Filling materials for permanent and temporary restorations and linings.

Lecture 3.  Filling materials for permanent and temporary restorations and linings. General requirements. Classification. Modern filling materials (cements, amalgam, composite materials). Composition, properties. Indications for use.

There are several classifications of filling materials:

–                     Depending to which group the tooth belongs, filling materials are distinguished:

*                   For front group of teeth( filling materials should correspond to high esthetic requirements);

*                   For molars and premolars (filling materials  should  stand high occlusion press)

–                     According to the material from what restorative materials are produced, they are divided into:

1. Metals: amalgam, alloys, pure metals (gold);

2. Non metals: cements, resins, composite materials.

–                     According to the purpose, filling materials are divided for:

*                   Temporary fillings;

*                   Permanent fillings;

*                   Treatment(therapeutic) linings;

*                   Isolative(insulating) linings;

*                   Fillings for the root canal.

 

Separate group of filling materials consist from adhesives(bond system), sealants, varnishes. It’s not filling materials, but dentist can’t work without them.

From the point of view of functionality and peculiarities of their usage in the clinic, all filling materials are divided into 2 groups:

*                   1. Restorative (should provide complete restoration of the shape of the tooth, and also renew the function of the tooth for long time);

*                   2. Curative-prophylaxis  (should have good curative-prophylaxis qualities).

AVAILABLE MATERIALS FOR PERMANENT FILLINGS

The direct restoratives in current, general use are amalgam, composite, glass ionomer and combinations of the last two groups.

GLASS IONOMERS

*                   Glass ionomers contain poly(alkenoic) acid and fluoro-aluminosilicate glass which set by an acid–base reaction  to give a cement.

*                   They adhere directly to tooth substance and to base metal casting alloys.

*                   They release fluoride after placement, giving the materials cariostatic properties, although this may only be short term.

*                   They also have a low tensile strength which makes them brittle and unsuitable for use in load-bearing areas in permanent teeth.

*                   They are used as lining and luting materials and to restore abrasion and erosion lesions, cervical lesions and deciduous (primary) teeth and as interim restorations.

*                   It must be taken into account, however, that they are less translucent than resin composite restoratives and therefore their appearance is less acceptable.

Glass ionomer cements consists of powder (fluoro-aluminosilicate glass) and liquid – 47,5%  water sol. of copolymers of acrylic acid with itaconic or maleic acid. In some glassionomer cements dried copolymer is added to powder, and as a liquid for mixing, distillate water is used. (‘anhydrous’ type)

Classification (R. W. Phillips, 1991), divide GI cements into several types:

І type — cement for fixation of crowns, dentures, orthodontic devices-luting: (AquaCem, Fuji I, Ketac-Cem); main requirement for these materials, the formation of a thin film– with the thickness 11-13 mkm, powder-liquid ratio-1,5: 1.

II type — cements for restoration (Fuji II-GC, Ketac – fil, Chemfil Superior-dentsply). Have higher strength and lower solubility in comparison with other GI cements.

a) subtype — for aesthetic restorations and used for restoration of class III and V cavities, also for treatment of uncarious lesions.

b) subtype — (reinforced) with increased strength to the stress (Fuji IX). Used for filling primary teeth, postponed restoration of permanent teeth, the sandwich technique.

ІІІ type — cements for linings (Baseline, Aqua lonobond). They have shorter working time (manipulation) and hardening time which reduces the total time of restoration.

IV type –dual cure cement (both chemical and with the help of light-curing lamp)

GI cements for root canal obturation. Used in combination with gutta-percha pins. GI cements of this type has extended manipulation time up to 15-20 min, and hardening time up to 1 h.

