Minor connectors

Rests & Rest seats

Presented by:

Dr. Mujtaba Ashraf

MDS II

Dept of Prosthodontics

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Contents:

Minor Connectors

Functions

Form and location

Tissue stops Rests and Rest Seats

Role of Rests in Control of Prosthesis Movement

Form of the Occlusal Rest and Rest Seat

Lingual Rests on Canines and Incisor Teeth

Incisal Rests and Rest Seats

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Minor Connectors

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Definition

The connecting link between the major connector or base of a partial removable dental prosthesis and the other units of the prosthesis, such as the clasp assembly, indirect retainers, occlusal rests, or cingulum rests. –GPT8

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Connects components to the major

connector

Direct retainer

Indirect retainer

Denture base

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Functions of Minor Connectors

Unification and rigidity Transfers functional stress to the abutment teeth. This

is a prosthesis-to-abutment function of the minor connector.

Transfers the effects of the retainers, rests, and stabilizing components throughout the prosthesis. This is an abutment- to-prosthesis function of the minor connector.

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Form and location

Sufficiently bulky to be rigid Should be located in the

embrasure The deepest part of embrasure

should be blocked

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Should be thickest towards lingual surface and tapering towards the contact area.

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Types of minor connectors

Minor connectors that join clasp assemblies to major connectors

Minor connectors that join indirect retainers or auxiliary rests to major connectors

-Stewart

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Minor connectors that join denture bases to major connectors

Minor connectors that serve as approach arms for vertical projection or bar-type clasps

-Stewart

Types of minor connectors

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Minor connectors that join clasp assemblies to major connectors Join clasp assemblies to major connectors must be

rigid, because they support the active components of the removable partial denture, the retentive clasps.

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Support the rests, which prevent vertical movement of a prosthesis toward the underlying tissues.These minor connectors should be broad buccolingually, but thin mesiodistally.

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A minor connector should be positioned in the associated lingual embrasure. This results in a sufficient bulk of metal without encroaching on the tongue space.

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Minor connectors that join indirect retainers or auxiliary rests to major connectors

These minor connectors should form right angles with the corresponding major connectors, but junctions should be gently curved to prevent stress concentration

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Minor connectors that join denture bases to major connectors

These minor connectors must be strong enough to anchor a denture base to the removable partial denture framework.

They must be rigid enough to resist fracture and displacement.

These components must provide minimal interference with the arrangement of artificial teeth.

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In maxillary distal extension bases, it must extent to cover the tuberosity, andIn mandibular distal extension bases it should extend two-third the length of residual ridges,

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Minor connectors that join the denture base to the major connector may be described as follows

Open/ Lattice construction

Mesh construction

Bead, wire, or nail-head components on a metal

base

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Open/ Lattice construction

Open construction consists of longitudinal and transverse struts that form a ladder-like network.

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Placement of the longitudinal and transverse struts is a critical factor in prosthetic tooth arrangement.

In the mandibular arch, one longitudinal strut should be positioned buccal to the crest of the ridge and the other lingual to the ridge crest.

In the maxillary arch, one longitudinal strut should be positioned buccal to the ridge crest.

The border of the major connector generally will act as the second longitudinal strut.

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Positioning of a longitudinal strut along the crest of the ridge must be avoided. This not only interferes with the placement of artificial teeth, but also leads to the denture base to fracture.

Transverse struts also must be positioned to facilitate the placement of artificial teeth.

Ideally, transverse struts should be designed to pass between the necks of the artificial teeth. This aids in tooth arrangement and often results in improved esthetics.

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Advantages of open construction

Used for multiple teeth replacement Provides strongest attachment of acrylic resin Facilitates rebasing and relining

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Mesh construction

A mesh minor connector may be compared to a rigid metallic screen.

Channels that pass through the connector are intended to permit acrylic resin penetration.

This allows resin encirclement of the minor connector and mechanical retention of the denture base.

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Drawback

• During the packing of acrylic resin, increased pressure is needed to force resin through the small holes in the minor connector.

• Insufficient packing pressure may result in inadequate resin penetration and a weak attachment to the framework.

• Also interfere with the arrangement of prosthetic teeth.

The smaller the openings in this minor connector, the weaker the attachment.

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Bead, wire or nail-head construction

Bead, nail-head, or wire components are often used in conjunction with metal denture bases.

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Its is indicated on short-span, tooth supported with well healed ridges.

The primary advantage of a metal base is related toimproved hygiene and thermal stimulation.

DisadvantagesInclude difficulty in adjusting and relining cast metal bases.Furthermore, the attachment of resin is relatively weak.

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Tissue stop

Used in all distal extension partial dentures using lattice work or mesh retention.

Provide stability and prevent distortion to framework during packing of acrylic resin.

Engage buccal and lingual slopes for stability.

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Made by removing approximately 2 square mm of the relief wax used to create space beneath the lattice work or mesh from the point where the posterior end of minor connector crosses the centre of the ridge.During waxing this little depression is waxed as a projection of latticework or mesh.

