Recent advances in Dental materials

 Recent advances in Dental materials

 CONTENTS:

Ø INTRODUCTION

Ø CLASSIFICATION

Ø HISTORY

Ø IMPRESSION MATERIALS

Ø DENTAL LUTING AGENTS

Ø DENTAL CERAMICS

Ø DENTURE BASE RESINS

Ø CONCLUSION

Ø REFERENCES

 INTRODUCTION:

               The overriding goal of dentistry is to maintain or improve the life of the dental patient. The goal can be accomplished by preventing disease, relieving pain, improving masticatory efficiency, enhancing speech and improving appearance. The main challenges for centuries have been the development and selection of biocompatible, long lasting, direct filling tooth restoratives and indirectly processed prosthetic materials that can withstand the adverse conditions of the oral environment.

      Dental materials may be classified as

1.     preventive materials

2.     restorative materials

3.      auxiliary materials

1. Preventive materials include

Pit and fissure sealants

Sealing agents that prevent leakage

Liners and bases

2. Restorative materials:

Ø  Restorative materials can be classified as direct restorative materials and indirect restorative materials

Ø  Direct restorative materials indicated to use intra orally to fabricate restorations or prosthetic devices directly on the tissues.

Ø  Indirect restorative materials which are to use extra orally in which the materials are formed indirectly on casts or other replicas of the teeth and other tissues.

3. Auxiliary dental materials are substances that are used in the process of fabricating dental prosthesis and appliances but do not become part of these devices.

Eg: etching materials, impression materials, casting materials, dental waxes, acrylic resins, gypsum cast and model materials, finishing and polishing abrasives.

Historical use of restorative materials:

§  Dentistry as a speciality is believed to have begun about 3000 B.C Gold bands and wires were used by the Phoenicians (After 2500 B.C).

§  Around 700B.C The Etruscans carved ivory or bone for the construction of partial denture teeth that were fastened to natural teeth by means of gold wires and bands. The gold bands were used to position the extracted teeth in the place of missing teeth.

§  Around 600 A.D The Mayans used implants consisting of sea shell segments that were placed in anterior teeth sockets.

§  The Incas performed tooth mutilations using hammered gold but the material was not placed for decorative purposes.

§  Fauchard (1678-1761) the father of modern dentistry used tin foil and lead cylinders for filling the tooth cavities.

§  Gold shell crowns were described by Monton in 1746.

§  In 1756 Phlipp Pfaff of Germany described a method for making impressions of the mouth in wax from which he constructed a model with plaster of Paris.

§  In 1774 Duchateau a French pharmacist designed a process for producing hard, decay proof porcelain dentures.

§  In 1808 Fonzi an Italian dentist developed an individual porcelain tooth form that was held in place with an embedded platinum pin.

§  Planteau, a French dentist first introduced porcelain teeth in 1817.

§  Ash further developed an improved porcelain tooth in England around 1837.

§  In 1839 Charles Goodyear has invented vulcanized rubber denture bases, and in 1935 polymerized acrylic resin was introduced as a denture base material to support the artificial teeth.

§  In 1907 Taggert developed a more refined method for producing cast inlays.

§  Mason developed a detachable facing to a crown to hold an artificial tooth.

In prosthetic dentistry auxiliary materials play a major role in fabrication of removable and fixed prostheses.

                         IMPRESSION MATERIALS

These materials can be classified according to the mode through which the ingredients react (set or harden) to solids, their mechanical properties, and their uses.

Based on setting mechanism

-         Materials set by irreversible reaction eg: alginate, zinc oxide eugenol impression paste, impression plaster, and elastomeric impression materials.

-         Materials set by reversible reaction eg: agar hydrocolloid, and impression compound.

Based on the mechanical properties:

-         Rigid material eg: ZOE impression paste, impression plaster and impression compound

-         Elastic materials eg: non aqeous elastomers, hydrocolloid impression materials.

Based on the uses;

For preliminary impressions

-         impression compound (if patient doesn’t have undercuts)

-         alginates (if patient have undercuts)

For final impressions

-         Impression plaster, ZOE impression paste, elastomeric impression materials, alginate hydrocolloids, mouth temperature waxes, soft acrylic resins.

Based on their use in dentistry

-         For edentulous: for C.D eg; impression compound, ZOE paste, alginate, elastomers.

-         For dentulous: for both FPD and RPD eg; agar hydrocolloid, alginate and elastomers

Based on amount of pressure applied

-         muco compressive eg; impression compound

-         mucostatic eg: impression plaster

-         selective pressure eg: ZOE IMPRESSION PASTE

Based on the manipulation

-         Kneading eg:Imression compound and putty consistency elastomers

-         Circular motion eg: ZOE impression paste and Polysulphides.

Based on tray used for impression

-         Special trays or custom made trays eg; ZOE impression paste, elastomeric impression materials.

-         Stock trays  ; rim lock- alginate; water cooled- agar hydro colloid

Desirable properties of the impression materials:

·        A pleasant odor, taste, and acceptable color

·        Absence of toxic and irritant constituents

·        Adequate shelf life for requirements of storage and distribution.

·        Economically commensurate with the results obtained.

·        Easy to use with the minimum of equipment.

·        Setting characteristics that meet clinical requirements.

·        Satisfactory consistency and texture.

·        Readily wet oral tissues

·        Elastic properties with freedom from permanent deformation after strain.

·        Adequate strength so it will not break or tear or removal from the mouth.

·        Compatibility with cast and die materials

·        Accuracy in clinical use.

·        No release of gas or other by products during the setting of the impression or cast and die materials.

HYDROCOLLOIDS:

Agar hydrocolloids:

Composition:

Agar- 13-17% is an organic hydrophlilic colloid (polysaccharide) extracted from certain types of sea weed.

Borates- 0.2-0.5% strengthens the gel

Sulfates- 1-2% accelerator

Diatomaceous earth, clay, silica, wax, rubber can be used as fillers

Thymol- bactericidal agent

Glycerin- plasticizer

Manipulation:

Ø  The hydrocolloid is usually supplied in two forms syringe material and tray material.

Ø  The manipulation includes liquefying the gel, placing it in the impression tray, tempering it to a lower temperature that the patient can tolerate and maintaining it in its fluid state to capture the details of the oral structures.

Ø  The equipment includes 3 compartments for liquefying the material, storing after boiling  and tempering the tray hydrocolloid.

Making the impression in conventional technique:

Ø  The syringe material is taken directly from the storage compartment and applied to the prepared teeth.

Ø  Then the tray material is tempered and the tray is filled and immediately brought in to position and seated with light pressure and held with a very light force.

Ø  Gelation is accelerated by circulating cool water (appx18-21 degree c) through the tray for 3-5 min.

Recent techniques:

Laminate technique:

·        A recent modification of the conventional procedure is the combined agar alginate technique. The hydrocolloid in the tray is replaced with a mix of chilled alginate that bonds with the agar expressed from a syringe.

·        The alginate gels by a chemical reaction whereas the agar gels by means of contact with cool alginate rather than with the water circulating through the tray. Since the agar not the alginate is in contact with the prepared teeth maximum detail is reproduced.

