Сraig. Dental Materials

the four groups, the bond strength was higher to polymerized acrylic than to unpolymerized acrylic. Most manufacturers recommend processing the soft liner and the denture base acrylic at the same time to improve the bond strength. Only the plasticized vinyl acrylic had a higher bond strength when processed against unpolymerized acrylic. Recently, bonding agents have been introduced that significantly increase the bond strength of several addition-silicone elastomers to denture base resin.

Dynamic viscoelastic measurements are effective for evaluating the elastic behavior of elastomers over time. In one study evaluating a plasticized acrylic, a fluoroelastomer, a heat-cured silicone, and a self-curing addition silicone, the acrylic and fluoroelastomer demonstrated viscoelastic behavior whereas the silicones exhibited elastic behavior. The silicones were more stable over time than the other materials.

Growth of Candida albicans and other microorganisms continues to challenge the use of shortand long-term soft denture liners. Unfortunately, researchers and manufacturers have yet to develop materials that will not support the growth of microorganisms. At this time, only excellent oral and denture hygiene and the use of antimicrobial agents are effective in minimizing this problem.

Unfortunately, many of the shortand longterm resilient denture liners contain significant amounts of plasticizers, mmy of which have questionable biocompatibility. Phthalate esters have caused epithelial changes when polymer disks containing these esters have been implanted in the hamster cheek pouch. The potentially premalignant changes are a cause for concern because the amount of this plasticizer leached from a typical soft liner may be between 10 and 40 times greater than environmental and food uptake. Although even this amount of plasticizer is low, demonstrated biocompatibility of soft denture liners should be considered.

Soft liners have an important place in denture prosthetics, but require improved strength, improved adhesion to the denture base, and the ability to inhibit the growth of microorganisms before they will be considered permanent.

Plastic teeth are prepared from acrylic and modified acrylic materials similar to denture plastics. Different pigments are used to produce the various tooth shades, and usually a cross-linking agent is used to improve strength and prevent crazing. Plastic teeth are prepared in layers of different colors so the shade is gradually lightened toward the incisal and occlusal portions to give these areas a translucent appearance. The gingival or body portion may not be as highly cross-linked as the incisal or occlusal portion. This is done to improve the chemical bond between the teeth and the denture base. Fillers may be added to increase resistance to wear. Recently, the use of composite materials as denture tooth materials has received some interest and new, modified tooth materials that appear to have increased wear resistance have been introduced.

As indicated in the discussion on the physical properties of dental plastics, poly(methy1 methacrylate) has satisfactory chemical properties for use as plastic teeth. It is nontoxic and insoluble in oral fluids but is soluble to some extent in ketones and aromatic hydrocarbons. The mechanical properties of compressive strength (76 MPa), abrasion resistance, elastic modulus (2700 MPa), elastic limit (55 MPa), and hardness (18 to 20 kg/ mm2) are low when compared with other restorative materials or with human enamel and dentin.

Of necessity, a discussion of the merits of plastic teeth includes a general comparison of the properties of plastic and porcelain teeth, a number of which are listed in Table 21-16. Certain properties may be listed as a disadvantage or an advantage depending on the particular purpose and point of view involved. For example, the softness and low abrasion resistance of plastic teeth may be cited as a disadvantage, because the teeth are more easily abraded and the occlusion and vertical dimension of the denture may be altered. On the other hand, low wear resistance also has been cited as an advantage, because plastic teeth are easy to grind and polish and tend to be self-adjusting in service. The hardness of porcelain teeth compared with plastic teeth is an advantage, whereas the resulting brittleness of

Plastic Teeth

High resilience

Tough

Soft-low abrasion resistance

Insoluble in mouth fluids-some dimensional change

Low heat-distortion temperature, cold flow under pressure

Bond to denture base plastic

Natural appearance

Natural feel-silent

Easy to grind and polish

Crazing and blanching-if non-cross linked

*Does not apply if silane-treated.

porcelain compared with the toughness of plastic teeth is a disadvantage.

