Current perspectives on dental adhesion: (1) Dentin adhesion - not there yet

Abstract

The essential goal of any adhesive restoration is to achieve a tight and long-lasting adaptation of the restorative material to enamel and dentin. The key challenge for new dental adhesives is to be simultaneously effective on two dental substrates of conflicting nature. Some barriers must be overcome to accomplish this objective. While bonding to enamel by micromechanical interlocking of resin tags within the array of microporosities in acid-etched enamel can be reliably achieved and can effectively seal the restoration margins against leakage, bonding effectively and durably to organic and humid dentin is the most puzzling task in adhesive dentistry. Much of the research and development of dental adhesives has focused on making the clinical procedure more user-friendly by reducing the number of bottles and/or steps. Although clinicians certainly prefer less complicated and more versatile adhesive materials, there is a trade-off between simplification of dental adhesives and clinical outcomes. Likewise, new materials are launched with claims of being novel and having special properties without much supporting evidence. This review article discusses dental adhesion acknowledging pioneer work in the field, highlights the substrate as a major challenge to obtain durable adhesive restorations, as well as analyzes the three adhesion strategies and their shortcomings. It also reviews the potential of chemical/ionic dental adhesion, discusses the issue of extensively published laboratory research that does not translate to clinical relevance, and leaves a few thoughts in regard to recent research that may have implications for future adhesive materials.

Fig. 1

(a) SEM micrograph of human enamel etched with 35% phosphoric acid for 15 s. Original magnification = 5,000X. (b) SEM micrograph of a replica of an interface of etched enamel with a dental adhesive. After curing the adhesive, the specimen was left in 6N HCl for 12 h to dissolve the enamel. The enamel prisms at the interface were not dissolved because the etched enamel was impregnated with polymerized adhesive, creating a hybrid layer. H – hybrid layer; Ad – adhesive; E – residual enamel. Original magnification = 5,000X.

Fig. 2

(a) SEM micrograph of fractured superficial dentin. Int – intertubular dentin; P – peritubular dentin; T – dentin tubule; Arrows – bacteria in the tubule lumen. Original magnification = 10,000X. (b) SEM micrograph of fractured deep dentin 75 μm from the pulp of the same tooth in Fig. 2a. Original magnification = 10,000X.

Fig. 3

(a) SEM micrograph of fractured middle dentin showing an odontoblastic process extending from the tubule Int – intertubular dentin; P – peritubular dentin; T – dentine tubule. Original magnification = 10,000X. (b) SEM micrograph of fractured deep dentin showing intratubular collagen (asterisk) with the characteristic 64 nm collagen banding pattern. Int – intertubular dentin; P – peritubular dentin; T – dentin tubule. Original magnification = 25,000X.

Fig. 4

SEM micrograph of fractured dentin-enamel junction (DEJ) area. Note the morphological differences between enamel and dentin hydroxyapatite. Original magnification = 5,000X. Arrows – DEJ; E – enamel; Int – intertubular dentin; T – dentin tubule.

Fig. 5

Non-carious cervical lesion (NCCL) with sclerotic dentin.

Fig. 6

Sequence of SEM micrographs to illustrate the morphological characteristics of the bonding substrate in a NCCL of a recently extracted mandibular canine. (a) SEM micrograph depicting a general view of the NCCL. The incisal aspect is on the right side (E-enamel), with the cervical aspect on the left side (R-root). The white arrows point to the natural incisal cavo-surface margin. The dark arrows point to the natural cervical cavo-surface margin. Original magnification = 30X. (b) SEM micrograph of the area included in the rectangle in Fig. 6a. The horizontal dotted line separates the unetched area (upper half) from the area that was etched with 35% phosphoric acid for 15 s (lower half). Original magnification = 100X. (c) SEM micrograph of the area included in the rectangle of Fig. 6b (etched area). Note the sclerotic casts in the tubules (circles) and, overall, hypermineralized dentin. Original magnification = 1,000X. (d) SEM micrograph of a sclerotic cast (asterisk) obliterating the tubule (T). Note how intertubular dentin (Int) is densely mineralized. Original magnification = 15,000X. (e) SEM micrograph of bacteria (arrows) ‘fossilized’ into the mineralized area of intertubular dentin (Int). Original magnification = 15,000X. (f) SEM micrograph of a longitudinal fracture of dentin in a NCCL. Note how the tubule is obliterated with rhombohedral mineral crystals, which were elegantly described in 1989 [173]. Int – intertubular dentin; P – peritubular dentin; T – dentin tubule. Original magnification = 15,000X.

