June 2013

RAJIV GANDHI UNIVERSITY OF HEALTH SCIENCES, Karnataka II.B.D.S Degree Examination - JUNE 2013DENTAL MATERIALS(RS-3)QP CODE: 1185

LONG ESSAYS1. Classify dental cements. Add a note on the composition, types, chemistry of setting reaction and uses of glass ionomer cements.2. Classify denture base resins. Discuss their properties, manipulation, advantages and disadvantages

SHORT ESSAYS3. Manipulation of agar impression material 4. Expansion in gypsum products 5. Abrasives and polishing agents 6. Write about materials used for dental implants 7. Phosphate bonded investments 8. Write briefly about cavity varnishes 9. Porosity in dental castings 10. Calcium hydroxide

SHORT ANSWERS11. Separating media 12. Delayed expansion 13. Sandwich technique 14. Gold foil 15. Galvanism

ANSWERSLONG ESSAYS1. Classify dental cements. Add a note on the composition, types, chemistry of setting reaction and uses of glass ionomer cements.CLASSIFICATION OF DENTAL CEMENTSAccording to the ReactionI. Acid base cements 1. Zinc oxide eugenol2. Zinc polycarboxylate cement3. Zinc phosphate 4. Glass ionomer5. Resin modified glass ionomer

II. Resin based cements1. Resin cements2. Compomers3. Self-adhesive resin cements

According to Bonding mechanism

GLASSIONOMER CEMENTCompositionPowder It is an acid soluble calcium fluoroalumino silicate glass ion leachable glass. Silica (SiO2)- Does not take part in the reaction but increases hardness & translucency Alumina (Al2O3)- Reacts with Polyacrylic acid to give aluminium polyacrylate matrix Aluminium phosphate (AlPO4) - Controls the setting time Aluminium fluoride (AlF3) - Act as flux & reduces fusion temperature Calcium fluoride (CaF2) - Improves the translucency Sodium fluoride - Provides strength & mechanical properties & gives Anticariogenic property

Liquid An aqueous solution of polymers and copolymers of Acrylic acid In most of the current cements, the acid in the form of a co-polymer with itaconic, maleic or tricarboxylic acids Polyacrylic acid Most important acid contributing to formation of the cement matrix Water Reaction medium and essential part of the cement structure Serves to hydrate the siliceous hydrogel and the metal salts formed If water is lost from the cement by desiccation while it is setting, the cement-forming reactions will stop Plays a role in transporting calcium and aluminium ions to react with poly acids Types1. Loosely bound water 2. Tightly bound water With the aging of cement, the ratio of tightly bound to loosely bound water increases Accompanied by an increase in strength, modulus of elasticity and decrease in plasticity

Modifiers - viscosity, inhibits gelation, shelf life Itaconic acid Promotes reactivity between the glass and the liquid Prevents gelation of the liquid which can result from hydrogen bonding between two polyacrylic acid chainsMaleic acid Causes the cement to harden and lose its moisture sensitivity fasterTricarboxylic acid A hardener that controls the pH of the set cement during setting process, which in turn controls the rate of dissolution of the glass. It facilitates extraction of ions from the glass. It typically increases the working time and also aids in snap test

TypesClassification I1. Type I Luting cement2. Type II Restorative cement3. Type III-Cavity liner ,cement base4. Type IV Pit & fissure sealant5. Type V Orthodontic cement6. Type VI-Core build up material7. Fuji VII-Glass ionomer stabilization and protection (high fluoride non resin containing auto cure GIC) 8. Fuji VIII-Highly viscous GIC for ART (anterior)9. Fuji IX-Highly viscous GIC for ART (posterior)10. Metal modified GIC 11. Resin modified GIC Compomer dicure, tricure system

Classification II (Phillip's) 1. Type I Luting crown, bridges & orthodontic brackets 2. Type IIa Esthetic restorative cement3. Type IIb Reinforced restorative cement4. Type III Lining cements, base

