Abstract
Objectives
Nickel chromium is widely used as a restorative material in dentistry but its biocompatibility is of concern as there are reports of patients suffering adverse effects caused by exposure to nickel-based restorations. The aim of this work was to quantify the amount of nickel released into solution from commercially available nickel-based alloys with varying compositions and to identify the potential use of thin films in further understanding the role of chromium in reducing nickel release.
Methods
Six commercially available nickel-based alloys were cast using the lost wax technique. Nickel chromium thin films were deposited onto silicon substrates by ion beam assisted physical vapor deposition. Both types of alloys were immersed into solution representative of saliva at pH 5 for 7 days. The amount of nickel released into solution was quantified by graphite-furnace atomic absorption spectrophotometry.
Results
The amount of nickel released from commercially available cast alloys did not correspond to the amount of nickel within the alloy. The total amount of chromium and molybdenum within the alloys proved to be of greater influence. Increasing the amount of chromium in the thin film alloys reduced the amount of nickel released but this was not linear.
Significance
Differences in the composition of commercial cast alloys highlighted the importance of other elements within the alloy on reducing the amount of nickel released. The use of thin film alloys can be useful in further understanding how the composition of an alloy can affect the amount of nickel released.
1
Introduction
Alloys are widely used as dental restorative materials because of their physical and mechanical properties. There are several different commercially available alloys used in dentistry. Nickel (Ni) based alloys are mostly used to form copings that support porcelain when constructing a porcelain-fused-to-metal crown or bridge work and is widely used by the National Health Service in the UK. Although Ni is commonly found in our daily diets there are concerns surrounding its biocompatibility in the oral cavity, so much that Sweden’s National Board of Health and Welfare has advised that no dental material containing over 1% Ni should be placed into a patients’ mouth . There are a high proportion of people who are sensitized to nickel and it has been found that up to 20% of females and 2% of males are allergic to Ni . Reports do show incidences of sensitivity that relate to Ni restorations placed in the mouth . This highlights the importance in understanding variables that could reduce the amount of Ni released into the oral environment.
A number of works have been published investigating the amount of Ni released from Ni-based alloys under simulated oral conditions . Most previously published investigations are restricted to using commercially available alloys and, as a result, are limited to the number of compositions available. Most Ni-based dental alloys contain between 60 and 84 wt% Ni with chromium (Cr) as the second highest component, which is usually between 8 and 26 wt% .
A useful technique in developing surfaces of varying concentrations is that of sputtering by an ion beam ( Fig. 1 ). This allows thin films of bulk materials to be grown on a substrate material under vacuum. Energetic ions bombard a target material and have enough energy to remove surface atoms, which are then deposited onto a substrate to form a thin film with composition representative of the target material . The target comprises two adjacent plates, one of Ni and one of Cr ( Fig. 2 a ). By changing the position of the target material relative to the ion beam, a thin film of any desirable composition can be deposited. This allows surfaces to be created with a wide range of composition that could not easily be obtained using conventional casting procedures . In order to achieve a larger area of thin film with reproducible composition, Ni strips can be used as a grid formation over a Cr plate ( Fig. 2 b). A previous study found that comparing a deposited thin film to a cast alloy with nominally the same composition showed that they had different microstructure but this did not affect their corrosion properties .
The first aim of this study was to compare the amount of Ni released from commercially available Ni-based dental alloys of varying composition during exposure to a solution representative of saliva. The second aim was to use the ion beam sputtering technique to create NiCr thin film alloys and to quantify the amount of Ni released during exposure to the same treatment solution as used for the cast alloys. This information will allow us first to show if the thin film alloys of specified compositions are representative of ion release from bulk cast alloys of similar composition. Once this is established we can then use thin film coatings to enable us to further understand the effectiveness of Cr, as well as other elements present in commercial dental alloys, in reducing the amount of Ni release.
2
Materials and methods
2.1
Preparation of test pieces
Test pieces were prepared from six different commercially available Ni-based dental alloys ( Table 1 ) following the same method as used in dental laboratories. The test pieces were initially prepared in wax (Metrodent Limited, West Yorkshire, UK) and measured 20 mm × 10 mm × 1.4 mm each. A maximum of five wax test pieces were placed into a single casting ring and positioned so that they were approximately 10 mm from the top of the casting ring, as this is where optimum heat is achieved. In order to minimize casting defects, the inside of the ring was lined with finoflask casting ring liner (Fino, Bad Bocklet, Germany) to buffer the effect of inevitable expansion during setting of the investment material. The wax was painted with a tension release agent, Sheramaster (Shera, Lemförde, Germany) to allow the investment material to freely flow over the wax. Phosphate bonded investment material (Sherafina-Rapid, Shera, Lemförde, Germany) was prepared following the manufacturers guidelines and this was carefully poured over the test pieces. The investment material was left to set under ambient conditions for 20 min. Following this, the top layer of the investment material was removed to expose a clean surface and the casting ring immediately placed into a cold furnace. The temperature of the furnace was gradually heated up to 800 °C over 2 h. This allowed the wax to completely burn away providing a mould for the metal to be cast. The casting was carried out using an induction-casting machine (Modular III, ASEG Galloni, Milan, Italy). After casting, the casting ring was left to cool under ambient conditions prior to thorough removal of the investment material. Any surface defects were removed using a series of grinding burs before being polished with silicon carbide paper with a grit size of 15 μm under a weight of 9.1 kg. The six different alloys used, as well as their compositions are highlighted in Table 1 .
