Dentures: Materials, Techniques, and Milestones

History Of Dentures

• Replacements for decaying or lost teeth have been made for thousands of years.
• Skillfully designed dentures were made as early as 700 B.C.E using ivory and bone.
• During Medieval times, dentures were seldom considered.
• Gaps between teeth were expected, even nobles had them.
• Queen Elizabeth, I filled the holes in her mouth with a cloth to improve her appearance in public.

Read And Learn More: Basic Dental Materials Notes

• When dentures were installed, they were hand-carved and tied in place with silk threads.
• Retention of false teeth became more diffilt as the number of teeth diminished in the mouth and those that wore full sets of dentures had to remove them before eating.
• The upper and lower plates fit poorly and were held together by steel springs.
• Many including George Washington suffered from tooth loss and unfit dentures.
• The major reason that the level of technology didn’t increase is that suitable materials for false teeth were hard to find.

• In ancient times, the most common material for false teeth were animal bone or ivory, especially from elephants or hippopotamus.
• Human teeth were also used; pulled from the dead or sold by poor people from their own mouths.

• These teeth soon rotted or decayed. Rich people got dentures made of silver, gold, Mother of pearl, or agate.
• In 1774, Duchateau and Dubois de Chemant designed a full set of dentures that would not rot. They were the first porcelain dentures.

• Giuseppangelo Fonzi created a single porcelain tooth held in place by a steel pin in 1808.
• Claudius Ash made an improved porcelain tooth in 1837.
• Porcelain dentures moved to the United States in the 1800s.
• They were marketed on a large scale. Porcelain dentures were prone to chip and also tended to appear too white to be convincing.
• Porcelain was tolerated by denture-bearing mucosa but was diffilt to fabricate and easily broken.
• In the 1700s plaster of Paris was introduced as an impression material.
• It was used to make a mold of the patient’s mouth.
• This helped the shape of the dentures to be more precise.

• Swaged gold was used as a denture base for those who could afford it.
• Other metals like German silver were also used as denture bases.
• There was a real breakthrough when vulcanized rubber was discovered by Charles Goodyear in 1839.
• This is a cheap, easy-to-work-with material that could be shaped to fit the mouth and hold the denture.
• The casting machine was invented by Bean in 1870 and aluminum bases with teeth set in vulcanite were used.
• Vulcanite and other dental products were supplied in India under British rule primarily by the Bombay Burma Trading Company which was set up in 1863 by the Wallace Brothers.
• After independence, the company was taken over by the Wadia group, the dental branch of which continues today as the Dental Product of India
  (DPI).
• A little later plastics were introduced and celluloid and phenol formaldehyde resins were tried in place of rubber.
• The discovery of acrylic resins in the early 20th century and improved impression techniques have since revolutionized denture treatment.
• Prior to 1937, the materials used for denture bases were vulcanite, nitrocellulose, phenol formaldehyde, vinyl plastics, and porcelain.

Vulcanite

• Vulcanite was discovered by Charles Goodyear in 1839.
• It was the first material that could be molded rather than the traditional method of swaging or carving.
• It contains rubber with 32% sulfur and metallic oxides for color.

Vulcanite Advantages

• Nontoxic and nonirritating
• Excellent mechanical properties
• Easy to mold and fabricate

Vulcanite Disadvantages

• It absorbs saliva and becomes unhygienic due to bacterial proliferation. Unpleasant odor, when processed.
• Poor esthetics due to the opacity of rubber.
• The material is quite hard to polish.
• Dimensional changes occur due to
  • Thermal expansion during heating in the vulcanizer
  • Contraction of 2 to 4% by volume during the addition of the sulfur to the rubber.

Nitrocellulose

1. Celluloid was invented by Hyatt in 1868 and used as a denture base material from about 1890.
2. Its color was better than vulcanite. It used camphor as a plasticizer which gave it an unpleasant taste and odor.
3. Though short-lived, this material indicated a general evolution towards plastics as the denture base material of choice.
4. Some of their disadvantages are enumerated below.

• Dimensionally unstable and distorted in service
• High water absorption
• Poor color stability (turns from pink to green with time)
• Contains unpleasant tasting plasticizers (Camphor)
• Highly flammable.

Phenol-Formaldehyde

• Phenol-formaldehyde resin (also known as Bakelite) was discovered in 1909.
• By 1929 they were introduced to dentistry and soon came to be used for denture bases.
• They were technique sensitive and had wide variations in color, dimensional stability, and strength.
• They become discolored and unesthetic and being thermosetting it is diffilt to repair.

Polymers

• Denture bases and other resins used in dentistry are made up of polymers.
• Therefore an understanding of the nature and chemistry of polymers is essential.
• Although work on synthetic polymers were started in the 18th century the nature of these unique materials were not clearly understood.
• In 1922 Hermann Staudinger (Germany) was the first to propose that polymers consisted of long chains of atoms held together by covalent bonds.
• He also proposed to name these compounds macromolecules.
• Before that, scientists believed that polymers were clusters of small molecules (called colloids), without definite molecular weights, held together by an unknown force.

Nature Of Polymers

1. Polymer

• A polymer is a large and often complex macromolecule that is made from smaller molecules.
• A macromolecule is any chemical possessing a molecular weight greater than 5000.
• Some polymers have weights in excess of 50 million.
• The mer ending represents the simplest repeating chemical structural unit from which the polymer is composed, for example, poly (methyl methacrylate) is a polymer having chemical structural units derived from methyl methacrylate.

2. Monomer

• The molecules from which the polymer is constructed are called monomers (one part).
• Polymer molecules may be prepared from a mixture of different types of monomers and they are called copolymers.

3. Molecular Weight

• The molecular weight of the polymer molecule equals the molecular weight of the various mers multiplied by the number of mer units.
• They may range from thousands to millions of units depending on preparation conditions.
• The molecular weight of polymers plays an important role in determining its physical properties.
• The average molecular weight for various denture polymers ranges from 8000 to 39000.
• Cross-linked resin teeth may have weights in excess of 600,000.

4. Degree Of Polymerization

Defied as the total number of members in a polymer.

• The higher the molecular weight of the polymer made from a single monomer, the higher the degree of polymerization.
• The strength of the resin increases with an increase in the degree of polymerization until a certain molecular weight is reached. Above this there is no change.

5. Molecular Weight Distribution

• A narrow molecular weight distribution gives the most useful polymers.
• However, most polymers have a wide range of molecular weights and so vary widely in their properties.
• For example the higher the molecular weight, the higher the softening and melting points and the stiffer the plastic.

Structure Of Polymers (Spatial Structure)

The physical structure of the polymer molecule is also important in determining the properties of the polymer.

There are three basic structures.

Linear

• Here the ‘mer’ units are connected to each other in a linear sequence. They can be further divided into
• Linear homopolymer It has mer units of the same type.

…– M – M – M – M – M – M – M – …

• The random copolymer of linear type It has two types of mer units, randomly distributed along the chain.

…– M – M – M – Y– M – Y– M – M – Y– Y– M – M –…

• Block copolymer It has two types of members distributed in segments or blocks.

…– M – M – M –…– M – M – Y– Y–…– Y– Y– M – M – …

• Branched
• Linear molecules are rarely realized. In practice, the mer units are arranged in a branched fashion or cross-linked.
• Branched homopolymer The mer units are of the same type.

• The random copolymer of the branched type It has two types of mer distributed randomly.

• The graft copolymer of branched type It has one type of mer unit on the main chain and another mer for the branches.

Cross Linked Polymer

It is made up of a homopolymer cross-linked with a single cross-linking agent. It is a network structure.

Polymerization—Chemistry

• The term polymerization refers to a series of chain reactions by which a macromolecule or polymer is formed from a group of smaller single molecules known as a ‘monomer’.
• These structural units are connected to each other within the polymer molecules by bonds.
• Polymerization is a repetitive intermolecular reaction that is capable of proceeding indefinitely.

Polymerization Types

Most polymerization reactions fall into two basic types.

