Dental Materials

Removable Prostheses and Acrylic Resins

Acrylic Resins

 

A. What Is an Acrylic Resin?

Acrylic resins are hard, brittle, glassy polymers.

The commercial plastic called Plexiglas is an acrylic resin product.

– Acrylic resin is clear and colorless, making it an excellent replacement material for glass in storm doors.

– Acrylic resins are easily colored.

– Technically, acrylic resins are classified as thermoplastic materials, and many commercial products are made by injection molding acrylic materials.

– In dentistry, however, acrylic resins are handled more like a thermoset material; after it sets, it is not heated and molded.

The most common acrylic monomer is methyl methacrylate.

–  Acrylic resins are long chains of such monomers.

– The chains have side groups that inhibit chain slippage and result in mechanical properties that provide more strength than polymer chains without side groups.

Polyethylene (the same material used in “plastic” sandwich bags) is an example of a polymer without side groups.

B. Acrylic Resins as Biomaterials

Acrylic resins were developed in the 1930s and were first used in dentistry in the 1940s.

– They quickly replaced materials previously used in the construction of dentures.

– They were also used (without success) as a direct restorative material.

– They have been adapted for many other uses in dentistry.

Their handling characteristics and mechanical properties are quite satisfactory for a variety of applications.

– These uses include temporary crowns, custom trays, and base plates for
denture construction
.

It is interesting to note that a dental biomaterials scientist (Dennis Smith) suggested to an
orthopedic surgeon (John Charnley) that acrylic resin would be a good material to cement artificial joints in place.

– In the 1960s, acrylic resin was introduced as the first bone cement and was used in hip
replacements.

– Today, acrylic resin is still commonly used to cement hip prostheses and other orthopedic
devices to bone.

Acrylic Resin Systems Used in Dentistry

Acrylic resin systems set by addition polymerization in the same manner as dental
composites.

– The same terms and classifications are used. They are based on the method used to initiate polymerization.

A. Cold-Cure or Chemically Activated Acrylic Resins

Cold-cure or chemically activated acrylic resin systems are supplied as a powder and
a liquid.

These are the same materials used for the “brush-bead” buildup technique for artificial
fingernails.

The typical components are listed in the next Table.Acrylic resin system

Components of an Acrylic Resin System

 

 Cold (Chemical)-Cure

Heat-Cure

 

 

 

Liquid

 

Methyl methacrylate

 

Methyl methacrylate

Hydroquinone

Hydroquinone

Ethylene glycol

Ethylene glycol

Tertiary amine

 

 

Powder

Acrylic resin powder

Acrylic resin powder

Benzoyl peroxide

Benzoyl peroxide

Fibers and colorants

Fibers and colorants

 1. Liquid

The liquid is mostly monomer, methyl methacrylate.

A cross-linking agent, such as glycol di-methacrylate, is added.

– An inhibitor is always added to methyl methacrylate to prevent premature polymerization; hydroquinone is most commonly used.

Methyl methacrylate is a powerful solvent. It can dissolve permanent marker (used for
labeling things) and even some polymers.

The ability of methyl methacrylate to dissolve acrylic resin, polymethyl methacrylate resin,
is an important property that has a significant effect on the handling properties of acrylic resin systems.

2. Powder

The powder is predominantly polymethyl methacrylate resin with added colorants and
benzoyl peroxide.

– It is usually composed of very small beads of acrylic resin.

When activated, benzoyl peroxide forms free radicals to initiate polymerization.Acrylic resin powder

3. Physical Changes During Setting

When the powder and liquid of an acrylic resin system are mixed, several stages in the
setting
process occur.

These stages can be noticed when a sufficient mass of material is mixed as in
the constr
uction of a custom tray. During the initial stages, the changes are physical.

– The mixed powder and liquid have a “grainy” or “sandy” feel.

The powder and liquid are separate phases.

As some powder dissolves, the mixed material becomes thicker and less “runny.”

– As more powder is dissolved, the material reaches the “dough” stage.

