PERMEABILITY CHARACTERISTICS OF DENTIN

Dentin represents the living, sensitive, porous mineralized compartment
that separates cavities prepared in teeth from the underlying dental
pulp. The permeability characteristics of dentin are central to
understanding dentin sensitivity and pulpal reactions to dental
procedures or materials. Much of the pulpal irritation of cavity
preparation is probably due to fluid shifts across dentin caused by
thermal or evaporation stimuli during cutting. The proposed experiments
are designed to measure the magnitude of these fluid shifts and reduce
them to a common denominator, a physical force (cm H2O). Once
identified, these forces will be applied to cavities to determine the
critical physical force (negative hydrostatic pressure) necessary to
produce odontoblast displacement in normal dog dentin and in dentin
treated with a drug which disrupts odontoblast microtubules. As part of
the reaction to cavity preparation may be due to neurogenic inflammation,
we also plan to measure the spontaneous rate of dentinal fluid flow,
pulpal pressure (PP) and pulpal blood flow (PBF) in innervated versus
denervated dog teeth following normal cavity preparation to examine the
influence of nerve stimulation on these variables. In another set of
experiments, PP and PBF will be manipulated pharmacologically using both
vasodilators and vasconstrictors, to determine how they influence fluid
shifts across dentin. Finally, dentinal fluid will be physically forced
across dentin (as may occur when crowns are seated) to determine what
that does to PP and PBF. Some of the pulpal irritation seen following cavity preparation may be
due to capillary forces acting on dry dentin. We will compare pulpal
reactions to normal cavity preparations in dog teeth, to that done
atraumatically in teeth were the dentin remains under saline solution
that will prevent the expression of capillary forces. Other teeth will
be subjected to mild thermal and chemical irritation for comparative
purposes. A series of experiments will be done in vitro in an attempt to seal
dentin with superior smear layers or adhesive resins to prevent fluid
shifts from occurring. The best method of sealing will then be used in
dog teeth in vivo to prevent fluid-induced pulpal irritation. Throughout
the grant period, the data we collect will be used to develop a series of
mathematical expressions of hydraulic conductance, diffusion and the
interaction of outward convective fluid movement on inward diffusion in
the presence or absence of smear layers. This will permit the
development of computer simulations of dentin and the pulpodentin complex
which should be extremely useful to many laboratories around the world. Overall, these experiments should provide much needed information
regarding the effects of cavity preparation on the pulpodentin complex.
The results will permit us to make a series of recommendations regarding
how clinicians can protect the pulp and avoid pulpal irritation.