7. Filling materials. Classification. Dental cements, their classification and characteristics. The concept of the contact point; the value of its improper restoration in the development of periodontal pathology.  Dental instruments for its restoration. Polishing of fillings: instruments, methods.

7. Filling materials. Classification. Dental cements, their classification and characteristics. The concept of the contact point; the value of its improper restoration in the development of periodontal pathology.  Dental instruments for its restoration. Polishing of fillings: instruments, methods.

Direct restoratives: clinical properties, handling and placement

Teeth may require repair as the result of dental caries, tooth wear, trauma or developmental defects. Despite decreases in the occurrence of primary dental caries in most industrialized countries, the restoration of teeth continues to be necessary. Maintenance and replacement of existing restorations represent a large component of this dental treatment, particularly in the older adult.

Several factors have to be taken into account when choosing the most appropriate restorative method and material for a clinical situation. The limiting factors include:

■ Patient motivation and suitability.

■ The number of remaining teeth and their relative positions.

■ The condition of their supporting tissues.

■ The amount of remaining tooth structure.

■ The restorative materials available, and their longevity as restoratives.

■ The occlusion and opposing teeth and restorations.

■ Aesthetic and other wishes of the patient, including cost factors.

When any method or material is chosen, it must be used in the most appropriate clinical situation.

Classification of filling materials

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).

*                    Temporary filling materials.

*                    1) Zinc-sulfate cements(dentin-powder, dentin-paste)

*                    3) Zinc-eugenol cements

*                    5) Poly–carboxylate cements

*                    6) Zinc-phosphate cements

*                    7) Glass-ionomer cements

*                    Filling materials for linings

*                    Isolative linings:

*                    1) Zinc-phosphate cements

*                    2) Glass-ionomer cements

*                    3) Poly–carboxylate cements

*                    Treatment (therapeutic) linings:

*                    1) Materials based on calcium hydroxide

*                    2) Zinc-eugenol cements

*                   3) Combined therapeutic pastes(not setting, are prepared ex tempore)

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

 

Conventional Glass-Ionomer Cements

As with other dental materials, the chemistry of dental cements has undergone continual evolution. The conventional glass-ionomer cements combine the technologies and chemistries from silicate and zinc polycarboxylate cements in order to incorporate the desirable characteristics of both. Thus they contain the ion-leachable fluoroaluminosilicate (FAS) glass of silicate cements but avoid their susceptibility to dissolution by substituting the phosphoric acid and its salts with the polymeric carboxylic acids of the zinc polycarboxylate cements. Like their predecessors, the zinc polycarboxylates, these materials are primarily acid–base cements.

Glass ionomer cements (GICs) were first introduced in the 1970s. In dentistry they have many applications including restorations, lining and bases, endodontics sealers and fissure sealants. Glass ionomers have two significant advantages: they adhere chemically to enamel and dentine and they release fluoride. They also adhere to base metals and so may be used as luting cements. They do not have the compressive strength of amalgam, and do not have the flexural strength or aesthetics of resin composite, and so, as a primary restorative material, their use is limited to that of a temporary restoration material, or they can be used to restore class V and class III cavities, where aesthetics are not important. They are also widely used as a restorative material for primary (deciduous) teeth and in the atraumatic restorative technique (ART), as they do not require any cavity modification other than caries removal. There have been many modifications to the standard glass ionomer; however, at the heart of each type of glass ionomer is the same basic setting reaction.

Biocompatability

GICs are very biocompatible materials with little evidence of toxic or allergenic effects. Some concerns over the possible toxic effects of aluminium have lead to attempts to produce aluminium-free glass ionomers, but these have so far proved unsuccessful in terms of producing a material suitable for clinical use.

Bonding to tooth substance

There are several theories on how glass ionomers bond to tooth substance. What is generally accepted is that the carboxyl groups bioreact with apatite in the tooth structure in a similar way to how they react with the glass particles: by causing dissociation of calcium and phosphate ions from the apatite and then the negatively charged carboxyl groups bonding with the charged surface of the enamel or dentine, so called ‘ion exchange’. In dentine, however, there is a much lower inorganic content, and adhesion is thought to be mainly due to the creation of hydrogen bonds with collagen within the dentine. The bond strength to dentine is relatively weak and has been estimated to be about 5 MPa. The failure of the glass ionomer bond, however, tends to be cohesive, i.e. there is a failure within the glass ionomer itself. Thus, if a restoration is lost, there is usually a thin layer of  GIC left on the dentine, and the pulp–dentine complex will remain protected. The bond to dentine has been described as a dynamic bond, which breaks and reforms as the organic component of the dentine itself undergoes natural turnover. This is in contrast to that of resin-based adhesive materials that adhere to tooth via micromechanical interlocking, which is not able to adapt to natural collagen turnover within the dentine, and which tends to fail adhesively. The failure occurs between the resin and the dentine and therefore the pulp–dentine complex is not protected.

