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

Resistance
In order to increase resistance to displacement due to lateral or 
rotational forces, the preparation requires minimal taper and also
increased height. The increased height of preparation must have a
constant diameter as parallel walls that are not on the same base do
not provide stability (Fig. 5.7).
When the clinical crown is short, or it is not possible to obtain near
parallelism, additional features such as grooves, slots or boxes can
enhance the resistance form significantly by reducing the radius of
rotation of the final crown
17
. These features should be prepared in the
long axis of the preparation and not just placed in line with the axial
wall. Axial grooves should, if possible, be placed into sound tissue of a
cusp and not into core material, which may be inherently weak. There
should be a definite wall perpendicular to the direction of the force in
order to limit the freedom of displacement and provide adequate
resistance. Axial grooves effectively improve the height:diameter
ratio, and enhance retention as well as resistance.
In some situations even the use of additional features will not pro-
vide enough resistance and in such cases surgical crown lengthening
may be utilised to increase the available clinical crown height. This
scenario, where the need to create interocclusal space by preparation
would result in short crowns with limited resistance, is discussed in
Chapter 6.
Principles of indirect restoration


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Fig. 5.7
Resistance provided by parallel walls at the same level.
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Chapter 5
Marginal integrity
The margin of the restoration (or finishing line) is the area at which the
restoration ends and presents a junction of restorative material and
tooth substance at the tooth surface. Finishing lines should not rou-
tinely be placed subgingivally, but should be placed where they may
be easily finished by the operator, cleaned by the patient and duplic-
ated by the impression. They should be placed on sound tooth sub-
stance, not the core, as this would result in concentration of stress on
the core and predisposition to failure. If possible, finishing lines
should be placed on enamel (though it is common for margins to 
end on dentine or even cementum) and end 2 mm below the core
6
in
order to support and protect the core through a ferrule effect (Fig 5.8).
A ferrule will decrease the incidence of core fracture and will also
decrease the incidence of root fracture with intra-radicular posts.
A sub-gingival margin may be inevitable when:
• Caries or a crack extends subgingivally.
• A restoration extends subgingivally.
• Extra axial length is required for retention.
• Aesthetics are essential – however a large number of margins
placed in the sulcus become supragingival because of gingival 
maturation or recession. It has been suggested that ideal placement
is at the level of the retracted gingivae
18
.
In determining the shape of tooth reduction at the margin, the 
aim must be to produce as small a marginal gap or discrepancy as 
possible. Apart from the increased potential for caries with an open