and mechanical behaviour of passivated and TiN/AlN-coated CoCrMo and Ti6Al4V alloys. Biomaterials 25(5): 851e864, 2004 Guarda-Nardini L, Manfredini D, Ferronato G: Temporomandibular joint total replacement prosthesis: current knowledge and considerations for the future. Int J Oral Maxillofac Surg 37(2): 103e110, 2008a Guarda-Nardini L, Manfredini D, Ferronato G: Total temporomandibular joint replacement: a clinical case with a proposal for post-surgical rehabilitation. J Craniomaxillofac Surg 36(7): 403e409, 2008b
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An electrochemical investigation of TMJ implant metal alloys in an artificial joint fluid environment: The influence of pH variation Dmitry Royhman a, b, Rashmi Radhakrishnan b, Judy Chia-Chun Yuan a, Mathew T. Mathew a, b, Louis G. Mercuri b, c, Cortino Sukotjo a, * a University of Illinois at Chicago College of Dentistry, Department of Restorative Dentistry (Head: Dr. Stephen Campbell, DDS, MMSc), 801 S. Paulina Ave, Chicago, IL 60612, USA b Rush University Medical Center, Department of Orthopedics (Head: Dr. Joshua Jacobs, MD), 1611 W. Harrison St., Chicago, IL 60612, USA c TMJ Concepts, 2233 Knoll Drive, Ventura, CA 93003, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Paper received 18 July 2013 Accepted 6 January 2014
Objective: To investigate the corrosion behaviour of commonly used TMJ implants alloys (CoCrMo and Ti6Al4V) under simulated physiological conditions. Methods: Corrosion behaviour was evaluated using standard electrochemical corrosion techniques and galvanic corrosion techniques as per ASTM standards. Standard electrochemical tests (Ecorr, Icorr, Rp and Cf) were conducted in bovine calf serum (BCS), as a function of alloys type and different pHs. Galvanic corrosion tests were conducted in BCS at a pH of 7.6. Alloy surfaces were characterized using white-light interferometry (WLI) and scanning electron microscopy (SEM). Results: The potentiodynamic test results exhibited the enhanced passive layer growth and a better corrosion resistance of Ti6Al4V compared to CoCrMo. Electrochemical impedance spectroscopy measurements demonstrated the influence of protein as a function of pH on corrosion mechanisms/kinetics. Galvanic coupling was not a major contributor to corrosion. SEM and WLI images demonstrated a significantly higher in surface roughness in CoCrMo after corrosion. Conclusions: The results of this study suggest that Ti6Al4V shows superior corrosion behaviour to CoCrMo due to its strong passive layer, simulated joint fluid components can affect the electrochemical nature of the metal/electrolyte interface as a function of pH, and the galvanic effect of coupling CoCrMo and Ti6Al4V in a single joint is weak. Published by Elsevier Ltd on behalf of European Association for Cranio-Maxillo-Facial Surgery.
Keywords: Electrochemistry TMJ Titanium Alloy CoCrMo alloy Corrosion Artificial joint fluid
1. Introduction Temporomandibular dysfunction (TMD) is a generic term used to describe the extra-articular and intra-articular disorders associated with this joint. It may be caused by injury to the mandible, temporomandibular joint (TMJ), muscles of mastication, or by disc displacement, arthritic disease, developmental TMJ abnormalities, neoplasia or ankylosis (Ingawale and Goswami, 2009; Stegenga et al., 1991). TMD is most common between the ages 20 and 40, having a higher occurrence in women (Warren and Fried, 2001). Twenty to twenty-five percent of the population have symptoms of TMD (Detamore and Athanasiou, 2003; Van Loon et al., 2002; Warren
* Corresponding author. University of Illinois at Chicago College of Dentistry, Department of Restorative Dentistry, 801 S. Paulina Ave. Room #365B, Chicago, IL 60612, USA. Tel.: þ1 312 355 0360; fax: þ1 312 996 3535. E-mail address: [email protected] (C. Sukotjo).