| Alloy | Ni (wt%) | Cr (wt%) | Mo (wt%) | Mn (wt%) | Nb (wt%) | Si (wt%) | Other (wt%) |
|---|---|---|---|---|---|---|---|
| HeraniumNA (Heraeus Kulzer, UK) | 59.3 | 24 | 10 | <2 | <2 | <2 | Fe < 2, Ta < 2 |
| Wiron99 (Bego, Bremen, Germany) | 65 | 22.5 | 9.5 | – | 1.0 | 1.0 | Fe 0.5, Ce 0.5, C 0.02 |
| VeraSoft (AalbaDent, CA, USA.) | 53.6 | 14.5 | – | 19.5 | – | 1.6 | Cu 9.5, Al 1.6 |
| VeraBond2V (AalbaDent, CA, USA.) | 71.85 | 12.8 | 9 | – | 4 | 0.5 | Al 2.5, Ti 0.35 |
| Wirolloy (Bego, Bremen, Germany) | 63.5 | 23 | 3 | 0.5 | – | 1 | Fe 9 |
| Remanium G-Soft (Dentaurum, Ispringen, Germany) | 66 | 26.5 | 5 | < 1 | – | 1.5 | Fe, C, B < 1 |
Argon (Ar) ions at 1.25 kV were used to sputter and deposit thin film samples with nominal composition onto a mirror-finish Silicon (Si) substrate (p-type, 〈1 0 0〉, Montco Silicon Technologies Inc., PA, USA). Pure Ni and Cr (Goodfellows, Cambridgeshire, UK) were used as target materials. The amount of each metal, as well as the geometry of the target, was adjusted to obtain films of varying composition. The Si substrate measured approximately 100 mm × 20 mm and was positioned at an angle of 60° and approximately 150 mm away from the target in a way that optimized the amount of sputtered atoms that would reach the substrate. The deposition time was a minimum of 2 h as this was sufficient to ensure that the substrate surface was completely covered. Each substrate was cut into individual test pieces and their composition determined using scanning electron microscopy coupled with an energy dispersive X-ray detector (SEM-EDX) (Oxford Instruments, Bucks, UK; Jeol, Herts, UK). The film was required to be thick enough so that the EDX beam would not penetrate into the substrate causing Si interference.
2.2
Immersion of test pieces
Phosphate buffered saline (PBS) solution was used as the immersion solution (0.4 g L −1 NaCl, 0.4 g L −1 KCl, 0.218 g L −1 KH 2 PO 4 , 1.192 g L −1 NaHPO 4 ) and the pH was adjusted to pH 5 using lactic acid. A sterile environment was created by autoclaving the PBS at 121 °C for 20 min and burning alcohol on the surface of each test piece prior to immersion into 4 mL of the PBS solution. The test pieces were positioned so that they were completely submerged with both sides exposed to the solution. All test pieces were agitated at 37 °C for 7 days in a sealed container.
2.3
Elemental analysis
After 7 days the test pieces were removed from the PBS solution and aliquots of the solution were analyzed to determine the amount of Ni released using a Varian Spectra AA 400 Series Zeeman Graphite-Furnace Atomic Absorption Spectrophotometer (GFAAS) (Varian, Oxford, UK). In some cases it was required that the solution was diluted to within the working range of the GFAAS (<250 ng mL −1 ). The value obtained in ng mL −1 was converted into ng of Ni released per cm 2 of sample area.
2.4
Statistical analysis
Significant differences between the amounts of Ni released from each cast alloy were calculated using the Mann–Whitney U test as this does not make assumptions about homogeneity of variances or normal distribution of the data sets. Significantly different values are quoted when α = 0.05.
2.5
Surface characterization
Each of the cast alloys were electrolytically etched with 10% oxalic acid and viewed under SEM to identify its grain structure. A comparison of the distribution of elements throughout the Heranium cast alloy test piece and the thin film alloy was determined using EDX mapping.
