J. Nihon Univ. Sch. Dent., Vol. 35, 161-170, 1993

Review

Dental Composites: A Review

Rozaidah TALIB

(Received 24 August 1992 and accepted 20 April 1993)

Key words: composites, properties, review

Abstract

Dental composite resins are widely used in dental practice and are continually being

developed in order to obtain better products. To gain full benefit from these materials, it is important for the clinician to understand their properties. The following is a review of the more common characteristics of composites in current use.

Overview

Composite resins were originally developed by Dr. Rafael Bowen of the National Bureau of Standards, U.S.A. [1] and introduced commercially to the dental profession nearly two decades ago. In an extremely short period of time they virtually replaced their predecessors, acrylic resin and silicate cement. Silicate cement, although anticariogenic and initially satisfactory esthetical-ly, undergoes rapid acid erosion in the mouth, loss of translucency and surface crazing, and has inadequate mechanical properties. Direct-filling acrylics are not satisfactory because of their

high polymerization shrinkage and a ten-fold disparity between their coefficient of thermal expansion and that of the tooth, resulting in marginal leakage and consequent unsightly stains and secondary caries [2]. The rapid acceptance of composites was due to a number of factors. These included superior esthetics, improved wear resistance, reduced polymerization shrinkage, improved mechanical properties, higher resistance to abrasion, a greater range of application and ease of manipulatio [3]. The polymerization shrinkage of methylmethacrylate is 21% and that of Bis-GMA is 5%, whereas the value is only 2% for composite resins.

In recent years, composite materials have been increasingly applied for a variety of clinical

procedures. At present they are used in some situations which would not have been considered feasible with "plastic" tooth-colored restorative material. Their use in such situations has been made possible, not only by the development of acid-etch bonding techniques developed by Buonocore as early as 1955, but also by apparent improvements in the strength characteristics of the more recently introduced materials. In addition to the restoration of cavities, composite resins have also been used to repair anterior teeth, cement orthodontic brackets, cover staining and erosion, to build up cores for crowns, and as a crown and bridge cement [4]. Numerous manufacturers and some dentists now advocate the use of composite resins for amalgam replace-ment in class I and Il cavity preparations, in addition to their common use in class III and IV

preparations. Three so-called 'posterior composites' have gained full acceptance by the Amer-ican Dental Association. The three-year findings of multicenter clinical trials provide substantial evidence [5] that one of these materials (Occlusin) satisfies the clinical requirements of a posterior composite over a period of three years, and recent abstracts have indicated that the five-year findings are similarly satisfactory.

General Composition

PHILLIPS [6] defined a composite as a "three-dimensional combination of at least two chemi-

To whom all correspondence should be addressed: Dr. Rozaidah TALIB, Faculty of Dentistry, University of Malaya, 59100 Kuala Lumpur, MALAYSIA

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cally different materials with a distinct interface separating the components". Composite resins used in dentistry comprise a blend of hard, inorganic particles bound together by a much softer resin matrix [7]. In general, a composite resin encompasses three phases, namely: A) The organic phase (matrix)

During the development of a suitable matrix for dental composite filling materials, numer-ous difficulties were encountered such as a lack of suitable curing agents and poor color stability[61. Experiments done by Bowen in the late 1950s designed to overcome these problems led to a compromise between an epoxy resin and a methacrylate resin. In 1958, he found that the reaction product of bisphenol A and glycidylmethacrylate thinned with TEGDMA (trieth-

yleneglycol dimethacrylate) would cure with an appropriate catalyst system within 3 min at room temperature with only 5% polymerization shrinkage [8]. A composite formulated with this resin as a matrix showed very good physical and mechanical properties. Many composites today utilize this monomer system, which is called "Bis-GMA", an abbreviated term for bisphenol A-glycidyl methacrylate. It is a high-molecular-weight monomer and serves to provide a solidifiable liquid for composite resins [9]. However, this liquid is very viscous. In order to obtain a reasonable consistency for convenient use, it is diluted by addition of other methacrylate monomers of low molecular weight and low viscosity. Manufacturers add about 25% trieth-