Condensation polymerization (step-growth)

Condensation resins are divided into two groups

1. Those in which polymerization is accompanied by repeated elimination of small molecules, i.e. the primary compounds react with the formation of byproducts such as water, halogen acids, and ammonia. The process can repeat itself and form macromolecules.
2. Those in which functional groups are repeated in the polymer chains. The members are joined by functional groups (like amide, urethane, ester or sulfide linkages). Formation of a byproduct is not necessary, for example, polyurethane.

• In the past, several condensation resins have been used to make denture bases.
• At present, condensation resins are not widely used in dentistry.

Addition polymerization

• Most resins employed extensively in dental procedures are produced by addition polymerization.
• Here, there is no change in chemical composition and no by-products are formed.
• In this type of polymer, the structure of the monomer is repeated many times in the polymer.
• Starting from an active center, one molecule at a time is added and a chain rapidly builds up, which can grow almost indefiitely as long as the supply of building blocks is available.

Chemical Stages Of Polymerization

This occurs in four stages

Polymerization Induction

• The induction or initiation period is the time during which the molecules of the initiator become energized or activated and start to transfer the energy to the monomer.
• Any impurity present increases the length of this period.
• The higher the temperature, the shorter is the length of the induction period.
• The initiation energy for activation of each monomer molecular unit is 16000-29000 calories per mol in the liquid phase.

There are three induction systems for dental resins

• Heat activation Most denture base resins are polymerized by this method, for example, the free radicals liberated by heating benzoyl peroxide will initiate the polymerization of methyl methacrylate monomer.
• Chemical activation This system consists of at least two reactants, when mixed they undergo a chemical reaction and liberate free radicals, for example, the use of benzoyl peroxide and an aromatic amine (dimethyl-p-toluidine) in self-cured dental resins.
• Light activation In this system, photons of light energy activate the initiator to generate free radicals, for example, camphor quinone and an amine will react to form free radicals, when they are irradiated with visible light.

Propagation

• Once the growth has started only 5000 to 8000 calories per mole are required, the process continues rapidly and is accompanied by the evolution of heat.
• Theoretically, the chain reactions should continue with the evolution of heat until all the monomer has been changed to the polymer.
• In reality, however, the polymerization is never complete.

Termination

The chain reactions can be terminated either by the direct coupling of two chain ends or by the exchange of a hydrogen atom from one growing chain to another.

Chain transfer

• The chain termination can also result from chain transfer.
• Here the active state is transferred from an activated radical to an inactive molecule and a new nucleus of growth is created.
• An already terminated chain can be reactivated by chain transfer resulting in continued growth.

Inhibition Of Polymerization

Such reactions are inhibited by

• Inhibitors These react with the activated initiator or any activated nucleus, or with an activated growing chain to prevent further growth, for example, hydroquinone (0.006%) is added to prevent polymerization of the monomer during storage.
• Oxygen Presence of oxygen (air) also inhibits polymerization.

Copolymerization

• The macromolecule may be formed by the polymerization of a single type of structural unit.
• However, in order to improve the physical properties, it is often advantageous to use two or more chemically different monomers as starting materials.
• The polymers thus formed may contain units of these monomers.
• Such a polymer is called a copolymer and its process of formation is known as copolymerization.

Types Of Copolymers

There are three different types of copolymers

Random type

• In the random types of copolymer, the different members are randomly distributed along the chain, such as

– M – M – M – M – Y – M – Y – M – Y – Y – M – M –

Block type

• In a block copolymer, identical monomer units occur in relatively long sequences along the main polymer chain.

… – M – M – M – M – … – M – M – Y – Y – Y – … – Y – Y – M – M – …

Graft type

• In graft copolymers, the sequence of one of the monomers are grafted onto the ‘backbone’ of the second monomer species.

Importance Of Copolymerization

Copolymerization is used to improve the physical properties of the resin. Many useful resins are manufactured by copolymerization.

• Small amounts of ethyl acrylate may be copolymerized with methyl methacrylate to alter the flexibility.
• Block and graft polymers show improved impact strength.
• In small amounts, they modify the adhesive properties of resins as well as their surface characteristics.

Cross-Linking

• The formation of chemical bonds or bridges between the polymer chains is referred to as cross-linking.
• It forms a three-dimensional network.

Cross-Linking Applications

1. The more recent acrylic resins are of cross-linked variety. Cross-linking increases rigidity and decreases solubility and water sorption.
2. Acrylic teeth are highly cross-linked to improve their resistance to solvents, crazing, and surface stresses.

Plasticizers

These are substances added to resins

1. To increase the solubility of the polymer in the monomer.
2. To decrease the brittleness of the polymer.

However, it also decreases strength, hardness, and softening points.

Plasticizers Types

They are of two types

1. External It penetrates between the macromolecules and neutralizes the secondary bonds or intermolecular forces. It is an insoluble high-boiling compound. It is not so widely used as it may evaporate or leach out during normal use of the resin.
2. Internal Here, the plasticizing agent is part of the polymer. It is done by copolymerization with a suitable comonomer.

Classification Of Denture Base Materials

3. ISO 20795-1:2013 Classifiation

Type 1 – Heat-polymerizable polymers

• • Class 1 – Powder and liquid
  • Class 2 – Plastic cake
• Type 2 – Autopolymerizable polymers
  • Class 1 – Powder and liquid
  • Class 2 – Powder and liquid for pour-type resins
• Type 3 – Thermoplastic blank or powder
• Type 4 – Light-activated materials
• Type 5 – Microwave cured materials

Synthetic Resins

• Modern living has been greatlyinflenced by synthetic plastics.
• Originally they were laboratory nuisances – waxy, sticky residues left after certain organic reactions.
• These resinous materials composed of giant molecules attracted the attention of chemists giving rise to the field of plastics.
• Synthetic plastics are nonmetallic compounds that are molded into various forms and then hardened for commercial use (for example clothing, electronic equipment, building materials, and household appliances).
• These materials are composed of polymers or complex molecules of high molecular weight.
• A variety of resins are used in dentistry. These include acrylics, polycarbonates, vinyl resins, polyurethanes, styrene, cyanoacrylates, epoxy resins, etc.

Classification Of Resins

Due to their heterogeneous structure and complex nature, they are diffilt to classify. Based on their thermal behavior, they are classified as

1. Thermoplastic
2. Thermosetting

1. Thermoplastic

• These are resins that can be repeatedly softened and molded under heat and pressure without any chemical change occurring.
• They are fusible and are usually soluble in organic solvents.
• Most resins used in dentistry belong to this group, for example, polymethyl methacrylate, polyvinyl acrylics, and polystyrene.

2. Thermosetting

• This category refers to resins which can be molded only once. They set when heated.
• These cannot be softened by reheating like the thermoplastic resins. They are generally infusible and insoluble.

Ideal Requirements Of Dental Resins

Dental resins, both restorative and denture base should

1. Be tasteless, odorless, nontoxic, and nonirritant to the oral tissues.
2. Be esthetically satisfactory, i.e. should be transparent or translucent and easily tinted. The color should be permanent.
3. Be dimensionally stable. It should not expand, contract or warp during processing and subsequent use by the patient.
4. Have enough strength, resilience, and abrasion resistance.
5. Be insoluble and impermeable to oral fluids.
6. Have a low specific gravity (light in weight).
7. Tolerate temperatures well above the temperature of any hot foods or liquids taken in the mouth without undue softening or distortion.
8. Be easy to fabricate and repair.
9. Have good thermal conductivity.
10. Be radiopaque (so that dentures/ fragments can be detected by X-rays if accidentally aspirated or swallowed and also to examine the extensions of the resin restoration in a tooth).
11. When used as a filling material it should bond chemically with the tooth.
12. Have a coefficient of thermal expansion which match that of tooth structure.
13. Be economical.

