Strategies to improve dentin bonding

In many medium- and large-sized direct restorations, the surface area of exposed dentin may exceed that of enamel. To adequately resist the stresses generated by composite polymerization shrinkage and to transfer occlusal and thermal forces throughout the restoration, a durable dentin bond is essential. Dentin bonding is achieved through the formation of both a hybrid layer and resin tags. The hybrid layer forms when the outer layer of dentin is demineralized, exposing a network of collagen fibrils that are subsequently infiltrated with adhesive resin and polymerized. Resin tags are created when adhesive penetrates into the dentinal tubules.¹ Bonding to dentin is considerably more technique sensitive than enamel because it is a hydrated, organic, and histologically variable structure that requires precise control of moisture, infiltration, and solvent evaporation to achieve stable hybridization and avoid incomplete resin penetration and long-term degradation.² This article reviews several critical steps involved in optimizing dentin bonding.

Surface preparation prior to bonding

The instrument used to prepare dentin influences bond strength due to its effect on the smear layer. The smear layer consists of debris and particles of ground dentin that remain on the tooth surface following preparation with rotary instruments. Smear layer thickness is greatest with coarse diamond burs, followed by fine diamonds, and is least with carbide burs.³ The clinical significance of the smear layer depends on the bonding strategy employed. In a total-etch technique, phosphoric acid removes the smear layer entirely, resulting in similar bonding outcomes regardless of the instrument used.⁴ In contrast, with self-etch systems, the acidic adhesive must penetrate and solubilize the smear layer. Thick smear layers may inhibit adequate penetration, potentially reducing bond strength.⁴ Therefore, when using self-etch adhesives, final preparation with carbide burs or fine diamond instruments is preferred.

Air abrasion (i.e., sandblasting) is another method to prepare the surface of dentin for bonding. This method may be effective for cleaning any contamination from the surface of the dentin prior to bonding; however, laboratory testing indicates that air abrasion does not affect dentin bond strength negatively or positively.⁵ Recent research suggests that air abrasion used with concomitant irrigation improves bond strength.⁶

Single vs. two-bottle bonding agents

In a two-bottle bonding system, the first bottle contains the primer, which typically consists of hydrophilic resin monomers (such as 2-hydroxyethyl methacrylate, HEMA) dissolved in volatile solvents such as ethanol, acetone, or water. The hydrophilic monomers allow penetration into the wet environment of the exposed collagen network to form the initial hybridized zone. The solvents facilitate transport of the monomers into demineralized dentin by reducing the primer viscosity and acting as a hydrophilic carrier phase that is miscible with the water-rich collagen matrix, allowing monomers to diffuse into the interfibrillar collagen spaces; they also displace residual moisture and promote evaporation because of their high volatility.¹ The second bottle contains the more hydrophobic adhesive resin, composed primarily of dimethacrylate monomers (such as bisphenol-A glycidyl dimethacrylate, Bis-GMA), viscosity regulators (triethylene glycol dimethacrylate, TEGDMA), photoinitiators, and, in some formulations, fillers.¹ Upon light curing, this adhesive polymerizes to form a resin matrix that encapsulates collagen fibrils and copolymerizes with the restorative resin. In these systems, thorough solvent evaporation is critical during the priming step, whereas the adhesive layer primarily requires prevention of pooling and adequate light polymerization.

Single-bottle systems combine the primer and adhesive into one solution to simplify clinical procedures and reduce chairside time. However, because hydrophilic and hydrophobic components coexist in a single formulation, these systems may demonstrate increased water sorption over time.⁷ Additionally, solvent evaporation may be more challenging due to the mixed composition.

Total-etch vs. self-etch

In total-etch (etch-and-rinse) techniques, phosphoric acid is applied to dentin to remove hydroxyapatite crystals and expose a collagen matrix. In self-etch systems (or selective-etch techniques, in which enamel is etched but dentin is not), the bonding agent (the primer in a two-bottle system) contains acidic functional monomers (i.e., MDP, 10-methacryloyloxydecyl dihydrogen phosphate). These acidic monomers condition and prime dentin simultaneously, demineralizing the underlying dentin, dissolving the smear layer, and incorporating it into the hybrid layer.⁸ Universal adhesives are mildly acidic self-etch bonding agents that can also be used following phosphoric acid etching.