Fig. 7

(a) SEM micrograph of a fractured human dentin specimen with smear layer and a smear plug created with diamond bur. Int – intertubular dentin; P – peritubular dentin; T – dentin tubule; Sp – smear plug; Oc –occlusal surface; Dotted circle – another tubule plugged with smear layer. Original magnification = 10,000X. (b) SEM micrograph of human dentin etched with liquid phosphoric acid for 15 sec. Int – intertubular dentin; P – peritubular dentin; T – dentin tubule; Oc – occlusal surface; Pc – exposed peritubular collagen from dissolution of the peritubular dentin; Dd – dentin demineralized by the etching agent; arrows – other tubules. Original magnification = 10,000X. (c) SEM micrograph of human dentin treated with the Clearfil SE primer (Kuraray) from the 2-step SE adhesive Clearfil SE Bond. The asterisk marks the area of dentin partially decalcified by the primer (pH = 1.8–2.0). Upon application of the respective hydrophobic bonding resin, this 0.5 μm deep area will become the hybrid layer. Int – intertubular dentin; T – dentin tubule; Oc – occlusal surface; Sp – primer-infiltrated smear plug. Original magnification = 15,000X. (d) SEM micrograph of occlusal view of human dentin treated with GC Cavity Conditioner (20% polyacrylic acid with 3% aluminum chloride hexahydrate) for 10 sec, and rinsed with water for 15 sec. Note residual smear layer (ovals) and some patent tubules (T). The intertubular dentin does not have morphological characteristics of demineralization (no visible collagen fibers). Original magnification = 5,000X.

Fig. 8

TEM micrograph of the adhesive-dentin interface formed by the 3-step ER adhesive OptiBond FL (Kerr). The particle-filled hydrophobic bonding adhesive resulted in filled resin tags (Rt). Ad – adhesive; H – hybrid layer; D – dentin. Original magnification = 6,000X.

Fig. 9

(a) SEM micrograph of human dentin etched with 32% phosphoric acid (Scotchbond Universal Etchant, 3M). Original magnification = 7,000X. (b) SEM micrograph of human dentin etched with 35% phosphoric acid (Scotchbond Etchant, 3M). Original magnification = 7,000X. Int – intertubular dentin; P –peritubular dentin; T – dentin tubule; Oc – Occlusal surface; Pc – exposed peritubular collagen from dissolution of the peritubular dentin; Dd – dentin demineralized by the etching agent; Circles – silica thickening agent; Arrows – intertubular anastomoses.

Fig. 10

(a) TEM micrograph of the adhesive-dentin interface formed by the 2-step SE adhesive OptiBond SE (Kerr). Hydroxyapatite crystals are visible inside the entire length of the 1 μm-thick hybrid layer (dotted circle). Int – intertubular dentin; T – tubule; Ad – adhesive; Cr – composite resin; H – hybrid layer. Original magnification = 12,000X. (b) TEM micrograph of the adhesive-dentin interface formed by the 1-step SE adhesive G-ænial Bond (GC), a HEMA-free adhesive. Hydroxyapatite crystals are mixed with smear layer remnants in the 0.4 μm-thick hybrid layer. Int – intertubular dentin; T – tubule; Ad – adhesive; H – hybrid layer; SP – smear plug. Arrows – empty areas corresponding to droplets in the transition between the adhesive layer and the hybrid layer. Original magnification = 40,000X.

Fig. 11

Selective enamel etching with 35% phosphoric acid in an occlusal preparation of a mandibular molar.

