Classification of dentin adhesive systems
7.4.3 Total-Etch Systems
The total-etch system completely removes the smear layer using an acidic gel (usually phosphate acid) and demineralizes hydroxyapatite under the smear layer of the dentin. The adhesive of the one bottle etching system is synthesized by dissolving the resin monomer to form an organic solvent. After etching and rinsing, the adhesive was applied to the treated tooth surface; then, the resin monomers penetrate into the demineralized dentin through the gap, which is filled with water between the dentin collagen (this gap was originally occupied by the hydroxyapatite) to form a “hybrid layer”. The hybrid layer is between the adhesive and dentin and is composed of collagen, resin monomer, residual hydroxyapatite, and water. The hybrid layer helps to reduce postoperative sensitivity and form a better marginal seal; at the same time, buffering the shrinkage stress produced by the polymerization of the resin composites as an “elastic buffer”.
7.4.4 Self-Etch Systems
A self-etch system combines the etching and pre-treatment as one step when processing enamel and dentin without the separate etching step. The mixture combined with etch and primer penetrates into and dissolves a part of the smear layer to form a “hybrid zone” with hydroxyapatite so that the “hybrid zone” is composed of the hybrid layer and the residual smear layer.
For both the etch and the primer in the two-step system and the self-etch adhesive in the one-step system, the composition is substantially the same, i.e., a mixture of water and the acidic monomer. The acid monomer is typically phosphate ester or carboxylic acid ester, and its pH is above that of the phosphoric acid gel. Water is a key component of self-etch systems because it is involved in the ionization of acidic components.
According to its acidic strength, self-etching adhesive can be divided into three categories: mild (pH > 1.5), medium (1.0 < pH < 1.5), and strong (pH < 1.0).
7.4.5 Enamel Bonding
Etching is a key step in enamel bonding. There are large quantities of hydroxyapatite in enamel, in which the surface layer turns into water-soluble monocalcium phosphate during phosphoric acid treatment. Meanwhile, the dental plaque, material alba, and food residues attached to the tooth surface are removed, thereby exposing a clean fresh surface layer. Owing to the orientation of hydroxyl and amino on the tooth surface after phosphoric acid treatment, a polarized surface is formed. The increased surface energy of enamel is beneficial for wetting and penetrating of the adhesive. It is generally accepted that 30–50 % of phosphoric acid enables even demineralization of the enamel surface; thus, total-etch systems are considered a better adhesive system for enamel bonding.
After the etching process, the wettability of the rough enamel surface is improved so that the adhesive more easily penetrating the micro-structure of the tooth surface, thus strengthening the interaction of adhesive and fresh enamel. When the adhesive is cured, the bonding interface generates a considerable mechanical interlocking force. It can be observed by SEM that a large quantity of cured adhesive filled in the demineralized interprismatic area of the enamel, and countless micro-protuberances (usually called resin tags) are formed on the enamel side of the bonding interface. These mechanical anchored structures formed by resin tags and enamel provide the prime bonding force for the materials and enamel.
Etching technology is the general method for performing enamel bonding. In this method, mechanical interlocking is obtained by etching treatment, and the formation of resin tags is the main adhesive mechanism. The etched enamel prisms and interprismatic areas are demineralized, and the low-viscosity adhesive penetrates into the micropores of the enamel via the capillaries. Then, the polymerization of the adhesive occurs, which forms the resin tags that can generate the micromechanical interlocking system.
For the two-step self-etch system, the demineralized enamel layer is thinner than that treated by phosphoric acid gel because of the higher pH of adhesive. There are two methods of improving the bonding strength of self-etch systems:
Removing the rodless enamel to obtain a rough enamel surface.
Using phosphate acid to pre-etch the enamel before self-etching. The bonding strength is clinically acceptable for dentin and etched enamel treated using the self-etch method, but it is insufficient for untreated enamel and sclerotic cementum.
For the one-step self-etch system, the bonding strength is very low and there is no enamel pre-etching. However, the bonding strength is acceptable when the enamel is beveled and prepared.
In spite of the self-etch system gaining in popularity, phosphoric acid etching of the enamel is still the gold standard for testing new bonding material.
7.4.6 Dentin Bonding
The surface property and interior structure of the dentin is sophisticated. The dentin contains a lot of organic components, water, dentinal tubule connected with pulp, and liquid effused from the tubules. In addition, there is a smear layer caused by instrumental cutting of the dentin. Therefore, it is difficult to perform the treatment of dentin.
The smear layer is caused by the formation of metamorphic organics and inorganics during the bonding surface preparation, and its thickness is 0.5–15 μm. The degree of treatment of the smear layer directly influences the bonding effect of dentin. In the total-etch system, the smear layer can be thoroughly removed, but probably leads to excessive opening of the dentinal tubule, which will increase the postoperative sensitivity.
In the case of the self-etch system, the weak acid can partially remove the smear layer and lead to the appropriate opening of the dentinal tubules, thus bringing about the benefit for dentin bonding. Consequently, whether the one-step or the two-step self-etch system is used, the treatment of dentin includes three aspects: removal of the smear layer; improving the surface activity of dentin; promoting the penetration and bonding strength of the adhesive. It is widely believed that the prime mechanism of dentin bonding is the formation of a hybrid layer or hybrid zone.