Advantages

·        syringe agar records tissues more accurately

·        Water cooled tray is not required

·        Sets faster.

Disadvantages:

·        Agar – alginate bond failure can occur

·        Viscous alginate may displace agar

·        Technique sensitive

Wet field technique:

·        This is a recent technique

·        The oral tissues are flooded with warm water. The syringe material is then injected in to the surface to be recorded.

·        Before syringe material gels tray material is seated.

·        The hydraulic pressure of the viscous tray material forces the fluid syringe material down in to the areas to be recorded.

·        The motion displaces the syringe material as well as blood and debris through out the sulcus.

ALGINATE HYDROCOLLOID

 The word alginate comes from the term algin. The term was coined by a scotttish chemist S.Williams received the first patent to use alginate as an impression material. It is a mucous extract obtained from certain brown sea weed. The substance is called anhydro-beta –d-manuronic acid or alginic acid.

Composition:

Ø  Potassium alginate 18%- to dissolve in water and react with calcium ions

Ø  Calcium sulfate dehydrate 14% - to react with potassium alginate to form an insoluble calcium alginate gel.

Ø  Potassium sulfate, potassium, zinc flouride, silicates or borates 10% - to counteract the inhibiting effect of the hydrocolloid on the setting of gypsum, giving a high quality surface to the die.

Ø  Sodium phosphate 2% - to react preferentially with calcium ions to provide working time before gelation.

Ø  Diatomaceous earth or silicate powder – 56% to control the consistency of the mixed alginate and the flexibility of the set impression.

Ø  Organic glycols in small amounts to make powder dustless.

Ø  Winter green, peppermint, pigments in traces to present a pleasant taste.

Ø  Pigments in traces to produce colour.

Ø  Disinfectants like quarternary ammonium salts and chlorhexidine 1-2% to help in the disinfection of various organisms.

Recent developments:

1. Dustfree alginates:

·        Inhaling fine airborne particles from alginate impression material can cause silicosis and pulmonary hypersensitivity.

·        Dustless alginates were introduced which give off or no dust particles so avoiding dust inhalation. This can be achieved by coating the material with glycerine or glycol. This causes the powder to become more denser than in uncoated state.

2. Siliconised alginates:

·        It is a two component system in the form of two pastes, one containing the alginate sol and the second containing the calcium reactor.

·        The components incorporate a silicone polymer component which makes material tear resistant compared to unmodified alginates. However the dimensional stability is reported to be poor.

3. Low dust alginate impression material:

Ø  Introduced by Schunichi, Nobutakwatanate in 1997.

Ø  This composition comprises an alginate a gelation regulator and a filler as major components which further comprises sepiolite and a tetraflouroethylene resin having a true specific gravity of from 2-3.

Ø  The material generates less dust, has a mean particle size of 1-40microns.

4. Antiseptic alginate impression material:

Ø  Introduced by Tameyuki Yamamoto, Maso Abinu patented in 1990.

Ø  An antiseptic containing alginate impression material contains 0.01 to 7 parts by weight of an antiseptic such as glutaraldehyde and chlohexidine gluconate per 100 parts by weight of a cured product of an alginate impression material.

Ø  The antiseptic may be encapsulated in a microcapsule or clathrated in a cyclodextrin.

5. CAVEX  Color change:

Ø  The alginate impression material with color indications avoiding confusion about setting time.

Ø  Color changes are visualizing the major decision points in impression making

Ø  end of mixing time

Ø  end of setting time ( tray can be  removed from mouth)

Ø  it indicates two color changes

Ø  Violet to pink indicates the end of mixing time.

Ø  Pink to white indicates end of setting time.

Ø  Other advantages of this material are

Ø  -improved dimensional stability (upto 5 days)

Ø  Good tear and deformation resistance

Ø  Dust free

Ø  Smooth surface, optimum gypsum compatibility.

NON AQEOUS ELASTOMERIC IMPRESSION MATERIALS:

Ø  Elastomers refer to a group of rubbery polymers which are either chemically or physically cross linked.

Ø  Chemically there are four kinds of elastomers used as impression materials.

Ø  -poly sulfide ; introduced in 1950

-         condensation silicones ; introduced in 1955

Ø  -addition silicones ; introduced in 1965

Ø  -poly ethers ; introduced in 1975

These impression materials are typically supplied in several consistencies

 - low (syringe or wash)

 - medium (regular)

 - high (tray)

Addition silicones are available in these three viscosities plus

--extra low

--monophase and putty (extra high)

Condensation silicones are usually supplied in

--loe

--putty consistencies

Poly ethers were available in

--low

--medium

--high consistencies.

Mixing systems

Three types of mixing systems are available to mix catalyst and base

--hand mixing

--Static auto mixing

--dynamic mechanical mixing

Hand mixing:

Ø  Equal lengths of catalyst and bases are dispensed on a paper pad, initial mixing is accomplished with a circular motion and final mixing to produce a mix free from streaks.

Automixing systems:

·        The base and the catalyst are in separate cylinders of the plastic catridge.

·        The plastic catridge is placed in a mixing gun containing two plungers that are advanced by a ratchet mechanism to extrude equal quantities of base and catalyst.

·        The base and catalyst are forced from the static mixing tip containing plastic internal spiral, the two components are folded over each other resulting in a uniform mix at the tip end.

Dynamic mechanical mixer:

·        It’s the newest system the catalyst and base are supplied in large plastic bags housed in a catridge, which is inserted in to the top of the mixing machine.

·        A new plastic mixing tip is placed on the front of the machine and when the button is depressed parallel plungers push against the collapsible plastic bags, thereby opening the bags and forcing the material in to the dynamic mixing tip.

·        The mixing tip has rotating internal spiral accomplishes rotation plus forward motion of the material through the spiral.

·        In this manner thorough mixing can be ensured and high viscosity material can be mixed with ease.

Disadvantages:

The equipment is expensive

There is slightly more material retained in the mixing tip.

Composition and reactions:

Polysulfide:

Ø  They are supplied in tubes of base paste and catalyst paste.

Ø  They are available in low , medium and high viscosities

Composition:

Base paste: 

Poly sulfide polymer- 80-85%

Titanium dioxide

Zinc oxide, copper carbonate or silica- 16-18%

Accelerator paste:

Lead dioxide – 30%

Dibutyl or dioctyl – 17%

Phthalate

Sulfur – 1-4%

Other substances such as magnesium stearate and deodorants – 2%

Reaction:

·        The lead dioxide catalyzes the condensation of the terminal and pendant –SH with –SH groups on other molecules, resulting in chain lengthening and cross linking. In the process the material changes from a paste to a rubber.

·        This reaction is accelerated by increase in temperature and by the presence of moisture.

·        Water is the byproduct in this condensation reaction.

Manipulation:

·        These materials are mixed on a mixing pad with a spatula

·        Adequate mixing time is 45-60sec; the working time is about 5-7min.

·        They stain clothing permanently, they can be electroplated, and some products can be silver plated.

·        Polysulfides must be poured within 1hour and cannot be repoured.

