DENTURE BASE CONSIDERATIONS AND STRESSBREAKERS IN REMOVABLE CAST PARTIAL DENTURES

Denture Base Considerations& Stress-Breakers in Removable Partial Dentures RUCHI GOELJR Final Year1Contents IntroductionFunction of denture baseIdeal denture base materialsAdvantages of metal basesDesign of denture baseMethods of attaching denture baseMethods of attaching artificial teeth

Contents (contd.)Stress breakersDefinitionConcept Mechanism of actionType Various attachmentsAdvantages and disadvantagesConclusion and references.

Denture base Denture base is the part of a denture that rests on the foundation tissues and to which teeth are attached.Glossary of Prosthodontic Terms , J Prosthet Dent 2005;94(1):10-92

Functions of denture baseSupports the artificial teeth mastication Transfer the functional occlusal forces to the supporting oral structures stabilityEstheticStimulation of the underlying tissues of the residual ridge.

Functions of denture base(contd)Prevent vertical and horizontal migration of remaining natural teeth.Provides support of the denture.Prevents undesirable food traps maintains oral cleanliness.

Ideal Denture BaseThe requirements for an ideal denture base are as follows:Accuracy of adaptation to the tissues, with low to minimal volume change.Dense, non- irritating surface capable of receiving and maintaining a good finish.Thermal conductivity.Low specific gravity, lightness in mouth.

Ideal Denture Base(contd)Sufficient strength; resistance to fracture or distortion.Self- cleansing.Esthetic acceptabilityPotential for future relining Low initial cost Such an ideal denture base material does not exist, nor is it likely to be developed in near future. However, any denture base, whether resin or metal and regardless of method of fabrication should come as close as possible to this ideal as possible. Denture base materialAcrylic resin MetalCombination Acrylic resin : Polymethyl methacrylate Poly vinyl resins Polystyrenes Metal base : Gold alloys Co Cr alloysMetal v/s Acrylic Base

Metal bases Tooth -supported partial dentures

Acrylic base Distal extension ridges as they can be relined frequently with greater ease

Accuracy and permanence of formMetal bases

Greater accuracy & better duplication of finer details.

Greater intimacy of contact provides for direct indirect retention.

Absence of internal strains & resistance to abrasion to cleaning agents

Need for additional PPS is eliminated.

Acrylic bases Volumetric shrinkage & distortion with less accurate adaptation Abrasion due to cleaning agents and constant brushing of tissue surface.

Need for additional recording and depth of PPS is necessary to aid in retention. Comparative tissue responseMetal base Inherent cleanliness due to greater density and bacteriostatic activity contributed by the ionization and oxidation of metal

Maintaining health of oral tissues.Acrylic base Porous surface of acrylic resin tend to accumulate mucinous deposits containing food particles as well as calcareous deposits bacterial colonization.

Unfavorable tissue response. Thermal conductivity

Temperature changes are transmitted through the metal bases thus maintaining the health of underlying tissues via stimulation.Moreover improving patient's acceptance by allaying the feel of a foreign body.Acrylic resins act as insulators thus depriving the tissues of the natural feel. Weight and BulkCast in much thinner sections than resin and still have adequate strength and rigidity

Acrylic base - if extreme loss of residual ridge bone occurs, in order to restore normal facial contours & fill the buccal vestibule to prevent the food being lost into the cheek & from going beneath the denture.Acrylic resin base

Where denture contours may be utilized for retention.Restoration of facial contours for esthetic reasons ideal polished surfaces.Avoid accumulation of food at denture borders. Frequent relining

Metal base Patient's preference Acrylic sensitivity Reduce chances of breakage Thinner sections are required to provide tongue space

Choice of material for denture base

Need of the situation

Metal resin bases : total metallic coverage with resin borders to avoid the display of metal & add buccal fullnessDesign of the denture baseA time-honored principle: base should cover as wide as area as the limiting structures will permit & that the patient can comfortably tolerate

Supported by the Snow shoe principle : broader coverage furnishes the best support with the least load per unit areaTooth supported v/s tooth - tissue supportedTooth supported

Support of the denture is obtained from abutment teeth at each end of edentulous space. Less chances of relining Metal bases can be used. Tooth - Tissue supported Major part of support is obtained from the Distal extension denture base. Changes are likely to occur due to residual ridge resorption need for relining. Resin bases are preferred.Tooth supported v/s tooth - tissue supported

Extension of tooth -supported base

Maxillary as well as mandibular - restricted to the edentulous span b/w abutments without peripheral borders extending into the vestibular fornix.Maxillary distal extensionFull coverage base extending to cover the tuberosity & hamular notches Posterior border: taper towards tissues; beveledTermination on tissues that are resilient but not movableTissue surface should be lightly beaded if in metal & post - dammed if in acrylicMetal finished lines- sharp straight junction with no overlapping of acrylicBuccal flange should extend into vestibular fornix Anterior border of labial flange taper posteriorly; beveled2mm in thickness, rounded and smooth Labial flange property contoured; festooning

Extension of mandibular distal extension baseT.Fischer and W.D. Sweeney total area of maxilla capable of support 1.6 times mandible

Should extent to cover retromolar pads distally and laterally to include the buccal shelf

Lingual flange : vertically downwards into alveolingual sulcus.