Uses Of Resins In Dentistry

1. Fabrication of dentures (denture base resins).
2. Artificial teeth (cross-linked acrylic resins).
3. Tooth restoration, for example, filings, inlays, and laminates (composite resins).
4. Cementation of orthodontic brackets, crowns, and fixed dental prostheses (FDPs) (resin cement).
5. Orthodontic and pedodontics appliances.
6. Crown and FDP facings (tooth-colored acrylic or composite resins).
7. Maxillofacial prostheses (for example. obturators for cleft palates).
8. Inlay and post-core patterns (pattern resins).
9. Dies (epoxy resins).
10. Provisional restorations in fixed prosthodontics (tooth-colored resins, provisional composites) and polycarbonate crowns.
11. Endodontic obturating materials sealers.
12. Core filling material.
13. Athletic mouth protectors.
14. Custom impression trays.
15. Splints and stents.
16. Models.

Acrylic Resins

The acrylic resins are derivatives of ethylene and contain a vinyl group in their structural formula. The acrylic resins used in dentistry are the esters of

1. Acrylic acid, CH₂ = CHCOOH
2. Methacrylic acid, CH₂ = C(CH₃ )COOH

The acrylic resins were so well received by the dental profession thatby1946, it largely replaced the earlier denture base materials like vulcanite.

Poly  Resins

• These are widely used in dentistry to fabricate various appliances.
• One of the reasons for its wide popularity is the ease with which it can be processed.
• Although it is a thermoplastic resin, in dentistry it is not usually molded by thermoplastic means.
• Rather, the liquid (monomer) methylmethacrylate is mixed with the polymer(powder).
• The monomer plasticizes the polymer to a dough-like consistency which is easily molded.

Poly Resins 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.

Heat Activated Denture Base Acrylic Resins are Available as

1. Powder and liquid.
2. Gels—sheets, and cakes.

• The powder may be transparent or tooth-colored or pink-colored (to simulate the gums, some even contain red fiers to duplicate blood vessels).
• The liquid (monomer) is supplied in tightly sealed amber-colored bottles (to prevent premature polymerization by light or ultraviolet radiation on storage).
• Commercial names Stellon (DPI), Lucitone(Bayer), and Trevelon (Dentsply).

Heat Activated Denture Base Acrylic Resins  Composition

Powder:

Liquid:

 

Polymerization Reaction

Polymerization is achieved by the application of heat and pressure. The simplified reaction is

Technical Considerations

Dentures are usually fabricated by one of the following techniques

1. Compression molding technique (usually heat-activated resins).
2. Injection molding technique (heat-activated resins).
3. Fluid resin technique (chemically activated resins).
4. Visible light curing technique (VLC resins).

Compression Molding Technique

This is the commonly used technique in the fabrication of acrylic resin dentures.

The resin used is Usually heat-activated acrylic resin.

Steps

1. Preparation of the waxed denture pattern.
2. Preparation of the split mold.
3. Application of separating medium
4. Mixing of powder and liquid.
5. Packing.
6. Curing.
7. Cooling.
8. Divesting, finishing, and polishing.

Preparation of a Waxed Denture Pattern

• Many structures in dentistry are constructed using a wax pattern.
• The structure to be created (in this case a denture) is first constructed in wax.
• The wax portions will be replaced later with acrylic.

Preparation of the split mold

• The waxed denture is invested in a dental flask with dental stone or plaster (also called flicking) using a 3-pour technique.
• After the stone or plaster sets, it is dewaxed by placing the flask in boiling water for not more than 5 minutes.
• After dewaxing the two halves of the flask are separated and the molten wax is flushed out with clean hot water.
• Removal of the wax leaves us with an empty space or mold into which soft acrylic is packed.
• Acrylic replaces the wax, assuming the shape of the final denture.

Application of separating medium

The resin must not contact the gypsum surface while curing.

1. To prevent water from the mold from entering into the acrylic resin. This may affect the rate of polymerization and color. It can also result in crazing.
2. To prevent monomer penetrating into the mold material, causing plaster to adhere to the acrylic resin and producing a rough surface.
3. Helps in easier retrieval of the denture from the mold.

Types of separating media

Various separating media used are

• Tinfoil
• Cellulose lacquers
• Solution of alginate compounds
• Calcium oleate
• Soft soaps
• Sodium silicate
• Starches

Tin foil

• Tin foil was the material used earlier and was very effctive.
• However, its manipulation is time-consuming and diffilt.
• It has been largely replaced by other separating media known as ‘tin foil substitutes’.

Sodium alginate solution

• This is the most popular separating medium. It s water-soluble.
• It reacts with the calcium of the plaster or stone to form a film of insoluble calcium alginate.

Sodium alginate solution Composition

2% sodium alginate in water, glycerine, alcohol, sodium phosphate, and preservatives.

Precautions to be taken

• Waxes or oils remaining on the mold surface will interfere with the action of the separating medium.
• Mold should be warm, not hot. The continuity of the film will break the mold is steaming hot.
• Avoid coating the teeth as it will prevent the bonding of teeth with the denture base.

Application

• Separating media is applied using a brush, coating only the plaster surfaces, and not the acrylic teeth.
• One or two coats may be applied.

Mixing of powder and liquid

• Polymer—monomer proportion = 3:1 by volume or 2:1 by weight.
• The measured liquid is poured into a clean, dry mixing jar.
• The powder is slowly added allowing each powder particle to become wetted by monomer.
• The mixture is then stirred and allowed to stand in a closed container.

If too much monomer is used(Lower polymer/monomer ratio)

• There will be greater curing or polymerization shrinkage.
• More time is needed to reach the packing consistency.
• Porosity can occur in the denture.

If too little monomer is used(Higher polymer/monomer ratio)

• Not all the polymer beads will be wetted by monomer and the cured acrylic will be granular.
• Dough will be diffilt to manage and it may not fuse into a continuous unit of plastic during processing.

Physical stages

• After mixing the material goes through various physical stages.
• No polymerization reaction takes place during these stages.
• A plastic dough is formed by a partial solution of the polymer in the monomer.

Stage 1: Wet sand stage

The polymer gradually settles into the monomer forming a fluid, incoherent mass.

Stage 2: Sticky stage

• The monomer attacks the polymer by penetrating into the polymer.
• The mass is sticky and stringy (cobweb-like) when touched or pulled apart.

Stage 3: Dough or gel stage

• As the monomer diffuses into the polymer, it becomes smooth and dough-like.
• It does not adhere to the walls of the jar.
• It consists of undissolved polymer particles suspended in a plastic matrix of monomer and dissolved polymer.
• The mass is plastic and homogenous and can be packed into the mold at this stage.

Stage 4: Rubbery stage

• The monomer disappears by further penetration into the polymer and/or evaporation.
• The mass is rubberlike, non-plastic, and cannot be molded.

Stage 5: Stif stage

The mass is totally unworkable and is discarded.

Working time

The working time is the time elapsing between stage 2 and the beginning of stage 4, i.e. the time the material remains in the dough stage (according to ADA Sp. No. 12, the dough should be moldable for at least 5 minutes).

The working time is affected by temperature.

• In warm weather when the working time is insufficient, the mixing jars are chilled to prolong the working time.
• Care is taken to avoid moisture.

Packing

The powder-liquid mixture should be packed into the flask at the dough consistency for several reasons.

• If it is packed at the sandy or stringy stages, too much monomer will be present between the polymer particles, and
• The material will be of too low viscosity to pack well and will flow out of the flask too easily.
• Packing too early may also result in porosity in the final denture base.
• If packed at the rubbery to the stiff stage, the material will be too viscous to flow, and metal-to-metal contact of the flask halves will not be obtained.
• Delayed packing may result in movement or fracture of the teeth, loss of detail, and an increase in the vertical height of the denture.

Trial closure

• The acrylic dough is packed into the flask in slight excess.
• The excess is removed during trial packing with damp cellophane or polyethylene film used as a separator for the upper half of the flask.
• A hydraulic or mechanical press may be used to apply pressure.
• The closing force is applied slowly during the trial packing to allow the excess dough, known as ‘flesh’ to flow out between the halves of the flask.
• The flask is opened and the flash is trimmed away. Before final closure, the separating film is removed and discarded.
• The final closure of the flask or metal-to-metal contact of the flask halves is then completed in the press.
• The flasks are then transferred to a holding clamp which maintains the pressure throughout the curing process.

