Journal of Dentistry, 9, No. 2, 1981, pp. 95-100 Printed in Great Britain

Basic techniques and materials for conservative dentistry

2. Recent advances in restorative materials P. H. Jacobsen, MDS, FDS

Department of Conservative Dentistry, Welsh National School of Medicine, Cardiff

P. B. Robinson, BDS, FDS

Department of Conservative Dentistry, King’s College Hospital Dental School, London

ABSTRACT Significant developments in direct-filling restorative materials are reviewed. These include high-copper amalgams, conventional and microfine composite resins and cavity varnishes.

CEMENTS AND LINING MATERIALS Bases

These are bulk lining materials intended to protect the pulp from chemical and physical irritants and to replace lost dentine.

The main considerations in the choice of a base are that it should have an adequate early compressive strength which is able to withstand the forces of amalgam condensation, and be non-toxic to the pulp. Since the commonly available bases may also be used as luting agents, the additional properties of insolubility, adhesion to tooth substance and low film thickness are required.

Zinc phosphate cement has been the most commonly used base for many years in spite of reports of pulp damage following its use (Plant, 1972). The pulp damage may arise from the heat produced on setting, or from its initial low pH. The material’s advantages are its good handling characteristics and high early compressive strength. Whenever the material is used as a base it must be mixed thickly.

The alternatives to zinc phosphate are zinc polycarboxylate, or a fortified, accelerated set zinc oxide-eugenol cement.

The zinc polycarboxylate cements e.g. Poly F (AD International Ltd, London), Durelon (Espe GmbH, Oberhay) developed by Smith (1968) have the advantages of low pulp toxicity, possible adhesion to tooth substance and low solubility. The early compressive strength is similar to that of zinc phosphate and one brand (Poly F) contains fluoride which may be an advantage when the material is used for luting. The disadvantage of the polycarboxylates is their short working time, shown by early ‘stringing’ of the mix. It is caused by zinc ions inducing polymerization of the polyacrylic acid prior to the acid-base reaction, which is the main setting reaction, beginning in earnest. If this occurs when the material is being used for luting, the mix should not be used, and a fresh one made.

Fortified, accelerated set zinc oxide-eugenol materials (now referred to as ZOE cements or zinc eugenates) have the advantage of containing eugenol with its well-known obtundent

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effect on the pulp. These ZOE cements are preferred to non-fortified or non-accelerated materials because of their superior handling and physical properties.

The ethoxybenzoic acid (EBA) reinforced materials have compressive strengths equal to those of the phosphates and polycarboxylates. They may also be used for luting and root canal sealing. Hand-mixing, however, is somewhat tedious, since the materials are thixotropic at anything below the correct powder : liquid ratio. They are available in capsulated form.

Polystyrene reinforced materials, e.g. Kalzinol (AD International Ltd), have compressive strengths approximately one-third to one-half those of phosphate and polycarboxylate, but this does not seem to be a disadvantage. In relationship to this, it is difficult to interpret the mechanical properties of lining materials since their compressive strength is tested routinely by crushing cylindrical specimens. However, if the material is used in a film and is also under constraint by the cavity shape and perhaps the first increment of amalgam, the actual strengths will be considerably higher than laboratory results might suggest.

Liners and varnishes

The principal liners are based on calcium hydroxide in a resin matrix, e.g. Dycal (Caulk Manufacturing Co. Ltd, Ontario), Procal (3M Dental Products, Minnesota), MPC and Hydrex (Kerr Manufacturing Co. Ltd, Michigan). The exception is Cavitec (Kerr Manufacturing Co. Ltd) which is a film ZOE material. The major use of the calcium hydroxide type is under composite materials where ZOE will cause discolouration of the restoration. They are also used for direct and indirect pulp capping procedures. Certain components of Hydrex have been implicated in the poor pulp response to this material (Paterson, 1976).