The main difference between porcelain and plastic teeth is that plastic teeth are softer but tougher than porcelain teeth. Other differences are the low elastic modulus, low resistance to cold flow, low abrasion resistance, and high impact strength of plastic teeth. Both plastic and porcelain teeth are insoluble in oral fluids. Porcelain teeth are resistant to organic solvents such as ketones and aromatic hydrocarbons, which will react with non-cross linked plastic teeth. Plastic teeth composed of poly(methy1 methacrylate) show small dimensional changes when placed in water. As expected, vinyl acrylic teeth do not show as much dimensional change as the totally acrylic teeth. Porcelain teeth show no dimensional change when stored in water and exhibit no permanent deformation from forces exerted on them in the mouth.

Friable

Hard-high abrasion resistance

Inert in mouth fluids-no dimensional change

High heat-distortion temperature; no permanent deformation under forces of mastication

Poor bond-to-denture base plastic*;mechanical retention provided in tooth design

Natural appearance

Possible clicking sound in use

Grinding removes surface glaze

/Occasional cracking

For denture teeth, the coefficient of friction of acrylic against acrylic in the presence of water or saliva is lower than the coefficient for porcelain against porcelain. The lowest coefficients of friction, however, are obtained when porcelain and acrylic specimens oppose each other.

In Fig. 21-16, the frictional behavior of acrylic denture teeth is shown for central incisors from two manufacturers. The tangential force on a diamond slider being drawn across the teeth was measured under various loads in distilled water. The width of the wear track was also measured. In general, low values of track width occurred with low tangential force values, and higher track widths and more severe surface failure were associated with higher values of tangential force. The "enamel" surfaces, which were the labial surfaces of the teeth, were more resistant to penetration and surface damage than the "dentin" surfaces, which were prepared by removing

a

A

Normal load (N)

Fig. 21-16 Tangential force versus normal load for a diamond slider on "enamel"and "dentin"surfaces

of two manufacturers' acrylic teeth (A and 6)in water.

(Adapted from Raptis CM, Powers JM, Fan PL: J Dent Res

60:908, 1981.)

1 mm of acrylic from the ridge lap area of the tooth.

Plastic teeth possess low heat-distortion temperatures, although the use of cross-linked plastics has improved this property considerably. Care should be taken not to flame plastic teeth during the preparation of a waxed denture. Porcelain teeth have high heat-distortion temperatures; however, sudden temperature changes may cause cracking or fracture. As indicated previously, plastic teeth exhibit cold flow, or permanent deformation, under stresses below their elastic limit, and their dimensions may be altered slightly during use. Plastic teeth are of a similar material to the denture base and may be chemically bonded to the base. The bond strength between chemically accelerated acrylics and plastic teeth is lower, and the use of mechanical retention on the underside of the teeth may

For periodic updates, L

prove useful. Highly cross-linked acrylic denture teeth may be treated with 4-methacryloxyethyl trimellitic anhydride to improve the bond strength to denture base resins. An adhesive containing this compound is also available for bonding acrylic to nickel-chromium and cobaltchromium denture bases or partial dentures. This allows acrylic posterior palatal seals to be added to metal bases and metal base dentures to be relined, when necessary.

Porcelain teeth are usually fabricated with mechanical retention for bonding to the denture base. It has been shown that treating porcelain teeth with a silane coupling agent, such as y-methacryloxypropyl-trimethoxysilane, provides a surface treatment that allows the teeth to be chemically bonded to the denture base material. When the bond was tested in tension, the porcelain ruptured rather than the bond. Because the use of cross-linked acrylic teeth has made the chemical bonding to the denture base more difficult, mechanical retention is often provided for plastic teeth. The chemical bond produced between plastic and porcelain teeth and the denture base has the distinct advantage of preventing capillary spaces around the teeth, which are difficult to clean and in which microorganisms may grow.

