Each year U.S. dentists replace an estimated 15 million amalgam restorations that have failed because of bulk fracture and another 10 million restorations that have failed because of occlusal marginal caries. These replacements cost the public $1.3 billion. When caries activity is high, fracture of amalgam near restoration margins contributes to the development of marginal caries. This research's major goal is to develop amalgams that resist bulk and marginal fracture. Two hypotheses will be explored: 1) that the rate limiting process that leads to bulk and marginal fracture is the nucleation and growth of cracks by low-frequency fatigue and 2) that alloying elements that dissolve in the gamma-one matrix phase Will improve amalgam's resistance to fatigue fracture. Amalgams with and without small amounts of zinc, palladium, or indium will be subjected to low-frequency fatigue. Amalgam near the margins of simulated restorations will be fatigued: The number of cycles to crack nucleation will be measured as a function of compressive load (30, 24 and 18 MPa), frequency (0.5, 1.5, 3.0 Hz), and amalgam margin angle (45 degrees, 60 degrees, and 75 degrees). Using plane strain fracture toughness specimens, fatigue crack velocity in amalgam will be measured as a function of change in stress intensity factor [delta Ks between 0.38 and 1.4 MPa(m) 1/2], frequency (0.5, 1.5, 3.0, and 6.0 Hz), and environment fair, water, and aqueous solutions of NaCl). Microstructural damage leading to the nucleation of fatigue cracks and damage associated with crack growth will be followed as a function of the test variables. Scanning electron microscopy will be used to characterize matrix deformation, crack paths, and fracture surfaces. Transmission electron microscopy will be used to characterize fatigue damage within gamma-1 Ag-Hg grains and to investigate the effect of zinc, indium, and palladium on this damage. To investigate the origins of the cyclic stresses that lead to bulk and marginal fracture, clinical trials will be conducted. Amalgam restorations will be placed in denture teeth of subjects who have removable partial dentures. These trials will compare amalgam marginal fracture: 1) in cavity preparations with flexible walls and preparations with stiffer walls and 2) in restorations with and without the cusps that may protect amalgam near the margins. A third clinical trial will explore the role of low-frequency fatigue in nucleating bulk fracture. The durability of amalgam restorations made with fatigue resistant (zinc-containing) and fatigue susceptible (zinc-free) versions of an amalgam will be compared.
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