Background and Significance: The development of porcelain crowns for the predictable restoration of teeth has been brought about by the development of bonding techniques to enamel and dentin with the use of multi-step total etch adhesive systems along with the evolution in small particle hybrid resin composites as adhesive luting agents. One of the most common post-operative concerns of patients following delivery of full coverage crowns and bridges is sensitivity to thermal conditions in the mouth (cold water, ice cream, etc.) (Goodacre, Bernal et al. 2003). In order to understand the biological basis for these materials it is important to understand the anatomy and neurophysiology of dentin. The following is adapted from a paper provided by Dr. Charles Cox (Professor, University of Alabama School of Dentistry) concerning dentin structure and mechanisms for reduction of dentin hypersensitivity .

Dentin is a vital tissue, harder than bone but weaker than enamel with a physiological elasticity preventing tooth fracture. Dentin is approx. 74% mineral; the organic phase being approx. 26% type-1 collagen with small amounts of proteins and water. The collagen matrix is secreted as a biologically "plastic" material, which hardens by a variety of mechanisms. Normal dentin is composed of millions of tubules or canals, running from the pulpal wall to the Dentin-Enamel Junction (DEJ). The dimensions of these tubules or canals varies depending on the position within the dentin structure. For instance, tubule diameter at the DEJ is 0.06 µm, and 3.0 µm at the pulpal wall (approx. 60,000 tubules / mm2). Most tubules are filled with fluid, an odontoblast cell process, collagen, and occasional non-myelinated pulpal nerves coursing approx. 150 µm towards the DEJ (Ten Cate 1998).

In normal (non-anesthetized) patients, Gysi (1900) reported that fluid distortion on the cavity floor caused sharp pain. Brännström et al. (1966, 1969, 1976, 1979, 1980, 1996) demonstrated dentin hypersensitivity (sharp pain) is the result of rapid fluid movement in the tubule complex, aggravated by aggressive air-drying. Physiological testing in various animal studies (Heyeraas 1985, Narhi 1983, 1985) confirmed the fluid pulsing or hydrodynamic mechanism (Brännström 1966) as the prime cause of sudden dentin pain. Physiologically, any rapid bi-directional fluid flow is the result of a sudden shift in tubule fluid from stimuli (i.e. cold or rapid airflow; Brännström 1996).

Management of dentin hypersensitivity is only one part of a successful dental complete coverage restoration (dental crown or bridge). In evaluating the clinical service-life of porcelain crowns there are three aspects of the bonding environment that needs to be considered. These consist of the tooth preparation, the adhesive and the tooth or crown preparation and the fit of the porcelain restoration to the tooth preparation. All ceramic restorations need enamel and dentin reduction of at least 1.5 mm along with a smooth shoulder and avoidance of sharp edges to avoid crack formation on the intaglio surface, proper esthetics, physiological contours and strength of the final restoration.(Goodacre, Campagni et al. 2001) Dental preparation for full coverage restorations (1.5 to 2 mm clearance which often means 1.5 - 2 mm loss of tooth structure) are inherently non-conservative and with a reduced amount of remaining tooth structure there is an elevated risk of post-operative sensitivity.

In today's society, having a dental restoration that provides minimal post-operative sensitivity is only part of the demand from patients. There is an increasing expectation for esthetic, "life-like" restorations that simulate if not replicate the natural tooth. This is complicated especially in patients with heavy occlusion or are missing teeth. The development of CAD/CAM milling technologies has laid the groundwork for use of high strength heat-pressed zirconia \[tetragonal zirconia polycrystals (TZP) a yttrium partially stabilized polycrystal structure\] as a supporting framework for supporting an esthetic, "life-like" Lucite-base veneering ceramic material to provide an all ceramic form of full coverage dental restorations for individual teeth or replacement of a single missing tooth with a 3-united fixed partial denture ("dental bridge"). The use of press porcelain crown technology has allowed the improvement of fit of refractory die porcelain crown fabrication relative to conventional Pt foil techniques and may lead to greater predictability and reduced microleakage (with enhanced service life of the dental restoration). The potential for an enhanced service life may depend on a combination of silination with a bifunctional bis-GMA primer and dual cured composite resin luting agent which may seal microcracks within the intaglio surface of the porcelain and a sufficient bond to tooth structure for a reasonable service life.