History of denture base materials
1.1 History of denture base materials
Wooden bases have been fabricated and used by Japanese in 16th, 17th and 18th centuries in an attempt to replace the missing teeth and fill the edentulous spaces. Human teeth and teeth carved from either wood, animal bones or marble were attached to those carved wooden dentures in that time. [1]
Gold, ivory and porcelain were the materials of denture bases in Europe in about the mid of 18th century. [1] Pieces of ivory were held together and attached to teeth by silk or springs. Complications such as discomfort, plaque accumulation and stain retention pushed the developers in that time to develop porcelain and use it instead of Ivory. They called it "mineral paste dentures". The dissatisfaction from porcelain was because of the fracture, roughness of the occlusal surfaces over time and the difficulties of its fabrication. Both dentists and patients shared their dissatisfaction with ivory and porcelain but were not until the end of 18th century when the alternative became available.
1.2 Prosthetic applications of denture base materials:
1.2.1 Vulcanite:
The introduction of vulcanite, hard rubber was in 1839 when Nelson Goodyear discovered the vulcanisation of rubber. Vulcanisation is a term used to describe the cross linking in rubbers. It was the first material which could be accurately moulded into a cast of the denture bearing area. [2]. Fillers were added to improve the mechanical and cosmetic properties of vulcanite. [3] However, it had some limitations such as porosity and absorption of saliva which enhanced the bacterial growth and proliferation. It was dimensionally unstable due to thermal expansion during heating in the vulcanizer and contraction during vulcanisation. There was also no chemical bond with denture teeth and some difficulties in cleaning. It was not acceptable aesthetically due to its colour which doesn't resemble the natural colour of human tissue. [4]
1.2.2 Phenol formaldehyde:
In 1909, Baekeland developed a thermosetting denture base resins constructed from phenol - formaldehyde and subsequently, became known as Bakelite. It was a result of condensation polymerisation reaction which was accompanied by liberation of water. Its application as denture bases was superior to that of vulcanite because it was semi translucent. However, it was short lived because it was quite brittle material, dimensionally unstable, short shelf life, difficult to repair and difficult to process under dental conditions.
1.2.3 Celluloid:
This material was made from cellulose nitrate and plasticised camphor and it was the first thermoplastic material used for denture base construction. It was found to be superior to vulcanite in both strength and aesthetic. However, it had some disadvantages as it soon discoloured, didn't adapt well, exhibited high water absorption, high degree of elastic recovery, difficult to repair and patients often complained of constant smell and taste of camphor. [4]
1.2.4 Vinyl resins:
They are one of the families of glass like plastics which include vinyl, styrol products, esters of acrylic acid and ketone condensation resins. [5]
Vinyl polymers of dental interest are derivatives of ethylene C2H4 monomer. For example:
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H2C = CH - C = O |
|
|
|
H CH3 |
|
|
|
H CH3 |
The copolymers of vinyl acetate (PVA) and vinyl chloride (PVC) were used as denture base materials. As a result of moulding, many permanent stresses and strains were included into the material causing fractures in the denture. [5]
1.2.5 Metals:
Magnesium and aluminium were used in 1960's as metal denture base materials. [6, 7] They fit accurately in mouth. However, base metal alloys such as aluminium are not very biocompatible. Titanium is an exception from the base metal alloys in term of biocompatibility. Precious metal alloys were also used but they are expensive. Furthermore, metal base dentures are difficult to rebase and don't have good cosmetic properties.
The development of denture base materials after 1960's, has yielded better acrylic resins in terms of the mechanical properties and treating methods of material. To date, PMMA continues to be the material of choice for denture base materials
1.3 Classification of polymers:
Polymers may be classified on the basis of either of the following: origin, thermal behaviour and structural configuration (spatial arrangement).
1.3.1 Classification on the basis of their origin:
1.3.1.1 Natural polymers:
Those polymers which occur naturally such as:
- Proteins (polyamides of polypeptides
- Polyisoprenes, e.g. rubber and gutta percha.
- Polysaccharides, e.g. starch, cellulose, agar and alginates.
