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A Novel Concept: The Introduction of Cross-Linking Monomers into a Self-Etch Adhesive to Create a More Hydrophobic & Durable Bond

March 1, 2011
by Byoung In Suh, PhD, Liang Chen, PhD, Douglas J. Brown, DDS, FAGD


Dental bonding agents (adhesives) have been rapidly evolving since the 1950s.1 Based on the adhesion strategy, today’s adhesives can be classified as “etch-and-rinse” (4th and 5th generations) and “self-etch” (6th and 7th generations). The novel incorporation of cross-linking monomers within today’s bonding agents is allowing “smart” manufacturers to significantly improve current adhesives that may soon lead to the creation of the “dream adhesive.”

The addition of cross-linking monomers to total-etch adhesives began early-on as a means to improve adhesive physical properties.2,23 BisCoverTM (BISCO) was the first dental glaze-polish to incorporate monomers with high cross-linking capacity for the purpose of “instantaneous cure”. If a resin/glaze could polymerize without a “sticky layer” in the presence of oxygen, the result would be a perfect polish.

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Self-etch adhesives use acidic primers to etch and prime the tooth surface simultaneously.3 Current self-etch adhesives offer both advantages and disadvantages to the clinician. The ease of application onto dentin and the anecdotal reporting of reduced post-operative sensitivity is in part due to the preservation of smear plugs; limiting fluid movement within dentinal tubules. The disadvantages of most are due to their acidic primers causing incompatibility with dual cure dentin restoratives and increased water permeability leading to plasticizing. This is the reason why it is generally recommended that the clinician apply a separate resin layer to achieve optimal durability and compatibility. The need to apply a separate thicker resin layer is time consuming for direct restorations and is contraindicated for indirect final restoration margin adaptation. As a result, the clinician must have a separate adhesive material for direct and indirect restorations.

Self-etch adhesives come as 2-step or 1-step/1-drop formulations. The 2-step self-etch adhesives (such as ALL-BOND SE® with liner, BISCO; Clearfil® SE, Kuraray; OptiBondTM XTR, Kerr) etch and prime the tooth first, followed by the application of a hydrophobic adhesive resin layer. Both OptiBondTM XTR (Kerr) and Clearfil® SE primers are not light curable requiring the need for a light curable resin liner. One-step/1-drop self-etch adhesives (such as ALL-BOND SE®, BISCO; AdperTM Easy Bond, 3M ESPE; Clearfil® S3, Kuraray) combine etching, priming and bonding resin into one single step.

Questioning the Durability of 1-Step Self-Etch Adhesives

 

Clinicians choose products based on claims of long-term bond stability. Researchers are challenging many of these claims. One of the most challenging issues of all current dental adhesives is to improve the bond durability.4 The long-term performance of dental adhesion is not only related to the clinical components of dentist application and patient interaction, but also dependent on the quality of the adhesive materials, incorporation of chemically active monomers to protect tooth structure, quality of dentin/enamel substrate and bonding strategies (total-etch or self-etch).

The tooth-resin bond may degrade due to chemical and physical stresses. Physical stresses include occlusal stress, thermal expansion/contraction, and polymer water swelling. The chemical degradation includes hydrolysis of chemical bond and plasticizing effect of resin polymer by water. The hydrolysis process is accelerated by enzymes such as matrix metalloproteinases (MMPs).5 MMP inhibitors and antimicrobial agents such as chlorhexidine6,7 and benzalkonium chloride (BAC)8 have been shown to reduce bond degradation and stabilize the hybrid layer. “Bioactive Etchants” such as BAC-containing etchant, (UNI-ETCH® with BAC, BISCO), will continue to be a focus in the dental industry.

