Self-etching adhesives: possible new pulp capping agents to vital pulp therapy

Abstract

Pulp capping is one of the solving for keeping vital pulp in the case of dentin caries, reversible pulpitis or traumatic pulp exposure. The presence of bacteria on the cavity walls or in the pulp was the major factor that leads to the failure of pulp capping. Traditional pulp capping agent, calcium hydroxide, may not prevent microleakage. Self-etching system is a newly developed adhesive system, which could provide less microleakage and would not break down or dissolve, preventing the oral fluids and bacteria from the pulp along the cavity wall. This may reduce such clinical problems as postoperative sensitivity, secondary caries and marginal discoloration. Researches showed that some kinds of self-etching adhesives induced the mild to moderate inflammatory pulp response, with negative bacterial staining. Inclusion of antibacterial components into self-etching system, such as 12- methacryloyloxydodecylpyridinium bromide (MDPB) may inhibit bacteria and provide better clinical effects. It is speculated that using the self-etching adhesive system containing the antibacterial agent, such as MDPB, to the dental pulp directly or indirectly, may inhibit bacteria after the placement of restoration as well as residual bacteria in the cavity.

Keywords

dental pulp / pulp capping / self-etching adhesives / antibacterial components

Introduction

Dental pulp is the only soft tissue in the tooth, and it contains nerve fibers, arteries, veins, and lymph vessels. The pulp tissue is surrounded by the dentin and the enamel. Its circulation extends into the tubular dentin and ensures that the dentin maintains its resilience and toughness. Dental pulp has the ability to form a dentin-like matrix as part of the repair in the dentin-pulp organ. A non-vital tooth could lack natural protection against an overload, thereby increasing the probability of fracture. Attempting to retain the vitality of the pulp of a tooth affected by caries, trauma, or other causes is desirable. Maintaining the integrity and health of teeth is essential. Pulp capping, which involves direct and indirect pulp capping, is one solution to maintain the vital pulp in case of dentin caries, reversible pulpitis, or traumatic pulp exposure [1]. The success of vital pulp therapy is dependent on the extent and the degree of the injury, capping agents, and bacterial microleakage [2].

The pulp capping agent is a kind of material that should have the following characteristics: (1) no stimulation to dental pulp, (2) stimulating the formation of reparative dentin, (3) highly permeable, and (4) preventing bacteria from the dental pulp [3]. In the past years, various materials have been used to cap the dental pulp directly or indirectly. The traditional pulp capping agent is calcium hydroxide, a kind of alkaline chemical compound that has antibacterial effects and the ability to induce new dentin formation. However, the hard-setting calcium hydroxide medication, such as Life (Kerr, USA) and Dycal (Dentsply, USA), do not prevent microleakage fully because the Ca(OH)₂ itself does not adhere to the dentin [4]. Moreover, the Ca(OH)₂ may disappear within the time span. The dentin bridge formed by Ca(OH)₂ is porous, with numerous tunnel defects. After 1-2 years, the Ca(OH)₂ dressing dissolves. Clinically, it is not an effective long-term seal [5]. Therefore, a good number of investigators researched on alternative pulp-capping materials that could reduce the microleakage and would not break down or dissolve.

Introduction to the self-etching system

The self-etching system is a newly developed adhesive system based on a current bonding strategy using non-rinsing, acidic, and polymerizable monomers on dentin and enamel simultaneously [6]. According to clinical practice, self-etching systems can be classified as a “one-step” system and a “two-step” system [7]. The “one-step” system includes the etchant, primer, and resin in one bottle. This so-called “all in one” adhesives combine the bonding procedures (i.e., etching, priming, and dispersing resin) into a single-step application. The two-step system contains self-etching primers that require an additional bonding step. The self-etching system requires a reduced number of steps and chairside time. It is easy to use for operators. It eliminates both the rinsing and the drying step. The self-etching primer simplifies the bonding technique and reduces the technique sensitivity of adhesive systems [8,9]. Therefore, the self-etching adhesive system is in the mainstream of current and future bonding techniques.