– At this point, the material is easy to handle and mold, and up to this point, the changes are mainly physical.

4. Polymerization Reaction

A cold-cure or chemically activated system has an activator, typically a tertiary amine, added to the liquid.

– When the powder and liquid are mixed, the benzoyl peroxide and the tertiary amine react to produce free radicals.

– The inhibitor in the liquid destroys the free radicals that are initially produced, and working time results.

This occurs while the material goes from a grainy to a dough stage.

When the inhibitor is used up, typically during the dough stage, chemical changes occur, and the polymerization reaction proceeds.

– The doughy material thickens and becomes stiffer.

The reaction generates heat as well, and the material becomes warm.

Many times when a mass of material is mixed as in the construction of a custom tray,
the material becomes hot to the touch.

– The material becomes rigid and solid as polymerization reaches completion.

5. Residual Monomer

Initially, the set material contains some residual monomer.

– Any monomer that does not polymerize soon evaporates, leaving little or no monomer
or unreacted double bonds in the set material.

6. Cross-Linking

Cross-linking the resin improves mechanical properties.

A linear resin without any crosslinking agent is brittle.

Addition of a crosslinking agent improves the toughness of the material.

B. Heat-Activated Acrylic Resins

1. Heat-activated acrylic resin systems are very similar to chemically activated systems.
The major exception is that no chemical activator is present in the liquid.
A minor difference is that less inhibitor is present in the liquid.
The inhibitor is not needed to provide working time; it functions as a preservative, reacting with free radicals to prevent polymerization during storage.

2. Heat-activated systems are supplied as powder/liquid systems similar to those of cold-cure resins. When the powder and liquid are mixed, they go through the same initial stages of the setting process. Because no chemical activator is present, the mixed material stays in the dough stage for an extended period of time.
Therefore, working time is much longer than it is for heatcure acrylic resins.
After the material is formed into the desired shape (to be explained later), the material is heated in a water bath.
The heat breaks down the benzoyl peroxide, forming free radicals.
Polymerization proceeds by changing the dough into a rigid material.
Products that are properly heat-cured are a bit stronger and tougher than cold-cure acrylic resins.

C. Light-Activated and Dual-Cure Acrylic Resins

Light-activated and dual-cure acrylic resin systems are available, but they are not as popular as light activated and dual-cure composites.

– Recently, light-activated and dual-cure composite materials for temporary crowns, custom trays, and other acrylic resin uses have been introduced.

– Because these composite materials are stronger, they are gaining acceptance.

– As prices decrease, they may completely replace acrylic resins for some uses.

D. Acrylic Resin Systems and Porosity

Regardless of the type of activation of an acrylic resin system, porosity is a major concern.

– Methyl methacrylate and other monomers evaporate easily at room temperature.

– If monomer evaporates during handling or processing, the resulting material will be porous.

– Porosity weakens the material.

Also, the denture is likely to collect debris in pores and develop an offensive odor and taste.

A great deal of effort is made to prevent porosity when acrylic resins are processed.

– Pressure and temperature controls are used to minimize porosity.Acrylic resin Porosity

Complete Dentures

A complete denture or full denture replaces an entire arch of missing teeth.

A complete denture also replaces alveolar bone, which is resorbed when teeth are missing.

– Dentures are made with acrylic materials that are colored to simulate the missing tissues.

Complete dentures are held in place by suction, which is the result of surface tension and
atmospheric pressure.

– Therefore, a complete denture requires precise adaptation to the supporting tissues and
a peripheral “sea
l” for adequate retention (like that of a suction cup).

– Saliva helps to achieve the seal and improves suction, just as water improves the
effectiveness of a suction cup.

– An impression for a complete denture involves much effort to determine and record the
supporting tissues and the proper extension of the denture borders.

– The borders of the denture that are recorded by the impression are reproduced in the
denture.

– Proper extension of the borders of a denture determines the seal and much of the success of a denture.