and Fried, 2001). In cases where intra-articular TMD demonstrates end-stage arthrosis, TMJ total joint replacement (TMJ TJR) may provide the best management option (Guarda-Nardini et al., 2008a, b; Ingawale and Goswami, 2009; Landes et al., 2013; Mercuri, 2000; Schuurhuis et al., 2012; Wolford et al., 2003). In order to achieve the best outcome, any material used to make a joint prosthesis should have biocompatibility, functional compatibility, low wear coefficients when loaded under functional conditions, adaptability to anatomical structures, rigidity to stabilized components, corrosion resistance, and non-toxic effects (Wolford et al., 2003). Currently, there are 2 types of total TMJ replacement devices approved by the Food and Drug Administration (FDA) for use in the United States: (1) stock or “off-the-shelf devices”: the cast CoCr-onall polyethylene device (Biomet Microfixation, Jacksonville, FL), and the cast all CoCr-on-cast CoCr device (Nexus CMF, Salt Lake City, UT); and (2) or “custom” devices: the wrought Ti6Al4V/CoCrMo-onwrought CP Ti backed polyethylene patient-fitted device (TMJ
1010-5182/$ e see front matter Published by Elsevier Ltd on behalf of European Association for Cranio-Maxillo-Facial Surgery. http://dx.doi.org/10.1016/j.jcms.2014.01.029
Please cite this article in press as: Royhman D, et al., An electrochemical investigation of TMJ implant metal alloys in an artificial joint fluid environment: The influence of pH variation, Journal of Cranio-Maxillo-Facial Surgery (2014), http://dx.doi.org/10.1016/j.jcms.2014.01.029
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Nomenclature Ecorr Icorr Ipass Rs Cdl Rp: CPE SEM WLI EIS OCP SCE
corrosion potential (Tafel extrapolation) corrosion current (Tafel extrapolation) passivation current solution resistance double layer capacitance polarization resistance constant phase element scanning electron microscopy white-light interferometry electrochemical impedance spectroscopy open circuit potential saturated calomel electrode
Concepts, Ventura, CA), and the cast all CoCr-on-cast CoCr custom device (Nexus CMF, Salt Lake City, UT). A custom made, patient fitted device is available for markets outside US (Biomet, Jacksonville, FL, US), which is designed by a virtual planning company (Medical Modeling, Golden, CO, US). Ti6Al4V and CoCrMo are the metal materials of choice for implants due to their biocompatibility, strength, ductility, corrosion resistance, and rigidity (Chaturvedi, 2009; Guarda-Nardini et al., 2008a; Langford and Frame, 2002; Songür et al., 2009; Virtanen et al., 2008). In the body, the TMJ implant is exposed to extremely complex and variable conditions from the electrolytic environment. Such factors include: temperature, biofilm, protein, joint fluid, and pH changes due to the severe pathological changes in the surrounding tissues from the surgical procedure or the presence of implant itself (Chaturvedi, 2009). Furthermore, there is often an inflammatory response, which can lead to the generation of hydrogen peroxide and increased levels of protein and cytokines (Katou et al., 1996; Virtanen et al., 2008). All of these factors may accelerate the electrochemical nature of the metal/electrolyte interface and lead to pitting corrosion (Virtanen et al., 2008). Furthermore, galvanic coupling (from mixed metal implants) may play a significant role as well (Reclaru and Meyer, 1994). Corrosion can lead to roughening of the surface, weakening of the restoration, liberation of elements from the metal or alloy, and toxic reactions (Cohen, 1998; Schiff et al., 2002; Songür et al., 2009; Virtanen et al., 2008). Corrosion can severely limit the strength and lifespan of the material leading to mechanical failure of the implant (Cohen, 1998; Schiff et al., 2002; Songür et al., 2009; Virtanen et al., 2008). Additionally, it can cause the release of metal ions into the surrounding tissues, which can cause several cytotoxic responses including, a decrease in enzyme activity, interference with biochemical pathways, carcinogenicity, mutagenicity, localized tissue irritation, and toxicity leading to skeletal and nervous system disorders (Westesson et al., 1987). Corrosion products have been implicated in causing local pain or swelling in the region of the implant in the absence of infection (Chaturvedi, 2009). Fig. 1a and b shows the fossa component of a TMJ total joint replacement device (Christensen (TMJ, Inc. Golden, CO) Metal on Metal prosthesis), which was retrieved due to implant failure. There is evident damage at the articulating surface, most likely attributed to wear and corrosion. There is a limited amount of information regarding TMJ implant material degradation and the associated mechanisms that may cause it. The objective of this study was to investigate the corrosion behaviour of the two most commonly used TMJ total joint replacement device metals (Ti6Al4V and CoCrMo) using simulated
Fig. 1. (a) Image of the fossa component of a failed TMJ total joint replacement device shows degradation at the articulating surface and (b) SEM image shows a 1000 magnification of the region inside the white box from (a).
in vivo environmental conditions. An artificial joint fluid environment (Bovine Calf Serum; BCS) was chosen, with a varying pH range to simulate the pH values at physiological tissues that interface surgical implants (pHs from approximately 4.0 to 7.7) (Konttinen et al., 2001). In addition, since corrosion is a function of the nobility of the exposed surfaces, especially when considering the use of implants with more than one metal type or alloy (Chaturvedi, 2009; Ciszewski et al., 2007; Virtanen et al., 2008), possible effects of galvanic corrosion were also examined. 2. Materials and methods 2.1. Sample preparation Ti6Al4V and CoCrMo were chosen for this experiment due to their implementation in current TMJ joint replacement devices. The chemical composition of the Ti6Al4V and CoCrMo alloy discs are shown in Table 1. Fifteen Ti6Al4V discs (Mac-Master Carr, Elmhurst, IL, USA) and fifteen wrought high-carbon CoCrMo discs (ATI Allvac, Pittsburgh, PA, USA) were milled from rods. The discs were then brought to a mirror finish (Ra 25 nm) in order to obtain standardized surfaces for which the measurements could be compared. In order to obtain the mirror finished surfaces, the Ti6Al4V discs were wet-ground using a series of abrasive discs (#320, #400, #600, and #800) (Carbimet 2, Buehler, Lake Bluff, IL, USA), followed by polishing with diamond paste (MetaDi 9-micron, Buehler, Lake Bluff, IL, USA) and lubricant (MetaDi Fluid, Buehler, Lake Bluff, IL,
