Rests & Rest seats in removable partial Dentures

Rests & Rest Seats
Asian Dental Academy
RPD Notes
www.asiandentalacademy.org

• Support (GPT) to hold up or serve as a
foundation or prop for.

Rests (Stewart) are the components of RPD
that serve primarily to transfer forces
occurring against the long axis of the
abutment teeth.
McCracken Any unit of a partial denture
that rests on a tooth surface to provide
vertical support is called a rest.
Rest seat is a prepared surface of a tooth or
fixed restoration into which a rest fits.

Functions
• To transfer forces on abutment teeth and in
distal extension cases to prevent over
displacement of soft tissues.
- Prevents PDL breakdown
- Prevents loss of occlusal relationship

• Maintain proper position of clasps

• Also help to transfer lateral forces –
Only tooth surface supported
(Not Distal extension)

• To restore continuity of arch in spaced
teeth.

• Rest along with its minor connector used
for reciprocation
• To reestablish plane of occlusion by onlay
rest.

• Rest also placed on proximal surfaces of
teeth adjacent to edentulous span to prevent
impaction of food between minor connector
and tooth.
• Serve as Indirect Retainers.

General Considerations
• Effect of occlusal rest on tooth.
Centre
of
rotation

The rotatory effect is reduced in three ways :

Types of Rests:
I) Primary / Auxiliary
II) Depending on surface:
i) Occlusal Rest.
ii) Cingulum Rest
iii) Incisal Rest.
III) Other types:
i) Internal rest.
ii) Ledge rest

Literature Review:
• Tsao D.H. ( 1970) described designing
occlusal rests using mathematical formulae.
• Ivanhoe (1985) described a new design for
cingulum rest.
• Meinig (1994) did a retrospective study on
RPD without rests.
• Rudd et al (1999) described tooth
preparation to receive a RPD
• Culwick et al (2000) in a survey studied the
size of occlusal rest seats.

• Kancyper et al(2000) described the use of
all ceramic crowns on RPD abutments.
• Davenport et al (2001) described rest seat
preparation.
• Sato et al (2003) studied the effect of OR
size and shape on yield strength. Found that
increasing width and thickness increased
YS while overrounding or sharp line angles
decreased it.
• Gapido et al (2003) studied the fatigue
resistance of cast OR in Co-Cr and Ag-Pd-
Cu- Au alloys.

Always do guide plane
preparation before rest seat
preparation.

Occlusal Rest
• Basically triangular outline.
• Apex of triangle rounded.
• Following the outline of mesial or distal
fossa.
• Length- 1/3rd
to ½ the mesio distal width-
average- 3-4 mm
• Facio lingual width- ½ distance between
buccal and lingual/ palatal cusp tips or 1/3rd
faciolingual width of tooth. 2-2.5 mm

• At least 0.5 mm thick in thinnest area. And
1-1.5 where it crosses marginal ridge.
• Floor inclined towards centre with the
deepest portion at the centre making the
enclosed angle acute.(for axial force
transmission and framework and tooth
forced together under occlusal force for
mutual support)
• Concave/ spoon shaped – smooth with no
sharp line angles.
• OR placed in line with crest of residual
ridge.

Occlusal rest in amalgam.
Material considerations – Creep
Design to have wider rests than the cavosurface
margin. Replace with cast restoration if minor
connector contacts proximal margin
Not routinely placed – only when questionable
prognosis of tooth. Use No. 4 round bur (not
diamond) and on reverse to polish.

• OR on rotated tooth – Ideally restore the tilting
by other means or cast restoration. If not place OR
in Mesial or distal fossae even though they are
slightly buccal or lingual due to rotation. If this is
not feasible the rest should be placed anywhere on
the surface, where a properly designed recess can
be prepared to support it.
• OR on mandibular bicuspid with rudimentary
lingual cusp - it is difficult to create an ideal
recess. Most satisfactory is to make a cast
restoration building a quasi- cingulum rest like in
canines.
• Rest Recess in Abraded Tooth – Enamel
thickness is compromised. Cast restoration is
recommended.