– Most patients function reasonably well with an upper denture.

– The same cannot be said for lower complete dentures.

– The peripheral seal of a lower denture is much less effective than that of an upper denture.

– In addition, an upper denture has a larger surface-bearing area (the palate), and it usually has a better alveolar ridge to support the denture.

A. Components of a Denture

A complete denture has two major components: the white denture teeth and the pink
denture base.

Denture teeth are purchased from a manufacturer.

– The denture base is made in the dental laboratory following the dentist’s prescription; it is made on the master cast, which is a positive reproduction of the patient’s alveolar ridge.Acrylic Teeth

B. Denture Teeth

Denture teeth come in a variety of shapes, sizes, and shades.

The shape is chosen to match that of the patient’s natural teeth, usually as judged from an old photograph.

Another technique is to use the shape of the face to select the tooth shape.

The size is determined by the size of the patient’s arch.

The shade of the teeth is chosen to match the patient’s natural complexion.

– Often, the patient desires white teeth and must be counseled as to the true color of natural teeth, because bright white teeth will look artificial.

1. Acrylic Resin Teeth

Today, most denture teeth are made from acrylic resin much like that used to construct the denture base.

– Denture teeth have more cross-linking agents added.

– Because the teeth are constructed under tightly controlled conditions at a manufacturing plant, they are stronger than the acrylic material used for the denture base.

Acrylic denture teeth are “chemically” bonded to the acrylic denture base during processing of the denture.

2. Porcelain Teeth

Porcelain teeth are made by manufacturers in much the same shapes, sizes, and shades as acrylic teeth.

– Porcelain teeth are much harder and more stain-resistant compared to acrylic teeth.

Porcelain teeth are rarely used, however, both because they excessively wear the opposing teeth and because it is generally believed they cause trauma and bone loss in the
supporting and opposing alveolar ridges.

Porcelain teeth are held in the denture by the mechanical undercuts of pins that are
embedded in the back of the denture tooth.Porcelain teeth

3. Composite Teeth

Composite denture teeth are made from materials similar to dental restorative composite materials.

The resin is slightly different, but fillers are added to improve strength and wear resistance.

C. Denture Base

The denture base is constructed on the master cast made from the final impression.

– The denture base is the pink part of the denture that sits on the alveolar ridge.

Constructing a Complete Denture

 

A. Impressions and Casts

Typically, denture construction requires a number of dental appointments and laboratory procedures.

Many dentists make a preliminary impression, preliminary cast, custom tray,
final impression, and
master cast (in that order).

– The denture is directly constructed on the master cast.Construction of a denture.

B. Recording the Maxillary and Mandibular Relationship and Arranging Teeth

The relationship of the maxillary and mandibular ridges, or the “bite of the patient,” is very important to the success of the prosthesis.

– An improper relationship can result in overworked muscles of mastication, poor phonetics, and unsatisfactory esthetics.

1. First, a “baseplate” and “wax rims” are constructed from chemically activated acrylic (or similar material) and baseplate wax.
They are made to precisely rest on the patient’s ridge.Construction of a denture.Construction of a denture.

2. The wax rims on the baseplates are used to determine the patient’s midline, “plane of
occlusion,” and size of the denture teeth.
The plane of occlusion is the plane where the upper.Construction of a denture.

3. At the same appointment, the patient’s bite is recorded.
This bite is used to position the upper and lower casts in the same relationship as the
supporting tissues in the mouth.Construction of a denture.

4. With the aid of baseplates and rims, the casts are mounted on an articulator using dental plaster.Construction of a denture.

5. The denture teeth are “set” in wax. Wax is easily softened with a hot instrument to allow the denture teeth to be partially set in wax and then easily moved into the proper position. When all the denture teeth are set in wax, it is called a denture setup.
A setup simulates the proper bite (relationship of the two arches), the vertical dimension (distance between the two arches), and the esthetics of the final denture.Construction of a denture.

