Research on dental amalgam: 1982-1986

The effects of amalgam contamination and different surface modifications on microleakage of dentin bonded to bulk fill composite when using different adhesive protocols

BACKGROUND

To evaluate the effect of amalgam contamination, different surface treatments, and adhesive protocols on dentin microleakage to bulk-fill composite resin material.

METHODS

Forty teeth were fixed in (polyvinyl siloxane) PVS molds, and the Class II cavities were placed on mesial and distal aspects. Thirty teeth were restored by amalgam and thermocycled to 10,000 cycles (5 and 55 °C, 30-s dwell time). The rest were restored with Filtek one Bulk Fill composite without amalgam predecessor. Samples were divided into: G1 (dentin pretreated with 2% chlorhexidine gluconate), G2 (0.5 mm of dentin was removed), G3 (no surface modification), and G4 (control, where composite was bonded to sound dentin without amalgam predecessor.). Single Bond Universal Adhesive system was used to bond the composite material, by using the etch-and-rinse protocol in the mesial cavity preparation and self-etch protocol in the distal. Specimens underwent thermocycling for 5000 cycles, then embedded in silver nitrate and sectioned for stereomicroscope examination. Descriptive statistics, Mann-Whitney U test, and Kruskal-Wallis test were used to analyze the results at p < 0.05.

RESULTS

The highest microleakage score values (4.00) were found in the G2, and G4 in etch-and-rinse protocol. While the lowest scores were found in G2 when using self-etching protocol (1.5). Lower microleakage values were associated with the chlorhexidine treatment group for both adhesive protocols. No significant differences were found between amalgam contaminated and non-contaminated groups.

CONCLUSIONS

Amalgam contamination did not affect microleakage. Self-etching adhesive protocol significantly reduced microleakage for all groups irrespective of the surface treatment. Chlorhexidine pretreatment improved microleakage for both adhesive protocols but had no significant effect.

Influence of Intracanal Irrigants on Coronal Fracture Resistance of Endodontically Treated and Bleached Teeth: An In vitro Study

Background:

Irrigation has a key role in the success of endodontic treatment. Intracanal irrigant solutions have adverse effects on the physical properties of dentin.

Aim:

The present study aimed to evaluate the effect of different irrigation protocols on coronal fracture resistance of endodontically treated teeth undergoing bleaching treatment.

Design and Materials and Methods:

Access cavities were prepared in 120 maxillary premolars which were divided into two groups (n = 60) – Group A: nonbleached, Group B: bleached (B). Each group was subdivided into five subgroups based on irrigation protocol (n = 12); G1: normal saline (NS), G2: 2.5% sodium hypochlorite (NaOCl), G3: 10% citric acid (CA), G4: 17% ethylene diamine tetra acetic acid, and G5: NaOCl plus CA. In Group B, the teeth were bleached using 38% hydrogen peroxide and 20% carbamide peroxide gels as in-office and at-home bleaching techniques for 3 weeks. All the teeth were restored with composite resin, thermocycled, and incubated for 24 h. The specimens underwent fracture resistance tests. Data were analyzed with ANOVA, Tukey honestly significant difference test, t-test, and Chi-squared test (α =0.05).

Results:

T-test showed significant differences between each two corresponding subgroups (P < 0.0001). In Group A, NS demonstrated significantly higher fracture resistance compared to others; however, minimum fracture resistance recorded in G2. In Group B, the maximum fracture resistance was recorded in G1, with the minimum being recorded in G5. Samples irrigated with NaOCl and NaOCl plus CA exhibited significantly lower fracture resistance compared to NS subgroup (P < 0.05).

Conclusions:

Within the limitations of this study, it can be concluded that the irrigation protocol used during endodontic treatment with/without bleaching can affect the coronal fracture resistance.

