Dental Composite Biomaterials

[[Introduction | Physical Characteristics | Types Of Composites | Radiospacity | Posterior Composites | Concerns About Composite Materials | Use Of Composite Materials]

Dr. Jones's Reply:

Introduction

The proliferation of composite materials on the market has been confusing for the practising dentist. However, in spite of the large number of dental composite products that have appeared during the past 15 years, they all have simple, common characteristics. They are all combinations of silane-coated inorganic filler particles with dimethacrylate resin, either BISGMA or urethane dimethacrylate (UDEMA). In some cases, a proportion of a lower molecular weight monomer such as TEGDMA is introduced to lower the viscosity. The filler particles used are either barium silicate glass, quartz or zirconium silicate, usually combined with 5-10% weight of very small-sized (0.04 µm) particles of colloidal silica. Modern dental composite materials are thus a blend of glass or ceramic particles dispersed in a photo-polymerizable synthetic organic resin matrix. The polymer materials are blended together with the finely divided inorganic material such as a barium aluminosilicate glass or other glass compositions having an effective amount of radiopaque oxide that renders the resultant glass radiopaque to x-rays. Many practitioners are perplexed by such terms as: conventional, traditional, all-purpose, universal, anterior, posterior, small particle, fine particle, hybrid, microfilled, compomer, flowable and packable. The terms "conventional" and "traditional" relate to materials that were developed in the 1970s, and although they have been slightly modified over the years, they tend to be less used in 1998. The terms "all-purpose," "universal," "anterior" and "posterior," which describe the intended uses of the composites, are more easily understood by the dentist.

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Physical Characteristics

Often the mean particle filler size is given in an attempt to characterize the type of material. Unfortunately, this can also be misleading, since the mean particle size cannot give any indication of the range of particle size. However, even giving the range does not indicate the distribution of the particles within the range.

An important characteristic is the surface area of the filler particles. The smaller the particle size, the larger the surface area. For example, a cube of filler material with dimensions of 2 x 2 µm would have a surface area of 24 µm². By cutting the cube into two halves, the surface area would increase by 8 µm², giving a surface area of 32 µm². The surface area of filler particles is usually given in square metres per gram (m²/g). The surface area of submicron-sized fumed silica that is incorporated in small amounts into most composites is very high about 130 m²/g. The surface area measured for filler particles for a number of commercial composites ranged from 4.4 to 65.6 m²/g.

In order to achieve a higher filler loading, a wide distribution of filler particle size must be present. In most cases, the filler weight percentage is given, which is unfortunately not as valuable as filler volume percentage, since it is the volume of exposed resin matrix to abrasion and the volume of matrix resin that has to polymerize that are important. It is obvious that a heavier (denser) filler would occupy less volume for a given weight of filler. For the same filler loading by weight, for example, if filler "A" had a density of 2 g/cc and filler "B" had a density of 3 g/cc, then the less dense filler "A" would occupy almost 50% more volume than filler "B". Thus, simply quoting the filler loading by weight percentage can be misleading. The filler content by volume does in fact characterize composite materials quite well.

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Types Of Composites

A number of composite materials are described as all-purpose or universal, the implication being that they can be used for both posterior as well as anterior use. However, some are indicated as being suitable only for small posterior restorations. Confusion arises when some of these same materials are also described as hybrid composites. Charisma, Herculite XRV, Prodigy, Tetric, TPH (Total Performance Hybrid), and Z-100 are examples of this type. Some materials have been described as posterior composites, for example, Adaptic II, Clearfil, Heliomolar RO, Marathon, Occlusin, P-50, Post Com II. Examples of materials described as anterior composites include Silux Plus and Durafill. The microfilled composite was developed to satisfy the need for a polishable composite. These materials have a very fine particle size of colloidal or fumed silica (0.04 µm) dispersed in a resin matrix. However, the very large surface area of the fumed silica filler (130 m²/g) significantly limits the volume of filler that can be incorporated. The manufacturers have attempted to overcome this problem by dispersing the filler in a resin that is heat cured, then ground to produce particles of about 25 µm. These heavily filled composite particles are then dispersed into a lower filled resin matrix of relatively low viscosity. These microfilled resins have lower filler loading than the hybrid, all-purpose or universal materials. The microfilled composites have lower mechanical properties due to the larger volume of resin. However, some microfilled composites have demonstrated wear resistance comparable to more heavily filled wear-resistant composites. Microfilled materials include Denta-colour, Durafill, Heliomolar RO, and Silux Plus. The wearing mechanism of composites is not fully understood, but the breakdown of composites is known to be related to degradation resulting from hydrolytic, mechanical and enzymatic phenomena. The term compomer, a combination of COMPosite and ionOMER, has been introduced in the past few years. It was intended to suggest a combination of composite and glass-ionomer technology. Although these materials contain a reactive fluoride glass and an unsaturated acidic constituent, they are anhydrous. No acid-base reaction takes place during the initial setting. As a result, the initial setting reaction for these materials is the same as for the regular light-curing composites. Any reaction between the ion-leachable glass and the acidic monomer can take place only when the compomer takes up water. These materials have strength values that are superior to conventional glass ionomer materials, but lower than the hybrid, all-purpose or universal composite materials. Compoglass and Dyract are examples of compomer materials.

