Strategies to Ensure the Bond Strength of Core Buildups

The ABCs of Dual-Cure Resin Composite Chemistry

To achieve complete polymerization throughout the entire volume of the resin composite material placed while allowing the convenience of cure on command, dual-cure resin composite core materials are formulated to contain at least two curing methods: self-curing and light-curing. Although these polymerization modalities are separate, the chemistries behind them feature some notable similarities. These similarities can be easily remembered using a simple mnemonic—the “ABCs” of dual-cure resin composite chemistry. Central to both chemical- and light-curing polymerization reactions is a common co-initiator. This co-initiator is a tertiary or aromatic tertiary amine, which is the “A” in the mnemonic. To summarize, tertiary amine co-initiators, such as N,N-dimethyl-p-toluidine (DMPT) and N,N-dihydroxyethyl-p-toluidine (DHPT), play a crucial role in resin composite polymerization in both self-cure and light-cure chemistry because they are the reducing agents (ie, electron donors) in these reactions. During self-curing polymerization, these tertiary amines participate with a second co-initiator in an oxidation-reduction reaction in the formation of free radicals, which triggers polymerization. The second co-initiators in self-curing polymerization are heat sensitive peroxides, such as benzoyl peroxide, which is the “B” in the mnemonic. Benzoyl peroxide and dibenzoyl peroxide are frequently utilized in dual-cure resin composite materials for chemical curing; however, these peroxides are known to be susceptible to heat decomposition, which is why most manufacturers recommended refrigeration of their core buildup materials during prolonged storage. Finally, photoinitiators such as camphorquinone, which is the “C” in the mnemonic, initiate resin composite photopolymerization by reacting with tertiary amines when exposed to activating wavelengths of light. The activated photoinitiator is reduced by a tertiary amine co-initiator, such as DMPT or DHPT, via transfer of a hydrogen atom, and the formation of radicals leads to polymerization.¹

Incompatibility With Single-Bottle Self-Etching Adhesives

The other essential property of dual-cure core buildup materials is their ability to achieve sufficient bond strength to both enamel and dentin. Recent advances in adhesive dentistry have championed simplified bonding systems; however, a material incompatibility between dual-cure core buildup materials and self-etching universal adhesives has been reported in the literature.^2-6 It is thought that this incompatibility is due in part to the increased acidity of single-bottle self-etching adhesives. In brief, early generations of bonding systems achieved dentin bonding via a total-etch (ie, etch-and-rinse) protocol, which removed the smear layer and exposed approximately 3 to 5 µm of demineralized collagen. Alternatively, the newer generations of self-etching bonding systems achieve dentin bonding by using acidic primers to partially dissolve the smear layer rather than remove it, which exposes approximately 1 µm of collagen.⁷ To accomplish this, these adhesives are formulated to be more acidic.⁸ It is theorized that the increased acidity of contemporary adhesives may prevent complete polymerization of self- and dual-cure materials at the adhesive-restorative interface.³ This is thought to be caused by remanent acidic components of these adhesives that participate in an undesired reaction with the tertiary amine co-initiators frequently contained in self- and dual-cure resin composites. It is also theorized that the increased hydrophilic nature of modern simplified adhesives can result in water being absorbed and retained, which can lead to osmotic stresses not only at the adhesive-restorative interface but also within the bulk of the core buildup material.^3,6

Optimizing the Bond Strength of Core Buildups

Despite the observed incompatibility between dual-cure core buildup materials and self-etching universal adhesives, there are several ways in which restorative dentists can circumvent it, including the following:

Incorporate a Dual-Cure Activator

One method involves the use of dual-cure activators. Dual-cure activators, which contain yet another co-initiator (typically an aromatic sodium salt of sulfinic acid), can be added to self-etching universal adhesives prior to their application to the tooth surface. The co-initiator present in the dual-cure activator reacts with the acidic monomers present in the adhesive, producing aryl sulfinic acid (ArSO₂H) and a sodium salt of the acidic monomer.¹ This “side reaction” between the dual-cure activator and the acidic components of the self-etching adhesive prevents the undesired degradation of the tertiary amine by the acidic monomers from occurring, and thus, the tertiary amine is still available to participate in radical polymerization via self-curing with the peroxide co-initiator.^1,9 In this manner, dual-cure activators allow self-etching adhesives to become compatible with dual-cure core buildup materials. It should be noted that dual-cure activators do not chemically initiate the polymerization of universal adhesives and that the layer of adhesive placed on the tooth must still be light cured prior to placement of the core buildup material.¹⁰