Composition

Although the original systems developed by Wilson and Kent (1971) have undergone several modifications, the essential components of this class of materials are unchanged and consist of water sol. of polycarboxylic acid, FAS glass, tartaric acid, pigments and radio-opacifiers. The polymeric components of most commercial embodiments are copolymers of acrylic acid with itaconic or maleic acid. Tartaric acid is added to the liquid to control the viscosity as well as the working and setting properties of the cement. The basic component is an acid-reactive FAS glass containing additional multivalent ions such as calcium, strontium and lanthanum, some of which are added to provide radiopacity to the set cement. The molar ratio of aluminum to silicon in the glass network has to be carefully controlled (usually 2 : 3 to 1 : 1) to provide the right balance of reactivity versus stability. These cements are supplied as bulk powder–liquid formulations as described for the zinc polycarboxylates. Both hydrous and anhydrous forms are available, depending upon the manufacturer. In the latter, the freeze-dried polyacid is mixed with the powder and water is supplied as the liquid.

In order to make it convenient for the dentist, manufacturers have developed unit-dose capsules, which are activated just before use by breaking a seal that allows the powder and liquid to come into contact. An amalgam triturator is then used to mix the components mechanically while specially designed delivery guns are provided to extrude the mixed paste directly into the oral cavity. The glass-ionomer luting cements contain a lower powder/liquid ratio, and hence are less viscous upon mixing than the restorative products. Furthermore, the particle size of the glass has to be quite small in order to meet the ISO specifications of film thickness.

Mechanism of action

Setting reaction

The essential constituents of glass ionomer cements are a poly-alkenoic acid and an ion-leachable glass. The glass particles can vary in size, and the size can determine the properties of the material, and are therefore graded for the various applications for glass ionomers. The acids involved, are originate from the polyalkenoic family of acids, which includes, amongst others, poly-acrylic acid, poly-itaconic acid and poly-maleic acid. The glass particles commonly are a calcium-alumino-silicate glass and contain calcium or strontium and fluoride. Restorative glass ionomers tend to contain larger particles than luting cements or lining materials; the setting reaction is, however, essentially similar. The setting reaction undergoes three main stages: dissolution, gelation and hardening. In the dissolution stage, the poly-alkenoic acid, in the presence of moisture, releases protons which attack the surface of the glass, causing the release of fluoride, calcium and aluminium ions. The outer surface of the glass becomes gel-like as the lost ions are replaced by hydrogen from the carboxylate groups from the poly-acid chains. The gelation reaction occurs over a few minutes, where the freed divalent calcium ions form ionic bonds with the negatively charged carboxyl ions on the poly-acid chains. They form cross-links if they connect to two adjacent chains or they may join two adjacent carboxyl groups on the same chain. During this stage, the material firms to the touch. Shaping of the restoration should be done quickly, as the material becomes friable as it hardens, making it very difficult to achieve a smooth shape. The hardening phase may take several days to complete, and only then does the material reach its final strength. During this phase, trivalent aluminium ions form more effective cross-links between adjacent chains.

Other components of the glass ionomer cement include tartaric acid, pigments and radio-opacifiers. The tartaric acid, when used in the correct concentration, will cause the setting to be improved; that is, the material remains workable for a sufficient amount of time to be placed and shaped, and then hardening occurs more rapidly. The final set material is porous enough to allow the free movement of small molecules, such as hydroxyl and fluoride ions, in and out of the material.

 

When the powder and liquid are combined, an acid–base reaction occurs between the FAS glass and the polycarboxylic acid in the presence of water eventually leading to a hardened mass. As with the zinc carboxylate cements, water plays several important roles in the overall setting of glass-ionomer cements.

