Requirements of Dental Cements for Lining, Base and Luting Applications
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.
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 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 10°C or more for a small cavity, whilst some light-activated composite materials can show a transient increase of 15°C 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.
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.
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 setti/>