·        Polysulfide impression materials are low to moderately hydrophilic and make an accurate impression in the presence of saliva or blood. Because the material has a low wetting angle it makes impressions more easily than poly ether and poly vinyl siloxanes.

Poly ether impression material:

Ø  It was introduced in Germany in the late 1960’s.

Ø  Available as two paste system and available in different viscosities light, medium, heavy bodies and putty consistencies.

Commercial names;

Impregum(F), Permadyne.

Composition:

Base paste;

·        Imine terminated polymer (polyether) – crosslinks to form the set material

·        A colloidal silica as the filler gives bulk

·        Glycol ether or phthalate acts as a plasticizer.

Accelerator:

Ø  Alkyl aromatic sulfonate – initiates cross linkage.

Ø  Colloidal silica as a filler – to form the paste

Ø  Plasticizers such as glycoether or phthalate.

Setting reaction:

·        When the base paste is mixed with the catalyst paste ionic polymerization occurs by ring opening of the ethylene – imine group and chain extension.

·        It sets by additional polymerization and no byproduct is formed.

·        Cross linking occurs by cationic polymerization via the imine end groups

·        The set material is hydrophilic. It can absorb water and swell resulting in dimensional change

·        Setting time – 8.3min

·        Mixing time -30sec

·        Improved polyether formulations such as “soft” polyethers are easier to remove, maintain proper rigidity for a wide range of applications nad capture fine details even in moist conditions.

·        This material taste bitter, currently it’s flavoured to offset the taste.

Condensation silicones:

·        It was the first type of silicone impression material

·        These materials are available two paste or paste-liquid-catalyst systems or putty in jars.

·        Multi phase materials available in different viscosities

·        Monophase- available in a single viscosity.

Composition:

Base paste;

Ø  Poly dimethyl siloxane

Ø  Colloidal silica or microsized metal oxide filler

                                      -  Putty viscosity- 60-70%

                                       -  Medium viscosity – 35-75%

                                       -  Low viscosity – 5-15%

             Color pigments

Accelerator paste:

Ø  Alkyl silicate such as orthoethyl silicate – cross linking agent

Ø  Stannous octoate – catalyst

Ø  Inert filler

Ø  Setting reaction:

                                                                    stannous                                                       

Dimethyl siloxane + ortho ethyl silicate                          silicone rubber +ethyl alcohol

                                                                 Octoate

·        It’s a condensation reaction

·        Cross linkage occurs between orthoethyl silicate and the terminal hydroxyl groups of dimethyl siloxane.

·        Ethyl alcohol forms as a byproduct which results in shrinkage.

·        Setting time 8-9 min, mixing time- 45 sec.

·        The setting occurs at room temperature and so called as (room temperature vulcanization) RTV silicones.

·        They are ideal for single unit inlays.

·        Electroplating is possible. Because of the high polymerization shrinkage the cast or die must be poured as soon as possible.

Addition silicones (poly vinyl silicones)

They were introduced in 1975.

They were available as

1.      Two paste systems

2.      Putty in jars

3.      Multiple materials available in different viscosities

4.      Monophase – available in a single viscosiy.

Commercial names:

Multi phase materials – Reprosil, Provil, President

Monophase materials – Imprint, Blue mouse

Composition:

Base paste:

·        Poly (methyl hydrogen siloxane)

·        Other siloxane prepolymers

·        Fillers to give bulk and viscosity

Accelerator paste:

·        Divinyl poly siloxane

·        Inert oils  and fillers – forms the bulk of the paste

·        Palladium salt – catalyst (chlorplatinic acid)

·        Palladium or hydrogen absorber

·        Retarder

·        Filler

Polyvinyl siloxane          Pt salt

              +                                                              silicone rubber

Silane siloxane

·        It’s an addition polymerization reaction.

·        The vinyl groups of the base paste reacts with the silane groups of the accelerator paste and cross linking occurs.

·        There is no production of by product.

·        If the pastes are in improper proportion, hydrogen gas may be liberated during the setting mechanism.

·        Palladium is added to absorb hydrogen to prevent dimensional change.

·        Latex gloves have been shown to adversely affect the setting reaction of addition silicones.

·        Sulfur compounds that are used in vulcanization of latex rubber gloves can migrate to the surface of stored gloves.

·        These compounds can be transferred on to the prepared teeth and adjacent soft tissues during tooth preparation.

·        These compounds can position the platinum containing catalyst which reacts in retarded or no polymerization in the contaminated area of the impression.

·        Vinyl and nitrile gloves donot have such an effect.

·        Residual monomer in acrylic resin provisional restorations and resin composite cores has a similar inhibiting effect on the set of addition silicone materials.

 

Recent advancements:

·        Surfactants have been added to addition silicones by manufacturers to reduce the contact angles, improve wettability, and simply pouring of gypsum models, known as hydrophilized addition silicones.

·        The hydrophilization of addition silicones is gained with the incorporation of non ionic surfactants as micelles. The molecules consist of a hydrophilized part and a silicone compatible hydrophilic part.

·        The mode of action of these surfactants is thought to be a diffusion controlled transfer of surfactant molecules from the poly vinyl siloxane in to the aqeous phase. In this manner the surface tension of the surrounding liquid is altered.

·        This increased wettability allows the addition silicones to spread more freely along the surface. (ref: Craig pg298.)

·        Miller and coworkers reported a significant reduction in the number of voids and an overall increased quality of polyvinyl siloxane impression when a modified polydimethyl siloxane wetting agent  (extrinsic surfactant) was applied to the prepared tooth surface before impressions made.

·        Recently radiofrequency glow discharge has been advocated for use as a didinifecting procedure for polyvinyl siloxane impressions. Whilst this procedure is claimed to clean and improve the wettability of the impression surface, it’s not clear if glow discharging results in sterilization.

(ref: Polyvinylsiloxane impression materials: An update in clinical use, Australian dental journal, 1998, 43(6),428-434)

Monophase impression materials:

·        Impression materials are available as single viscosity pastes called monophase materials.

·        These materials can be used as both light bodied and heavy bodied materials.

·        The amount of pressure given during mixing determines the viscosity. The greater the shear the thinner the viscosity.

·        If more pressure is used it can be used as a lightbodied material if less pressure is used it acts as a heavy bodied material.

Visible light cured polyether urethane:

The composition of the resin matrix is similar to that of light cured composites.

These materials are available as

                           -light bodied

                           - heavy bodied

Composition includes:

               -polyether urethane dimethacrylate

               - diketone – photo initiator

                - Transparent silica – filler (40-60%)

Manipulation:

·        The undercuts should be blocked out before making the impression. Transparent stock trays are available.

·        The light bodied material is syringed and the heavy body material is placed above it.

·        Blue light is used for curing. The exposure should be done from the posterior to anterior region. Each region should get an exposure of 30sec.

·        After removal the impression can be filled and re exposed to light.

Advantages:

·        Long working time, but short setting time.

·        Impressions can be corrected.

·        Dimensional stability, flow, detail reproduction.