6. Before processing, the setup (the baseplates with the teeth set in wax) is placed in the
patient’s mouth. This procedure is called the “wax try-in”.
The wax try-in allows patients to see the arrangement of teeth that their denture will have. It also allows the dentist to check function, occlusion (bite), and phonetics
before the
actual denture is processed.

C. Processing the Denture

1. Processing a denture involves embedding the master cast and the denture setup in
a denture flask filled with gypsum materials.
It is done in such a way as to create a split mold. The mold is opened, and the wax and the baseplate are removed. Acrylic resin in the dough stage is placed in the mold space.
The mold is then closed, and the acrylic resin replaces the wax and base plate, forming the denture base.Processing a denture.

2. The closed mold is heated in a water bath to activate the heat-cure acrylic resin.
The heating rate is controlled to reduce porosity. The application of pressure minimizes loss of the volatile monomer. Loss of monomer results in porosity and weak spots.

3. After processing, the denture is removed from the mold by breaking the surrounding
gypsum materials. It is then finished, polished, disinfected, and delivered to the patient. Photographs of completed dentures

 

Partial Dentures

Many patients are partially edentulous, and the remaining teeth are frequently used to
support and retain a prosthesis.

– If the prosthesis can be removed by the patient, it is called a removable partial denture or simply a partial denture.

– Most partial dentures are supported by both natural teeth and the alveolar ridge.

A lower partial denture with a few remaining natural teeth to hold it securely functions far better than a complete lower denture.

– The dental hygienist should stress to the patient the critical need to maintain the remaining natural teeth so that wearing a mandibular complete denture is avoided.

Patients with a mandibular complete denture often have problems chewing.

– The mandibular denture will lack suction or retention and will float and move around in the mouth.

– The lack of stability of a mandibular denture may cause “sore spots” (ulcerations of the oral mucosa) and make speech difficult.

Helping the patient to retain several mandibular teeth will allow a patient to wear and use
a mandibular partial denture.

– This is an important preventive responsibility of the dental hygienist.

A. Frameworks

A partial denture uses a cast metal “framework” for retention.

– The framework is composed of clasps, connectors, and mesh.

1. The framework has clasps that rest on and go around the abutment teeth.
Because the clasps are metal, they can be bent to adjust their fit.

2. Connectors are the thicker parts of the framework that connect the mesh and clasps
together.

3. The framework also has an area of mesh that the acrylic resin flows into and around when the partial denture is processed. This results in the mechanical attachment of the teeth and the denture base to the framework.Components of a partial denture.

B. Denture Base

The denture base is constructed and used in much the same way as a complete denture.

– However, the mesh areas of the partial denture framework are embedded in acrylic resin.

C. Teeth

The same teeth used to construct complete dentures are used to construct partial dentures.

Teeth are chosen with the proper shape, size, and shade to replace the missing natural teeth.

D. Processing a Partial Denture

A partial denture is processed much like a complete denture.

– However, the acrylic resin must flow through and around the mesh of the framework

Relining a Denture

Dentures lose their “fi t” after a period of time, because the alveolar ridge atrophies and
resorbs when teeth are not present.

Most resorption occurs when the teeth are first extracted, but resorption continues slowly throughout a patient’s life.

– Therefore, as the alveolar ridge changes, the fi t of the denture changes as well.

Indications that a denture is not fitting as well as it should include frequent use of denture
adhesives or over-the-counter denture “cushion” materials or liners.

Use of these materials makes cleaning the denture difficult and can result in a foul smell.

Long-term use of adhesives may cause tissue inflammation.

Liners may change the bite or occlusion of a denture, causing trauma to the supporting
tissues.

The dental hygienist must be aware of these problems and educate patients about the proper use of over-the-counter products and treatment options.

An “old” denture can have its fit improved by a process called relining.

Relining adds a small amount of new material to the inside tissue area of the denture base to replace the additional alveolar ridge that has been lost since the denture was made.

Relining a denture can be done either using a dental laboratory or at chairside.