 A very important by-product of the setting reaction is the release of fluoride ions from the glass matrix. This fluoride release process is sustained and occurs over a long period of time. It is important to realize that this fluoride ion release is a result of the setting reaction and the ion-exchange process in the cement. In this process, the fluoride from the glass is replaced by carboxylate groups and water. Hence, if properly formulated, there is little chance of the cement losing its strength with time. Considerable research has proved that no loss of strength of the glass-ionomer cement occurs over extended storage in water.

Role of tartaric acid

Tartaric acid is added to prolong the working time of the cement mix, improve the manipulative characteristics and narrow the range of the setting time. In the absence of this component, the mix becomes rubbery within a few seconds and is rather difficult to work with. The mechanism by which tartaric acid operates is believed to be a temporary suppression of the ionization of the polyacid as well as a preferential extraction of the cations from the glass so that polyacrylate complexes cannot form immediately, thus leading to an increased working time. Later, it sharpens the set and accelerates the hardening of the glass-ionomer mix.

 

Manipulation

Handling notes

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.

Setting and working time

The factors controlling the setting and working times are similar to those described for the polycarboxylate cements except that here the reactivity and the particle size of the FAS glass, rather than zinc oxide, are of relevance.

Physical properties

The conventional glass ionomers are rather brittle materials with high modulus of elasticity, low diametral tensile strength and low fracture toughness. They are susceptible to desiccation and hence must be protected with a varnish or a resin bonding agent during the setting process. Compressive strengths are similar to those of the zinc phosphate cements.

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.

•  Initial low pH of the material (although it increases rapidly during the setting process) may irritate the pulp.

•  Moisture contamination leading to poor sealing of the tooth and improper margins, hence leakage to bacteria and their by-products.

•  Too low a viscosity of the mix causing fluid to be forced down the dentinal tubules during seating of the prosthesis.

 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. According to some manufacturers the use of polyacrylic acid conditioner is contraindicated in crown and bridge procedures so that the smear layer on the abutments remains to act as a protective layer. Their prolonged maturation time, however, allows for early water sensitivity and they are also more susceptible to hydrolytic degradation than the more insoluble RMGIs or composite luting cements. For this reason, it is advisable to use protective agents such as varnishes or resin-glazes along the crown margins or cover the fillings made from glass-ionomer with the varnish.

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.(Fig. 7)

 

 

Indications for glass ionomer cements

GICs generally are indicated for use in the following circumstances:

■ Liners and bases for direct and indirect restorations.

■ RMGICs are indicated for bonded-base restorations and temporary restorations, especially between appointments in endodontic therapy.

■ For rapid stabilisation of a dentition where there are multiple cavities in a patient with a high caries risk.

 ■ Atraumatic restorative technique (ART).

■ Cementation of cast indirect restorations.

■ Traditional GICs are useful for the restoration of root caries in the elderly patient.

 

Resin-modified glass ionomers

Resin-modified glass ionomers have a resin (monomer) component as well as the poly(alkenoic) acid and fluoro-aluminosilicate glass of conventional glass ionomers. They set by two mechanisms: acid–base reaction and curing of the monomer (chemically, by light or both).

They have improved appearance and physical properties compared with conventional glass ionomers. They are used in similar situations to glass ionomers and may also be used for small core build-ups.

RMGIs, developed in the late 1980s, are more recent entrants into the dental cement arena, having been first introduced commercially as a luting cement in 1994. This class of cements is less technique-sensitive than the conventional glass-ionomer materials and possesses some very favorable physicomechanical properties compared with conventional GI materials yet releases similar levels of fluoride. Since the RMGI luting cements also allow easy removal of excess cement, show minimal post-operative sensitivity and, so far, have exhibited good clinical performance and durability, this class of material has rapidly become one of the most popular materials for routine crown and bridge applications. Improvements in delivery format have now consolidated their use.

 Composition

The first-generation materials were supplied as powder–liquid configurations and are available in hand-mix or encapsulated versions. Vast differences do exist between products from different manufacturers; hence care has to be taken in choosing a particular commercial product for clinical use. The essential components of a true RMGI are:

• polycarboxylic acid copolymer often modified with pendant methacrylate groups;

• FAS glass;

• water;

• water-compatible methacrylate monomers;

• free-radical initiators.