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Relief

Mechanical retention of denture base resinAllows the acrylic resin to flow under the constructionRelief wax is placed in the edentulous areas of 1 mm

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Finish Lines

The planned junction of different materials. (GPT-8)

In RPDs it denotes the junction of acrylic resin denture bas and metal major connector.

InternalExternal

If they are located on the outer surfaces of major connectors, they are called external finish lines.If they are positioned on the inner or tissue surfaces, they are termed internal finish lines.

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Internal finish lines are formed as a result of relief wax placed on the edentulous ridges of a master cast prior to duplication.The relief wax (24 to 26 gauge) creates an elevated area on the resultant refractory cast. The margins of the relief wax establish internal finish lines in the completed metal framework. The margins of the relief wax should be sharp and well defined.

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External finish lines also must be sharp and should be slightly undercut to help lock the acrylic resin to the major connector.The internal angle formed at the junction of the major or and minor connectors should be less than 90 degrees.

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When viewed from an occlusal perspective, the external finish line should follow an arc established by the lingual surfaces of the remaining teeth.This permits the development of contours that are not irritating to the patient.

The contour of the external finish line should be consistent with the contours of the major connector. Transition from the external finish line to a denture base should be smooth and flowing.

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Approach Arm Minor connector This serves as an approach arm for a bar clasp, supports

a direct retainer and engages an undercut from the gingival margin.

Only minor connector that is not required to be rigid. Should be smooth, even and taper from its origin to its

terminus.

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Rests and Rest Seats

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Definition

Rest: A rigid extension of a fixed or removable dental prosthesis that prevents movement toward the mucosa and transmits functional forces to the teeth or dental implant. GPT-8

The components of a removable partial denture that transfer forces down the long axes of the abutment teeth are called rests. -Stewart

Rest seat: The prepared recess in a tooth or restoration created to receive the occlusal, incisal, cingulum or lingual rest. GPT-8

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Functions Support: Prevent movement of prosthesis toward the

tissues. Force transfer: To direct the forces of mastication

parallel to the long axis of the abutment tooth.

Occlusal rests and rest seats, Albert Seiden, JPD 1958

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To act as a stop, preventing injury and over displacement of the soft tissues under partial denture bases.

To maintain the clasp, which is usually attached to the occlusal rest, in its properly surveyed position.

To function as an indirect retainer in a free-end denture base partial denture.

Occlusal rests and rest seats, Albert Seiden, JPD 1958

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Functions

To help maintain the partial denture in position. This is done when the rests contact or occlude with opposing teeth during mastication and deglutition.

To transmit to the abutment teeth some of the lateral forces applied to the partial denture during mastication.

To prevent impaction of food between the clasp and the proximal surface of the abutment tooth.

Occlusal rests and rest seats, Albert Seiden, JPD 1958

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Functions

To close a small space between teeth by bridging that space with occlusal rests.

To build up an abutment tooth with a large occlusal onlay rest. This is necessary if there has been a loss of occlusal contact due to the drifting of teeth

To act as a rigid reciprocal arm of a retaining clasp. This function is most pronounced when the rest is set into a tapered box-lock seat in a gold crown or inlay.

To prevent extrusion, of abutment teeth.

Occlusal rests and rest seats, Albert Seiden, JPD 1958

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Classification

Based on Relation to Direct Retainer Primary rests Secondary rests

Based on Area of Placement Occlusal rests Lingual or cingulum rests Incisal rests

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Primary rest A rest that is part of a retentive clasp assembly is referred to as a

primary rest. Prevent vertical movement of a prosthesis toward the tissues and also

help transmit applied forces to the supporting teeth. Should be shallow and saucer shaped, and should function as ball-

and-socket joint. This geometry permits dissipation of potentially harmful lateral forces.

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Auxiliary or secondary rests A rest that is responsible for additional support or indirect

retention is called an auxiliary rest or secondary rest. Are used as indirect retainers in extension base removable partial

dentures (Class I, Class II, and long-span Class IV applications). These rests are placed anterior or posterior to the axis of rotation

to prevent the extension bases from lifting away from the underlying ridges.

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Occlusal rest

A rigid extension of a removable dental prosthesis that contacts the occlusal surface of a tooth or restoration, the occlusal surface of which may have been prepared to receive it. –GPT8

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Form of the Occlusal Restand Rest Seat

The outline form of an occlusal rest seat should be a rounded triangular shape with the apex toward the center of the occlusal surface.

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It should be as long as it is wide, and the base of the triangular shape (at the marginal ridge) should be at least 2.5 mm for both molars and premolars.

Rest seats of smaller dimensions do not provide an adequate bulk of metal for rests, especially if the rest is contoured to restore the occlusal morphology of the abutment tooth.