Distolingual flange extended laterally into retromylohyoid space; beveled.

Concave to allow adequate tongue spaceLabial and buccal extensions : mucosal reflections.

Methods of attaching denture basesResin bases are attached to partial denture framework by minor connector designed such that space exists b/w it & underlying tissues.

Relief of 20 gauge thickness over master castraised platform on investment cast on which the pattern for minor connector frame is formed

After casting, the retentive frame is returned to master cast where it stands away from the tissue surface sufficiently to permit the flow of resin beneath the surfaceMinimum thickness of resin - l.5mm - to allow relieving during adjustment & relining - avoid weakening and subsequent fracture of resin surrounding metal framework. Investment cast on which pattern for minor connector is formed.

Various designs of minor connectors used

Latticework Meshwork Nail head Beaded Open ladder- like framework (12 ~ l4 gauge half round wax or 18 gauge round wax) extending buccally as well as Iingually preferred to finer latticework mesh pattern - excellent attachment - sufficient thickness of resin thereby minimizing distortion due to release of internal strains & fracture due to weakening - avoid interference in artificial teeth placement

Open construction can be used whenever multiple teeth are to be replaced.This form of minor connector provides the strongest attachment of acrylic resin to the removable partial denture framework. It also facilitates relining and rebasing of removable partial dentures.

MeshworkA mesh minor connector may be compared to a rigid metallic screen. Channels that pass through the connector are intended to permit acrylic resin penetration.Relief and border extension for a mesh minor connector should be identical to open type.The main drawback of a mesh minor connector is the difficulty it presents during the packing of acrylic resin.Insufficient packing pressure may result in inadequate resin penetration and a weak attachment to the framework.The smaller the openings in this minor connector, the weaker the attachment.Mesh construction also may interfere with the arrangement of prosthetic teeth.Mesh construction may be used whenever multiple teeth are to be replaced.

Bead, nailhead, or wire constructionBead, nailhead, or wire components are often used in conjunction with metal denture bases. The metal bases are cast to fit directly against the underlying soft tissues.Hence, no relief is provided beneath these minor connectors.Resin is attached to the free surface of such bases, and retention is gained by encompassment of surface projections.Bead, nailhead, and wire construction should be limited to short-span, tooth-supported applications in patients with well-healed ridges.Methods of attaching artificial teethPorcelain & resin teeth attached with resin Porcelain teeth are mechanically retained. Anterior : lingually placed retentive pins Posterior : diatoric holes.

Resin teeth are chemically united with the acrylic resin of denture base.Metal bases : resin attachment is accomplished by nail head, retention loops or diagonal spurs placed at random.

Junction of resin to metal should be an undercut finishing line so as to avoid separation and seepage.Porcelain or resin tube teeth and facingsCemented directly to metal base - tube teeth have hole on underside / side groove may be present for cementation with resin.

Modification : ready -made resin teeth attached to metal base with resin of same shade - pressing on resin tooth.TUBE TEETH

DisadvantagesDifficulty in obtaining satisfactory occlusionLack of adequate contours for functionaltongue & cheek contactUnaesthetic display of metal at gingivalmarginsResin teeth processed directly to metalAdvanced crosslinked copolymers allow teeth tobe processed in acrylic at an established occlusal relationship therefore less occlusal adjustments are neededSufficient hardness & abrasion resistanceAdv: - occlusion can be created - limited space - occlusion can be reestablishedMetal teethCast as a part of frameworkIndicated - limited space available for the attachment of artificial tooth - usually when replacement of second molar is desirable to prevent migration of the opposing tooth Disadvantage - cast in chrome alloy which is difficult to adjust

Recent advancements direct chemical bonding of teeth to metal2.Tribochemical coating: Fusing a microscopic layer of ceramic to the metal by:

How a distal extension base differs from tooth borne partial denture?Support of Distal extension denture baseDistal extension base - dual support mechanism - abutment and residual ridge - major part of it provided by edentulous ridgeTooth - supported - solely by abutment teethSo in a free end partial denture clinician must decide how the function and parafunction forces can be best distributed b/w the edentulous and dentulous areas.Movement of distal extension base

Conditions confronted by dentistDetermine the selection of type of direct retainer system & load distribution b/w abutment & ridge

1. Good PDL support with favorable ridge : any retainer with equal distribution of load2. Good PDL support with unfavorable ridge: >stress on abutment3. Poor PDL support with favorable ridge : >stress on ridge therefore a stress releasing type of retainer4Poor PDL & ridge support : stress releasing type of retainer to preserve the remaining, teeth as long as possibleStrain on abutment teeth can be minimizedFunctional basingBroader coverageHarmonious occlusionCorrect choice of direct retainersStress-breakersSTRESS-BREAKERS

DefinitionA device which relieves the abutment teeth of all or part of the occlusal forces. -- GPT. A Stress-breakers is a device that allow movement between the denture base and the direct retainer which may be intracoronal or extracoronal.Also called "Stress director" or "Stress equalizer".