 Setting and working time

The working time is typically greater than 2.5 min from the start of mix at ambient temperature of 23 °C. Higher temperatures and vigorous spatulation shorten the working time while lower temperatures prolong it. Excess material can be removed when the cement reaches a waxy stage after placement in the mouth (2–3 min at 37°C) using a suitable instrument. The restoration should be finished and the occlusion checked when the material has completely set (about 5 min from placement, depending on brand).

 Post-operative sensitivity

Perhaps the most attractive reason why this class of cements was readily adopted in clinical practice is because the users of RMGI luting cements observed remarkably low post-operative sensitivity in their patients (Christensen, 1995; Hilton et al., 2004). This is perhaps because these self-adhesive materials do not need the removal of the dentinal smear layer and are not as technique-sensitive as the conventional materials. The seal at the dentinal margin is very good; hence, hydrodynamic fluid flow or the ingress of bacteria do not appear to be a problem. Another factor that may play a role (though a minor one) is that the RMGIs start out at a higher pH (lower acidity) and, hence, do not cause an adverse reaction on the pulp.

 

Polyacid modified resin composites

 

Polyacid modified resin composites are also known as compomers.

*                   Their properties are more like those of composites than glass ionomers.

*                   They have limited fluoride release but are stronger and have a better appearance than glass ionomers.

*                   Their wear resistance is less than that of composite restoratives.

*                   They do not adhere directly to tooth substance, they require an adhesive agent to create a micromechanical bond to tooth structure.

*                   They may be utillised to restore cervical and anterior proximal cavities and for primary teeth.

Advantages

Compomers and resin composites share the same advantages. Additional advantages include fluoride release and ease of handling. The only situation where the use of a more hydrophilic resin system (i.e. a compomer) might be advantageous is in the restoration of non-carious and carious cervical lesions where moisture control can be problematical. The hydrophilic nature does not mean that the material can tolerate excess moisture in the form of saliva.

 

POLISHING METHODS FOR GLASS IONOMER CEMENTS RESTORATIONS

 

Type II.1 restorative aesthetic
Dual cure
–         Contour and polish immediately after light activation, working from restoration to tooth only. –         Begin with fine polishing diamonds at intermediate high speed (40 000 revolutions/min) under air/water spray. –         Continue with ever-finer diamonds at lower speeds, still under air/water spray. –         Finally complete using aluminium oxide graded polishing discs at slow speed under air/water spray, then seal with a low-viscosity resin glaze.
Auto cure
–         Because of the slow-setting chemistry, do not attempt to contour or polish the cement for at least 24 hours. –         Gross contour can be achieved with very fine sintered diamonds under air/water spray at 20 000 revolutions/min. –         Refine the surface with graded rubber polishing points and cusps at 5 000 revolutions/min under air/water spray. –         Finish to a gloss with graded polishing discs at 3000 revolutions/min under air/water spray, then seal with a low-viscosity resin glaze.
Type II.2 restorative reinforced
–         Because of the rapid-setting chemistry, these cements can be contoured and polished beginning at 6 minutes from the start of mix. –         Gross contour can be achieved with very fine sintered diamonds under air/water spray at 20 000 revolutions/min. –         Refine the surface with graded rubber polishing points and cusps at 5 000 revolutions/min under air/water spray. –         Interproximal surfaces can be contoured and polished with the Profin Directional System equipment using diamond or polishing blades.
A selection of polishing cusps and points, finishing and polishing discs.	A selection of graded fine diamond polishing stones. Note the colour coding from moderately coarse to extra fine, allowing for rapid development of a fine surface.

 

Zinc phosphate cement

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.

 Zinc polycarboxylate cements

Zinc polycarboxylate cements, sometimes referred to simply as ‘poly-carboxylate’ or ‘polyacrylate’ cements, are based on the reaction of zinc oxide with polycarboxylic (same as polyalkenoic) acid in water (Smith, 1968). Like the zinc phosphate cements these are also supplied as powder–liquid compositions.

