Handling characteristics of gallium alloy for dental restoration

Marginal adaptation and micro-porosity of class II restorations of a high copper amalgam and a palladium-free gallium-based alloy

The aim of the current investigation was to compare the marginal adaptation and internal porosity of a gallium (Ga)-based alloy (Galloy) with a high copper amalgam (Permite C DP) when used in moderately sized conventional class II cavities. Ten dentists placed two restorations of each material in standardized class II cavities in typodont teeth set in a phantom head. The proximal surfaces of the restored teeth were subsequently examined using an optical microscope and colour photographs were taken. The teeth were then serially sectioned before being re-examined microscopically and re-photographed. Three dentists rated the photographs of the restorations on two occasions, 2 weeks apart, for marginal adaptation and internal porosity using a six and five point scoring criteria, respectively. Inter- and intra-examiner agreements were assessed with weighted kappa statistics. The Ga-based alloy exhibited inferior marginal adaptation and a significantly higher level of porosity and internal defects compared with the dental amalgam. Marginal defects were mainly concentrated at the gingival third of the proximal boxes for both alloys. The poor marginal adaptation and extensive internal porosity detected for the Ga-based alloy was attributed to the difficulty in the alloy condensation related mainly to the 'stickiness' of the alloy to the condensers and to the rapid change in the plasticity of the alloy during condensation. This could possibly be a factor in the post-operative complications reported with the clinical use of this alloy.

A 3-year longitudinal, controlled clinical study of a gallium-based restorative material

AIM

The aim of this three-year longitudinal controlled study was to compare the clinical performance of Galloy versus a high copper, mercury based Dispersalloy filling material.

METHODS

Moderate to large class I and class II cavities or replacement restorations were selected and 25 Galloy restorations and 25 Dispersalloy controls were placed in 14 adult patients by a single operator. Restorations were photographed and a silicone impression recorded at baseline, 6 months, 1 year, 2 years and 3 years.

RESULTS

At 3 years all 22 Dispersalloy restorations but only 4 Galloy restorations were still in situ. Three Dispersalloy restorations were lost to follow-up. Tooth fracture was observed in 15 (60%) of the Galloy restorations by the end of the 3 years, compared to one (4%) Dispersalloy restoration, which failed due to tooth fracture. A further six Galloy restorations had to be removed due to complaints of persistent pain. Four teeth restored with Galloy required endodontic treatment but none of the Dispersalloy restored teeth required endodontics. Of the four Galloy restorations remaining in situ, three were relatively small restorations and the fourth a moderate sized restoration required a marginal repair.

CONCLUSION

The clinical performance of Galloy restorations was so grossly inferior to the Dispersalloy controls that Galloy cannot be recommended for clinical use in moderate to large or multi-surface cavities.

A palladium-free gallium-based alloy: analysis of composition and microstructure

OBJECTIVES

To study the microstructure of a palladium-free gallium-based alloy (Galloy) before and after mixing to identify the metallic phases produced by the setting reaction and assess the quantitative elemental analysis of the phases present.

METHODS

The Galloy powder was analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM) and the particle size distribution was determined using a high resolution particle size analyzer. Cylindrical test specimens were mechanically condensed in accordance with ISO 1559: 1986 for XRD, energy dispersive spectroscopy (EDS) and SEM analyses of the set alloy. Disc shaped specimens were prepared by manual condensation to compare the width of the reaction zone with specimens condensed mechanically.

RESULTS

The Galloy powder consisted of spherical particles varying between 0.1 and 40 microm in diameter which contained a network of grooves and clefts extending deep within the substance of the particles. The reaction zone appeared 'grainy' and 'fragmented' compared with the well defined reaction zone previously observed for Pd-containing gallium-based alloys. The student t-test indicated the width of reaction zone varied significantly (P<0.001) with method of specimen fabrication.Significance. The significantly larger reaction zone in the manually condensed specimens was possibly due to failure of the condensation technique to force out excessive liquid alloy present in the triturated mass. The surface topography of the Pd-free Galloy powder particles could be responsible for the characteristically fragmented and ill-defined reaction zone observed in the set material compared with the well defined reaction zone previously observed for Pd-containing gallium-based alloys.