ConceptConcept of stress breakers came in existence in relation to free- end partial dentures to reduce the torque & load on abutment teethDesigned to separate the action of retaining elements from the movement of denture base by allowing some movement b/w the two As these devices transfer load from one structure to another - "Load distributors" or Stress directors (Menson-1972).Broken stress philosophySuggests that it is more appropriate to mechanically isolate the abutment from extension base movement during functional loading.

Mechanical device or attachments must be positioned between abutments and extension bases within the RPD framework.

Permit horizontal, vertical and/or rotational movement of the extension base relative to abutment.

aimTo direct occlusal forces in the long axis, of the abutment teeth. To prevent harmful forces being applied to the remaining natural teeth. To share the forces as evenly as possible between the natural teeth and distal extension area according to the ability of these different tissue to accept the forces.Mechanism of action

In a tooth tissue supported partial denture, when an occlusal load is applied, the denture tends to rock due to the difference in the compressibility of the abutment and soft tissues.A stress breaker is a hinge like joint placed with in the denture framework, which allows the two parts of the framework on either side of the joint to move freely.typesType IType IIWith movable joints b/w Direct Retainers and denture base that permit vertical movement, or hinge-type or a combination.Devices with hinges, sleeves, cylinders or ball and socket joint.E.g. Dalbo attachment, Crismani attachment, ASC 52 attachment.

Flexible connection b/w the Direct Retainers and denture base is provided.Wrought wire connectorsDivided or split major connectorsMovable joints b/w two major connectors.

Intracoronal attachment

Introduced in the late 19th century by Dr. Herman E.S. Chayes.

Consists of matrix/keyway and patrix.

These two components interlock in a sliding joint configuration.

Crismani combined attachmentFemale section is housed within the abutment intracoronal retainer.Hinge connector located within the male section embedded in the denture.Allow for vertical movement of the base by hinge action.Spring mechanism help return base to original position when out of occlusal contact.

Extracoronal resilient attachmentIntroduced in the early 1900s by Henry P Boos and modified by F. Ewing Roach.Provides rigid, movable or resilient connection between the abutment and the denture base.Sliding joint.Alternatives to intracoronal attachments - Short clinical crowns. - High pulp horns Extracoronal attachments(contd.)Dalbo: Most PopularResilient hinge type of extracoronal attachmentBoth rotational and vertical movement

ASC-52Universal resilienceExtracoronal slide attachment with a spring activated universal joint hinge & adjustable retentionSmallest of the stress directors

Extracoronal attachments

Hinge type attachmentExtracoronal attachments(contd.)

Stud attachmentBar and clip attachmentType IIMovable joint b/w two major connectors- dual casting techniqueEg. Double lingual bar of wrought metal; one supporting the clasps and other components whereas the others supporting and connecting distal extensions

Split major connector - Made flexible, by separating portion of its length either by saw cut or casting to thin shim

Advantages of stress directorsVertical forces acting on the abutment teeth are minimized and alveolar support of abutment teeth is preserved.Intermittent pressure of denture bases massage the mucosa thus providing physiologic stimulation, which prevents the bone resorption and eliminates need for relining.Minimal requirement of direct retention.Weak abutment is well splinted even during the movement of the denture base.Disadvantages of stress directorsDesign is complicated and expensive.The assembly is very weak and tends to fracture very easily.Difficult to repair.Precise and structurally demanding tooth preparationReduced stability against horizontal forces.Effectiveness of the indirect retainers is reduced or eliminated all together.

ConclusionThe controversy of whether to employ a stress-director in distal extension RPD has not been resolved. Successful prostheses can be fabricated by both methods. However the key to success lies in intelligent treatment planning and meticulous restorative procedures. Routine recall and maintenance is perhaps the most important consideration for the longevity of the prostheses & maintenance of the health of the supporting oral structuresW.L. McCracken: Unless a partial denture is made with adequate abutment support, with optimal base support, and with harmonious and functional occlusion, it should be clear to all concerned that such a denture should be considered only a temporary treatmentPrieskel concludes"It seems more important to concentrate on the production of stable prostheses than to enter the controversy over broken stress and rigid connectors "References McCrakens removable partial denture. 12th edition.Clinical removable partial prosthodontics. 3rd ed. by Stewart.Precision attachments in dentistry. H.W. Preiskel. 3 rd ed.Removable partial prosthodontics by Grasso and Miller. 3rd ed.