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

Integrity of the remaining tooth structure
The preparation should be planned to maximise the preservation 
and protection of remaining tooth structure. Increasing cavity depth
and width increases the potential for outward flexion of buccal and 
lingual walls
7
. Preparations with a curved floor show less cuspal
movement than those with a flat floor and a flat floor with its sharp
angles and stress concentrations may lead to fracture. This flexure
may also have effects on subsequent buccal restorations
8
. If caries has
undermined the remaining tooth structure to a significant degree, 
the tooth may fracture during function. The planned removal of such
healthy tissue may, in fact, preserve tooth structure in the long term
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by minimising the subsequent risk of fracture, which may otherwise 
lead to loss of a large quantity of strategic tooth structure. Also, it 
has long been established that there is increased fracture incidence 
in teeth with restorations of a wide isthmus and having three or more
surfaces. The provision of cuspal protection should be considered 
in such cases.
Placement of margins
Black originally proposed that margins should be placed well into the
embrasures in cleansable areas, but the degree to which this has been
adopted has slowly reduced over the years with the acceptance that
good oral hygiene is sufficient. Cervically, Black recommended that
margins should be placed in a caries-free zone subgingivally, but 
this zone is the area of gingival attachment! It is now accepted that
margins should be kept free of the gingivae to avoid periodontal
problems and that incidence of overhangs and marginal gaps must be
avoided. It may be necessary to extend the preparation if the margin
(i.e. interface between tooth structure and restorative material) is
close to a contact with an opposing tooth as there is the potential for
early breakdown at this weak interface. This emphasises the need to
mark the occlusal contacts before preparation is commenced, espe-
cially if rubber dam is being used. Similarly, for cavities involving the
proximal surface it may be necessary to extend the gingival margin in
an apical direction to allow placement of a matrix band. This differs
significantly from Black’s ‘ideal’ preparation with predefined place-
ment of margins (Fig. 2.1).
Elderton
9,10
has argued that many amalgam restoration failures are
due to marginal breakdown owing to a low amalgam marginal angle
(AMA) and high cavo-surface angle (CSA). He has suggested that
preparations with AMA of at least 70° (ideally 90°) will yield longer
lasting restorations (Fig. 2.2). In a 2-year clinical study of amalgam
restorations in preparations with such margins, Stratis and Bryant
11
commented on the difficulty of consistently achieving these angles.
They showed that utilising these angles (together with finishing pro-
cedures) resulted in fewer marginal fractures although the short-term
nature of the study was noted.