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particularly in molar teeth, and must therefore have adequate
mechanical properties to resist these. Although a full-coverage crown
may afford some protection to the core if the margins are extended
gingivally beyond the core
6
, this protection is limited. The strongest
materials available at present are amalgam (for a direct core) or a cast
metal such as gold (for an indirect core in an endodontically treated
tooth).
Essentially, there are four types of direct core material available;
amalgam, resin composite, glass ionomer or hybrid materials such as
light-cured resin-modified glass-ionomer cement (RmGIC).
Amalgam
Amalgam has perhaps the best track record when used for substantial
posterior core build-ups. Amalgam has good contrast with tooth sub-
stance and is easy to prepare. The long time to full set may predispose
to early fracture, which is unfortunate, as the preparation cannot 
usually be prepared at the same visit, although newer high-copper
amalgam alloys have high strength within a short time and may be
prepared at the same visit after a short delay. The thermal expansion
of amalgam is quite dissimilar to dentine, and this factor may predis-
pose to failure after a period of time. In addition, amalgam cannot be
bonded to tooth substance without resorting to proprietary products
for amalgam bonding.
Resin composite
The use of resin composite as a core material has advantages and 
disadvantages. The composite does not require a two-visit crown
preparation technique and, when necessary, the crown preparation
can be commenced immediately. However, against this, the resin
composite is difficult to prepare to the correct form because it may be
difficult to differentiate between tooth tissue and core substance,
though resin composites of contrasting colour are available. Light-
cured resin composites should be used with caution, as full depth of
Principles of indirect restoration