Self-etching adhesive as a pulp capping agent

Studies on pulp healing indicate that the presence of bacteria on the cavity walls or in the pulp is a major factor leading to the failure of pulp capping [10]. If mircroleakage is prevented, and bacteria are controlled outside the pulp, the pulp may recover by itself and the pulp capping treatment may obtain a good result. Theoretically, the primers of self-etching systems decalcify the inorganic component and infiltrate the collagen fibers in situ at the same time. Thus, the collapse of air-dried and demineralized collagen is prevented. A good seal is achieved because the smear plugs are left intact [11]. The inflammation of dental pulp could be minimized or even avoided by controlling the invasion of microbes into the pulpal chamber, therefore protecting the pulp dentinal complex from the penetration of bacteria and their products. The self-etching system reduces microleakage at the restoration-tooth interface. It prevents oral fluids and bacteria from entering the pulp along the cavity wall. This may reduce such clinical problems as postoperative sensitivity, secondary caries, and marginal discoloration [12].

Considering that self-etching systems could solidify immediately and seal the dentin tubule, they provide the pulp a favorable and long-time obturation. Self-etching systems have been proposed as candidates for pulp capping agent. When using the self-etching adhesive to the human pulp in vivo, de Souza Costa et al. reported that one kind of self-etching adhesive system, Clearfil Liner Bond 2, evoked a mild to moderate inflammatory response in the short-term [13]. Medina et al. compared the effect of three single-bottle and four self-etching primers on calcium hydroxide in exposed monkey pulps and found that some of the self-etching adhesives induced the persistent moderate to severe inflammatory pulp response with positive bacterial staining [14]. The inclusion of antibacterial components in the self-etching system has also been attempted using several methods. The results of the in vitro tests indicate that some of the trials seem promising. Imazato et al. identified the bactericidal effect of the primer incorporating the antibacterial monomer 12-methacryloyloxydodecylpyridinium bromide (MDPB) against bacteria in carious lesions [15]. They also applied indirectly the antibacterial primer solution to cavities infected with Streptococcus mutans. Restoration with the experimental primer resulted in little or no pulpal inflammation for all periods tested [16]. It is presumed that some modifications are needed to the self-etching systems to achieve clinically successful effects.

Cavity preparation and tooth preparation are common operations during dentistry outpatient service. With the resin composite and self-etching adhesive used extensively, an adhesive resin with antibacterial MDPB could be used as a pulp capping agent. The antibacterial MDPB has the bactericidal effect and has the capability to disinfect cavities containing residual bacteria. Imazato et al. investigated the antibacterial effects of the dentin primer of Clearfil Protect Bond, which contains MDPB, against Streptococcus mutans, Lactobacillus casei, or Actinomyces naeslundii. They found that the bonding system, including the MDPB-containing primer and the adhesive, should be highly effective in its antibacterial effect before and after curing [17]. For a deep cavity with less residual dentin thickness, the MDPB-containing primer obstructs dentin tubules and cuts off the pulp from stimulation from the outside. At the same time, it inhibits the growth of bacteria, thus allowing the dental pulp to recover as soon as possible. Recently, Tziafas et al. evaluated the effect of the new antibacterial adhesive system to pulp vitality and primary odontoblastic function in infected unexposed and exposed cavities. The results showed that there was neither severe inflammation nor tissue necrosis in the dentinal cavities or in the pulpal exposures treated with the self-etching adhesive containing MDPB [18]. The MDPB-containing primer may have the ability to maintain pulp vitality and primary odontoblastic function, and using the self-etching adhesive containing the antibacterial monomer to the pulp is possible. It inhibits bacteria after the placement of the restoration as well as the residual bacteria in the cavity. Indirect pulp capping is recommended for self-etching adhesive because of its chemical properties.

References

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