Varnishes are intended primarily to reduce or to eliminate microleakage and seal the open ends of dentinal tubules prior to cavity filling. They may also be used as the sole lining for shallow cavities. Essentially, microleakage is the passage of fluid, ions and bacteria in the space between the cavity wall and the restorative material. The space is made up of two components. First, irregularities in the cavity wall fmish, which restorative material cannot fill, produce microscopic pathways. Secondly, the shrinkage of the restorative material on setting increases the gap. Cavity varnishes, by virtue of their low viscosity, are intended to penetrate areas of cavity roughness to produce a smooth cavity ‘wall’ against which the restorative material is packed. Studies have shown that varnishes reduce early microleakage before they are lost by dissolution. This is valuable since following cavity preparation the pulp is often hypersensitive and should be protected from saliva, and chemicals from the restorative material. In theory, the dissolution of the varnish is matched by the corrosion of amalgam and the gap remains sealed.

The hypothesis that bacterial contamination of the material/cavity wall interspace is a major cause of pulpal inflammation has been proposed by Brannstrom and Nyborg (1971) and Beagrie and Brannstrom (1977). They suggest that the cavity walls should be sterilized by materials which are harmless to the pulp but which are microbicidal. However, recent work by Vlietstra et al. (1980) has shown that exposure to an air/water spray is effective in removing bacteria.

Brannstrom (1969) has developed Tubulitec cavity varnish which will prevent micro- leakage and stop organisms deriving nutrient from the dentine. The dry material is polystyrene containing fluoride, calcium hydroxide and zinc oxide. The formulation of a suitable substitute is given in Table I, and may be prepared by a pharmacy.

Jacobsen and Robinson: Conservative dentistry 2 97

Tab/e 1. Formulation of ‘Tubulitec Substitute’ Cavity Varnish

Materials %

Zinc oxide 5 Calcium hydroxide 5 Sodium monofluorophosphate 2 Polystyrene 1 Chloroform to 100

Other varnishes are usually a single natural resin (e.g. Copal) in a volatile vehicle. However, the correct viscosity is important. The type of copal ether varnish that was used to protect silicate cement restorations is too viscous for use as a cavity varnish. All varnishes are un- suitable for thermal insulation and, therefore, should not be used without a base in deep cavities.

The recommended procedure for amalgam restorations is to place a base, such as a fortified, accelerated set ZOE, and then disperse a drop of varnish over the cavity walls. Care must be taken to avoid pooling of the varnish in the line angles by the gentle use of an airstream to disperse and dry it. The amalgam is then packed in the usual way.

For shallow anterior cavities, the varnish may be used alone, or for deeper cavities, following the placement of a calcium hydroxide liner. Then the varnish on the enamel of the cavity wall should be removed with an excavator so that the polymeric material may be seated directly on to tooth substance. Of course, the cavity walls should not be coated prior to using an adhesive materal, such as glass-ionomer cement, since adhesion will be blocked.

AMALGAM

In spite of the amalgam war of the nineteenth century and the more recent evidence of mercury toxicity, the amalgam restoration remains the mainstay of conservative dentistry. Surveys of amalgam restorations have blamed errors in cavity preparation for approximately 50 per cent of failures. In those failures which could be attributed to the amalgam alone, the predominant finding was loss of marginal integrity. Other criticisms which have been made of amalgam include its susceptibility to corrode in oral fluids and its inability to restore large occlusal areas because of creep. Creep and corrosion are also implicated in the marginal fracture of amalgam since the wedge of material at the cavity margin may be fatigued by occlusal movement and corrosion will accelerate cracking and its final fracture.

Significant reductions in creep and corrosion have been achieved in the development of the high-copper amalgam alloys. Innes and Youdelis (1963) introduced the metallurgical concept of dispersion strengthening to amalgam by incorporating silver-copper eutectic spheres with conventional silver-tin filings. It is only amalgam alloys of this type that are correctly referred to as ‘dispersed phase’. Other methods of increasing the copper content have been developed and the various systems are shown in Table II. Essentially, these alloys contain more than 6 per cent copper, the maximum currently permitted by national and international specifications.