 Composition

The powder is quite similar to that of zinc phosphate cements and consists mainly of sintered zinc oxide ground to a fine particle size. The sintering process reduces the reactivity of the amorphous zinc oxide and helps in the manipulation of the cement. Small amounts of magnesium oxide (1–5%) are added to aid the sintering process while the incorporation of fumed silica helps in the mixing and flow of the cement. Fluoride salts, e.g. stannous fluoride, may also be incorporated in small amounts to improve mechanical strength and to serve as a source of leachable fluoride. In some commercial embodiments the powder is coated with 5–20% of anhydrous polyacrylic acid to make it less technique-sensitive during the mixing process. The liquid consists of an aqueous solution of a polycarboxylic acid, generally a homopolymer of polyacrylic acid or a copolymer of acrylic acid with itaconic or maleic acids. The average molecular weight (weight average) is usually in the range 20 000–50 000. The viscosity of the solution may be controlled by the addition of small amounts of tartaric acid. For products where the powder is coated with polycarboxylic acid, the liquid is either a dilute solution of the polyacid or simply water.

 Mechanism of action

Setting reaction

The polycarboxylic acid reacts with the basic zinc oxide in a neutralization reaction forming a zinc polycarboxylate complex salt (Smith, 1983). This results in the formation of a cross-linked polycarboxylate hydrogel reinforced by the oxide particles. The cross-linked gel is bound to the polyanion chains by electrostatic interaction. The setting reaction has been studied by infrared spectroscopy which shows that the carboxylic acid groups (COOH) are progressively converted to carboxylate (COO−) groups as the cement hardens.

Water plays several important roles in controlling the chemistry and properties of all acid–base cements. These are outlined below.

1.   As a diffusion medium. Water is needed for the acids to ionize so that the protons can be dissociated and solvated and the acidic property can be manifested. It is also needed for the diffusion of the metallic ions of the bases (e.g. zinc oxide) so that these can enter the liquid phases and thus react with the acid. In addition, it is essential for the diffusion of fluoride ions, where present, out of the set cement.

2.   As a stabilizer of the carboxylate complexes. A portion of the water coordinates with the zinc carboxylate complexes by electron donation to stabilize them. This is known as bound water and cannot be easily removed from the stabilized matrix.

3.   As a plasticizer. The residual water helps to plasticize the set cement and makes it more resilient and less prone to failure by fracture.

 Properties

Setting and working time

Setting and working time are two important clinically relevant parameters for any cement. The working time is the time available for the manipulation of the unset cement while the setting time is the time required by the material to set or harden from a fluid or plastic state to a rigid one. There is no standard value for the working time, but it should be reasonably long so that adequate time is available for mixing and placing the cement. Since both the working time and setting time are dependent on the same chemical reaction the former should not be too long to ensure the setting of the cement within a reasonable time.

Adhesion to tooth structure

Polycarboxylate cements can bond to enamel and dentine by ionic bond formation with the calcium ions of the hydroxyapatite mineral. In order to achieve this it is essential to have the prepared tooth surface free of debris and contaminants. Bond strength values as measured in the laboratory are dependent on the cohesive strength of the cement, particularly its modulus. Measured values are therefore low (enamel 3–13 MPa; dentine 2–4 MPa) compared with RMGIs and resin cements. However, it is debatable whether in vitro bond strength measurements are a true reflection of clinical performance. In actual usage, the service life of prostheses cemented with polycarboxylates is quite acceptable clinically.

Clinical usage

Zinc polycarboxylate cements have a fairly long record of clinical success when used for specified indications. The most frequent use is for cementation of cast alloy- and metal-supported inlays, onlays and single-unit crowns. Because of the lower values of compressive modulus these materials should not be used for permanent cementation of long-span bridges. Less frequently they are also used as cavity liners and orthodontic band cementation but in such cases a higher powder/liquid ratio is recommended by manufacturers. The comparatively high long-term solubility and low hardness limit the utility of this class of materials for permanent cementation, resulting in a decline in their use in recent times.

 

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.

 

Temporary coronal restorations during endodontic treatment

 

Root canal treatment may involve multiple visits. Also, unless it is limited to a routine access cavity, the final restoration is usually not completed in the same appointment as the root canal treatment. A temporary restoration, normally for 1 to 4 weeks, is then required. In special situations when definitive restoration must be deferred, the temporary must last for several months.

Objectives of Temporization in endodontology

 

The temporary restoration must do the following:

1. Seal coronally, preventing ingress of oral fluids and bacteria and egress of intracanal medicaments.

2. Enhance isolation during treatment procedures.

3. Protect tooth structure until the final restoration is placed.

4. Allow ease of placement and removal.

5. Satisfy esthetics but always as a secondary consid-eration to providing a seal.

These  objectives  depend  on  the  intended  duration  of  use.  Thus  different  materials  are  required,  depending  on  time, occlusal load and wear, complexity of access, and loss of tooth structure.