Please cite this article in press as: Royhman D, et al., An electrochemical investigation of TMJ implant metal alloys in an artificial joint fluid environment: The influence of pH variation, Journal of Cranio-Maxillo-Facial Surgery (2014), http://dx.doi.org/10.1016/j.jcms.2014.01.029
D. Royhman et al. / Journal of Cranio-Maxillo-Facial Surgery xxx (2014) 1e10 Table 1 Chemical composition of Ti6Al4V and CoCrMo. Composition (in wt%) Ti alloy CoCrMo alloy
Ti 89.62 C 0.241
Al 6.1 Co 64.60
V 4.0 Cr 27.63
C 0.004 Mo 6.04
Fe 0.16 Si 0.66
O2 0.106 Mn 0.70
N2 0.008 Al 0.02
H2 0.0022
USA) on polishing cloth (TexMet Polishing Cloth, Buehler, Lake Bluff, IL, USA), and finally brought to a mirror finish using colloidal silica polishing suspension (MasterMet, Buehler, Lake Bluff, IL, USA) on chemomet polishing cloth (Chemomet I, Buehler, Lake Bluff, IL, USA). CoCrMo alloy discs were wet-ground using a slightly different series of abrasive discs (#240, #320, #400, and #600) (Carbimet 2, Buehler, Lake Bluff, IL, USA), followed by polishing with 6 mm Polycrystalline Diamond Suspension (MetaDi, Buehler, Lake Bluff, IL, USA) on polishing cloth (TexMet Polishing Cloth, Beuhler, Lake Bluff, IL, USA), and finally brought to a mirror finish using 1 mm Polycrystalline Diamond Suspension (MetaDi, Buehler, Lake Bluff, IL, USA) on nylon cloth (Premium Cloths, Buehler, Lake Bluff, IL, USA). Before testing, the samples were ultrasonically cleaned with 70% isopropanol and deionized water, and finally dried with nitrogen gas stream. 2.2. Corrosion tests For corrosion tests, the samples were divided into 8 groups (n ¼ 3) as a function of different disc types (Ti6Al4V or CoCrMo) and pH (3.0, 4.5, 6.0, or 7.6) as shown in Fig. 2a. Electrochemical behaviour of Ti6Al4V and CoCrMo was studied by analysing open circuit potential (OCP), corrosion current density (Icorr), passivation current (Ipass), and corrosion potential (Ecorr). All electrochemical tests were conducted using a custom-made electrochemical cell (made of acrylic) using a standard 3-electrode configuration in which the potentiostat (G300, Gamry Inc., Warminster, PA, USA) was used to perform the corrosion measurements. A saturated calomel electrode (SCE) was used as the reference electrode, a graphite rod as a counter-electrode, and the metal alloy as a working electrode (exposed surface (0.38 cm2)). All tests followed the American Society for Testing for Metals (ASTM) guidelines (G61 and G31-72). The corrosion behaviour of Ti6Al4V and CoCrMo alloy
3
in various artificial joint fluid pHs was examined by modifying the pH of a bovine calf serum (BCS) solution (electrolyte volume ¼ 10 mL). The BCS solution contained: BCS (30 g/L protein content), NaCl (18 g/L), Propylene Phenoxetol (30 mL/L), Tris (54 g/ L), and HCl (approximately 55 mL/L), or enough to bring the pH of the solution down to 7.6. Once the initial pH of 7.6 was reached, lactic acid was added to attain the rest of the pH levels (3.0, 4.5, and 6.0). The samples were mounted into the electrochemical cell and the temperature of the test solution was maintained at 37 C. The sequence of tests included open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and finally a cyclic polarization test. Open circuit potential was monitored for a period 3600 s. The EIS measurements were performed in the frequency range from 100 kHz to 5 MHz, with AC sine wave amplitude of 10 mV applied to the electrode at its corrosion potential. EIS results were used to propose an equivalent circuit to represent the electrochemical process in relation to the observed electrochemical phenomena. A simple model of this circuit is presented in Fig. 2b, which consists of solution resistance (Rs), polarization resistance (Rp), and capacitance of the electrochemical double layer (Cdl). Under the exposure to complex solution like BCS and to account the heterogeneity of surface, Cdl was replaced by constant phase element (CPE). The modelling was done with Echem Analyst (Gamry Inc., Warminster, PA, USA). The cyclic polarization scan was carried out from 0.8 V to 1.8 V (2 mV/s scan rate). Corrosion measurements Ecorr, Icorr, and Ipass were obtained from the polarization curves. Ecorr and Icorr values were estimated using Tafel extrapolation. 2.3. Galvanic corrosion tests The galvanic corrosion of coupled Ti6Al4V/CoCrMo (n ¼ 3) was measured at pH ¼ 7.6 as is shown in Fig. 2a. Measurements were taken using a 3-electrode configuration with Ti6Al4V as the working electrode, CoCrMo as a counter-electrode, and a saturated calomel electrode (Gamry Inc., Warminster, PA, USA) as the reference electrode for 24 h. For each trial mean values were taken of the total current density, potential, and integration throughout the experiment. 2.4. Surface characterization In order to obtain surface roughness measurements (Ra and rms), three-dimensional images of the corroded surfaces were taken using a white-light interferometry microscope (Zygo New View 6300, Zygo Corporation, Middlefield, CT, USA). Six measurements were taken per disc. Further surface characterization was performed using a scanning electron microscope (SEM) (Jeol JSM6490 LV, Oxford Instruments, Oxford, UK) in order to examine surface changes on the metal surfaces due to the corrosion process (passive films and pitting behaviour). 2.5. Statistical analysis
Fig. 2. (a) Schematic diagram of the design of experiment and sample groups and (b) schematic diagram for the three-element Randle’s equivalent circuit used to determine the polarization resistance (Rp). Rs represents the solution resistance and CPE represents the constant phase element used to replace the double layer capacitance (Cdl).
