RPD Manual 11 - Removable Prosthodontics - Dalhousie University

Direct and Indirect Retainers - 57
Rotation of a partial denture about the fulcrum line is prevented by indirect retainers placed
against definite rest seats on the opposite side of the fulcrum line from the distal extension base.
Indirect retainers should be placed as far as possible from the distal extension base so as to gain
the best possible leverage advantage against lifting of the distal extension base.
Although, the most effective location of an indirect retainer is frequently in the vicinity of an
incisor tooth, these teeth may not be strong enough to support an indirect retainer. In addition,
incisors often have steep lingual inclines that cannot be favourably altered to support a rest. In
such cases, the nearest canine tooth or the mesial occlusal surface of the first premolar may be
the best location, despite the fact that it is not as far removed from the fulcrum line.
The factors influencing the effectiveness of an indirect retainer include:
1. Proper seating of the denture. Unless the principle occlusal rests are held in their seats by
the action of the direct retainers, rotation about the fulcrum line will not occur and,
therefore, an indirect retainer cannot act to prevent lifting of the distal extension base away
from the tissues.
2. Distance from the fulcrum line.
3. The rigidity of the connectors supporting the indirect retainer. All connectors must be
rigid if the indirect retainer is to function as intended.
4. The effectiveness of the supporting tooth surface. The indirect retainer must be placed on a
definite rest seat,, on which slippage or tooth movement will not occur. Tooth inclines and
weak teeth should never be used for the support of indirect retainers.
In addition to preventing movement of a distal extension base away from the tissues, an indirect
retainer may serve the following auxiliary functions:
1. It tends to reduce torquing leverages on the principle abutments. This is important when an
isolated tooth is being used as an abutment (a situation that should be avoided whenever
possible). Ordinarily, proximal contact with the adjacent tooth prevents such tilting of an
abutment as the base lifts away from the tissues.
2. Contact of the indirect retainer’s minor connector with vertical tooth surface aids in
stabilization against horizontal movement of the denture. Such tooth surfaces, when made
parallel to the path of placement, may also act as auxiliary guiding planes.