Effect of recasting on element release from base metal dental casting alloys in artificial saliva and saline solution.

AIM: The aim of this study was to quantitatively estimate the concentration of ion release from recasted base metal alloys in various pH conditions using atomic absorption spectroscopy (AAS).
MATERIALS AND METHODS: Specimens of commercially available dental casting alloys (cobalt [Co]-chromium [Cr] and nickel [Ni]- chromium [Cr]) were prepared using lost-wax casting techniques and were stored in the test solution for 1 week and 4 weeks, and ions released during chemical corrosion were detected using AAS.
RESULTS: An increase in the quantity of ion release was observed with recasting. These changes were higher after twice recasting in Ni-Cr alloy.

INTRODUCTION

The natural resources are getting depleted due to the extensive usage of metallic and nonmetallic materials. The lack of awareness on recycling is one of the reasons for exploitation of the natural resources. At present, the public and industry are more concerned about conservation of natural resources. Hence, dentistry must contribute by reusing the excess alloys such as sprues and buttons in the rehabilitation of oral tooth structure.[12]

Biocompatibility and corrosion resistance of alloys are closely interconnected. The toxicity of dental casting alloys depends on the quantity of metal ions released into the oral cavity due to corrosion which may leave an undesirable metallic taste. Given these unpleasant circumstances, the patient may request for the removal of the metallic restoration.[3]

There are many factors contributing to the corrosion of metallic restorations. The inherent factors that favor the corrosions are microstructure, chemical composition, fabrication technique, and even its galvanic contacts with other existing metallic restorations.[3]

Many studies have reported on the metallic ion released from high noble, noble, and base metal dental casting alloys under different pH conditions which simulated the oral cavity.[45] It was observed that a low pH environment (i.e., acidic conditions) increases the release of metallic ions from dental alloys. This effect is especially pronounced for nickel (Ni)-based alloys.[6]

Several studies have investigated on the release of metallic ions from dental cast alloys.[6789] For many Ni-based dental casting alloys, Ni was the main element released while the other major elements such as chromium (Cr) and molybdenum (Mo) are released at much lower concentrations.[9]

In commercially available alloys, Cr and Mo range from 11–25 wt% to 0–10 wt%, respectively. In a study published recently, it was reported that the corrosion resistance of alloys is mainly influenced by Cr and Mo content. Alloys with lower amounts of Cr and Mo were reported to be more susceptible to corrosion.[10] To measure a lower corrosion rate in dental alloys, in vitro electrochemical techniques have proved to be sufficiently sensitive methods.[11] In a previous investigation, recasting has been found to affect the elemental composition, hardness, and corrosion behavior of silver-palladium alloys and the yield strength and percentage elongation of type III gold alloys.[12] However, recasting base metal alloys about twenty times, without the addition of new alloys, have not disclosed any significant change in mechanical properties.[13]

To date, no studies have investigated the effect of pH on ion release from recasted excess base metal alloys from casting sprues and buttons. The aim of this study was to quantitatively estimate the release of ions from two commercially available dental casting alloys in three different pH media for 1 week and 4 weeks.

MATERIALS AND METHODS

Two commercially available metal-ceramic dental casting alloys representing different compositions were selected for this study [Table 1]. Castings were carried out as per the manufacturer's recommendations.

Table 1

Composition and weight percentage of four commercially available casting alloys used in the study

For the specimen preparation, wax patterns were sprued and invested using phosphate-bonded investment material. The alloys were cast into specimens of area 2.028 cm2, simulating the approximate area of metallic restorations in the oral cavity. The second, third, fourth, fifth, and sixth generation specimens were prepared by casting the alloy into a rod form after which the rod was sandblasted and electropolished to remove surface impurities. These rods were then cut into small pellets of size 15 mm in length, which represents manufacturer's alloy pellet size. These procedures were repeated once, to get specimens for the second generation, and twice, three times, four times, and five times for the third, fourth, fifth, and sixth generations, respectively.

After the casting, the specimens were subjected to a metallographic polishing technique using 180–2000 grit silicon carbide abrasive paper, and final polishing was done with 0.05 μm Al2O3 suspension. Subsequently, each specimen was degreased with acetone, followed by ultrasonic cleaning with deionized water and dried under a hot air stream.[141516]

The tests were carried out with specimens immersed in solutions of 0.9% saline solution at pH 7.3 and artificial saliva at pH 6.7 and 2.3. The artificial saliva was freshly prepared using the chemicals (Merck, India) containing 0.7 g/L NaCl, 1.2 g/L KCl, 0.26 g/L Na2 HPO4, 0.33 g/L KSCN, 1.5 g/L NaHCO3, 0.2 g/L K2 HPO4, and 1.5 g/L urea and lactic acid to adjust the pH.[12] Specimens were immersed in polypropylene tubes containing 10 mL of immersion solutions of varying pH. All test tubes were stored at 37°C using humidity cabinet.

Atomic absorption spectroscopy

After 1 week or 4 weeks of immersion, quantities of metallic ions such as Ni, Co, Fe ions released into the immersion solutions were measured by atomic absorption spectroscopy (AAS) (GBC 932 Plus TIFAC, USA) (n = 3). This technique can quantify metal ions in very dilute concentrations in the range of parts per million (mg/mL). Ion release concentrations obtained from atomic absorption measurements were expressed in mg/mL. According to ISO 16744 – dentistry - base metal alloys for fixed dental restorations, the maximum quantity of released corrosion products is 1 mg/cm2/week.[17] To calculate the ion release in mg/cm2/week, amounts in mg/mL were converted to mg of mass released from the alloy per square centimeter of alloy surface exposed to the immersion solution for total ions released.