The most important general characteristics of Glass-ionomer cement:

– ability to form chemical bonding with dental hard tissues;

– anticarious activity;

– sufficient mechanical strength and elasticity;

– satisfactory esthetic features;

– radiopaque during x-ray examination;

– no irritating action on the pulp of the tooth;

Disadvantages of Glass-ionomer cement

– Sensitivity to the presence of moisture during hardening

– Overdrying of the cement surface in the setting stage leads to worsening of its properties and may cause postoperative sensitivity

– Long setting time of the material (24 hours)

– Risk of irritating action on the pulp in deep cavities

Manipulation

The prepared cavity should first be conditioned with a dentine conditioner, the most common one being 10% polyacrylic acid. The conditioner removes the smear layer and debris from the prepared cavity walls, allowing a clean surface to bond to, but leaves smear plugs intact so as to prevent contamination of the cleaned dentine surface with dentinal tubular fluid. The acids used for the bonding of resin composite should not be used for conditioning prior to glass ionomer placement, as the low molecular weight of these acids would demineralize the dentine, leaving less calcium available for the ion-exchange bonding mechanism. The polyacrylic acid conditioning agent should only be applied for a short time, about 10 seconds, and should be washed off thoroughly before drying, without dehydrating the dentine.

Care must be taken when handling this class of materials since they are quite technique-sensitive and errors could lead to a potentially compromised clinical outcome. Manufacturers of these cements recommend that the powder and liquid are dispensed in defined ratios and then mixed rapidly with a spatula within 30–45 s. The general rule is to first incorporate half the amount of the powder into the liquid and then mix in the other half. The stainless steel cement spatula is most often used, although in some practices a plastic spatula is preferred to avoid potential discoloration. As with all powder–liquid formulations care must be taken to dispense the powder and liquid accurately so as to prevent operational variability. The working and set times are dependent on the powder/liquid ratio. The cement should be used immediately after mixing because the working time is only a few minutes at room temperature. If the ambient temperature is high, working time is even further decreased. If a skin forms over the cement, the mix should be discarded – otherwise adhesion will be compromised. The manipulation is easier for encapsulated versions although it is important to follow the manufacturers’ recommended speed and times for trituration of the capsules. Conventional glass ionomers are quite susceptible to moisture contamination. After initial set and removal of excess cement, it is often advisable to coat the cement margins with a sealing agent supplied by the manufacturer.

Fluoride release

Glass-ionomer materials exhibit a sustained release of fluoride over a long period of time. The uptake of the released fluoride ion in human saliva and its incorporation into human enamel have been reported. Although considerable debate exists about the ‘clinical proof’ of the benefits of fluoride, occurrence of recurrent caries in the teeth where these cements have been used is reported to be rare.  This can be attributed to the ability of glass-ionomer cements to inhibit demineralization and enhance remineralization through release of fluoride to the adjacent tissue and surrounding fluid. The rate of fluoride release depends on a particular product brand. However, as a general class, glass ionomers release more fluoride than other types of fluoride-releasing materials.

Adhesion

Like the polycarboxylate cements, the conventional glass ionomers can bond to the calcium of hydroxyapatite of tooth tissue.

Post-operative sensitivity

In the past, one of the common complaints against conventional glass-ionomer cements has been the high incidence of post-operative sensitivity with greater incidence reported for the ‘anhydrous’ type. The most frequent reasons cited for post-operative dentinal hypersensitivity are as given below.

• Desiccation of the tooth. The glass ionomers require water for their setting and may absorb water from the dentinal tubules during this process. This phenomenon is thought to be more prevalent in the anhydrous cements where the dried polycarboxylic acid also needs water for rehydration.

Clinical usage

Although somewhat technique-sensitive, conventional glass-ionomer luting cements have a good clinical history and are employed primarily for the cementation of permanent metal and porcelain-fused-to-metal (PFM) crowns, for cementation of posts and as orthodontic band cement. The prolonged release of fluoride makes them particularly attractive for conditions where the risk of secondary caries is high.