Water sorption of self-etch adhesive interfaces is still considered the most important factor relating to bond degradation.7,9 The 1-step self-etch adhesives are specifically formulated to etch (i.e. high concentration of hydrophilic acidic monomers), prime and bond; all in one single step. Contributing to water sorption of these 1-step self-etch adhesives is the higher concentration of acidic monomers (lower pH) in an attempt by researchers and manufacturers to achieve improved etching of enamel. The simplicity of these adhesives comes at the expense of their high affinity for water, even after cure (i.e. polymerization). Such hydrophilicity turns them into a semi-permeable membrane,10-15 which allows for water transfusion across the cured adhesive system. This is evidenced by Tay et al. who showed lower bond strength, water bubbles and water tree formation.10-15 It is documented that 1-step self-etch adhesives generally cannot provide durable bonds, especially on dentin. AdperTM PromptTM L-Pop (3M ESPE) was shown to be very sensitive to water-aging effects.16 Clearfil® S3 (Kuraray) had a significant decrease in microtensile bond strengths following as little as 5000 thermocycles17 or 1 year water of storage.18 Pisani-Proenca showed all of the five 1-step self-etch adhesives tested (Etch & Prime 3.0; Degussa, AdperTM PromptTM L-Pop; 3M ESPE, Solist; DMG, Futurabond®; Voco America, and AQ Bond; Sun Medical) had significantly reduced microtensile bond strength after 12 months aging in water with Futurabond® (Voco America) showing pre-testing failures.19 HEMA-free 1-step self-etch adhesive (G-BondTM, GC America) also produced significantly lower tensile bond strengths after 1 year water of storage.18

Despite the great convenience of 1-step adhesives, the long-term bonding durability continues to be questioned. If the past is a predictor for the future (as with total-etch adhesives), improvement of 1-bottle/1-drop self-etch adhesives durability will be the result of chemistry taking on improved hydrophobic characteristics of the cured adhesive resin.

The Importance of Hydrophobic Chemistry to Long-Term Success

 

Long-term durability of 3-step total-etch adhesives attribute their success to the placement of a final hydrophobic resin layer. These materials have been proven with excellent long-term clinical performance and are still considered by many as “gold standard”.4 Similarly, 2-step self-etch adhesives add a separate hydrophobic resin layer (i.e. bonding agent or liner) to improve the performance of self-etching primers. The addition of the hydrophobic resin layer acts as a barrier in preventing permeability at the bonded interface. The paper, “A Critical Review of the Durability of Adhesion to Tooth Tissue”,4 compared contemporary adhesive systems and concluded that the 2-step self-etch adhesives seemed more durable than 1-step self-etch adhesives. The paper suggested that only 2-step self-etch adhesives closely approach the “gold standard”. This conclusion has been supported by many research studies. With the application of an additional hydrophobic resin layer, the 1-step self-etch adhesives, AdperTM PromptTM L-Pop (3M ESPE), Xeno® III (Dentsply), and iBond® (Heraus Kulzer), had significantly reduced degradation of resin bonds over six month aging, although the application of an additional hydrophobic resin layer was not required by the manufacturers.20 In one study, the 2-step self-etch adhesive, Clearfil® SE (Kuraray), showed stable microtensile bond strengths after aging for one year.17 An important study comparing Clearfil SE® (2-step self-etch) to Clearfil® S3 Bond (1-step self-etch) both manufactured by Kuraray, showed the 2-step self-etch adhesive having significantly improved bonding durability.16

The Dream Adhesive: Hydrophilic on application – Hydrophobic upon polymerization in 1-Step/1-Drop

 

The challenge, as previously stated, is to create an all-in-one adhesive which is chemically stab
le, protective of collagen fibers, sufficiently wetting of dentin and quickly polymerizes forming a hydrophobic barrier to water penetration. Creating this formula using current 1-step/2-step models would be the approach.

ALL-BOND SE® (BISCO, Fig. 1) is a 2-component system, specifically formulated to contain highly cross-linking monomers (similarly contained in BisCoverTM), which will form highly cross-linked polymers. It was postulated that highly cross-linked polymers could effectively block water molecule’s attack, and exhibit lower water sorption, resulting in improved clinical durability. This concept is supported by a 2 year clinical study by Reis et al. where ALL-BOND SE® (BISCO), used in both the 1-step or 2-step protocol, resulted in high retention rates.24 A recent study by Hass et al. also noted that although it has been well documented that 1-step self-etching systems behave as permeable membranes, their performance depends on material composition, the acidity alone cannot explain the differences in performance. This study evaluated the association between resin-dentin bond strength (microtensile bond strength) and the physical properties (ultimate tensile strength and microhardness) of the cured adhesive using five 1-step self-etching adhesives (Futurabond® NR; VOCO America, Go; SDI, G-BondTM; GC America, ALL-BOND SE®; BISCO, and Clearfil S3; Kuraray).25 When the materials were compared, the 1-step ALL-BOND SE® (BISCO) showed higher microtensile, ultimate tensile strength, and micro-hardness than the other materials.25

