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

Preparation design features
In general, once carious dentine has been removed from a cavity, the
resulting shape will be undercut; however this general shape may not
be sufficient to retain a restoration when the preparation/restoration
is particularly large. In order to increase the retention and resistance
form of a preparation, the placement of well-defined preparation 
features such as undercuts, slots and grooves will often suffice 
(Fig. 2.4). These include parallelism or relative undercuts of all prepa-
ration walls, proximal box form, retention grooves in the proximal
line angles and box form in buccal and lingual groove areas of molars.
Although healthy tooth structure should be retained whenever 
possible, the careful and judicious removal of dentine to create 
retentive features will result in enhanced service of the restoration
and ultimately greater longevity of the tooth itself due to fewer inter-
ventions over the lifespan of the tooth. Defined preparation features 
can be easily and safely cut into remaining dentine with a variety of
small burs. In order for these features to have maximum benefit, they
should be placed in opposing dentine walls.
Principles of direct intervention


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Fig. 2.4
Supplementary features for retention of direct restorations.
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In a large preparation with one or more missing cusps, the resulting
preparation floor is often fairly flat in its entire profile. If this prepara-
tion were filled with no additional preparation features, even when
there are significant undercuts in the remaining dentine walls, the
ability of the restoration to withstand lateral forces will be limited.
The placement of a circumferential groove or shelf will provide a
significant increase in the resistance form of the preparation.
The preparation of circular chambers cut vertically into dentine 
of about 1–1.5 mm diameter and 2 mm deep, into which restorative
material is placed, can provide resistance and retention. These 
features have also been termed amalgam inserts or amalgapins.
A disadvantage of slot-retained amalgam restorations is that they
are particularly sensitive to displacement during matrix removal, 
and great care must be taken not to dislodge the restoration when
removing the matrix.
Dentine pins
The use of dentine pins is well established as a method of providing
additional retention (Fig. 2.5). Three types of pin (cemented, friction-