Amalgam Margin Breakdown Caused by Creep Fatigue Rupture

Abstract

This study was designed to determine whether the mechanical cyclic stressing that occurs during normal mastication contributes to margin breakdown of dental amalgam restorations. The method used appears to duplicate the mechanical stresses developed in vivo during mastication, as the result of tooth flexing. We evaluated one low-copper alloy--NTD--and three high-copper alloys--Dispersalloy, Phasealloy, and Tytin. We prepared simple amalgam restorations in a cavity centrally located in an aluminum beam. Each specimen received five periods of three-point cyclical loading (1.7 hertz, 4200 cycles at 37 degrees C). The margin area was subjected to SEM examination prior to and at the completion of each period of cycling. At the beginning of each period of cycling, beam deflection was set to establish a maximum theoretical stress of 1, 2, 4, 6, or 8 MPa. For all brands, cycling resulted in margin gap formation and surface wrinkling. Wrinkling in Dispersalloy occurred as a wide band of shallow wrinkles, whereas that in NTD occurred as a narrow band of deep wrinkles. At 21,000 cycles, very little void formation and fracturing had occurred in the Dispersalloy or NTD restorations. In contrast, the Phasealloy and Tytin restorations developed extensive fracturing even after 4200 cycles. Fracture surface analyses of Phasealloy and Tytin indicated that creep fatigue rupture was the fracture mechanism responsible for margin breakdown in these amalgam restorations, when subjected to cyclic compressive loading similar to that experienced during mastication.

Indentation Creep Behavior of a Direct-filling Silver Alternative to Amalgam

“…Most of the previous studies on amalgam creep focused on the measurement of creep rates (Espevik, 1977;Mahler et al, 1977Mahler et al, , 1982Gjerdet and Espevik, 1978;Jordan ct al., 1978;Osborne et al, 1980;Gale et al, 1982;Averette and Marek, 1983;Okabe et al, 1985;Port and Marshall, 1985;Williams and Cahoon, 1989;Watkins et al, 1995). These studies provided valuable information on relationslhips among creep, corrosion, and microstructure of amalgam.…”

“…The creep of dental amalgam has been extensively studied. Amalgam creep has been observed to contribute to marginal breakdown due to creep-induced fracture at the margins (Jordan et al, 1978;Marshall et al, 1980;Osborne et al, 1980;Gale et al, 1982;Williams and Cahoon, 1989) and corrosion due to creep-induced rupture of surface-protective films (Gjerdet and Espevik, 1978;Mahler et al, 1982;Averette and Marek, 1983). The material microstructure and phases were shown to determine the creep rates of amalgam (Espevik, 1977;Mahler et al, 1977;Okabe et al, 1985;Port and Marshall, 1985;Watkins et al, 1995).…”

Creep as a Mechanism for Sealing Amalgams

“…This hypothetical theory was originally proposed by Professor Peter Winchell in 1975 and published in 1978 (Osborne & others, 1978b) to explain another dental amalgam phenomenon, marginal fracture. Although this theory did contain a mechanism to partially explain the marginal breakdown of amalgam (Williams & Hedge, 1985), other concepts have expanded and given better explanations of this phenomenon (Jokstad, 1991;Williams & Cahoon, 1989;Sutow & others, 1985).…”

“…Creep occurs at temperatures closer to the melting temperature of the metallic compound, and dental amalgam is within 20% of its fusion temperature (Williams & Cahoon, 1989). The creep of dental amalgam is confined to the lower temperature phase-that is, the gamma-1 matrix (Ag 2 Hg 3 ) (Espevik, 1977;Sarkar & Eyer, 1987;Mahler & Adey, 1991).…”

Influence of cavity depth on marginal degradation of amalgam restorations

“…If the time factor for condensing the amalgam is important, the differences between the large and small restorations would be especially obvious for the fast-set alloys. A third proposed mechanism is that the masticatory forces induce stresses at the restoration margins in the cavities with flexible axial walls and cusps (4)(5)(6)(7)(8). lf this biomechanical mechanism is correct, it was assumed that the margins would show increased degradation in cavities in which high cusp movements are possiblethat is, in cavities with deep or wide occlusal parts and with cavity walls diverging in the occlusal direction.…”