GICs as an adhesive cavity lining (the sandwich technique)

GICs have a number of advantages as a cavity lining as they bond to dentine and release fluoride which may help to reduce recurrent decay. They can be used beneath either a composite resin or an amalgam. The so called sandwich technique involves using a GIC as a dentine replacement and a composite to replace enamel.

Zinc phosphate cement has been a mainstay in dentistry for crown and bridge applications for well over a century and has undergone many refinements in formulation and compounding. In therapeutic dentistry it is used as lining material (ADHESOR – SpofaDental). However, with the advent of newer chemical technologies its use has gradually declined over the last decade. This type of cement is supplied as a powder–liquid formulation.

Composition

The powder consists of mainly amorphous zinc oxide as the major ingredient with small amounts of oxides of magnesium and bismuth added mainly to facilitate the calcining process for manufacture of the powder. Small quantities of silica (typically fumed) are often added to aid in providing the right viscosity and yield stress. Minor amounts of barium and calcium compounds may be added by various manufacturers to provide a smooth, creamy mix, which is desirable for easy flow during cementation of the restoration. Some products also contain tannin fluoride although it is doubtful if the amount of fluoride released is of clinical significance.

The liquid consists of an aqueous mixture of orthophosphoric acid with small amounts of aluminum and zinc phosphates which act as buffers in reducing the reactivity of the free acid. The amount of water used in the formulation is quite critical since it dictates the setting time – too little prolongs the setting time whereas too much shortens it.

Manipulation

The proper proportioning of the powder and liquid components and the mixing technique are critical to ensuring adequate clinical success with these materials. The proper amount of powder should be slowly incorporated in incremental amounts into the liquid on a cool slab (at approximately 21 °C) since the mixing temperature has a profound influence on the setting characteristics. Aggressive mixing results in too high an exotherm for this class of technique-sensitive material. Spreading the mix over a large area on the cool glass slab helps to dissipate the heat. The liquid is rather hygroscopic, absorbing water if exposed to excessive humidity, and losing water if kept in a dry atmosphere or if the vial is not kept properly sealed.

 Properties

The clinically significant properties of zinc phosphate cements include setting time, solubility and film thickness. The mechanical properties of importance are strength, modulus of elasticity and hardness. In comparison with the more recent types of cements, the modulus of elasticity is quite high, which causes them to be rather brittle materials that are prone to fracture.

The solubility of the set cement is relatively high at about 0.2% in 24 h compared with some other classes of cements. Furthermore, these materials have virtually no adhesion to the dentinal core of the tooth preparation.

Positive qualities of these cements are good heat-insulating properties, low toxicity and correspondence of the cement to the coefficient of  thermal dilatation of dental hard tissues. However, they also have some disadvantages: considerable solubility and shrinkage, a small mechanical and chemical resistance compared to silicate, silica, phosphate and other types of cements. Recently, to composition of the zinc–phosphate cements  were added salts of silver and other substances that give cement anti-microbial and cariostatic properties.

Phosphate cement. In dental practice phosphate cement is used frequently for isolative linings, sometimes as a permanent filling material – for deciduous teeth on the stage of root resorption.

TEMPORARY RESTORATIVE MATERIALS AND THEIR PLACEMENT

Temporary restorations are placed for the following reasons:

■ To improve patient comfort by:

1.                Preventing sensitivity.

2.                Preventing food packing.

3.                Restoring appearance.

4.                Covering sharp margins of a cavity.

■ To provide a sedative effect on an infllamed pulp.

■ As an interim restoration before placing the final restoration; perhaps to allow improvement in gingival condition or to assess the patient’s response to diet and oral health advice.

■ As a planned procedure prior to placing a laboratory-made restoration.

■ To assess the prognosis of the tooth and/or pulp.

■ To prevent drifting, over-eruption, tilting or gingival overgrowth.

■ For caries prevention: by using a fluoride leaching material, such as glass ionomer.

Choice of material

This depends on:

■ The size and shape of the cavity: a self-adhesive  material such as a glass ionomer may be required if the cavity has no inherent retentive form.

