Operative dentistry aje qualtrough, jd satterthwaite la morrow, pa brunton

Current concepts
Most authorities now recognise that the presence of bacteria is the
most important determinant factor of pulp inflammation and ultimately
pulp death. Bacterial contamination may be derived from the initial
carious lesion, cavity preparation and restoration placement, the
smear layer or microleakage. Hilton
14
stated that ‘an understanding 
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of the properties of the currently available materials, and how they
interact with the pulpal tissue, can help the practitioner decide when
to use bases and liners and which products to choose.’ The routine
placement of a preparation liner or base is now not advocated. All
preparations should have some form of preparation sealer and some
preparations (usually deep) will require a liner and/or base.
The pulp may be damaged during the restorative procedure by
inadequate water cooling of the burs, use of worn burs or by accidental
entry into the pulp chamber (pulpal exposure) by hand or rotary
instruments. Accurate knowledge of the anatomy of each tooth is
therefore essential to ensure that tooth preparation is completed with
the minimum of iatrogenic damage. An important consideration here
is the age of the patient, in that younger patients have larger pulp
chambers than older patients.
To prevent further noxious stimuli reaching the pulp it has been
usual practice to protect further the pulp by applying therapeutic
materials to the floor and/or the pulpo-axial wall of the preparation.
These materials were commonly placed under amalgams and resin
composites to prevent thermal stimulation of the pulp and acid 
contamination of dentine respectively. It has now been demonstrated
that thermal stimulation of dentine is not a problem clinically and that
routine basing of amalgams, to prevent thermal stimulation, inher-
ently weakens the restoration. It is also now accepted that dentine 
can be etched and therefore routine lining for resin composites is now
contraindicated.
Sealer
Traditionally, cavity varnishes have been routinely used to provide a
protective coating for freshly cut tooth structure. A cavity varnish is 
a natural gum, such as copal or rosin, or a synthetic resin dissolved 
in an organic solvent such as acetone, chloroform or ether, which
evaporates and leaves a protective film behind. Many studies support
the view that the application of a cavity varnish under amalgam
restorations provides a temporary seal, decreasing microleakage until
corrosion products are deposited. Doubts have been expressed as to
the effectiveness of Copalite varnish to seal teeth restored with high
copper amalgam long enough for corrosion products to be deposited
at the interface
19
. The increased microleakage seen with some high
copper amalgam restorations may be due to the fact that the varnish
dissolves before the corrosion products are fully formed. Recent
advances in dentine bonding agents have led to recommendations for
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their use under amalgam restorations to seal the dentinal tubules,
eliminate dentinal fluid movement, decrease microleakage and post-
operative temperature sensitivity.
In recent years various desensitising agents have been used in the
management of tooth hypersensitivity. These agents are reported to
be effective by reducing the diameter of the dentinal tubule and limit-
ing fluid movement
20
. It has been postulated that the application of
the same mechanism allows desensitising agents to be equally effective
in preventing postoperative sensitivity when amalgam restorations
are placed. These materials may be of value in the treatment of cavity
surfaces before amalgam placement.
Liner
Cavity liners are placed to a thickness of typically less than 0.5 mm.
They act as cavity sealers and may have the additional therapeutic
benefits of fluoride release, adhesion to tooth structure and antibacter-
ial properties. Liners may not have sufficient thickness or strength to
be used alone in deep preparations; therefore they are frequently
overlaid by a base material. The most popular currently used cavity
liners are calcium hydroxide and glass-ionomer cements. Resin-
modified light-activated glass-ionomer liner materials have gained
increased popularity and have the added advantage of ease of place-
ment, command set and early resistance to moisture contamination.
Eugenol based materials are contraindicated as liners for resin com-
posite restorations, because the eugenol may be absorbed into the
resin composite, act as a plasticiser and decrease bond strength. This
view has, however, been disputed over recent years
21,22
.
Base
The ideal base material is a thermal insulator, non-toxic, cariostatic,
has persistent antibacterial properties, is able to stimulate reparative
dentine formation, and is strong enough to withstand the forces of
amalgam condensation and masticatory forces. Bases are traditionally
dentine replacement materials, and may also be used to block out
undercuts for indirect restorations. All cement bases dissolve slowly
and disintegrate with time in the oral environment. They act as a
mechanical barrier between the restorative material and the under-
lying pulp. The remaining dentine thickness overlying the pulp is the
single most important factor when deciding whether or not to place a
base. In vitro studies have shown that a remaining dentine thickness of
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between 0.5 and 1 mm reduces the toxicity levels of materials by 75%
and 90% respectively
23
. Dentine is said to be the most effective base
and should not be removed to accommodate a proprietary material.
The most commonly used bases have been zinc polycarboxylate,
glass-ionomer cements, zinc oxide eugenol and zinc phosphate
cements.
On the basis of research, the philosophy of basing a preparation 
to an ideal form has fallen into disrepute. Bases have few benefits 
and make the restoration more prone to fracture. The main question
today has to be whether cement bases under amalgam restorations 
are necessary and have any value in current operative dentistry 
techniques.
Materials that are used for bases can sometimes be used for tempor-
ary dressings. If a patient has lost a restoration or when tooth prepara-
tion is not completed, it is usual to insert a temporary restorative
material (temporary dressing). This is designed to seal the preparation
and prevent pain from exposed tooth substance and to preclude 
further carious activity until a permanent restoration can be placed.