Fluoride release from giomers is comparable to that from acid-modified composites (compomers). The fluoride available for release is that which is liberated from the glass during the initial reaction with acid prior to blending with resin. Hence, the fully reacted glasses produce products with significantly greater fluoride release than the surface reacted glasses. Furthermore, the presence of HEMA in the fully reacted glass products facilitates more rapid diffusion of water and fluoride through these materials.

The modified composite materials and giomers may have an ability to be recharged with fluoride in a similar manner to that described for conventional glass-ionomers. This may be another example of the potentially ‘smart’ behaviour of materials with some glass ionomer structure.

The effectiveness of fluoride release as a form of prevention or protection is a matter of some debate. After the first few days of initial ‘fluoride burst’ which is seen for conventional glass ionomers and some resin-modified materials, the rate of release becomes very low and it is debatable whether this long-term release can be beneficial. However, a sustained long-term release promoted by a continual release, recharge and re-release may well provide the positive therapeutic effect which dentists require. Hence, it is likely that the long-term pattern of recharging and re-release is much more important than the high initial release. On the other hand there is some evidence that the materials which have the greatest initial fluoride release also have the greatest ability to be recharged.

Fluoride release of materials changes in response to changes in pH. Lower pH generates greater levels of fluoride release and this may be another example of ‘smart’ behaviour since the benefit of fluoride is particularly required in regions subjected to plaque acids. However, the mechanism of fluoride release at low pH is likely to involve some dissolution of the material and so the increased fluoride release may be at the expense of material durability and longevity.

25.8 Clinical handling notes

The handling characteristics and clinical techniques used for these materials are different to those for the conventional GICs, not least because they become hard, as a consequence of the visible- light-activated reaction, well before the acid–base reaction is complete.

Matrix techniques are similar to those used for composites with transparent cellulose acetate or polyester strips on proximal surfaces. Again, cervical cavities pose a problem because of the need for transparent matrix to permit light activation. One possibility is to use a small piece of kitchen film (cling film) as a physical separator that will allow contouring using hand instruments prior to light activation.

These materials are not as susceptible to desiccation as the chemically setting types and consequently surface finishing can be undertaken immediately after placement.

25.9 Suggested further reading

Berg, J.H. (1998) The continuum of restorative materials in pediatric dentistry: a review for the clinician.

Pediatr. Dent. 20, 93.

Garcia-Godoy, F. (2000) Resin-based composites and compomers in primary molars. Dent. Clin. North Am. 44, 541.

Hse, K.M., Leung, S.K. & Wei, S.H. (1999) Resinionomer restorative materials for children: a review.

Aust. Dent. J. 44, 1.

ISO 9917-2 (1998) Dental water-based cements Part 2: light activated cements.

Itota, T., Carrick, T.E., Yoshiyama, M. & McCabe, J.F. (2004) Fluoride release and recharge in giomer, compomer and resin composite. Dent. Mater. 20, 789.

McCabe, J.F. (1997) Resin-modified glass ionomers.

Biomaterials 19, 521.

Sidhu, S.K. & Watson, T.F. (1995) Resin-modified glass ionomer materials. A status report for the American Journal of Dentistry. Am. J. Dent. 8, 59.

26.1 Introduction

Temporary crown and bridge resins are used to provide immediate temporary coverage following tooth preparation for crowns or bridges.

The usual technique is to record an initial impression in an alginate material prior to tooth preparation. The major impression may be recorded then or subsequently using an elastomeric impression material. The mixed temporary crown or bridge resin is applied to the prepared areas by placing it into the desired area of the alginate impression which is reseated in the patient’s mouth. After initial setting, the impression and the resin are removed and final hardening occurs outside the mouth. Temporary crowns can also be fabricated by placing the resins on to prepared teeth in clear plastic crown formers. In addition, the fit of prefabricated crowns can be improved by relining them with one of the temporary crown and bridge resins.

The temporary crowns and bridges are cemented into place with temporary cements, normally of a zinc oxide – eugenol composition.

26.2 Requirements

The product should ideally be non-injurious to oral tissues since it comes into direct contact with freshly cut dentine and the oral mucosa. During setting, it should not give an unduly large temperature rise whilst in contact with dentine, as this could damage the pulp. It should not undergo a large setting contraction which could make removal of the temporary crown or bridge difficult, particularly if the set material is rigid.

The material should also have convenient setting characteristics, including the following.

(1)Sufficient working time to allow mixing, placement into the impression and seating into the mouth.

(2)After seating in the mouth, rapid attainment of a ‘rubbery’ stage which facilitates its easy removal without distortion.

(3)Rapid hardening outside the mouth, enabling the trimmed crown or bridge to be cemented into place after a short time.

It should be strong and tough enough to resist fracture and wear in use and should, ideally, be tooth-coloured. Factors which affect durability and appearance are not of prime importance in view of the temporary nature of its use.

