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CHAPTER VI.

RESISTANCE FORM.

Definition.— Extension for resistance is a term applied to that
procedure Avhich has for its sole object the carrying of the cavity
outline from localities subjected to great stress, to localities not
frequently subjected to the crushing strain. This is often mistaken
for extension for prevention, whereas it has reference only to re-
sistance to stress.
A proper application of this procedure Avill involve a careful
study of occlusion and articulation in each individual case.
Resistance form involves a consideration of the management of
weakened enamel walls and a study of the flow and edge strength
of the filling material used with a view of so shaping the cavity
as to minimize the effects of the crushing strain.
Its importance is in direct proportion to the exposure of the fill-
ing in occlusion and articulation, and the strength of the closure
of the jaws.
The force to provide for is from one to two hundred pounds and
in some cases even more, particularly in mid-jaw locations.
Weakened enamel walls are those which through decay, or un-
necessary cutting, have been robbed of much of their supporting
dentine. All such unsupported enamel should be cut away with a
chisel, particularly if by any chance the wall of enamel under con-
sideration Avill receive much stress in the process of mastication,
or the introduction of the filling.
Stress from within should be avoided by not allowing such weak-
ened walls to remain and form any part of the retention of the
filling.
Weakened walls are sometimes allowed to remain, or a portion
of them, when they can be so protected by a layer of rigid filling
material as to prevent all stress, but this is permissible only when
their presence will screen unsightly metal fillings and when the
kind of filling used can be introduced without injury to the walls.
Before applying the rubber dam each case should be inspected
for the surface contact in occlusion and articulation and then the
margin so laid as to occupy the least exposed position. Many times
all stress cannot be avoided, but the amount of stress a margin is
liable to receive should have due consideration and good judgment
exercised in the placing of the margin.
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