Disadvantages

·        Expensive

·        Requires special equipment

The effect of disinfection and a wetting agent on the wettability of addition silicone impression materials

·        Paul J.Milward et al. had conducted a study on the effect of disinfection procedure and the use of surface wetting agent on the wettability of 4 addition polymerized silicone impression materials.

·        They use testing specimens made from 4 addition silicone materials (light bodied president, light bodied Extrude, medium bodied Extrude, and Aquasil)

·        Two disinfection solutions (actichlor and perform) were used.

·        They concluded that application of an external disinfectant actichlor is recommended in preference to Perform the wettability of materials. Treatment with a surface wetting agent after disinfection is recommended to obtain accurate and void free casts and dies.

                      (Ref: JPD 2001, 86,165-7)

                             LUTING CEMENTS

A dental cement used to attain indirect restorations to prepared teeth is called a luting agent . Luting agents may be definitive or provisional depending on their physical properties and the planned longevity of the restoration.

REQUIREMENTS:

·        It must not harm the tooth or tissues.

·        It must allow sufficient working time to place the restoration.

·        It must be fluid enough to allow complete seating of the restoration.

·        It must not dissolve or wash out and must maintain a sealed intact restoration.

·        It must quickly form a hard mass strong enough to resist functional forces.

·        It must not dissolve or wash out and must maintain a sealed intact restoration.

Classifications:

Ø  Craig’s classification based on the chief ingredients eg: zinc phosphate, zinc silicophosphate, zinc oxide eugenol, zinc polyacrylate, glass ionomer, and resin.

Ø  O’Brien classified dental cements by matrix and bond type (eg: phosphate, phenolate, poly carboxylate, resin, resin modified glass ionomer)

Ø  Donovan classified cements into conventional (eg:zinc phosphate,polycarboxylate, glass ionomer) and contemporary (eg:resin modified glass ionomer, resin )

Contemporary definitive luting agents:

Resin modified glass ionomer (RMGI):

·        Introduced in 1980’s.

·        In the original glass ionomer  cement, part of the water component of glass poly alkenoate cement was replaced with a water hydroxyl methyl methacrylate (HMMA) mixture plus an initiator/ activator for the added resins.

·        Resin modified glass ionomer is a dual cure hybrid, because setting occurs by a combination of the long term, complex acid- base reaction typical of glass ionomer cement and chemical or light iniated polymerization of the added resin.

·        The acid base reaction continues to develop a polysalt hydrogel matrix which hardens and strengthens the existing polymer matrix.

Compomers:

·        The compomers , also known as polyacid- modified composite resins appeared in the late 1990’s, and were described as being a combination of composite resin (comp) and glass ionomer (omer), offering the advantages of both.

·        Compomers are anhydrous resins that contain ion leachable glass as part of the filler and dehydrated poly alkenoic acid.

·        The physical behaviour of the compomers is more like composite resins than glass ionomer, with higher compressive and flexural strength than RMGI, but inferior to unmodified composite.

·        Tooth addition is very little, fluoride release is very limited and it’s less than that of conventional glass ionomer .

Resin:

·        Resin cements are methyl- methacrylate, Bis- GMA dimethacrylate or Urethane dimethacrylate based with fillers of colloidal silica or barium glass 20-80% by wt.

·        They are available as powder/liquid, encapsulated or paste/paste systems and may be auto, dual or light cured to form the polymer matrix.

·        Resin bonding to enamel is by mechanical interlocking into an acid etched surface. Bonding to dentin is also micromechanical but is more complex usually requiring  multiple steps that include removal of the smear layer and surface demineralization, then application of unfilled resin bonding agent or primer to which the resin commercially bonds.

·        Non eugenol provisional cement is recommended for provisional restorations, when resin will be used for definitive restoration. Since the residual eugenol from provisional cement can interfere with the setting reaction of the bonding agent.

·        Many new resin luting systems have recently appeared that reduce luting procedures by including the use of a self etch primer built in. eg:Unichem by 3M ESPE,Maxcem by Kerr Orange ,California.

·        Light cured resin cements are cured more completely after initial placement. Where as auto and dual cured resins slowly gain  strength.

·        Resin cements chemically bond to etched silane treated prcelainit has been postulated that resin cement bonded to considered tooth on oneside and etched /silane coated porcelain on the other helps diffuse stresses across the tooth.

·        Compressive and tensile strength, toughness and resilience of resin cement equal or exceed those of other luting agents.

·        The resin luting cement offers no fluoride release or uptake, film thickness maybe relatively high, removal of restoration may require total destruction.

·        They are more technique sensitive and expensive.

Adhesive resin:

·        In the early 1980’s conventional Bis GMA resin cement was modified by adding a phosphate ester to monomer component to improve the degree of chemical bonding as well as micromechanical bonding to tooth structure and base metal alloys.

·        Eg; Panavia – contained the bifunctional adhesive monomer 10- methacryloyloxy deci dihydrogen phosphate (MDP) and was a powder/ liquid system.

·        In 1994, Panavia was modified to include a dentin/enamel primer containing hydroxyethyl methacrylate (HEMA), N-methanyloyl 5- aminosalicylic acid and MDP intended to improve bond strength to dentin.

·        Eg;Panavia 21, marketed as a two paste system offered 3 shades, tooth colored 9T.C, translucent), white (EX,semitranslucent) and opaque (OP).

·        The current product Panavia F is a two paste system that is dual cured , self etching and self adhesive plus fluoride releasing.

·        C&B metabond – modified Bis GMA composite by decreasing filler and adding 3% 2 hydroxy-3b – naphthoxypropyl methacrylate in methyl methacrylate with 4- methacryloyloxy ethyl trimellitate anhydride (4-META) and tri-n-butyl borane.

·        It’s a powder liquid autocuring system and may be used for resin bonded prostheses.

(Ref: DCNA, July 2007, vol51, No.3)

Development of a novel comonomer free light cured glass ionomer cement for reduced cytotoxicity and enhanced mechanical strength (Ref: Journal of Dental materials 23(2007), 994-1003).

Ø  Dong xie, Youfun yang et al had developed a novel comonomer free light cured glass ionomer system based on 4 arm star shape poly acrylic acid.

Ø  The mechanical strengths and invitro cytotoxicity of the formed system were evaluated and compared with those of several representative commercial glass ionomer cements.

Ø  The 4- arm poly (acrylic acid) was synthesized using ATRP and tethered with glycidyl methacrylate (GM). The GM tethered polymer was formulated with water, photoinitiators and fujiII, fuji II LC and vitremer were used for comparision. Compressive strength (CS) and MTT assay were used as tools to evaluate the mechanical strengths and invitro cyto toxicity of the cements respectively.

Ø  They concluded that this novel comonomer free light cured glass ionomer cement has significantly improved mechanical strength, and no invitro cytotoxicity observed.

Fuji CEM Automix;

Ø   GC America announces Fuji CEM Automix , the first automix delivery      system available ina resin modified glass ionomer.

Ø   Fuji CEM  Automix requires no hand mixing and dispences a consistent mixing ratio directly into the restoration.

Cavity liner:

Calcium hydroxide:commonly employed as adirect or indirect pulp capping agent.