Curing (polymerization)

After final closure, the flasks, are kept at room temperature for 30 to 60 minutes (sometimes called ‘bench curing’).

Purpose of bench curing

• Permits an equalization of pressures throughout the mold.
• It allows time for more uniform dispersion of monomer throughout the mass of dough.
• It provides a longer exposure of resin teeth to the monomer in the dough, producing a better bond of the teeth with the base material.

Curing cycle

• The curing or polymerization cycle is the technical name for the heating process used to initiate, control and complete the polymerization of the resin in the mold.
• The curing cycle selected depends on the thickness of the resin.
• Long curing cycles are recommended for thicker resins to avoid internal porosity.
• The processing is done in a time-temperature-controlled water bath.

Recommended curing cycles

• Long cycle
  • 74 °C for 8 hours
  • 74 °C for 8 hours, then boil for 1 hour
• Short cycle – 74 °C for 2 hours, then boil for 1 hour of the thinner portions (short cycle).

Cooling

• The flask should be cooled slowly, i.e. bench cooled. Fast cooling can result in a warpage of the denture due to differential thermal contraction of the resin and gypsum mold.
• Cooling overnight is ideal. However, bench cooling it for 30 minutes and then placing it in cold tap water for 15 minutes is satisfactory.

Deflsking

• The cured acrylic denture is retrieved from the flask. This is called deflating.
• The flask is opened and the mold is retrieved.
• The mold separates quickly because the surrounding plaster was poured in layers (3-pour technique).
• Plaster cutting forceps may be used to break up the plaster.
• Deflsking has to be done with great care to avoid flexing and breaking of the acrylic denture.

Finishing and polishing

• The denture is smoothened using progressive grades of sandpaper.
• Finely ground pumice in water is commonly used for final polishing (refer to chapter on abrasives).

Injection Molding Technique

The resin used A special thermoplastic resin (Ivocap – Ivoclar Vivadent) is used.

Sources of heat

Apart from the heated water bath, there are other methods of supplying heat.

• Steam
• Dry air oven
• Dry heat (electrical)
• Infrared heating
• Induction or dielectric heating
• Microwave radiation

Microwave Energy Polymerization Advantages

• It is cleaner and faster than the conventional hot water.
• The fit of the denture is comparable or superior.

Equipment

• This technique uses special equipment including a special bath for curing.
• A sprue hole and a vent hole are formed in the gypsum mold with the help of sprue formers).
• 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.
• Continuous feeding of the material under pressure compensates for shrinkage.
• There is no difference in accuracy or physical properties as compared to the compression molding technique.

Injection Molding Technique Advantages

• Dimensional accuracy (low shrinkage).
• No increase in the vertical dimension.
• Homogeneous denture base.
• Low free monomer content.
• Good impact strength.

Comparison of self cured and heat cured acrylic resin:

Injection Molding Technique Disadvantages

• Higher cost of equipment.
• Mold design problems.
• Less craze resistance.
• A special flask is required.

Chemically Activated Denture Base Acrylic Resins

• The chemically activated acrylic resins polymerize at room temperature.
• They are also known as ‘self-curing’, ‘cold-cure’ or ‘auto-polymerizing resins.
• In cold-cured acrylic resins, the chemical initiator benzoyl peroxide is activated by another chemical, instead of heat as in heat-cure resins.
• Thus, unlike heat-activated resins, polymerization is achieved at room temperature.

Acrylic Resins Available As

• Like heat-activated resins, chemically-activated resins are supplied as powder and liquid.
• The powder may be clear, pink, veined or tooth-colored.

Acrylic Resins Uses

• For making temporary crowns and FDPs.
• Construction of special trays (this type contains more filers).
• For denture repair, relining, and rebasing.
• For making removable orthodontic 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 (specialized material is available).

Acrylic Resins Composition Liquid

Acrylic Resins Composition Powder:

 

Polymerization Reaction

The simplified reaction is outlined

Advantages And Disadvantages

1. Better initial fi, which is because the curing is carried out at room temperature. Thus there is less thermal contraction.
2. Color stability is inferior to that of heat cure resin, due to subsequent oxidation of the tertiary amine.
3. Slightly inferior properties because the degree of polymerization of self-curing acrylics is less than that of heat-cured ones.
4. For repairing dentures, self-curing resins are preferable to heat-cured resins as heat curing causes warpage.

Manipulation Of Autopolymerizing Resins

1. Sprinkle on technique
2. Adapting technique
3. Fluid resin technique (special material is available for this)
4. Compression molding technique.

Sprinkle on technique

Synonym Also known as salt and pepper technique.

• Separating media is applied first on the cast.
• Powder and liquid is applied alternatively from droppers.
• The powder is sprinkled on the cast and then wet with monomer.
• The appliance or prosthesis is constructed section by section until completion.
• To improve the strength, the appliance is further cured in hot water under pressure for around 20 minutes using a pressure pot.

Adapting technique

• Powder and monomer liquid is proportioned and mixed in a glass or porcelain jar.
• When it reaches the dough stage, it is quickly removed and adapted onto the cast and manually molded quickly to the desired shape.
• An alternative technique uses a template.
• The resin is pre-shaped using a roller and template before adapting it to cast.
• Curing is completed in a pressure pot.

Fluid Resin Technique

• A special resin (Castdon by Dreve) is available for this technique.
• The chemical composition of the pour-type of denture resin is similar to the poly (methyl methacrylate) materials.
• The principal difference is that the pour-type of denture resins have high molecular weight powder particles that are much smaller and when they are mixed with monomer, the resulting mix is very fluid.
• Therefore, they are referred as fluid resins’.
• They are used with significantly lower powder-liquid ratio, i.e. it ranges from 2:1 to 2.5:1.
• This makes it easier to mix and pour.

Method of flaking and curing

• Agar hydrocolloid or silicone is used for mold preparation in place of the usual gypsum.
• A special flask and resin is used for the fluid resin technique.
• The technique involves preparing the mold with silicone, creating of channels for pouring and venting, and pouring the fluid resin through one channel in a thin stream till excess is seen through the vent.
• Polymerization is done under pressure at 0.14 MPa (20 psi) at a temperature of 45 °C for 25 minutes.

Fluid Resin Technique Composition

Advantages of fluid resin technique

1. Better tissue fi.
2. Fewer open bites.
3. Less fracture of porcelain teeth during deflating.
4. Reduced material cost.
5. Simple laboratory procedure for flaking (no trial closure), deflating, and finishing of the dentures.

Disadvantages of fluid resin technique

1. Air inclusion (bubbles).
2. Shifting of teeth during processing.
3. Infraocclusion (closed bites).
4. Occlusal imbalance due to shifting of teeth.
5. The incomplete flow of denture base material over the neck of anterior teeth.
6. Formation offers of denture material over cervical portions of plastic teeth that had not been previously covered with wax.
7. Poor bonding to plastic teeth.
8. Technique sensitivity.

In general, these types of resins have some what lower mechanical properties than conventional heat-cured resins.

Clinically acceptable dentures can be obtained when using any of the techniques, provided proper precautions are exercised.

Light Activated Denture Base Resins

• It consists of a urethane dimethacrylate matrix with an acrylic copolymer, microfine silica filers, and a camphor quinone-amine photoinitiator system.
• Commercial name VLC triad

Light Activated Denture Base Resins  Mode of Supply

• Triad products are available for various uses including denture bases, custom trays, orthodontic appliances, and temporary crowns and bridges.
• It is supplied in premixed sheets of having clay-like consistency.
• It is supplied in various colors including pink transparent depending on its intended use.
• It is provided in opaque light-tight packages to avoid premature polymerization.

