Les matériaux de restauration au gallium 

[ Introduction | Handling Characteristics | Physical Properties and Corrosion Resistance | Biocompatibility | Clinical Performance | Conclusions | Acknowledgements | References ] 

Introduction

The search for alternative direct-filling materials to replace silver amalgam, is intensifying. Despite assurances of relative safety, there is continuing controversy over the use of a restorative material containing mercury, and concern about the discharge of amalgam waste.

Tooth-colored resin-composite materials are viable alternatives in many situations, particularly for the post-fluoride dentition. However, they do not provide the necessary handling qualities, durability and longevity required as a replacement for silver amalgam in may posterior situations.

A metallic alternative containing gallium, instead of mercury, was suggested as early as 1928 in Germany, and has been under development since 1956. The Japanese government, sensitive to environmental mercury after the 1953 Minimata incident, when methyl mercury was discharged and contaminated local fish, approved gallium alloys as dental materials in 1990. The first commercial alloy, Gallium Alloy GF (Tokuriki Honten), was produced in Tokyo.

This liquid gallium alloy, composed of gallium, indium and tin, replaced mercury and was triturated with a spherical alloy powder similar in composition to that used for amalgam restorations, with the addition of 9% palladium. The resulting plastic mix was essentially similar to amalgam and could be condensed, carved and allowed to harden in a like manner. A major problem with this early product was extreme corrosion. Clinical studies showed substantial sensitivity, discoloration, surface roughness and marginal breakdown, within a short time period.

One study¹ documented 36% failure within eight months due to a combination of restoration fracture, tooth fracture and cracked teeth. An improved alloy, GF II, with a palladium content reduced to 2%, has since been developed, and recently, a new alloy called Galloy (Southern Dental Industries, Australia) of similar composition, was introduced. This was approved for sale in Canada and received FDA approval in the United States in 1995. Galloy uses a spherical, high-copper alloy powder, identical in composition to the company’s own silver amalgam powder for the alloy Lorvic. The liquid portion is a gallium alloy containing gallium (62%), indium (25%), tin (13%) and bismuth (0.05%).

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Handling Characteristics

Gallium alloys have been reported to be more difficult to handle because they adhere to the walls of the mixing capsule and instruments. The manufacturer of Galloy developed a novel mixing and delivery capsule which allows trituration and dispensing directly into the cavity preparation. Despite this, intra-oral insertion has its problems. Special condensers are recommended to avoid adherence of the alloy to packing instruments. Alcohol on the condenser tip has also been suggested.

In most other respects, the handling characteristics of gallium alloys are somewhat similar to those of a spherical amalgam. However, clinical use is extremely technique sensitive and any moisture contamination during placement has potentially serious consequences due to the resulting alloy expansion.

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Physical Properties and Corrosion Resistance

The mechanical properties of current gallium alloys are equal or superior to those of the high-copper amalgams when tested against the parameters of the International Standards Organization (ISO). For gallium alloys, all parameters tend to meet or exceed the requirements for silver amalgam. The compressive strength of Galloy is equivalent to that of Dispersalloy, while that of the spherical amalgam Tytin is higher at early test times. Diametral tensile strength, a more important factor in clinical fracture resistance, has been documented to be higher for gallium alloys.

The creep resistance of the current gallium alloys also complies with ISO requirements. Although this property may have an impact on the rate of marginal breakdown of silver amalgam, there is substantial debate about whether corrosion is a more important factor. Gallium alloys show considerably more corrosion^1,2 than the current high-copper silver amalgam alloys, which have benefited from many decades of intense research and refinement. Current high-copper zinc containing alloys, have the highest resistance to corrosion, with a 13-year survival rate of 85%, according to a recent analysis combining the results of 14 independent clinical trials.³

Under dry conditions, current gallium alloys appear to comply with traditional testing requirements for dimensional change during setting. However, moisture contamination causes marked expansion of the restoration. Broom et al⁴ reported that water contamination of Galloy during condensation produced dramatic dimensional changes. This resulted in a directive from the manufacturers for strict isolation of the operative field during clinical placement and use of a hydrophobic resin coating above and below the restoration. Excessive expansion of a setting gallium alloy can produce stresses sufficient to crack the tooth. Even exposure to external fluids up to three days after condensation, has shown expansion effects in a recent photoelastic stress study.⁵

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Biocompatibility

The major controversy concerning silver amalgam usage emerged after animal studies documented mercury release from restorations. This release is likely caused by disturbance of the surface passivation layer of the amalgam restoration during tooth-brushing and chewing.

Gallium alloys are very prone to corrosion, with the release of large amounts of the element gallium, as well as other metal ions. Immersion tests of various gallium alloys in dilute salt solutions and artificial saliva, shows substantial dissolution of gallium. Gallium alloys will be subjected to the same scrutiny as silver amalgam, regarding element release, absorption, excretion and body burden. Not a great deal is known about the disposition of elemental gallium. Environmental exposure is currently negligible as it has a low and diffuse distribution in the earth’s crust. In 1988, it was reported that the world production of gallium was in the order of approximately 20 tons per year.