Ø  Two paste system employed as a direct or indirect pulp capping agent.

Available as

Ø  Twp paste systems containing base and catalyst pastes in collapsible tubes

Ø  Light cured systems

Ø  Powder and liquid

Ø  Single paste system

Commercial names:

-         CRCS (calciobiotic canal sealer):

-         It’s essentially a zinc oxide eugenol sealer to which calcium hydroxide has been added for its osteogenic effect.

Sealapex (by manufacturing company):

-         The base is zinc oxide with calcium hydroxide as aell as butyl benzene sulfonamide and zinc stearate.

-         Calasept (by scania dental AB, Sweden ): it contains calcium hydroxide + potassium chloride + sodium chloride+ calcium chloride+ sodium bi carbonate + distilled water

Calen; it contains calcium hydroxide + zinc oxide + colophony + poly ethylene glycol.

                              DENTAL CERAMICS

The word ceramics is derived from the greek word Keramos meaning pottery or burnt stuff. Ceramics is an inorganic compound with non metallic properties typically composed of metallic or semi metallic and non-metallic elements (eg. Al2O3 CaO and Si3N4).

Def:

An inorganic compound with nonmetallic properties typically consisting of oxygen and one or more metallic or semimetallic elements (eg: aluminium, calcium, lithium, magnecium, potassium, silicon, sodium, tin, titanium and zirconium) that is formulated to produce the whole or part of a ceramic based dental prosthesis.

Ceramic:

Def:  an inorganic compound with non metallic properties typically composed of metallic (or semi metallic0 and non metallic elements (eg:Al2O3, and Si3N4)

Ceramics can be classified in one of four categories

Ø  silicate ceramics

Ø  oxide ceramics

Ø  non oxide ceramics

Ø  glass ceramics

Dental ceramics fall in category of silicate ceramics, which are characterized by an amorphous glass phase with a porous nature.

History:

Ø  The porcelain tooth material was patented in 1789 by French dentist de Chemant in collaboration with a French pharmacist Duchateau.

Ø  In 1808, Fonzi an Italian dentist invented a terrometallic porcelain tooth that was held in place by a platinum pin or frame.

Ø  Planteau, a French dentist introduced porcelain teeth to  united  states in 1817 and Peale an artist.

Ø  Ash developed an improved version of the porcelain tooth in 1837.

Ø  Dr Charles land introduced one of the first ceramic crowns to dentistry in 1903.

Ø  The first commercial porcelain was developed by Vita Zahnfabrik in about 1963.

Ø  A significant improvement in the fracture resistance of porcelain crowns was reported by Mclean and Hughes in 1965 when a dental aluminous core ceramic consisting of a glass matrix containing between 40 and 50 wt % Al2O3 was used.

Ø  Improvements in all ceramic systems developed by controlled crystallization of a glass (dicor) was demonstrated by Adan and Grossman (1984)

Ø  This glass was melted and cast in to a refractory mold and subsequently crystallized to form the dicor glass ceramic that contained tetrasilicic flouramina crystals in a glass matrix.

Ø  Pressable glass ceramic (IPS Empress) was introduced in early 1990’s, containing 34% vol. of leucite. A more fracture resistant, pressable glass ceramic (IPS Empress 2) containing appx. 70% vol. of Lithia disilicate crystals was introduced in the late 1990’s.

Classification:

Dental ceramics can be classified based in many factors.

1. Based on chemical composition

a)Silicate ceramics:

Silicate ceramics have oxides of silicon and other atoms of aluminium, potassium, magnecium calcium  eg:potash felds, sodium feldspar.

b).Non silicate ceramics:

Without silica the other ingredients being the same eg: alumina (Al2O3), Spinell (MgO, Al2O3)

c).Non oxide ceramics:

This includes silicon carbide, tungsten carbide or graphite.

2. Based on crystalline nature:

Ø  Crystalline ceramics; eg:feldspathic porcelain contains leucite (crystal phase)

Ø  Non crystalline ceramics eg:glass

3. Based on fusion temperature:

1.      High fusing- 1300degree C

2.      Medium fusing – 1101 – 1300 degree C

3.      Low fusing 850- 1100 degree C

4.      Ultra low fusing  less than 850 degree C

The medium fusing and high fusing types are used for production of denture teeth.

The low fusing and ultra low fusing porcelains are used for crown and bridge construction.

4. Based on type:

·        Feldspathic porcelain

·        Aluminous porcelain

·        Glass infiltered alumina

·        Glass infiltred spinell

·        Castable glass ceramic

·        Injection molded glass ceramics (IPS Impress: Optec)

·        Leucite reinforced porcelain.

5. Based on the method of fabrication:

·        Pressure moldingband sintering

·        Condesnsation and sintering

·        Casting and ceramming

·        Slip casting

·        Sintering and glass infiltration

·        Milling by computer control

·        Copy milling

6. Based on application

·        Core porcelain

·        Opaque porcelain

·        Dentine or body porcelain

·        Enamel porcelain

7. Based on sub structural material

1.      Cast metal poecelain

2.      Swaged metal porcelain

3.      Glass ceramic

4.      CAD-CAM porcelain

5.      Sintredceramic core

8. Based on use

·        Denture teeth

·        Metal ceramicsveneer, inlays, onlays , crowns

·        Fixed partial denture

9. Based on firing

·        Air fired porcelain

·        vaccum fired porcelain

·        Diffusible gas firing

10. Classification  based on recent types of ceramics

1.      castable glass ceramics eg: Inceram, alumina, Inceram, spinell

2.      pressable ceramics EG: Optec HSP, IPS empress

3.      CAD – CAM ceramics eg: cere vitablock markI, vitablock mark II

4.      Injection molded ceramics eg: Optec HSP

CASTABLE GLASS CERAMICS:

·        EG Inceram, alumina, Inceram, Spinell, Dicor  and Dicor MGC

·        Castable ceramic systems are used to cast crowns by the lost wax process.

·        Indicated in cases of single anterior and posterior crowns.

·        Tooth preparation is either90 degree shoulder with a rounded internal line angle or 120 degree chamfer with adequate tooth reduction from 1mm. Minimum on gingivo axial aurfaces to 1.5-2 mm incisally and occlusally.

·        The restoration is waxed on to the die and the wax pattern of the crown is invested in a phosphate bonded investment.

·        An ingot of the ceramic material is placed in a special crucible and melted and cast with a motor driven centrifugal casting machine at 1380 degree C.

Advantages:

·        Ease of fabrication

·        Improved esthetics

·        Minimal processing shrinkage

·        Good marginal fit

·        Low thermal expansion, near to the enamel

·        Minimal abrasiveness to tooth enamel

Disadvantages:

·        Limited use in low stress areas

·        Inability to colour internally

·        Low tensile strength.

Hot pressing:

Eg: IPS Empress, IPS Empress 2, IPS e max press, OPC

-         Pressure molding is used to make small intricate objects. This method used a piston to force a heated ceramic ingot through a heated tube in to a molod, where the ceramic form cools and hardens to the shape of the mold.