An in vitro Evaluation of Microleakage of Posterior Teeth Restored with Amalgam, Composite and Zirconomer - A Stereomicroscopic Study

BACKGROUND

Numerous restorative materials are being used in dentistry to achieve adequate strength and restore aesthetics. However, a perfect ideal restorative material has still eluded dentist. Dental amalgam is versatile material with self-sealing property, but is unaesthetic. Other restorative materials like, composites require conservative preparation, but exhibits polymerisation shrinkage resulting in microleakage. To overcome these drawbacks a high strength restorative material reinforced with ceramic and zirconia fillers known as zirconomer has been introduced. The aim of this study was to evaluate the micro-leakage of these three different restorative materials.

MATERIALS AND METHODS

Thirty non-carious human permanent first and second molars were utilized in this study. Class I cavities were prepared on the occlusal surface; cavities were then restored with amalgam, composite and zirconomer as per manufacture's instruction. All samples were stored for 24 hours in distilled water followed by thermocycling. The entire tooth surface was painted with two coats of varnish to within 1mm of the restoration margins. The teeth were immersed in dye. Teeth were sectioned and observed under stereomicroscope.

RESULTS

In this study the zirconomer exhibited the highest micro leakage as compared to composite and amalgam but composite having higher micro leakage as compared to amalgam and lower micro leakage as compared to zirconomer.

CONCLUSION

Even though composite and amalgam are being marketed aggressively and new material like zirconomer are on origin, amalgam still proves to be one of the best materials.

Determinants of quality in operative dentistry

The definition of quality in operative dentistry has often, at least in part, been related to how well a cut preparation compares with an ideal preparation. The ideal preparation follows well-defined design principles. These design principles have their roots in empirical dentistry and scientific evaluations, the latter often being conducted in vitro. Because of the complexity of following these design principles practically, a large portion of dental education consists of perfecting cavity preparations. By focusing on how to cut these cavity preparations as closely as possible to the ideal preparation, dentists with high psycho-motor skills have been able to provide the public with restorative procedures of high standards over the years. However, because of the tendency of relating quality in operative dentistry to the ideal preparation, we found it justifiable to review the literature dealing with the cavity design principles of the Class II amalgam preparation. What triggered this review was a request from the International Dental Federation (FDI) to start a process leading to a scientifically based quality definition of dental restorations, a definition that determines how different factors, including cavity design principles, affect the longevity of both tooth and restoration. From our review, we conclude that patient response and restoration performance over time, rather than how closely a cavity preparation compares with the ideal preparation, will be of more significance in determining the longevity of a Class II amalgam restoration.

Corrosion of amalgams under sliding wear

OBJECTIVES

During mastication, dental amalgams are simultaneously subjected to corrosion by the oral environment and to a sliding-wear process by biting forces. In the present study, the effect of sliding wear on the corrosion behavior of two high-copper dental amalgams was investigated.

METHODS

An experimental apparatus was utilized that allows electrochemical testing under sliding-wear conditions. Corrosion potential measurements and anodic polarization scans were conducted in 0.1 M NaCl solution under sliding wear to characterize the behavior of two commercial, high-copper, single composition dental amalgams. In addition, long duration tests were conducted to assess possible corrosion and wear synergistic effects.

RESULTS

The results showed that sliding wear caused a sharp reduction in the corrosion potential, a significant increase in the corrosion rate and a decrease in the repassivation rate of both amalgams. These effects are due to the mechanical removal by the wear process of the surface protective film formed on dental amalgams. The simultaneous action of sliding wear and corrosion can also induce embrittlement that leads to cracking.

SIGNIFICANCE

The present evidence suggests that this cracking may be one of the major contributors to marginal failures of dental amalgam restorations.

Setting reactions in dental amalgam. Part 1. Phases and microstructures between one hour and one week

The literature on the setting mechanisms of dental amalgams made from powders of silver-rich alloys of tin and/or copper has been critically reviewed. In Part 1 of the review, the microstructure and phase content of recently set amalgams are described. The composition, morphology, and location of product phases are emphasized, since these features are clues to the setting reaction. Thus, Part 1 provides the background needed to understand the kinetics of the setting reactions, which is the topic of Part 2 of the review.

Effect of mercury on corrosion of eta' Cu-Sn phase in dental amalgams

OBJECTIVES

The eta' Cu-Sn phase is the most corrosion-susceptible major component of the microstructure in high-copper dental amalgams, even though metallurgically prepared specimens of the phase are corrosion-resistant. The purpose of this study was to determine if mercury is the cause of the degradation.

