DENTAL RESIN/SYNTHETIC RESIN

SYNTHETIC RESINS Are non-metallic compounds which are molded

into various forms and then hardened for commercial use (e.g., clothing, electronic equipment, building materials and household appliances).

These are materials are composed of polymers or complex molecules of high molecular weight.

A variety of resins are used in dentistry which includes: Acrylics Polycarbonates Vinyl resins Polyurethanes Styrene Cyanoacrylates Epoxy resins

CLASSIFICATION OF RESINS

Based on the THERMAL BEHAVIOR THERMOPLASTIC –

resin that can be repeatedly softened and molded under heat and pressure without any chemical change occuring

They are fusible and are usually soluble in organic solvents

THERMOSETTING – Resin that can be molded only once. They set when heated They are generally infusible and insoluble

USES OF RESINS IN DENTISTRY

Fabrication of dentures (denture base resins)

Artificial teeth (cross-linked acrylic resins)

USES OF RESINS IN DENTISTRY

Orthodontic and pedodontic appliances

Provisional restorations in FPD

Tooth restorations (fillings)Inlay and post-core patterns (pattern resins)

Crown and FPD facings (tooth colored acrylic or composite resins)

Cementation of orthodontic brackets, crowns and FPDs (resin cements)

Custom impression trays Dies (epoxy resins)

Maxillofacial prostheses (obturators for cleft palates)

Endodontic and core filling material

Athletic mouth protectors

Splints and stents Models

IDEAL REQUIREMENTS OF DENTAL RESINS

Be tasteless, odorless, non-toxic and non-irritant to the oral tissues

Be esthetically satisfactory (color should be permanent)

Be dimensionally stable (not expand, contract or warp during processing and subsequent used by the patient)

Have enough strength, resilience and abrasion resistance

Be insoluble and impermeable to oral fluids Have a low specific gravity (light in weight)

Tolerate temperatures well above the temperature of any hot foods or liquids taken in the mouth without undue softening or distortion

Be easy to fabricate and repair Have good thermal conductivity Be radiopaque When used as filling material it should:

Bond chemically with the tooth Have coeeficient of thermal expansion which

match that of tooth structure Be economical

ACRYLIC RESINS

Are derivatives of ethylene and contain a vinyl group in their structure formula.

The acrylic resins used in dentistry are the esters of: Acrylic acid, CH2=CHOOH Methacrylic acid, CH2=C(CH3)COOH

POLY (METHYL METHACRYLATE ) RESINS

Widely used in dentistry to fabricate various appliances

Although, it is a thermoplastic resin, in dentistry it is not usually molded by thermoplastic means

The liquid (monomer) methylmethacrylate is mixed with the polymer (powder)

Types (based on the method used for its activation) Heat activated resins Chemically activated resins Light activated resins

HEAT ACTIVATED DENTURE BASE ACRYLIC RESINS

Heat activated polymethyl methacrylate resins are the most widely used resins for the fabrication of complete dentures

Available as: Powder and Liquid

Powder may be transparent or tooth colored or pink colored

The liquid (monomer) is supplied in tightly sealed amber colored bottles

Gels – sheets and cakes

COMPONENTS OF ACRYLIC RESIN Dispensing Form: Powder (polymer) Liquid

(monomer) Powder1. Polymethyl Methacrylate – main component, in

beads or pear form2. Benzoyd/Benzoyl Peroxide – an initiator, starts the

polymerization process3. Dibuthyl Pthalate – plasticizer, improves the

workability4. Talc or Gelatin – prevents cohesion of the beads

during storage/ prolong storage life5. Dyed Synthetic Fibers – improves aesthetics

appearance as it produce reddish and bluish small blood vessel appearance

6. Titanium Dioxide – fibers, hardening agents7. Coloring Material/Pigments

Liquid (Monomer)1. Methyl Methacrylate – main component,

transparent, colorless liquid with sweetish odor, toxic when inhaled in prolong periods of time

2. Hydroquinone – prevents premature polymerization during storage

3. Glycol Dimethacrylate – cross linking agent makes resin hard, stronger, serves as bridge that unites two polymerized chains, improves physical property

CUSTOM TRAY CONSTRUCTION With a red pencil, outlined the planned area of the

denture base in the diagnostic cast. Use the the following landmarks as basis for the design of the denture base:

Mucobuccal fold External oblique ridge Masseteric notches Retromolar pads Mylohyoid ridges frenae

With a blue pencil, outline the tray design. The tray design should be about 2 mm shorter the planned denture base.

Mix the resin , form it into a ball during the dough stage and flatten it to an even thickness using a roller or two wet glass slabs. (omit this step if using VLC resin).

Apply petroleum jelly on the diagnostic cast to prevent acrylic from sticking.

Adapt the resin to the cast and trim the excess with a sharp knife. Allow the material to set. (place in a curing unit for VLC).

Finish the tray by trimming the border. Border thickness should be about 2 to 3mm.

Attach a handle made of the same tray material. Position the handle at the anterior ridge crest area.

