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

Fig. 5.8
Ferrule provided by extension of 
crown margins beyond core.
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margin, larger marginal gaps may lead to periodontal problems and
increased bone loss
19,20
. It is common in general engineering to utilise
the principle of a slip joint; if the restoration has an acute edge at the
margin, the marginal gap will be minimal even when a restoration
fails to seat fully, i.e. the full occlusal discrepancy is not reflected at 
the margin. Production of such a ‘slip joint’ is not without problems,
and for this reason a number of margin designs, suitable for varying
applications, may be produced (Fig. 5.9). The margin design aims to
achieve minimal marginal discrepancies while considering factors
such as the mechanical properties of the material to be used and ease
of construction.
Feather edge
The feather edge is the closest to a slip-joint that can be produced. It
results in an acute angle of metal at the margin so that any failure to
seat occlusally is not reflected totally at the margin. The feather edge
margin is only applicable for a cast metal restoration, which is strong
in thin section and may be burnished. However, feather-edge finish-
ing lines should not be used as:
• They do not provide a definitive finishing line for the dentist.
• They do not provide a definitive finishing line for the technician.
• The axial wall of the casting may lack rigidity due to the fine edge
of metal.
• The restoration margin may be overcontoured as a compromise to
provide definition and rigidity.
Principles of indirect restoration


123
Fig. 5.9
Margin configurations.
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