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Biocompatible denture polymers--a review.


Several difficulties exist in producing a satisfactory denture material or designing a technique that is useful for its application. Conditions in oral cavity seem almost suited to annihilation. Biting stresses on dentures can be extremely high, temperatures may fluctuate between 25[degrees]C to 45[degrees]C [1] and pH may change instantaneously from acidic to alkaline. The warm and moist oral environment, which is also enzyme and bacteria rich, is conducive to further decay. The soft tissues and structures in contact with the denture polymers may be injured from the toxic leaching or breakdown of the material.

The very basic, initial denture material that was used with much success was vulcanite [2-3]. Vulcanite is an unsaturated polymer of isoprene, impregnated with 32% sulphur and used to be supplied as plastic sheets. The sheets were cut and packed in the denture mold space and polymerized under high heat (168[degrees]C) and pressure (620 KN/[m.sup.2]). This material could not sustain for long because of its aesthetic and dimensional change issues and lasted only for around 8 to 10 decades before getting replaced by newer resins (PMMA).

Towards developing a successful denture material, it is of prime importance to possess information on the chemical, physical and mechanical properties of the material together with the bacteriological, physiological and pathological responses of the material, which cannot be divorced from the former.

Polymers used in today's dentistry, commonly known as "Dental Resins", are used in all specialties of the profession, whether restorative (involving restoration of diseased teeth or structures), prosthodontics (involving dentures) or surgical (involving stents and supports). Polymers are, thus, an inevitable part of this modern vocation.

This article reviews various denture polymers for their properties and provides suggestions on the development of newer polymers for future clinical use. The first part of the article presents the general requirements for prosthetic polymers, with later emphasis on the individual polymers and structures used in modern dentistry. Materials have been classified according to their chemical structure in place of their dental applications, in order to make the correlation between structure, properties and uses, more wholesome.



General Dental Applications

Dentures (bases, liners, tissue conditioners, artifical teeth etc), Cavity Restorative Materials (composites- self cure/ light cure), Sealants (pulpal, cavity and margin sealants), Impression Materials (alginate, agar, elastomers, waxes etc), Cements (resin based cements), Others (gloves, rubber dam, mixing bowls, plastic spatulas etc)

Denture Applications

Complete and partial removable dentures, Denture liners, Tissue conditioners, Oral and maxillofacial appliances-cleft palate plates, maxillary supports etc

Orthodontic appliances--Habit breaking appliances (nail biting, thumb sucking etc)

Requirements for a denture polymer

Physical properties

A denture polymer should possess adequate resilience and strength to biting, chewing, impact forces and excessive wear under mastication. It should be stable under all conditions of service, including thermal and loading shocks [4-5]. It should also have reasonable specific gravity for certain special applications, making it lighter in weight.

Mechanical properties increase considerably with an increase in number average molecular weight, [bar.Mn], becoming independent as approaches infinity [6-7].

Aesthetic properties

The resin should exhibit sufficient translucency and transparency (hue, chroma and value) to match the adjacent structures and tissues [4-5]. It should be capable of being pigmented or tinted to camouflage the surroundings. Once fabricated, it should maintain the appearance and color and not change subsequently.

Chemical stability

The biomaterial should be chemically stable and not deteriorate inside the oral cavity by inducing some chemical reaction or an adverse event. It should preferably polymerize to completion, without leaching any residual monomers [4-5].

Rheometric properties

The flow behavior in polymers involves elastic and plastic deformation (viscous flow) and elastic recovery when stresses are released, as reported by Nielsen [8]. Molecular weight, chain length, number of cross linkages, temperature and applied force greatly determine the typical behavior. Plastic flow is irreversible and causes permanent polymer deformation, compared to elastic recovery in certain polymers, when applied stress is removed.

Biopolymers exhibit complex combined elastic-plastic deformation called as visco-elastic recovery.

Thermal properties

Polymers normally show a large variation in their properties with temperature. At sufficiently low temperatures, amorphous polymers are hard and glass-like, compared to softer and more flexible, when a critical temperature is reached-usually the glass transition temperature (Tg).

The Tg of a plastic is one of the very important set points in determining whether the polymer is thermosetting or thermoplastic and hence our desired clinical properties are affected.

Tg varies with molecular weight, as described by Fox and Flory [6-7] and thus modifies the material properties.

Tg = [Tg.sup.a] - K/

where, [Tg.sup.a] is the glass transition temperature of the polymer with infinite molecular weight. This suggests that Tg becomes independent of molecular weight at high values of.