Light Activated Denture Base Resins Manipulation

• The denture base material is adapted to the cast while it is in a plastic state.
• The denture base can be polymerized without teeth and used as a baseplate.
• The teeth are added to the base with additional material and the anatomy is sculptured while the material is still soft.
• It is polymerized in a light chamber (curing unit) with the blue light of 400-500 nm from high-intensity quartz-halogen bulbs.
• The denture is rotated continuously in the chamber to provide uniform exposure to the light source.

Microwave Cured Denture Resins

• Microwave-cured resins are available (Nature-Cryl by GC).
• The material should comply with the requirements of ISO 20795-1:2013 type 5 denture base.
• The denture is invested and cured in a unique plastic flask.
• Dewaxing is done in a microwave for 1.5 minutes.
• Following Curing time is 3 to 4 minutes (depending on the product) in a standard household microwave oven.

 

Specialized Poly (Methyl Methacrylate) Materials

• Several modified polys (methyl methacrylate) materials have been used as denture resins.
• These include hydrophilic polyacrylates, high-impact strength resins rapid heat polymerized acrylic, light-activated denture base material (described earlier), and pour-type acrylic resins (described earlier).

High-Impact Strength Materials

• These materials are butadiene-styrene rubber-reinforced poly (methyl methacrylate).
• The rubber particles are grafted to methylmethacrylate so that they will bond well to the heat-polymerized 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.
• These materials have twice the impact strength of conventional acrylic resins.
• They are indicated for patients who risk dropping their dentures repeatedly, for example, senility, and parkinsonism.

Comparison of resin and porcelain denture teeth:

 

Rapid Heat-Polymerized Resins

• These are hybrid acrylics (example QC 20) that are polymerized in boiling water immediately after being packed into a denture flask.
• After being placed into the boiling water, the water is brought back to a full boil for 20 minutes (reverse cure).
• After the usual bench cooling to room temperature, the denture is deflated, trimmed, and polished in the usual manner.
• The initiator is formulated to allow for rapid polymerization without the porosity that one might expect.

Resin Teeth

• The composition of resin teeth is essentially poly (methyl methacrylate) copolymerized with a cross-linking agent.
• A greater amount of the cross-linking agent is used in resin teeth in order to reduce the tendency of the teeth to craze upon contact with the monomer-polymer dough during construction.
• The gingival ridge-lap area may not be as highly cross-linked as the incisal in order to facilitate chemical bonding to the denture base.
• Various pigments are utilized to produce a natural esthetic appearance.
• The bond between the resin teeth and denture base resin is chemical in nature, unlike porcelain teeth which require mechanical locking.
• A comparison of resin and porcelain teeth is presented in Table.
• Failure may occur if the ridge lap area is contaminated with residual wax or separating media.
• The mold should be finished well with a detergent solution in order to remove the wax completely.
• Use of a flme for smoothing of the wax during teeth setting should be done carefully since the teeth surfaces may melt or burn.
• The resultant stresses induced during cooling may contribute to crazing in service.

Special Tray Acrylic Resins

• These are specialized chemically activated resins and are available in powder and liquid form.
• Some are available as light-activated materials.
• They contain a high amount of inorganic filers (for example French chalk) which increases the plasticity and workability of the material.
• They come in colors like green and blue. They are used to fabricate custom trays for making facial impressions using zinc oxide eugenol, elastomeric materials, and alginate, etc.
• The custom-made acrylic resin trays may not be dimensionally stable until 20 to 24 hours after fabrication.
• Therefore it is advisable to use the tray after this period.

Pattern Resins

Commercial Name Duralay.

• It is a specialized resin intended for making patterns of inlays, posts, and other structures in the mouth.
• Like wax, these materials burn of completely before casting.
• But unlike wax, they are dimensionally very stable after setting and are not affected by small temperature variations.
• The inlay cavity is lubricated. The powder and liquid are mixed and inserted into the cavity.
• It is shaped quickly into the desired form. Further shaping is done after it sets using a bur.
• The pattern is removed, invested, and reproduced in metal.

Properties Of Methylmethacrylate Denture Resins

• Dentures are subjected to large stresses in the mouth.
• Acceptable denture resins must meet or exceed the standards specified in ISO 20795-1:2013.

Methyl Methacrylate Monomer

• It is a clear, transparent, volatile liquid at room temperature.
• It has a characteristic sweetish odor.
• The physical properties of monomers are
  • Melting point                                            –   – 48 °C
  • Boiling point                                             –  100 °C
  • Density                                                     –  0.945 gm/ml at 20 °C
  • The heat of polymerization                       –  12.9 Kcal/mol
  • Volume shrinkage during polymerization  –  21%

Poly

Taste and odor

• Completely polymerized acrylic resin is tasteless and odorless.
• On the other hand, poorly made dentures with a high amount of porosity can absorb food and bacteria, resulting in an unpleasant taste and odor.

Esthetics

• It is a clear transparent resin which can be pigmented (colored) easily to duplicate oral tissue.
• It is compatible with dyed synthetic fiers. Thus esthetics is acceptable.

Density

The polymer has a density of 1.19 gm/cm3.

Strength

• These materials are typically low in strength.
• However, they have adequate compressive and tensile strength for complete or partial denture applications.
  • Compressive strength  –  75 MPa
  • Tensile strength            –  48–62 MPa

Self-cured resins generally have lower strength values.

The strength is affected by

1. Composition of the resin
2. Technique of processing
3. Degree of polymerization
4. Water sorption
5. The subsequent environment of the denture.

Impact strength

• It is a measure of energy absorbed by a material when it is broken by a sudden blow.
• Ideally, denture resins should have high impact strength to prevent breakage when accidentally dropped.
• Unmodified acrylic resins are generally brittle. Plasticizers increase the impact strength.
• However, a significant improvement in impact strength is observed when the resin is modified with rubber.
• The Izod test values* are shown below.
  • Chemically activated resin                    – 13 (J/m)
  • Conventional heat-cured acrylic resin   – 15 (J/m)
  • Rubber-modified acrylic resin                – 31 (J/m)
  • Polyvinyl resins                                      – 30 (J/m)

Fatigue strength

• Fatigue strength refers to the ability of the denture to withstand a large number of small cyclic loading such as during mastication over a period of time.
• Most current dental plastics have sufficient fatigue strength.

Hardness and wear resistance

• Acrylic resins are materials having low hardness.
• They can be easily scratched and abraded.
• Polyvinyl acrylics have the best wear resistance and pour-type acrylics has the least.
  • Heat-cured acrylic resin           – 18 KHN
  • Self-cured acrylic resin            – 16 KHN
  • Rubber-modified acrylic resin  – 14 KHN
  • Light-cured resin                      – 18 KHN

Modulus of elasticity

• They have sufficient stiffness for use in complete and partial dentures.
• Self-cured acrylic resins have slightly lower values.

Creep

• Denture resins exhibit creep. When a load is applied an initial deflection is observed.
• If the load is sustained additional deformation is observed over time.
• The additional deformation is called creep. Chemically activated resins have higher creep rates.

Dimensional stability

• A well-processed acrylic resin denture has good dimensional stability.
• The processing shrinkage is balanced by the expansion due to water sorption.

Polymerization shrinkage

Acrylic resins shrink during processing due to

1. Thermal shrinkage on cooling
2. Polymerization shrinkage

• During polymerization, the density of the monomer changes from 0.94 gm/cc to 1.19 gm/cc.
• This results in shrinkage in the volume of monomer-polymer dough.
• However, in spite of the high shrinkage, the fi of the denture is not affected because the shrinkage is uniformly distributed over all surfaces of the denture.
• Thus, the actual linear shrinkage observed is low.
  • Volume shrinkage  –  8%
  • Linear shrinkage    –  0.53%
  • The self-cured type has a lower shrinkage (linear shrinkage— 0.26%).

Water sorption

• Acrylic resins absorb water (0.7 mg/cm²) and expand.
• This partially compensates for its processing shrinkage.
• This process is reversible. Thus on drying they lose water and shrink.
• However, repeated wetting and drying should be avoided as it may cause warpage of the denture.