• OR on mesially tipped molar: Floor
perpendicular to the long axis to prevent
further tipping of tooth.
• Occlusal rest in cast restorations –
Tooth preparation should include the
recess.Wax pattern should be carved on the
surveyor to maintain guide planes.
Occlusal rest in existing cast restorations.
Inform patient about need to replace cast
restoration if perforation occurs

No. 4 bur or same
size round diamond
point.
Start in floor of
fossa and cut half
depth of bur and
extend on facial
side over marginal
ridge. Repeat on
lingual side to make inverted V shape on MR. Check
depth and if adequate remove island of enamel, blend
walls and flare facially and lingually.

Verify the depth of
Preparation by asking
The patient to bite on
wax and using a wax
gauge.
Round line angles and
polish by reverse
rotating No. 4 SS bur
at moderate speed.

EMBRASURE REST
Extends over the occlusal embrasure of 2
approximating posterior teeth from mesial
fossa of one to the distal of another.
- Reduce marginal ridges by equal amount. If
uneven : no attempt should be made to get
them to same level
- NEVER BREAK CONTACT POINT
- Generally – Clearance over the buccal
slopes of preparation is less giving rise to
fracture of clasp arm

Embrasure rest:

As the preparation
passes over the
buccal and lingual
embrasures – it
should be 1.5 –
2 mm wide and
1-1.5 mm deep.
-CHECK FOR
CLEARANCE IN
EXCURSION
Buccal inclines must
be rounded as
chances to create
undercuts.

ONLAY REST
On abutment tooth that has occlusal surface
that is partially below the plane of occlusion
as a result of being tipped or rotated below
the occlusal plane.
- Used only in caries resistant individuals,
high motivation for oral hygiene
- Should be removed from mouth for at least
8 hours
Preparation involves widening deep fissures
to make it self cleansing and fluoride
application.

INTERNAL REST/ MILLED REST/
SEMIPRECISION ATTACHMENT
In tooth supported RPD these may be used for
both occlusal support and horizontal stabilization.
Flat floor and near parallel walls.
It is NOT an retainer and should not be confused
with internal attachment.
- It facilitates elimination of visible clasp arm
buccally. Retention is provided by a lingual clasp
arm, either cast or wrought wire.
- Use of machined mandrel made in Co-Cr which is
waxed in pattern parallel to path of insertion. After
casting the mandrel is tapped out, leaving the rest
in the casting.

Long box occlusal rest
– in rotational path RPD
Must be deep enough to resist rotation
but not deep enough to contact dentine.
More than half mesio distal width
Nearly parallel facial & lingual walls.www.asiandentalacademy.org

- Prepare first on cast and verify path of
insertion.
- Extend more than half mesio distal width.
- 1.5 to 2 mm deep with lateral walls nearly
parallel or slightly divergent occlusally.
- Floor flat – at right angles to long axis.
- Occlusal outline may be dovetailed or
straight.
- Cavosurface margin slightly beveled.

Lingual Rest/Cingulum Rest
A definite rest seat is
required to direct
forces axially.
Usually on maxillary
canines as sufficient
enamel and the
favourable slope is
seen.
Or on multiple
maxillary incisors

Better than incisal rest – as
closer to centre of rotation.
More aesthetic.
Less subject to distortion
and breakage.

• Half moon shaped extending from MR to another
over the cingulum. 2.5 – 3 mm long, 2mm labio
lingually.
• Deepest point over the cingulum. At least 1.5 mm
• Rest seat is V shaped and ALWAYS GINGIVAL
TO CONTACT OF OPPOSING TOOTH.

- No. 37 inverted cone
carbide, green stone
or small knife edged
diamond wheel used.
- Start preparing incisal
to cingulum. The flat
side of the instrument
should follow the
lingual surface of the
tooth cutting towards
the apex of the tooth.
- Polish with
carborundum
impregnated rubber
wheel.