1.3.1.2 Synthetic polymers:
Synthetic polymers are produced industrially or in the laboratory by chemical reactions. Dental examples include acrylic resin and elastomeric impression materials which were developed in the early twentieth century in an attempt to improve materials whose properties duplicate or at least simulate those of natural rubber. [5]
1.3.2 Classification on the basis of structural or spatial configuration into:
1.3.2.1 Linear:
In linear polymers the structure units are connected to one another in linear sequence.
1.3.2.2 Branched:
In which side branch chains are connected to the main ones.
1.3.2.3 Cross linked polymers:
In which the adjacent linear chains are joined one to another at various positions by covalent bonds.
1.3.3 Classification on the basis of their thermal behaviour:
1.3.3.1 Thermoplastic resin:
They are polymers which can be shaped by heat and after cooling they maintain their shape.
The polymer chains in thermoplastics are bonded to each other by secondary bonds which make them relatively soft and their mechanical properties are sensitive to heat. E.g. Non cross linked polymethyl methacrylate.
1.3.3.2 Thermosetting polymers (thermoset):
They are formed into a permanent shape and set by a chemical reaction. They can not be remelted and reformed into another shape but degrade or decompose upon being heated to high temperature.
The polymer chains in thermoset polymers forms network, with cross links between them (primary covalent bonds). E.g. cross linked polymethyl methacrylate.
1.4 Preparation of polymers:
Polymers are prepared by a process called polymerisation which is the chemical reaction by which monomer units become chemically linked and form polymer.
1.4.1 Types of Polymerization:
Polymerisation can occur either by condensation which is a series of localized reactions often with by product such as halogen, water or ammonia or by addition which is a simple addition reaction. [5]
1.4.1.1 Condensation polymerisation:
Polysulphide rubber impression material is an example of condensation polymerisation reaction used in dentistry. The low molecular weight polysulphide paste is converted to a high molecular weight material by a condensation reaction. Water and lead sulphide are by products of this reaction.
1.4.1.2 Addition polymerisation:
Polyethylene, poly acrylic acid, poly meth acrylic acid and poly methyl meth acrylic acid are all examples of addition polymerisation reaction used in dentistry. Addition polymerisation results in the formation of large molecules "polymers" without the formation of by products. The structure of the monomer is repeated many times in the polymer.
Important addition polymerisation reactions are free radical, ring opening and ionic reactions.
1.5 The stages of polymerisation reaction of acrylic resins are:
1.5.1 Induction:
Free radicals are necessary to start the reaction which can be generated by activation of monomer molecules (Benzoyl peroxide) by either:
- I. Physical means
- Heat.
- Light: This may be ultraviolet, visible light or laser.
- II. Chemical:
Where chemical accelerator such as tertiary amine with organic peroxide are used to produce free radicals at room temperature.
Activation of initiator takes place by any of the mentioned methods leading to the decomposition of the initiator to give up free radicals. The free radical encounters a monomer and transfers it into an activated monomer and the polymerisation reaction is initiated.C6H5COO•→ C6H5 + CO2
1.5.2 Propagation:
In this stage the activated monomer which is considered the free radical is rapidly added to other monomer molecules and the free radical is shifted to the end of the growing chain.
Once the chain growth has started, the polymerisation reaction continues with considerable velocity. Theoretically, the chain reactions should continue with heat evolution, until all the monomer has been converted to polymer. However the polymerisation reaction is never complete. [5]
R• + M → R M•, and R M• + M → R M M• and so on.
Where R• represents a free radical and M a molecule of monomer.
1.5.3 Termination:
The chain reactions can be terminated either by
- Direct coupling of two activated chains, to form one longer chain.
- Transfer of a hydrogen atom from growing chain to another.
IIMm • + IIMn • → IIMmMnII
Where IIMm represent a polymer of m monomer units and IIMn represent of a polymer of n monomer units.
1.6 Factors affecting properties of polymers.
1.6.1 Molecular weight and degree of polymerisation:
The molecular weight of polymer molecule equals the molecular weight of the various mers multiplied by the number of the mers and may range from thousands to millions of M.W units, depending on the preparation conditions.
