Minor Connectors Rests Rola M Shadid BDS MSc

Minor Connectors & Rests Rola M. Shadid, BDS, MSc

Lecture Outline Discuss functions of minor connector Discuss types of minor connectors Discuss functions of rests Discuss forms of rests & prep of rest seats

Minor Connectors Connects components to the major connector – Direct retainer – Indirect retainer – Denture base

Functions of Minor Connectors Unification and rigidity Stress distribution * Bracing through contact with guiding planes Maintain a path of insertion via contact with guiding planes

Types of Minor Connectors Embrasure Minor Connectors – Between two adjacent teeth

Embrasure Minor Connectors Should have sufficient bulk to be rigid but in the same time unobjectionable as possible Should be located in an embrasure not be located on a convex surface Should be tapering toward the contact area

Embrasure Minor Connectors – Joins major connector at right angles – Relief placed so connector not directly on soft tissue

Embrasure Minor Connectors It should be thickest toward the lingual surface, tapering toward the contact area Triangular shaped in cross section

Embrasure Minor Connectors – Contact teeth above height of contour – Prevents wedging & tooth mobility

Gridwork Minor Connectors Connect the denture base and teeth to the major connector

Gridwork Minor Connectors Adjacent edentulous spaces Usually connect major connector to direct retainers Open lattice work or mesh types

Gridwork Minor Connectors Minor connector for mandibular distal extension base should extend posteriorly about 2/3 the length of the edentulous ridge Never on the ascending portion of the ridge

Gridwork Minor Connectors Minor connectors for maxillary distal extension denture bases should extend the entire length of the residual ridge

Gridwork Minor Connectors Mesh type – – – Flatter Potentially more rigid Less retention for acrylic if openings are small

Gridwork Minor Connectors Lattice Type • Potentially superior retention • Interferes with setting of teeth, if struts are too thick – Both types are acceptable if correctly designed

Gridwork Relief Mechanical retention of denture base resin Allows the acrylic resin to flow under the gridwork

Gridwork Relief wax is placed in the edentulous areas 1 mm of relief

Relief Under the Gridwork Should begin 1. 5 - 2 mm from the abutment tooth

Relief Under the Gridwork Creates a metal to tissue contact – Preferable since it wears less – Less porous, (hygiene)

Junction With Major Connector Butt joint with slight undercut in metal Maximum bulk of the acrylic resin Prevents thin, weak edges fracturing

Gridwork Design Facially just over the crest of the residual ridge

Position of Major Connector Junction Should be ≈ 2 mm medial to lingual surface of denture teeth Ensures bulk of resin around teeth

Mandibular Tissue Stops Contact of metal with cast at posterior of distal extension gridwork Prevents distortion at free end during hydraulic pressure of processing

No Tissue Stops In Maxillary major connector acts as a tissue stop (no relief)

Proximal Plates Minor connectors originating from the gridwork in an edentulous area Broad contact with guiding planes May or may not terminate in an occlusal rest

Proximal Plates

Proximal Plates Shifted slightly lingually – – – Increases rigidity Enhances reciprocation Improves esthetics Often a triangular space below the guiding plane (an undercut)

Proximal Plates Rigid, cannot be placed in undercut Block-out placed in undercuts prior to waxing and casting the framework

Rests & Rest Seats

o The most effective resistance can be provided if the tooth is stressed along its long axis o Prosthesis should engage the tooth in a manner that encourages axial loading

Rest A rigid component of RPD resting in a recessed preparation on the occlusal, lingual or incisal surface of a tooth. Provides vertical support

Rest Seat Portion of a tooth prepared to receive a rest *

Functions Direct occlusal forces along axis of abutment

Functions SUPPORT - Prevents impingement of soft tissue * Fractured abutment, no rest seat Slides down incline

Major connector imbeds into tissue

Functions Maintains established occlusal relationships by preventing settling of the denture *

Functions Maintain components in their planned positions (maintains a clasp -tooth relationship)

Functions Provide reference for relines or impressions

Functions Act as indirect retainer – Prevents rotation (Class I or II RPD’s only)

Form of Occlusal Rest Rounded triangular shape with the apex toward center of occlusal surface As long as it is wide

Occlusal Rest Seat Form Base of triangle should be one third the bucco-lingual width of the tooth *

Rest Seat Form Smooth flowing outline form (i. e. no sharp line angles)

Occlusal Rest Seat Deepest portion is central

Occlusal Rest Seat § Floor should be apical to marginal ridge § Angle formed by occlusal rest & vertical minor connector from which it originates should be less than 90°

“Positive” Rest Seats An explorer tip will not slide off the rest seat

Occlusal Rest Seat Directs the occlusal forces along the long axis of the tooth Prevent orthodontic movement

Occlusal Rest Seat Floor should be concave or spoon shaped (ball-&-socket joint) Prevents horizontal stresses & torque

Occlusal Rest Seat Marginal ridge must be lowered and rounded 1 -1. 5 mm – Bulk of metal to prevent fracture *

Occlusal Rest Seat Adjacent Tooth – Rest not flared to facial line angle – Lingual flared more - space for minor connector

Secondary Occlusal Rest * § To prevent slippage of the primary rest § To prevent orthodontic movement of abutment tooth § To direct forces over greatest root mass of abutment

Extended Occlusal Rest * § To minimize further tipping of the abutment § To ensure that forces are directed down the long axis of abutment