Corrosion analysis was performed by evaluating 5 parameters from standard corrosion tests (Icorr, Ecorr, Ipass, Cdl and Rp), 3 parameters from galvanic corrosion tests (current density, potential, and integration), and surface roughness measurements (Ra and rms), taken from white-light interferometry. One-way analysis of variance was used to determine the effects of pH (factor 1, four levels) and its effect on Icorr, Ecorr, Ipass, Cdl and Rp in each alloy type (factor 2, two levels). Tukey’s post hoc analysis was used for pairwise comparisons within these groups. A two-sample independent t-test was performed to compare the differences between Ti6Al4V and CoCrMo for Icorr, Ecorr, Ipass, Cdl, Rp, as well as surface
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Fig. 3. Electrochemical results (a) cyclic polarization curves, (b) Icorr, (c) Ecorr, and (d) Ipass.
roughness parameters (Ra and rms) with a critical value of p ¼ 0.05 (Statistical Package for the Social Sciences, version 17.0; SPSS Inc., Chicago, IL, USA). 3. Results 3.1. Cyclic polarization scans The potentiodynamic curves for CoCrMo and Ti6Al4V in BCS are presented in Fig. 3a. The values for OCP, Icorr, Ipass, and Ecorr are presented in Table 2, Fig. 3bed respectively. In general, all four parameters decreased with increasing pH in both alloys. Significant differences among pHs (p < 0.05; ANOVA) were noted within both metals for Ecorr, Icorr and Ipass values. In the CoCrMo group, there was a significant difference among all pHs for Ecorr, Icorr, and Ipass (p < 0.05; Tukey HSD test), except for Icorr pH 4.5 vs. pH 6.0 (p ¼ 0.071) and Ipass pH 6.0 vs. 7.6 (p ¼ 0.151). In the Ti6Al4V group, significant differences were seen at all pHs for all 3 parameters (p < 0.05; Tukey HSD test), except for Icorr pH 6.0 vs. 7.6 (p ¼ 0.113) and Ipass pH 4.5 vs. 3.0 (p ¼ 0.104), pH 4.5 vs. 6.0 (p ¼ 0.658), and pH 4.5 vs. 7.6 (p ¼ 0.527). Ti6Al4V shows significantly lower Icorr values and higher Ipass values than CoCrMo in all pHs (p < 0.0001; t-test), the Ecorr value for Ti6Al4V was significantly higher in pH 3.0 (p ¼ 0.004; t-test) and lower in 6.0 (p ¼ 0.035; t-test) compared to CoCrMo.
3.2. EIS measurements The EIS circuit parameters, polarization resistance (Rp), and capacitance (Cdl) are presented in Fig. 4c and d for both alloys. Rp is the polarization resistance at the metal/electrolyte interface. Cdl is the specific double-layer capacitance at the working electrode/ electrolyte interface. In general, Cdl was significantly different among pHs for both metals (p < 0.05, ANOVA); however, Rp only showed significant values for Ti6Al4V (p ¼ 0.14, ANOVA). In CoCrMo, Cdl pH 3.0 was significantly lower compared to other pHs (p < 0.01, Tukey HSD test). In Ti6Al4V, pH 3.0, Cdl was significantly higher only when compared to pH 7.6 (p ¼ 0.013, Tukey HSD test) and Rp was significantly lower only when compared to pH 7.6 (p ¼ 0.01, Tukey HSD test). When comparing between the metal alloys, Rp for Ti6Al4V was significantly higher in all pHs compared to CoCrMo (p < 0.05, ttest). Ti6Al4V only showed significantly higher value of Cdl in pHs 3.0 (p ¼ 0.000, t-test) and 6.0 (p ¼ 0.002, t-test) compared to CoCrMo. 3.3. Galvanic corrosion The mean values for potential coupling Ti6Al4V and CoCrMo were 1.02 to 0.46 (V vs. SCE) and the deviations of the mixed potentials are within the range of 0.01e0.04 (V vs. SCE). The mean
Table 2 Evolution of open circuit potential as a function of pH with standard deviation for tested conditions. Sample type
Ti alloy CoCrMo alloy
Parameter
OCP (Eq¼0) (V vs. SCE) OCP (Eq¼0) (V vs. SCE)
pH level pH 3.0
pH 4.5
pH 6.0
pH 7.6
0.42 (0.00) 0.28 (0.02)
0.50 (0.01) 0.31 (0.08)
0.66 (0.01) 0.52 (0.02)
0.76 (0.00) 0.79 (0.00)
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Fig. 4. EIS results: (a) Nyquist plot, (b) Bode plot, (c) polarization resistance, and (d) capacitance.