The treatment of dentin is described as follows. At first, the dentin surface is treated by the etch so that the smear layer can be partially removed. After demineralization of the intertubular dentin, the microporous stent of collagenous fiber is exposed to form a porous belt and the opening of dentinal tubules; then, the primer is used. It can become wet and penetrate into the micropores of collagen fibers and dentinal tubules to facilitate the subsequent penetration of the adhesive; at last, the adhesive is coated. After the primer and the adhesive are cured in situ, they form the hybrid layer (or hybrid zone) with dentin collagen fibers, which will obtain a solid bond with the dentin because this zone contains many resin micro-protrusions and large resin protrusions of the dentinal tubules. Meanwhile, the residual unsaturated ethylene of the adhesive copolymerizes with the resin monomer; thus, the resin composites can be bonded to dentin. Along with the development of the dentin bonding system, the processes mentioned above are simplified into a one-step or two-step procedure.
The durability of the self-etch bonding system is a significant issue. At the early stage, the bonding strength is acceptable. As time goes on, the bonding strength is continuously decreased, especially for the one-step self-etch system, owing to the hydrophilicity of the acidic monomer in addition to the high water content to maintain ionization of the acidic monomer, and this may ultimately even affect the bonding to enamel. Meanwhile, the inadequate penetration of the resin into the tooth structure may also accelerate the degradation of the bonding interface. Shrinkage stress caused by the polymerization of resin composites, which act on the bonding interface, results in reduced dentin bond strength if the dentin bond strength cannot resist it. This will bring about the formation of gaps or edges resulting in secondary caries and dentin sensitivity.
7.5 Resin Composite Bonding Restoration Technique
7.5.1 Indications and Contraindications
Currently, resin composite has been widely used indirect restorative dentistry. Almost all the dental defects can be repaired via a resin composite, which can also be used for an abnormal shape or the color of teeth in cosmetic restoration, in addition to the restoration of endodontic treatment teeth.
The following situations should be taken into account: in the anterior restoration of a class IV cavity, except for a crossbite and clenching, the teeth deficit, which does not exceed one half, can be considered a direct composite restoration. For posterior teeth restoration, a severe attrition and cusp defect need to be excluded. We do also not use resin composite if the cavity cannot be completely isolated from saliva, gingival crevicular fluid, and blood.
7.5.2 Requirements for Restoration Design
Acid etching followed by bonding provides retention, while increasing the resistance of the remaining tooth structure. The tooth types, the position of the teeth in the dental arch, the size and type of defect, whether the treatment is for the placement of the original prosthesis, the occlusal function, and the relationship between the edges of the tooth preparation need to be considered in bonding restoration. What is more, the quantity and quality of the remaining hard tissue also need to be considered, the mechanical force of remaining tooth structure is exposed to the defect, and the reserve area extends to the range of the sound dental tissue.
7.5.3 Cavity Preparation
The principles of cavity preparation for resin composite restoration are based on the principles of amalgam cavity preparation, combined with the characteristics of bonding restoration. The principles emphasize preserving as much of the tooth structure as possible in the premise of removal of infected tissue and caries staining. Cavity shape is determined by the area of the lesion, and retention of the restoration relies on the etching. The extension for prevention is not needed.
The cavity shape only involves the carious parts and developmental defects. For enamel caries, the depth of the cavity should be limited to the enamel, without proceeding to the dentin, or increasing the supporting retention; to remove a large shallow dish caries, the cavity should be extended at the buccal and tongue groove, and then prepare the bevel at the edge of the cavity, adding the auxiliary retention ditch at the bottom and side walls. However, at the occlusal contact points in the occlusal cusp the edge bevel is not needed.
The abrasion of composite resin material is not as good as that for the silver amalgam. Therefore, the occlusal factors should be considered, especially the functional occlusal tip occlusion. On the occlusal surface, the cavity preparation should embody the preservation principle, and the cavity edge and line angle should be more obtuse than with silver amalgam, to facilitate closing together. For the occlusal cavity, the beveled edge could increase the cavity width, which means the wear of the prosthesis is greater than for the conventional cavity; however, at these parts of the restoration, especially the edges, flakes often form and breakage is easy by force. Therefore, the preparation of the bevel at the non-occlusal contact at the occlusal surface remains controversial, and in contact with occlusal cusp, the bevel should be avoided.
For the proximal cavity, buccal and lingual walls should be introverted, and the enamel bevel edge can be prepared, without extending to the self-cleaning area. An additional retention groove could be prepared at the axiofacial and axiolingual line angles, in the same way as the silver amalgam. There are pulp walls at the occlusal surface and an axial wall in the proximal surface, and a large part is involved in dentin; thus, less enamel is available in the proximal cavity, which is not conducive to bonding restorations. Therefore, to preserve as much tooth tissue as possible, especially the enamel thickness, for the gingival wall parts, carious tissue should be removed without extension to the root side.