Timelines in the Development of Denture Base Materials and a Review of Current Ones

Dr. Silas M. TokaMDS I (PROS)

12th November, 2015

Introduction•The earliest complete upper and lower denture

appear to date from circa 1500 and was dug up in Switzerland (Rath 1958).

•It consists of two arch shaped templates of bone, crudely carved from an ox’s femur and tied together at their posterior extremities to form a hinge.

•There are no bases as such and retention must have been from the remaining natural teeth.

•Highly likely the prosthesis must have been cosmetic rather than functional.

Historic Finds•Dentures carved from wood and ivory are claimed

to have been discovered by Belzoni in Egyptian tombs, but no proof of any of these discoveries exists (Guerini 1909, Ambler 1909).

• Guerini and Ambler quoted the Roman poet Martial (80AD), to support the view that complete dentures were provided in ancient Rome.

•Ambler (1906) stated that a complete set of false teeth, fabricated in gold, was found in a Roman tomb a few years before his writing (an unsubstantiated report).

Historic Finds•The fabrication of a denture was first described

by Purmann in 1684 (Guerini 1909).

•A lump of wax was partly carved, and partly moulded to the desired shape and contour, so that it reproduced the missing teeth, fitted snugly against those remaining, and adapted to the palate and residual alveolar ridge.

•This model was then used by a craftsman for reproduction in bone or ivory.

Historic Finds•However, such a denture is unlikely to have

derived its retention from a denture base, as a large wax pattern would have been impossible to withdraw from the mouth without distortion.

•The pattern was therefore kept as small as was reasonably possible.

•Springs were introduced as means of retention most probably by Fauchard (1728), but these dentures probably caused soft tissue injuries and were not functionally optimum.

Complete Denture Impression•What was yet to be appreciated, was that a

broad denture base to distribute occlusal loads, and one which, by its extension, would also assist in stability and retention.

•According to Guerini (1909), Frederick the Great’s dentist, Philip Pfaff (1756), developed a more effective way of impression taking.

•His method was to take a wax impression of the entire jaw in two separate pieces, which are re-assembled outside the mouth to make a plaster cast.

Complete Denture Ivory Bases•With a reasonably accurate, dimensionally

stable cast, the carver proceeded to fabricate an ivory denture without frequent recourse to the patient.

•To give the denture a more lifelike appearance, human teeth were added to the front on the ivory base (Guerini 1909, Ash 1920, Woodforde 1968).

•Both ivory and human teeth however, deteriorated in the mouth.

Complete Denture Ivory BasesA letter of 1798 to George Washington from his dentist, contained detailed advice on countering the stains and for plugging holes in the President’s dentures.

The dentist seemed to believe that the stains were coming from portwine and “other acids” (Woodforde 1968).

Complete Denture Porcelain Bases•Alexis Duchateau, a French apothecary wore

hippopotamus ivory dentures that had become stained due to occupational repercussions.

•He succeeded in fabricating a denture made of porcelain in one piece, both the base and the teeth, after partnering up with Paris Dentist, Dubois de Chémant.

•Duchateau’ s efforts were acknowledged by the Royal Academy of Surgeons, Paris, in 1776, despite the fact that he was never able to fabricate a similar denture for anyone else.

Complete Denture Porcelain Bases•Meanwhile, Dubois de Chémant perfected the

porcelain technique.

•By 1804, he claimed that 12,000 of his sets were in use, denoting a wide patient acceptability of his methods.

•However, his methods were not reproducible by other dentists, given the unpredictable shrinkage of porcelain during firing.

•Porcelain was however found to be ideal for making individual teeth separate from the base (Ash1920, Bremner1954).

Complete Denture Vulcanite Bases•By 1814, porcelain bases had fallen out of

favour (Bremner1954).

•Greater accuracy could be achieved by hand carving of ivory.

•Vulcanite was discovered by Charles Goodyear in 1839 and he was granted a patent for using it as a denture base material in 1855 (Lufkin1948).

•Ivory bases however still continued to be favoured even after the discovery of vulcanite.

Complete Denture Vulcanite Bases•Vulcanite is the product of vulcanization of

rubber to yield very hard rubber.

•The occurring reaction is basically addition of sulfur at the double bonds, forming intramoleculer ring structures, so a large portion of the sulfur is highly cross-linked in the form of intramoleculer addition.

•High sulphur content up to 40% may be used for greatest resistance to swelling and minimal dielectric loss.