In general those polymers made up of large and longer molecules are stronger and more resistant to thermal and mechanical stresses than those composed of small molecules. It should be also noted that the polymer chains grow in random manner in which some chains may grow faster than the others and some are terminated before the others. Thus, not all the chains within the polymer have the same length. Each chain has its own molecular weight.
1.6.2 Cross linking:
Cross linking occurs where adjacent linear chains are joined together at various positions by chemical bonds (covalent bonds). This process is achieved either by non reversible chemical reaction or by the addition of cross linking agent. It was found
1.6.3 Co polymerization:
Copolymers are polymer chains containing two or more different types of monomeric units.
Copolymers are of three types: random, block and graft.
Random type is when the different monomer units are randomly distributed along the chain, such as:
… M - M - M - Y - M - Y - M - M - Y - Y - M - M …
Block, when the monomer units occur in relatively long sequences along the chain such as:
… M M M …M M Y Y Y … Y Y Y M M - M …
Graft, when the sequence of one of the monomer is grafted onto a backbone of the second monomer such as:
… M - M - M - M M ... M - M - M - M M …
׀ ׀
Y Y
׀ ׀
Y Y
Co polymerisation process was introduced to enhance the physical properties of the material; a customization process involves combining two or more chemically different monomers to form copolymer. [5]
1.6.4 Plasticizers:
They are compounds which are added to partially neutralize the secondary bonds or intermolecular forces that normally prevent the resin molecules from slipping past one another under stresses.
There are mainly two types of plasticizer:
- External plasticizer: in which the plasticizer penetrates between the polymer chains and separates them apart making the lesser forces between them.
- Internal plasticizer: co monomer can be considered as a plasticizer. In this case the plasticizer is a part of polymer main chain.
Plasticizers are often added to resins to reduce their strength and hardness as well as the softening point (glass transition temperature).
This principle is used to produce acrylic soft lining materials.
1.6.5 Spatial structure:
The spatial structure of polymers has an effect on their properties. In general, the cross linked polymers flow at higher temperature than linear or branched polymers. They have lower water sorption and they are stronger, more rigid and more resistant to the action of solvents.
1.6.6 Rate of loading:
Polymers are also sensitive to the rate of loading. They behave in a ductile manner at slow rate of loading and they respond in a brittle manner at high rates of loading.
1.6.7 Temperature:
Their properties are sensitive to temperature. They soften as they are heated near their glass transition temperature.
1.6.8 Addition of fillers:
Addition of fillers to the polymers to form composite structures will improve their properties. E.g. Composite restorative materials.
1.7 Acrylic denture base materials
1.7.1 Heat cured resins:
The loss of rubber supplies to the United Kingdom in 1942 gave the chance to use the acrylic resins which had really begun in 1935 with the introduction of Kallodent, in its injection moulded form, by Imperial Chemical Industries Ltd. Dough moulding technique has been introduced by Kulzer shortly after Kallodent and now the powder/liquid form of the material is being used in dentistry.