-         IPS Empress is a glass ceramic provided as core ingots that are heated and pressed until ingot flows in to a mold. It contains a higher conc. Of leucite crystals that increase the resistance to crack propagation (fracture).

Machinable ceramics:

Computer aided design / computer aided manufacturing:the evolution of CAD –CAM systems for the production of machined inlays, onlays and crowns led to the development of a generation of machinable porcelains.

There are two popular systems available for machining all ceramic restorations

·        CEREC System (siemens, Bensheim, Germany)

·        Celay system (Mikrona technologies, Switzerland)

CEREC System:

·        CEREC is a dental restoration product that allows a dental practitioner to produce an indirect ceramic dental restoration using a variety of computer assisted technologies including 3D photography and CAD/CAM.

·        The cavity preparation is first photographed and stored as a three dimensional digital model and proprietary software is then used to approximate the restoration shape using biogenic comparisions to surrounding teeth.

·        When the model is complete a milling machine carves the actual restoration out of a ceramic block using Diamond Head cutters under computer control.

·        CEREC is an acronym for chairside economical restoration of esthetic ceramics.

HISTORY:

·        It was introduced by Werner H.Mormann (1980) at the University of Zurich.

·        The first chair side CEREC introduced in 1985.

·        In 1994 CEREC -2 was introduced.

·        In 2000 CEREC -3 was introduced.

·        In 2003 , 3D soft ware version is released, allowing users to see 3D views of teeth and models

·        In 2008, Sirona release the MCXL milling unit, this milling unit can produce a crown in 4 minutes.

CEREC –I:

·        introduced in 1985

·        chief indications are single and dual surface inlays and the material is vitablocs markII

·        The concept of grinding inlay bodies externally with a grinding wheel along the mesiodistal axis suggested itself.

·        In this arrangement we could turn the ceramic block on the block carrier with a spindle and feed it against the grinding wheel which ground from the full ceramic and new contour with a different distance from the inlay axis at each feed step.

CEREC -2

-         introduced in 1994

-         Additional cylinder diamond enabling the firm grinding of partial and full crowns.

-         An upgraded 3D camera was provided.

CEREC -3

§  Skipped the wheel and introduced the two bur system.

§  It’s a compact windows based CAD- CAM system.

§  In 2006 a step bur was introduced, reduced the diameter of the top one third of the cylinder bur to a small diameter tip enabling high precision form grinding with reasonable bur life.

§  The three dimensional virtual display of the preparation of the antagonist and of the functional registration became available with the introduction of the three dimensional version of the soft ware in 2003.

§  The current CEREC – 3 System can fabricate inlays, onlays and posterior crowns as well as anterior crowns and veneers.

§  Two materials can be used with this system:

§  Vita mark II (VIDENT, BALDWIN PARK ca)

§  Dicor  MGC (Dentsply international, York, PA)

§  Vita mark IIcontains sanidine (KAl Si3O8) as a major crystalline phase within a glassy matrix.

§  Dicor MGC is a machinable glass ceramic similar to Dicor, with the exception that the materials cast and cerammed by the manufacturer.

Celay system:

§  The celay system (Mikrona technologie, spreitenbach, Switzerland) uses a copy milling technique to manufacture ceramic inlays or onlays from resin analogs.

§  The Celay system is a mechanical device based on pantographic tracing of a resin inlay or onlay fabricated directly on to the prepared tooth or on to the master die (Eidenbenze U/1994).

§  One ceramic system material available for use with the celay system is vita – celay (vident, Baldwin park, CA). this material contains sanidine as the major crystalline phase within a glassy matrix.

§  Recently, n-ceram presintered slip cast alumina blocks (vident, Baldwin park, CA)  have been machined with the celay copy milling system used to generate coping for crowns and fixed partial dentures.

§  (mclare and Sorensen)

Review of new materials:

Sintered porcelains:

Alumina based ceramics:

§  Aluminous core porcelain is a typical example of strengthening by dispersion of a crystalline phase (mclean and kedge, 1987). Alumina has a high modulus of elasticity (350 GPa) and high toughness (3.5-4Mpa).

§  Its dispersion in a glassy matrix of similar thermal expansion coefficient leads to significant strengthening of the core. Hiceram is a more recent development in this system.

Magnesia based core porcelain:

§  Magnesia core ceramic wad developed as an experimental material in 1985 (O’Brien, 1985). Its high thermal expansion coefficient (14.5 x 10-6 /degreeC) closely matches that of the body and incisal porcelains designed for bonding to metal (13.5x 10-6 ).

§  The flexural strength of unglazed magnesia core ceramic is twice as high (131 MPa) as that of conventional feldspathic porcelain.

Zirconia based porcelain:

§  Mirage II (Myron international, Kansas city, KS ) is conventional feldspathic porcelain in which tetragonal zirconia fibres have been included.

§  Zirconia undergoes a crystallographic transformation from monalinic to tetragonal at 1173 degree C)

§  Partial stabilization can be obtained by using various oxides such as CaO, MgO, y2o3 and Ceo which allows high temperature tetragonal phase to be retained at room temperature.

§  The result of this transformation is that compressive stresses are established on the crack surface, there by arresting its growth. This mechanism is called transformation toughening.

§  The addition of yttria – stabilized zirconia to conventional feldspathic porcelain has been shown to produce substantial improvements in fracture toughness, strength and thermal shock resistance.

Leucite reinforced feldspathic porcelain:

§  Optec HSP  material is a  feldspathic porcelain containing up to 45 vol% tetragonal leucite (Schmid et al 1992, Pinche etal 1994,  Demy and Rosensteil , 1995)

§  The greater leucite content of Optec HSP porcelain compared with conventional feldspathic porcelain for metal ceramics leads to a higher modulus for rupture and compressive atrength.

§  The larger amount of leucite in the material contributes to a high thermal contraction coefficient. In addition the large thermal contraction mismatch between leucite (22-25x10-6/degreeC ) and the glassy matrix (8x10-6 /degreeC) results in development of tangential compressive stresses in the glass around the leucite crystals when cooled.

§  These stresses can act as crack deflectors and contribute to increase the resistance of the weaker glassy phase to crack propagation.

Slip cast all ceramic materials:

§  Slip casting involves the condensation of an aqueous porcelain slip on a refractory die. The porosity of the refractory die helps condensation by absorbing the water from the slip by capillary action. The piece is then fired at high temperature on the refractory die.

§  The fired core is later glass infiltered a unique process in which molten glass is drawn in to the pores by capillary action at high temperature.

§  Advantages include reduced porosity, fewer defects from processing and higher toughness than conventional feldspathic porcelains.

§  Disadvantages include high opacity and long processing times.

§  Materials used in this technique are

                 -Alumina based materials

                 -Spinell and zirconia based materials. 

Glass ceramics:

Mica based :

§  Glass ceramics obtained by controlled devitrification of glasses with a suitable composition including nucleating agents. Depending on the composition of the glass, various crystalline phases can nucleate and grow within the glass.

§  The advantage of this process is that dental restorations can be cast by means of lost wax technique, thus increasing the homogeneity of the final product compared with conventional sintered feldspathic.

