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

117
Fig. 5.4
Anatomical reduction.
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Chapter 5
Reduction is often overestimated. The use of depth grooves, or a
bur tip of known diameter, is a useful aid
12
, though a silicone index,
either of the tooth before reduction or taken from a diagnostic wax-up
of the intended shape, is the most reliable guide (Fig. 5.5). The most
common errors are non-anatomical reduction or a lack of a functional
bevel.
Poorly controlled reduction can lead to both technical and biological
problems. Typical problems arising as a result of under-preparation
include:
• Aesthetic failure
• Metal flexure
• Poor emergence profile
• Occlusal interference or perforation of the restoration
• Over-contoured restorations at the margin, which lead to plaque
retention and associated periodontal problems and increased risk
of marginal caries
Excessive preparation can lead to:
• Pulpal damage
• Ceramic fracture
• Core fracture
Fig. 5.5
Silicone index showing occlusal reduction.
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Types of extra-coronal restoration
The type of extra-coronal restoration should be as conservative as 
possible while achieving the aims of treatment. Restorations may be
simply categorised on the basis of the type of material to be used and
on the amount of tooth covered (full or partial coverage).
The mechanical characteristics of the material chosen will dictate
the amount of tooth substance removed. Extra-coronal restorations
can be broadly categorised into three traditional categories, veneer
metal (gold) restorations, all ceramic restorations and metallo-
ceramic restorations.
The amount of reduction required will largely depend on the type
of material to be used for the final restoration (Fig. 5.6), for example:
• Gold – 1 mm coverage occlusally with 1.5 mm over supporting
cusp, may be 0.8 mm on palatal aspect of anterior teeth.
• Metallo-ceramic – 1.4 mm minimum on the facial surface of an
anterior tooth.
• Ceramic – 1–1.5 mm on anterior teeth for traditional porcelain, less
if the ceramic can be laminated to the tooth structure as with a
resin-bonded crown, or up to 2.5 mm if a high-strength crystalline
ceramic core is utilised.
An indirect restoration that covers all or most of the clinical crown
is termed a full-coverage restoration, a partial-coverage restoration
being one which leaves some of the clinical crown intact (e.g. onlay or
capped cusp inlay, 
3
/
4
or 
7
/
8
crown, adhesive veneers). If retention
(described later) can be maintained, intact surfaces of tooth structure
should be saved and not removed for convenience or speed.
Principles of indirect restoration