yleneglycol dimethacrylate (TEGDMA) to produce a "workable" resin [7]. According to BRADEN,[2] methylmethacrylate can be used as a thinner, but it is not satisfactory because of its volatility and it produces higher polymerization shrinkage. Other components of the matrix are [9]: (i) polymerization inhibitor, e.g. monomethylether of hydroquinone to extend the storage stability (shelf life) and working time, (ii) a catalyst, e.g. benzoyl peroxide to initiate polymeriza-tion, (iii) a tertiary aromatic amine, also called "co-catalyst" which is present in the chemically cured composite only, e.g. N, N-dihydroxyethyl-p-toluidine to accelerate polymerization, (iv) a UV-activator, e.g. methylether of benzoin which is present in light-curing composites only to

photoinitiate polymerization and (v) a UV absorber, e.g. 2-hydroxy-4-methoxybenzophenone to improve color stability, i.e. by minimizing the color change in the material when exposed to sunlight.

Another commonly used resin system is urethane dimethacrylate. This is a dimethacrylate with no aromatic rings in its structure. It plays the same role as Bis-GMA, but it has a lower

polymerization shrinkage and is more viscous. REES AND JACOBSEN [10] reported that urethane dimethacrylate has improved toughness due to the greater flexibility of the urethane linkage. B) The dispersed phase (filler)

It was discovered as early as 1905 that plastics and elastomers can be improved by addition of small particles or fillers [11]. For dental purposes, besides reinforcement, the main reason for adding fillers to resin is to reduce polymerization shrinkage and the coefficient of thermal expansion [8, 12]. Inorganic fillers impart desirable physical properties such as rigidity, surface hardness, low shrinkage and a low coefficient of thermal expansion [7]. The higher the proportion of the dimensionally stable filler relative to the dimensionally unstable resin, the lower will be the coefficient of thermal expansion of the composite. PHILLIPS [6] stated that dispersed hard

particles must be present at a high concentration if they are to inhibit deformation of the matrix. Usually the matrix is able to behave viscoelastically and may creep under prolonged application of load. Fillers reduce the degree of creep over a given period of time for an identical applied stress.

Examples of the fillers used in dental composites are ground quartz, pyrolytic silica, aluminum silicate, lithium-aluminum silicates, borosilicate glasses and various other types of

glass, including some containing oxides of heavy metals such as barium which render the composite material radiopaque. The fillers are treated with vinyl silane (coupling agent) to

promote adhesion between resin and filler. BOWEN AND REED [13] proposed the use of etchable glass with a porous surface to improve the bond between the filler and the polymeric binder.

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With such a filler, the monomer can penetrate the porous surface and enhance the mechanical

retention of the resin to the filler particles. In addition, they claimed that the monomer will react

with the coupling agent which is chemically bonded to the filler surface, thus forming a chemical

bond between matrix and filler. However some solubility problems with the filler suggested poor

behavior in the oral environment. SODERHOLM [14] reported that some glass fillers are slightly

water-soluble, and that water degrades the silane coupling agent. Some components of the glass

may therefore leach out of the composite material.

By filling a resin, its physical and mechanical properties are markedly affected [15]. Within

certain limits, the higher the filler load, the better the mechanical properties. However, when

formulating a composite, one must be careful not to overpack it with fillers, since the wetting

of all particles is essential. A material containing only partially wetted filler particles will show

poor physical and mechanical properties. The concentration of matrix required just to coat the

filler per unit of its surface area will vary from one matrix to another and from one filler to

another [11].

Fillers used in dental composites may be produced by grinding and milling, precipitation

or by condensation. The particle size may vary, depending on the method of manufacture, from

10 nm for some of the pyrolytic silica fillers to 100 ƒÊm for some quartz or glass fillersm. There

is a trend towards the use of blends of fillers having different particle sizes in order to maximize

the filler content.

