P2-RPD CHAPTER 6

- Provides efficient resistance to functional chewing forces

- Achieved by engaging remaining teeth through proper use of rest seats and rests

What is the importance of rest and rest seat in RPDs?

Natural teeth possess sophisticated support mechanisms and show better displacement and recovery.

What supports RPD better: teeth or oral mucosa?

Anyy partial denture component that sits on a tooth surface to provide vertical support.

What is a rest in RPD?

The prepared tooth surface designed to receive the rest.

What is a rest seat in RPD?

• Occlusal rest

• Lingual rest

• Incisal rest

What are the types of rests based on location?

Generally restore the original anatomy of the tooth before the rest seat was prepared.

What should the topography of a rest do?

To provide vertical support for the partial denture.

What is the primary purpose of a rest?

• Maintain components in their planned positions

• Maintain established occlusal relationships by preventing denture settling

• Prevent impingement of soft tissue

• Direct and distribute occlusal loads to the abutment teeth

What are the additional functions of rests?

• Support the position of the denture

• Resist movement toward the tissue

• Transmit vertical forces to abutment teeth

• Direct forces along the long axis of teeth

What functions do rests serve in tooth-supported RPDs?

• Must be rigid

• Must receive positive support from abutment teeth

• Must stay in stable contact with the tooth

• Must not move independently or slip

How must a rest behave under occlusal load?

More load is supported by tissue rather than the abutment tooth.

What happens in distal extension RPDs as the load is farther from the abutment?

More occlusal load is transmitted through the rest to the abutment tooth.

What is transmitted closer to the abutment in distal extension RPDs?

• Prevent apical movement

• Maintain retentive clasp arm’s position in the undercut

• Keep clasp arm passive but in contact when at rest

• Allow clasp arm to engage under dislodging forces

• Prevent settling and maintain vertical stability

How do rests affect clasp arm function?

• Eliminate compression of soft tissues

• Control vertical movement of the denture base

• Eliminate or alter fulcrum lines

• Increase support and stability of the prosthesis

How can implants be used as rests?

• As long as it is wide

• Base (at marginal ridge) at least 2.5 mm wide

• Smaller dimensions are inadequate for metal bulk and occlusal anatomy restoration

What is the outline form of an occlusal rest seat?

Approximately 1.5 mm

How much reduction is needed for the marginal ridge?

• Apical to both the marginal ridge and occlusal surface

• Must be concave or spoon-shaped

• Sharp edges or line angles must be avoided

Where should the floor of the rest seat be?

Less than 90 degrees

What should be the angle between the rest and the minor connector?

• Directs occlusal forces along the long axis

• Prevents slippage

• Avoids orthodontic-like forces causing tooth movement

Why should the rest-minor connector angle be <90°?

Unfavorable loading and potential tooth movement

What is the effect of an angle >90° between the rest and minor connector?

When the existing rest seat is too shallow or already inclined apically and cannot be deepened

When is a secondary occlusal rest used?

• On the opposite side of the tooth from the primary rest

• Slightly apically inclined from the marginal ridge if possible

Where should a secondary occlusal rest be placed?

Prevent unfavorable forces if connectors are sufficiently rigid

What is the function of opposing occlusal rests on diverging inclines?

Shallow ball-and-socket joint to prevent horizontal stress transfer

What is the ideal relationship between rest and abutment in tooth-tissue supported RPDs?

Provide only vertical (occlusal) support

What is the function of an occlusal rest?

They do not stabilize horizontal movement

What is a limitation of occlusal rests?

Locking effects that can cause harmful leverages

What must be avoided in occlusal rests?

• In Kennedy Class II, Mod 1

• In Kennedy Class III

• When the most posterior abutment is a mesially tipped molar

When is an extended occlusal rest used?

• Minimize further tipping of the abutment

• Direct occlusal forces down the long axis

What is the purpose of an extended occlusal rest?

• Extends more than one-half of the mesiodistal width of the tooth

• Covers approximately one-third of the buccolingual width

• Must allow at least 1 mm of metal thickness

• Rounded and free of undercuts or sharp angles

What are the design features of an extended occlusal rest?

Onlay to restore the occlusal plane

What is the form of extended rest in tilted abutments?

• Remove or restore pits, fissures, grooves

• Bevel buccal and lingual occlusal surfaces (1–2 mm)

• Restore occlusal contour and function

• Include a 1–2 mm guiding plane on the mesial surface

What are the steps in onlay-type extended rest preparation?

When a direct retainer requires interproximal rest placement

When are interproximal occlusal rest seats used?

• Extended farther lingually than usual

• Use of adjacent (not combined) rests

What are the design features of interproximal rest seats

• Prevent framework wedging

• Shunt food away from contact points

Why used of Adjacent rest instead of single rest?

Eliminating natural contact points

What must be avoided during prep of interpromiximal occlusal rest?

• Ensure strength of rest component

• Avoid alteration of occlusion

Why is sufficient tooth reduction needed in interproximal occlusal rest?

• To evaluate interocclusal contact areas

• Ensure adequate rest space

• Avoid occlusal interference

Why must mounted diagnostic casts be used?

Vertical grooves that create torquing forces

What must be avoided in lingual interproximal prep?

In tooth-supported partial dentures with cast retainers on all abutment teeth

When are internal occlusal rests used?

• Occlusal support

• Horizontal stabilization

What support do internal occlusal rests provide?

Retainers or attachments (which they are not)

What are they commonly confused with?

Floor of the rest seat

Where is occlusal support derived from?

Near-vertical walls of the rest seat

Where is horizontal stabilization derived from?

• Parallel to the path of placement

• Slightly tapered occlusally

• Slightly dovetailed to resist proximal dislodgment

What are the design features of internal rest seats?