Both types of artificial teeth have a realistic appearance, but the noise or clicking of porcelain teeth against each other is about three times greater than plastic against porcelain or plastic against plastic. The processing of a single tooth replacement and the characterization of plastic teeth are considerably easier than for porcelain teeth. In addition, plastic teeth may be ground and polished with ease, whereas grinding of porcelain teeth removes the surface glaze, making repolishing more difficult. Porcelain teeth should not be used opposing natural teeth or gold restorations because excessive wear will occur.

The choice between plastic and porcelain teeth depends to a great extent on the preferences of the dentist and the patient. In the last few decades acrylic teeth have become much more popular than porcelain. Plastic teeth are used opposite natural teeth or gold restorations

and in patients with poor ridge conditions or limited space. Porcelain teeth may be used for patients with good ridge support, adequate space, and those with both maxillary and mandibular dentures. There is no ANSVADA specification available for porcelain teeth, although one is available for plastic teeth.

10.Dimensional stability: When tested as outlined, the dimensional change of a tooth shall be within -t2% of its original mesial-distal dimension.

ANSIIADA SPECIFICATION NO. 15 (IS0 3336) FOR SYNTHETIC POLYMER TEETH

ANSVADA Specification No. 15 presents the desired properties for plastic teeth. Only an abstract of this specification is included here, because a large number of requirements are listed.

Dimensions of teeth: The dimensions shall be within 5% of those stated by the manufacturer.

Color and blend: Sets of anterior and posterior teeth, representing each shade from the same manufacturer, shall exhibit no perceptible color differences between each other and the manufacturer's shade guide.

Freedom from biologic hazard: Refer to IS0 10993-1,Biological Evaluation of Medical Devices.

Surface finish: No stain will occur in service.

Retention of finish: After processing and reprocessing, the teeth shall be capable of being polished to restore the original finish.

Repolishing: The teeth shall be capable of being ground and repolished to a finish equivalent to their original appearance.

Quality of bonding to denture-base polymers: The teeth shall be capable of being bonded to heat-polymerized denturebase materials.

Color stability: There shall be no perceptible color change in the exposed teeth. Resistance to blanching, distortion and crazing: When tested as outlined, no tooth shall exhibit blanching or distortion.

Maxillofacial materials are used to correct facial defects resulting from cancer surgery, accidents, or even congenital deformities. Noses, ears, eyes and orbits, or any other part of the head and neck may be replaced by these prostheses (Fig. 21-17).

Maxillofacial prostheses are difficult to fabricate and have a relatively short life of 6 months to several years in service. They fail as a result of inherent problems with static and dynamic properties over varying periods and because of color degradation. The prostheses are expensive to fabricate, and many patients cannot afford frequent replacement.

The several types of materials for maxillofacial prostheses vary considerably in ease of preparation and physical properties.

POLY(METHYLMETHACRYLATE)

Poly(methy1 methacrylate) was once commonly used for maxillofacial prostheses, and is still used occasionally to make artificial facial parts. When properly pigmented, these prostheses can look quite realistic. The main disadvantages are that the acrylic is hard and heavy, does not flex when the face moves, and does not have the feel of skin. Stone molds are generally used with poly(methy1 methacrylate), and these are sacrificed when the prosthesis is deflasked after processing.

PLASTICIZED POLWINYLCHLORIDE

Polyvinylchloride has been used widely for maxillofacial applications, but it has been replaced by newer materials with superior properties. Polyvinylchloride is a rigid plastic with a glass transition temperature higher than room temperature. For maxillofacial applications, plasticizers are added to produce an elastomer at room

temperature. Other ingredients added to polyvinylchloride include cross-linking agents for added strength and ultraviolet stabilizers for color stability. There is no chemical reaction involved when the material is processed. The product is supplied as finely divided polyvinylchloride particles suspended in a solvent. When the fluid is heated above a critical temperature, the polyvinylchloride will dissolve in the solvent. When the mix is cooled, an elastic solid is formed. Plasticized polyvinylchloride is processed at 150" C, and metal molds are generally used.

