Despite the weight of scientific evidence, as well as assurances from international health advisory groups, that mercury release from dental amalgam poses no health threat to the general population (Mackert and Berglund, 1997a, b), a number of countries have already placed restrictions on the use of dental amalgam. There is, therefore, a need to perfect biocompatible dental materials such as ceramics so that they can serve as amalgam substitutes in the event of further restriction, or an outright ban, on dental amalgam. The research proposed in this application is directed toward developing stronger, less abrasive, and more thermally stable dental ceramics for use in porcelain-fused-to- metal (PFM) and all-ceramic restorations. Ceramics have many desirable properties that make them useful for dental restorations - translucency, chemical durability, and biocompatibility. However, two major problems arise in their use as replacements for tooth structure lost to disease or trauma: their potential for brittle fracture and their capacity to cause abrasive wear of opposing tooth structure (Rosenblum and Schulman, 1997). When porcelains are fused to metal or to a high-strength ceramic core as a means of obviating their shortcomings in the area of strength, the potential for a third problem is introduced - porcelain thermal expansion instability and the resulting thermal incompatibility between the porcelain and the metal or ceramic core.
Five specific aims are proposed: 1. To develop firing schedules for various porcelain metal systems to minimize the thermally induced changes in leucite content that lead to thermal expansion changes; 2. To continue modification of porcelains to render them more resistant to thermal expansion changes; 3. To develop a leucite-reinforced machinable ceramic that can be strengthened via heat treatment following machining in the CEREC 2 CAD/CIM machine; 4. To develop experimental porcelain with a leucite particle size of less than 4 um as a means of minimizing microcracking, increasing strength, and reducing wear of opposing enamel; and 5. To evaluate the release of Al, Si, and other elements from the experimental ceramics to be developed in the proposed research, compare these release value to those of existing ceramics, and relate them to dietary and other sources of intake of these elements. New ceramic formulations and combinations of nucleating agents will be investigated to reduce the leucite particle size as means to effect these physical properties improvements.

National Institute of Health (NIH)
National Institute of Dental & Craniofacial Research (NIDCR)
Research Project (R01)
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Oral Biology and Medicine Subcommittee 1 (OBM)
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Medical College of Georgia (MCG)
Schools of Dentistry
United States
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Twiggs, S Warren; Mackert Jr, J Rodway; Oxford, Amalia L et al. (2005) Isothermal phase transformations of a dental porcelain. Dent Mater 21:580-5
Mackert Jr, J Rodway; Sheen, Geoffrey W; Williams, Amalia L et al. (2003) Effects of local cooling rate and processing variables on leucite in dental porcelain. Int J Prosthodont 16:647-52
Mackert Jr, J R; Twiggs, S W; Russell, C M et al. (2001) Evidence of a critical leucite particle size for microcracking in dental porcelains. J Dent Res 80:1574-9
Mackert Jr, J R; Williams, A L; Ergle, J W et al. (2000) Water-enhanced crystallization of leucite in dental porcelain. Dent Mater 16:426-31
Mackert Jr, J R; Twiggs, S W; Williams, A L (2000) High-temperature X-ray diffraction measurement of sanidine thermal expansion. J Dent Res 79:1590-5
Khajotia, S S; Mackert Jr, J R; Twiggs, S W et al. (1999) Elimination, via high-rate laser dilatometry, of structural relaxation during thermal expansion measurement of dental porcelains. Dent Mater 15:390-6
Mackert Jr, J R; Russell, C M (1996) Leucite crystallization during processing of a heat-pressed dental ceramic. Int J Prosthodont 9:261-5
Mackert Jr, J R; Williams, A L (1996) Microcracks in dental porcelain and their behavior during multiple firing. J Dent Res 75:1484-90
Denry, I L; Mackert Jr, J R; Holloway, J A et al. (1996) Effect of cubic leucite stabilization on the flexural strength of feldspathic dental porcelain. J Dent Res 75:1928-35
Mackert Jr, J R; Khajotia, S S; Russell, C M et al. (1996) Potential interference of leucite crystallization during porcelain thermal expansion measurement. Dent Mater 12:8-12

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