The introduction of high-copper alloys has prompted considerable work on the

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Tab/e II. High-copper amalgam systems

Type Composition Particle shape Example Copper content

%

Dispersed phase Ag-CU eutectic/Ag, Sn intermetallic 71%-29%

spheres/lathe cut particles

spheres/spheres

Dispersalloy Amalcap non yz 12

Cupralloy 21

Ternary alloy Ag-Cu-Sn spherical

spheroidal

Sybralloy 29

Aristalloy CR 13

13 Quaternary alloy Ag-Cu-Sn-In spheroidal lndiloy plus 5

lndium

correlation of marginal fracture with creep, zinc content and other factors. All clinical trials so far reported show that highcopper amalgams as a group have superior marginal integrity over conventional amalgams. However, laboratory studies have shown that the label ‘high-copper’ does not guarantee superior physical properties. In fact, several high-copper alloys have compressive strength, flow and creep values comparable with good conventional alloys.

Care must be taken in interpreting data from flow and creep tests. Sometimes the terms have been incorrectly interchanged. Flow is determined by static loading on a cylindrical specimen between 3 and 24 hours after mixing. It is, therefore, a very slow, early compressive strength test and perhaps not clinically relevant, since patients are usually asked to avoid chewing on a new amalgam. Creep is determined by static loading for 4 hours on a specimen 7 days old, and is more readily related to clinical circumstances.

The good high-copper amalgams have creep values below O-3 per cent in comparison with the best conventional amalgams at around 1 per cent (Eames and Macnamara, 1976). The copper concentration and form of alloy alone are not a reliable guide to performance.

In a clinical study of two alloys, Mjijr and Espevik (1980) have determined significantly better marginal integrity with the alloy having the lowest creep value. Mahler et al. (1980) related clinical observations of marginal integrity to creep and zinc content in conventional and high-copper amalgams using multiple regression analysis. They predicted that the high- copper amalgams with low creep values would exhibit the best clinical behaviour and would not be influenced significantly by the presence or absence of zinc. Within the conventional amalgams those with low creep and high zinc content would exhibit better marginal integrity than the non-zinc alloys.

A further factor in the better performance of high-copper amalgams is the virtual elimin- ation of the tin-mercury (gamma -2) phase from the matrix. The corrosion resistance is increased, thereby removing a further factor in the promotion of marginal fracture.

Clinically, the handling of high-copper amalgams is superior to conventional lathe-cut materials, since they are either predominantly or totally spherical in type. They require one-half of the optimum condensation pressure for conventional alloys, carve easily without tearing and provide a smooth finish. The use of mechanical condensers is contraindicated

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and matrix bands should be firmly wedged to avoid cervical overhangs which are more prone to occur with spherical alloys (Friend and Jacobsen, 1972).

TOOTH-COLOURED MATERIALS Composites There are now over fifty brands of composite materials on the world market together with a series of bonding agents and finishing kits. The majority of the materials are based on the concept of Bowen’s formulation of a difunctional acrylate resin and a silane coupled particulate reinforcing filler. Variations between materials are essentially differences in the molecular backbone of the resin; the type and concentration of filler; and the presence and type of stabilizing and catalysing compounds.

There is no doubt that the clinical performance of the composites, though not without criticism, has shown them to be the best toothcoloured direct-filling material so far developed. The two major disadvantages of composites are their critical handling character- istics and their potential for wear and discolouration.

The difunctional acrylates and the amine-peroxide catalyst system combine to achieve a very rapid ‘snap’ set. This apparent advantage brings with it a correspondingly short working time, usually under one minute, permitting only one cavity to be filled successfully from each mix of material. Rapid manipulation is therefore essential. It is also necessary to use a matrix for all classes of cavity; a cellulose acetate strip for Class III and a cervical foil for Class V. This produces the pressure necessary to achieve flow of the material into the cavity detail and create a smooth surface on the set material. These points are discussed in detail elsewhere (Jacobsen, 1975; 1976; 1981).

Colour instability is caused by deterioration of the resin and, more importantly, by its abrasion, which leads to exposure of filler particles above the surface of the resin and a rough surface. This surface then attracts plaque and other intra-oral stains. Microleakage, causing staining of the margins, is a minor problem in comparison. There is no way to avoid these surface problems; they are inherent in the composite’s formulation. The application of a ‘glaze’ material is a temporary expedient, since this too has low abrasion resistance. The low abrasion resistance of composites makes them totally unsuitable for the restoration of occlusal surfaces.