Complete Denture Vulcanite Bases•Vulcanite had advantages over all other materials

then available despite its largest drawback of poor taste and smell because of its sulphur component.

•However, it was cheap and could easily be adapted to a cast of the mouth.

•It could accurately reproduce the dimensions and surface details of the cast when cured.

•Ash (1920) reports that after 1868, vulcanite dentures became the order of the day and a property of the multitude.

Complete Denture Vulcanite Bases•Vulcanite gained popularity when plaster became

available as an impression material in the mid 19th century.

•Up to then, only wax had been used.

•Plaster now offered a reliable impression technique, vulcanite: a cheap and accurate base material, and porcelain: realistic and durable teeth.

•And when in 1844, Wells discovered nitrous oxide as an inhalational anaesthetic, extractions were now more tolerable and patients could seek denture care.

Complete Denture Vulcanite Bases•Vulcanite however, despite being a considerable

improvement from other denture base materials was still not readily available to all.

• It was protected by patents and dentists had to pay royalties to use it, with whoever it was that infringed on the laws, being ruthlessly prosecuted.

•Alternative materials were thus still being sought.

•Tin and aluminium were tried but none superseded the properties of vulcanite, despite the fact that vulcanite was sometimes porous, absorbent, opaque and lifeless in appearance (Miner1973).

Complete Denture Vulcanite Bases•Vulcanite use intensified after 1879, when, on

Easter Sunday, an irate dentist, Dr. Samuel Chalfant DDS, murdered Josiah Bacon, the treasurer of the Goodyear Dental Vulcanite Company, who had been the patent enforcer (Woodforde1968).

•Vulcanite became the standard denture base material after the patent expired.

Complete Denture Vulcanite Bases

Complete Denture Gold Bases•Before the advent of vulcanite, the other

principal material present was gold, which was both intrinsically expensive and demanding of lengthy and skilled technique.

•Richardson described the process of swaging gold dentures and attaching teeth on them.

•It is expected in all likelihood that both gold and ivory dentures were for the very rich.

•Corbett in 1888 pointed out that it took 6 weeks to make ivory denture bases with natural teeth.

Wood and Gutta Percha Denture Bases•Attempts were made to carve cheap dentures

from wood, which was easily worked but was unaesthetic and subject to rapid fouling in the mouth.

•Gutta percha was also tried but was not durable.

Celluloid Denture Bases•Invented in 1868 by Hyatt.

•Employed from about 1890 as a denture base material (Peyton 1975).

•Its colour was better than vulcanite but the need to use camphor as a plasticizer gave it an unpleasant taste and odour.

•It also lacked dimensional stability tending to distort in service, and so whatever popularity it enjoyed was brief.

Celluloid Denture Bases•Celluloid marked the introduction of plastics

whose history dates from 1832, when Braccanot developed Xyloidine from starch cotton and wood fibres (Schwartz 1950).

•Various other modifications of cellulose nitrate were found wanting because of poor dimensional stability.

•Cellulose was however a precursor to inventions of materials better than vulcanite.

Bakelite Denture Bases•A phenol-formaldehyde resin discovered in

1909.

•By 1924, such resins were being produced commercially, and between then and 1939, as many as fifteen products of this kind were introduced in dentistry.

•They were technique sensitive and quality control was difficult, with wide variations in colour, dimensional stability, and strength, dependent on processing conditions (Sweeney 1939).

Methacrylate Denture Bases•In 1901, Rohm produced commercial products

of acrylic acid.

•There is a conflict in dates but consensus seems to suggest the following:

1. 1935: First appearance of acrylics for dental use.

2. 1936: Polymethyl methacrylate (PMMA) in the form of transparent sheet introduced by Rohm and Haas (Schwartz 1950).

3. 1937: Sheet and powder PMMA introduced by Du Pont De Nemours.

Methacrylate Denture Bases•In 1937, Vernonite, a methacrylate acrylic

resin was clinically evaluated by Wright (Peyton 1975), and was found to fulfill virtually all the requirements of an ideal denture base material.

•By 1945, it was estimated that 95% of all dentures were constructed of methacrylate polymers with porcelain teeth (Peyton 1975).

Methacrylate Denture Bases•PMMA denture bases initially presented with

difficulties in accurate processing, encouraging a brief stint of use of metals and “mucostatic techniques” (Bohannan 1954).

•However, PMMA had too many obvious advantages and the early problems were soon overcome.