26.3 Available materials

Five types of material are available, as outlined in Table 26.1. The auto-cure acrylic material is essentially identical with that discussed on p. 195 as a restorative resin. A dual cure methacrylate material is now available. The liquid contains both chemical and light-sensitive initiators. The material undergoes a chemical setting reaction initially to achieve a partial set, the provisional restoration can be removed from its matrix (and the mouth) at this stage and final setting is achieved by visible light curing either in the mouth or at the chair-side. Care is required as the heat generated during this latter process is substantial.

The higher methacrylate resin is similar in many ways to the product sometimes used as a reline material for dentures and described on p. 124.

The composite material is different from those used as filling materials, which are not suitable for temporary crown and bridgework due to their unfavourable setting characteristics.

26.4 Properties

Setting characteristics: The composite materials have the advantage of exhibiting a distinct rubbery stage, during which the temporary crown or bridge can be removed from the patient’s mouth without distortion or damage.

This rubbery phase is achieved in the composite material by the use of a multifunctional acrylic monomer which produces a relatively high crosslink density early on in the setting reaction. Normal filling composites do not possess this characteristic and set rapidly to a hard rigid solid.

After removing the provisional crown during its rubbery stage, final setting can be accelerated by immersion in hot water for a few seconds.

The two types of acrylic material do pass through a rubbery stage, when removal is facilitated, but this stage is not as distinct as in the other two products. Automated mixing of provisional crown and bridge resins should enhance their properties by eliminating mixing porosity. The dual-cure composite material once again relies upon its chemical set to give the rubbery stage to facilitate removal of the provisional restoration. Final curing is achieved out of the mouth by visible-light activation.

None of the materials should be allowed to set completely in situ since they all undergo a significant setting contraction. This is greatest for the polymethylmethacrylate material where a volumetric shrinkage of about 5% occurs.

Each of the materials exhibits an exothermic reaction on setting, the temperature rise being greatest in the acrylic material. This may have important biological consequences, when a con-

siderable bulk of resin is present, in constructing a large temporary crown or bridge on freshly cut vital dentine. One should ensure that the materials are removed from the patient’s mouth well before their maximum temperature rise has occurred. This relies on being able to identify the commencement of the rubbery stage after which the maximum temperature is normally reached within a minute or two.

Biocompatibility: Certain components of some of the products are known to have an irritant effect when placed on freshly cut vital dentine. Methylmethacrylate monomer, for example, present in the acrylic material, falls into this category. When using this product it is necessary to either varnish the preparations or apply a surface layer of petroleum jelly as a protective measure. The isobutylmethacrylate monomer used in the higher methacrylate product is far less irritant than methylmethacrylate.

Mechanical properties: The mechanical properties of the materials become most significant when minimal tooth tissue removal or shoulderless crown preparations are used. There is a danger of fracture occurring in the thin areas at the tapered margin of such a temporary crown. This is most likely in the acrylic material which is weaker and more brittle than the other products.

Appearance: In terms of appearance the acrylic, higher methacrylate and composite materials are all available in a range of shades and a good match with tooth substance can be achieved.

27.1 Introduction

Cements are widely used in dentistry for a variety of applications. Some products are used primarily for cavity lining whilst others are primarily used for luting applications. Other, more specialist products, are used for sealing root canals as part of a course of endodontic treatment.

Some cements are specifically formulated as filling materials. These products are discussed in Chapters 20, 24 and 25. Some of the materials described previously in those chapters are used for a variety of applications and some of the discussion related to their use as fillings will be relevant to other applications of the same or similar products.

27.2 Requirements of cavity lining materials

Certain filling materials are not suitable for placing directly into a freshly prepared cavity. In such circumstances, a layer of cavity lining material is placed in the occlusal floor of the cavity, and on the pulpal axial dentine wall for class II cavities, prior to placement of the filling. The requirements of the cavity lining material chosen for any specific application depend on the depth of the cavity, which determines the thickness of residual dentine between the base of the cavity and the dental pulp, and the type of filling material which is being used to restore the tooth.

The purpose of the cavity lining, or cavity base, is to act as a barrier between the filling material and the dentine which, by virtue of the dentinal tubules, has direct access to the sensitive pulp. Depending upon the specific circumstances, the lining may be expected to provide a thermal, chemical and electrical barrier as illustrated in Fig. 27.1. In addition, the cavity lining or base must have sufficient mechanical strength to resist

disruption during the placement of fillings and provide a firm, rigid base which will adequately support the filling above it.

Thermal barrier

The cavity lining or base is often expected to form a thermally insulating barrier in order to protect the pulp from sudden intolerable changes in temperature. The insulating properties of the cement are characterised by its value of thermal conductivity or thermal diffusivity (see Chapter 2).

A thermally insulating cavity lining is particularly required when a metallic filling, such as amalgam is used. Table 21.5 shows that the thermal diffusivity value for amalgam is about 40 times greater than that for dentine. In deep cavities, having only a thin residual layer of dentine, there is a danger of ‘thermal shock’ to the pulp when the patient takes hot or cold food. A layer of insulating cavity lining material of sufficient thickness helps to prevent this. Unfortunately this protective effect can be negated if a dentine pin is used to help to provide retention for a metallic restoration. The interface between the pin and the restoration cannot be insulated hence conduction of thermal change can occur rapidly into the tooth. In shallow cavities, where there is a relatively thick layer of residual dentine, it becomes less important to use a lining with thermal insulating properties. Indeed, for amalgam restorations in shallow cavities, the use of a thick layer of cavity lining may reduce the thickness of the overlying amalgam to such an extent that it becomes weakened and liable to fracture. Hence, in such shallow cavities, the base and walls of the cavity can be lined with a varnish.

