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

119
Fig. 5.6
Reduction required for metallo-ceramic and cast metal restorations (Pc 
=
porcelain, Au 
= gold).
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120


Chapter 5
Retention and resistance
The ability of an indirect restoration to resist dislodging forces relies
primarily on the retentive and resistance form of the preparation.
• Retentive form – those features of the preparation that resist removal
of the restoration in its long axis.
• Resistance form – those features of the preparation that resist 
dislodgement due to forces outside the path of withdrawal of the
restoration, i.e. lateral or rotational forces.
Resistance is probably the more important of the two. There exists a
relationship between the two but this is not direct. The role of the
cement lute should also be considered; traditional cements are strong
in compression but weak in shear loads, therefore good resistance
form (and retention) is necessary to minimise the shear loading on 
the luting cement. Adhesive cement lutes offer large increases in 
resistance and retention, but they should not be used simply to com-
pensate for poor preparation. Both traditional and adhesive lutes 
may undergo fatigue failure, and uncertainty exists regarding the
longevity of adhesive bonds. Therefore whichever type of lute is 
used, attention should still be given to achieving good retentive and
resistance form.
Retention
Retention is primarily a surface area effect, depending on:
• Height of preparation
• Diameter
• Surface texture
• Taper
Of these, taper is the most critical factor. Theoretically, the more
nearly parallel the opposing walls of a preparation, the greater the
retention
13
. In order to avoid production of undercuts and to allow
seating of the crown, a slight taper is cut. Various suggestions have
been made regarding optimal taper, commonly 6° (5–10°) is quoted,
though higher figures are often given for molars. The rationale for 
this is that full seating of a restoration is more important than a tight
casting for good retention
14
(due to the role of the cement lute) and
long teeth may need a greater taper in order to allow seating of the
final crown. Resistance and retention will be excellent with a long
crown. Less taper should be produced on short teeth when retention
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and resistance will be poor. Despite these arguments, the use of 
die-spacer (to provide space for the cement lute) will largely reduce
problems with seating of restorations. Also, in order to maximise
retention, it would be wise to aim for near parallelism in all cases,
especially as most clinicians underestimate the amount of taper that
has been produced
15,16
. When aiming to achieve near-parallelism, it
should be remembered that burs commonly used for tooth prepara-
tion are tapered and simply need to be held in the long axis of the
preparation in order to produce a taper.