Biocompatibilty (Biological-stability)

The biological compatibility of a material is a complex phenomenon, involving interactions from biology, patient risks, trials, clinical experience and engineering expertise. Though ignored for several years, it is the fundamental requirement for any biological material today.

Biocompatibility is the ability of a polymer material or a device to remain biologically inert during its functional period [9]. Toxicity is usually manifested by the release of several chemical constituents from the material (as shown in Table 1), which induces an allergic response in terms of localized or generalized stomatitis/ dermatitis, severe toxicological reactions or carcinogenic/mutagenic effects.

The dental resins should be non-toxic, nonirritating and otherwise non-detrimental to oral tissues. To fulfill these requirements, they should be preferably insoluble in saliva and all other body fluids. They should not become insanitary or disagreeable in taste, odor or smell and should be highly stable.

In general, no single test is used to evaluate the biological efficacy of a material. Several invitro, animal and usage tests are required, complementing one another in the overall testing scheme. However, ANSI/ADA approved document No. 41 recommended standard practices for evaluation of dental materials [10-11] and ISO standard 10993 [12] have been followed across the globe to standardize the biocompatible testing scheme. Several other standards, complementing for the safe application of the biopolymer are EN ISO 4049:Dentistry-polymer filling, restorative and luting materials; EN ISO 10477:Dentistry-polymer based crown and bridge materials; EN ISO 1567: Dentistry-Denture base polymers and EN ISO 6847:Dentistry-resin based pit and fissure sealants.

Denture Materials

Acrylic resins- Poly methyl methacrylates (PMMA)

Acrylic resins are the most commonly used polymeric materials in denture dentistry, majority of which comprise the Poly methyl methacrylate (PMMA). These are used in individual impression trays and orthodontic devices, in addition to dentures and artificial crowns. PMMA is well known for its property of being a bone cement (for fixing hip implants) and of its use in making acrylic glass (as a base for artificial fingernails and varnish).

According to their mode of chemical reaction (free radical generation), they are classified as heat curing, chemical (auto) curing, light curing or microwave curing. Methylesters of methacrylic acid are the basic constituents of PMMA but several additions are made as per their properties and applications to get the desired characteristics.

The polymerization mechanism involving conversion of monomers to polymer, in both the heat and auto curing reactions has been discussed in detail by Trommsdorf et al. [13] and Phillips et al. [4] in 1973. The composition and properties of heat and auto curing resins has been summarized in Table 2 & 3 respectively.

Decomposition of the initiator (primarily dibenzyl peroxide) into free radicals under heat, initiates chain propagation for a heat activated reaction [14-15].

Polymerization of chemically curing acrylics is triggered via a redox reaction occuring at the oral temperature, mainly under the influence of an accelerator-primary amine, sulfinic acid or substituted barbituric acid, comprising the amine-peroxide redox system [14-15].

Light curing and microwave curing acrylics are derived partly from the PMMA and partly from urethane dimethacrylate (UDMA) [16] and ethylene glycol dimethacrylate (EGDMA) [17].

Two important considerations for biopolymers are the monomer to polymer conversion and residual monomer content because of their application in approximation to oral tissues. Heat polymerized poly methyl meth acrylate and thicker areas of the denture show significantly fewer residual monomers [18-19]. Vallitu et al. [14] demonstrated that when polymerization was done at a higher temperature for a longer period of time, the residual leachable monomer content decreased from 1 wt% to less than 0.1 wt%. It has been documented in literature that further application of a light polymerizable resin over the surface, additionally reduces the leachable monomer content. Several studies have been conducted using advanced diagnostic tools like HPLC, GC-MS and IR [20-24] to identify the components that leach during the polymerization process.

Baker et al demonstrated 45 mg/ml (ml of saliva) release of MMA in saliva of patients with inserted dentures over a week, compared to dentures polymerized at 70[degrees]C for 3hrs with no MMA leach [24]. In-vitro studies have however shown that there is a release of PMMA from heat polymerized resin, though in significantly smaller quantities [25-26].



Furthermore, formaldehyde release from the heat polymerized resin is relatively high in-vitro and in-vivo, in comparison to the microwave and light polymerized specimens. Two mechanisms of formaldehyde release, as suggested, include oxidation of methacrylate group and finally copolymerization of methacrylate with oxygen, followed by final decomposition to formaldehyde resin [27-28].

Lygre et al. reported the release of certain other constituents like phenylsalicylate, biphenyl and phenyl benzoate [23].