values for current were 5.29 to 121.42 (nA/cm2) and the range of deviations of the mixed current was between the range of 10.72e 0.99 (nA/cm2). The galvanic current gradually increased towards zero as can be seen in Fig. 5. The galvanic current was integrated during the last 6 h. The mean integrated value change was 1.98 C. 3.4. Surface characterization The surface parameters (Ra and rms values), before and after corrosion, are displayed in Table 3. In the CoCrMo group, Ra and rms values were significantly different among the 5 groups tested (p < 0.0001, ANOVA) and all corroded groups yielded significantly higher Ra and rms values compared with the control (p < 0.05, Tukey HSD test). When comparing metals, CoCrMo yielded significantly higher Ra (p < 0.05, t-test) and rms (p < 0.001, t-test) compared to Ti6Al4V within each pH. WLI images showed a higher surface roughness after corrosion in both alloys, but CoCrMo alloy surfaces demonstrated higher surface roughness compared to the Ti6Al4V counterpart at the same pHs (Fig. 6a). SEM inspection demonstrated an increase in surface roughness and overall pitting
Fig. 5. Galvanic coupling of Ti6Al4V and CoCrMo with potential (Left Y-axis) and current (Right Y-axis) over time.
behaviour after corrosion in both alloys, especially in CoCrMo (Fig. 6b). 4. Discussion The findings in this study were consistent with previous findings obtained from this laboratory (Barão et al., 2012; Mathew et al., 2011), which show that lower pH levels increased the rate of transfer of electrons at the metal/electrolyte interface, BCS can be used as a suitable electrolyte to simulate joint fluid in an electrochemical cell, and that dissolved biomolecules can influence corrosion kinetics in the test system. 4.1. EIS analysis The EIS technique was utilized for characterizing electrochemical reactions at the metal/electrolyte interface and assessing the interfacial changes from the effect of protein at the metal surface. EIS data is commonly analyzed through two stages. The first stage (stage a) is analysis via data plots, namely Nyquist plot (imaginary component of impedance vs. real component) and Bode plots (impedance and phase angle vs. frequency). The second stage (stage b) is analysis by using the EIS data to develop an equivalent electrical circuit or model for the electrochemical process. This is used to quantify electrochemical kinetic parameters such as polarization resistance (Rp) and capacitance (Cdl). The EIS data is presented as Nyquist plot and Bode plot in Fig. 4a and b respectively. These plots show the variation of impedance as a function of frequency of the double layer formed at the interface of the electrolyte and the metal during the corrosion process. The Nyquist plot (imaginary component of impedance vs. real component of impedance) demonstrates the corrosion resistance of the metal surface. A decreased semicircular diameter of capacitance loop indicates a decreased corrosion resistance. In the second stage
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Table 3 Representative surface roughness parameters (Ra and rms), mean and standard deviation, of Ti6Al4V and CoCrMo as a function of corrosion in different BCS pH levels. Sample type
Roughness parameter
Corrosion process Control (non-corroded)
Corroded (pH 3.0)
Corroded (pH 4.5)
Corroded (pH 6.0)
Corroded (pH 7.6)
CoCrMo
Ra (mm) rms (mm)
0.007 (0.001) 0.023a (0.020)
1.035 * (0.817) 1.270b* (0.956)
0.626 * (0.237) 0.782b* (0.252)
0.929 * (0.138) 1.116b* (0.157)
1.175b* (0.007) 1.394b* (0.090)
Ti6Al4V
Ra (mm) rms (mm)
0.009 (0.003) 0.013 (0.003)
0.009* (0.003) 0.014* (0.007)
0.014* (0.002) 0.018* (0.002)
0.011* (0.003) 0.014* (0.004)
0.009* (0.002) 0.012* (0.003)
a
b
b
b
For each variable, small letters compare different pH values at the same metal alloy type. Different letters indicate significant differences among the groups (a ¼ 0.05). Asterisk (*) indicates a statistical difference between Ti6Al4V with CoCrMo in each corresponding pH level (a ¼ 0.05).
Fig. 6. (a) White-light interferometry microscopy 3D images of Ti6Al4V and CoCrMo before and after corrosion. X and Y scales are in mm and Z scale is in mm. The Z scale covers 0.2 mm in amplitude. Colours represent the peaks (from w0 to þ0.1 mm of amplitude in the Z axis) (red and yellow) and valleys (from w0 to 0.1 mm of amplitude in the Z axis) (green and blue) on the titanium and CoCrMo surfaces topographies. (b) Surface characterization SEM surface profile images for Ti6Al4V and CoCrMo before and after corrosion as a function of pH.
of EIS analysis, the data was employed to develop an equivalent electrical circuit of the corrosion process. The electrical circuit of this particular electrochemical process was similar to Randle’s circuit (Fig. 2b); however, Nyquist results appear to show more complicated behaviour than that which could be modelled with the simple Randle’s circuit. The semicircles appeared to be depressed (vertical radius is shorter than horizontal radius). To compensate for the peculiar nature of this Nyquist plot, the simple capacitance component in the Randle’s circuit was replaced by constant phase element (CPE). CPE behaviour is related to the heterogeneous properties of the surface and models the behaviour of the double layer as an imperfect capacitor. 4.2. The influence of pH on Ti6Al4V Under air-exposure, an oxide layer is spontaneously formed on the Ti surface, which protects the metal against corrosion (LavosValereto et al., 2004; Pan et al., 1996; Tamura et al., 2002). As this layer is subjected to chemical attack, it can break down and cause the Ti surface to be exposed, which will lead to an increased
corrosion rate on the Ti surface. This layer is represented by the Ipass value. Even though the protectiveness of this oxide layer is reduced as pH is decreased, the Ipass values in Ti exhibited relatively slight changes as acidity increased (Fig. 3c); which demonstrates the stability of the oxide layer in Ti. The cyclic polarization scan (Fig. 3a) also showed that the passive region remained constant with increasing current. The Icorr values exhibited a relatively small change in current as a function of pH (Fig. 3b), which also demonstrated a stable oxide layer in Ti. This is confirmed by the SEM images (Fig. 6b), which do not show any pitting behaviour. The EIS measurements showed the increased rate of corrosion in an acidic environment due to the fact that Rp values decreased with increasing acidity and Cdl values increased (Fig. 4c and d respectively). This observation was further verified by examining the classical Pourbaix diagram, which explains the corrosion mechanisms as a function of electrode potential and pH level of the environment. The diagram for Ti is shown in Fig. 7a, which indicates the efficiency of the Ti to form an oxide layer, as it has a large passivation zone.