Incisal Rests
Indicated in anterior
teeth which are
sound and cast
restoration not
indicated & where
cingulum rest can
not be used.
Used on mandibular canines. Used on incisors to
stabilize them along with lingual plate (when fixed
Splinting not done due to poor prognosis)

• Incisal rest is small V shaped notch located 1.5 to
2.0 mm from proximo-incisal angle of tooth made
by knife edged diamond wheel or green stone.
Round edges with carborundum impregnated
rubber wheel. 2.5 mm wide mesio distally.
• Deepest part towards centre of tooth. –1.5 mm
• Notch should be rounded and slightly extend on
facial surface to provide positive seat.
• Enamel on lingual surface should be prepared to
provide space for minor connector to make it
unobtrusive as its rigidity has to be maintained.
(due to length)

Justification for incisal rests:
- If natural faceting is present.
- Tooth morphology does not permit
cingulum rest
- These rests restore defective or abraded
tooth anatomy.
- Incisal rests provide stabilization
- Full incisal rests may restore or provide
incisal guidance.

Additive Mouth Preparation
• Bonded metal contours into which contain
prepared rest seats.

• Lyon (JPD,1985) described resin bonded
etched metal rest seats
Tooth preparation – involves using fine
flame shaped bur to scribe a light gingival
finish line 1mm supragingival or at CEJ
(gingival recession). A small cingulum
rest seat using no.35 bur is prepared to
provide definite stop. Fabrication done on
refractory cast.

Advantages:
1. Luting medium is insoluble.
2. Supragingival margins- cleansable.
3. No involvement of dentine.
4. Economical than full crown.
5. Maintains original aesthetics.
Leupold(JPD,1985) described etched castings to
adjunct mouth preparation for RPD. He
described mouth preparation to include
horizontal notches 2-3 mm long and 1-1.5 mm
deep and fabricate palatal veneers 0.3 mm thick
in non bulk areas with duralay pattern resin.

• Seto & Caputo (JPD,1986) did the
photoelastic stress analysis of resin bonded
cingulum rest seats. found that it is better to
have preparation of horizontal grooves

• Traditional surveyed Crowns and Pontics
with proper guide planes and rest seats.

• Kancyper (JPD,2000) described the use of
surveyed all ceramic crowns made by
Procera system in which the areas under
stress were in dense Aluminum oxide core
while other areas including those in contact
with retentive terminal end of retentive
clasp in veneering porcelain.

Ledge rest
• On the surveyor the wax
pattern for FVC or 3/4th
crown can be carved to
provide a guide plane.
• A gingival ledge can be
created which acts as an
occlusal rest and allows
use of porcelain on
occlusal surface for better
aesthetics.

• It enables the reciprocal arm to make
maintain contact tooth surface well before
the retentive arm can exert whiplash effect.
• Reciprocal arm less obtrusive- can be
inlaid.
• Acts as a rest.
Placed at junction of occlusal and middle
third. (Miller) while according to Brudvick
placed around 3-3.5 mm from gingival
margin.

Composites: To use or not to
use?
Brudvick and Kratochvil advocate placement of
lingual rest seats in composite
Literature Review:
1. Hebel (JPD,1984) studied and found that a cast
Co-Cr clasp terminal caused wear of enamel of
20 microns and 50 microns on composite over a
period of 3 years.
2. Toth et al (JPD,1986) studied the bond strength
of lingual rest seat prepared in composite.

Criteria for successful lingual rest seats
1. Shear resistance equal or greater than
expected forces.
2. On the basis of earlier data at least 12 kg
force has to be withstood.Found to be
21.95 kg.
3. Should survive for at least 3 years under
repetitive forces with chemical
degeneration of material as well.
4. Acceptable abrasion resistance.
5. Ability to repair.

• Bonded resin contours
• Techniques-
1. Brudvick
2. Direct build up
and preparation.
(Latta,JPD 1988)
3. Using framework
(Yard et al, JPD 1988)

ThankYou

Rests & Rest seats in removable partial Dentures

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Rests & Rest seats in removable partial Dentures