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:
1- Rigid connectors.
2- Direct retainer design.
3- Denture base coverage (support and indirect retention).
4- Denture tooth placement.
5- Contour of the denture base.
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.
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.
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These notes are now available online http://randwick.sydneyinstitute.wikispaces.net/Dental+Studies