Solubility

• Acrylic is virtually insoluble in water and oral fluids.
• They are soluble in ketones, esters, and aromatic and chlorinated hydrocarbons, for example, chloroform and acetone.
• Alcohol causes crazing in some resins. ISO stipulates solubility should not exceed 1.6 μg/mm³ for type 1 (heat-cured) and 8.0 μg/mm³ for type 2 (self-cured).

Thermal properties

• Stability to heat Poly (methyl methacrylate) is chemically stable to heat up to a point.
• It softens at 125 °C. However, above this temperature, i.e. between 125 °C and 200 °C it begins to depolymerize.
• At 450 °C, 90% of the polymer will depolymerize to monomer.
• Thermal conductivity They are poor conductors of heat and electricity.
• This is undesirable because patients wearing acrylic complete dentures often complain that they cannot feel the temperature of food or liquids they ingest, thus reducing the pleasure.
• Replacing the palatal portion with metal is one solution because the metal is a better conductor of heat.
• The inclusion of sapphire whiskers improves conductivity.
• Thermal conductivity for acrylic resin is 5.7 x 10^-4cal/sec/cm²) ( °C/cm).
• Coefficient of thermal expansion These materials have a high coefficient of thermal expansion (CTE).
• The CTE for poly(methylmethacrylate) resin is 81 × 10^-6/ °C. The addition of filers reduces CTE.
• Heat distortion temperatureThis is the measure of the ability of a plastic to resist dimensional change when loaded under heat.
• It is measured by observing the temperature at which a specimen under a 1.8 MPa load deflects 0.25 mm.
  • Heat distortion temperature for PMMA – 71 to 91 °C.
  • Heat distortion temperature for vinyl resin – 54 to 77 °C.
• Distortion is of concern during procedures like the repair or polishing of dentures.
• Temperatures should be kept low to avoid distortion.

Color stability

• Heat-cured acrylic resins have good color stability.
• The color stability of self-cure resins is slightly lower (yellows very slightly).

Biocompatibility

• Completely polymerized acrylic resins are biocompatible.
• True allergic reactions to acrylic resins are rarely seen in the oral cavity.
• The residual monomer (approximately 0.4% in a well-processed denture) is the usual component singled out as an irritant.
• A true allergy to acrylic resin can be recognized by a patch test.
• Direct contact of the monomer over a period of time may provoke dermatitis.
• The high concentration of monomer in the dough may produce local irritation and serious sensitization of the fingers.
• Inhalation of monomer vapor is avoided.

Precautions to be taken are

1. Minimize residual monomer content by using proper processing techniques.
2. Avoid direct handling of acrylic dough with bare hands.
3. Work in well-ventilated areas to avoid inhalation of the monomer vapor.

Residual monomer

• During the polymerization process the amount of residual monomer decreases rapidly initially and then later more slowly
• The highest residual monomer level is observed with chemically activated denture base resins at 1 – 4% shortly after processing.
• When they are processed for less than one hour in boiling water the residual monomer is 1 -3%.
• If it is processed for 7 hours at 70 °C and then boiled for 3 hours the residual monomer content may be less than 0.4%.
• In heat-cured acrylic before the start of curing the residual monomer is 26.2%.
• In 1 hour at 70 °C it decreases to 6.6% and at 100 °C it is 0.29%.
• To reduce the residual monomer in heat-cured dentures it should be processed for a longer time in boiling water.
• The temperature should be raised to boiling only after most of the polymerization is completed otherwise porosity may result.

Adhesion

• The adhesion of acrylic to metal and porcelain is poor and mechanical retention is required.
• Adhesion to plastic denture teeth is good (chemical adhesion).
• Adhesion to metal or ceramic can be improved by treating with silane coupling agents.

Radiopacity

• There have been instances of broken pieces of dentures being aspirated or swallowed.
• Radiopacity is a desirable property to enable easy location of the fragments.
• Most denture base materials are radiolucent. However, a few radiopaque materials are being manufactured.
• Radiopacity is obtained by the inclusion of heavy metal salts like bismuth or uranyl at concentrations of 10 to 15%.

Shelf life

• Acrylic resins dispensed as powder/liquid have the best shelf life.
• The gel type has a lower shelf life and has to be stored in a refrigerator.

Processing Errors

Porosity

Porosity presents many problems

1. It makes the appearance of a denture base unsightly.
2. Proper cleaning of the denture is not possible, so denture hygiene and thus, the oral hygiene suffers.
3. It weakens the denture base and the pores are areas of stress concentration, thus the denture warps as the stresses relax.

Porosity may be

1. Internal porosity
2. External porosity.

1. Internal porosity

• Internal porosity appears as voids or bubbles within the mass of the polymerized acrylic.
• It is usually not present on the surface of the denture.
• It is confined to the thick portions of the denture base and it may not occur uniformly.
• Cause Internal porosity is due to the vaporization of monomer when the temperature of the resin increases above the boiling point of the monomer (100.8 °C) or very low molecular weight polymers.
• Exothermic heat of the surface resin dissipates easily into the investing plaster.
• However, in the center of the thick portion, the heat cannot be conducted away.
• Therefore, the temperature in the thick portionsmayrise above the boiling point to fmonomercausing porosity.
• Avoided by Dentures with excessive thickness should be cured using a long, low-temperature curing cycle.

2. External porosity

It can occur due to two reasons

• Lack of homogeneity If the dough is not homogenous at the time of polymerization, the portions containing more monomers will shrink more.
• This localized shrinkage results in voids. The resin appears white.
  • Avoided by Using proper powder/liquid ratio and mixing it well.
  • The mix is more homogenous in the dough stage, so packing should be done in the dough stage.

Lack of adequate pressure Lack of pressure during polymerization or inadequate amount of dough in the mold during final closure causes bubbles which are not spherical. The resin is lighter in color

• • Avoided by Using the required amount of dough. Check for excess or flash during trail closure. Flash indicates adequate material.

Crazing

• Crazing is the formation of surface cracks on the denture base resin. The cracks may be microscopic or macroscopic in size.
• In some cases, it has a hazy or foggy appearance rather than cracks. Crazing weakens the resin and reduces its esthetic qualities. The cracks formed can cause fractures.

Crazing Causes

Crazing is due to

1. Mechanical stresses
2. Attack by a solvent
3. Incorporation of water

• In poly (methyl methacrylate) crazing occurs when tensile stresses are present.
• The cracks are at right angles to the direction of tensile stress.
• Crazing is a mechanical separation of the polymer chains or groups under tensile stress.
• Crazing is visible around the porcelain teeth in the denture and is due to the contraction of the resin around the porcelain teeth during cooling after processing.
• Weak solvents like alcohol result in randomly placed cracks.
• Water incorporation during processing will form stresses due to the evaporation of water after processing, causing crazing.

Avoided by

1. Using cross-linked acrylics
2. Tin foil separating medium
3. Metal molds

Denture Warpage

• Denture warpage is the deformity or change of shape of the denture which can affect the fit of the denture.
• Warpage can occur during processing as well as at other times.
• It is caused by a release of stresses incorporated during processing.
• Some of the stresses are incorporated as a result of the curing shrinkage while other stresses may be a result of the uneven or rapid cooling.
• Packing of the resin during the rubbery stage can also induce stress.
• Some stresses may be incorporated during improper deflating.
• These stresses are released subsequently
  • During polishing, a rise in temperature can cause warpage
  • Immersion of the denture in hot water can cause warpage
  • Re-curing of the denture after the addition of relining material, etc.

Repair Of Acrylic Resin Dentures

An acrylic denture fractured in service can be repaired. Repair resins may be

1. Heat-cured, or
2. Self-cured.

1. Heat-cured resin

• These resins are cured at 74 °C for 8 hours or more.
• The use of a heat-cured resin will tend to warp the denture during processing.

2. Self-cured resin

Although the repair with a self-cured resin invariably has a lower transverse strength than that of the original heat-cured denture base resin, self-cured resins are usually preferred because warpage is insignificant as curing is done at room temperature.

