RPD Manual 11 - Removable Prosthodontics - Dalhousie University

I. Retention
Direct and Indirect Retainers
Direct and Indirect Retainers - 43
Retention is the ability of a removable partial denture to resist dislodging forces during function.
Retention depends upon several factors:
a. adhesion, cohesion, interfacial surface tension and atmospheric pressure
b. gravity
c. frictional retention (guiding planes, bracing elements)
e. indirect retention
f. direction of dislodging force relative to the path of placement.
g. mechanical retention
Mechanical Retention
Mechanical retention is obtained by placing portions of the partial denture into tooth or soft
tissue undercuts. Maximizing the other retentive factors can minimize the need for mechanical
retention.
Most mechanical retention is derived from the use of direct retainers (clasp assemblies) utilizing
tooth undercuts. There are two classes of mechanical retainers: intracoronal and extracoronal.
Intracoronal retainers (precision attachments) are mechanical devices set into the casting of a
full crown. These are generally reserved for removable partial denture therapy that requires
exceptional effort in producing ideal esthetics. There are many contraindications and
disadvantages to precision attachments.
Extracoronal retainers engage an external surface of an abutment in a natural undercut or in a
prepared depression. There are two main classes of clasps: 1) those that approach the undercut
from above the height of contour (suprabulge retainers) and 2) those that approach the undercut
from below (infrabulge retainers).
II. Extra-Coronal Direct Retainers
Definition - A direct retainer is a unit of a removable partial denture that engages an abutment
tooth in such a manner as to resist displacement of the prosthesis away from basal seat tissues. It
is usually composed of a retentive arm, a reciprocal (bracing) element or arm, a rest and a minor
connector.
Retention is derived by placing a clasp arm into an undercut area so that it is forced to deform
upon vertical dislodgment. Resistance of the clasp to deformation generates retention.
Resistance is proportionate to the flexibility of the clasp arm. Non-flexible portions of clasp
arms must be placed occlusal to the height of contour (suprabulge area).