Positive Traits of PMMA1. It has good stable colour which can be made

to correspond to either the gingiva or the teeth.

2. It is inert and translucent.3. PMMA chemically bonds to teeth that are

very life-like also made of PMMA.4. It is robust.5. Can be simply repaired or added to.6. Is inexpensive.

Methacrylate Denture Bases•Pure poly(methyl methacrylate) is a colorless,

transparent solid.

•To facilitate its use in dental applications, the polymer may be tinted to provide almost any shade and degree of translucency.

•Its colour and optical properties remain stable under normal intraoral conditions, and its physical properties have proven adequate for dental applications.

PMMA Presentation•Poly(methyl methacrylate) denture base material

usually is supplied as a powder-liquid system.

•The liquid contains nonpolymerized methyl methacrylate.

•The powder contains prepolymerized poly(methyl methacrylate) resin in the form of small beads.

•When the liquid and powder are mixed in the proper proportions, a workable mass is formed.

•Subsequently, the material is introduced into a mold cavity of the desired shape and polymerized.

TRIAL DENTURE BASESSynonyms: Record Base, Base Plate

Definition

•A record base or base plate is a temporary form representing the base of a denture.

•It is used in recording maxillo-mandibular relations and in the arrangement of the teeth.

Requirements•Should be rigid. •Should be accurate. •Should be dimensionally stable. •The borders should be round and smooth

as the borders of finished dentures. •Should be thin at the crest, labial and

buccal slopes to provide space for tooth arrangement.

TypesTEMPORARY RECORD BASES•They are discarded and replaced by denture

base material, once their role in establishing jaw relation, teeth arrangement and try in is complete.

PERMANENT RECORD BASES•They are not discarded and become part of the

actual base of the finished complete denture.

Types of Temporary Record Bases•Shellac base plate •Reinforced shellac base plate •Cold cure acrylic resin •Visible light cure acrylic resin •Vacuum formed vinyl and polystyrene •Base plate wax

Types of Permanent Record Bases•Heat cure acrylic resin •Gold •Chromium-cobalt alloy •Chromium-nickel alloy

COLD CURE ACRYLIC RESINS•Heat and microwave energy may be used to

induce denture base polymerization.

•The application of thermal energy leads to decomposition of benzoyl peroxide, and the production of free radicals.

•The free radicals formed as a result of this process initiate polymerization.

•Chemical activators also may be used to induce denture base polymerization.

Composition of Cold Cured PMMA• Chemical activation is completed at room temperature.

• They are also referred to as cold-curing, self-curing, or autopolymerizing resins because chemical activation is accomplished through the addition of a tertiary amine, such as dimethyl-para-toluidine, to the denture base liquid (i.e.,monomer).

• Upon mixing powder and liquid components, the tertiary amine causes decomposition of benzoyl peroxide.

• Consequently, free radicals are produced and polymerization is initiated.

• Polymerization progresses in a manner similar to that described for heat-activated systems.

Cold Cure Acrylic ResinsPOSITIVE ATTRIBUTES•Good strength (no need for reinforcement ) as

compared to materials like shellac and baseplate wax.

•Display slightly less shrinkage than heat-activated resin, therefore have greater dimensional

Cold Cure Acrylic ResinsDRAWBACKS• The degree of polymerization achieved using

chemically activated resins is not as complete as that achieved using heat-activated systems.

• Thus, there is a greater amount of unreacted monomer in chemically cured denture bases.

• This unreacted monomer acts as a plasticizer that results in decreased transverse strength of the denture resin.

• The residual monomer also serves as a potential tissue irritant, thereby compromising the biocompatibility of the denture base.

Fabrication of Cold Cure Acrylic Bases

METHODS•Sprinkle on method •Finger adapted dough method •Flasking method •Stone mold method •Wax-confined method

Sprinkle-on Method

Finger adapted dough Method

Flasking Method

•A wax pattern is constructed over the cast with a thickness and contour desired for complete denture.

•The cast and pattern are flasked, the wax is eliminated, acrylic resin is packed and allowed to set under the pressure of clamps or press.

•The processed base is removed from the cast, smoothed, and polished at the borders.

Visible Light Cured Acrylic Resin

•This material is a composite having a matrix of urethane dimethacrylate, microfine silica, and high molecular weight acrylic resin monomers. Acrylic resin beads are included as organic filler.

•Visible light is the activator, whereas camphorquinone serves as the initiator for polymerization.