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Fig. 7. Classical Pourbaix diagrams for (a) Ti and (b) Cr, and schematic diagrams of the electrochemical process and ion exchange in presence of protein content solution for, (c) Ti6Al4V, (d) CoCrMo.
4.3. The Influence of pH on the CoCrMo alloy The polarization resistance values from the Rp graph (Fig. 4c) for CoCrMo seem to remained relatively steady despite the change in pH. This result is also supported by Nyquist and Bode plots (Fig. 4a and b respectively), which show only slight changes in resistance to polarization. The Ipass values in CoCrMo exhibited larger changes than Ti6Al4V as acidity increased (Fig. 3d); which demonstrates a less stable oxide layer than Ti6Al4V. Additionally, the higher Icorr values in CoCrMo alloy also indicated a less stable oxide layer (Fig. 3b). The cyclic polarization scan also showed that the passive region did not remain constant with increasing current (Fig. 3a), which further demonstrates a less stable passive layer. From these results, it can be concluded that the formation and growth of the passive film in CoCrMo was decreased with decreasing pH, but this decrease in the passive layer did not have a significant role in the corrosion kinetics of the material. In the corrosion process of CoCrMo, Cr is the electrochemically active metal and dominates the oxide film formation and passivation process (Ciszewski et al., 2007; Cohen, 1998; Virtanen et al., 2008). This process mainly forms Cr2O3, which acts as a protective barrier between the metal surface and the corrosive environment. Fig. 7b shows the classical Pourbaix diagram for the Cr. This diagram demonstrates how the passive region is shifted to higher pH, which implies corrosion behaviour at low pH environments. Hence, the reason for the decreased affinity of CoCrMo could be coupled with the low tendency of passive film formation of Cr and its relatively low presence in the alloy (w27%) as shown in Table 1. Additionally, even though Co is generally considered less active (The oxidation of Cr is preferential to that of Co.), it still dominates in terms of its presence in the alloy (w64%) (Lewis et al., 2005). 4.4. The possible influence of protein on corrosion kinetics Synovial fluid contains mucin, fat, albumin, leukocytes and cytokines (Kim et al., 2012), all of which may affect the corrosion
kinetics of the implant material (Yang and Black, 1994). Additionally, some of these biomolecules may attach to the implant surface and affect the surface reactivity of that implant. For example, it was previously reported that the formation of a protein-containing biofilm on a metal surface increased corrosion of the base alloy in CoCrMo (Yan et al., 2007). Another study determined that albumin significantly altered the surface properties of the Ti6Al4V by affecting the repassivation process (Khan et al., 1999). Furthermore, it was also discovered that metal alloys exhibit higher polarization resistance in the presence of synovial fluid than NaSO4 solution on several implant metal alloys, and that this may be due to the adsorbed layer of organic molecules on the metal surface (Contu et al., 2003). Still, it is unclear whether biomolecules inhibit or accelerate electrochemical reactions. The presence of protein can induce deviation from classical Pourbaix observations on the corrosion behaviour. It is likely that different molecules, metals, and combinations therein may have different ranges of effects. 4.5. The comparison between Ti6Al4V and CoCrMo 4.5.1. Difference and similarity in basic corrosion behaviour The polarization curves explicitly indicate the superior corrosion behaviour of Ti6Al4V compared to CoCrMo (Fig. 3a). The passivation behaviour and corrosion kinetics of Ti6Al4V was very distinct from CoCrMo. In the case of Ti6Al4V, the vertical passivation plateau after a potential of 0.5 V vs. SCE showed no evidence of pitting behaviour; however, CoCrMo demonstrated an initial passivation zone, ranging from 0.5 to þ0.5 V vs. SCE, and after that, the surface was subject to pitting behaviour. With both alloys, the Icorr value decreased as pH increased (Fig. 3b). This indicates that as the solution shifts from an acidic environment to a more basic nature, the corrosion rate declines. A similar trend was observed in the Ti6Al4V and CoCrMo. Fig. 3d shows that Ecorr trends and values were very similar in both metal types; however, there were significant differences in electrochemical parameters (Icorr and Ipass) between Ti6Al4V and CoCrMo (Fig. 3b and c respectively).
