5. Corrosion Characteristics of Dental Magnetic Attachments

M. Koike,¹ Y. Takada,² O. Okuno,² and T. Okabe¹

¹Department of Biomaterials Science, Baylor College of Dentistry, Texas A&M Health Science Center

²Division of Dental Biomaterials, Tohoku University Graduate School of Dentistry

Introduction

In dental magnetic attachment systems, a removable denture in which a magnet is embedded is held by the magnetic attraction of a keeper bonded to the root cap. Since the corrosion resistance of rare-earth magnetic alloys is very poor, a magnet is needed to seal using a corrosion-resistant stainless steel. The keeper is prepared using a ferritic stainless steel. On the other hand, the root cap is usually made of a dental precious alloy. Since such a magnetic system is used in a corrosive oral environment, various alloying elements may dissolve out of the stainless steel and casing made of dental precious alloys. Metal from the dental magnetic systems is released into the surrounded environment by various mechanisms, including corrosion, wear, and mechanically accelerated electrochemical processes, such as galvanic corrosion, crevice corrosion, stress corrosion, corrosion fatigue, and fretting corrosion. These metal releases have been associated with clinical failure and allergic reactions in dental applications. The increasing necessity for prolonged use requires a dental magnet with less metal release. In selecting adequate materials based on use and duration, not only corrosion characteristics but also the behavior of the ion release of alloying elements of metallic biomaterials should be examined. In the oral cavity, the environmental fluids include the dental plaque, which consist of organic acids, such as lactic, formic, and inorganic acids, and protein. As for pH, human saliva is reported to have pH of 6.2-7.6 in a static condition. It has also been reported that the pH increases to approximately 4 when sucrose is consumed and that the pH of foods and beverages can range from 2.0 to 11. On the other hand, the reduction of pH in the oral cavity by eating and drinking is not considered to persist for prolonged periods because of the buffering action of components in saliva and plaque. Okazaki et al. reported the corrosion resistance of dental alloys in a pseudo-oral or simulated body environment. In the various solutions, the elements of the alloys were released and affected by the constituents and the pH of the solution and composition of the alloys.

Objective

In the present study, the corrosion characteristics of the component of the dental magnetic system, some stainless steels, and dental precious alloys were determined to obtain basic data for their corrosion behavior using the procedures specified in ISO standard 10271 (2001).

Materials and Methods

1. Alloys used

Four different stainless steels (SUS 444, SUSXM27, SUS447J1, and SUS316L) and three conventional dental casting alloys (Au-Ag-Pd alloy, Type 4 gold alloy, and multi-purpose dental gold casting alloy) were tested. Their nominal chemical compositions are listed in Table 1.

2. Specimen preparation

After all four kinds of stainless steels were rolled into a 1 mm thickness, they were cut into a 15 mm x 10 mm sheets. These stainless steel sheets were heat-treated at 1,050�‹C for 30 min in argon and quenched in ice water in order to relieve the strain built up in the deformation process. For static immersion tests, approximately 100 ƒÊm of the surface layer from all six surfaces of each cut specimen was removed using ASTM 120 grit SiC paper. Each surface was then polished up to ASTM 600 grit SiC. Before the experiments, specimens were cleaned ultrasonically for 2 min in ethanol. Five specimens were prepared in each type of stainless steel. In addition, for all electrochemical tests, the specimens were embedded in an epoxy resin disk similar to that described in a previous study (Koike et al., 2003). Before testing, the surface of each embedded specimen was polished with 1 mm diamond paste and cleaned ultrasonically for 2 min in ethanol. Three specimens for each type of stainless steel were prepared.

Three types of the dental precious alloys were cast into the same size as stainless steel specimens with a gypsum-based investment material (CRISTQUICK II, GC. Co., Japan) at 1,020�‹C using an air pressure casting machine (CASPAC C602, Dentronics Co., Japan). After bench-cooling to room temperature, the six surfaces of each cast sheet (10 mm x 15 mm x 1 mm ) were polished and cleaned in a similar manner to that used with the stainless steel specimens for the immersion and electrochemical tests.

