Dental Crowns Clinical Trial
Official title:
Outcome Evaluation of All Ceramic Crowns and Fixed Partial Dentures Bonded With Two Different Dental Cements.
The restoration of indirect partial and full coverage restorations in dentistry necessitates
the use of a luting agent to act as a means of mechanical and potentially, chemical retention
of the restoration. Upon the introduction of the first generation of glass Ionomer cements
for use as a luting agent, an elevated short-term post operative hypersentivity was reported.
In part, this adverse event was considered to be due to a desiccating effect of the cement as
water is utilized in the setting reaction and thus a desiccation of the dentinal tubules was
proposed as a potential mechanism leading to disruption of the neurosensory odontoblasts.
Further materials development in the field of dental cement luting agents has lead to the
introduction to the current market of resin formulations that have an alternative mechanism
of setting. Consequently, there are anecdotal reports of a decrease in the incidence of
post-operative sensitivity but no comparison with the effect of conventionally used dentin
desensitizing agents as a part of the bonding process of the restoration.
The purpose of this trial is a single blinded parallel design randomized clinical trial to
evaluate the perceived incidence of post-operative sensitivity when full coverage all ceramic
crowns or short-span three unit fixed partial dentures (dental bridge) is fabricated in the
posterior part of the mouth. The dental crowns or bridges will be bonded with conventional
glass ionomer cement or a Urethane dimethacrylate / Bis-GMA composite resin dental cement.
Outcome measures will be both objective clinical criteria and the use of a calibrated pain
survey instrument.
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.
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