No visible buccal clasp arm

What is an advantage of internal rests?

A lingual clasp arm in a natural or prepared infrabulge area

What provides retention in internal rest setups?

• Carved in wax

• Spark eroded into casting

• Using plastic rest patterns waxed into crown/partial veneer patterns

How are internal rest seats fabricated?

Dental cast surveyor to maintain parallelism

What helps orient rest seats during fabrication?

Only in tooth-supported RPDs

Where are internal rests currently used?

Resist tissue-ward (vertical) movement and support retention

What is the role of implants as rests?

Stress from vertical resistance affecting the attachment

What must be considered in dual-function (rest + retention) implant design?

• Resist vertical movement

• Provide positive support

• Allow low-profile connection close to ridge

• Reduce torque

• Allow strategic placement for biomechanical advantage

What are the advantages of using implants as rests

• By eliminating fulcrum lines

• Decreasing the effective lever arm

How can implants reduce denture rotation?

• Future use for implant-fixed prosthesis

• Current placement at the distal-most tooth to resist functional forces

What are considerations for implant placement?

• Sound enamel

• Restorative materials proven to resist fracture and distortion under stress

What surfaces can support rests?

• When the patient has low caries risk

• When oral hygiene is well maintained

When is enamel safe for rest support?

Occlusal surface

Which surface is less prone to caries for rest placement?

Restore them without requiring full crowns

What should be done with precarious fissures in sound teeth?

Full-coverage crowns properly contoured for RPD support

What provides best protection against caries in RPD?

• Future vulnerability of the tooth

• Patient compliance with hygiene

What must be assessed when using unprotected enamel for rest seats?

• Clinical judgment

• Economic constraints

• Patient’s oral hygiene and recall ability

What factors affect the decision to use unprotected enamel?

• Prepare proximal surfaces first to create guiding planes and remove undercuts

• Prepare occlusal rest seats afterward to ensure proper position

What is the correct sequence of tooth preparation for rest seats?,

• Ensures correct relationship with marginal ridge

• Prevents shallow or excessively deep seats

Why is the occlusal rest seat prepared after proximal surfaces?

• Use large round bur to define outline and reduce marginal ridge

• Deepen floor with smaller round bur

• Finish with polishing points

What is the technique for preparing enamel occlusal rest seats?

• Finish rest seat

• Restore defect flush with the floor

What is done if minor enamel defects are found during prep?,

Flouride gel

What is appliead after the enamle recountering?

• Slightly widen the seat

• Add a secondary rest on the opposite side

What if rest seat is too shallow in a restoration?

They are usually deeper and larger

How do rest seats in crowns differ from those in enamel?

• Evaluate lingual and incisal contacts

• Ensure or create space

Why analyze mounted casts before placing anterior rests?

• When posterior abutments are missing

• When indirect retainers or auxiliary rests are needed

When are anterior teeth used for rest seats?

canines

Which anterior teeth are most suitable for rests?

Use multiple incisor rests instead of a single one

What is done when no canines are available for rests?,

• Root length and form

• Tooth inclination

• Ratio of clinical crown to alveolar support

What must be evaluated before placing anterior rests?

Lingual rest

Which rest type is preferable in anterior teeth: lingual or incisal?

• Closer to tipping axis

• Better esthetics

Why are lingual rests preferred over incisal?,

• More visible

• Less favorable mechanics

Why are incisal rests less ideal?

It has gradual lingual incline and prominent cingulum

Which canine is suitable for enamel lingual rests?

They have steep inclines and thin enamel

Why are mandibular anterior teeth less suitable for lingual rests?

• Create V-shaped rest at junction of gingival and middle thirds

• Apex points incisally

What is the first method for lingual rest prep?,

• Inverted cone diamond

• Finished with tapered round-end stone

What instruments are used for the first method?

• Sharp line angles

• Enamel undercuts

What should be avoided during lingual rest prep?,

Toward the cingulum (not the axial wall)

Where should the floor of a lingual rest seat be directed

Create rest seat in a restoration (cast)

What is the second method of anterior lingual rest prep?

Allows accurate wax pattern and stronger support

Why is the second method preferred?

• Restore lingual anatomy

• Provide smooth contour

• Be unnoticeable to the tongue

What must the framework do with cast lingual rest?

• Cast veneer crowns

• Three-quarter crowns

• Inlays

• Laminate veneers

• Composite restorations

• Etched metal restorations

What restorations can receive cast lingual rests?

When labial surface is sound and contours are acceptable

When are three-quarter crowns used for rest seats?,

• Labial contour is inadequate

• Labial surface has decalcification or caries

When are three-quarter crowns contraindicated?

• Poor biomechanics

• Esthetic concerns

Why are incisal rests least desirable?

Auxiliary rest

• Indirect retainer

When are incisal rests used?

Mandibular arch

Which arch more commonly uses incisal rests?

Mandibular canine

Which anterior tooth is most ideal for incisal rests?,

• Good support

• Minimal enamel reduction

• Less visible metal

What are the advantages of incisal rests on canines?,

Rounded notch at incisal angle of canine or edge of incisor

What is the shape of an incisal rest seat?

Apical to incisal edge

Where is the deepest part of the incisal rest?

• Labial bevel

• Lingual bevel

What bevels are required in incisal rest prep?,

• Width: ~2.5 mm

• Depth: ~1.5 mm

What are the dimensions of an incisal rest seat?,

• No other rest location possible

• Anterior guidance needed

• Defective or abraded incisor anatomy

• Added stabilization needed

When are multiple incisal rests used on mandibular incisors?

• Thick enamel areas

• Restorations with thin enamel

What are ball-type rests used for