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

SUPPLEMENTARY RETENTION FOR DIRECT RESTORATIONS
Retention is the ability of a restoration to resist forces that would 
dislodge it in the long axis of the tooth. Resistance of a restoration 
is the ability of a restoration to resist forces that would dislodge it in 
a lateral or rotational direction. In general terms, features of a prepara-
tion that provide resistance form will also provide some degree of
retention, and the two terms are often interchanged.
Retention of a restoration within the tooth relies primarily on there
being sufficient coronal tooth tissue which can provide:
• Adequate bulk of dentine to form an undercut preparation or allow
for placement of undercuts without resulting in weakened tooth
structure.
• Sufficient coronal tooth tissue to provide ‘bracing’ to lateral 
forces and hence provide some resistance to displacement of the
restoration.
Often with extensively broken down teeth it is impossible to develop
appropriate retention and resistance form with the remaining tooth
Principles of direct intervention


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tissues and alternative methods to retain the restoration must be 
considered. A variety of techniques may be employed (perhaps in
combination) to provide the extra retention.
Bonding
The principle of acid etching of enamel and the use of resin-based
adhesives with resin composite materials is well established and 
the continuing development of multi-purpose bonding systems has
allowed such materials to be bonded to tooth structure in a broad
range of situations without the need to sacrifice healthy tooth struc-
ture in order to increase retention and resistance. A more difficult 
situation arises when the material being used does not bond to tooth
structure. Retention is normally provided by undercuts and prepara-
tion features; however at times it may not be possible to create these
features. This is classically the case with amalgam restorations in large
preparations. With any operative procedure, there is a fundamental
need to preserve tooth structure wherever possible and this equally
applies to situations in which additional retention is required. In this
respect, the ability to achieve additional retention through bonding
restorative materials to tooth structure (and rely less on mechanical
means of resistance) offers obvious advantages.
A variety of propriety adhesives that are specifically for use in
bonding amalgam to tooth structure are now available
27
. These are
principally bi-functional polymeric resins, for example phosphonated
esters of bis-GMA, 4-META and HEMA. Most of these adhesives
bond to enamel and dentine in a similar way to resin composite bond-
ing systems, though the bond between amalgam and adhesive is
thought to be purely micromechanical
28
.
Numerous in vitro studies related to bonded amalgam restorations
have been reported in the literature. Despite there being few long-term
clinical studies, there are definite short-term advantages, including:
preservation of tooth structure, decreased immediate postoperative
sensitivity, increased retention and increased fracture resistance of
remaining tooth structure
29–31
. The long-term benefits are, however,
less certain, reflecting uncertainty regarding the durability of the resin
bond. In addition it should not be forgotten that bonding an amalgam
restoration requires a well-controlled operating field and may be
more time consuming.
When the short-term benefits of bonding amalgam restorations are
of use, for example to reinforce weakened cusps before providing 
a cuspal coverage restoration (large cores, or endodontically treated
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teeth), there seems little excuse for not adopting such a procedure.
The long-term benefit of bonding amalgam restorations is uncertain.
Therefore, given the lack of long-term clinical data, increased cost 
and technique sensitivity, the use of adhesive liners under amalgam
restorations cannot yet be advocated as a routine procedure.