Extended Occlusal Rest § Extends more than one half mesiodistal width of tooth § One third the buccolingual width of the tooth § Allow for minimum of 1 mm thickness of metal § Rounded with no undercuts

Extended Occlusal Rest (Onlay) If the abutment is severely tilted, the extended occlusal rest may take the form of an onlay to restore the occlusal plane

Extended Occlusal Rest (Onlay) * § Provide stabilization § Restore the contour & occlusion of natural tooth § Directs forces down the long axis of tooth

Double Embrasure (Interproximal) Rest Seat § Adjacent rests are used to prevent interproximal wedging by framework § Joined rests are designed to shunt food away from contact points

Double Embrasure (Interproximal) Rest Seat Form Flared more dramatically on facial and lingual line angles *

Interproximal Rest Seat Prep. § Avoid reducing or eliminating contact points of abutment teeth § Sufficient tooth structure must be removed * § Ensure all line angles are smoothed

Prep. For Rest Seats Rests may be placed on sound enamel or on any restoration material Rests placed on sound enamel are not conducive to caries in mouth with a low-caries index provided that good oral hygiene is maintained

Prep. for Rest Seats Patient should be advised that future susceptibility to caries is not predictable and that much depends on oral hygiene and possible future changes in caries susceptibility.

Prep. For Rest Seats A fluoride gel should be applied to abutment teeth following enamel recontouring If the master cast will be fabricated from an irreversible hydrocolloid impression, application of the gel should be delayed until after impressions are made.

Prep. For Rest Seats Light pressure High-speed handpiece +/- Waterspray Minimal heat is generated

Prep. For Rest Seats Keep in enamel No anesthesia

Prep. for Occlusal Rest Seats The larger round bur is used first to lower the marginal ridge and to establish the outline form of rest seat A slightly smaller round bur is then used to deepen floor of rest seat The preparation is smoothed by polishing point

Prep. for Occlusal Rest Seats When a small enamel defect is encountered in the preparation of an occlusal rest seat, it is usually best to ignore it until the rest preparation has been completed.

Prep. for Occlusal Rest Seats When perforation in restoration does occur, it may be repaired, but occasionally the making of a new restoration is unavoidable.

Occlusal Rest Seats in Crowns Occlusal rest seats in crowns and inlays are generally made somewhat larger and deeper than those in enamel *

Lingual Rests on Canines & Incisor Teeth A canine is much preferred over incisor * When a canine is not present, multiple rests that are spread over several incisors are preferable to the use of single incisor

Cingulum Rest Seat Form Inverted “V” at junction of gingival & middle one third of tooth < 900

Cingulum Rest Seat Form Rest seat prep. broadest at most lingual aspect of canine As preparation approaches proximal surfaces of tooth, it is less broad than at any other areas

Cingulum Rest Seat Form Slightly rounded to avoid sharp line angles Test as ‘positive’ with explorer tip (floor of rest seat should be toward cingulum rather than axial wall)

Cingulum Rest Seat 2. 5 to 3 mm mesiodistal length 2 mm labiolingual width 1. 5 mm incisal-apical depth *

Correct Preparation

Preparation Too High

Preparation Too Low

Cingulum Rest Prep. Do not create an enamel undercut Cylindrical bur along the long axis of the tooth

Maxillary Cingulum Rest Seats Avoid opposing occlusion

Maxillary Cingulum Rest Seats 1. 5 - 2. 0 mm clearance for metal Check with articulated models *

Cingulum Rest Seat Placement Place in sound tooth structure* Not on amalgam restorations If cingulun not prominent or large pulp – Use different tooth – Composite restoration – Onlay or crown – Bonded rest seat

Bonded Rest Seat Cast chromiumcobalt alloy rest seat forms Attached to lingual surfaces of anterior teeth by use of composite resin cements with acid-etched tooth prep.

Ball (Round) Lingual Rest Mesial of the canine teeth when typical cingulum rest contraindicated – Large restoration – Lack of clearance with the opposing teeth – Poor cingulum

Ball (Round) Lingual Rest Spoon shaped, similar to occlusal rest seat More difficult due to the incline of the lingual surface Easily incorporated into crowns

Ball (Round) Lingual Rests § Rest seats prep. in -tooth surface if sufficient enamel thickness exists -restorations placed in teeth where enamel thickness is inadequate § More conservative than cingulum lingual rest seats

Incisal Rest The least desirable placement of a rest seat Incisal rests are used predominantly as - Auxiliary rests - Indirect retainers

Incisal Rest

Lingual Rest Preferable to Incisal Rest § Lingual rest placed nearer horizontal axis of rotation (tipping axis) of abutment - Less tendency to tip the tooth * § More esthetically acceptable than incisal rest

Incisal rest More applicable to mandibular canine -Definite support with little loss of tooth structure -Little display of metal

Incisal Rest Prep. 2. 5 mm wide 1. 5 mm deep

Incisal Rest Prep. Rounded notch at the incisal angle with the deepest portion apical to the incisal edge The notch should be beveled labially & lingually *

Multiple Incisal Rests o Take advantage of natural incisal faceting. o Tooth morphology does not permit other designs. o Can restore defective or abraded tooth anatomy. o Provide stabilization. o May restore or provide anterior guidance.

References 1. Mc. Cracken’s Removable Prosthodontics, 11 th Edition 2005 by Mc. Givney GP, Carr AB. Chapter 5 and 6 2. Dalhousie continual education