■ The position in the mouth: tooth-coloured material should be used for anterior teeth. Stronger materials should be used for the occlusal surfaces of posterior teeth.

■ How long the temporary restoration is to be in place: this depends on the wear characteristics of the material used.

■ The choice of eventual restoration: eugenol plasticizes composite resin restoratives so there is a risk that any eugenol remaining from the temporary restoration could adversely affect a subsequent composite resin restoration, although recent research suggests this is not a problem.

Ideal temporary material

The ideal temporary material should be easy and quick to mix, place and shape. It should set quickly and  have appropriate strength and wear characteristics. The material used should be non-toxic and be non-irritant to the pulp, preferably with a sedative effect on the pulp. It should also have an acceptable colour, taste and smell and be cheap and readily available. It is essential that it is easy to remove and is compatible with other materials.

Available materials

■ Zinc oxide eugenol based materials: these are quick and easy to insert and remove, but are unaesthetic, lack compressive strength and the taste is sometimes considered unpleasant.

■ Polycarboxylates.

■ Glass ionomers.

DENTAL AMALGAM

Dental amalgam is a mixture of mercury and an alloy containing silver and tin with added copper and zinc. The alloy and mercury are held together in a capsule, with the two components separated by a plastic diaphragm. When the diaphragm is broken and the capsule is placed in the mixing machine (amalgamator), the two components are mixed together (triturated) to form a silver-coloured paste. This paste is then condensed into the cavity. This is a very important stage: well-condensed amalgams are stronger than poorly condensed ones, as more of the weaker, mercury-rich γ2-phase is removed during carving. The final set material should contain 45–50% mercury; however, when there is less than 50% mercury in the amalgam mix it can prove too dry and difficult to work with. The more mercury in the mixed material, the softer the material is and the easier it is to pack and carve, but the set restoration will be weaker and more prone to corrosion. To reduce the amount of mercury in the final restoration, the amalgam should be vigorously condensed, as this causes excess mercury to rise to the surface where it can be carved away and discarded in a safe manner. For this reason amalgam should always be placed to overfill a cavity.

Amalgam is weak in thin section so cavities have to be cut suitably deep (2 mm) and because amalgam does not adhere to tooth tissue, the cavity must be undercut. Dental amalgam continues to be used despite concerns about health and the environment because it has high clinical success, known performance, relatively low cost and is easy to manipulate. Despite the high usage of this material it is not ideal and suffers from several problems including marginal breakdown, fracture and poor appearance. Secondary caries is the most common reason given for the replacement of amalgam restorations but this diagnosis may not necessarily always be correct.

There is a well-recognized need for effective alternatives, not only because of its less than ideal properties but also because of public and political concerns about its use, the changing patterns of dental disease and patient expectations of dental care.

Properties

Dimensional changes: The setting reaction for amalgam involves a dimensional change.

Strength: The strength of dental amalgam is developed slowly. It may take up to 24 hours to reach a reasonably high value and continues to increase slightly for some time after that. At the time when the patient is dismissed from the surgery, typically some 15–20 minutes after placing the filling, the amalgam is relatively weak. It is necessary, therefore, to instruct patients not to apply undue stress to their freshly placed amalgam fillings.

Plastic deformation (creep): Amalgam undergoes a certain amount of plastic deformation or creep when subjected to dynamic intra-oral stresses.

Corrosion: The term corrosion should be distinguished from the often misused term tarnish. Tarnishing simply involves the loss of luster from the surface of a metal or alloy due to the formation of a surface coating. The integrity of the alloy is not affected and no change in mechanical properties would be expected. Amalgam readily tarnishes due to the formation of a sulphide layer on the surface.

Corrosion is a more serious matter which may significantly affects the structure and mechanical properties. The heterogeneous, multiphase structure of dental amalgam makes it prone to corrosion. Electrolytic cells are readily set up in which different phases form the anode and cathode and saliva provides the electrolytes. Corrosion produces a restoration with poor appearance and may significantly affect mechanical properties.