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Radiospacity

One of the requirements of using a composite as a posterior restorative is that it should be radiopaque. In order for a material to be described as being radiopaque, the International Standard Organization (ISO) specifies that it should have radiopacity equivalent to 1 mm of aluminium, which is approximately equal to natural tooth dentine. However, there has been a move to increase the radiopacity to be equivalent to 2 mm of aluminium, which is approximately equal to natural tooth enamel. A majority of the composites described as all-purpose or universal have levels of radiopacity greater than 2 mm of aluminium

Posterior Composites

Although composite materials have significantly improved in recent years, their use as routine posterior restoratives is not generally recommended. Clinical trials have shown typical wear rates for the modern posterior (all-purpose or universal) composite materials of between 7 and 12 µm/year. However, the clinical trials reported in the scientific literature have been carried out by academics under specially controlled conditions, and may not be typical of the results obtained in a busy general dental practice. Occlusal wear will take place in areas of occlusal contact. Occlusal wear can cause over-eruption of the opposing tooth. Analysis of two-year clinical wear data indicates a strong correlation with the flexural strength, the stronger materials exhibiting the least wear. In vitro tests have indicated that the composite materials which cause the most wear of opposing tooth enamel are also the materials which exhibit the greatest abrasion loss themselves. Dental amalgam has been shown to cause the least wear of tooth enamel and undergoes less wear itself than any composite.

The placing of posterior composites is regarded as a difficult operative procedure because of the difficulty in attaining ideal proximal contacts. Pulp protection is essential. Incremental build-up is mandatory; the material must be placed in several layers to minimize polymerization shrinkage during light curing. However, polymerization shrinkage _ 1.5-2.5% by volume _ is inevitable. The occlusion must be adjusted meticulously. It is exceedingly difficult to finish composites in the gingival proximal areas. Composite materials are technique sensitive and require about twice the time to place compared to amalgam.

Composite materials do not undergo complete polymerization; about 40-60% of the carbon bonds remain unsaturated. Polymerization is compromised by oxygen inhibition, but this problem has been minimized to some extent by the development of photo-cured systems, which reduce the potential for oxygen incorporation that occurred during mixing of chemical setting type materials. Most composite resin restorative materials are now photo cured. The elastic modulus of the stiffest posterior composite materials is typically about half that of tooth enamel and other restorative materials such as dental amalgam and dental porcelain.

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Concerns About Composite Materials

The full potential of glass and glass/ceramic/resin composite biomaterials may not yet have been achieved since the current composite materials cannot completely withstand the aggressive environment of the oral cavity. Composite materials have been criticized, by those who see amalgam as the material of choice, for having inadequate toughness, low modulus, insufficient ductility and poor wear resistance. The procedure for composites, unlike amalgam, requires very careful moisture control. It is also accepted that composite materials do not fully polymerize. Some polymer degradation products such as bis phenol A (an estrogenic substance), TEGDMA, formaldehyde, methacrylic acid and benzoic acid, have been eluted from dental resins. It has also been shown that the filler particles result in degradation products such as silicon, boron and aluminium. However, most of these criticisms are not major concerns.

The main problems with composite materials such as posterior restoratives are: lack of wear resistance, low modulus and incomplete polymerization, and polymerization shrinkage that adversely affects long-term survival.

The modulus of elasticity of dental composites is regarded as an important fundamental property, since a material with a low modulus will more readily elastically deform under functional stresses. Excessive elastic deformation under functional stress may result in the fracture of surrounding brittle tooth enamel structure, or alternatively, in an increase of micro-leakage. A composite restoration with a higher modulus of elasticity will be able to provide support at the interface with tooth enamel to protect the enamel rods at the margin from fracturing.

The use of polymer/glass or ceramic composite systems as posterior dental restoratives that will be subjected to much higher levels of stress than anterior restorations might suggest the use of materials with higher modulus of elasticity to minimize the risk of cusp fracture. Higher modulus of elasticity and lower polymerization shrinkage are achieved with higher filler loading combined with acceptable long-term silane coupling between the filler and the resin matrix. The main advantages of composite materials are good esthetics and the use of adhesive systems that can bond the composite to enamel and dentine.

It is extremely important to select with care and caution the clinical situation that can be treated with a posterior composite restoration. The long-term wear of posterior composite restorative materials is influenced by a wide range of factors, such as:

• chemistry of the resin matrix (most materials are very similar);

• chemistry of the filler (most materials are very similar);

• effectiveness of the silane coupling agent (most     materials are reasonably similar);

• size, distribution, shape and volume of the filler (considerable variation);

• size and location of the dental cavity preparation       (considerable variation);

• magnitude, location and direction of the occlusal forces (can vary widely);

• degree of polymerization (similar but also influenced by clinical technique); and

•   clinician's technique (most important factor).

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Use Of Composite Materials

Materials placed in high-stress situations should be highly filled materials (or materials that have several years of clinical trial success in posterior restorations) possessing good radiopacity preferably equal to dental enamel. Any anterior composites should exhibit good translucency, which is found with the less highly filled composites. However, several of the so-called "hybrid" materials containing very translucent fillers can produce excellent results. n

Dr. Jones is professor of biomaterials, at Dalhousie University, Halifax, Nova Scotia.

The author has no declared financial interest in any company manufacturing the types of products mentioned in this article.

Editor's note: I invite readers to send me questions about clinical problems experienced in everyday practice. I will seek answers to these questions from recognized Canadian experts. You can send me your questions by e-mail, fax or regular mail. I look forward to hearing from you.

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