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

Bevelled shoulder
A shoulder with a bevel can also be used to create an acute edge 
of metal at the margin but should not be used routinely for veneer
metal crowns as in this scenario it is unnecessarily destructive. It can
however be used as a finishing line for partial coverage restorations.
Structural durability
For an indirect restoration to be able to withstand functional forces, 
it must be rigid enough to resist flexure and thick enough to resist
wear and fracture. Rigidity is especially important at the margins of
restorations – if the axial portion is too thin, for example with an acute
feather-edge finishing line, the restoration is at risk of deformation
and may result in breakage of the cement lute. The restoration must
also be thick enough to resist wear and when reducing a tooth,
sufficient reduction must be made in order for the restoration to have
adequate thickness, for example, 1 mm occlusal and 1.5 mm functional
cusp coverage for gold, 1.75–2 mm for a metallo-ceramic crown.
Structural durability requires control of stresses
22
. Chamfers con-
centrate stresses less than shoulders. As mentioned previously, when
Principles of indirect restoration


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Chapter 5
a shoulder is to be used, the junction between the base of the axial 
wall and the gingival margin of the preparation should be rounded 
to minimise stress in this area. Rounding of the reduced cusps, near
parallel preparations and multiple point contacts on the final occlusion
also reduce stress.
Occlusal stability
As discussed in Chapter 1, any new restoration must be in harmony
with the existing occlusion if this is satisfactory, but may be used to
create a new occlusal relationship in situations when the existing 
pattern is not satisfactory. In order for this to be determined, pre-
operative examination of the occlusion is obviously essential. This may
involve the use of study models mounted with a face bow record on
an articulator, especially if multiple units or units involving guiding
surfaces are to be restored. Note must be taken of existing rela-
tionships, both static (intercuspal position) and excursive (canine
guidance or group function, anterior guidance and the presence of
excursive interferences or gross discrepancies in movement from a
retruded position to the intercuspal position).
The new restoration should be constructed such that it has
sufficient contacts with the opposing tooth/teeth to maintain a stable
relationship, which will also depend upon the durability of the 
material from which it is constructed. In addition, if the restoration 
is to provide a guiding surface, then the nature of this guidance (for
example, whether to copy or change the existing relationship) must be
carefully planned before the preparation is commenced.
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