Cytotoxicity testing is one of the most fundamental tests for biocompatibility and is one of the first tests performed on a new dental material. In one study using mercury, gallium and indium nitrates, gallium and indium nitrate had no cytotoxic effect on mitochondrial activity of mouse fibroblasts. Another study examined the effects of corrosion products. In the short term, the gallium alloy demonstrated a lower cytotoxicity than the amalgam. After 10 weeks, gallium and silver amalgam displayed similar cytotoxicity. Overall, the cytotoxicity of gallium alloys was found to be less than or similar to that of amalgams.⁶

The human pharmacokinetics of gallium show substantial differences from those of mercury. Gallium is not a normal constituent of body tissues though it may be present in bone for which it has a predilection, in trace amounts. Gallium has a low vapor pressure and is not absorbed through the lungs, nor through the skin. It is however, retained in the lung alveoli. Although the main route of excretion is through the kidneys, the chief storage depot of gallium is bone. Gallium citrate shows slow absorption when placed subcutaneously, with 30-50% eventual urinary excretion and 30-48% skeleton retention.

Gallium has been used in medicine as a tumor scanning indicator and even to treat tumors, due to its ability to disrupt cellular metabolism. It has also been used to treat hypercalcaemia in malignancies, by decreasing calcium content in blood and increasing it in bone, with no evident toxicity to bone cells. Gallium dental alloys have the advantage of not containing mercury but, as stated by Hero and Okabe⁷ in a 1994 review, their biocompatibility is still a controversial matter and more studies are needed. Similarly, more information is needed on the potential environmental impact of gallium, should usage increase significantly.

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Clinical Performance

Few clinical studies have been conducted. One, two and three-year results of a pilot clinical study of the gallium restorative alloy, Galloy, in modestly sized Class I restorations, were published by Osborne and Summitt.⁸ A resin sealant was placed above and beneath the restoration to prevent moisture from contacting the restorative material before total set. Tarnish and corrosion were very evident at one year, but the restorations were judged clinically adequate. By two years, 25 of the original 27 restorations were evaluated in eight patients. All were intact and functioning, but 45% exhibited tarnish and 60% had surface corrosion.

One lingual cusp of a maxillary premolar fractured at 22 months. Although this occurrence was isolated, it was noted that the fractured cusp was next to an extremely small Class I restoration and that this particular restoration was sealed with a dentin bonding agent, not the company’s recommended hydrophobic sealer. Patients with existing silver amalgam restorations in the same mouth did not report any galvanic activity. After three years, two more teeth had experienced fracture and the restorations were replaced. The remaining functioning restorations demonstrated a high incidence of tarnish and corrosion.

Use of the same gallium restorative in larger restorations has proved disastrous. In one study,⁹ routine moderate to large Class II restorations, were placed in 48 molars by dentists specially trained in the use of Galloy. Technique followed all manufacturers’ recommendations, including a resin sealant for moisture protection. By 15 months, 60% of the Galloy restorations had failed. Of these, 44% of teeth had fractured and 8% of restorations. A further 4% required endodontic therapy and 4% had to be replaced due to persistent post-operative sensitivity.

The control silver amalgam restorations showed a failure rate of 6.4%, all due to restoration fracture. The authors concluded that the clinical performance of the gallium alloy was unacceptable. Similar results have been reported in a clinical study conducted in Singapore.

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Conclusions

Gallium restorative alloys have been developed in an attempt to provide a mercury-free metallic direct restorative material. When placed with meticulous attention to moisture control, small restorations can function reasonably well in short-term clinical trials. However, tarnish and corrosion are pronounced and the long-term effects of this are unknown.

Sealing of restorations, before and after placement, with a hydrophobic resin sealant is mandatory to prevent excessive expansion, post-operative sensitivity and tooth fracture. Even with such sealing procedures, recent clinical studies have shown a totally unacceptable level of untoward clinical sequelae. This demonstrates the importance of thorough in-vitro analysis and appropriate clinical trials before market distribution.

While the profession requires a less technique-sensitive alternative to silver amalgam than resin composite, for difficult and larger posterior situations, gallium alloys in their present state of development do not provide the answer. Indeed, the manufacturer of Galloy has voluntarily withdrawn the product from the market, pending reformulation and development of new delivery systems.

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Acknowledgements

Acknowledgment: The contribution of Dr. Ileana Martino, a former MRC summer research student is gratefully acknowledged.

Dr. Dorothy McComb is professor and head, Restorative Dentistry, Faculty of Dentistry, University of Toronto.

Reprint requests to : Dr. Dorothy McComb, Restorative Dentistry, Faculty of Dentistry, University of Toronto, 124 Edward St., Toronto, ON M5G 1G6.

The author has no declared financial interest in any company manufacturing the types of products mentioned in this article.

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