Setting Reaction Acid Base Reaction Between polyacrylic acid & the aluminosilicate glass

Glass powder + Polyacrylic acid Ca & Al Polyacrylate gel + Silica gel (poly salt gel)

Three stages1. Dissolution When the powder & liquid are mixed, Surface of glass particles are attacked by acid. Then Ca, Al, Na, & fluorine ions are leached into aqueous medium.2. Gelation During early stages of mixing Ca2+ ions released more rapidly & cross-links the polyacrylic acid chains to form calcium polyacrylate (initial set) & are being carvable like amalgam & are highly susceptible to moisture contamination or water sorption . Ca ions will be gradually replaced by Al ions over the next 24 hours. A hard surface, more resistance to dissolution is produced only after the aluminum polyacid (aluminum polyacrylate ) is formed 3. Hardening Can take as long as 7 days. Aluminum ions provide the final strength to the matrix. Na & Fluorine ions do not participate in the cross linking of the cement

Uses1. Anticariogenic property due to the release of fluoride ions 2. Chemical adhesion to enamel & dentin3. Excellent biocompatibility with the pulp4. Good initial esthetic5. Porcelain like translucency, can be used for anterior 6. Pseudoplastic in nature i.e. freshly mixed cement exhibit shear thinning 7. Co-efficient of thermal expansion similar to that of tooth structure 8. Good thermal insulator9. Easy to manipulate10. Excellent flow characteristic of Type I GIC11. Favorable bioactive property

2. Classify denture base resins. Discuss their properties, manipulation, advantages and disadvantages

CLASSIFICATION OF DENTURE BASE RESINSA. According to the Materials used 1. Metallic materials Cr-Ni, Gold alloys, Ti alloys, 18-8 stainless steel etc 2. Non-metallic materials Earlier: Vulcanite, Bakelite, Polycarbonates, Nylon, Cellulose nitrite, PVC, Polystyrene & Epoxy resin Present: PMMA unmodified, PMMA modified with carbon fibres, nylon & butadiene styrene rubber

B. According to the Fabrication technique 1. Compression moulding 2. Injection moulding 3. Fluid pour & cure technique

C. According to the Method of polymerization (curing)1. Thermal energy 2. Chemical /auto-polymerization 3. Microwave energy4. Light energy

PROPERTIESMethyl Methacrylate monomer It is clear, transparent, volatile liquid at room temperature. It has a characteristic sweetish odor The physical properties of monomer are Melting point: 480C Boiling point: 100.80C Density: 0.945gm/ ml at 200C Heat of polymerization: 12.9 K cal / mol Volume shrinkage during polymerization: 21%21%

Polymethylmethacrylate1. Taste and odor Completely polymerized acrylic resin is tasteless and odorless

2. Esthetics It is a clear transparent resin, which can be pigmented easily to duplicate oral tissues. It is also compatible with dyed synthetic fillers. 3. Density Polymer has a density of 1.19 gm/cm3

4. Compressive and tensile strength These materials are typically low in strength. However they have adequate compressive and tensile strength for complete or partial denture application

5. Transverse strength Under clinical conditions, heat cure polymethyl methacrylate is stiff enough not to flex unduly during function and recovers well from this degree of bending

6. Impact strength The impact strength of polyvinyl acrylic is about twice that of poly (methyl methacrylate) Addition of plasticizers may increase the impact of plastics

7. Hardness and abrasion resistance Heat cured acrylic resin: 18-20 KHN Self-cured resin: 16-18 KHN Cross-linked poly (methyl methacrylate) is only slightly harder than regular poly (methyl methacrylate). The incorporation of fillers may alter resistance to abrasion but the hardness of plastic matrix remains unchanged.