METHODS

Specimens of metallurgically prepared eta' Cu-Sn phase were tested in synthetic saliva using electrochemical methods. Mercury was adsorbed on the surface by exposure to mercury vapor, and the changes in the corrosion potential and anodic polarization characteristics were determined. Galvanic currents between eta' specimens and a high-copper dental amalgam were measured for both Hg-free and Hg-contaminated eta' electrodes.

RESULTS

The Hg-free eta' phase showed a wide region of passivity. Exposure to mercury made the eta' phase electrochemically more active, and current peaks appeared on the polarization curves. The Hg-free eta' electrode was a cathode to a high-copper dental amalgam. The Hg-contaminated eta' electrode started also as a cathode, but became anode after a period of exposure.

SIGNIFICANCE

It was concluded that mercury destabilizes the protective passive film on the eta' phase, making the phase more susceptible to corrosion in the oral environment. The results of the galvanic current measurements seem to indicate that the galvanic interaction between eta' and the other phases facilitates the mercury-related degradation.

A new technology for direct restorative alloys

OBJECTIVES

The purpose of this study was to develop a new technology for preparing mercury-free metallic dental restorative materials.

METHODS

The novel approach relies on the cold welding of surface treated silver particles. At ambient temperature, intermetallic compound formation takes place spontaneously at the silver-tin interface. The ability of a loose powder to consolidate at ambient temperature under moderate pressure and within a short time duration was investigated for various mixtures of elemental silver, tin, and pre-alloyed silver-coated powders. Surface treatment aimed at removing silver surface oxide layers is done with a dilute acid. The compressive and the transverse rupture strengths of several of the consolidated powder mixtures were determined.

RESULTS

Cold-welding of surface-treated powder particles takes place across the silver-silver interface and is promoted by exposure to a mild acid. Powder mixtures containing approximately 50% silver powder, 40% silver-coated silver-tin intermetallic compound particles, and a small silver-tin fraction deposited from an aqueous solution display very good condensability, depending on the amount of silver-silver interface area available. The attained rupture strength values (200 Mpa) were higher than those of amalgams; the compressive strengths (120 MPa) and hardness values (100 KHN), however, were lower than those found for amalgams.

SIGNIFICANCE

Mercury-free silver-tin powder mixtures can be processed for use as metallic composite dental restorative materials.

Influence of amalgams, bases, and varnish on seal composition at restoration tooth interfaces

Eighty extracted, noncarious premolars were restored with one of 20 different combinations of restorative materials: a high- or low-copper amalgam, unlined or lined with one of five bases, and varnished or unvarnished. The teeth were aged in 1% NaCl and after 3 months and 1 year the seal material deposited on the cut tooth cavity surface and the surface of the amalgam restoration was analyzed by use of energy dispersive x-ray analysis. Atomic weight percentages of marginal seal elements were statistically examined with ANOVA and Tukey's test with significance set at p < 0.05. The results indicated that only one type of seal was formed centered around tin. More marginal seal material was present in the 1-year specimens. Restoration materials used affected the deposition rate of marginal seal material. Low-copper amalgam-restored teeth formed more seal material with significantly greater amounts of chlorine and tin. Less marginal seal material was found in varnished cavities. A calcium hydroxide base produced a thick marginal seal with a significantly different composition of elements compared with the unlined and zinc-based specimens. Tin in Poly-F Plus polycarboxylate cement did not significantly increase the tin content of the marginal seal. Base materials can play a role in determining crevice pH. The findings have bearing on improved longevity of amalgam restorations.