CUSTOM TRAY CONSTRUCTION With a red pencil, outlined the planned

extent of the denture base. Use the following landmarks as basis for the design:

Mucobuccal fold Coronomaxillary space Hamular notches Vibrating line Frenae

With a blue pencil, outline the tray design. The tray must be designed about 2 mm shorter along the muccobuccal fold, except along the posterior palatal seal area.

An apron about 3-4mm wider than the planned postdam area, should be made instead.

Apply petroleum jelly on the diagnostic cast to prevent acrylic from sticking.

Adapt the acrylic tray material to the cast following the same procedure for mandible and attach a handle.

VISIBLE LIGHT CURED RESIN(POLYMERIZING RESIN)

STEPS IN MAKING DENTURE BASE

1. Impression Taking2. Cast Construction3. Wax Pattern Construction4. Investing of Wax Pattern5. Wax Elimination/Burnout Procedure6. Preparation of Mold Space7. Packing of Resin Dough8. Curing9. Deflasking10. Trimming and Poishing

IMPRESSION TAKING

Preliminary impression-modeling compound, plaster of paris Outlining of the study cast

Secondary impression – to construct individual tray for working cast Working cast with outline

CAST CONSTRUCTION

Working cast

SHELLAC BASE PLATE

WAX PATTERN CONSTRUCTION

Use pink wax and pass over flame to soften

Place the softened wax over the cast and press it hard to get the exact shape of the cast

Then place another wax over the 1st wax Pour melted wax (using the carver) into

the periphery of wax pattern and allow it to flow to prevent Plaster of Paris from going inside wax pattern during investing

Wax pattern should be stable to the working cast

WAX PATTERN CONSTRUCTION

INVESTING THE WAX PATTERN1. Prepare the flasks for investing

the wax pattern by applying petroleum jelly on the inside part of the upper and lower halves, knock out plate and cover of the flask.

2. Soak the cast in water until it is entirely wet. Do not over soak, as this will cause the cast to etch. Apply separating medium to the base

3. Mix plaster of paris and then fill the sides of the lower half of the flasks, leaving the center portion with an amount of plaster mix just enough to accommodate the cast.

4. Press the cast at the center of the lower half of the flask. The bottom of the cast must touch the base of the flask. See to it that the land area is slightly above or at level with the rim of the flask

5. Using the plaster spatula, work around the cast so the plaster is smoothed even with the base of the cast. Contour the plaster so that no undercut around the cast is present.

6. Allow to partially set. Moisten a finger and complete the smoothening of the lower half. Let the plaster set completely.

7. Apply separating medium all around the plaster investment and allow it to dry.

8. Secure the upper half of the flask to the lower part. Be sure that there is metal-to-metal contact between them. Clean the rim with a sharp knife if the plaster investment interferes with this or re-contour the investment to accommodate the upper half.

9. Mix plaster of paris and pour it onto the upper half. Place the flask on a vibrator to ensure that plaster flows to the crevices, thus reducing the chances of air bubble formation. Add plaster until the flask is full, to complete the investment. Put the flask cover and firmly tap it to allow the excess plaster to escape. Allow the plaster to completely set

TWO METHODS OF POURING THE PLASTER OF PARIS TO THE UPPER HALF OF METAL FLASK

Single Pour Technique Mix the plaster then pouring it to the upper half

then place the cover Double Pour Technique/Two Capping

Technique Pouring is done twice, 1st pour of the investing

medium is only to the level of the wax pattern. This is good in the presence of artificial teeth

WAX ELIMINATION/BURNOUT PROCEDURE AND MOLD SPACE PREPARATION

1. Prepare two metal water containers. One of these should be big enough to be filled with water and hold two flasks.

2. Fill the containers with water and bring it to a running boil. One container will be used to heat-soak the flasks and the other to hold clean, hot water.

3. Soak the flasks in boiling water for about 4-5 minutes. The wax should have softened by this time.

4. Separate the upper and lower halves of the flasks by inserting a blunt knife at the slot at the back of the flask. Peel off the softened wax and discard.

5. Flush the mold space with hot water coming from the boil-out container. Use a dipper with a hole at the bottom. Do not use all the water in the dipper. Discard it because the top portion contains wax that can contaminate the old

6. Apply detergent and brush all the areas to remove any trace of wax.

PACKING THE RESIN(STAGES OCCUR ONCE THE RESINS ARE MIXED)

Damp/Sandy Stage

Sticky/Stringy Sage

Gel/Dough Stage

Rubbery Stage

Stiff Stage

DAMP/SANDY STAGE

Mixture is cloudy and sandy

No reaction yet between monomer and polymer

STICKY/STRINGY STAGE

Monomer attacks the surface of polymer

The mixture becomes viscous and sticks to the stirring rod

GEL/DOUGH STAGE

Mixture is easily molded into different forms and shapes

Ideal for compression molding, no longer sticky

RUBBERY STAGE STIFF STAGE

Mixture is rubber-like in character and no longer flows freely

Cannot be molded anymore

Mixture is dry and resistant to mechanical deformation

PACKING THE RESINPrepare the acrylic mixing jar and soak a piece of polyethylene sheet in water.