C) The interfacial phase (coupling agent)

This phase consists of either a bipolar coupling agent, usually an organo-silane, connecting

the organic resin matrix and the inorganic filler, or a co-polymeric or homopolymeric bond

between the organic matrix and partially organic filler[161. Stable adhesive bonding of the filler

to the resin is essential for strength and durability of the composite. For example, lack of an

adequate bond will permit dislodgment of the filler from the surface or ready penetration of

water along the filler-matrix interface. Thus, the manufacturer coats the surface of the filler with

a suitable 'coupling agent', normally 3-methacryloxy propyltrimethoxy silane [6]. Such agents

may also act as a stress absorber at the filler-resin interface. A study done by BOWEN [1] showed

that the incorporation of vinylsilane-treated silica powder into an organic polymer reinforced

the material. The material had lower solubility and disintegration in water and was less affected

by desiccation. The same resin filled with silica not having vinylsilane surface treatment showed

inferior properties and wear resistance after equivalent immersion in water.

Polymerization of Composite Resins

Acrylates are polymerized by addition polymerization. In this type of reaction, no by-

products are obtained [17]. There are three stages in the polymerization process: initiation,

propagation and termination. The polymerization reaction must be initiated by a free radical,

which may be created by heat, chemical or photochemical reactions. In the initiation stage, the

free radical reacts with the monomer by opening the unsaturated double bond and transferring

the free electron to the end of the chain being formed. The monomer molecule is thus activated

and able to bond to another monomer molecule. Propagation continues until all free radicals

have been joined or until no adjacent linkages are available.

Two changes occur during polymerization: Polymerization shrinkage and polymerization

exotherm, with a consequent rise in temperature. Most composites shrink between 0.5% and

0.7%, compared with about 21% for a direct-filling acrylic. These figures seem to be clinically

acceptable, and some of the shrinkage may be offset by swelling through water uptake. The

setting exotherm is not clinically significant because the duration of the temperature rise is

reasonably short and most manufacturers recommend the use of lining material [2].

GOLDMAN [18] reported that polymerization shrinkage of resin-based restorative materials

ranges from 1.67 to 5.68%. The highest overall shrinkage tends to occur with powder-liquid

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materials, whereas photo-polymerized materials appear to shrink the least , with paste-paste materials showing intermediate shrinkage . The polymerization shrinkage continued for 3 to 4 weeks in a dry specimen studied by JACOBSEN [19], but immersion in water caused the material to expand, thus compensating for the setting shrinkage. Polymerization by Chemical Activation

Chemically activated composite materials are supplied as two components; one containing a chemical initiator (benzoyl peroxide) and the other containing a chemical activator (an amine , usually a tertiary aromatic amine). These materials may be in the form of two pastes , i.e. a paste and liquid or a powder and liquid system. On mixing the two components , free radicals are formed to initiate polymerization. Once polymerization starts , there is no control of the setting of the composite. Polymerization into molecular chains continues until termination of the reaction. At the end, the catalytic constituents (tertiary amine) remain and are a potential source of problems. In the oral environment the tertiary amine may undergo chemical change that results in a color shift of the restorations, seen as darkening , known as "amine discolora-tion" [20]

The Bis-GMA materials have a fast setting reaction , which commences as soon as the components of the materials are intermixed (two-paste system). During the manipulation there is a very slight increase of viscosity in the period up to the end of the working time[19]. This slight change in viscosity is clinically important , since if the insertion of the material into the prepared cavity is delayed, the adaptation of the material deteriorates . The increase in viscosity and rapid setting inevitably produce a relatively high proportion of unpolymerized methacrylate or acrylate groups. For chemically activated materials these are distributed almost uniformly throughout the mass of the material [7]. Polymerization by Light Activation

The development of light-activated composite resins was an important advance in dentistry , enabling "command setting" of single-paste materials . The mechanism of polymerization still involves the generation of free radicals, but instead of a chemical source of energy this system uses photon energy from lamps.

In the 1970s, materials activated by ultraviolet light with a wavelength of about 365 nm were introduced. Under appropriate conditions of exposure to radiation , a compound such as benzoin methylether breaks down to form free radicals which can initiate polymerization [21]. Despite its potential benefits , the above system is not without problems. Limited depth of polymerization has been reported, with depth values as low as 1.5 mm after 60 s exposure to ultraviolet light [22]. A second problem with the ultraviolet system is related to safety . Harmful effects of near-ultraviolet radiation (320 to 400 nm) have been listed , including skin cancer and eye damage [23].