Copper-enriched amalgams contain little or no γ2 phase. The copper–tin phase, which replaces γ2 in these materials, is still the most corrosion-prone phase in the amalgam. The corrosion currents produced, however, are lower than those for conventional amalgams.

Thermal properties: Amalgam has a relatively high value of thermal diffusivity, as would be expected for a metallic restorative material. Thus, in constructing an amalgam restoration, an insulating material, dentine, is replaced by a good thermal conductor. In large cavities it is necessary to line the base of the cavity with an insulating cavity lining material prior to condensing the amalgam. This reduces the harmful effects of thermal stimuli on the pulp.

Biological properties: Certain mercury compounds are known to have a harmful effect on the central nervous system. The patient is briefly subjected to relatively high doses of mercury during placement, contouring and removal of amalgam fillings.  A lower, but continuing, dose results from ingestion of corrosion products.

Another potential problem concerns allergic reactions to mercury in dental amalgam. Such allergic reactions, usually manifested as a contact dermatitis or lichenoid reaction, are well documented and caormally be explained by previous sensitization of the patient with mercury-containing medicaments.

Serious problems can be avoided by ensuring that the surgery is well ventilated and that flooring  of a suitable type is chosen such that accidental spillages can be readily dealt with. Excess, waste or scrap amalgam should be stored, under water or chemical fixative solution, in a sealed container in order to prevent another possible source of contamination. Mercury or freshly mixed amalgam should never be touched by hand. Mercury is readily absorbed by the skin, a fact which was obviously not appreciated in the days when it was normal practice to ‘mull’ the material in the hand before condensation. In addition to being hazardous this practice leads to contamination of the amalgam.

Despite the increased exposure of dental personnel to mercury vapour, examinations of the health, mortality and morbidity rates for dentists have shown that they are not significantly different from those of the general population, a fact which should go a long way towards reassuring those who harbor fears over mercury toxicity.

DIRECT RESIN COMPOSITES

Direct resin composites are the material of choice for anterior restorations and they are increasing in use and popularity for posterior restorations, mainly because of their appearance. Composites do not adhere directly to tooth tissue and rely on the acid-etch technique and the use of dental adhesives for adhesion to enamel and dentine.

The physical characteristics of resin composite materials are much improved from their initial forms, and methods of handling them have developed considerably and are no longer copies of amalgam techniques. However, it can be more difficult to generate good proximal contour and contact with these materials than with amalgam. Polymerisation shrinkage of the resin during curing (in the order of 2–3%) still occurs and may contribute to marginal defects, cuspal distortion and crack formation in the enamel or dentine, and may therefore contribute to postoperative pain or sensitivity for the patient. There are, however, a number of clinical techniques available to overcome these problems and the longevity of restorations using the newer resin composites is much improved over that of the original materials.

Reducing the effect of polymerisation shrinkage may be achieved by incremental packing of the  curing composite. Each increment should touch as few walls of the cavity as possible. The stress induced by polymerization shrinkage is highest in cavities with more bonded than unbonded surfaces: the occlusal cavity has the potential for the most stress. The final outer layer of a restoration is oxygen-inhibited – superficial soft, sticky layer, on freshly polymerized resin; this free monomer layer remains uncured. To prevent its formation it must be covered to eliminate contact with air and complete composite polymerization. Coating composite with glycerin and light curing is a popular technique. Alternatively, composite is overbuilt and the outer uncured layer is removed.

Classification and composition of composites

Resin matrix

The setting reaction of a resin composite involves the polymerisation of the resin matrix. This is the process whereby small components, termed monomers, combine to form large-chain molecules. The longer the chain, the more viscous the material becomes until it reaches a solid state. The polymerisation process is often described as the  setting reaction or  curing process. During storage and the placing of restorations, it is a requirement that the monomers are prevented from polymerising, other-wise the material would be difficult to handle. In order for polymerisation of resins to occur when desired, free radicals, highly reactive charged substances, need to be created within the material. These initiate the chain reaction of polymerisation. Free radicals can be created in one of two main ways with modern resin composites: by chemical reaction or by light.