Visible Light Cured Acrylic Resin

Visible Light Cured Acrylic Resin

The single-component denture base resin is supplied in sheet and rope forms and is packed in light-proof pouches to prevent inadvertent polymerization.

Vacuum-formed Vinyl and Polystyrene

Any undercuts are relieved, and separating medium is applied to the cast. A sheet of base plate material placed over the cast and inserted in the vacuum chamber. Electric heater switched to heat the sheet. Turn on the vacuum. The sheet will adapt closely to the cast. Switch of the heater and allow the record base to cool. Remove the record base and cut the excess material.

Vacuum-formed Vinyl and Polystyrene Advantages •Easy to fabricate •Uniform thickness •Accurate adaptation to the master cast •Good rigidity

Disadvantages •Expensive •Difficult to form smooth rounded borders

HEAT-PROCESSED DENTURE BASE RESINS

•Heat-activated materials are used in the fabrication of nearly all denture bases.

•The thermal energy required for polymerization of such materials may be provided using a water bath or microwave oven.

•Available information indicates that the physical properties and fit of microwave resins are comparable to those of conventional resins, despite their fast speed of polymerization.

Heat Processed Acrylic Resin

Composition of Heat Cured PMMAPOWDER•Consists of prepolymerized spheres of

poly(methyl methacrylate) and a small amount of benzoyl peroxide.

•The benzoyl peroxide is responsible for starting the polymerization process and is termed the initiator.

Composition of Heat Cured PMMALIQUID• The liquid is predominantly nonpolymerized methyl

methacrylate with small amounts of hydroquinone.

• Hydroquinone is added as an inhibitor and it prevents undesirable polymerization or “setting” of the liquid during storage.

• A cross-linking agent also may be added to the liquid.

• Glycol dimethacrylate is commonly used as a cross-linking agent in PMMA denture base resins.

• Glycol dimethacrylate is chemically and structurally similar to methyl methacrylate and therefore may be incorporated into growing polymer chains.

Composition of Heat Cured PMMALIQUID• The liquid is predominantly nonpolymerized methyl

methacrylate with small amounts of hydroquinone.

• Hydroquinone is added as an inhibitor and it prevents undesirable polymerization or “setting” of the liquid during storage.

• A cross-linking agent also may be added to the liquid.

• Glycol dimethacrylate is commonly used as a cross-linking agent in PMMA denture base resins.

• Glycol dimethacrylate is chemically and structurally similar to methyl methacrylate and therefore may be incorporated into growing polymer chains.

Polymerization Cycle•One technique involves processing the denture

base resin in a constant temperature water bath at 74˚ C (165˚ F) for 8 hr or longer, with no terminal boiling treatment.

•A second technique is consists of processing in a 74˚ C water bath for 8 hr and then increasing the temperature to 100˚ C for 1 hr.

• A third technique involves processing the resin at 74˚ C for approximately 2 hr and increasing the temperature of the water bath to 100˚ C and processing for 1 hr.

Polymerization Cycle•Polymerization is an exothermic process.

•If the temperature of the resin exceeds the boiling point of unreacted monomer and/or low molecular weight polymer(s), these components may boil.

•Clinically, boiling yields porosity within the completed denture base.

•Such porosity usually will not be seen at the surface of the denture base.

Polymerization Cycle• The heat generated as a result of polymerization is

conducted away from the surface of the resin and into the surrounding dental stone.

• Consequently, heat is dissipated, and the surface temperature of the resin does not reach the boiling point of the monomer.

• Because resin is an extremely poor thermal conductor, heat generated in a thick segment of resin cannot be dissipated.

• As a result, the peak temperature of this resin may rise well above the boiling point of monomer.

• This causes boiling of unreacted monomer and produces porosity within the processed denture base.

Polymerization Cycle• Following completion of the chosen polymerization

cycle, the denture flask should be cooled slowly to room temperature.

• Rapid cooling may result in warping of the denture base because of differences in thermal contraction of resin and investing stone.

• The flask is removed from the water bath, bench cooled for 30 min, then immersed in cool tap water for 15 min.

• The denture base is then deflasked, prepared for delivery, and stored under water to decrease the probability of unfavourable dimensional changes.

References1. Philips’ Science of Dental Materials by

Kenneth J. Anusavice, 11th Edition, 2003,Elsevier

2. Murray MD, Darvel BW; The Evolution of the Complete Denture Base. Theories of Complete Denture Retention – A Review. Part 1 Aust Dent J 1993 Jun;38(3):216-9