RESULTS

Two dental alloys representing different compositions and different casting groups were tested for ion release in artificial saliva at 6.7% and 2.3% and 0.9% saline solution at pH 7.3 for 1 week and 4 weeks.

Atomic absorption spectroscopy analysis

Tables 2 to 7 show the results of released ions from the two commercially available dental alloys in three different media at 1 week and 4 weeks period. For all the dental alloys in all the three media, the ion concentration of each element increased with decreasing pH and also with the storage time from 1 week to 4 weeks.

Table 2

Quantities of ion released from cobalt-chromium Wirobond C alloy in artificial saliva at pH 2.3

Table 7

Quantities of ion released from nickel-chromium Wirolloy in 0.9% saline solution at pH 7.3

The quantity of ion released for the two tested alloys was lower in 0.9% saline solution at pH 7.3 and higher in artificial saliva at pH 2.3. Among the alloys, ion release was higher with Ni-Cr Wirolloy at all the test solutions than Co-Cr Wirobond C alloy.

The quantities of ion released were found to increase with the recasting. The first-generation alloys released lower quantities of ions and the sixth-generation recasted alloys showed a higher amount of ion releases.

The amount of ion release observed with Co-Cr Wirobond C alloy is very less and may not induce any adverse biological reaction. On the other hand, 3–6 times recasted Ni-Cr Wirolloy at pH 2.3 for 1-week storage released higher quantities of metallic ions, i.e., 1.052, 1.120, 1.202, and 1.263 mg/cm2 respectively. These values were much higher in 4 weeks storage.

DISCUSSION

In this study, three artificially prepared solutions that mimicked the pH variances routinely encountered in the oral environment were selected as immersing solutions, and two commercially available dental base metal alloys representing different compositions were tested for ion release for 1 week and 4 weeks.

According to Barrett et al.,[18] the average dietary intake of Ni is 200–300 μg/day. The amounts of Ni necessary to induce allergy after a single exposure have been calculated to be 0.6–2.5 g.[8] In the present study, the quantities of Ni released from Ni-based Wirolloy in the three different immersion media were markedly below the average dietary intake, i.e. 227 μg/day up to the sixth casting at pH 2.3. Cr is an essential nutrient of human beings in amounts of 50–200 mg/day in glucose metabolism. The average dietary intake of Cr is 280 μg/day. However, chromate salts – which result from the corrosion of base metal alloys – could cause skin sensitivity and dermatitis. The incidence of Cr allergy was reported to be 10% in males and 3% in females. The estimated lethal dose for Cr in humans is about 50–70 mg/kg body weight. In the present study, the quantities of Cr released from Wirobond C and Wirolloy were also marked below the average dietary intake of 280 μg/day.

Intraoral corrosion is a very complex process. The rate of corrosion is affected by a variety of factors such as composition, metallurgical state, surface conditions, and environment.[19] A multiphase alloy is more prone to corrosion.[7] A higher percentage of Cr2O3 and MoO3 in the passive film could lead to a higher resistance to metal ion transfer through the passive film. When compared to Cr2O3, the oxide of Ni is more porous and has less protective ability to corrosion. Hence, the zones of a passive film rich in NiO will act as weak regions for localized corrosion and cause localized dissolution of Ni-rich phases.[20] Among the selected alloys in the present study, Co-Cr alloys have higher Cr concentration which helped in the formation of the highly impervious passivating layer, and hence, in the present study, Co-Cr alloys showed lower ion release than Ni-Cr alloys. As the recasting numbers increase, the quantity of ion release increases. This may be due to their change in chemical composition due to the loss of few elements during remelting procedures.

Wirolloy used in the present study contains higher amounts of Fe and carbon (C) in their composition and results show that these alloys were severely affected by artificial saliva and saline solution. At pH 2.3, all the tested alloys showed higher ion release. This was due to the acidic condition, containing lactic acid, as expected. Three to six times recasted Ni-Cr Wirolloy released higher quantities of metallic ions, at pH 2.3 for 1-week storage, i.e., above the permissible limit as recommended by ISO 16744.[17] Hence, Wirolloy is not suitable for recasting, i.e., above twice.

The dental metallic restorations invariably release metal ions into the oral environment which have the potential to interact with oral tissues. Although the amounts of metal ions released are not proportional to their relative weight content in alloys composition, they may still cause contact dermatitis and other biocompatibility problems.[19] The conventional method to determine corrosion rate is the weight loss method which requires long-term exposure due to very low corrosion rate.

The results of the present study indicate that the amount of ion release from dental base metal alloys depend not only on the composition and recasting numbers but also on the pH conditions as well as the duration of immersion time. The quantitative estimates for the ion release found to be less than the normal dietary intake for the Co-Cr alloy after recasting several times. Wirolloy after twice recasting can be shifted to industry for other applications, where the biocompatibility is not a major requirement.

CONCLUSIONS

Within the limitations of this study, the following conclusions were drawn:

The in vitro corrosion behavior of Wirobond C alloy was better than that of Wirolloy. The total ion release of Ni-Cr Wirolloy alloy after 1-week immersion in artificial saliva at pH 2.3 was approximately seven times higher than Co-Cr Wirobond C alloy

Since the amount of ion release did not increase much with recasting numbers in WBC, it can be used for recasting

The Ni-Cr Wirolloy, on the other hand, showed an increase in ion release with recasting and hence is not suitable for recasting above twice recasting.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

Table 3

Quantities of ion released from cobalt-chromium Wirobond C alloy in artificial saliva at pH 6.7

Table 4

Quantities of ion released from cobalt-chromium Wirobond C alloy in 0.9% saline solution at pH 7.3

Table 5

Quantities of ion released from nickel-chromium Wirolloy in artificial saliva at pH 2.3

Table 6

Quantities of ion released from nickel-chromium Wirolloy in artificial saliva at pH 6.7