Many modifications and methods have been used through out the recent years to improve the physical properties of PMMA. These methods include addition of cross linking agents or fillers in an attempt to over come its strength deficiencies, using different initiator systems such as heat and chemical or visible light polymerisation or using alternative processing methods such as compression, injection and fluid resin/pour) [8]
Nylon and Styrene are two different materials which were used in dentistry for construction on denture base materials. Nylon was developed as a result of the classic research by Carruthers and associates of the Du Pont Chemical Co. [9] Hargreaves then evaluated a different polymer of nylon and developed guidelines for optimum properties of nylon fabricated for dental use [10]
Styrene was also studied to some extent in 1950s but very little research has been conducted on it as denture base material in recent years. [11]
Poly (methyl meth acrylic) (PMMA) is the current material of choice for the construction of denture base materials. [12] However, it is not the ideal material [5] but it keep getting its popularity due to its working characteristics, ease manipulation with minimum expense and equipment, stability, accuracy of fit and acceptable appearance in mouth. The aesthetic can be further improved by incorporating aesthetic fibres to simulate blood vessels in denture base materials. This incorporation of fibres could be used to gain more than aesthetic as what previous studies have concentrated on their reinforcing effect on mechanical properties of denture base resins [13], [14], [15], (Braden et al, 1988), [16] [17]
|
Component |
Quantity (weight %) |
Purpose |
Powder |
PMMA |
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Benzoyl peroxide |
2 3 |
Initiator |
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Dimethyl p toludine |
|
Activator |
|
EGDMA |
1 - 2 |
Cross linking agent |
|
1,4 BDMA |
1 - 2 |
Cross linking agent |
|
Ethylacrylate |
5 |
Copolymer |
|
Dibutylphthalate |
8 - 10 |
Plasticizer |
|
Polybutadiene |
5 - 10 |
Impact strength modifier |
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Cadmium sulphide/sulphate |
<0.02 |
Colour pigment |
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Organic pigments with iron oxide |
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Colour pigment |
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BiCl3, BiBr3, zirconyldimethacrylate |
|
Radiopacity agent |
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Short fibres |
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Reinforcement/simulated arterioles |
Liquid |
MMA |
|
|
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Hydroquinone |
<0.006 |
Inhibitor |
Poly (methyl methacrylate) usually mixed with methyl methacrylate with initiator to form acrylic resin dough. This dough is polymerised in two part metallic flask. Either using a water bath or dry heat oven at 70ºC for up to 7 hours followed by a terminal boil is the optimum resulting in maximum MMA conversion. [19] Dough is polymerised in fibre reinforced plastic flask rather than metallic flask in microwave curing technique.
Powder: liquid ratio:
The proper monomer/polymer ratio is usually 3 to 3.5:1 by volume, or 2.5:1 by weight. Not using the correct ratio while mixing the polymer and monomer will result in variations in the final mass of PMMA such as granular effect with high polymer to monomer ratio and excessive volumetric shrinkage with high monomer to polymer ratio. [5]
Variations in polymer/monomer mixing ratios from 1.5:1 to 4.5:1 by volume have been evaluated to determine the effect on dimensional accuracy and impact resistance of PMMA homo polymer. Two polymerisation cycles were used in the study and it was shown that a wide variation in mixing ratios had no effect on dimensional accuracy or impact resistance of the final resin and the polymerisation cycle that includes a final boiling period is preferred. [20]
1.7.2 Alternative method of PMMA fabrication:
1.7.2.1 Injection moulded cured resins:
The technique of injecting PMMA to fabricate dentures has been known for many years. [21] The introduction of injecting moulded technique gives birth to modified acrylic resin in an attempt to improve not only the physical and chemical properties but also the working properties to aid the laboratory in processing of complete dentures. It allows directional control of the polymerization process through the flask design. A constant flow of new material from the sprue compensates for the polymerization shrinkage. Pryor used a spring mechanism applying continuous hydraulic pressure to reservoir of unpolymerised resin to compensate for the polymerisation shrinkage. Grunewald et al.[11], independently investigated injection moulding and found no significant advantages over conventional packing, Pryor's technique did not gain popularity.
Various injection moulded denture base materials and processing techniques are now available, one of these systems is the SR Ivocap (Ivoclar AG, Schaan, Liechtenstein). It has been first introduced in mid 1970's. Many studies have been carried out on the material and in general, it has been said that the SR Ivocap system has less linear dimensional change compared to conventional PMMA. [22, 23]
Another new injection system is used called Success system produced by DENTSPLY International. It claims to eliminate changes in vertical dimension of occlusion to produce denture base materials that require few, if any, adjustments in the laboratory.