POLYURETHANE

Polyurethanes are used as maxillofacial materials. The formation of polyurethane is the result of the direct addition of diisocyanate to a polyol in the presence of an initiator, as shown in the

equation above. Isophorone polyurethane has also been used as a maxillofacial material.

The reaction must be carried out in a dry atmosphere, or carbon dioxide will be produced and a porous elastomer will result. The diisocyanates are very toxic and must be handled with extreme care. The processing temperature of 100" C is reasonable, and stone molds can be used.

HEAT-VULCANIZED SILICONE

Heat-vulcanized silicones are used occasionally for maxillofacial prostheses. The vulcanization mechanism is achieved by an addition reaction. The components of heat-vulcanized silicones are a polydimethylvinyl siloxane copolymer with approximately 0.5% vinyl side chains, 2,4- dichlorobenzoyl peroxide as an initiator, and a silica filler obtained from burning methyl silanes

(see below). Vulcanization results from thermal decomposition of the initiator to form free radicals that cross-link the copolymer into a threedimensional structure. The processing temperature is 220" C, and metal molds are used. The copolymer is supplied as a rubbery solid with a high viscosity. The pigments are incorporated into the polymer with roller mills. Although this material is more difficult to pigment and process, excellent results can be obtained.

Polydimethylvinylsiloxane

2,4-Dichlorobenzoyl peroxide

ROOM TEMPERATURE-VULCANIZED SILICONES

Because of their good physical properties and favorable processing characteristics, room temperature-vulcanized (RW) silicones have become popular as maxillofacial materials, and are now used more often than any other material. They have good physical and mechanical properties, are easy to color and process, and allow the use of stone molds. There has been a steady improvement in the physical and mechanical properties of these materials. They are affected by accelerated aging, but not to a degree that compromises their usefulness as maxillofacial materials. These R W silicones are similar to addition silicone impression materials in that they consist of vinyland hydride-containing silox-

anes and are polymerized with a chloroplatinic acid catalyst.

OTHER ELASTOMERS

Several elastomers have been investigated for use as maxillofacial materials: aliphatic polyurethanes, chlorinated polyethylene, silphepylene polymers, organophosphazenes, butadienestyrene, butadiene-acrylonitrile, and siliconePMMA (polymethyl methacrylate) block copolymers.

FABRICATION OF THE PROSTHESES

The method for fabricating a prosthesis is similar for most materials. An impression is made of the affected area with alginate. A master cast is poured, duplicating the defect on the patient. The artificial part (such as a nose) is then carved in wax or clay on the master cast and tried on the patient to see if it fulfills the esthetic requirements for form.

The pattern is then invested in a manner similar to that used for complete dentures. Denture flasks are often used for this purpose. When the prosthesis is quite complex (such as an eye and orbit), threeor four-part molds are made. With some materials, metal molds are required because of high processing temperatures. After the pattern is invested, it is removed from the mold by use of a boiling water bath.

The mold is now ready to make the prosthesis. The patient should be present so pigments may be added to the elastomer to give a realistic appearance and match the patient's skin color. Generally, dry mineral earth pigments or artist's oil-based pigments are used. Color matching is done by mixing small amounts of the pigments into the elastomer. Some clinicians use color tabs and predetermined pigment formulations to match skin color. When a color match is achieved, the elastomer is compression molded and processed according to the manufacturer's instructions.

After processing, the prosthesis is removed from the mold and the excess flash is removed.

The prosthesis is then delivered to the patient. Surface pigmentation is generally done to give the prosthesis a more lifelike appearance. Medical grade silicone adhesives are often used to carry and bond surface colorants to the prosthesis. When mechanical undercuts are not present for retention, the patient may use adhesives to keep the prosthesis in place.

PHYSICAL PROPERTIES

The static and dynamic properties of the more popular maxillofacial materials are shown in Table 21-17. Values are representative of commercial products in each category. Heat-vulcanized silicone has the highest tensile strength at 5.87 MPa, and polyurethane the lowest at 0.83 MPa. The remaining materials have tensile strengths about 30% lower than the heatvulcanized material. Tensile strength is important, because when the patient removes the prosthesis, high tensile forces are applied, particularly in thin areas.