3. Static immersion test

Each polished specimen was immersed by suspension in a 10 mL test solution for 7 days at 37 �} 1�‹C. The test solution consisted of 5.85 �} 0.005 g NaCl and 10.0 �} 0.1 g lactic acid dissolved in 1000 mL deionized water (pH 2.3�}0.1) followed by ISO10271. In the test, each sheet specimen was suspended in a polyethylene vial (Liquid Scintillation Vials, Weat Chester, PA, USA) using a finishing line so that the specimen did not touch any of the inside walls of the vials. The vial was covered with a lid to prevent evaporation. For each metal, five specimens were prepared. A solution without a metal specimen was incubated in a similar condition and used for the blank test.

At the end of the above immersion period, a 1.5 vol% of the concentrated HNO₃ solution was added to the test solution to avoid the concentration changes. Prior to the determination of elements dissolved in the immersion solution of each container, the test solution was further diluted 10 times using a 2% HNO₃ solution. The measurements of elements dissolved in the immersion solution were analyzed quantitatively using an inductively coupled plasma mass spectrometer (ICP-MS: ELAN DRC II, Perkin-Elmer Institute, Norwalk, CT, USA). The unit of the results was converted to mg/cm2 from ppb. These data were analyzed using ANOVA/Tukey's HSD (ƒ¿=0.05).

4. Electrochemical measurements

The corrosion characteristics of the alloys were evaluated using the determination of the open-circuit potential (OCP) and the potentiodynamic anodic polarization. Three specimens were tested for each metal. For all the corrosion evaluation, 9.0 g NaCl was dissolved in approximately 950 mL deionized water and adjusted to pH 7.4 �} 0.1 using 1% lactic acid or 4% NaOH. The test solution was diluted with water to 1,000 mL and used at 37�‹C.

Measurements were performed using a potentiostat (Potentiostat/galvanostat Model 273A; EG&G, Princeton Applied Research, Princeton, NJ, USA) controlled by a personal computer with dedicated software (352 Soft Corr III, Princeton Applied Research). Before starting the measurements, argon gas (99.999%) was introduced into the electrolyte for at least 30 min at a rate of about 100 cm³/min while the magnetic stirrer was activated. The working electrode (specimen) was immersed in the electrode, and the reference electrode (saturated Calomel Electrode) was adjusted. The OCP measurement continued up to 2 h �} 6 min with slight bubbling of the argon gas flow rate. The OCP measurement determined the corrosion potential of a metal in an electrode. The anodic polarization scan was started within 5 min after finishing the OCP measurement at 150 mV below the OCP. The potentiodynamic sweep rate was 1 mV/sec up to 1,000 mV above the OCP.

The data yielded the open circuit potential (E_ocp: mV vs. SCE); the zero current potential (E_z: mV vs. SCE); the breakdown potential (E_p: mV vs. SCE) with the corresponding current density (I_p: A/cm²); the active peak potentials (E_c: mV vs. SCE) between E_z and E_p with the corresponding current density (I_c: A/cm²); and the current density (I₃₀₀: A/cm²) at potential of (E_z +300) mV vs. SCE. E_ocp is the electrical potential difference between metals and an electrolyte when no electrical current flows between them, which can be used to predict the long-term service lifetime of metal structures (Tait, 1994). Ez is the potential at the inflection point, which occurs because potentials at around this point are no longer at the magnitude that causes ion reduction, re-oxidation of metal ions, and re-crystallization of the film or oxide. E_p is the potential at which the current increases with increasing potential. E_c is the potential observed when a metal produces visible quantities of corrosion after a brief exposure to electrolytes. General and pitting corrosion often occur together when a metal exhibits active corrosion behavior. I₃₀₀ was determined by the current density at E_z+300 mV vs. SCE.