The main advantages of the injection moulded technique over the conventional compression pack technique including less material manipulation, more accurate reproduction of vertical dimension occlusion and more accurate denture fit but it fails to show improvement in porosity, transverse and impact strength. [24]
In a pilot clinical evaluation of injection denture base system, the only difference noted by the examiners between the conventional compression pack denture resin and the injection pack prosthesis was that the fibres tended to be straighter and more ordered in the conventionally packed prosthesis, and more curled and more randomly oriented in the injection processed prosthesis. [25]
1.7.2.2 Microwave cured denture resins:
The use of microwave energy to polymerise denture base materials was first reported by Nishii in 1968[26]. Kimura et a[27]l. reported that it was possible to cure acrylic resin in a very short time using this technique. In 1984 and 1985, the light fiber reinforced plastics (FRP) flask was substituted for the heavy brass flask and compress, and the water bath curing tank gave way to microwave oven. [28], [29]
The main two advantages of microwave over conventional curing technique are, first, the heat is almost equally distributed over the inside and outside of the substance. Second, temperature rises rapidly. However, no significant differences were observed between microwave and conventional water bath curing technique regarding porosity, hardness and transverse strength. [29]
A recent study taken by Azzari MJ et al, 2003[30] concluded that the mechanical properties of the acrylic denture base microwave polymerised depend on both the exposition time and microwave power. The exposition time appears to be the most relevant factor to make compatible mechanical properties and toxicity levels for applications.
1.7.3 Chemically cured denture base resins:
The chemically activated (self curing) materials were first used in Germany during the Second World War (Sweeney, 1958), containing a tertiary amine activator such as dimethyl para toluidene, require less energy but the resultant resin has a relatively high residual monomer content, high creep values, lower strength and stiffness, poorer colour stability, and is not usually free of porosity. (Combe)
1.7.4 Light activated denture base resins:
It has been available commercially for several years. This material has been described as 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, and camphoquinone serves as initiator for polymerisation. The denture base resin is supplied in sheet and rope forms as single component and is packed in light proof pouches to prevent inadvertent polymerisation. [5]
1.8 Physical properties of denture base resins:
1.8.1 Polymerisation shrinkage:
When methyl mathacrylate monomer is polymerised to form poly (methyl methacrylate), the density of the mass changes from 0.94 to 1.19 g/cm³. This change in density results in volumetric shrinkage of 21%. The agreed percentage of volumetric shrinkage which has been observed in laboratory and clinical investigations is 7% for conventional heat activated resin mixed at suggested powder: liquid ratio. [5]
The finished denture may have contracted as much as 0.4% linearly [31]. Linear shrinkage affects the adaptation of the denture and may result in distortion. The greater the linear shrinkage, the greater the discrepancy observed in the initial fit of denture. [5]
Contraction within the conventional compression moulding technique can be compensated or balanced by expansion due to water sorption. [32] Less dimensional change was observed with dentures constructed by using RS (Rafael & Saide) tension system than those processed with conventional packing technique. [33]
Injection moulding technique has been used to overcome the dimensional inaccuracies of conventional compression moulding [21]. However, there is a study concluded that there is no difference in clinical adaptation that could be demonstrated between conventionally packed and injected prosthesis. [25]
1.8.2 Porosity:
The presence of voids in denture base materials may affect adversely the physical and aesthetic properties of denture. [5] It may have effect on the hygienic properties of the denture base materials as porosity is related to microorganism growth. [34]
Contraction, gaseous and granular porosity are three main types of porosity in denture base materials. Contraction porosity caused by either or both insufficient acrylic dough placed in the mould or insufficient pressure applied on the material during curing. It is stated that the resin porosity may be caused by inclusion of air and it is more likely happens with injection moulding system. [35] Porosity of microwave polymerised resin was not different from the injection moulded and conventional heat polymerised resins. [29, 36]
The second cause of voids is the vaporisation of unreacted monomer and low molecular weight polymers when the temperature of resin reaches the boiling point of the species. Granular porosity could be a result of inadequate mixing of powder and liquid components specially, when most of the resin mass contains more monomer that shrinks after polymerisation producing voids. [5]
Controlling temperature accurately and ensuring correct timing is important to minimise porosity and retain a smooth, clean and polishable denture base surface especially when microwave polymerisation is used.