8. Fatigue strength and fracture toughness Acrylic polymers show sensitivity. Over stressing of dentures on deflasking must be avoided. Otherwise a small crack may be produced which will later propagate through the material. Similarly surface of a denture must be smoothly polished. In an upper denture stress accumulation during chewing may occur at a point between central incisors. Here a notch to accommodate labial frenum frequently weakens the facial flange. Fatigue or midline fracture may occur if the masticatory forces are relatively large. Fatigue strength increases with a higher molecular weight and an increase in the plasticizer content.

9. Modulus of elasticity Acrylic resins have sufficient stiffness (modulus of elasticity 2400 MPa) for use in complete and partial dentures. However when compared with metal denture bases it is low.

10. Polymerization shrinkage When methyl methacrylate monomer is polymerized to form polymethyl methacrylate the density of the mass changes from 0.94 to 1.19g/cm3. This change in density results in volumetric shrinkage of 21% when a conventional heat activates resin is mixed at a suggested powder: liquid ratio about 1/3rd of resultant mass is liquid. Consequently the volumetric shrinkage exhibited by polymerized mass should be approximately 7% In addition to volumetric shrinkage, linear shrinkage occurs. Greater the linear shrinkage greater is the discrepancy observed in the initial set of dentures. Based on a projected volumetric shrinkage of 7% the linear shrinkage exhibited should be 2%. But the observed linear shrinkage is less than 1% Processing shrinkage has been measured as 0.26% for a representative chemically activated resin, compared with 0.53% for representative heat activated resins

MANIPULATION Proportioning: P/L ratio 3:1 by volume or 2.5:1 by weight: Mixing: Done with a stainless steel spatula. Allow the monomer to react physically with polymer in a sealed jar to avoid monomer evaporation. The mixture goes through the following physical stages:1. Wet sandy stage A wet sand like structure is formed. Little or no interaction occurs on the molecular level. Polymer beads remain unaltered & the consistency of the mixture is coarse or grainy

2. Sticky stage (stringiness) Material becomes tacky (sticky) as the monomer attacks surface of individual polymer & absorbed into the beads. Polymer chain starts to uncoil, thereby increasing the viscosity.

3. Dough stage An increased number of polymer chains enter the solution. Thus monomer dissolves polymer, forming homogeneous soft dough like mass. This is non-sticky & mouldable This is the correct stage for packing the material

4. Rubbery or elastic stage Monomer will dissipated by evaporation & penetrate into remaining polymer bead. Mass rebounds when compressed or stretched

5. Stiff stage If the mix is left too long it becomes too rubbery & stiff. Can be due to continued evaporation of unreacted monomer In both rubbery & stiff stage material cannot be molded/ packed.

ADVANTAGES

DISADVANTAGES

SHORT ESSAYS3. Manipulation of agar impression material Manipulation The temperature lag between the gelation temperature and the liquefaction temperature of the gel makes it possible to use agar as a dental impression material Once in the mouth, the Material is cooled below mouth temperature to ensure gelation

Preparation and Conditioning The first step in material preparation is to liquefy the hydrocolloid gel in boiling water The material must be held at this temperature for a minimum of 10 minutes At high altitudes, the boiling point of water is too low to liquefy the gel. Propylene glycol can be added to the water to obtain a liquefaction temperature of 100C After the agar hydrocolloid material has been liquefied, it may be stored in the sol condition at 65C until it is needed for injection into the cavity preparation or for filling a tray.

Tempering the material Since 55C is the maximum tolerable temperature, a storage temperature of 65C would be too hot for the oral tissues, especially for the bulk of the tray material. Therefore, the material used to fill the tray must be tempered. The tempering time is short (3 to 10 min), just sufficient to ensure that all the material has reached a lower temperature (55C). In any case, the loaded tray should never be left in this bath for more than 10 minutes, because gelation may have proceeded too far, thereby malting the material unusable. Tempering of tray material also increases the viscosity so that it will not flow out of the tray. The syringe material is never tempered, since it must be maintained in a fluid state to enhance adaptation to the tissues.