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The marginal ridge of the abutment tooth at the site of the rest seat must be lowered to permit a sufficient bulk of metal for strength and rigidity of the rest and the minor connector. This means that a reduction of the marginal ridge of approximately 1.5 mm is usually necessary.

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The floor of the occlusal rest seat should be apical to the marginal ridge and the occlusal surface and should be concave, or spoon shaped.Caution should be exercised in preparing a rest seat to avoid creating sharp edges or line angles in the preparation.

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The angle formed by the occlusal rest and the vertical minor connector from which it originates should be less than 90 degrees. Only in this way can the occlusal forces be directed along the long axis of the abutment tooth.

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An angle greater than 90 degrees fails to transmit occlusal forces along the supporting vertical axis of the abutment tooth. This also permits slippage of the prosthesis away from the abutment, which can result in orthodontic-like forces being applied to an inclined plane on the abutment, with possible tooth movement.

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An occlusal rest seat should occupy one third to one half the mesiodistal diameter of the abutment and approximately one half the buccolingual width of the tooth measured from cusp tip to cusp tip.

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Lingual or cingulum rests

A metallic extension of a partial removable dental prosthesis framework that fits into a prepared depression within an abutment tooth’s lingual surface. –GPT8

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Lingual or cingulum rests are used primarily on maxillary canines. The normal morphology of a maxillary canine permits preparation of a satisfactory rest seat with minimal tooth reduction.

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The thickness of enamel on the lingual surface of a mandibular canine rarely allows a lingual rest to be used.Lingual rests on incisors are also rare.

The main indication for their use is missing canines. In this instance, multiple incisors should receive lingual rests to distribute the stresses over a number of teeth because a single incisor seldom offers adequate support.

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A lingual rest is located closer to the rotational center of the supporting tooth and does not tend to tip the tooth.

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Form

Outline form of lingual rest is half-moon shaped, making a smooth curve from one marginal ridge to the other, crossing the centre of the tooth above the cingulum with deepest point over the cingulum.

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The rest is is V-shaped and has two inclinesLabial incline and lingual inclineLabial incline is parallel to the labial surface of toothLingual incline begins at the top of cingulum and converges labiogingivally towards centre of the tooth, to meet the labial incline at the apex of rest seat.

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The mesiodistal length of the preparation should be a minimum of 2.5 to 3 mm,labiolingual width about 2 mm,incisal-apical depth a minimum of 1.5 mm.

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Round Lingual Rest Seat Form

Mesial of the canine teeth when typical cingulum rest contraindicated Large restoration Lack of clearance with the opposing teeth Poor cingulum

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Spoon shaped, similar to occlusal rest seatMore difficult due to the incline of the lingual surface Easily incorporated into crowns

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Lingual rest seats on cast restorations When a crown is to be placed on an anterior tooth and

a rest seat is required, the rest seat should be placed in the wax pattern

The floor of the rest seat is readily carved to be the most apical portion of the preparation. A saddle-like shape, which provides a positive rest seat located favorably in relation to the long axis of the tooth, is formed.

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The framework of the denture is made to fill out the continuity of the lingual surface so that the tongue contacts a smooth surface without the patient being conscious of bulk or irregularities.The lingual rest may be placed on the lingual surface of a cast veneer crown a three-quarter crown, an inlay, a laminate veneer, a composite restoration.

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Incisal rests Incisal rests are placed at the incisal angles of anterior

teeth and on prepared rest seats. Less desirable than lingual rests and rest seats for

anterior teeth. Nevertheless, they may be used successfully if abutments are sound and cast restorations are not indicated.

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The incisal rest may be used on a canine abutment in either arch, it is more applicable to the mandibular canine. This type of rest provides definite support with relatively little loss of tooth structure and little display of metal.Esthetically it is preferable to the three-quarter crown.

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Three views of an incisal rest seat preparation on the mandibular canine adjacent to a modification space.The labial view demonstrates inclination of the floor of the rest seat, which allows forces to be directed along the long axis of the tooth as nearly as possible.

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The proximal edge of the rest seat is rounded rather than straight. The lingual view shows that all borders of the restseat are rounded to avoid sharp line angles.It is especially important to avoid a line angle at the junction of the axial wall of the preparation and the floor of the rest seat

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Should appear as a small , V-shaped notch located approximately 1.5 to 2.0 mm from the proximal-incisal angle of the tooth.

The deepest part of the preparation should be toward the center of the tooth mesiodistally.

The notch should be rounded and should extend slightly onto the facial surface to provide a positive seat for the rest.

A=2mmB= 1.5mm

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References

Stewart’s clinical removable partial prosthodontics, 3rd ed McCrackens removable partial prosthodontics, 12th ed Removable partial dentures-A Practitioners’ Manual, Olcay

Şakar Seidin A. Occlusal rests and rest seats. J Prosthet Dent

1958;8: 431-440. Dunny JA, King CIE. Minor connector design for anterior

acrylic resin bases: A preliminary study. J Prosthet Dent 1975;34:496-502.

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