Porosity is likely to develop in thicker portions of denture base and can reduce the frequency and size of the porosity by 30% in both thick microwave and water bath specimen by a longer polymerisation time at lower wattage [29]
In a comparison between two different curing techniques, it has been found that the conventional polymerised denture base resin has less porosity than the injection moulded resin. [24]
1.8.3 Water sorption:
Poly (methyl methacrylate) is like other polymers; absorb water slowly over a period of time. [5] This is because of polar properties of molecules. Water molecules diffuse into denture base resin and take positions in between polymer chains forcing these chains a part. This process causes expansion of polymerised resin and interferes with the entanglement of polymer chains. According to ISO 1567 standard, water sorption of denture base polymer should not exceed 32 μg/mm³ for heat cured or chemical cured materials.
Water sorption affects the flexural strength and thermal modulus of dentures. (Vallittue 1998) Flexural modulus decreased significantly and also the transverse strength decreased.
By adding some rigid rod polymer fillers thewater sorption of denture base polymer decreases due to hydrophobic nature of rigid rod polymer. In a study taken by Vourinen, [37], it has been shown that water sorption was reduced by adding rigid rod polymer fillers to denture base polymer and it had shown decrease in flexural strength and increase in both micro hardness and flexural modulus of denture base polymer.
1.8.4 Solubility:
One of the requirements of an ideal denture base material is that the denture base resin should be biologically acceptable and in order to fulfil this requirement, the denture base resin should be completely insoluble in saliva or in any other fluids taken into the mouth. [5] According to ADA specification no. 12, weight loss of denture base resin must not exceed 0.04 mg/cm².
1.8.5 Crazing:
Small surface flows can be produced by either stress relaxation or by the effect of solvent agent. [5] Crazing caused by solvent action is usually occurs from prolonged contact of denture base resin with liquids such as ethyl alcohol.
Crazing in denture base resins usually caused by tensile stresses which act on the polymer chains at right angels trying to separate these chains apart.
In 1975 Causton studied fracture in denture base materials and found that the denture base materials have lower strength than commercial poly (methyl methacrylate) (Perspex), and also observed the crack propagated through the beads. [38] In 1976, Koblitz reported the proportion of crack propagation around the beads increased relative to the proportion passing directly through the beads with the increase in volume fraction of beads from 60% to 76%.
A study conducted to measure the effect of aesthetic fibres on the flow properties of acrylic resin denture base materials taken by Katsikas NG et al.1994[39] indicates that moulding of acrylic resin denture base materials will not be affected by fibre inclusion up to 0.2%, however, at higher levels, 0.8% to 3% viscosity is increased which will influence flow properties that could, in turn, adversely affect the mould process. Thus, addition care should be taken when moulding acrylic resin denture base materials with high levels of aesthetic fibre content.
1.8.6 Strength:
Composition of denture base resin, method of processing and conditions presented by oral environment are three main factors that govern the strength of denture base resin. Peyton, 1950 [40], reviewed the curing methods and concluded that when all factors were considered it was doubtful that any of these methods had any real advantage over the water bath method and the important factor for consideration was the careful control of the temperature during processing.
In a study taken by Ganzarolli SM et al, 2007[24], conventional polymerised resin presented the lowest transverse and impact values compared to microwave polymerised and injection moulded resin because it didn't contain any cross linking agents in its composition in that study.
It has been reported by Hyden [41] that the transverse strength is related to polymerisation efficacy and subsequent formation of short chains of polymers with low molecular weight.
As the degree of polymerisation increase, the strength of resin also increases. [5] Chemically activated resins generally display lower degree of polymerisation and as a result they exhibit increased levels of residual monomer and decreased strength and stiffness values compared to heat activated resins. [5]
The flexural moduli for both microwave and water bath polymerised resins are higher compared to with that of injection moulded material. [24]
1.8.7 Creep:
Denture resins display visco elastic behaviour. [5] With slow application of stress, material behave like a viscous liquid [42] and deformation will continue with time.
The rate of creep may be elevated by increase of temperature, applied load, residual monomer and presence of plasticizers. [5]
Indentation creep, recovery and cycle loading - deloading were found similar in the resins polymerised either by microwave irradiation or heat (microwave curing cycle of 3 minutes at 500 W was used). [43]