Measure enough amounts of polymer and monomer, following manufacture’s instruction.

EMPTY THE MEASURED MONOMER INTO THE MIXING JAR. ADD POLYMER IN A SPRINKLING FASHION. MAKE SURE THAT THE POWDER IS COMPLETELY WET WITH THE MONOMER. A STIRRING ROD MAY BE USED FOR THIS PURPOSE, TAKING CARE NOT TO STIR, AS THIS MAY INTRODUCE AIR INTO THE MIXTURE

COVER THE MIXING JAR TO PREVENT EVAPORATION OF THE MONOMER.

Constantly monitor the polymerization process. The resin is ready for packing during the “dough” stage.

During the “dough” stage, wet your hand with water and knead the acrylic material. Form it into a ball and then roll it. Pack it in the mold space using your fingers.

GET THE POLYETHYLENE SHEET AND LINE IT OVER THE ACRYLIC. THIS WILL ENABLE YOU TO DO A TRIAL CLOSURE. CLOSE THE FLASK WITH THE LOWER HALF AND APPLY A PALM PRESSURE.

Put the assembled flask into a press. This could be accomplished by either of the following: Bench press Pneumatic press

Pneumatic Press Bench Press

Release the pressure and gradually separate the lower and upper halves using a dull knife. Lift away the polyethylene sheet and see to it that the mold is totally filled with resin.

The flash on the investment is cut away using a sharp knife.

TRIAL-CLOSURE TECHNIQUE

Reassemble the flask. Clamp this using a spring clamp and bench cure for about 30 minutes to allow excess monomer to evaporate.

Repeat the procedure for other investment.

CURING Heating process of resin to

allow complete polymerization.

Rapid curing: 74°C/165°F water bath for 2 hours then increase to 100°C for 1 hour.

Slow curing: 65 -70°C water bath for 8-9 hours

Turn the curing unit on. Set the temperature at 65-70°C and the timer to about 8-9 hours.

Place the flask in the water bath and leave it to cure for the time duration.

BENCH COOLING

After curing cycle, retrieve the flasks from the water bath. Allow them to bench-cool before divesting the denture base. Rapid temperature change alters the dimensions of the base.

DIMENSIONAL CHANGES DURING CURING

Thermal Expansion – change from room temperature to temperature of water bath

Contraction – polymerization shrinkage of resin

Thermal Contraction – change from room temperature of water bath to room temperature

DEFLASKING Remove the lid of the flask

and put the flask into a flask ejector. Carefully separate the halves of the flask from the investment by inserting levers at the sides of the ejector and engaging the flask at the recess provided for this purpose.

An alternative way would be to use a mallet. Remove the flask cap and invert the flask. Tap the knock-out plate until the lower half of the flask separates. Re-invert the flask and tap the stone cap until the investment is completely free from flask.

Saw through the investment, making several linear cuts. The cut must be deep enough, without damaging the denture base, so that the parts will be done easily if the cast has been prepared well with the separating medium.

Slowly lift the denture base from the cast. Do not force it out if there is difficulty or it may break. Saw the cast and pry the pieces out until the base is free from the investment.

TRIMMING AND POLISHING Trim the “flash” (excess

resins at the sides of denture base) from the processed denture and clean off the investing plaster.

Polish it using a wheel rag and acryluster.

It is important to remove the residual monomer from the finished denture base because it can cause irritation to the oral tissues.

DEFECTS OF DENTURE BASE Presence of porosities

External Porosities – caused by improper manipulation of investing medium

Internal porosities – caused by too rapid curing process and improper manipulation of resin and packing too soon

Crazing – linear small cracks caused by too much pressure exerted during deflasking

Fracture – caused by too much pressure during packing and by accidental dropping of denture

base during deflasking.

INJECTION MOLDING TECHNIQUE Used a special

thermoplastic resin (Ivoclar)

This technique uses special equipment including a special bath for curing

A sprue hole and a vent hole are formed in the gypsum mold

The soft resin is contained in the injector and is forced into the mold space as needed

It is kept under pressure until it has hardened

Advantages: Dimensional accuracy (low

shrinkage) No increase in vertical

dimension Homogenous denture

base Low free monomer

content Good impact strength

Disadvantages: High cost of equipment Difficult mold design

problems Less craze resistance Special flask is required

CHEMICALLY ACTIVATED DENTURE BASE ACRYLIC RESINS

Also known as “self-curing” “cold cure” or “auto-polymerizing resin”

Supplied as powder (clear, pink, tooth colored) and liquid

Polymerize at room temperature

USES For making temporary

crowns and FPDs Construction of special

trays For denture repair,

relining and rebasing For making removable

othodontic appliances For adding a post-dam to

an adjusted upper denture For making temporary and

permanent denture bases For making inlay and post

core patterns

LIGHT ACTIVATED DENTURE BASE RESINS

Consists of a urethane dimethacrylate matrix with an acrylic coplymer, microfine silica fillers, and a camphoroquinone-amine photoinitiator system

Supplied in premixed sheets having a clay like consistency