Abstract
Objective
The aim of the study was to assess the effect of individual metallic elements within experimental Au–Pt-based dental alloys for porcelain veneering on ion release.
Methods
A binary Au–10 at% Pt alloy (AP10) was designed as a parent alloy. Six ternary AP10–X (X = In/Fe/Sn/Zn) and four quaternary (AP10–In2)–Y (Y = Fe/Sn/Zn) alloys containing oxide-forming elements, X and Y, up to 2 at% were prepared and ion release from the experimental alloys in deionized water and commercial soft drink was examined. For ion release determination samples with size 10 mm × 10 mm × 0.5 mm were immersed in 20 ml of deionized water for 5 min. Samples were then removed and immersed in 20 ml of Sprite Light ® for a further 5 min, and 2 h at 37 °C. The amounts of ions released in the test solutions were analyzed by inductively coupled plasma-mass spectrometry.
Results
When looking at individual elemental ion release, the order of the amount of dissolved ions was Fe > Zn > In > Sn. Among the base metal elements examined, Fe showed significantly higher levels of ion release than the other base metal elements for all three testing conditions ( P < 0.05). When looking at the effects of test solution on ion release from the alloys, Sprite Light ® caused significantly higher level of ion release than deionized water, with the exception of In in the ternary AP10–In1.0 and AP10–In1.7 alloys and the quaternary (AP10–In2)–Sn1.0 alloy, which showed similar or slightly greater amounts of ion release into deionized water.
Significance
Significant ion release was only observed from the Fe element. Sn and In elements showed less ion release than the Fe and Zn elements. Accordingly, Sn and In elements should be recommended as oxide-forming elements in Au–Pt-based metal–ceramic systems.
1
Introduction
Porcelain-fused-to-metal (PFM) restorations are widely used in dentistry because of their excellent clinical properties. Gold alloys for PFM restorations contain small amounts of oxide-forming elements such as In, Sn, Fe and Zn. Oxide layers, formed during the degassing process, are known to improve the bond strength between the metallic frame and the veneering porcelain . Although much of the metallic frame is covered with veneering porcelain it is usual for a small collar of alloy to be left uncovered. This is usually highly polished and partially sub-gingival. It is therefore in contact with the tissue and is also open to attack from oral fluids.
Metal ions released from dental alloys interact with metabolic pathways and cell structures causing damage . Cation release can provide inflammatory reactions and may modulate the immune response by activation or inhibition of T- and B-cells . These responses can be in the form of oral mucositis, gingivitis/periodontitis and alveolar bone resorption .
The UK adverse reactions reporting project showed that reactions to precious metals accounted for about 5% of the reactions caused by metals and the number of allergic causes attributed to metals appears to be small. Another study found that in not more than 10% of patients was allergy diagnosed as contributing to a complaint or symptom. However, metal components from almost all cast dental alloys can be detected in adjacent tissue .
The single most important property of a dental casting alloy to its biological safety is its corrosion potential . Multiple phase alloys increase the risk of elemental release . Labile elements within dental alloys have been found to be more likely to be released regardless of the alloy composition with elements such as Zn being more labile than Au . Other work has found that ion release is not generally correlated with the concentration of the individual metal in the alloy or the nobility of the alloy .
Most of the alloys used for metal–ceramic systems are high Au and Pd-free alloys based on ternary systems of Au (80–86%), Pt (10–15%) and In (1–2%) . Good biocompatibility is obtained by the inclusion of the two high content noble metal elements (Au and Pt) and strength by the In elements . Palladium-based alloys have been found to have side effects such as allergies . Many case reports exist describing Pd sensitivity and recovery after removal of dental restorations . Further, because Pd-containing dental alloys have been identified as a possible source of sensitization, the public should be protected from possible adverse effects by minimizing the use of Pd-containing alloys or the release of Pd from alloys . When looking at the elements to be investigated in this study only Zn and In have been cited in the literature as causing adverse reactions to oral mucosa . Therefore, we are attempting to develop new Pd-free Au–Pt-based high noble dental alloys for PFM restorations to avoid possible side effects caused by Pd. To make clear the effects of the inclusion of oxide-forming elements, In/Fe/Sn/Zn, on various properties of the above-mentioned Pd-free PFM alloys, we are performing systematic studies including optical properties of a series of experimental alloys .
The aim of the current study was to assess the effect of individual oxide-forming metallic elements added to experimental Au–Pt-based porcelain-fused-to-metal (PFM) alloys on ion release. The hypothesis being that the oxide-forming elements will show more ion release when compared to the Au–Pt precious metal elements and that the oxide-forming elements would show varying degrees of ion release when compared to each other.
2
Materials and methods
2.1
Sample preparation
The compositions of alloys are either expressed as weight percentage (wt%) or atomic percentage (at%). Although wt% is the more commonly used description, biological and chemical properties are best understood by knowing the at% as it better predicts the number of atoms available to be released and affect the body . Therefore, chemical compositions of the experimental alloys were designed on the atomic percentage basis in the present study. A binary Au–10 at% Pt alloy (referred to as AP10) was designed as a parent alloy. Six ternary AP10–X (X = In/Fe/Sn/Zn) alloys and four quaternary (AP10–In2)–Y (Y = Fe/Sn/Zn) alloys were designed and the amount of oxide-forming elements X and Y were restricted up to 2 at%.