Techniques of Placement

 

The quality of the coronal seal depends on the thickness of the material, how it is compacted into the cavity, and the extent of contact with sound tooth structure or restoration. A minimum depth of 3 to 4 mm is required around the periphery, preferably 4 mm or more to allow for wear. In anterior teeth, the access is oblique to the tooth surface; care must be taken to ensure that the material is at least 3 mm thick in the cingulum area. Temporary restorative material (dentine paste) is placed as follows. Chamber and cavity walls should be dry. Dentine paste can be placed over thin  layer  of  cotton that is  placed  upon  the  canal  orifices  to  prevent  canal blockage or upon orifices of obturated root canals.  (Figure 15-28). Care must be takeot to incorporate cotton fibers into the restorative material, which can promote rapid  leakage.  Dentine paste  is  packed  into  the  access  opening  with  a  plastic  instrument  by one portion from  the  bottom  up  and pressed against the cavity walls. (Figure 15-29). Excess is removed, and the surface smoothed with moist cotton. The patient should avoid chewing on the tooth for at least an hour.

 

Figure 15-28 Techniques for temporization. The left two are correct techniques. Either minimal space is occupied by cotton or no cotton pellet is

used,  particularly  if  the  proximal  is  to  be  restored.  The  right  diagram  is  incorrect.  Most  of  the  chamber  is  packed  with  cotton,  which  leaves

inadequate space and strength for the material (3 to 4 mm are required), and cotton fibers may promote bacterial leakage.

 

 

 

 

Figure 15-29 Techniques for placing temporary material.

A, Single large “blob” placed in the access opening will not seal the walls.

B,  The incremental technique, which adds successive layers pressing each against the chamber walls, is correct.

 

Subsequent  removal  using  a  high-speed  bur  requires  care  to  avoid  damage  to  the  access  opening. Alternatively,  a slow-speed handpiece can be used.

 

 

The concept of the contact point; the value of its improper restoration in the development of periodontal pathology. 

Dental instruments for its restoration.

When a restoration involves an interproximal surface, it is not possible to achieve a properly adapted restoration without a matrix band. A matrix band creates a temporary interproximal surface, and, when appropriate, a matrix retainer secures the matrix band in place.

There are some features of the II class cavities treatment. One of the main tasks in the treatment of such cavities is to restore the contact point between adjacent teeth. The restoration of contact point will prevent from further developing of periodontal diseases. To restore the contact point clinician used matrices.

Matrix band: a matrix band is placed to help retain the restorative material during placement, to give shape to the proximal surface of the restoration and to allow close adaptation of the restorative material to the cavity. The band should be closely adapted to the cervical margin and should be burnished against the adjacent tooth to help formation of a good contact. There are many types of matrix bands and holders (Fig. see below), but commonly used ones are:

Fig. 10 Matrix bands and holders:

(a) Siqveland; (b) Tofflemire; (c) Ivory; (d) cellulose strip; (e) incisal corner; (f) opaque cervical matrix; (g) clear cervical matrix; (h) circumferential matrix; (i) circumferential matrix.

● Siqveland: this system uses a straight band and the holder and band are removed from the tooth simultaneously. This can sometimes result in removal of part of the newly packed amalgam.

●  Tofflemire: this system has the advantage that the holder is removed before the band and this may  prevent removal of the restoration with the band.

●  Circumferential: a number of systems exist that have no retainer/holder. The band is tightened by  a spring mechanism.

●  Ivory: this has a holder which engages into a selection of holes in a metal band. The metal band replaces only one proximal wall and therefore  cannot be used for cavities involving both proximal walls.

■ Wedge: the next stage is to place a wedge at the cervical margin of the band, normally from the buccal aspect. The wedge has several functions:

●  It separates the teeth slightly so that when the matrix band is removed there is no space between the adjacent teeth and a tight contact is formed. Wooden wedges swell slightly by absorbing moisture in the mouth so are preferable to plastic wedges.

●  It prevents excess material at the cervical area of the cavity forming a ledge.

●  It shapes the band at the cervical margin of the tooth.

●  It can help retain the band in place.

 

Information was prepared by Levkiv M.O.