§  Dicor is a mica based glass ceramic

§  Micas are classified as layer type silicates.

§  Cleavage planes are situated along the layers and this special crystal structure dictates the mechanical properties of the mineral itself. Crack propagation is not likely to occur across the mica crystals and is more probable along the cleavage planes of these layered silicates.

§  In the glass ceramic material the mica crystals are usually highly interlocked within the glassy matrix, achieving a “house of cards” microstructure (Grossman 1972). The interlocking of the crystal is a key factor in the fracture resistance of glass ceramic.

Hydroxyl apatite based:

Cera pearl (Kyocera, Sandiego CA) is a castable glass ceramic in which the main crystalline phase is oxyapatite transformable into hydroxyapatite when exposed to moisture (Hobo and Iwata 1985).

Future directions:

§  The future of ceramics for dentistry is clearly open to new technologies. Research is now focusing on fractrographic analysis of clinically failed restorations, measure of fatigue parameters and lifetime prediction of ceramic restorations.

§  The metal ceramic technique is still the most commonly used procedure in restorative dentistry and the success of new all ceramic systems will depend as much on developmental as on analytical research.

                          DENTURE BASE RESINS

Poly (methyl methacrylate) polymers were introduced as denture base materials in 1937.

Previously materials such as vulcanite , nitrocellulose, phenol formaldehyde, vinyl plastics and porcelain were used. Later other polymers vinyl acrylic polysterene epoxy, nylon, vinyl sterene, poly carbonate, poly sulfone- unsaturated polyester, polymethane, polyvinylacetate ethylene, hydrophilic polyacrylate, silicones, light activated urethane dimethacrylate, rubber reinforced acrylics and butadiene reinforced acrylic were used.

Ideal requirements of denture base materials:

1.      Strength and durability

2.      Satisfactory thermal properties

3.      Processing accuracy and dimensional stability

4.      Chemical stability

5.      Insolubility in and low sorption of oral fluids

6.      Absence of taste and odour

7.      Biocompatibility.

8.      Natural appearance

9.      Color stability

10. Adhesion to plastics , metals and porcelains

11. Ease of fabrication and repair

12. Moderate cost.

Classification of denture base materials:

1.      Based on the duration of use

2.      Based on the material used

3.      Based on the chemical composition of the resins

4.      Types of acrylic resins

           -based on their mode of activation

           -based on filler particles.

1. Based on duration  of use:

§  Temporary : it’s used to construct occlusal rims for jaw relations

§  Permanent: it’s a final prosthesis

It’s made in heat cure resin or in casting alloys

2. Based on the material used

§  Non metallic- acrylic resins and waxes

§  Metallic - base metal alloys, type IV gold alloys.

3. Based on the chemical composition of the resins:

§  Type1: acrylic

§  Type2: dimethacrylate

§  Type3: composites

Types of acrylic resins:

Based on their mode of activation:

§  Heat activated

§  Chemically activated or self cure or cold cure or autopolymerized resins

§  Light activated resins.

Based on the filler particles

§  Unfilled resins for direct filling eg:acrylic resins

§  Filled resins for direct filling eg: composites

Evolution of acrylic resins:

§  Acrlic acid and its derivatives came to be well known by the 1890’s

§  Dr.ottorohm is considered as the father of Recent acrylic. He introduced polymers of acrylic acid in 1901.

§  1927 acryloid and plexigum both polymers of polymethylmethacrylate were introduced by Rohm and Haas.

§  In 1931 commercial production of harder poymethyl methacrylates occurred with the introduction of plexiglass (also known as organic glass, leucite I plexite)

§  Acrylic resins came in to use in dentistry between 1930 and 1940. they are used in dentistry as denture base materials.

ANSI/ADA specifications No: 12 (1567) for denture base resins:

Categories include the following types and classes:

Type 1: heat polymerizable polymers (class 1 powder and liquid; class 2: plastic cake)

Type 2: auto polymerizable polymers (class1: powder and liquid, class2: powder and liquid pour type resins)

Type3: thermoplastic blank or powder

Type4: light activated materials

Type5: microwave cured materials

The ADA specifications for non processed materials are

§  The liquid should be as clear as water and free of extraneous material and the powder, plastic cake or procured blank should be free of impurities such as dirt and lint.

§  A satisfactory denture base results when the manufacture’s instructions are followed. The denture base should be nonporous and free from surface defects.

§  The cured plastic should take a high gloss when polished

§  The processed denture should not be toxic to a normal healthy person

§  The color should be as specified

§  The plastic should be translucent

§  The cured plastic should not show any bubbles or voids.

Specific requirements:

§  Water sorption shall not be more than 0.8mg/cm2 after immersion for 7days/ at 37degreeC.

§  Stability shall not be more than 0.04mg/cm2 after water soaked specimen is dried to constant weight.

§  Plastic shall show no more than a slight color change when exposed   to a 24hr specified UV lamp test.

Recent advances in denture base materials:

Pour type acrylics:

§  The chemical composition of the pour type denture resins is similar to poly (methyl methacrylate) materials that are polymerized at room temperature.

§  The principle difference is in the size of the polymer powder or beads. The pour type denture base resins commonly referred to as fluid resins, have much smaller powder particles, when mixed with monomer the resulting slurry is very fluid.

§  The mix is quickly poured in to an agar hydrocolloid or modified plaster mold and allowed to polymerize under pressure at 0.14MPa.

§  Centrifugal casting and injection molding are technique s used to inject the slurry into the mold.

High impact strength acrylics:

§  Denture base materials that have greater impact strength have been introduced.

§  These polymers are reinforced with butadiene –styrene rubber. The rubber particles are grafted to methyl methacrylate to bond to the acrylic matrix.

§  These materials are supplied in a powder- liquid form and are processed in the same way as other heat accelerated methyl methacrylate materials.

Rapid heat polymerized acrylics;

§  These hybrid acrylics are polymerized in boiling water immediately after being packed in to a denture flask.

§  The initiator is formulated from both chemical and heat activated initiators to allow rapid polymerization without the porosity.

§  After placing the denture in boiling water the water is brought back to afull boil for 20min.

§  After bench cooling to room temperature, the denture is deflasked, trimmed and polished in the conventional manner.

Light activated resins:

§  This denture base material consists of a urethane dimethacrylate matrix with an acrylic copolymer, microfine silica fillers and a photoinitiator system.

§  It’s supplied in premixed sheets having clay like consistency.

§  The denture base material is adapted to the cast while it’s still pliable

§  The denture base can be polymerized in a light chamber with blue light of 400-500nm.

§  The denture base can be polymerized in a light chamber without teeth and used as a record base.

§  The teeth are processed to the base with additional material and the anatomy is sculptured while the material is still plastic.

§  The denture rotates in chamber to provide uniform exposure to the light source.

Reinforced denture base with glass fillers:

(Ref: JOP1999, 18-26, vol-8, no.1)

·        Mona K.Marie has conducted a study to evaluate the effect of short glass fibers on the transverse strength of a heat polymerized denture base material.

·        In their study they incorporated glass fibers (Sio2- 54%, Al2O3-14%, B2O3-9%, MgO-5% and CaO-18%) that were 3.8 micrometers in diameter.

