05. rests & rest seats

Rests & Rest SeatsRests & Rest Seats

RestRest• A rigid component resting in a recessed A rigid component resting in a recessed

preparation on the occlusal, lingual or preparation on the occlusal, lingual or incisal surfaceincisal surface

• Provides vertical supportProvides vertical support

Rest SeatRest Seat

• Portion of a tooth prepared to receive a Portion of a tooth prepared to receive a restrest

FunctionsFunctions

• Direct forces along long axis of abutmentDirect forces along long axis of abutment

FunctionsFunctions• SUPPORT - prevents denture base from SUPPORT - prevents denture base from

moving cervically & impinging gingivamoving cervically & impinging gingivaSlides down inclineSlides down incline

Fractured abutment, no rest seat

Major connector imbeds into tissue

FunctionsFunctions

• Maintain a clasp -tooth relationshipMaintain a clasp -tooth relationship

FunctionsFunctions

• Prevent extrusion of abutmentsPrevent extrusion of abutments• Provide reference for relines or Provide reference for relines or

impressionsimpressions

FunctionsFunctions

• Act as indirect retainerAct as indirect retainer− Prevents rotation (Class I or II RPD’s Prevents rotation (Class I or II RPD’s

only)only)

Preparation of Rest SeatsPreparation of Rest Seats

• Light pressure Light pressure • High-speed handpieceHigh-speed handpiece• +/- Waterspray+/- Waterspray• Minimal heat is generatedMinimal heat is generated

Preparation of Rest SeatsPreparation of Rest Seats

• Keep in enamelKeep in enamel• No anesthesiaNo anesthesia

Preparation of Rest SeatsPreparation of Rest Seats

• Occlusal Rest SeatsOcclusal Rest Seats− Diamonds Diamonds

• 8801- 0148801- 014• 38006 -13538006 -135

− Medium round Medium round carbide burs carbide burs • #2, #2, #4#4

8801- 0148801- 014

Preparation of Rest SeatsPreparation of Rest Seats

• Cingulum Rest Seats & Cingulum Rest Seats & Guiding PlanesGuiding Planes− Long, medium Long, medium

diameter cylindrical diameter cylindrical bur or diamond (e.g. bur or diamond (e.g. #57L; #57L; 8837K-0148837K-014))

8837K-0148837K-014

Rest Seat FormRest Seat Form

• Smooth flowing outline form (i.e. no Smooth flowing outline form (i.e. no sharp line angles)sharp line angles)

Occlusal Rest Seat FormOcclusal Rest Seat Form

• Rounded triangular shape Rounded triangular shape − Apex near center of toothApex near center of tooth

Occlusal Rest Seat FormOcclusal Rest Seat Form

• Base of triangle should be one third the Base of triangle should be one third the bucco-lingual width of the toothbucco-lingual width of the tooth

Occlusal Rest Seat FormOcclusal Rest Seat Form

• Marginal ridge must be lowered and Marginal ridge must be lowered and rounded 1-1.5mmrounded 1-1.5mm− Bulk of metal to prevent fractureBulk of metal to prevent fracture

Occlusal Rest Seat FormOcclusal Rest Seat Form

• Floor inclined towards the center Floor inclined towards the center • Angle formed by rest and minor Angle formed by rest and minor

connector should be less than 90connector should be less than 9000

Occlusal Rest Seat FormOcclusal Rest Seat Form

• Deepest portion is centralDeepest portion is central

Occlusal Rest Seat FormOcclusal Rest Seat Form

• Floor should be concave or spoon shaped Floor should be concave or spoon shaped ball-&-socket jointball-&-socket joint

• Prevents horizontal stresses & torquePrevents horizontal stresses & torque

Occlusal Rest Seat FormOcclusal Rest Seat Form

• Directs the occlusal forces along the long Directs the occlusal forces along the long axis of the toothaxis of the tooth

• Prevent orthodontic movementPrevent orthodontic movement

““Positive” Rest SeatsPositive” Rest Seats

• An explorer tip will not slide off the rest An explorer tip will not slide off the rest seatseat

a

Explorer willNOT slip off restseat if pulledproximallly Explorer WILLslip off rest seatif pulledproximallly‘Positive’ Rest Seat(< 90°) Rest Seat not ‘Positive’(>90°)