Microscopic evaluation demonstrates that total-etch systems typically produce a hybrid layer approximately 3-4 µm thick, whereas self-etch adhesives generally produce a thinner hybrid layer of approximately 1 µm.¹ Despite this shallower penetration, self-etch adhesives may achieve comparable dentin bond strengths⁹ due to chemical interaction between functional monomers (e.g., MDP) and residual hydroxyapatite-bound calcium. The choice of etching technique may be affected by the type of dentin. For example, a clinical trial of noncarious cervical lesions indicated improved retention when a universal adhesive was applied in total-etch technique rather than self-etch or selective-etch technique, possibly a result of bonding to sclerotic dentin.¹⁰

Length of time for etching

With total-etch techniques, dentin should not be etched beyond the manufacturer’s recommended time (typically 15 seconds). Overetching may create a zone of demineralized collagen that exceeds the depth of adhesive infiltration, leaving unsupported collagen fibrils beneath the hybrid layer.¹¹ These areas are susceptible to hydrolytic and enzymatic degradation, compromising long-term bond durability.

Moisture content after etching

Following etching in total-etch techniques, convention has been for dentin to remain visibly moist. This moisture prevents collapse of the exposed collagen network (desiccation) and allows adhesive monomers to infiltrate effectively. Many contemporary adhesives, including universal adhesives, contain water/ethanol solvent rather than acetone solvents and can rehydrate partially collapsed collagen, making precise moisture control somewhat less critical.^12,13 Self-etch systems do not require premoistened dentin, and laboratory research suggest that application to gently air-dried dentin is acceptable.¹²

Active agitation of the bonding agent

Active agitation during adhesive application enhances infiltration between collagen fibrils and improves adaptation to the dentin surface. In self-etch systems, agitation also promotes dissolution and incorporation of the smear layer. Studies consistently demonstrate higher bond strengths when adhesives are actively scrubbed onto the dentin surface rather than passively applied.14,15

Air evaporation of the bonding agent

Air thinning is essential to evaporate residual solvent from the adhesive layer prior to polymerization. Incomplete solvent removal can lead to void formation and reduced bond strength.¹⁶ In two-bottle systems, solvent is typically present only in the primer; therefore, the priming step requires thorough air drying to ensure solvent evaporation. The adhesive layer primarily requires gentle air thinning to prevent pooling.

Adhesive layer thickness

Excessive pooling of adhesive should be avoided, as overly thick adhesive layers (several hundred microns) may act as a weak mechanical interface and appear radiolucent on radiographs.¹⁷ Conversely, an adequately thick adhesive layer can function as a stress-absorbing intermediate layer during composite polymerization. A flexible adhesive layer may accommodate polymerization shrinkage stresses by slight elastic deformation.¹⁸ Additionally, because the outermost portion of resin (around 10 to 20 µm)¹⁹ is subject to oxygen inhibition, a thin adhesive layer may experience incomplete polymerization. Studies have shown improved bond strength when certain thin adhesives are applied in two layers (with intermediate curing)²⁰ or covered with the adhesive (second bottle) of a two-bottle adhesive or a flowable composite.^21,22

Light polymerization of the adhesive layer

Adequate light energy is required for complete adhesive polymerization. Energy delivery depends on both curing light intensity and exposure time. Research within the author’s lab has demonstrated that curing an adhesive for three seconds with high-intensity light may achieve similar bond strength to curing for 20 seconds at lower intensity, whereas a one-second exposure is insufficient even at higher power. Proper positioning of the curing light is also essential to ensure adequate energy delivery to the adhesive layer. It is also preferrable to place and polymerize overlaying composite soon after adhesive polymerization to prevent time for osmosis of water through the hybrid layer from underlying dentin.²³

Prevention of hydrolytic and enzymatic degradation of the hybrid layer

Long-term durability of dentin bonding remains a challenge due to enzymatic and hydrolytic degradation. Enzymatic degradation results from matrix metalloproteinases (MMPs) present within dentin, which are activated during acid etching and degrade exposed collagen fibrils. Application of 2% chlorhexidine following etching has been shown to inhibit MMP activity.²⁴ Hydrolytic degradation occurs when water penetrates the hybrid layer from dentinal tubules and causes hydrolysis of chemical groups, particularly in adhesives with high hydrophilic content. Therefore, minimizing voids, ensuring complete resin infiltration, and limiting excessive hydrophilicity within the hybrid layer are essential for improving long-term bond durability.

Editor's note: This article appeared in the April 2026 print edition of Dental Economics magazine. Dentists in North America are eligible for a complimentary print subscription. Sign up here.

References

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