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

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Chapter 5
Shoulder
The shoulder (or heavy chamfer/rounded shoulder) is the margin
type necessary for ceramic restorations due to their brittleness and 
liability to fracture other than in compressive loads. The margin
ensures that a relatively wide ledge provides support for the ceramic
to resist occlusal forces and minimise tensile stresses that may lead to
fracture of the ceramic. The shoulder should be produced to form a
90° angle to the external preparation margin. The sharp, 90° internal
line angle classically associated with this margin concentrates stress
in the tooth and sharp edges of ceramic may be rounded during firing,
resulting in reduced accuracy of fit. Thus this internal angle should be
rounded (Fig. 5.10), hence terms such as heavy chamfer (though this
may lead to confusion) and rounded shoulder are used.
The shoulder is generally not used for metal restorations as it will
not provide the acute margin that, as described above, will minimise
marginal gaps and allow the margin to be burnished. However, it has
been suggested that the shoulder finish may, in fact, give less of a
marginal gap than a feather edge despite the theoretical advantages of
the ‘slip-joint’
21
. This is primarily because of problems of expressing
Fig. 5.10
Detail of shoulder margin.
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cement lute from inside the casting during seating, which is more
difficult with increased speed of seating and an acute margin. A more
horizontal margin will result in the marginal gap not closing until 
the casting is nearly fully seated. A casting made to fit a shoulder 
margin may therefore have better fit, but the shoulder preparation is
more destructive than other margin types and should be avoided
when possible.
Chamfer
A chamfer is a compromise between the feather and shoulder finish.
The chamfered finishing line is that of choice for metal restorations. 
It provides an acute margin, which is desirable, yet allows for escape
of the cement lute. It also results in a restoration that has adequate
axial bulk to provide rigidity without the need to overcontour and
without being overly destructive of tooth substance, as a chamfer is
more conservative of tooth substance than is the shoulder, as less axial
reduction is necessary. It also exhibits the least stress (the underlying
cement will have less likelihood of failure) and is readily identified 
on the die.