More recently, systems activated by visible light (wavelength about 470 nm in the blue region of the spectrum) have become available . These retain the merit of "command setting" while avoiding the problems associated with earlier ultraviolet-setting materials . The visible light-curing systems are also claimed to have greater depth of cure [24]. The first material to employ this type of activation was introduced in 1977. In light-curing composites, an alpha-diketone and an amine are used as the catalyst system. These do not react except in the presence of light with a wavelength of about 470 nm. The ketone then absorbs energy and reacts with the amine to produce free radicals which initiate polymerization [25].

It was reported that the emission of ultraviolet lamps tended to decrease with use, whereas the lamps used for the visible light systems maintain a fairly constant output throughout their lives, thereby reducing curing problems due to changes in light intensity [26]. Light-activated materials differ substantially from chemically activated products with regard to setting character -istics. No mixing is required and the viscosity does not increase significantly until the material is exposed to an activating light source. They should not, however, be considered to have an

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unlimited working time [7]. Slow polymerization may be activated by sunlight, room lights and

particularly, by the surgery operating light. Unlike the case of chemically activated materials, the

degree of polymerization is not uniform throughout the mass of the material. The concentration

of unpolymerized groups is lowest near the surface, which is close to the light source, and highest

at the base of the cavity [27]. For optimal use of these light-activated composites, it seems that the

best way to maintain adhesion to the cavity floor is incremental placement.

Polymerization shrinkage

The setting shrinkage, which is outside the clinician's control, is a major drawback of

polymeric materials. This allows gaps to form between the cavity wall and the restoration

(despite the use of a matrix), permitting microleakage of oral fluids, bacterial toxins and soluble

ions of all kinds, which may give rise to pulpal sensitivity, staining of restoration margins and

secondary caries [19]. Light-activated composites contract toward the external surface of the

restoration closest to the light source, in contrast to chemically activated resins, which shrink

toward the center of the material. In both cases, internal stresses develop in the restoration1281.

Although a higher filler content might reduce the shrinkage to some extent, the maintenance of

suitable viscosity requires more TEGDMA diluent, which in turn results in more polymerization

shrinkage [29]. Dentine bonding agents are used for bonding composites to dentine in order to

overcome microleakage, but it has been reported [30] that if the tooth-restoration interface remains

intact, polymerization shrinkage may pull the cusps inwards, resulting in cuspal flexure. It has

been suggested that this movement is probably responsible for postoperative sensitivity and

oblique cuspal fracture.

All present-day restorative resins shrink during polymerization, and no commercial dentine

bonding agent is able to prevent this shrinkage [31, 32]. As long as polymerization shrinkage cannot

be reduced substantially, and adhesion to dentine is still lacking, the use of composite resin for

direct restorations demands careful and thoughtful clinical application.

Some Mechanical Properties

The elastic modulus of the composite to be used in a stress-bearing area is an important

property to consider, since a material with a low modulus will deform under masticatory forces [33].

The findings of one study [34] have suggested that a composite restoration will show three to six

times more deformation than an amalgam restoration under a given load. Mechanical deforma-

tion can lead to marginal breakdown [35]. Breakdown is most likely to occur in products having

a lower modulus of elasticity. In composites, the modulus depends primarily on the inorganic

filler content. Those with a low filler content are more prone to mechanical deformation.

Materials which have high strength and/or hardness do not necessarily have high resistance

to wear [36]. Individual differences in the hardness of composite materials arise from their matrix

composition, the type of filler used and the filler loading. It has been reported that the hardest

material is that with the greatest filler loading [37]. The change in length per unit length of a

material for a 1•Ž change in temperature is termed the linear coefficient of thermal expansion.