The varnish consists of a solution of a natural or synthetic resin in a volatile solvent. It is painted

Fig. 27.1 Diagram illustrating the way in which a cavity lining protects the dental pulp.

into the cavity and the solvent evaporates to leave a very thin layer of resin which helps to seal the ends of the dentinal tubules. The varnishes do not provide adequate thermal protection in deep cavities since they form only a thin layer. The resin in the varnish will tend to swell in the presence of water; this may help to establish a marginal seal around an amalgam restoration immediately after its placement.

Another potential cause of thermal injury to the pulp is through the considerable amount of heat liberated by certain filling materials during setting. The acrylic resins, for example, can give a temperature rise of 10ºC or more for a small cavity, whilst some light-activated composite materials can show a transient increase of 15ºC for an average-sized cavity. Temperature rises of this magnitude may cause injury to the pulp and one function of the cavity lining is to form an insulating barrier against such stimuli.

One complicating factor is that many of the lining cements themselves set by an exothermic reaction.

Chemical barrier

Cavity lining materials may be required to form a protective barrier against potential chemical irritants present in some filling materials. Phosphoric acid in silicate materials, and acrylic monomers in some resin-based materials, are two such potential irritants. The situation may again be complicated if the cement itself contains irritants. Some cements

may be suitable for use in shallow to medium depth cavities but totally unsuitable in deep cavities where they may be placed adjacent to the pulp.

Set against these conventional arguments is a gathering wealth of knowledge which suggests that conventional wisdom relating to the effects of some chemicals, particularly acids, on the pulp has been misguided. It is now accepted by most authorities that the dentine and pulp are able to survive contact with quite powerful acids (e.g. 37% phosphoric acid) providing that access to the pulp is effectively sealed at the end of the course of treatment. Hence, creating a chemical barrier is now seen in terms of generating an adhesive bond at the tooth/restorative material interface so that leakage at the margins can be reduced or eliminated.

Electrical barrier

When two dissimilar metals are placed adjacent to or opposing each other (e.g. amalgam/gold) it is possible to set up a galvanic cell which not only accelerates corrosion but can cause pain. The use of an electrically insulating lining material helps to prevent such activity. Unfortunately, most of the lining materials used are either water-based or contain polar organometallic compounds. They are not, therefore, ideal electrical insulators.

Varnishes consisting of less polar resins, such as polystyrene, may be used to provide some electrical resistance. These are sometimes painted onto the surface of metallic restorations giving temporary relief to the symptoms of ‘galvanic pain’.

Strength and flow

The vast majority of cavity lining materials are supplied as two components which are mixed together, initiating a setting reaction. The setting characteristics should allow sufficient time for mixing and placing in the cavity followed by rapid setting in order that the filling material can be placed without too much delay.

The lining should remain intact during the placement of the filling material. The integrity of the lining depends on several factors.

(1)The degree of set achieved at the time the filling material is placed.

(2)The strength of the set material and its thickness.

(3)The type of cavity.

(4)The pressure exerted during the placement of the filling material.

(5)The degree of support from surrounding structures.

(6)The choice of correct operative techniques.

When an amalgam restoration is being placed it is important to ensure that the lining is fully set or there is every possibility that the material will undergo considerable flow due to the high pressures used. This is illustrated in Figs. 27.2 for a class I cavity and 27.3 for a class II cavity. It can be seen that in both cases the insulating layer of lining is lost and the amalgam comes into close contact with dentine. For the class II cavity, a further danger is the production of a thin section of amalgam as the lining flows upwards during the packing of the inter-proximal box (Fig. 27.3a) and is forced further upwards during the filling of the remainder of the cavity (Fig. 27.3b).

If the lining material has set at the time of amalgam condensation there is little chance of flow. The strength of the set material should be sufficient to resist fracture. For class I cavities there is little chance of fracturing a set lining, even though it may have relatively low strength, since it is supported on all sides by a rigid cocoon of tooth substance (Fig. 27.4). For class II cavities the situation is different, as illustrated in Fig. 27.5. The axial wall of lining is unsupported and attempts to condense amalgam directly onto this may cause fracture at the exposed corner. This technique not only destroys the integrity of the lining but may produce an incompletely filled cavity, having voids as shown.

The correct technique is to condense amalgam into the interproximal box first, as shown in Fig. 27.6. This provides support for the lining of the axial wall which can then withstand direct forces.

The placement of other filling materials does not present problems as severe as those encountered with amalgam condensation since much smaller forces are used for adaptation. If the lining material is properly set, there is little chance of flow and fracture is also unlikely at lower pressures.

Fig. 27.3 Flow of an unset cement lining in a class II cavity caused by the high pressure of condensation. (a) Whilst filling the box. (b) Whilst filling the occlusal part of the cavity.