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

Protection from fracture and wear
Restorations providing cuspal coverage provide preservation of 
tooth structure by protecting the remaining axial walls from stresses
that may subsequently lead to fracture of weakened, susceptible
cusps. Also, a well-made indirect restoration that can control loads on
the tooth by maintaining stable occlusal contacts may minimise 
load and wear on the tooth and restoration itself. Intra-coronal
restorations, such as inlays, may also provide a similar degree of 
protection if they are adhesively bonded to the tooth, though an inlay
that spans from one proximal surface to the other (i.e. mesio-occluso-
distal) will create a wedging effect that inherently predisposes the
tooth to fracture.
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Pattern of tooth substance removal
Reduction of tooth structure should be appropriate and yet not 
excessive. There needs to be sufficient reduction to provide enough
space to accommodate the required thickness of restorative material
without necessitating overcontouring of the restoration. If too much
tooth structure is removed, the health of the tooth will be com-
promised. Reduction must be anatomical, that is the reduction planes
should broadly follow the contours of the tooth (or planned final
shape) – a flat over-reduced occlusal surface will shorten the prepa-
ration, reducing retention as well as reducing the resistance form 
(Fig. 5.4). Conversely, inadequate reduction in the occlusal grooves
will not provide adequate space for good functional morphology.
Also, a flat single plane of reduction on the axial portions of the tooth
will remove more tooth substance than is necessary, and will result in
an uneven space for the restorative material
11
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There should be greater tooth substance removal over the func-
tional cusp (i.e. palatal maxillary cusps or buccal mandibular cusps).
This is usually achieved by means of a wide bevel. If this is not done
and the crown is constructed to a normal contour then the resulting
restoration will be too thin in this area. If adequate bulk is provided 
by over-contouring the final restoration, then the occlusal contacts
will be incorrect, either too high or result in interferences during
excursive movements. In addition, a lack of a functional bevel leads to
uneven and uncontrolled distribution of stress and tensile forces on
the cement lute.
Principles of indirect restoration