RPD Manual 11 - Removable Prosthodontics - Dalhousie University

Direct and Indirect Retainers - 53
The I-bar and the proximal plate disengage the abutment tooth and thereby reduce torquing of
the tooth. This is an accurate deduction. Therefore the RPI design be used wherever possible.
However, an I-bar cannot be used routinely due to the fact that a tissue undercut frequently exists
in the region of the abutment.
A distal rest concept has drawbacks if designed incorrectly. If the guiding plane on the distal
surface of the abutment covers the entire length of the tooth, loading of the denture base will
cause the minor connector contacting the guiding plane to act as a "wrench" and torque the tooth.
However, if a short guiding plane is prepared with a relief area between the converging surface
of the tooth and the minor connector, the distal tilting of the abutment can be prevented or
minimized. During distal loading, the minor connector could move into the relief area and the
rest would be permitted to escape from the rest seat. This would change the fulcrum point from
point A to point B, driving the abutment tooth mesially against adjacent tooth.
However, if the retentive arm is place into a mesiobuccal undercut, torquing might still occur,
since the retentive tip would tend to rotate in an occlusal direction, thereby engaging the tooth.
To minimize the activating effect of the retentive arm, a wrought wire arm should be used
(greater flexibility, less ability to torque). In addition and where possible, the retentive undercut
should be place in the mid-buccal of the tooth, with the mesial clasp tip placed above the height
of contour. Thus occlusal movement of the clasp tip will disengage rather than engage the tooth.
Similarly, lingual bracing arms should not be carried too far into embrasures in distal extension
cases, since tissueward movement could also result in torquing forces being placed on the
abutments.