Infection Control For Dentures

• Care should be taken to prevent cross-contamination between patients and dental personnel.
• New appliances should be disinfected after construction.
• Items such as rag wheels often can be steam-autoclaved.
• Appliances can be sprayed with disinfectants before they leave the operatory.
• Since polymeric materials can absorb liquids, toxic agents such as phenol or glutaraldehyde are avoided.
• Ethyleneoxidegasisa suitable method.

Care Of Acrylic Dentures

Denture treatment is time-consuming and labor-intensive, besides being expensive. Therefore proper care and maintenance of the denture is important.

• Dentures should be stored in water when not in use since dimensional changes can occur on drying.
• Acrylic dentures should not be cleaned in hot water, since processing stresses can be released and can result in distortion.
• Abrasive dentifrices (regular toothpastes) should not be used, since the plastic is soft and can be easily scratched and abraded.
• The tissue surface should be brushed carefully with a soft brush since any material removed alters the fi of the denture.

Besides physical brushing various cleaning agents are commercially available (see denture cleansers).

Denture Cleansers

• A wide variety of agents are used by patients for cleaning artificial dentures.
• They include dentifrices, proprietary denture cleansers, soap and water, salt and soda, household cleansers, bleaches, and vinegar.

Dentures are cleaned by either

1. Immersion in an agent or
2. By brushing with the cleanser.

The most common commercial denture cleansers are the immersion type, which are available as a powder or tablet.

Their composition usually includes

1. Alkaline compounds
2. Detergents
3. Flavoring agents
4. Sodium perborate

• When the powder is dissolved in water, the perborate decomposes to form an alkaline peroxide solution, which in turn decomposes to liberate oxygen.
• The oxygen bubbles then act mechanically to loosen the debris. Vinegar is effctive in dissolving calculus.
• The household cleansers are contraindicated, as they affect the fi of the denture and produce rough surfaces on prolonged use.

CAD/CAM Dentures

• ComputerAided Design – ComputerAided Manufacturing (CAD/CAM) technology has already made significant strides in the field of dentistry, especially in the field of ceramic crowns and bridges.
• Recently, CAD/CAM technology has become commercially available for the fabrication of complete dentures.
• It is a system by which impressions, interocclusal records, and tooth selection can be completed in one appointment.
• The dentures are then fabricated using CAD/ CAM technology and placed in the second appointment.
• Commercial names: AvaDent digital dentures (Global Dental Science, LLC), Dentca CAD/CAM, Nobilium CAD/CAM.

CAD/CAM Dentures Technique

• The denture is fabricated in 2 visits.
• First visit Impressions, jaw relation records, occlusal plane orientation, tooth mold and shade selection, and maxillary anterior tooth positioning record.
• Second visit Placement of dentures. The Avadent denture system involves proprietary equipment which makes it possible to fabricate the dentures in 2 visits.
• One of these includes the anatomical measuring device (AMD)which helps to establish a maxillomandibular relationship, perform gothic arch tracing, establish lip fullness, and assist in teeth selection.
• Mold tabs are used for teeth selection. The impressions together with the customized AMD and the selected mold are sent to the specialized labs where they are digitized and a virtual denture designed.
• The denture base without teeth is milled from blanks and the teeth are bonded subsequently using a proprietary glue.

CAD/CAM Dentures Advantages

1. Less number of appointments.
2. Less chair time.
3. Less a waiting period for the final denture.
4. Low shrinkage as no polymerization process is involved.
5. Duplicate dentures are easy to make as records are stored digitally.

Denture Relines

Recliners may be classified as

1. Hard or soft (resilient)
2. Heat-cured or self-cured
3. Short-term or long term
4. Resin-based or silicone-based.

Heat Cured Acrylic Resin

• The new resin is cured against the old denture by a compression molding technique.
• A low curing temperature is necessary for the relining process to avoid distortion of the denture.

Heat-Cured Acrylic Resin Disadvantages

There is a tendency for it to warp toward the relined side due to

• Diffusion of the monomer from the recliner before curing, and
• Processing shrinkage of the liner. For this reason, rebasing is preferred to relining.

Chairside Reliners

• These materials are used for relining resin dentures directly in the mouth.
• Some of them generate enough heat to injure oral tissues. According to ADA Sp. No. 17, the peak temperature reached during curing should not be more than 75 °C.
• Generally, the specifications are far less demanding for these materials than for regular denture base resins.
• On the whole, their properties are inferior to laboratory-processed acrylic resins.
• They have higher porosity and water sorption. They often contain low molecular weight polymers, plasticizers or solvents to increase their fluidity while seating the denture.
• They tend to discolor, become foul-smelling, and may even separate from the denture base.
• Thus, these materials have many disadvantages and are therefore considered as short-term materials.
• Light-activated resins are also available for relining.

Soft Or Resilient Denture Liners

International Standards organization describes two categories of soft liners.

1. Short-term soft liners (also known as tissue conditioners) (ISO 10139 Part 1).
2. Long-term soft liners (ISO 10139 Part 2).

Long-Term Soft Liners

• The purpose of the permanent soft liner is to protect the soft tissue by acting as a cushion.
• They are used when there is irritation of the mucosa, in areas of severe undercut, and congenital or acquired defects of the palate.

Long-Term Soft Liners Requirements

• Good bonding to the denture base.
• Should be biocompatible.
• Should be hygienic and not become foul-smelling.
• Should maintain its resilience for a long period.
• Should have good dimensional stability.
• Should inhibit bacterial growth.
• Low water sorption (max. 20 μg/mm3).
• Easy to process.

Long-Term Soft Liners Classifiation (ISO 10139-2:2009)

Based on the depth of penetration

• Type 1—soft
• Type 2—extra soft
• Based on their method of processing they are further divided into
  • Mouth-cured or chairside soft liners – do not last beyond a few weeks.
  • Processed soft liners– lasts up to a year.

Long-Term Soft Liners Types

Several soft lining materials are available commercially

• Plasticized acrylic resin
• Plasticized vinyl resins
• Silicone rubbers
• Polyphosphazine

Plasticized Acrylic Resin

• This is most commonly used. It may be self-cured or heat-cured.
• In self-cured type, poly(ethyl methacrylate), poly(methyl methacrylate) or acrylate copolymers mixed with an aromatic ester-ethanol liquid containing 30–60% plasticizer, such as dibutyl phthalate.
• The heat-cured resin may be supplied in a sheet form or powder-liquid form.
• The powder is composed of selected acrylic resin polymers and copolymers so that when they are mixed with the appropriate monomer and plasticizer liquid, the glass transition (softening) temperature of the cured resin will be below mouth temperature.

Disadvantages They lose plasticizers and harden with use.

Vinyl Resins

The plasticized poly (vinyl chloride) and poly (vinyl acetate) resins, like the plasticized acrylic resins, lose plasticizer and harden during use.

Silicone Rubbers

These materials retain their elastic properties but may lose adhesion to the denture base.

• Room temperature curing chairside silicone.
• Heat-cured silicones are generally a one-component system.
• They are supplied as a paste or gel containing an oxygen catalyst.
• It is heat polymerized against acrylic resin using a compression molding technique.
• For adhesion between silicones and the denture base, a rubber poly (methyl methacrylate) graft polymer solution cement may be used (one brand does not require adhesive as it contains a copolymer of silicone and a second polymer that achieves adhesion to the acrylic resin).
• Other polymers are Polyurethane and polyphosphazene rubber.

Problems associated with soft liners

1. Inadequate bonding to dentures, especially silicone liners.
2. Some silicone liners and hydrophilic acrylics undergo a high volume change (up to 40%) with gain and loss of water.
3. The heat-cured soft acrylics bond well to the hard denture base but lose their softness as a plasticizer is leached from the liner.
4. It reduces the denture base strength, not only because of reduced base thickness but also by the solvent action of the silicone adhesive and the monomer.
5. Trimming, cutting, adjusting and polishing of a soft liner is diffilt. The silicone surface is abrasive and irritating to the oral mucosa when compared to that of hard acrylic resin.
6. The greatest disadvantage of the permanent soft liner, as well as the tissue conditioner (temporary soft liner) is that they often have a characteristic disagreeable taste and odor and they cannot be cleaned as effectively.
7. The debris that accumulates in pores in the silicone liner can promote fungal growth (Candida albicans).