Ensure Photopolymerization at the Interface

Although many manufacturers have brought complementary dual-cure activators to market for their respective self-etching adhesives, attaining desirable bond strength to dentin via self-curing remains a challenge. Studies have shown variable success in obtaining sufficient shear bond strength when using a dual-cure activator.^11-13 Other studies examining dual-cure resin composite core buildup materials have demonstrated that light-curing often results in a higher degree of conversion (ie, polymerization) than self-curing alone.¹⁴ Therefore, one potential way in which clinicians may achieve greater bond strength when using a self-etching adhesive with a dual-cure core buildup material would be to ensure that adequate photopolymerization occurs at the adhesive-restorative interface. Practically speaking, this may mean placing the dual-cure resin composite buildup material in more than one layer. Photopolymerization at the interface immediately following placement may not only achieve a higher degree of conversion but also prevent the undesired degradation of the tertiary amine from occurring. Alternatively, restorative clinicians can place a layer of a light-cure flowable material following curing of their self-etching adhesive’s bonding resin, provided they are able to achieve sufficient radiant light energy in all areas of the restoration to ensure adequate curing.

Use a Two-Step Self-Etching Adhesive

Another method by which restorative dentists can circumvent the observed incompatibly between self-etching adhesives and dual-cure resin composite buildup materials is to avoid it entirely by using a two-step, self-etching sixth-generation adhesive rather than a one-step, self-etching seventh- or eighth-generation adhesive. Because the bonding resins (ie, second bottle) of two-step, self-etching adhesives do not contain acidic monomers or solvents, they form a neutral and hydrophobic adhesive layer. In contrast, the single-bottle seventh- and eighth-generation adhesives are more acidic and hydrophilic, and because both the etching and bonding components are contained in the same bottle, the adhesively bonded restorative surface is more likely to adversely react with the co-initiators present in dual-cure resin composite core buildup materials.¹

Implement an Amine-Free Dual-Cure Buildup Material

One final approach to dealing with this material incompatibility is to switch to a new core buildup material with a different chemistry. Several new commercially available dual-cure core buildup materials use amine-free resin polymerization chemistry. These materials replace the tertiary amine with cumene hydroperoxide and a thiourea as novel co-initiators. In this reaction, the hydroperoxide is the oxidizing agent, and the thiourea is the reducing agent. In addition, a copper(II) metal salt is used as a catalyst, and alterations in the amount of the metal salt present can change the speed at which polymerization occurs.¹⁴ Significantly, both hydroperoxide and thioureas are stable at room temperature, so there is no need for refrigeration when storing these materials.¹⁵ Amine-free core buildup materials exhibit increased compatibility with self-etching adhesives; however, in vitro shear bond strength studies conducted at the authors’ laboratory have suggested that these materials can still be slightly adversely affected by the increased acidity of self-etching universal adhesives. Despite this observation, amine-free core buildup materials are adversely affected at a much lower level than traditional amine-containing dual-cure materials.

Conclusion

In summary, dual-cure core buildup materials provide restorative dentists with important clinical properties in the restoration of endodontically treated and compromised teeth—most significantly their unlimited depth of cure and their ability to bond adequately to dentin. Although dual-cure core buildup materials afford convenience, it is imperative for restorative dentists to understand that their bond strength may be compromised when they are used in conjunction with a self-etching universal adhesive because the increased acidity of such adhesives can adversely affect the co-initiators present in their dual-cure formulations. Restorative dentists should take care to ensure that adequate polymerization occurs at the adhesive-restorative interface by following any of the aforementioned approaches. Taking these steps may help clinicians realize their ultimate goals of providing patients with more predictable and durable restorative outcomes. 

About the Authors

Zachary K. Greene, DMD, MS
Second-Year Resident
Biomaterials Residency Program
University of Alabama at Birmingham
School of Dentistry
Birmingham, Alabama

Nathaniel C. Lawson, DMD, PhD
Director, Division of Biomaterials
Program Director, Biomaterials Residency Program
University of Alabama at Birmingham School of Dentistry
Birmingham, Alabama
General Dentist
UAB Faculty Practice
Birmingham, Alabama

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