CHAPTER 6: REST & REST SEATS

rest

provides vertical support on tooth surfaces

located on properly prepared tooth surfaces

defined by tooth surface to receive them (occlusal rest, lingual rest, incisal rest)

supports partial denture position and resists tissue movement

transmits vertical forces to abutment teeth and directs forces along long axes

must be rigid and receive positive support from abutment teeth for stable contact under occlusal loading

rest seat

The prepared surface of an abutment to receive the rest

role of rests in control of prosthesis movement

prosthesis frame should engage the tooth in a way that encourages axial loading for effective resistance

in rpd with distal extension bases, the denture becomes more secure as distance from the abutment increases

rests prevent denture movement in an apical direction, allowing the retentive portion of the clasp arms to maintain a designated position relative to the tooth undercuts

despite being passive, the retentive portion should remain in contact with the tooth to resist vertical dislodging force.

result when denture settles and clasp arms are no longer in contact with the tooth

vertical displacement can occur before the clasp arm becomes functional, reducing retention

role of rests in preventing denture settling

by maintaining stable contact with the abutment tooth under occlusal load, preventing downward movement toward the tissue

req for a rest to function & achieve stability

It must be rigid and receive positive support from the abutment tooth, with no slippage under load

this is how occlusal load distributed in a distal extension partial denture

Closer to the abutment → more load to the abutment tooth via the rest;
Farther from the abutment → more load to the supporting residual ridge tissue.

primary purpose of the rest

to provide vertical support for the partial denture

maintains components in their planned positions

maintains established occlusal relationships by prevent- ing settling of the denture

prevents impingement of soft tissue

directs and distributes occlusal loads to abutment teeth

4 benefits of using an implant as a rest

implant eliminates compression of supporting soft tissues

controls vertical movement of the den- ture base

eliminates or alters fulcrum lines

serves to increase support and stability of the prosthesis.

ideal outline shape of an occlusal rest seat

rounded triangular shape

apex toward the center of the occlusal surface

length & width of an occlusal rest seat compare

it should be as long as it is wide

2.5 mm

how much is the minimum base width of the marginal ridge for both molars & premolars?

1.5 mm

how much should the marginal ridge be lowered for a proper occlusal rest seat?

—lowering the marginal ridge is necessary to allow enough bulk of metal for strength and rigidity of the rest and its minor connector

floor of the occlusal rest seat be positioned

it should be apical to the marginal ridge and the occlusal surface

ideal floor shape for an occlusal rest seat

concave or spoon-shaped

less than 90°

what is the ideal angle between the occlusal rest and its vertical minor connector?

—this is to direct occlusal forces along the long axis of the abutment tooth and prevent slippage

consequence greater than 90°

fails to transmit forces along the long axis

may cause slippage

can create orthodontic-like tipping forces on the abutment tooth

relationship of the occlusal rest to the abutment

a shallow ball-and-socket joint

this is to avoid transferring horizontal stresses to the abutment

in tooth–tissue supported partial dentures

secondary occlusal rest

Used when primary rest cannot be modified due to enamel or restoration perforation

Passes over lowered marginal ridge and slightly apically inclined

Two opposing occlusal rests on diverging tooth inclines prevent unfavorable forces

Occlusal rest to abutment should be a shallow ball-and-socket joint to prevent horizontal stresses transfer

Provides only occlusal support

[ no horizontal stabilization, it should be provided by other components; can cause locking effect ]

danger of a “locking” occlusal rest design

It can transfer horizontal stresses

create leverage forces on the abutment tooth

extended occlusal rest typically used

Kennedy Class III

Kennedy Class II, modification 1

when the most posterior abutment is a mesially tipped molar

main purpose of an extended occlusal rest

to minimize further tipping of the abutment and direct occlusal forces down the long axis of the tooth

more than ½ of the mesiodistal width of the tooth

how far mesiodistally should an extended occlusal rest extend?

1/3 of the buccolingual width of the tooth

what should be the buccolingual width of an extended occlusal rest?

at least 1 mm

What is the minimum metal thickness needed for an extended occlusal rest?

shape/surface characteristics of an extended occlusal rest preparation

rounded, with no undercuts or sharp angles

modification made to an extended occlusal rest for severely tilted abutments

the form of an onlay to restore the occlusal plane

tooth preparation steps needed for an onlay-type extended occlusal rest

remove or restore pits, fissures, and grooves

place a 1–2 mm bevel on buccal and lingual occlusal surfaces for stabilization

shape to restore contour and occlusion of the natural tooth

ensure forces are directed down the long axis of the tooth

interproximal occlusal rests

when the design of a direct retainer assembly requires rest seats in an interproximal position

prepared like individual occlusal rests but extended farther lingually than usual

they are designed to shunt food away from contact points.

reason why adjacent rests used instead of a single rest in interproximal designs

to prevent interproximal wedging by the framework

mounted diagnostic cast analysis

important before preparing interproximal occlusal rests bc:

to assess interocclusal contact areas

ensure sufficient space for rest placement without altering occlusion

indication of internal occlusal rest

for totally tooth supported only

—just a rest not a retainer, should not be confused with attachment

form of the internal occlusal rest seat

parallel to the path of placement → ensures proper seating and stability.