4. Expansion in gypsum products Normal Setting Expansion Dental gypsum products when mixed with water and allowed to set in air exhibit an expansion of their peripheral boundaries that are known as "normal setting expansion." Under ordinary conditions, plasters have 0.2% to 0.3% setting expansion, low- to moderate-strength dental stone about 0.15% to 0.25%, and high-strength dental stone only 0.08% to 0.10%. The setting expansion of high-strength/high-expansion dental stone ranges from 0.10% to 0.20%. (AP-05) Mechanical mixing decreases setting expansion. A vacuum-mixed high-strength stone expands less at 2 hours than when mixed by hand. Power mixing appears to cause a greater initial volumetric contraction than is observed for hand mixing. The W/P ratio of the mix also has an effect, with an increase in the ratio reducing the setting expansion. The addition by the manufacturer of sodium chloride (NaCl) in a small concentration increases the setting expansion of the mass and shortens the setting time. The addition of 1% potassium sulfate, on the other hand, decreases the setting time but has no effect on the setting expansion.

Hygroscopic Setting Expansion When additional water is brought into contact with the setting material, an increased expansion is observed. This latter expansion has been termed "hygroscopic expansion." A typical, high-strength dental stone has a setting expansion of about 0.08%. If during the setting process the mass is immersed in water, it expands about 0.10%.

5. Abrasives and polishing agents Refer to Q. No. 11 of Year June 2014

6. Write about materials used for dental implants 1. Metals & Alloysi. Stainless Steel- Austenitic steel implants Most commonly used materials The austenitic steel mainly contains 18% of chromium; provides corrosion resistance, 8% of nickel; stabilizes the austenitic structure, 80 percent iron and 0.05%-0.15% of Carbon.ii. Cobalt-Chromium Molybdenum Alloys Used as cast and casted in annealed conditions. Composition is Co Cr Mo alloys Advantages1. These alloys possess an outstanding resistance to corrosion and they have high modulus.2. Low cost and long - term clinical success. Limitation: The cobalt alloys are least ductile and bending must be avoided.iii. Titanium and titanium Alloys Commercially pure titanium (Cp) is a very highly reactive metal. It oxidizes (passivate) on contact with air or normal tissue fluids. This reactivity is favorable to the implant devices because it minimizes biocorrosion. Cp titanium is available in four different grades such as Cp grade 1,Cp grade 2,Cp grade 3 and Cp grade 4.C p titanium contains oxygen and minor amounts of impurities such as nitrogen, carbon, hydrogen, etc.iv. Titanium-aluminum-vanadium (Ti AI V alloys) Has modulus of elasticity is slightly greater than Cp Ti

2. Ceramics Oxides ceramics were introduced for surgical implant Two types of ceramic materials arei. Bioactive materials Hydroxyapatite They are rich in calcium phosphate This is used as an implant material for augmenting alveolar ridges & filling bony defectsii. Bioglass A dense ceramic material made from calcium oxide, sodium oxide, phosphorous pentoxide & silicon dioxide

3. Polymers Have been fabricated in porous & solid forms for tissue attachment & replacement augmentation & as coating for force transfer to soft & hard tissues

7. Phosphate bonded investments Phosphate-bonded investments are much stronger and withstand much higher temperatures than do gypsum bonded investments Used for casting high fusing alloys, e.g. high fussing noble metal alloys metal ceramic alloys, and base metal alloys like nickel chromium and cobalt-chromium.

Classification (ADA specification No: 42) Type I: For inlays, crowns and other fixed restorations. Type II: For partial dentures and other cast removable restorations.

