ContentsDefinition of denture baseRequirementsFunctionsTypes a,Plastic b,Metal
Definition –Is that part of the removable partial denture which rests on oral mucosa and to which teeth are attached.
REQUISITES FOR IDEAL DENTURE BASE• Accuracy of Adaptation To Tissues With Low Volume Change• Dense non irritating surface capable of receiving & maintaining a good finish.• Biologically Compatible with tissues• Thermal conductivity• Low specific gravity – Lightness in mouth• Esthetics
Functions –EstheticsSupport and retain artificial teethAssist in transfer of occlusal forces directly to abutment teeth by rests.Prevent vertical and horizontal migration of remaining natural teeth.Eliminate undesirable food traps.Stimulates the underlying tissue.
Types – Plastic Acrylic - hot cure - cold cure - light cure - pour and cure Polystyrene Valplast Metal Gold Co-Cr Titanium Vitallium
Metal Base-Indications Tooth supported partial dentures Inadequate interarch space Structural details Designed with optimum extension Thinner base than plastic resin Avoid sharp margins
Metal Base - Advantages • Accuracy & performance of form • Comparative tissue response • Thermal conductivity • Weight & Bulk
Comparitive tissue response-Inherent cleanliness of cast metal base contributes to health of oral tissue.Bacteriostatic activity – ionization and oxidization of metal base.Metal base naturally cleaner than an acrylic resin base.
Thermal conductivity-Temperature changes transmitted though metal to the underlying help to maintain health of that tissue.Patients acceptance.Denture resins – insulating property
Weight and bulk-Metal alloy may be cast thinner than acrylic resin and still have adequate strength.Certain situations demands use of acrylic denture base-
Metal Base-Disadvantages • Esthetics • Enhancement of retention not possible – lack of weight of metal base • Relining difficult • Restoration of normal facial contour can not be achieved
PorosityTwo types of porosity are recognized :--Contraction porosity-Gaseous porosityContraction porosity :- occurs because the monomer contracts bysome 20% of its volume during processing .-gaseous porosity :- occurs due to the exothermic reaction ofpolymerization which could make the temperature of the resin torise above 100c which is the boiling temp. , if this temp. isexceeded before the polymerization process is completed ,gaseous monomer will be formed which is the cause of gaseousporosity.It can be avoided by alllowing the temp. to be raised in a slow andcontrolled fashion.-polymerization should be carried out slowly to prevent gaseousporosity and under pressure to prevent contraction porosity.
Mechanical properties :-The tensile strength of acrylic resin is typically no more than 50mpa , the elastic modulus is low , the flexure modulus being in theregion of 2200 – 2500 mpa .When this is combined with lack of fracture toughness , it perhapsnot surprising that dentures are prone to fracture .An alternative approach for strengthening of acrylic dentures isincorporation of fibers so as to produce a fiber – reinforcedcomposite and these include :- carbon fibers , aramid , uhmpe ( ultra high molecular weight polyethelene ) and the glassfibers.
Physical properties-Thermal conductivity :- acrylic has low coefficient of thermal conductivity , froma patient point of view , it will isolate the oral soft tissues from any sensation oftemperature.-Coefficient of thermal expansion :- its quite high about 80 ppm / c , in generalit does not present any problem except that there is possibility that porcelain teeth setin denture base resin may gradually loosen and lost due to different expansion andcontraction.-Water sorption and solubility :- resin molecule absorbs water due to itspolar nature , in practice this helps to compensate for the slight processing shrinkage .-Biocompatibility :- acrylic is highly biocompatible and patients suffer fewproblems , nevertheless some patients will show an allergic reaction and this is mostprobably associated with the various leachable components in the denture such as anyresidual monomer or benzoic acid .
Valplast1950,sValplast – flexible base resin ideal for partial dentures.Esthetic yet fully functional alternative to traditional cast metal based removable partial dentures.
Biocompatible nylon and thermoplastic resin-flexibility and stability.Color, shape and design of valplast partials blend seamlessly with natural appearance of gingiva making prostheses nearly invisible.Strenght of valplast resin doesn’t require a metal framework-eliminates metallic taste.Enables partial to be fabricated thin enough with non metallic clasps.
Adequate occlusal clearance b/w arches for tooth placement.No tooth preparation required.Vitallium combination-better support and clasp esthetics
Need for reliningDistal extension base differs from toothsupported base-made of material-relined.Acrylic resin denture base materials that can be relined are generally used.
Loss of support - distal extension bases- changes in residual ridge-loss of occlusion-heavy occlusal contact b/w remaining natural teeth.Relining necessary.Rotation of fulcrum line with indirect retainers lifting from their seats as distal extension base is pressed against ridge tissue-relining
Loss of occlusal contact and rotation-relining.Loss of occlusal contact with no evidence of rotation -restablish occlusion.Loss of support assessed clinically-hydrocolloid, wax,tissue conditioning material.
ReferencesCarr A B, Mc Givney G P, Brown D T, Major connector in McCraken’s Removable partial Prothodontics. 11th ed, st louis: Mosby; 2008Stewart K L, Rudd K D, Kuebker W A, Major connector in Stewart’s Clinical Removable Partial Prosthodontics. 2 nd, Chennai; 2004Miller E L, Grasso J E, Major connector in Removable Partial Prosthodontics. 2nd ed, Baltimore: Williams & Wilkins; 1979 Development of in vivo measuring system of the pressure distribution under the denture base of removable partial denture. J Prosthodont Res. 2009 Jan;53(1):15-21