slightly tapered occlusally → allows for insertion and removal without binding.

slightly dovetailed → prevents proximal dislodgment of the denture component.

main advantage of an internal occlusal rest

it eliminates a visible buccal clasp arm

allows the rest seat to be in a more favorable position relative to the tipping axis.

where retention usually obtained in internal occlusal rest design

from a lingual clasp arm (cast or wrought wire) in a natural or prepared infrabulge area

ways how to create internal occlusal rests in abutment castings

can be carved in wax, spark eroded, or made using ready-made plastic rest patterns that are waxed into crown/partial-veneer patterns

2 components of internal occlusal rests

reciprocal arm & support — inside

lingual retentive — outside

disadvantage of internal occlusal rest

invasive and extensive

must be considered when an implant serves both as a rest and for retention

how vertical movement resistance affects the retentive function of the attachment; vertical stiffness

a low-profile connection (close to the ridge)

beneficial in implant rest use as it imparts less torque to the implant

how an implant placement affect rotation around a fulcrum line

t can eliminate rotation at an unsupported end or reduce it by shortening the lever arm.

2 main placement considerations for implants as rests

future use as support for a fixed implant prosthesis

current placement at the distal-most tooth position.

reason why proximal prep done before occlusal rest seat prep

to maintain proper marginal ridge height and rest seat floor position

result if occlusal rest seat is done before proximal prep

marginal ridge too low/sharp

floor too close to ridge

large round bur #8

first bur used in enamel prep for occlusal rest seat

reducing the marginal ridge

establish the outline form

smaller round bur #6

second bur used to shape the floor of the occlusal rest seat

deepening the floor establishing the spoon shaped form

to do if rest seat is shallow due to avoiding perforation

widen the seat

plan rest seat location before tooth prep

preferred site for an external rest

occlusal surface of a molar or premolar

must be considered when placing rests on incisors

root form

root length

tooth inclination

crown-to-alveolar support ratio

reason why lingual rest preferred over an incisal rest

closer to the horizontal axis of rotation → less tipping tendency and more esthetic

reason why maxillary canines ideal for lingual rests

has gradual lingual incline and prominent cingulum

2 outline forms of lingual rests on canines & incisor teeth

inverted V — lingual

saddle-like — proximal

measurement of mesiodistal length

2.5-3mm

measurement of labiolingual width

2mm

measurement of incisal-apical depth

1.5mm

location of lingual rest seat prepared

at the junction of the gingival and middle 1/3 of the lingual surface

smaller tapered stones with round ends

burs used to complete the lingual rest preparation after starting with an inverted cone diamond

location of enamel lingual rest seat relative to the tooth anatomy

toward the cingulum rather than the axial wall

polishing sequence recommended for an enamel lingual rest seat

shaped abrasive rubber polishing points followed by flour of pumice

main advantage of accentuating the cingulum in the wax pattern for a lingual rest seat

allows carving the floor to be the most apical portion

creating a saddlelike shape that provides a positive rest seat in line with the tooth’s long axis

4 restorations on which a lingual rest can be placed

cast veneer crown

three-quarter crown

inlay

laminate veneer

composite restoration

etched metal restoration

chromium-cobalt alloy rest seat

attached with composite resin cements and acid-etched tooth preparations.

can be used for individually cast rest seats bonded to lingual surfaces

sapphire ceramic orthodontic brackets

bonded to the lingual surfaces of mandibular canines and shaped as rest seats

advantages over the metal acid-etched retained rests:

• a laboratory step is avoided

• increased bond strengths are achieved

disadvantage:

• removal requires grounding off rest seats

• posing heat generation & pulpal damage risks

three-quarter crown

may be used if the labial surface of the tooth is sound and if the reten- tive contours are satisfactory

veneered complete coverage restoration

if the labial surface presents inadequate or excessive contours for placement of a retentive clasp arm or if gingival decalcification or caries is present

main uses of incisal rests

mostly as auxiliary rests or indirect retainers

most frequently used on MN canines

not indicated on incisors except under unusual situations

advantages of incisal rests

definite support

minimal tooth reduction

little display of metal

better esthetics than a three-quarter crown

disadvantage of incisal rests compared to lingual rests

more likely to cause orthodontic tooth movement due to unfavorable leverage

shape of an incisal rest seat

a rounded notch at the incisal angle or edge, deepest portion apical to incisal edge

ideal dimensions of an incisal rest seat

2.5mm wide and 1.5mm deep

use of incisal rests may be justified by the following factors

may take advantage of natural incisal faceting

tooth morphology does not permit other designs

can restore defective or abraded tooth anatomy

provide stabilization

may restore or provide anterior guidance

contouring of incisal rest

it should be slightly overcontoured

to allow labial and incisal finishing to the adjoining enamel, similar to finishing a three-quarter crown or inlay margin

to minimize metal display while maintaining effectiveness

inverted cone

half of this bur can form the ideal floor of a lingual rest seat

tapered stones with round end

this bur is to round off line angles and accentuate the preparation