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Ti6Al4V showed superior corrosion properties compared to CoCrMo. CoCrMo had much larger Icorr values, and much smaller Ipass values. These differences may be caused by presence of a more stable oxide layer on the Ti surface, which offers superior corrosion resistance (Chaturvedi, 2009; Cohen, 1998; Khan et al., 1999; LavosValereto et al., 2004; Pan et al., 1996; Tamura et al., 2002). The impedance responses in phase angles were similar for both alloys; however, the Nyquist plot (Fig. 4a), demonstrated the higher corrosion resistance of Ti6Al4V compared to CoCrMo. Similarly, the Bode impedance plot (Frequency vs. Impedance) and phase plot (Frequency vs. Phase angle) (Fig. 4b) show that in the high frequency region, CoCrMo exhibited a lower impedance value compared to Ti6Al4V. A close observation to the phase angle plot indicates that both alloys exhibited similar maximum relative angle value. Generally, the highest delay of current with respect to potential (i.e. maximum value of angle) specifies better corrosion behaviour. In contrast, the greatest width of the capacitive behaviour at the intermediate frequency zone implies the better corrosion behaviour by a decrease of susceptibility to corrosion. The Ti6Al4V shows higher width compared to CoCrMo (Fig. 4b). This behaviour is possibly caused by the protection of the metal surface due to the presence of passive film, which retards the relaxation time. This affects the electrochemical characteristics of the surface and creates an overlapping effect of such relaxation periods. In doing so, this influences the charge transfer process. Polarization resistance showed a steady trend for CoCrMo and a linear increment in Ti6Al4V. Furthermore, in Ti6Al4V, the values exhibited much more dispersion compared to CoCrMo. From this, it can be concluded that acidity in BCS accelerated the ion exchange between the metale electrolyte interface in both metal types, but to a much higher extent in CoCrMo. Despite having a higher polarization resistance, Ti also showed a higher capacitance compared to CoCrMo. This could be due to the fact that Ti is considered to be a more electrochemically active metal and has a high tendency to form the passive oxide film (Chaturvedi, 2009; Cohen, 1998; Khan et al., 1999; Lavos-Valereto et al., 2004; Pan et al., 1996; Tamura et al., 2002). Although Ti6Al4V exhibited better corrosion behaviour, its strength-to-weight ratio is significantly lower than CoCrMo (Cohen, 1998; Virtanen et al., 2008). For this reason, CoCrMo is employed for heavy bearing applications, such as with hip implants, more often than Ti6Al4V (Cohen, 1998; Kawalec et al., 1995). The TMJ joint is considered to be low load-bearing and highly active (Van Loon et al., 1999, 2002). The choice between Ti6Al4V and CoCrMo alloy might be critical. The spontaneous ability of Ti6Al4V to form the oxide layer is an advantage if the joint is active. The sliding action under low contact pressure might support this selection; however, under high impact pressure, the selection of CoCrMo alloy might be favourable (Medley et al., 1996; Virtanen et al., 2008). 4.5.2. Surface conditions and schematic diagram WLI and SEM images (Fig. 5a and b respectively) showed that there is a drastic increase in roughness of CoCrMo compared with the relatively modest increase of roughness of Ti6Al4V. Acidity levels in BCS seemed to play a much larger role in CoCrMo corrosion than it did in Ti6Al4V and this may be due to a more stable oxide layer which forms at the Ti surface. As reported earlier in many studies (Goldberg and Gilbert, 2004; Songür et al., 2009) SEM images from this study showed evident pitting behaviour on CoCrMo (Fig. 6b). Ti6Al4V exhibited good immunity to pitting behaviour in all pH environments compared to CoCrMo (Fig. 6b). A schematic diagram of the surface of Ti6Al4V and CoCrMo after or during the corrosion exposure is provided in Fig. 7(c and d). The
Ti6Al4V surface exhibited a more stable passive film and the surrounding protein played a larger role on the corrosion kinetics. Metal ions were released to the surrounding electrolyte and their role in corrosion kinetics is very difficult to predict. It should be noted that during the corrosion exposure, the surface is exposed to protein and may have an adhered protein layer. Protein adhesion to the surface under electrochemical exposure could be explained through two main mechanisms, chemical adsorption and physical adsorption (Ivarsson et al., 1985; Kasemo, 1998; Lausmaa et al., 1990; Rosengren et al., 1996; Vojvodic et al., 2006; Wälivaara et al., 1994). CoCrMo exhibits a higher protein affinity than Ti6Al4V. This could be attributed to the stable passive film generated on the surface that inhibits both chemical adsorption and physical adsorption process; which prevents further adhesion of protein on the surface (Hodgson et al., 2004; MetikoHukovi c and Babi c, 2007; Valero Vidal et al., 2010). 4.6. Galvanic corrosion influence Galvanic corrosion results from coupling dissimilar metals in direct electrical contact in corrosive solutions; such as the TMJ implant joint, which often utilize Ti6Al4V and CoCrMo alloy together to make a single joint. In this study, the potential pathway that leads to galvanic corrosion was not a strong contributor to corrosion. This is most likely attributed to the passivation behaviour of Ti6Al4V and to a lesser extent, the passive layer in CoCrMo as well. This passivation acts as a protective barrier to prevent galvanic interaction (Kummer and Rose, 1983; Mears, 1975; Virtanen et al., 2008). 