A 3-year clinical evaluation of a gallium restorative alloy

Several alternative materials have been suggested to take the place of amalgam, because of the environmental toxic effects of its mercury component. One such material is gallium-based alloy restoratives. The aim of this in vivo study was to compare the long-term clinical performance of a commercial gallium alloy with an admixed high copper amalgam alloy. For this purpose, 32 gallium and 32 amalgam restorations were placed in molar teeth in 14 human subjects. All the selected patients had at least two molar teeth that required restoration. In this way both restoratives were used in the same oral cavity. The restorations were examined at baseline, 6 months, 1, 2 and 3 years. At baseline, six teeth restored with gallium alloy showed post-operative sensitivity, whereas none of the amalgams were sensitive. At the end of 3 years, only a few amalgam restorations showed slight surface tarnish and marginal integrity loss. None of them needed replacement. Of the 32 gallium restorations placed, five had to be removed because of sensitivity, corrosion and tooth fracture. Also dramatic surface roughness and corrosion were noticed in 12 gallium restoration. According to the results of this clinical study, gallium-based restoratives should not be used before their physical properties are improved.

A comparison of the mechanical properties of a gallium-based alloy with a spherical high-copper amalgam

OBJECTIVES

The aim of the present study was to investigate how the mechanical properties of a palladium free gallium-based alloy (Galloy) compare with a leading spherical high-copper amalgam (Tytin).

METHODS

Cylindrical specimens were mechanically condensed, according to the ISO 1559:1986 standard, to measure compressive strength, Vickers hardness, static creep and dimensional change on setting. Disc and beam shaped specimens were manually prepared to assess the diametral tensile and flexural strengths of the investigated alloys.

RESULTS

The mean hardness, 1h compressive fracture strength, 24 h diametral tensile and 24h flexural strengths of Galloy were significantly lower (P<0.001) than Tytin. No significant differences in modulus of elasticity, creep, dimensional change on setting, 24 and 168 h compressive fracture strength for the two alloys were identified.

SIGNIFICANCE

The significant reduction in the 1 h mean compressive fracture strength and hardness identified for Galloy compared with Tytin possibly indicate a slower setting reaction in the gallium-based alloy. Manual condensation of the gallium-based alloy produced specimens with inferior mechanical properties possibly due to the increased likelihood of introducing voids within the test specimens. Previous reports indicating poor corrosion resistance and moisture sensitivity of gallium-based alloys highlight the need for further research to investigate the effect of the oral environment on the gallium-based alloy.

Long-term corrosion of a Ga-containing restorative material

OBJECTIVE

The aim was to simulate and characterize the long-term corrosion of a Ga-containing alloy (Galloy, SDI).

METHODS

To induce corrosion, cylindrical specimens, 8 x 4 mm, of the material were subject to potentiostatic polarization at -0.1 V (SCE) in a phosphated buffered saline (PBS) solution at 20 degrees C for d. The current-time transients during polarization were recorded and the corresponding anodic charge, Q, was calculated. Parallel potentiostatic corrosion tests in a Cl-free PBS solution were also conducted to demonstrate the significance of the Cl- ion in corrosion. In addition, potentiodynamic anodic polarization tests were performed to characterize the overall corrosion behavior of the alloy in both electrolytes. The external and internal corroded layers, formed during potentiostatic corrosion in PBS, were measured by optical microscopy. SEM and EDXA were used to characterize the morphology and composition of the potentiostatically polarized surfaces.

RESULTS

Galloy was passive in Cl-free PBS. The Cl- ion in PBS destroyed passivity and initiated a "dissolution-precipitation" type reaction during potentiostatic corrosion. The latter led to circumferential internal corrosion and growth of a layer of external corrosion products. The thickness of the internal and external corrosion layers was 0.77 +/- 0.07 and 0.86 +/- 0.37 mm, respectively. The Q value (89.3 +/- 13.7 C/cm2) in PBS was about two orders of magnitude higher than that (0.66 +/- 0.24 C/cm2) in Cl-free PBS. The corrosion products contained Sn, Ga, In, Cu, O and Cl.

