Compatibility and Composite Restorations with the Pulp
The dental nerve does not like plastic compounds, or so-called composite restorationsWithin the framework of dental filling treatment, composites – in combination with bonding agents (so-called primers/adhesives) – come in direct contact with the surface of the dental enamel and dentin of the teeth.
Since dentin is very closely connected to the dental pulp via so-called dentin tubuli (pulp/dentin system), there are two essential questions concerning the use of composites with regard to the protection of the pulp: To what extent is it possible, by means of composites and bonding agents, to protect the pulp long-term from the manifold influences present in the oral cavity (e.g. microbes)? Do substances released by composites and bonding agents themselves lead to a damaging of the pulp? Are they sufficiently tolerated by the body?
Shielding the Pulp/Dentin System
In all composites, a polymerization contraction takes place during the curing phase, which is not fully compensated by the subsequent swelling due to absorption of moisture. Besides that, composites are subject to permanent changes in their volume and shape due to mechanical and thermal effects occurring in the oral cavity. In practical application, attempts are made to achieve sufficient bonding and marginal sealing through special processing techniques (e.g. the layering technique) as well as the utilization of bonding agents.
In this context, the latter are accorded multiple importance. On the one had, they are to reduce the permeability of the dentin, and on the other hand, they guarantee adhesion between the composites and the hard dental substances (enamel and dentin). Furthermore, some composites release substances (e.g. fluorides), and a protective effect on the particularly threatened transitional zone between the restoration and the surface of the tooth is assumed.
Nowadays, undesirable effects such as oversensitivity after a composite insertion, fissure formation, or the development of structural tensions and micro fractures can be reduced through correct processing and the use of modern enamel/dentin adhesives. However, they cannot be completely eliminated. Among these are, for example, fatigue behavior, the flexural modulus, or the thermal expansion coefficient, which varies from that of the hard dental substances.
Tissue Compatibility of Composites and Adhesives
All dental restoration materials must fulfill safety standards regarding certain undesirable characteristics such as toxic, mutagenic, carcinogenic, and allergenic effects that are defined by national and international standards as well as by corresponding legal regulations. These requirements can be fulfilled by composites and bonding agents that are commonly used today. To date, the risk of undesirable side effects (poisoning, allergies, etc.) in patients is low, so that it is considered acceptable, according to current knowledge, to utilize these material groups without any limitations for persons of all ages – apart from proven allergies.
While the occurrence of undesirable reactions (especially allergies) in patients has been described very rarely up to the present, the dental team is at a greater risk in that regard. It follows that precautionary measures must be observed in the processing of these materials. Thus, composites and adhesives may only be used with the aid of suitable application instruments, since even protective gloves cannot safely prevent skin contact with the danger of a corresponding sensitization.
In accordance with the current scientific state of knowledge, the aspect of compatibility with the dental pulp can be outlined as follows: In case of very thin dentin layers (under 0.5 mm), a possibly clinically relevant diffusion of components of adhesives/composites in the dental pulp may occur. In case of greater thickness of the layer (above 0.5 mm) though, dentin seems to constitute an effective barrier against released ingredients from composites and bonding agents.
The danger of a damaging of the pulp due to acids or plastic components used within the course of the adhesive technique was probably overrated in the past. Today, completely “inert” behavior of the restoration materials is seen as less important for the maintenance of the pulp's health than its ability to shield the pulp/dentin system permanently (particular against microbial influences). Therefore, the risk of an irritation of the pulp caused by composites and bonding agents in the sense of undesirable chemical-toxic or physical effects is apparently low when there is a completely closed layer of dentin with sufficient thickness.
However, any direct contact of these substances with the pulp tissue should still be avoided. Although the direct capping of an exposed dental nerve with bonding agents and composites under certain test conditions (e.g. completely caries-free teeth in young patients) has been described as successful in some publications, this approach cannot be recommended as a routine procedure yet. First, further (long-term) studies must show whether and – possibly which – of the available techniques and compounds should be considered for such treatments.
This assessment comes in the wake of evidence of severe pulp damage after direct contact of composites or bonding agents with the pulp tissue in certain studies. In addition, an immune and genotoxic effect of substances of this kind has been proven.
The recommendations regarding the protection of dental pulp in composite restorations have been repeatedly subjected to corrections during recent decades. While it was generally tried in the past, e.g., to prevent direct, permanent contact between the plastic and the dentin by inserting a lining, the opposite goal is pursued today. What is desired is a bond between the plastic, enamel and dentin that is as close as possible. The bond is accomplished by means of bonding agents, the use of which require special surface conditioning both for the enamel as well as for the dentin – more on that in the video Teeth Bonding". The use of linings thus requires a new approach. Based on clinical-practical considerations, the following procedures present themselves:
Shallow Cavities/Dentin Away from the Pulp
In case of shallow cavities, the use of linings is largely omitted today, unless particular sections (e.g. heavily discolored areas) are to be specifically covered for aesthetic or other reasons. Thus, it is not necessary to sacrifice additional tooth substance during the preparation just to create room for a lining.
Deeper Cavities/Dentin Close to the Pulp
In cases of deeper or severely undermining cavities, however, a lining could still be indicated. This recommendation arises from, among other things, the fact that the question of irritations of the dental pulp after direct contact with primers/adhesives still has not been sufficiently answered. For safety reasons, sections of dentin in the immediate vicinity of the pulp should be specifically covered, if for no other reason that in such cases, even the smallest exposures of the pulp cannot always be ruled out in a clinical setting.
If, in such cases, one reverts to calcium hydroxide preparations, one should pay attention to perform a very delicate and thin application. This also applies to the use of hardening calcium salicylate cements, which, due to their limited chemical and mechanical stability, may not be utilized as “universal” lining materials to be applied in large areas and thick layers.