All dental composites consist of a blend of resin and inorganic filler. Methods used to characterise materials are based upon the method used to activate polymerisation of the resin and on the particle size distribution of filler.

Methods of activation: Polymerisation may be activated chemically, by mixing two components, one of which typically contains an initiator and the other an activator, or by an external ultraviolet or visible light source. The traditional method for delivering the blue visible light required for ‘visible light activation’ involves the use of a quartz tungsten halogen (QTH) lamp. Other systems including plasma arc, laser and light emitting diode (LED) systems are now also available.

For chemical activation, many different methods of dispensation are available. The most popular is the ‘two paste’ system. Each paste contains a blend of resin and filler. One paste contains about 1% of a peroxide initiator, such as benzoyl peroxide, whilst the other paste contains about 0.5% of a tertiary amine activator, such as N, N′ dimethyl-p-toluidine on p-tolyl diethanolamine. The ensuing reaction is a free radical addition polymerization.

Other systems which rely on chemical activation are as follows:

1.                Powder/liquid systems, in which the powder contains filler particles and peroxide initiator whilst the liquid contains monomer, comonomer and chemical activator.

2.                Paste/liquid materials in which the paste contains monomers, comonomers, filler and peroxide whilst the liquid contains monomers and chemical activator.

3.                Encapsulated materials in which the filler, mixed with peroxide, is initially separated within a capsule from the monomers and comonomers containing the chemical activator. On breaking the seal between the two parts of the capsule the reactive components come into contact and are mixed mechanically.

Hints and tips on the use of resin composites

Unlike amalgam, resin composites are technique sensitive. They are not tolerant of poor placement techniques, yet the results achieved are generally excellent in carefully selected cases. Success is more likely if attention is paid to the following:

■ Resin composites should not be packed or condensed into cavities. They are not ‘white’ amalgam and should not be handled in the same way as amalgam. They should be adapted and contoured to the preparation. Packing of resin-based products, particularly with a poorly adapted matrix band, is associated with an increased incidence of overhang formation that is often difficult to remove. Packing has also been shown to increase the formation of voids within the set material.

■ In proximal preparations it is advantageous to build up the marginal ridge of the restoration in the first instance. Further build-up can be as for an occlusal preparation, which simplifies the process.

■ Try not to connect two opposing walls when an increment is placed as it places the opposing walls under stress when the increment is cured. A clinical example of this is bulk curing of a large mesio-occlusal-distal resin composite placed in premolar teeth where cracks are ofteoted at the base of a cusp after curing. Oblique increments are recommended for this reason; they also facilitate easier contouring of the restoration.

■ Transparent matrix bands offer no advantage over metal matrices. Apart from the difficulty of using them, they are so thick that they may lead to under-contouring of the contact area. Metal matrix bands, especially sectional metal matrices, have been shown to be equally effective for the placement of proximal resin composites, provided additional trans-tooth curing is done in the form of 20-second curing from either side of the embrasure after the band is removed.

■ Use separate instruments for resin composite. Several manufacturers produce non-stick instruments especially for use with resin-based restorative materials. These should not be contaminated by using them with amalgam as this can lead to discolouration of the resin composite.

■ The curing of resin composite is inhibited by oxygen, which is helpful on the one hand, in that each cured increment has uncured monomer on the surface to which additional increments can be added. On the other hand, this is undesirable for the final increment placed and for that reason it is preferable to overbuild the restoration slightly and cut it back during finishing and polishing.

■ The use of a wedge is important when proximal preparations are restored. It creates a potential space for the interdental papillae and allows for tight contact point formation, especially when resin composite restorations are placed.