Topics to be covered today are:
 Fulcrum line
 Indirect Retention
The fulcrum line
Definition: The fulcrum line is a longitudinal axis of rotation around that a
partial denture may turn when forces are applied to one of its various
saddles.
A saddle may have one or more fulcrum lines affecting its movements. In
the case of tooth born saddle the fulcrum line is formed by the two rests
closest to it. In the case of free end saddles, a fulcrum line may be formed by
the crest of the residual ridge, as well as surrounding rests.
The occlusal forces (mastication or dislodging) on one side of the arch
cause lifting forces on the opposing side of the arch. This results in a tilting
or twisting action around the given fulcrum line for that force. This rotation
is counteracted by:
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Rigid connectors.
Direct retainer design.
Denture base coverage (support and indirect retention).
Denture tooth placement.
Contour of the denture base.
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INDIRECT RETAINERS.
If a removable partial denture is not supported by natural teeth at
each end of the edentulous saddle area, some provision must be made
for the prosthesis to resist rotational forces that are exerted. This may
also apply to long span bounded saddles (anterior especially).
Rotational movement around the fulcrum line, either toward the tissue
or away from the tissue, may occur as forces are applied to the denture
base. Movement toward the supporting ridge will be limited by the
amount of compressible mucosa, or the amount of bone resorbption
since the prosthesis was made. This movement to the ridge can only be
controlled by accurate denture base adaptation. An indirect retainer does
not control this movement.
Movement also occurs away from the tissue. This movement may be
caused by sticky foods or other matter lifting the denture base; the tongue or
buccinator muscles when they are activated by speech, mastication or
swallowing. Gravity may also exert dislodging forces on a maxillary
prosthesis. The reason for the use of the indirect retainer is to counter the
movement produced by these forces.
The indirect retainer in free-end prosthesis uses the mechanical
advantage of leverage by moving the fulcrum line farther from the force.
The indirect retainer may contribute to the support and stability of the
prosthesis.
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http://randwick.sydneyinstitute.wikispaces.net/Dental+Studies
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FACTORS INFLUENCING INDIRECT RETAINERS.
Several factors influence the effectiveness of the indirect retainer.
The greater the distance between the indirect retainer and the fulcrum
line, the more effective will be the indirect retainer. A line projected at
right angles from the fulcrum line and ending at a tooth that is capable
of supporting a suitable rest will indicate the most effective location for
the indirect retainer. The longer this line, the more favourable will be
the result.
In order for the indirect retainer to provide any function, the direct
retainer must prevent lifting of the denture base and the rests. This
lifting is one of displacement and not of rotation. The indirect retainer
does not resist displacement.
An indirect retainer must be rigid. A flexible indirect retainer will
allow greater movement than a rigid one.
The indirect retainer is most often an auxiliary rest. If incisor teeth are
to be used for an indirect retainer, it should be as a group and not
individually as they are not capable of withstanding pushing forces
individually.
Outcomes
You should now be able to define and explain
a fulcrum line and indirect retention in
relation to a RAPD.
These notes are now available online
http://randwick.sydneyinstitute.wikispaces.net/Dental+Studies
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