SIGNIFICANCE

Massive internal and external corrosion in a Cl-containing medium as in saliva, accumulation of corrosion products at the cavity wall, and the consequent stress build-up contribute to post-operative pain, tooth straining, marginal breakdown and fractured teeth reported with the clinical use of Galloy.

Clinical evaluation and microstructural analysis of a direct placement gallium restorative alloy

OBJECTIVES

The objective of this study was to assess the clinical performance of a direct placement gallium alloy sealed with an established dentine adhesive system. In addition, microanalysis of a few gallium restorations that failed in clinical service was performed. Clinical factors such as pulpal sensitivity, fracture of the restoration and of the tooth, marginal deterioration, and tarnish were assessed.

METHODS

Sixty-five restorations of Galloy and 62 of Tytin (49 and 51 Class II restorations, respectively) were placed according to a predetermined scheme for randomisation in 37 patients by two operators using rubber dam isolation. For the Galloy restorations, the enamel and dentine were etched, and then sealed with PAAMA 2 dentine adhesive according to the manufacturer's instructions. After carving, PAAMA 2 was applied to the Galloy and light-cured. Cavity preparations for Tytin received no adhesive sealer. All restorations were polished at least 24 h post-operatively. Microstructural analysis of retrieved fragments of failed restorations was conducted using electron probe microanalysis.

RESULTS

At 1 year, only one Tytin restoration was found to have failed due to an isthmus fracture. The remaining restorations of Tytin were intact with no reported sensitivity. Of the 65 Galloy restorations placed, 28 had to be removed, including restorations in teeth, which were symptomatic, non-vital and/or fractured, and teeth with fractured restorations. Tarnish was present on many of the Galloy restorations. Retrieved fragments of failed Galloy restorations exhibited a dark surface at the pulpal wall interface and small cracks were observed in that surface. Internal cracks and extensive corrosion was observed using the microprobe. Gallium oxides and chlorides were identified as the predominant corrosion products.

CONCLUSIONS

The gallium alloy, Galloy, sealed with PAAMA 2 dentine adhesive system demonstrated a high clinical failure rate.

In vitro evaluation of the effect of freshly mixed amalgam and gallium-based alloy on the viability of primary periosteal and osteoblast cell cultures

The effect of freshly mixed specimens of a gallium-based alloy Galloy and a high copper spherical amalgam Tytin on the viability of primary periosteal and osteoblast cells was investigated. The cells were grown from the parietal bones of 2-3 day old Albino Wistar rats and were seeded in multi-well plates and exposed to the test materials for 1 or 6 days. The number of viable cells in each test group was determined using the Trypan blue dye exclusion test and compared with the controls. The area of cell death around the test specimens was also measured. Statistical analysis (ANOVA, Tukey's pairwise comparisons) showed a significant effect of the test materials on cell viability (P < 0.01). The viability of cultures containing Tytin was significantly lower than cultures with Galloy and the controls (P < 0.05). The viability of cultures containing Galloy was significantly lower (P < 0.05) than controls except for periosteal cells after 1 day. The culture media was analysed using atomic absorption spectrophotometry (AAS) for metal ion content. The results suggested that mercury (Hg) was the main element released from the dental amalgam followed by copper (Cu) and silver (Ag) whilst gallium (Ga) was the main element released from the gallium-based alloy followed by indium (In), Ag and Cu.

The effect of water contamination on dimensional change and corrosion properties of a gallium alloy

OBJECTIVE

This aim of this study was to determine the effect of water contamination on the dimensional change over time and the corrosion properties of a gallium-based alloy (Galloy, SDI), compared to an amalgam (Tytin, Kerr).