Three of the materials are similar in maximum percent elongation with values from 422% to 445%. Plasticized polyvinylchloride has a lower percent elongation at 215%. Knowing the percent elongation is helpful, because different parts of the face have different requirements in terms of

how far the elastomer must stretch to accommodate facial movement.

Tear resistance is important for maxillofacial materials because prostheses may be torn when patients remove them. In the pants tear test, a thin sheet of the elastomer is made in the shape of a pair of pants. The legs of the pants are then pulled slowly apart and the energy required to propagate a tear is measured. Heat-vulcanized silicone and RTV silicone have excellent tear resistance, because the samples do not tear but stretch, as in tensile elongation. Plasticized polyvinylchloride and polyurethane have good tear resistance at 4.3 x 10"nd 6.7 x 10"~nes/cm, respectively.

The evaluation of maxillofacial materials should involve not only the static but also the dynamic physical properties, because the stressstrain curves for elastomers are nonlinear. Because of the nonlinearity of the stress-strain properties of these materials, they function differently at high and low rates of loading. The dynamic modulus is the ratio of stress to strain applied for small cyclic deformations at a given frequency at a specified point on the normal stress-strain curve, or it is the slope of the stress-strain curve at a given point corresponding to a given frequency.

In practical terms, elastomers with a high dy-

namic modulus are rather rigid materials, whereas materials with a low dynamic modulus are more flexible. In Table 21-17, the heatvulcanized silicone has the highest dynamic modulus at 4.66 MPa, and the RTV silicone has the lowest at 2.12 MPa.

During the last few years there have been many significant studies on the physical properties of unpigmented and pigmented maxillofacial elastomers as a function of accelerated aging. New elastomers are also being evaluated for maxillofacial applications. It is gratifying to see renewed interest in an area of biomaterials that is often overlooked.

Efforts have been made to modify the stressstrain properties of maxillofacial materials to match living facial tissues. Medical-grade silicone adhesive has been combined with an RTV silicone base in various ratios to control the elastic properties. The stress-strain profiles of human facial tissues and various ratios of the silicone elastomers are shown in Fig. 21-18. The curves demonstrate that it is possible to formulate silicones that will match the elastic properties of facial tissues.

Major advances have been made in the last few years in the area of maxillofacial materials.

New materials have made it possible to make lifelike maxillofacial prostheses that last longer in service.

PLASTIC FACINGS FOR CROW

AND BRIDGE APPLlCATtONS

'+

Before the introduction of porcelain-fused-to- metal crowns, acrylic facings were the only type of crown facing available. Porcelain facings have now replaced acrylic in all but a few applications. When used for facings, acrylic materials are similar in composition and properties to the best available denture teeth.

TEMPORARY CROWN AND B

RESTORATIONS

Chemically accelerated plastics have become popular as temporary restorations. They have decreased the use of aluminum shell and polycarbonate temporary crowns, because they are easy to fabricate and are esthetic. They are similar to chemically accelerated denture base plastics and are available in several shades to approxi-