Studies from the literature reveal that [LD.sub.50] of MMA in rats is 9 g/Kg of body mass [29-30]. This high concentration indicates a very low and acute systemic toxicity of MMA. There have not been any reported effects on alteration of organs like kidneys, spleen, pancreas, lungs and gut [30]. The half life of MMA is 20-40 minutes [31] and if injected into blood serum, is rapidly metabolized into pyruvate through the Kreb's energy transport process (Citric acid cycle) [32].

Nakamura and Kawahara [33] studied the cytotoxic effects of two weeks old aqueous extracts from two heat polymerized and three chemically cured polymers. There were no cellular alterations in the extract and the toxic activity was found to diminish with aging.

Cimpan et al. [34] also concluded similar results with emphasis on the fact that auto polymerizing resins were more toxic than heat polymerizing ones.

In addition to the toxic leach, microorganisms further complicate the situation. It has been observed that dentures with soft liners [35], that leach MMA and formaldehyde, synergistically promote microbial proliferation and cause stomatitis [35]. Several forms of allergies including type IV hypersensitivity, urticaria, allergic stomatitis, dermatitis and psoriasis have been reported in literature from various different polymer components [36-37].

Some of the older literature reports show the formation of fibrous malignancies after subcutaneous implantation of PMMA [38] but nothing has been confirmed till date.

Poly ethyl methacrylate (PEMA) & Poly butyl methacrylate (PBMA)

In respect to the diverse applications of polymers in denture science, higher molecular weight methacrylate polymers such as polyethylmethacrylate (PEMA) and polybutylmethacrylate (PBMA) have found uses as soft denture liners.

PEMA with Tg of 65[degrees]C and PBMA with Tg of 30[degrees]C are both soft at oral temperatures, with the addition of sufficient quantities of plasticizer (commonly butylphthalyl butylglycolate) [40]. These soft liners, made from highly plasticized acrylic processes under normal pressure molding techniques, remain permanently soft until the plasticizer is lost through leaching following which the material becomes rigid [40].

Bis-gma resins

Modification in the basic acrylic structure can produce resins with widely diverging properties.

Bowen et al. [41], in 1963, modified the existing acrylic structure, by combining one part of bisphenol A (Figure 1) with two parts of glycidyl methacrylate (Figure 2), in the presence of 0.5% N,N-dimethyl-p-toluidene, at 60[degrees]C, to form the rigid polymer known as Bowen's resin. Several additions were made to the final mixture to get a reasonable working viscosity with better properties [42]. Properties of BIS-GMA reinforced resin have been compared to PMMA acrylic resins in Table 4.

Copolymers of methylmethacrylate and bis phenol A dimethacrylate have been reported in literature for high impact strength crown and bridge usage [43].

Hydron (Hydrogels)

Sprinel et al. [44] contributed several reports to the development of soft tissue conditioners through hydrophilic and biocompatible polymers, primarily poly N-substituted methacrylamides. These biopolymers retain 35-90% water and act as cushions and conditioners for the oral tissues under masticatory stress. The primary monomer used is 2-hydroxy ethylmethacrylate (HEMA), cross linked with ethylene glycol dimethacrylate (EGDMA), to give a three dimensional swollen hydrogel. Figures 3 and 4 show the chemical structures of HEMA and EGDMA.


These are usually formed by cross linking bisphenol A with a di-substituted ketone group. They can be used for denture base construction, owing to their high impact strength, compared to the traditional acrylic resins.

Due to their difficult manipulation and processing under high pressure and temperature, they are not routinely used.

Future polymers

Polymerization is accompanied by the evolution of heat and volume contraction of the mixture, both of which affect the final outcome of the procedure. Several procedures [45] are now being considered, towards developing newer denture polymers with improved strengths and properties, by modifying the primary polymer matrix with several additions. Different fillers (glass, silica, borosilicates and fused quartz), binders and processing techniques (rods, fibers and matte) are being experimented to develop the ideal denture polymer, suiting our clinical requirements [46].

Several technological solutions have been proposed and are under testing. Polyethylene woven fibers, braided fibers and unidirectional fibers [47-51] are under experimentation. Fiberflex based on Kevlar, developed by Dupont, is under consideration, as a unidirectional fiber reinforced denture material [46]. Glass woven and braided fibers are also promoted for indirect system of fabrication by Swiss dental laboratories [52]. Fiberkor (glass unidirectional fibers) and Vectris (glass unidirectional fibers in mesh) are also under trial as future denture substitutes [53-58].