METHODS

Normal and water-contaminated (10 microL of dH2O during condensation) Galloy and Tytin samples were fabricated using a split metal mold and mechanical condenser. The normal samples were tested for dimensional change under dry and wet (immersed in dH2O) conditions. The water-contaminated samples were tested for dimensional change under only dry conditions. Linear measurements were made over a 7 day period using a stage-equipped dial micrometer. The corrosion potential (Ecorr), the corrosion current density (lcorr), the passive current density (lp), the protective scale formation current density (lps) and potential (Eps), and the breakdown potential (Ebd) were obtained from the polarization curves generated in oxygenated saline solution. Single factor ANOVA and Duncan's multiple range tests were used to determine differences between the various samples.

RESULTS

The water-contaminated Galloy samples contracted to a minimum of -3.42 +/- 1.73 microns cm-1 in 5 h followed by expansion to a maximum of 22.56 +/- 3.90 microns cm-1 at the end of 7 days. Normal Galloy and Tytin samples and water-contaminated Tytin samples exhibited rapid contraction during the first 24 h, followed by minimal change over the next 7 days. The Ecorr of normal and water-contaminated Galloy samples was more active than that of the normal and water-contaminated Tytin samples. The lcorr of the normal and water-contaminated Galloy samples was three orders of magnitude greater than the lcorr of normal and water-contaminated Tytin samples. The Eps for the water-contaminated Galloy and Tytin samples was more noble than for the normal Galloy and Tytin samples. The Ebd for the Tytin and water-contaminated Tytin samples was greater than the corresponding values for the Galloy and water-contaminated Galloy samples.

SIGNIFICANCE

The gallium-based alloy exhibited expansion if contaminated with water during the condensing and setting process. Post-setting exposure to water did not result in expansion of the gallium-based alloy. The alloy also exhibited a greater susceptibility to corrosion than the amalgam. Due to the possibility of delayed expansion, this material should be used cautiously, particularly in applications involving weakened tooth structure.

Marginal leakage of Gallium Alloy root-end fillings: an in-vitro assessment

The sealing ability of amalgam and Gallium Alloy Gallium Filling (GF) root-end fillings was evaluated in vitro using a highly uniform collection of sheep incisor roots. Following ultrasonic canal debridement and orthograde obturation with gutta-percha and sealer, root-end cavities were prepared in 100 roots and filled with amalgam (50 teeth) or Gallium Alloy GF (50 teeth), Twenty-five teeth from each group were subjected to immediate dye leakage assessment under vacuum conditions with methylene blue dye (2%), pH 7. Linear dye penetration was measured following longitudinal splitting. The other 25 teeth from each group were incubated in Ringer's solution for 12 weeks before leakage assessment by the same method. Control teeth were included in each component of the study. Mean linear dye penetration was: amalgam--5.17 mm at baseline, 2.33 mm after 12 week's incubation; Gallium Alloy GF--2.21 mm at baseline, 1.41 mm after 12 week's incubation. The apical marginal seal of both materials improved significantly following storage in Ringer's solution (P<0.001). Gallium Alloy GF provided a better apical seal than amalgam, both at baseline and following storage (P<0.001). Subjective evaluation of the general handling characteristics of Gallium Alloy GF revealed that it was a more difficult material to manipulate than amalgam, largely because of its wetting ability and consequent adhesion to dental instruments.

The physical properties of a gallium alloy restorative material

OBJECTIVES

This study was conducted to compare the physical properties of a gallium alloy restorative material and a widely used mercury-containing dental amalgam.

METHODS

Although the specimens were not prepared according to ISO specifications, specimens of both materials were subjected to ISO standard (ISO 1559; 1986) tests. The results were analyzed by ANOVA, Mann-Whitney U test and Student's t-test.

RESULTS

The gallium alloy showed setting expansion which was greater than that for the other amalgam and greater than the upper limit set by the ISO standard. For other properties, gallium alloy performed as well as, and in the case of creep, was superior to that of the amalgam.

SIGNIFICANCE

The gallium alloy proved difficult to manipulate even when using the PTFE coated instruments recommended by the manufacturer. It is suggested that while the material may prove to be a viable alternative to conventional dental amalgam, a considerable improvement in its handling characteristics would be required before it would gain widespread acceptance for clinical use.