mate the color of the patient's teeth. In one widely accepted technique, a thin polystyrene sheet is heated and then vacuum-formed over a wet gypsum model of the patient's teeth taken before tooth preparation. The thin template is then trimmed to include the teeth to be prepared and teeth on either side. In the case of a bridge, a denture tooth can be waxed to the model before vacuum forming, and this allows space in the template for the pontic of the temporary bridge. After the teeth are prepared for crowns and the final impression has been made, the thin polystyrene template is used to make the temporary restoration. The tooth preparations are lubricated, and the chemically accelerated plastic powder and liquid are mixed and placed into the template in the area of the restorations. Chemically activated temporary resins are now also available in automixing syringes and can be directly injected into the matrix. When the acrylic approaches the doughy stage, the template and acrylic are placed over the prepared teeth and seated. The patient is then asked to bring the teeth into occlusion. The temporary restoration may be removed and cooled in water several times during polymerization to control the heat and then reinserted into the mouth. When the restoration is still slightly elastic, it is removed from the mouth and allowed to continue polymerization at room temperature. The restoration is then trimmed, polished, and cemented to the prepared teeth with a temporary cement. The resulting restoration simulates the teeth as they were before preparation and has acceptable esthetics. Generally only minimal occlusal adjustment is required because the teeth are in contact during polymerization. The accuracy of various products should be considered when selecting a material. Marginal opening can be caused by polymerization shrinkage. There are reports in the dental literature of some allergic reactions to the temporary crown and bridge plastics; these reactions are believed caused by monomer or the amine accelerator. Heat generation from polymerization can also be harmful to the teeth. Using thermocouples and extracted teeth prepared for crowns, temperatures of 40" to 84" C have been recorded in the pulp chambers during

polymerization. With external cooling of a restoration, the temperature rise is minimal. Silanetreated glass fibers have been used to significantly increase the flexural strength of a provisional resin. Further evaluation needs to be done to evaluate biocompatibility.

Light-activated polymers used to make temporary restorations are available in several tooth shades. The method is similar to that used for chemically activated acrylics, but has several advantages. The material can be removed from the mouth in the template, the flash can be removed before curing, and the restoration can be reinserted several times to ensure easy seating after processing. The final polymerization is done in a light chamber identical to that used for processing light-activated denture bases. The lightactivated material allows fast and accurate fabrication of temporary restorations, and has the added advantage of no methyl methacrylate monomer in the formulation, thus reducing the potential for allergic reactions.

Light-activated and chemically activated composites are also gaining popularity for use as temporary restorations. A review of these materials is found in Chapter 9.

The use of occlusal splints in the treatment of patients with temporomandibular joint pain or excessive bruxism has become a routine procedure. These splints are made by the same technique used for dentures. The splint is waxed on a model of the patient's teeth, usually the maxilla. The model and wax pattern are invested in a denture flask, and the wax is boiled out. Af- ter cooling, alginate separator is painted onto the mold and allowed to dry. A clear, heataccelerated acrylic resin is packed into the mold and processed. The acrylic resin is mixed and packed when it has reached the doughy stage. Chemically accelerated acrylic resin is also used, but less often. The properties of acrylic splints are similar to those of heat-accelerated denture materials. Recently, a clear light-activated acrylic has been introduced that simplifies the construction

of occlusal splints. A major advantage is that the splint can be made quickly and without flasking.

Chemically accelerated acrylic is used to fabricate inlay patterns and direct posts and cores. Commercial products are brightly pigmented, have good dimensional stability, and are convenient to use. The pattern is made by painting the powderliquid onto the die or tooth in layers and allowing it to polymerize. After polymerization, the pattern can be modified with stones and burs. If necessary, inlay wax may be added to complete the pattern. Considerably longer burnout times must be used with acrylic patterns.

Chemically accelerated acrylic is commonly used to produce custom impression trays. These trays provide a nearly constant distance between the

tray and the tissues, allowing a more even distribution of the impression materid during the impression procedure, and improved accuracy. The same material may also be used to lnake record bases for prosthetic applications. The principal difference in the composition of the plastics for this purpose is the addition of substantial quantities of fillers, which decrease warpage of the material during setting. Shrinkage of chemically accelerated acrylic trays can occur for up to 7 days, although the greatest shrinkage takes place during the first 24 hours. The dimensional changes of five commercially available tray materials are shown in Fig. 21-19. Significant shrinkage occurs after the initial polymerization. The products differ considerably in the length of time required to reach 90% of the total dimensional change. Although the maximum shrinkage is less than 0.4%, chemically accelerated acrylic impression trays should be allowed to stand from 2 to 9 hours, depending on the product, if dimensional changes are to be minimized. In practice, many dentists have a dental laboratory fabricate the impression trays, and the time between fabrication and use will be adequate. If trays are to be used soon after fabrication, the tray should