The data were analyzed using non-parametric methods (ƒ¿=0.05).

Results and Discussion

1. Static immersion test

The quantities of metal released from the stainless steels and the dental alloys into the immersion solutions are shown in Figures 1 and 2, respectively. In all the stainless steels, all the alloying elements were found to dissolve. Fe dissolved the most of all other elements, including Cr, Mo, and Ni (p<0.001). 316L showed the highest dissolution among all stainless steels (p<0.001). On the other hand, in the dental precious alloys, Cu, Ag, Pd, Pt, and Ir were detected from each specimen. Au could not be detected because of a minimal amount of dissolution, which was below the detection limit. The amount of dissolved Cu in all types of alloys was the highest compared with other elements (p<0.001). The Au-Ag-Pd alloy showed the highest total amounts of dissolution of all precious alloys (p<0.001). The Type 4 gold and Au-Ag-Pd alloys showed a higher total amount of dissolution than any of the stainless steels tested, except for the Au-Pt-based alloy (p<0.001). It appeared that the overall corrosion resistance of stainless steels was better than that of dental casting alloys.

2. Electrochemical behavior

2.1. Open circuit potential measurements

Table 2 is a summary of the OCP values obtained for the metals tested. The OCP values show that the corrosion potential of all stainless steels, the Au-Ag-Pd alloy, and the Ag-Au alloy decreased initially with time. In contrast, in the Au-Pt alloy, an initial increase in the OCP values was observed in the first few minutes. For all metals, Kruskal-Wallis indicated that there were no significant differences in E_ocp among all the metals tested. The E_ocp of the stainless steels was lower than that of the precious dental alloys (p>0.05).

2.2. Potentiodynamic polarization measurements

Table 2 is a summary of the various corrosion parameters obtained for the metals tested. Kruskal-Wallis indicated no significant differences in E_z, E_p, and the current density at E_p and I₃₀₀ among all metals tested. The E_z and E_p of a series of stainless steels were lower than those of the precious dental alloys (p>0.05). No significant differences were observed in the current densities at Ep and I₃₀₀ among all metals (p>0.05). On the other hand, the I₃₀₀ of the 447 JI and XM 27 was significantly higher than that of the other metals (p>0.05). Ec was observed only in the dental precious alloys, and the Ec of the Au-Ag-Pd alloys was significantly lower than that of the Au-Cu and the Au-Pt alloys (p<0.05).

Figures 3 and 4 show the typical anodic polarization curves of the stainless steels and dental alloys in the NaCl and lactic acid solution. For all metals, no passivation regions were observed. The breakdown potentials for XM 27 and 447JI were significantly higher than those of the other metals in the 0.9% NaCl in the deaerated condition. On the other hand, for I₃₀₀, the values of stainless steels included the negative regions of the potential. However, in the dental precious alloys, the values were in the positive regions of the potential. No significant differences were observed between the stainless steel and the dental precious alloys regarding the I₃₀₀ values.

Conclusions

Fe ions were mainly released from each stainless steel. The total amount of ions released from each stainless steel was significantly smaller than those from the precious alloys tested in this study. In particular, the breakdown potentials of both XM 27 and 447J1 steels were also significantly higher than those of 316L steel as biomedical stainless steel and the precious alloys. Therefore, it can be concluded that the overall corrosion resistance of magnetic stainless steels, such as XM27 and 447J1, is sufficient for them to be used in the oral cavity as well as that of other dental metals.

References

1. Koike M, Cai Z, Fujii H, Brezner M, Okabe T (2003). Corrosion behavior of cast titanium with reduced surface reaction layer made by a face-coating method. Biomaterials 24:4541-4549.

2. ISO 10271 (2001). Dental metallic materials - Corrosion test methods. Geneva: International Organization for Standardization, 2001 (E).

3. Ewers GJ, Greener EH (1985). The electrochemical activity of the oral cavity - A new approach. J Oral Rehabil 12:469-476.

Discussion Board