Composition 1. Refractory material (80%) Cristobalite or Quartz or mixture of both withstand high temperature & gives large setting expansion2. Binder Magnesium ammonium phosphate (NH4MgPO4) increases strength, setting & thermal expansion3. Carbon Act as reducing agent

Setting Reaction NH4 H2 PO4 + MgO + 5H2O NH4MgPO4.H2O The final product is mono ammonium diacid phosphate or magnesium ammonium phosphate

Advantages1. High strength after heating to withstand high temperature. 2. Have sufficient green & fried strength 3. Can with stand impact forces & pressure of centrifugal casting.4. High setting and thermal expansion to compensate for thermal shrinkage of cast metal prosthesis or porcelain veneers during cooling

Disadvantages1. Using casting temperature greater than 1375C ,results in mold breakdown & rougher surface of the casting2. Because of high strength of phosphate bonded investment, it is very difficult to remove the casting from the investment

8. Write briefly about cavity varnishes Varnish is an organic gums or rosin suspended in organic solution like ether or chloroform. When applied on the tooth surface the organic solvent evaporates leaving behind a thin protective film. Provides a barrier against passage of irritants from the restorative material & reduces micro leakage. Supplied as liquid in dark colored bottles

Composition Natural gums such as copal or rosins or synthetic resins dissolved in an organic solvent, such as acetone, chloroform or ether

Functions 1. Reduces the marginal leakage around silver amalgam2. Protection against various restorative irritant chemicals3. Blocks the penetration of corrosion products of amalgam into the dentinal tubules4. In case of amalgam restoration, varnish prevents the discoloration of tooth against migration of ions into the dentin

Application Dipping a small cotton pellet (ball) held in pliers, in to the varnish and then thoroughly painting all cavity walls. Apply at least 2 thin layers of varnish. When first layer dries, small pinholes usually develop. A second or third application fills in voids & produces a continuous coating

Precaution Varnish is not indicated for adhesive materials, such as GIC and resin based composite.

9. Porosity in dental castings Porosity may internal or external. Porosities are classified as,I. Solidification defects1. Localized shrinkage porosity 2. Microporosity

II. Trapped gases1. Pin hole porosity 2. Gas inclusions 3. Subsurface porosity

III. Residual air

1. Localized shrinkage porosity During a correct cooling sequence, the sprue should freeze last. This allows more molten metal to flow into the casting (pattern area), to compensate for the shrinkage of the casting as it solidifies. If the sprue solidifies before the rest of the casting no more molten metal can be supplied from the sprue into the pattern (casting). The subsequent shrinkage produces voids or pits known as porosity.

Prevention Using sprue of correct thickness Attach sprue to thickest portion of wax pattern Flaring the sprue at the point of attachment or placing a reservoir close to the wax pattern

2. Suck back porosity If the temperature of the liquid entering is very high, it raises the temperature of the mold opposite to the sprue attachment Due to this hot spot , liquid solidify last near it Since the sprue has already solidified, no more molten material is available and the resulting solidification shrinkage localize near the hot spot & causes a suck back porosity.

Prevention By reducing the temperature of liquid By reducing the thickness of the sprue By flaring the point of sprue attachment ( 90 angle)

3. Microporosity These are fine irregular voids formed throughout the internal portion of the casting.

Causes Caused by casting the alloy at a temp lowered than recommended By use of low temp investment mold Under such condition, alloy solidify at a faster rate than the normal, thereby causing micro porosities throughout the casting

Prevention By increasing the mold temperature

4. Pinhole porosity Many metals dissolve or occlude gases when molten. Upon solidification the dissolved gases are expelled causing tiny voids E.g. platinum and palladium absorb hydrogen. Copper and silver dissolve oxygen

Prevention Liquid should not be kept for longer time before casting & its temperature should not be low

5. Gas Inclusion porosities Are spherical voids & larger than the pin whole type. Occurs due to entrapment of gases during casting procedure Larger spherical porosity can be caused by occluded gas from a poorly adjusted torch flame or use of mixing or oxidizing zones rather than reducing zone of the flame

Prevention Minimized by melting gold alloy on a graphite crucible & by correctly adjusting & positioning the torch flame during melting

6. Sub surface porosity Occurs just below the surface of casting Caused by simultaneous nucleation & release of gas bubbles in the liquid Minimised by controlling the rate at which molten metal enters the mold

