Corrosion Sealing of Amalgam Restorations In Vitro

Abstract

©Operative Dentistry, 2009, 34-3, 312-320 DB Mahler • BV Pham • JD Adey Clinical RelevanceAfter placement, amalgam restorations exhibit a gap between the restoration and tooth structure, which fills with corrosion products during time in clinical service. To reduce the time necessary to fill this gap, an amalgam that exhibits a small initial gap shortly after setting and an amalgam that contains zinc is recommended. SUMMARYAmalgam restorations, when first placed, have been shown to exhibit a gap at the amalgam/tooth interface. With time in service, this gap fills with corrosion products that have the potential to "seal" the restoration. With the advent of high-copper, more corrosion-resistant amalgams, there has been concern that the time required to create this seal would be increased significantly when compared with low-copper traditional amalgams. The current study was designed to address this concern. Amalgam was condensed into a MACOR mold, simulating a Class I cavity form and then immersed into a 1.0% NaCl solution to simulate oral conditions. Using an air pressure test, the sealing was monitored over time. The results showed that the sealing was influenced by the size of the initial gap prior to immersion as well as corrosion resistance of the amalgam and that a corrosion-resistant amalgam with a small initial gap size can seal as quickly as a corrosion-prone amalgam. Therefore, it is not possible to predict sealing behavior based on corrosion resistance, alone. Furthermore, the presence of zinc in the amalgam alloy has been shown to result in the formation of zinc corrosion products in the amalgam/mold margin, which contributes to more rapid sealing. Analysis of a tooth extracted after 16 years of clinical service that had been restored with an amalgam-containing zinc was also shown to contain zinc corrosion products in the occlusal marginal area. This could explain the reported reduction in marginal fracture of clinically placed amalgam restorations made from zinccontaining alloys.

Preliminary Evidence That Bioactive Cements Occlude Artificial Marginal Gaps

“…[11][12][13][14][15] The limited protective properties of current restoratives underscore the significant challenge that secondary caries still presents as a failure mode for restorative materials. Notwithstanding the long-standing observation that dental amalgam occludes marginal defects by either deposition of metallic corrosion products and/or material creep mechanisms; 16,17 no other currently available direct or indirect dental material has demonstrated an analogous ability to seal or reseal marginal defects. 18 Clearly, additional functionalities added to dental restorative materials could augment fluoride release to improve longevity of dental restorations, especially by improving long-term marginal stability of dental restorations.…”

Influence of Hand Instrumentation and Ultrasonic Scaling on the Microleakage of various Cervical Restorations: An in vitro Study

“…The microleakage was evaluated after 1 week and the corrosion products were observed in the margins of restorations. 43 Hence, low leakage scores in the amalgam restorations of the current investigation can be related to corrosion sealing of the zinc-containing alloy used in other studies. 29,43 Another reason for low amalgam microleakage scores may be due to low-dimensional changes of high copper alloy used in this study because high copper alloys show the lowest dimensional changes.…”

“…43 Hence, low leakage scores in the amalgam restorations of the current investigation can be related to corrosion sealing of the zinc-containing alloy used in other studies. 29,43 Another reason for low amalgam microleakage scores may be due to low-dimensional changes of high copper alloy used in this study because high copper alloys show the lowest dimensional changes. 44 These results should be interpreted with caution since it is difficult to extrapolate the findings of this and other similar in vitro investigations to the behavior of these restorative materials in the oral environment.…”

Secondary caries and microleakage

“…For metallic restorations, it is likely that spontaneous active crevice corrosion develop, that further lowers the local pH [83]. The most thermodynamically active components that penetrate and precipitate in the dentin is tin and zinc, and the main corrosion constituent in toothrestoration interfaces including in dentinal cavity walls has been identified as oxides, sulphides and chlorides of tin, and to a lesser extent zinc and copper [84,85]. Under which circumstances the metallic ions are transported superficially, or alternatively penetrate and precipitate deeper into the dentin remains unclear [86].…”