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

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Space
Obviously, interocclusal space (in the intercuspal position) is required
for the placement of any restorative material. Ceramic requires 
more bulk (and therefore space) than metal (gold), which is strong 
in thinner sections. Interocclusal space is routinely provided by
preparation of the tooth and removal of tooth structure. Generally, 
the degree to which reduction can be performed occlusally without
compromising retention and resistance or the health of the tooth 
will determine whether it will be possible to have ceramic on the
occlusal surface. Problems arsing from limited crown height are dealt
with later.
There is also a relationship between crown height and choice of
material, especially with anterior teeth. Restorations on ‘long’ teeth,
when the margin has to be placed gingivally, may have to have a
metal margin otherwise preparation for a ceramic margin may
encroach on pulp space (Fig. 6.1). This will also result in a better 
contour and emergence profile and lessen the chances of problems
associated with plaque retention. This also applies to bulbous molar
teeth when the margin often has to be placed on radicular dentine.
Aesthetics and patient wishes
There is an increasing demand for aesthetic restorations. The need for
an aesthetically acceptable restoration should never be overlooked. In
general, better aesthetics are possible with all ceramic restorations
Fig. 6.1
Preparation modification with labial metal collar for
long teeth. (Dotted line shows preparation for ceramic margin.)
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Chapter 6
owing to better optical properties and translucency. Although liked
by some, metal restorations are obviously unaesthetic but in some 
situations (e.g. limited space for the restorative material or need for 
a strong material) then no alternative exists. When deciding what 
material to use, it can be debated whether the wishes of the patient 
are the most important, or only important, factor if other factors are
equivocal. In situations in which there is a true indication for the 
use of (unaesthetic) metal and the primary intention of treatment 
is to provide a functional rather than an aesthetic result, then this
should be explained. All-ceramic restorations are more destructive.
However, if sufficient space can be provided for an aesthetic ceramic
restoration without undue removal of healthy tooth structure, and
wear and fracture are not a concern, then there is little argument
against providing such treatment.
In summary, when considering material selection, although vari-
ous materials are available, the choice is basically between metal
(gold), ceramic or a combination of both (metallo-ceramic).
Cast metal (gold) is regarded by many to be the most satisfactory
extra-coronal restorative material, has a hardness similar to enamel,
does not creep intra-orally, can be cast accurately, and wax carving
allows good detail and contour. It can be used in thin section with fine
margins and hence requires only a small amount of reduction. Gold 
is not aesthetic, though it is liked by some.
All-ceramic restorations are the most aesthetic yet are brittle and
liable to fracture, especially in thin sections, thus more bulk is needed
than for gold. Generally, ceramic restorations are not strong enough
to be used alone for posterior teeth or bridgework, though restora-
tions made with a high-strength crystalline core show promise for use
as single units or short-span bridges where there is adequate coronal
height for the extra bulk of connector
2,3
. Cracks may arise from micro-
pores on the surface, which may then open in tension or bending,
especially if not supported, and this influences margin design. Dental
porcelains are harder than enamel and if unglazed may abrade tooth
surface.
Metallo-ceramic restorations (typically for full coverage crowns)
allow good strength along with good aesthetics but require much
more tooth reduction than alternatives in order to provide space for
the metal substructure and sufficient ceramic for an aesthetic result.
Although ceramic is frequently used for the functional surface, ideally
this should be in metal in order to preserve tooth structure (no need
for extra reduction to accommodate the ceramic). A metal occlusal
surface will also avoid the overcontoured, wider occlusal table often
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present with ceramic occlusal surfaces and thus reduce the possibility
of producing a non-working interference and give a better functional
surface that is less likely to abrade the opposing dentition.