Journal of Dental Research, Vol 68, 1188-1193, Copyright © 1989 by International & American Associations for Dental Research Online Journals
Amalgam margin breakdown caused by creep fatigue rupture
P. T. Williams and J. R. Cahoon
Faculty of Dentistry, University of Manitoba, Winnipeg, Canada.
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.
