3. 1. Define a rest and list its functions.
2. Explain the different types of rests
for posterior and anterior teeth.
3. Describe the dimensions of all the
types of rests.
4. Components of a RPD
1. Major connectors
2. Minor connectors
3. Rests
4. Direct retainers
5. Indirect retainers
(CLASS I AND CLASS II ONLY)
6. Denture bases
7. Teeth
5. Forces that are
applied to a
removable partial
denture must be
transferred to the
supporting teeth and
tissues in an
atraumatic fashion.
6. The components of a
removable partial
denture that transfer
forces along the long
axes of the abutment
teeth are called rests.
8. The relationship between a rest and
a rest seat must be such that forces
transmitted from the prosthesis to
an abutment are directed apically
down the long axis of the tooth.
9. In this manner, stress
can be absorbed by
the fibers of the
periodontal ligament
without damaging the
ligament or the
supporting bone.
10. In the case of a tooth-supported
removable partial denture, all the
stresses are transferred to the
abutment teeth.
11. In a tooth-tissue-supported removable
partial denture, only a portion of the
stresses are transferred to the teeth,
while the edentulous ridge must absorb
the remainder of the load.
12. In addition to transferring forces, each
rest should serve as a vertical stop for
the prosthesis. Firm, positive contact
between a rest and rest seat minimizes
vertical displacement of the prosthesis
and prevents injury to the soft tissues.
13.
14. The primary purpose of the rest is to
provide vertical support for the
partial denture. In doing so, it also
does the following:
1. Maintains components in their
planned positions
15. 2. Maintains established occlusal
relationships by preventing settling
of the denture
3. Prevents impingement of soft
tissue
22. A rest that is responsible for
additional support or indirect
retention is called an auxiliary
rest or secondary rest.
23. Primary rests prevent vertical
movement of a prosthesis
toward the tissues and also help
transmit applied forces to the
supporting teeth.
24. Rest seats should be shallow and
saucer shaped, and should function
as ball-and-socket joints. This
geometry permits dissipation of
potentially harmful lateral forces.
26. Auxiliary or secondary rests are used
as indirect retainers in extension
base removable partial dentures
(Class I, Class II, and long-span
Class IV applications).
27. Three major forms of rests are:
1. Occlusal rests — so named
because they are seated on the
occlusal surfaces of posterior teeth.
28. 2. Lingual or cingulum rests — those
seated on the lingual surfaces of
anterior teeth, usually canines.
29. 3. Incisal rests — those seated on
the incisal edges of anterior teeth.
31. The outline form of an occlusal rest
seat should be triangular, with the
base of the triangle located at the
marginal ridge and the rounded apex
directed toward the centre of the
tooth.
32. The rest seat should occupy one
third to one half the mesiodistal
diameter of the tooth
33. and approximately one half the
buccolingual width of the tooth
measured from cusp tip to cusp tip.
34. The floor of the occlusal rest seat
must be inclined slightly toward the
centre of the tooth.
35. The angle formed by a line dropped
down the proximal surface of the tooth
parallel to the long axis of the tooth
and the floor of the rest seat must be
less than 90 degrees
36. so that the
transmitted occlusal
forces can be
directed along the
vertical axis of the
tooth.
37. An angle greater than 90 degrees not
only will not transmit the forces
vertically, but will create an inclined
plane effect.
38.
39. This inclined plane effect can produce
slippage of the prosthesis away from
the abutment teeth.
40. Also, it can cause orthodontic
movement of abutment teeth, with
pain and bone loss.
43. The deepest portion of an occlusal
rest should be located near the center
of the mesial or distal fossa. From this
point, the rest seat should rise gently
toward the marginal ridge.
44. An occlusal rest should be at least
0.5 mm thick at its thinnest point
and 1.0 to 1.5 mm thick where it
crosses the marginal ridge.
46. Lingual or cingulum rests are used
primarily on maxillary canines. The
normal morphology of a maxillary canine
permits preparation of a satisfactory rest
seat with minimal tooth reduction.
47. The form of a lingual rest seat should be
V shaped when viewed in cross section.
This geometry prevents migration of the
abutment away from the removable
partial denture framework.
48. The rest seat, from the lingual
aspect, takes the form of a broad
inverted V, maintaining the natural
contour sometimes seen in a
maxillary canine cingulum.
Dimensions: The
mesiodistal length of the
preparation should be a
minimum of 2.5 to 3 mm
49. The proximal view demonstrates
correct taper of the floor of the rest
seat. It also should be noted that
the borders of the rest seat are
slightly rounded.
Dimensions: incisal-
apical depth a minimum
of 1.5 mm
50. From the incisal view, the rest seat
preparation is broadest at the most
lingual aspect of the canine. As the
preparation approaches the proximal
surface of the tooth, it is less broad
than at any other area.
Dimensions: labiolingual
width about 2 mm
54. Incisal rests are most frequently
used on mandibular canines, but
may be used on maxillary
canines. They are not indicated
on incisors.
55. An incisal rest seat approaches from
the lingual surface, crosses the incisal
edge, and extends onto the facial
surface of an abutment (arrows)
56. An incisal rest seat
should appear as a
small , V-shaped
notch located
approximately 1.5 to
2.0 mm from the
proximal-incisal
angle of the tooth.
57. Three views of an incisal rest seat
preparation on the mandibular canine
58. Other types of rests
Extended Occlusal Rest
In Kennedy Class II, modification 1, and
Kennedy Class III situations in which the most
posterior abutment is a mesially tipped molar,
an extended occlusal rest should be designed
and prepared.
59. To minimize further tipping of the
abutment and to ensure that the forces
are directed down the long axis of the
abutment.