The coefficient of thermal expansion of composite resins depends on the quantity of the

inorganic filler content. However, if there is a significant mismatch between the resin and the

tooth substance, then thermal cycling may cause microleakage due to percolation of fluids down

the interface [33]. Most acrylic polymers absorb water. Self-polymerizing resins both absorb water

and contain water-soluble matter [17]. Excessive water absorption can adversely affect the

mechanical properties of the resin. Water can act as a resin plasticizer, and hydrolytic damage

to both filler and silane can result. Furthermore, leaching of ions from filler particles and the

presence of interfacial microcracks can be problematic [38]. Water sorption increases creep and

decreases the elastic modulus and ultimate strength of a range of composites [39]. Water sorption

may affect composite materials by reducing their wear resistance [14]. Microfilled resin has the

greatest water sorption due to its lower filler content and markedly smaller filler particle size [37].

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Wear is a loss of material, which , in the oral cavity, is characterized by loss of the original

anatomical form of the material [17] . Wear of restorative materials involves interaction between

at least three major mechanistic pathwaysr [40]:

( i ) Abrasive wear, caused by occlusal contact, coarse foodstuffs or tooth-brushing .

( ii ) Chemical effects on the filler, silane or resin.

(iii) Fatigue, caused by cyclic stresses from opposing teeth or restorations, which may result in

small surface or subsurface cracks, eventually propagating to cause material loss by fatigue

wear.

It has been documented clinically that the presence of subsurface and internal porosities

leads to an accelerated wear pattern . Wear of composites is related to internal crack formation

and crack propagation, and microcracks are present in the subsurface area of posterior compos -

ites under masticatory load . When cyclic loading is applied, these microcracks propagate step by

step in the subsurface area through areas of maximum shear stress . Other microcracks propagate

towards the surface and form loose particles [41] .

Classification

Each phase of composite resin components has been analyzed to form a basis for

classification [42]. The influence of resin type on the clinical behavior of composites is largely

unknown, and a classification based on organic matrices does not appear to be useful since most

composites contain mixtures of different resins . Bipolar molecules, mainly organosilanes , are usually used as coupling agents to connect the inorganic fillers and the organic matrix

. With

prepolymerized partially organic filler particles, the bond is polymeric in nature . Both these types of agents, and all current versions of interface adhesion

, are equally sensitive to chemical disintegration. Hence, it has been concluded that the use of coupling agents as a basis for

classification does not seem to be appropriate [42] . The fillers demonstrate distinct differences in

properties. The type, amount and size of the filler particles affect the properties of composites . Therefore, these may provide a basis for establishing a workable classification

. There are several classification systems in use to define composite resins on the basis of filler

particle size, distribution, and the quantity incorporated. One example of such a classifica-

tion [17] is given in Table 1. MCCABE [7] categorized composites into three groups according to

particle size and the quantity of inorganic filler. These are conventional, microfilled and hybrid

composites.

Conventional materials: These have filler particles within the size range 1-100 ƒÊm . A further subdivision is a group possessing filler particles mainly within the size range 1 -6 ƒÊm

, and another group has a particle size range of 15-20 ƒÊm with some large particles up to 50 ƒÊm or

even 100 ƒÊm in diameter. Conventional composites generally contain between 75 wt% and 80

wt% filler (50-60% by volume) . Particles larger ƒÊm are visible to the naked eye [11] and these

materials are therefore not recommended for use, since the esthetics of the material would be

jeopardized due to the very rough surface. In recent years, there has been a trend toward the use

of smaller, softer and more rounded macrofillers [24] .

Microfilled materials: These contain pyrolytic silica particles with a diameter range of

10-100 nm and a mean of around 40 nm . The filler loading is normally within the range 30-60

wt%. The incorporation of these extremely small particles into the resin is technically demand -

ing, owing to the very high surface area of the filler .

Hybrid materials: These contain a blend of both conventionally sized filler particles (1 -100

μm) and small particles of pyrolytic silica (10-100 nm) . The relative amounts of each and thetotal tiller content vary from one product to another , but most materials contain a total of 78-85 wt% filler. A typical product contains 76 wt% conventional particles and 7 wt% pyrolytic silica

. It is possible to make yet a further subdivision within this group depending on whether the

conventional particles are of the smaller or the larger type .