All the experimental alloys were prepared from high-purity component metals (Ishifuku Metal Industry Co., Ltd., Tokyo, Japan). Appropriate amounts of component pure metals were melted in a high-frequency induction furnace and the ingots obtained were subjected to cold rolling and homogenizing heat-treatments at high temperatures using exactly the same processes used to produce commercial dental alloys. A number of plate samples with size 10 mm × 10 mm × 0.5 mm were obtained. The analyzed composition in atomic percentage of the 12 alloys used in the study can be seen in Table 1 . A commercially produced Au–Pt-based alloy BiOcclus 4 ® (DeguDent GmbH, Postfash 1364 63403 Hanau, Germany) was used as a control.
| Alloys | Au | Pt | In | Fe | Zn | Sn | Rh and Ta |
|---|---|---|---|---|---|---|---|
| AP10 | 90.1 | 9.9 | 0 | 0 | 0 | 0 | 0 |
| AP10–In1.0 | 89.1 | 9.9 | 1.0 | 0 | 0 | 0 | 0 |
| AP10–In1.7 | 88.4 | 9.9 | 1.7 | 0 | 0 | 0 | 0 |
| AP10–Fe0.8 | 89.2 | 10.0 | 0 | 0.8 | 0 | 0 | 0 |
| AP10–Fe1.9 | 88.3 | 9.8 | 0 | 1.9 | 0 | 0 | 0 |
| AP10–Zn1.7 | 88.5 | 9.8 | 0 | 0 | 1.7 | 0 | 0 |
| AP10–Sn0.9 | 89.2 | 9.9 | 0 | 0 | 0 | 0.9 | 0 |
| (AP10–In2)–Fe1.0 | 87.3 | 9.7 | 2.0 | 1.0 | 0 | 0 | 0 |
| (AP10–In2)–Fe1.7 | 86.6 | 9.7 | 2.0 | 1.7 | 0 | 0 | 0 |
| (AP10–In2)–Zn2.1 | 86.3 | 9.6 | 2.0 | 0 | 2.1 | 0 | 0 |
| (AP10–In2)–Sn1.0 | 87.3 | 9.8 | 1.9 | 0 | 0 | 1.0 | 0 |
| BiOcclus 4 | 83.3 | 10.8 | 2.8 | 0 | 1.5 | 0 | 1.6 |
All 12 alloys were then lost wax cast into square plates 10 mm × 10 mm × 0.5 mm and ground smooth. Two samples of each of the alloys were tested. The pieces of alloy were put through the oxidizing, opaque and main porcelain firing cycles as would normally be performed during a metal–ceramic restoration construction. All the square plates were then polished to a clinically acceptable state on both sides and on the edges using fine stones (Meisinger, Germany), rubber wheels (Identoflex AG, Buchs SG, Switzerland) and bristle brushes and fine lambs wool mops (C&LE Attenborough Ltd., Nottingham, UK) loaded with universal polish (yellow and green polish for precious metals, Metrodent, Huddersfield, UK) to replicate the exposed palatal/lingual gingival collars of finished restorations.
2.2
Ion release
Each alloy sample was then immersed in 20 ml of deionized water (pH value 7.0) for 5 min. The samples were then removed from the water and immersed in 20 ml of Sprite Light ® (a popular, erosive, sugar free soft drink with a pH value of 2.91–2.98, The Coca Cola Co., Uxbridge, UK) for a further 5 min or 2 h at 37 °C. Each sample was placed in a tapered centrifuge tube, so that all the surfaces were exposed to the deionized water or Sprite Light ® .
All the test solutions were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS Agilent 4500, Agilent Technologies, Santa Clara, CA 95051, USA). ICP-MS detection limits for the target elements in the 12 alloys are given in Table 2 . All the test solutions were acidified with 200 μl of nitric acid (for Au determination) or hydrochloric acid (for all other ions). For each analysis the instrument performed five measurements and calculated the mean and relative SD (%) for each element. Thus, with the two samples tested in each group, the total number of measurements recorded per element was 10. The surface area of the plates to the volume ratio of Sprite Light ® solution was 0.055 cm 2 ml −1 , which is below the range 0.5–6.0 cm 2 ml −1 recommended by the ISO standard 10933 . As no biological studies were being performed for the present alloys, ratios of our experimental sample surface area to Sprite Light ® solution volume were considered acceptable .
| Elements | Detection limits (ng l −1 ) |
|---|---|
| Pt, Zn, Sn, Rh, Ta | 1 |
| In | 3 |
| Fe | 5 |
| Au | 6 |
2.3
Statistical analysis
The results were analyzed using two-way analysis of variance (ANOVA) at the 95% confidence level ( P = 0.05). The Newmans–Kuel multiple comparison summary was used to indicate significant differences. Individual comparisons were analyzed by using a paired t -test.
2
Materials and methods
2.1
Sample preparation
The compositions of alloys are either expressed as weight percentage (wt%) or atomic percentage (at%). Although wt% is the more commonly used description, biological and chemical properties are best understood by knowing the at% as it better predicts the number of atoms available to be released and affect the body . Therefore, chemical compositions of the experimental alloys were designed on the atomic percentage basis in the present study. A binary Au–10 at% Pt alloy (referred to as AP10) was designed as a parent alloy. Six ternary AP10–X (X = In/Fe/Sn/Zn) alloys and four quaternary (AP10–In2)–Y (Y = Fe/Sn/Zn) alloys were designed and the amount of oxide-forming elements X and Y were restricted up to 2 at%.
All the experimental alloys were prepared from high-purity component metals (Ishifuku Metal Industry Co., Ltd., Tokyo, Japan). Appropriate amounts of component pure metals were melted in a high-frequency induction furnace and the ingots obtained were subjected to cold rolling and homogenizing heat-treatments at high temperatures using exactly the same processes used to produce commercial dental alloys. A number of plate samples with size 10 mm × 10 mm × 0.5 mm were obtained. The analyzed composition in atomic percentage of the 12 alloys used in the study can be seen in Table 1 . A commercially produced Au–Pt-based alloy BiOcclus 4 ® (DeguDent GmbH, Postfash 1364 63403 Hanau, Germany) was used as a control.
