Intermediate restorative materials
Since the presence of a restoration may have an adverse effect on the pulp, a range of materials, termed intermediate restorative materials (IRMs), has been developed to be applied to the dentine prior to the placement of the restorative material. These materials include cavity varnishes, bases and liners, and, as they are intended to remain in place permanently, these materials should not be confused with temporary restorative materials.
The distinction between cavity bases and liners is that the former consist of a thick mix of material that is placed in bulk in the cavity, while the latter is only applied as a thin coating over the exposed dentine. A definition of a cavity liner is a dentine sealer that is less than 0.5 mm thick and is able to promote the health of the pulp by adhesion to the tooth structure or by antibacterial action. In contrast, a base is a dentine replacement used to minimize the bulk of the restorative or block out undercuts.
Their role may be protective, palliative or therapeutic when they are applied to vital dentine. The choice of a cavity varnish, base or liner requires an appreciation of the need for pulpal protection, and how the agents may interact with the restorative material chosen for a particular clinical situation.
In order to make the correct choice of which intermediate restorative material to use for a particular restorative procedure, it is important to understand the nature and mechanisms by which adverse factors affect the pulp. Three possible sources of pulpal irritation have been identified:
The importance of the first two factors has been well recognized for some time, but more recently it has been shown that the latter factor is probably the most important in producing pulpal irritation.
In the intact tooth, temperature changes are conducted through the enamel and dentine to the pulp. Here, nociceptive afferent fibres may be thermally stimulated, eliciting a painful response. Such a direct thermal stimulus of the pulp is, however, unlikely except when cutting a cavity or direct heat is generated due to an exothermic reaction on the part of the restorative material when in close proximity to the pulp. When dentinal tubules are exposed, it is possible for fluid to flow into and out of the tubules, commonly referred to as the hydrodynamic effect. This is the process responsible for exposed root surface sensitivity and is readily dealt with by sealing the root surface. The hydrodynamic effect is almost certainly also responsible for the short-latency pain produced by thermal stimulation of some minimal-amalgam restorations. If the dentinal tubules are patent and a small gap has been allowed to form under the amalgam restoration, possibly due to inadequate adaptation to the cavity wall, fluid movement down the tubules can occur because of the opening and closing action of this gap. This can happen as a consequence of the amalgam expanding or contracting when exposed to extremes of temperature or the application of an occlusal load. Thus, the placement of a cavity varnish or thin lining in a cavity is done in order to protect against fluid movement through the dentine, and not to act as a thermal insulator, as was thought at one time.
Acrylic resins have been cited as examples of materials that will cause a pulpal reaction when placed without a lining. However, toxicity tests suggest that these materials are well tolerated by the soft tissues. Acrylic resins are extensively used as bone cements in hip replacements without any adverse inflammatory reaction. This would suggest that other factors are responsible for the pulpal reaction associated with these materials.
Until recently, most studies of the pulpal toxicity of restorative materials have not considered the influence of bacterial contamination, which is now believed to play a major role in the production of pulpal inflammation, as considered below. This does not mean that we need not worry about chemical toxicity, as the low pH of some materials, such as zinc–phosphate cements and zinc–polycarboxylate cements, may well have an effect on the pulp.
A matter of considerable interest and debate is the effect of micro-leakage. This term loosely describes the penetration of oral fluids and small numbers of bacteria and their toxic by-products between the filling material and the cavity walls. This percolation has been shown to be a potential source of pulpal irritancy.
For example, zinc–phosphate cements do not show pulpal inflammation (and may even show some dentine bridge formation) when placed on exposed pulps in the absence of bacteria. In contrast, control animals showed severe pulpal inflammation and abscess formation. Other materials do show an inflammatory response, even in the germ-free animals, demonstrating that chemical toxicity may still be an important factor in some instances.
Our increased understanding of the mechanism of pulpal toxicity does not change the fact that some materials will damage the pulp if not separated from the overlying dentine by a suitable lining. However, whereas in the past it was thought that the primary role of an intermediate restorative material was to protect the pulp from the toxic action of restorative materials, this view has had to be modified to take account of the role of bacterial toxins. The use of intermediate restorative materials is now aimed at:
The adhesive approach is now so important that it has been dealt with separately in Chapter 2.5; here we will consider only the cavity varnishes, bases and liners. We will first discuss the chemistry of these materials and then consider which may be the most appropriate for various clinical applications.
Cavity varnishes consist of a clear or yellowish liquid that contains natural resins such as copal, colophony and sandarac, or synthetic resins such as polystyrene. The resins are dissolved in a solvent such as alcohol, ether or acetone, and are applied to the cavity floor with a brush or cotton pledget. The solvent is allowed to evaporate, leaving behind a thin coating of the resin. This process may have to be repeated up to three times to ensure a uniform coating of resin.
This material is supplied as two white or light yellow pastes. One paste consists of a mixture of calcium hydroxide (50%), zinc oxide (10%) and sulphonamide (40%). The other paste consists of butylene glycol disalicylate (40%) with varying amounts of titanium dioxide and calcium sulphate.
Equal volumes of the two pastes are mixed together for about 30 seconds; the cement will then set in approximately 2 minutes. The setting process for these materials has not been fully elucidated but is believed to involve a chelating reaction between the zinc oxide and butylene glycol disalicylate.
The freshly mixed cement is highly alkaline, with a pH of 11–12. It is believed that this is responsible for an important feature of calcium hydroxide cements: their ability to cause the pulp of the tooth to lay down secondary dentine. When the paste is placed in contact with the pulp, possibly in the presence of a microexposure, it will cause a three-layer necrosis of some 1.5 mm thickness. This eventually develops into a calcified layer.