None of the soft denture recliners can be considered entirely satisfactory. It is necessary to review the patients periodically and if necessary change the material.

Tissue Conditioners (Short-Term Soft Liner)

• Unlike the soft liners previously mentioned, tissue conditioners are soft elastomers used to treat irritated mucosa.
• Their useful function is very short, generally a matter of a few days.
• They are replaced every 3-5 days. Their hardness ranges from 14–49 Shore A hardness units 24 hours after mixing.
• They lose alcohol over time resulting in a weight loss of 5–9%.
• These materials show both viscous and viscoelastic behavior which help in both adaptation to tissue and cushioning of masticatory forces.

Tissue Conditioners Uses

1. Ill-fitting dentures can cause inflmmation and distortion of the oral tissues. Relining an ill-fitting denture with a tissue conditioner allows the tissues to return to normal at which point a new denture can be made.
2. As an impression material (this material is used in a special impression technique known as a functional impression).

Tissue Conditioners Composition

• These are highly plasticized acrylic resins, supplied as a powder and a liquid.
• Powder Poly (ethyl methacrylate) or one of its copolymers.
• Liquid Aromatic ester (butyl phthalate butyl glycolate) in ethanol or alcohol of high molecular weight.

Tissue Conditioners Manipulation

• The denture base is relieved on the tissue surface.
• Powder and liquid are mixed together to form a gel and it is placed on the tissue surface of the denture and inserted in the mouth.
• The gel flows readily to fill the space between the denture base and the oral tissue.

The properties that make tissue conditioners effctive are

• Viscous properties, which allow excellent adaptation to the irritated denture-bearing mucosa over a period of several days and bring it back to health.
• Elastic behavior which cushions the tissues from the forces of mastication and bruxism.

Denture Adhesives

Denture adhesives are highly viscous aqueous solutions which are often used to improve the retention of complete dentures.

Denture Adhesives Supplied As

Powders or Paste.

Denture Adhesives Composition

• Kerala gum
• Tragacanth
• Sodium carboxy methyl cellulose
• Polyethylene oxide
• Flavoring agents
• Some also contain antimicrobial agents and plasticizers.

Denture Adhesives Properties

• When applied to the denture base and inserted, the polymer portion absorbs water and swells.
• They improve the retention of the denture base through adhesion.
• It fills up the spaces between the denture and the tissue.
• The high viscosity also prevents displacement. They usually have a pleasant smell.

Biological considerations

• Most of the components are permitted food additives and are generally safe.
• However, if ingested in excess, they can cause gastrointestinal disorders.
• Kerala gum can cause allergic reactions to some patients.
• It is also acidic (pH 4.7-5) and can cause caries if natural teeth are present.
• Therefore, its use is contraindicated in partially edentulous patients.

Biological considerations Disadvantages

It has an unpleasant feel, is diffilt to clean, and is diluted easily by saliva.

Biological considerations  Indications

Considering its properties, its use should be limited to

1. Temporary retention of poorly fitting dentures.
2. Patients having poor neuromuscular control.

Rebasing Of Dentures

• Because of soft tissue changes that occur during the wearing of the denture, it is often necessary to change the tissue surface of the denture.
• Such a readaptation of the denture is done by either rebasing or relining the denture.

Difference Between Rebasing And Relining

• In rebasing, the original teeth are retained and a new denture-bearing area is constructed with heat-cure acrylic resin.
• In other words, the entire denture base is replaced with new material.
• In relining, only a part of the tissue surface of the denture is removed and replaced with new material.

Provisional Crown And Fdp Materials

• The fabrication of a crown or fixed partial denture (FDP) is a laboratory procedure, and several weeks may lapse between the preparation of the teeth and the cementation of the final restoration.
• A provisional restoration provides protection to the pulp from thermal and chemical irritation caused by food and liquids, maintains positional stability, aids mastication, and maintains esthetics during this interim period.

Provisional Crown Required properties

1. A temporary restoration must be nonirritating to soft tissues and pulp.
2. They should have adequate strength to withstand the forces of mastication for the interim period.
3. They should be esthetic, especially for the anterior teeth.
4. Low thermal conductivity.
5. Low dimensional change and low exothermic reaction.
6. Easy to manipulate.

Functions of provisional restorations

1. Maintenance of esthetics and function.
2. Protection of prepared dentine from thermal and chemical trauma.
3. Maintenance of gingival position, contour, and health.
4. Maintenance of good oral hygiene.
5. Maintenance of occlusal stability and prevention of supra eruption.
6. Prevention of drifting or tilting of adjacent teeth.
7. Allows the dentist to check that suffient interocclusal reduction has been prepared.
8. Assists the clinician in diagnosis and treatment planning.
9. Assists the clinician in the construction of a new occlusal scheme.
10. Allows the esthetic evaluation of replacements.

Materials

Provisional crown and FDP materials may be performed or custom-fabricated.

Preformed Crowns

1. Synonym Anatomic crowns.
2. A preformed crown forms the external contour of the crown.
3. These crowns can be luted directly to the prepared teeth after adjustment or they may be relined with a resin prior to cementation.
4. The various performed materials include

• Polycarbonate
• Cellulose acetate
• Aluminum
• Nickel-chrome
• Tin-silver.

Polycarbonate

• This is a polymer of high impact resistance.
• It has an esthetic natural appearance.
• It is available only in a single shade.
• They are supplied in incisor, canine, and premolar shapes.

Cellulose acetate crown formers

• Cellulose acetate is a thin transparent matrix available in all tooth shapes and a range of sizes.
• It is used in combination with a tooth-colored resin.
• It primarily acts as a template within which the provisional material is filed and placed over the prepared tooth.
• After the acrylic resin sets, the cellulose acetate is peeled of and discarded and the crown is trimmed and cemented.

Custom-Fabricated Provisional Restorations

Temporaryorprovisional crowns and FDPs can also be custom-made from various types of resin. The materials used are

• Polymethyl methacrylate resins (Gel)
• Polyethyl (isobutyl) methacrylate resins (Trim, Snap)
• Epimine resins
• Resin-based composite provisional(referchapteron Resin Based Composites).

Polymethyl methacrylate

• These resins are generally self-cured.
• Commercial name Gel.

Polymethyl methacrylate Advantages

1. Acceptable mechanical properties.
2. Color stability is better than that of poly ethyl methacrylate resins.

Polymethyl methacrylate Disadvantages

1. High polymerization shrinkage.
2. High heat liberation during setting.
3. High irritation to gingival tissues.

Polyethyl (isobutyl) methacrylate resins

• These are available as a two-component powder monomer system.
• The two components are mixed to form a dough and inserted into a template which is placed over the prepared tooth/teeth.
• When set the material assumes the shape of the crown or FDP.
• Heat accelerates the setting. The excess material is trimmed and the restoration is smoothened and polished.
• The restoration is tried intraorally and cemented with suitable temporary cement.
• Commercial name Trim, Snap.

Polymethyl methacrylate resins Advantages

1. Less polymerization shrinkage and heat liberation.
2. Flow better during adaptation.
3. Less irritation to soft tissues.

Polymethyl methacrylate resins Disadvantages

1. Less tensile strength.
2. Poor color stability.
3. Clogs bur if trimmed with high-speed turbines.

Epimines

• The examines are supplied as a two-component system.
• A paste containing a high-molecular-weight examine monomer combined with a polyamide (nylon) filer and a liquid containing a benzene sulfonate catalyst.

Epimines Advantages

1. Less polymerization shrinkage.
2. Less exothermic heat.
3. Good flow properties.

Epimines Disadvantages

1. Tissue irritation (caused by catalyst).
2. Poor impact strength.
3. Poor resistance to abrasion.
4. Expensive.