Occlusal Rest Seat FormOcclusal Rest Seat Form

• Conventional FormConventional Form

Occlusal Rest Seat FormOcclusal Rest Seat Form

• Adjacent ToothAdjacent Tooth− Rest not flared to facial line angleRest not flared to facial line angle− Lingual flared more - space for minor Lingual flared more - space for minor

connectorconnector

Double Embrasure Rest Seat Double Embrasure Rest Seat FormForm• Adjacent teeth, the form is also modifiedAdjacent teeth, the form is also modified• Flared more dramatically on facial and Flared more dramatically on facial and

lingual line angleslingual line angles

Double Embrasure Rest Seat Double Embrasure Rest Seat FormForm• Provides space for the retentive & Provides space for the retentive &

bracing armsbracing arms• Ensure all line angles are smoothedEnsure all line angles are smoothed

Lingual Rest SeatsLingual Rest Seats

• Usually the canine, due to its well-Usually the canine, due to its well-developed cingulumdeveloped cingulum

• When canine is not available, an incisor When canine is not available, an incisor may be usedmay be used

Cingulum Rest Seat FormCingulum Rest Seat Form

• Inverted “V”Inverted “V”• < 90< 9000

Cingulum Rest Seat FormCingulum Rest Seat Form

• Slightly rounded to avoid sharp line Slightly rounded to avoid sharp line anglesangles

• Test as ‘positive’ with explorer tipTest as ‘positive’ with explorer tip• 1mm depth1mm depth

Correct PreparationCorrect Preparation

Preparation Too HighPreparation Too High

Preparation Too LowPreparation Too Low

Cingulum Rest PreparationCingulum Rest Preparation

• Do not create an enamel undercutDo not create an enamel undercut• Cylindrical bur along the long axis of the Cylindrical bur along the long axis of the

toothtooth

Cingulum Rest Seat PlacementCingulum Rest Seat Placement

• Place in sound tooth structure or Place in sound tooth structure or restorationsrestorations

• Not on amalgam restorationsNot on amalgam restorations• If not prominent or large pulpIf not prominent or large pulp

− Use different toothUse different tooth− Bonded rest seatBonded rest seat− Onlay or crownOnlay or crown

Maxillary Cingulum Rest SeatsMaxillary Cingulum Rest Seats

• Avoid opposing occlusionAvoid opposing occlusion

Maxillary Cingulum Rest SeatsMaxillary Cingulum Rest Seats

• 1.5 - 2.0 mm clearance for metal1.5 - 2.0 mm clearance for metal• Check with articulated modelsCheck with articulated models

Bonded Cingulum RestsBonded Cingulum Rests

• Flat emergence profileFlat emergence profile• Pumice, rinse, etch, bond using rubber Pumice, rinse, etch, bond using rubber

damdam• Ensure cervical composite well adaptedEnsure cervical composite well adapted

Composite Bonded Rest Seat FormComposite Bonded Rest Seat Form

CorrectCorrect OvercontouredOvercontoured OvercontouredOvercontouredOpen marginOpen margin

Round Lingual Rest Seat FormRound Lingual Rest Seat Form

• Mesial of the canine teeth when typical Mesial of the canine teeth when typical cingulum rest contraindicatedcingulum rest contraindicated− Large restorationLarge restoration− Lack of clearance with the opposing Lack of clearance with the opposing

teethteeth− Poor cingulumPoor cingulum

Round Lingual Rest Seat FormRound Lingual Rest Seat Form

• Spoon shaped, similar to occlusal rest seatSpoon shaped, similar to occlusal rest seat• More difficult due to the incline of the More difficult due to the incline of the

lingual surface lingual surface • Easily incorporated into crownsEasily incorporated into crowns

Incisal Rests Incisal Rests • Inferior mechanically & estheticallyInferior mechanically & esthetically• Lingual rest is nearer to the center of Lingual rest is nearer to the center of

rotationrotation− Less torquing potentialLess torquing potential