A. Laboratory-Processed Reline

A laboratory-processed reline involves making an impression, much like the final impression that is made when a denture is constructed.

The existing denture functions as the impression tray.

A variety of impression materials are used depending on the clinician’s personal preference.

The denture containing the impression is sent to a dental laboratory.

A cast is poured.

The impression and cast are separated, and the impression material is removed from the
denture.

New acrylic material is added to fill the space between the denture and the cast (the space previously occupied by the impression material).

The acrylic material is cured, finished, and polished in the same manner as during the
construction of a new denture.

The denture is returned to the dentist and given to the patient.

A laboratory reline is generally superior to a chairside reline; however, patients must go
without their denture for a day or more when the reline is processed at a dental laboratory.

B. Chairside Reline

A chairside reline uses a different material.

– The same material that functions as the impression material also becomes the added
material.

– The material is mixed, placed in the denture, and then placed in the patient’s mouth.

– The material sets in the mouth and becomes rigid.

– The excess is trimmed, and the denture is polished and returned to the patient.

The advantage is that the patient does not go without his or her denture.

– The disadvantage is that the new material is more porous and, typically, not as smooth as that of a laboratory reline.

Immediate Dentures

Most dentures are made for patients who have had their teeth previously extracted.

An immediate denture is a denture that is placed at the same appointment during which the remaining teeth are extracted.

Typically, when an immediate denture is constructed, many of the teeth in the arch to receive the denture are already missing and most of the remaining teeth are hopeless.

– An impression is made by the dentist, and the denture is constructed in the laboratory,
similar to a complete denture.

At the delivery appointment, the remaining teeth are extracted, the denture is placed, and postoperative instructions are given to the patient.

An immediate denture not only restores function and esthetics, it can protect the extraction sites during healing. In most cases, an immediate denture is relined several months after it is placed. Immediate Denture

Repairing Acrylic Prostheses or Appliances

It is not unusual for a denture or other acrylic prosthesis or appliance to fracture.

– It seems that dogs love to chew on them.

Repairs are made by using new, chemically activated acrylic material as a glue to repair the fracture.

– In many offices, the dental hygienist may be the professional who repairs simple fractures of partial or complete dentures.

The process involves several steps.

A. Clean the Surface

First, clean the surfaces being repaired.

– Often, this involves grinding the surface to remove a thin layer of the surface and any
contaminants.

B. Apply Monomer

Next, monomer is applied to the clean surfaces to dissolve some of the set material.

One end of some of the polymer chains dissolves in the monomer; the other end remains
embedded in the solid acrylic resin.

C. Apply New Material

New material is then mixed, applied to the surface, and allowed to set.

– The polymer chains of the old material become entangled in the new material.

Because no residual double bonds are available for a chemical reaction, the bond
is quasi-chemical.

The same process is used to “chemically” bond acrylic denture teeth to the denture base
during denture processing.

D. Finish and Polish

Finally, the repaired prosthesis is finished and polished by using acrylic burs, pumice, and other materials as for a complete denture.

– This too can be done by the dental hygienist.

Handling Acrylic Devices

It is important that patients regularly clean their dentures.

Patients should be instructed to brush their prostheses with a denture cleaner and a denture brush or to use other denture cleaning procedures.

Calculus may form on the denture in similar areas as in natural teeth.

It is the dental hygienist’s responsibility during the recall appointment to
remove the
calculus in a manner that is not harmful to the acrylic.

Most dentists prefer that their patients do not sleep with dentures in the mouth.

– Therefore, patients should store their dentures in water at night.

During dental appointments, it is important to store acrylic appliances in water.

– Do not let them dry out, because they may warp.

– This applies not only to dentures but also to acrylic orthodontic retainers.

The dental hygienist should play an active role during recall visits and educate patients who wear complete and partial dentures.

 

ــــــــــــــــــــ ⒹⒺⓃⓉⒶⓁⓈⒸⒾⒺⓝⓒⒺ ــــــــــــــــــــ