The various chemical cavity bonding agents have been disappointing in clinical practice and the acid-etch technique is certainly the most reliable means of improving retention, even though it is just a mechanical bond.

Ultra-violet or visible light-cured materials overcome the problem of the short working time, but they have an unreliable depth of cure which varies according to the configuration of the cavity.

The microfine composite materials, so called after their filler particle size which is below 0.7nm, in principle offer considerable advantages over conventional materials. There are technical problems in the manufacture of these materials and early examples had very low filler loadings with consequently poor abrasion resistance and high water absorption. The technology is improving rapidly and materials are now available with loadings of up to 60 per cent by volume in comparison with the 75-80 per cent of conventional materials. By

virtue of the small filler, the materials are capable of being polished and do not stain as

readily as conventional materials. As yet, they do not have the abrasion resistance required for restoring the incisal edge.

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Glass- ionomer cements

These were developed for cavity restoration, fissure sealing and luting. They achieve true chemical adhesion to tooth following the application of a citric acid conditioner. They have two disadvantages which are a lack of translucency and a long setting time. Both of these are being overcome by changes in formulation of the cements. The proportioning and mixing are critical in order to produce the correct ‘wetness’ to allow the adhesion reaction to proceed, but predosed capsules have eliminated this problem. The diffusion of fluoride ions from the matrix to the surrounding tooth provides their cariostatic property.

At its present stage of development, this material is the best available for the restoration of Class V abrasion cavities.

REFERENCES AND FURTHER READING

Beagrie G. S. and Brannstrom M. (1977) Pulpal response to cavity treatment with micro- bicidal solution and silicate restorations in monkeys. J. Can. Dent. Assoc. 43, 239.

Bowen R. L. (1963) Properties of a silica reinforced polymer for dental restorations. J. Am. Dent. Assoc. 66, 57.

Brannstrom M. (1969) A new approach to insulation. Dent. Pratt. Dent. Rec. 19, 417. Brannstrom N. and Nyborg H. (1971) The presence of bacteria in cavities filled with silicate

cement and composite resin material. Svensk Tandlak -T. 64, 149. Brown D. (1976) The clinical status of amalgam. Br. Dent. .I. 141. Eames W. B. and Macnamara J. F. ( 1976) Eight high copper amalgam alloys and six conven-

tional alloys compared. Operative Dent. 1, 98. Friend L. A. and Jacobsen P. H. (1972) Handling properties of a capsulated spherical particle

amalgam. Br. Dent. J. 133, 247. Innes D. B. K. and Youdelis W. V. (1963) Dispersion strengthened amalgam. J. Can. Dent.

Assoc. 29, 587. Jacobsen P. H. (1975) Clinical aspects of composite restorative materials. Br. Dent. J. 130,

276. Jacobsen P. H. (1976) Working time of polymeric restorative materials. J. Dent. Res. 55,

244. Jacobsen P. H. (1981) Current status of composite restorative materials. Br. Dent. J. 150,

15. Mahler D. B., Marantz R. L. and Engle J. H. (1980) A predictive model for the clinical

marginal fracture of amalgam. J. Dent. Res. 59, 1420. Mjor I. A. and Espevik S. (1980) Assessment of variables in clinical studies of amalgam

restorations. J. Dent. Res. 59, 15 11. Osborne J. W., Gale E. N., Chew C. L., Rhodes B. F. and Phillips R. W. (1978) Clinical

performance and physical properties of twelve amalgam alloys. J. Dent. Res. 57, 983. Paterson R. C. (1976) The reaction of the rat molar pulp to various materials. Br. Dent. J.

140, 93. Plant C. G. (1972) An investigation into the mechanical, physical and biological properties

of lining materials. MDS thesis, University of Birmingham. Smith D. C. (1968) A new dental cement. Br. Dent. J. 125, 381. Wilson A. D. and Kent B. E. (1972) A new translucent cement for dentistry. Br. Dent. J.

132, 133. Vlietstra J. R., Sidaway D. A. and Plant C. G. (1980) Cavity cleansers. Br. Dent. J. 149, 293.