Fig. 12

(a) SEM micrograph of human enamel conditioned with the 1-step SE adhesive Prompt L-Pop (3M ESPE). The adhesive was not light cured. It was dissolved in acetone for 12 h in a rotator. Note that the etching pattern resembles that of phosphoric acid as a result of the low pH of this adhesive (pH ≈ 1). Original magnification = 5,000X. (b) TEM micrograph the adhesive-dentin interface formed by the ‘strong’ 1-step SE adhesive Prompt L-Pop (3M). Int – intertubular dentin; T – tubule; Ad – adhesive; H – hybrid layer; R – Sp-adhesive mixed with residual smear plug; C – composite resin. Original magnification = 20,000X.

Fig. 13

(a) SEM micrograph of a replica of human enamel conditioned with G-Bond Plus (GC). The adhesive was dissolved in acetone for 12 h in a rotator. The replica of droplets is depicted throughout the surface. Original magnification = 10,000X. (b) SEM micrograph of the adhesive-dentin interface formed with G-Bond Plus (GC) in self-etch mode. This particular area corresponds to the top of the hybrid layer below the adhesive layer. The asterisks depict residual droplets. Original magnification = 10,000X. (c) TEM micrograph of the adhesive-dentin interface formed with the G-Bond Plus (GC) in self-etch mode. H – hybrid layer; Int – intertubular dentin; Arrows – residual droplets The dotted lines mark the hybrid layer thickness; Ovals – hydroxyapatite crystallites visible inside the hybrid layer denoting mild decalcification; Asterisks – residual smear layer.

Fig. 14

(a) SEM micrograph (backscattered mode) of the adhesive-dentin interface formed between the universal adhesive Single Bond Universal (3M) and dentin. This specimen was part of a nanoleakage study using ammoniacal silver nitrate as the tracer. Note the deposition of silver ions as water-trees in the adhesive layer denoting residual water. AD – adhesive; D – Dentin. Original magnification = 2,500X. (b) SEM micrograph in backscattered mode of the adhesive-dentin interface formed between the universal adhesive Prime & Bond Elect (Dentsply) and dentin. This specimen was part of a nanoleakage study using ammoniacal silver nitrate as the tracer. Note the intense deposition of silver ions in the hybrid layer and into the wall of the dentin tubules. This universal adhesive has acetone as the organic solvent. Acetone-based adhesives usually need more applications than the number recommended by the respective manufacturer. This may explain the absence of an adhesive layer at the entrance of the tubules, allowing the penetration of the composite resin (C) into the tubules. C – composite resin; H – hybrid layer; D – dentin. Original magnification = 5,000X.

Fig. 15

(a) SEM micrograph of the adhesive-dentin interface formed between a universal adhesive applied in etch-and-rinse mode and dentin. Dentin was partially dissolved with 6N HCl for 30 sec and deproteinized with 5%NaOCl for 5 min. Note that the typical reticulation from the presence of collagen in the hybrid layer is only observed in a <1 μm deep area (H). The characteristic reticular pattern on the hybrid layer is absent (asterisks) below this area. The collagen is dissolved by the deproteinizing agent NaOCl when the collagen fibers are not fully enveloped by the resin from the adhesive. This phenomenon is known as hybridoid layer and ghost hybrid layer [62,104,174]. C – composite resin; Rt – resin tag. Original magnification = 10,000X. (b) SEM micrograph in backscattered mode of the adhesive-dentin interface formed between a universal adhesive applied in etch-and-rinse mode and dentin. Dentin was partially dissolved with 6N HCL for 30 sec and deproteinized with 5%NaOCl for 5 min. Note that reticulation from the presence of collagen in the hybrid layer is only observed in a 1 μm deep area (H). Below this area, the characteristic reticular pattern on the hybrid layer is absent (asterisks), for the same reason explained in Fig. 15a. C – composite resin; Rt – resin tag. Original magnification = 10,000X.

Fig. 16

TEM micrograph of the adhesive-dentin interface formed by Fuji II LC (GC) with dentin. G – glass particle; M – matrix; IZ – interaction zone; D – dentin. Original magnification = 40,000X.

Fig. 17

TEM micrograph of Fuji II LC (GC) after setting. G – glass particle; M – matrix; Asterisk – silica gel around the glass particles. Original magnification = 100,000X.