·        Optimal adhesion between the fibers and the polymer matrix can be obtained by mixing with silane coupling agents.

·        Incorporation of glass fibers in a continuous roving form increases the strength of dentures and enhances the fracture resistance.

·        Main disadvantage of this system is difficulty in handling the fibers and inadequate degree of impregnation of fibers with the resin.

Other materials used for reinforcement of acrylic resin materials are

Polymer-fiber composites:

·        Polymer fiber composites are materials that are composed of   a polymer matrix and reinforcement.

·        The fiber reinforcement is characterized by its length being much greater than it’s cross sectional dimensions.

·        In polymer free composites the fibers are embedded in a polymer matrix which binds the fibers and forms a continuous phase surrounding the fibers.

·        The polymer matrix transfers the loads to the fibers which are stronger component of the composite.

·        The composites with long fibers are called continuous fiber composites and those with short fibers are called short fiber composites.

·        The chemical bond between the polymer and the fibers should ideally be of a covalent nature.

·        proper adhesion makes it possible to transfer the stresses from the matrix to the fibers.

Carbon /graphite fibers:

Ø  The carbon / graphite fiber reinforcement of the denture base materials was published in the early 1970’s

Ø  The study reported a 100% increase of the transverse strength of PMMA.

Aramid fibers:

·        Aramid fiber is the generic name for aromatic polyamide fibers, which are more commonly called Kevlar fibers after the first commercially available AF produced by Du pont.

·        Fibers have been shown to significantly increase the impact strength of acrylic denture base material.

·        The Aramid yellow color of the aramid fibers might limit their use to certain intra oral applications.

Metal fillers:

Ø  They improve the thermal conductivity of PMMA and enhances it’s strength, but also contribute to poor esthetics for complete dentures.

Ultra high modulus polyethylene fibers (UHMP):

Ø  The effect of unidirectional UHMP fiber reinforcement on the transverse strength of the PMMA depends on the amount of fibers present.

Ø  Fiber contents as high as 40-70% wt considerably enhanced transverse strength of the composite.

Aluminium oxide addition:

·        Ayman E.Ellakwa et al. conducted a study on the effect of adding from 5.2% by wt Al2O3 powder on the flexural strength and thermal diffusivity of heat polymerized acrylic resin.

·        In their study aluminium oxide powder was added to polymer of heat polymerized acrylic resin.

·        The monomer and the polymer of the heat polymerized acrylic resin were proportioned, mixed packed and pressed in to the mold following manufacturer’s directions.

·        Aluminium oxide commonly referred to as alumina possess strong ionic, interatomic bonding giving rise to its desirable material characteristics.

·        Can exist in several crystalline phases which hexagonal alpha phase is the most stable form.

·        They conclude that the incorporation of Al2O3 powder from 5% to 20% by weight in to conventional heat polymerized denture base resin results in an increase in both it flexural strength and thermal diffusivity.

Introduction of a denture injection system for use with microwavable acrylic resins;

-         GC lab technologies (LOCK PORT IL) introduced a denture base processing system that combines injection molding and microwave activation techniques that accelerate the polymerization process.

-         In the GC INJECTION system a pneumatic press is used to force unpolymerized acylic resin into the mold cavity. A modified microwavable flask is used to facilitate this process.  The modified flask has a small channel in its lid that permits a small diameter sprue (7mm) to pass from the external surface of the flask in to the mold cavity.

Advantages:

The injection process eliminates the need for direct handling of resin during the packing process

Disadvantages:

Ø  The additional cost of the pneumatic press and associated flask components.

Ø  The necessity of adding and removing screws.

·        Ali pervizi (2004) et.al compared the three dimensional changes of 3 injection molded denture base materials to that of conventionally processed polymethyl methacrylate (PMMA) Resin.

·        They compare the dimensional accuracy of maxillary complete denture, which are processed in 4 types of materials. 1. PMMA (microlon). 2. Injection –molded PMMA (Northern) 3.injection-molded nylon (valplast). 4. Injection molded styrene.

·        They concluded that for all groups the greatest distortion occurred with nylon and the least with styrene.

Development of a radio opaque auto polymerizing dental acrylic resin:

-         There are many materials which can act as radio opaque additives

Eg: Barium sulfate, Barium acrylate, Bismuth bromide)

-         But these materials weaken the resin and decrease the transverse and impact strengths.

-         Patrik A.Mattie et al (19940 proposed a component that is Triphenyl Bismuth found to be soluble in avariety of monomers and polymers seems to overcome the problems of the bismuth trihalides and has a very low level of cytotoxicity which  indicates significant biocompatibility.

-         TPB doesnot leach from the resin and provides radioopacityequivalentto aluminium.

-         And the authors concluded that TPB doesnot significantly alter required performance and processing properties.

CONCLUSIONS:

                          It is the goal of medical procedure to provide the best treatment for the patient while following the Hippocratic oath: “First, do no harm”. As dentists, we are challenged to restore function while providing a highly esthetic result. The choices available for esthetic restorations are expanding continually as more private and public research is aimed at improving clinical results.

                         An examination of material properties should lead us to select those

systems engineered to provide the patient with best clinical out come with respect to esthetics , function , longevity and compatibility with surrounding natural tissues.

      REFERENCES

1.     Restorative Dental materials: G Craig & John M Powers-11th edition2002.

2.     Phillips science of dental materials: Anusavice;  11th edition

3.     DCNA, July 2007, 994-1003.

4.     O’Brien, Dental Materials & their Selection 1997

5.     Evolution of dental ceramics in the twentieth century, John W.Mclean, JPD VOL-85, NO.1, Jan-2001.

6.     A novel comonomer –free light-cured glass – ionomer cement for reduced cytotoxicity and enhanced mechanical strength. Dong Xie, J of  Dental materials 23 (2007) 994-1003.

7.     The effect of disinfection and a wetting agent on the wettability of addition silicone Impresion materials; Paul J.Milward, JPD 2001; 86.165-7.

8.     Introduction of a denture injection system for use with microwaveable acrylic resins; R,D Phoenix, JOP, V ol 6, No.4, DEC1997, pg286-291.

9.     JOP , 2004,VOL13, NO.2(june), pg 83-89

10.                        JOP; VOL-2, No.3 ,sept 1993; pg 174-177

11.                        JOP; VOL-3,No.4 DEC.1994;pg 213-218

12.                        Poly vinyl siloxane impression materials; an update on clinical use; Michael N.Mandiko; Australian dental journal ,1998, 43 (6); 428-434.

13.                        JOP; xx (2008) 1-6

14.                        JOP;  VOL-5, No.4 DEC,1996, PG 270-76

15.                        JOP; VOL-8, No.1 march 1999, pg 18-26

16.                        Clinical performance of chair side CAD/CAM restorations;JADA, VOL 137, 22-31

17.                        The evolution of the CEREC system; Werner H. Mormann, JADA, vol 137, 2006

18.                        Materials for chairside CAD/CAM produced restorations, Russell GIORDANO;JADA, vol 137 ,2006 14