2
Materials and methods
2.1
Preparation of test pieces
Test pieces were prepared from six different commercially available Ni-based dental alloys ( Table 1 ) following the same method as used in dental laboratories. The test pieces were initially prepared in wax (Metrodent Limited, West Yorkshire, UK) and measured 20 mm × 10 mm × 1.4 mm each. A maximum of five wax test pieces were placed into a single casting ring and positioned so that they were approximately 10 mm from the top of the casting ring, as this is where optimum heat is achieved. In order to minimize casting defects, the inside of the ring was lined with finoflask casting ring liner (Fino, Bad Bocklet, Germany) to buffer the effect of inevitable expansion during setting of the investment material. The wax was painted with a tension release agent, Sheramaster (Shera, Lemförde, Germany) to allow the investment material to freely flow over the wax. Phosphate bonded investment material (Sherafina-Rapid, Shera, Lemförde, Germany) was prepared following the manufacturers guidelines and this was carefully poured over the test pieces. The investment material was left to set under ambient conditions for 20 min. Following this, the top layer of the investment material was removed to expose a clean surface and the casting ring immediately placed into a cold furnace. The temperature of the furnace was gradually heated up to 800 °C over 2 h. This allowed the wax to completely burn away providing a mould for the metal to be cast. The casting was carried out using an induction-casting machine (Modular III, ASEG Galloni, Milan, Italy). After casting, the casting ring was left to cool under ambient conditions prior to thorough removal of the investment material. Any surface defects were removed using a series of grinding burs before being polished with silicon carbide paper with a grit size of 15 μm under a weight of 9.1 kg. The six different alloys used, as well as their compositions are highlighted in Table 1 .
| Alloy | Ni (wt%) | Cr (wt%) | Mo (wt%) | Mn (wt%) | Nb (wt%) | Si (wt%) | Other (wt%) |
|---|---|---|---|---|---|---|---|
| HeraniumNA (Heraeus Kulzer, UK) | 59.3 | 24 | 10 | <2 | <2 | <2 | Fe < 2, Ta < 2 |
| Wiron99 (Bego, Bremen, Germany) | 65 | 22.5 | 9.5 | – | 1.0 | 1.0 | Fe 0.5, Ce 0.5, C 0.02 |
| VeraSoft (AalbaDent, CA, USA.) | 53.6 | 14.5 | – | 19.5 | – | 1.6 | Cu 9.5, Al 1.6 |
| VeraBond2V (AalbaDent, CA, USA.) | 71.85 | 12.8 | 9 | – | 4 | 0.5 | Al 2.5, Ti 0.35 |
| Wirolloy (Bego, Bremen, Germany) | 63.5 | 23 | 3 | 0.5 | – | 1 | Fe 9 |
| Remanium G-Soft (Dentaurum, Ispringen, Germany) | 66 | 26.5 | 5 | < 1 | – | 1.5 | Fe, C, B < 1 |
Argon (Ar) ions at 1.25 kV were used to sputter and deposit thin film samples with nominal composition onto a mirror-finish Silicon (Si) substrate (p-type, 〈1 0 0〉, Montco Silicon Technologies Inc., PA, USA). Pure Ni and Cr (Goodfellows, Cambridgeshire, UK) were used as target materials. The amount of each metal, as well as the geometry of the target, was adjusted to obtain films of varying composition. The Si substrate measured approximately 100 mm × 20 mm and was positioned at an angle of 60° and approximately 150 mm away from the target in a way that optimized the amount of sputtered atoms that would reach the substrate. The deposition time was a minimum of 2 h as this was sufficient to ensure that the substrate surface was completely covered. Each substrate was cut into individual test pieces and their composition determined using scanning electron microscopy coupled with an energy dispersive X-ray detector (SEM-EDX) (Oxford Instruments, Bucks, UK; Jeol, Herts, UK). The film was required to be thick enough so that the EDX beam would not penetrate into the substrate causing Si interference.
2.2
Immersion of test pieces
Phosphate buffered saline (PBS) solution was used as the immersion solution (0.4 g L −1 NaCl, 0.4 g L −1 KCl, 0.218 g L −1 KH 2 PO 4 , 1.192 g L −1 NaHPO 4 ) and the pH was adjusted to pH 5 using lactic acid. A sterile environment was created by autoclaving the PBS at 121 °C for 20 min and burning alcohol on the surface of each test piece prior to immersion into 4 mL of the PBS solution. The test pieces were positioned so that they were completely submerged with both sides exposed to the solution. All test pieces were agitated at 37 °C for 7 days in a sealed container.
2.3
Elemental analysis
After 7 days the test pieces were removed from the PBS solution and aliquots of the solution were analyzed to determine the amount of Ni released using a Varian Spectra AA 400 Series Zeeman Graphite-Furnace Atomic Absorption Spectrophotometer (GFAAS) (Varian, Oxford, UK). In some cases it was required that the solution was diluted to within the working range of the GFAAS (<250 ng mL −1 ). The value obtained in ng mL −1 was converted into ng of Ni released per cm 2 of sample area.
2.4
Statistical analysis
Significant differences between the amounts of Ni released from each cast alloy were calculated using the Mann–Whitney U test as this does not make assumptions about homogeneity of variances or normal distribution of the data sets. Significantly different values are quoted when α = 0.05.
2.5
Surface characterization
Each of the cast alloys were electrolytically etched with 10% oxalic acid and viewed under SEM to identify its grain structure. A comparison of the distribution of elements throughout the Heranium cast alloy test piece and the thin film alloy was determined using EDX mapping.
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