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Commercially available composite resins have been categorized into four groups [43]:

1) Conventional: filler size 8-100 ƒÊm, occupying approximately 54% of the volume.

2) Microfilled: filler size 1 ƒÊm, occupying approximately 25% of the volume.

3) Hybrid: filler size 0.04-50 ƒÊm, occupying approximately 64% of the volume.

4) Resin-bonded ceramics: filler size 0.04-50 ƒÊm, occupying approximately 75% of the

volume. These are termed "resin-bonded ceramics" by the manufacturer, because of the

high ratio of filler to matrix achieved.

Clinical Performance

Factors of importance which may affect the longevity of composites include ability to resist

high stress without fracture, ability to resist smaller cyclic stresses without degradation, ability

to resist abrasive forces without undergoing wear and ability to resist chemical degradation [7].

Conventional composites: The presence of macrofillers in conventional composites makes

it difficult to give the resin a smooth surface, and trimmed restorations soon become rough,

leading to plaque accumulation and staining. Hydrolysis of the interfacial bond and rapid wear

of the organic matrix cause protrusion and dislodgment of the macrofillers. For this reason,

rather poor wear properties result in both stress-bearing and contact-free situations1421. When

macrofilled composites are abraded by food, only the polymer matrix between the filler particles

is worn down; the filler particles themselves are not abraded, but fall out with time as they lose

their grip in the surrounding polymer [44].

The use of conventional composites in posterior teeth may lead not only to overeruption of

the tooth or teeth in the opposing arch but also the mesial migration of teeth distal to those

undergoing loss of interproximal contact [45, 46]. In general, conventional composites are unsuit-

able for areas of heavy masticatory force, but their marginal adaptation seems to be superior to

that of amalgam [47-51].

Microfilled composites: The introduction of a microfine filler into a composite resin system

has overcome the finishing problems associated with conventional composites. The smooth

surface produced by abrasion of microfine filled composites reduces friction and therefore

eliminates the type of abrasion seen in conventional composites, which exhibit selective wear of

the soft matrix and not the inorganic filler, this producing a rough surface [52] The ability to

produce and maintain a smooth surface texture is a major advantage, and it is possible that the

extremely small filler particles impart some degree of matrix protection [53]. Some investigators,

based on the results of short-term clinical trials, have indicated that in terms of abrasion

resistance, microfilled materials are better than conventional composites [53, 54]. When compared to

amalgam, it was concluded that microfilled composites showed quantitatively similar wear

resistance to that of amalgam [55, 56]. Although considerable clinical success has been achieved, the

results obtained have been mainly from short-term investigations, and thus the use of microfilled

composites in posterior teeth is not yet justified [57].

Hybrid composites: These materials are increasingly advertised and promoted as an alterna-

tive to amalgam. It has been reported that compared to microfills, hybrid composites exhibit

superior abrasion resistance, a lower coefficient of thermal expansion, reduced polymerization

shrinkage, improved tensile strength and reduced water absorption [3]. Since they are heavily

loaded inorganically, they have a high degree of fracture resistance [20].

It has been reported that hybrid composites show better wear resistance than conventional

and microfilled composites, but lower resistance than Dispersalloy amalgam [58]. However, a

contradictory findings were reported by MCCOMB AND BROWN [59]. Using silver amalgam as a

control they found that the wear resistance of hybrid composites after two years of clinical

observations was lower than that of microfilled composites and amalgam; amalgam showed the

highest wear resistance. SETCOS et al. [60] evaluated about 400 posterior restorations using

Occlusin (light-activated, urethane-based radiopaque hybrid composite), with 100 amalgam

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restorations. Hybrid composites offer a structural compromise between wear-resistant conven-

tional composites and fatigue-resistant microfilled materials. This compromise results from an

improvement in the character, form, size and distribution of the filler phase. Generally, these

materials are as highly filled as possible, with a wide range of filler particle size. The upgraded

properties of the hybrids resemble those of dentine and lathe-cut amalgams. These changes favor the use of these materials in the premolar-molar region, and they are now commonly indicated

as the posterior composites of choice{331.

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