7. Back pressure porosity/Entrapped air porosity As the liquid enters the mold through the sprue, air trapped in the mold is compressed at the extremities ,which exert a back pressure preventing the alloy liquid to occupy this region

Causes This is caused by inadequate venting (air escape) of the mould. Excess investment material over the wax pattern at the open end of the casting ring

Prevention Using adequate casting force Use investment of adequate porosity Place pattern not more than 6 to 8 mm away from the end of the ring. Providing vents in large castings

10. Calcium hydroxide Refer to Q. No. 08 of Year December 2014

SHORT ANSWERS11. Separating media Functions1. To prevent the diffusion of water molecules from the mold (gypsum) into the unpolymerized packed dough 2. To prevent the diffusion of the monomer from the unpolymerized packed dough in the mold material3. To prevent direct contact between the denture base resin & the mold surface

One of the first widely used material was tin foil, which was found to be time consuming & inconvenient At present alginate sol (cold mould seal) is found to be more effective All parts of the gypsum cast in the flask should be painted by sodium alginate (by camel hair brush) except the exposed surface of the teeth

12. Delayed expansion Synonym Secondary Expansion

Definition Zinc-containing amalgam, if contaminated by moisture during trituration or condensation produce Delayed

MechanismZn+ moisture (H2O) ZnO + H2 Reaction takes place after amalgam is inserted into the cavity. Hydrogen gas that is evolved does not combine with amalgam but rather collects within the restoration & exert large internal pressure, causing abnormal expansion This may not appear within first 24 hr but may become evident few days (3-5 days) after insertion of amalgam.

Clinical Significance Causes severe pain due to pressure on the pulp Amalgam ditching - Fracture of thin cavity walls on biting Increased marginal leakage around the restoration

Remedy Use of non Zn containing alloys (unicompositional) Cavity restoration should be done in dry condition using rubber dams

13. Sandwich technique This technique , involves the bonding between composite resin & GIC GIC act as enamel or dentin bonding agent & composite resin as a restorative material Steps1. Conditioning of dentin: Acid etching using 37% phosphoric acid, makes the tooth surface rough 2. Application of bonding agent (GIC) & is polymerized by light activation 3. Insertion of composite resin into the cavity 4. Finally finishing of the restoration

Indication Class II & class V composite restoration

14. Gold foil Oldest form of gold and is the most durable. Standard No.4 gold foil is supplied in 4 x 4 inch (100 x 100 mm) sheets that weigh 4 grains (0.259 gm) and are about 0.51 m thick. The numbering system refers to the weight of a standard sheet and reflects the thickness. Thus, No. 3 foil weighs 3 grains (0.194 g) and is about 0.38 m thick. Gold foil is supplied in the following forms, 1. Plain gold foil2. Corrugated gold foil3. Platinum gold foil4. Laminated gold foil5. Gold foil cylinder

15. Galvanism The presence of metallic restorations in the mouth may cause a phenomenon called galvanic action, or galvanism. This results from a difference in potential between dissimilar fillings in opposing or adjacent teeth. These fillings, in conjunction with saliva or bone fluids such as electrolytes, make up an electric cell. When two opposing fillings contact each other, the cell is short-circuited, and if the flow of current occurs through the pulp, the patient experiences pain and the more anodic restoration may corrode. A single filling plus the saliva and bone fluid may also constitute a cell of liquid junction type. The galvanic currents developed from the contact of two metallic restorations depend on their composition and surface area. An alloy of stainless steel develops a higher current density than either gold or cobalt-chromium alloys when in contact with an amalgam restoration. As the size of the cathode (such as a gold alloy) increases relative to that of the anode (such as an amalgam), the current density may increase The larger cathode likewise can enhance the corrosion of the smaller anode.

Remedy Avoid the use of dissimilar metal opposing restoration Provide a good insulating base to protect pulp from galvanic shock

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