4.7. Clinical relevance of the finding with respect to TMJ implant In February 2011, the FDA issued a press release ordering postmarket surveillance studies on the three manufacturers of TMJ implants in order to determine the length of time before the implants are removed or replaced due to pain or failure (Press Announcements e FDA orders postmarket surveillance of certain TMJ implants, 2011). The FDA stated that a substantial number of patients required implant revision surgery within three years (Press Announcements e FDA orders postmarket surveillance of certain TMJ implants, 2011). Some studies that examined the tissue surrounding retrieved TMJ implants found wear debris from the breakdown of the implant due to wear and corrosion processes (Guarda-Nardini et al., 2008a; Mercuri, 2000; Wolford et al., 2003). This paper presents the results of an initial investigation into the corrosive behaviour of commonly used TMJ implants alloys (CoCrMo and Ti6Al4V) under simulated physiological conditions as a possible cause of early TMJ total joint replacement devices. This study demonstrates that increased acidity in artificial joint fluid has a negative effect on TMJ implant corrosion resistance. Increased acidity during inflammation process may increase the release of corrosion products into the TMJ joint, which over time, can cause adverse biological reactions (Westesson et al., 1987). Given these implications, it is important to find the best biomaterial for implantation with optimal corrosion properties. In general, this study demonstrates that of the currently used TMJ total joint replacement device alloys, Ti6Al4V shows superior corrosion behaviour to CoCrMo. 4.8. Future work and limitations In this study, only electrochemical tests were performed. Other factors, which may cause the degradation of the metals and alloys, such as wear, were not considered. Additionally, only one type of artificial joint fluid was chosen as the electrolyte and it contained
Please cite this article in press as: Royhman D, et al., An electrochemical investigation of TMJ implant metal alloys in an artificial joint fluid environment: The influence of pH variation, Journal of Cranio-Maxillo-Facial Surgery (2014), http://dx.doi.org/10.1016/j.jcms.2014.01.029
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only one protein concentration (30 g/L). Other artificial joint fluids and concentrations may yield different results. Finally, the testing conditions were meant to mimic the physiological environment in a TMJ, but the exact environment cannot be fully duplicated. There are no methods to duplicate the exact complex environment of the human TMJ in vitro. Still, the methods used in this experiment are standard methods designed to simulate the physiological environment in order to test for specific effects. In the future, attempts will be made to examine the effects of corrosion of TMJ TJR implants in a simulated TMJ environment in conjunction with the effect of wear debris on the system. Selection of the implant metal should be based on the mechanical conditions as well as the electrochemical nature for which it is designed. Further investigations are required in this area, addressing the current limitations. 5. Conclusions Within the limitation of this study, currently used TMJ TJR device alloys, Ti6Al4V shows superior corrosion resistance to CoCrMo. Increased acidity in artificial joint fluid has a negative effect on alloy corrosion resistance and the coupling of CoCrMo and Ti6Al4V in a single joint prosthesis causes a weak galvanic reaction. Ethical approval Not required. Funding This study was partially supported by Oral and Maxillofacial Surgery Foundation to C.S., M.T.M. and L.G.M. Conflict of interest None. Acknowledgements The authors would like to acknowledge Dr. M. A. Wimmer (Director, Wear Testing Lab, Rush University Medical Centre), Prof. R. Urban for the SEM facilities (Rush University Medical Centre), the College of Dentistry at University of Illinois at Chicago (UIC) for the financial support, and to ATI Allvac for the CoCrMo alloy specimens. References Barão VA, Mathew MT, Assunção WG, Yuan JC, Wimmer MA, Sukotjo C: Stability of cp-Ti and Ti-6Al-4V alloy for dental implants as a function of saliva pH e an electrochemical study. Clin Oral Implants Res 23(9): 1055e1062, 2012 Chaturvedi TP: An overview of the corrosion aspect of dental implants (titanium and its alloys). Indian J Dent Res 20(1): 91e98, 2009 Ciszewski A, Baraniak M, Urbanek-Brychczynska M: Corrosion by galvanic coupling between amalgam and different chromium-based alloys. Dent Mater 23(10): 1256e1261, 2007 Cohen J: Current concepts review. Corrosion of metal orthopaedic implants. J Bone Joint Surg Am 80(10): 1554, 1998 Contu F, Elsener B, Bohni H: Characterization of implant materials in fetal bovine serum and sodium sulfate by electrochemical impedance spectroscopy. II. Coarsely sandblasted samples. J Biomed Mater Res A 67(1): 246e254, 2003 Detamore MS, Athanasiou KA: Structure and function of the temporomandibular joint disc: implications for tissue engineering. J Oral Maxillofac Surg 61(4): 494e506, 2003 Goldberg JR, Gilbert JL: The electrochemical and mechanical behaviour of passivated and TiN/AlN-coated CoCrMo and Ti6Al4V alloys. Biomaterials 25(5): 851e864, 2004 Guarda-Nardini L, Manfredini D, Ferronato G: Temporomandibular joint total replacement prosthesis: current knowledge and considerations for the future. Int J Oral Maxillofac Surg 37(2): 103e110, 2008a Guarda-Nardini L, Manfredini D, Ferronato G: Total temporomandibular joint replacement: a clinical case with a proposal for post-surgical rehabilitation. J Craniomaxillofac Surg 36(7): 403e409, 2008b
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Please cite this article in press as: Royhman D, et al., An electrochemical investigation of TMJ implant metal alloys in an artificial joint fluid environment: The influence of pH variation, Journal of Cranio-Maxillo-Facial Surgery (2014), http://dx.doi.org/10.1016/j.jcms.2014.01.029