This paper focuses on the properties and uses of polymers that are being used and are under trial as denture polymers. PMMA resin continues to be the universal versatile polymer in denture dentistry. Depending upon the type of polymerization, PMMA resins may leach 0.1-5% of the residual monomer and additives, mainly MMA and formaldehyde, contributing to localized allergic reactions. Studies have reported a possible carcinogenic and embryotoxic potency of MMA. Though, severe complications have been reported rarely in literature, yet the frequency of allergic reactions has been observed to increase with the increased use of higher amounts of PMMA resins. There is a need for the development of newer high strength, radio-opaque acrylic denture materials employing polymer blends, heavy metals and block co-polymers. There is a need for the development of non-leachable plasticizer or plasticizer free soft denture liners that could retain their softness permanently.

Received 10 April 2009, Accepted 13 June 2009, Published online 27 December 2009


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Rahul Bhola *, Shaily M. Bhola, Hongjun Liang, Brajendra Mishra

Department of Metallurgical &Materials Engineering, Colorado School of Mines, Golden, CO 80401

* corresponding author:
Table 1: Percolating agents and ingredients of PMMA [15,19,39]

Function                       Substance

                               Methyl methacrylate (MMA),
Monomer                        Ethylene glycoldimethacrylate (EGDMA)
Degradation products of MMA    Methacrylic acids
Oxidation products of MMA      Formaldehyde
Stabilizer                     Hydroquinone, Resorcinol, Pyrogallol
Accelerator of auto            N,N-dimethyl-p-toluidine
  polymerizing resins          (tertiary amine)
                                Poly (ethyl methacrylate),
Matrix monomer of light
  curing resins                Ethoxylized bisphenol-A-dimethacrylate
Matrix monomer of light
  curing resins & microwave    Urethane dimethacrylate (UDMA)
  curing resins
Initiator                      Dibenzoyl Peroxide (DBP),
Reaction products of DBP       Biphenyl, Phenyl benzoate, Benzoic acid
Photoinitiator of light        Camphorquinone
  curing polymers
Plasticizers                   Dibutly phthalate, Dicyclohexyl
UV absorbers                   Phenyl salicylate
Coloring agents & fillers      CdS, CdSe, Inorganic fillers, Cu,

Table 2: Comparing compositions of heat activated & chemically
(auto) activated PMMA resins [4,8]

Heat activated PMMA resin
(Two component system)

Powder system

Poly (methyl             Main constituent

Benzoyl peroxide         Initiator

Mercuric Sulphide,       Dyes
Cadmium Sulphide

Zinc oxide, Titanium     Opacifiers

Dibutyl phthalate        Plasticizer

Dyed particles-          For aesthetics
glasses, beads

Liquid System

Methyl methacrylate      Plasticizes Polymer

Dibutyl phthalate        Plasticizer

Glycol dimethacrylate    Cross-linking agent

Hydroquinone             Inhibitor

Chemically (auto) activated PMMA resin
(Two component system)

Powder System

Poly (methyl             Main constituent

Benzoyl peroxide         Initiator

Mercuric Sulphide,       Dyes
Cadmium Sulphide

Zinc oxide, Titanium     Opacifiers

Dibutyl phthalate        Plasticizer

Dyed particles-glasses,  For aesthetics

Liquid System

Methyl methacrylate      Plasticizes

Dibutyl phthalate        Plasticizer

Glycol dimethacrylate    Cross-linking
(1-2%)                   agent

Hydroquinone             Inhibitor
Dimethyl-p-toludiene     Activator

Table 3: Comparison of properties of heat and chemically (auto)
cured PMMA resins [4,8]

Heat Activated Acrylic Resins          Chemically (auto) activated
                                       acrylic resin
High molecular weight                  Comparatively lower molecular
Heat is necessary for polymerization   Heat is not the primary source
                                       for polymerisation
Porsosity of the cured resin is less   Porosity in the cured resin is
                                       much gretaer
Stronger cured material                Strength is less compared to
                                       heat cured resins
Lower residual monomer content         Higher residual monomer content
Shows less distortion, creep and       Comparatively higher distortion,
  initial deformation                  increased creep, slow recovery,
                                       and greater distortion.

Table 4: Comparison of properties of BIS-GMA
reinforced resin with PMMA acrylic resin [41]

Properties               BIS-GMA         Acrylic
                         Resin           resin

Volume shrinkage (%)     2.7             6.2
Compressive Strength     110-160         75
Tensile Strength         28              28
Modulus of elasticity    11200           1800
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Author:Bhola, Rahul; Bhola, Shaily M.; Liang, Hongjun; Mishra, Brajendra
Publication:Trends in Biomaterials and Artificial Organs
Article Type:Report
Geographic Code:9INDI
Date:Jan 1, 2010
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