In vitro marginal leakage in varnished and lined amalgam restorations

Black's class I classic cavity preparations were completed in 440 extracted, intact human premolars and restored with two silver amalgams and five different base materials with and without cavity varnish, which resulted in 20 combinations. A standard fluorescent dye test was performed and the percentage of marginal leakage recorded after storage for 7 days, 3 months, and 1 year in 1% NaCl at 20 degrees C. The results were computed by use of a linear logistic statistical analysis with a critical level of statistical significance of p < 0.05. Sealing improved with time and there was consistently better sealing in the first months without varnish, but after 1 year this was not significant. The low-copper amalgam recorded better sealing than the high-copper amalgam. A calcium hydroxide base promoted sealing but base-free cavities performed second best. Sealing declined with 1 year if Dycal base was used. Zinc-containing bases exhibited poor sealing, even those containing tin. Base solubility can improve sealing in calcium-containing bases but not in zinc-containing bases.

Interactions between dental amalgams and the oral environment

Dental amalgam fillings interact in a complex way with the environment in the oral cavity as they are subjected to chemical, biological, mechanical, and thermal forces. These forces change the restoration's appearance and properties, while metal ions, amalgam debris, non-metallic corrosion products, and mercury vapor are released into the oral cavity. The phenomena and conditions that affect the amalgam/environment interaction include the chemistry and biochemistry of the environment, formation of biofilms on the amalgam surfaces, existence of localized corrosion cells, galvanic contacts with other metallic restorations, abrasion during mastication, and synergistic effects of the different forces. Corrosion processes result in a degradation of the functional amalgam properties, while tarnishing reactions cause discoloration. Corrosion degradation of amalgam fillings is due mainly to localized corrosion cells in pores and crevices. Corrosion on occlusal surfaces is accelerated by abrasion during mastication, which removes the protective surface films. The average total amounts of metal species, including mercury, released per day in vivo from a restoration have not been determined. Much of the reported indirect evidence for high mercury release rates is either unreliable or controversial. A more detailed investigation is needed and will require the development of more sophisticated techniques of sampling in vivo, as well as both experimental and theoretical modeling in vitro.

Valiant-PhD and Lojic N amalgam alloys: four-year clinical results

From 1984 to 1986, 97 similar pairs of Valiant-PhD and Lojic N amalgam restorations were placed at random by one dentist in Class II cavities prepared in 105 molar and 89 premolar permanent teeth of 73 patients treated in a private practice. Assessments were made of the different handling properties of the two alloys and of the effects of polishing, or otherwise, on the clinical behaviour of the restorations. Valiant-PhD alloy had more 'body' for condensing and was easier to carve and polish than was Lojic N alloy. After periods of up to four years, there was only one restoration failure (from bulk fracture) for each alloy although both materials deteriorated over time. Both the polished and the unpolished restorations showed similar deterioration for marginal fracture and staining, and for surface tarnish, and their initially-different surface textures became more similar during the study. Statistically, Lojic N restorations showed significantly more surface tarnish, but less marginal fracture than did Valiant-PhD restorations, and the tarnishing did not appear to be related to the effects of corrosion.

Comparison of the sealing ability of four cavity varnishes: an in vitro study

The purpose of this study was to test and to compare, in vitro, the sealing ability of four different cavity varnishes used to minimize the marginal leakage of amalgam fillings. For this purpose, a model was developed in order to avoid quality-deteriorating effects (such as condensation forces used during placement of amalgam) and temperature changes. In this model, raw ceramic disks were used instead of dentin. The four varnishes showed significant reducing effect on the penetration of the dye (Methylene Blue) compared with the control. There was no cavity varnish with optimal sealing.

Deterioration of restorative materials and the risk for secondary caries

This article reviews the literature related to marginal deterioration of amalgam restorations and the risk for secondary caries. Background information on this characteristic material failure is provided, including how it is measured clinically, its rate of progress with time, and the responsible microstructural mechanisms. The association between degree of marginal fracture and recurrent or secondary caries is reviewed in clinical and in vitro studies. Earlier clinical investigations and a more recent laboratory study demonstrated a strong correlation between gap size and recurrent caries for a caries-active environment, supporting replacement of defective restorations for preventive reasons. However, more recent clinical data show the complex and equivocal nature of this association. Poor specificity and validity for detection of recurrent caries with an explorer and mirror further complicate interpretation of some trials and decision-making in clinical practice. Improved methods of diagnosing recurrent caries and further studies of the factors influencing its initiation and progression are needed.