DeMunck and others have previously shown, following three month aging, all classes of adhesives showed evidence of degradation.4 BISCO scientists evaluated the effect that the addition of highly cross-linking monomers would play on improving bond durability of self-etch adhesives (i.e. ALL-BOND SE®; BISCO). Microtensile bond strengths on dentin were evaluated up to three year aging in 37°C water. Testing microtensile bond strengths (~1mm2 bonding area to occlusallyoriented dentin) is considered the standard validation method to assess adhesion durability. The collection of BISCO’s three year research data has recently been completed and shows a significant adhesive advancement. The incorporation of highly cross-linking multifunctional monomers into the primer helps to greatly improve the adhesive’s bond durability similar to separate application of hydrophobic resin layer. What this means to the clinician is when using ALL-BOND SE® (BISCO), no additional resin layer is needed: a time-saving reduction vs. previous self-etch protocol, universal application in both direct/indirect procedures and significantly improved durability.

The following three-year microtensile study was conducted to determine if the incorporation of highly cross-linking monomers into ALL-BOND SE® (BISCO) would eliminate the need for a separate resin layer while preserving long-term durability.

In this study, microtensile specimens were prepared as previously described in the literature. The resin layers were either HEMA-containing resin (i.e. ALL-BOND SE® Liner, BISCO) or HEMA-free resin (i.e. ALL-BOND 3® Resin, BISCO). Light-cured composite (LIGHT-CORETM Blue, BISCO) was placed in 1-2mm increments and cured for 60s (up to 4-6mm build-up). The samples were stored as beams in 37°C water for time periods of 24 hours, three months, six months, and three years. Following water storage, bond strengths were tested using the Microtensile Tester (BISCO). The results showed that microtensile bond strengths were stable remaining around 45 MPa during the three year aging (Fig. 3). No significant bond strength reduction was found for all the categories (with and without resin layers) and time frames. Aging of micro-specimen beams (4-side exposure) (Fig. 2) in 37°C water is very aggressive, and typically results in significant bond strength reduction as previously reported.4

The results of the ALL-BOND SE® (BISCO) study show stable, high bond strengths with and without a resin layer. Previous research has indicated that a separate resin application is always beneficial and recommended. This study clearly demonstrates that the addition of highly cross-linking monomers in ALL-BOND SE® (BISCO) eliminate the need for additional resin layers to promote bond durability. AdperTM Easy Bond (3M ESPE), was included in this study and showed a 30% bond strength reduction after three years of aging.

Adhesion is the Foundation of Aesthetic Restorative Dentistry

 

The addition of highly cross-linking multifunctional monomers within ALL-BOND SE® (BISCO) is shown to eliminate the need for a separate placement of a resin layer. ALL-BOND SE® (BISCO) with highly cross-linking monomers can effectively eliminate or reduce water degradation. This is an adhesive breakthrough for clinicians; simpler can be better! (See case pictures.)

ALL-BOND SE® (BISCO) is a light-cured, self-etching adhesive that combines etching, priming and bonding in one application. ALL-BOND SE® (BISCO) can be used as a single-step bonding agent, or with the radiopaque liner. Unlike most current self-etch adhesives, ALL-BOND SE is fully compatible with light-, self- and dual-cured materials without the use of additional activators.

ALL-BOND SE® (BISCO) is also available in proprietary cartridges for use with the patented ACE® dispenser. ACE®’s fast and easy application takes the guesswork out of every procedure by accurately dispensing the correct proportion of material with the click of a button. The ability to produce uniform drops with a color-changing feature provides dentists and dental assistants with complete confidence of a properly mixed product. ACE® ALL-BOND SE® makes self-etch dental bonding more reliable, more predictable and more compatible with today’s restorative options. OH

Dr. Byoung Suh is the President of BISCO, Inc. He founded the company in 1981 and continues to conduct research in dental materials.

Dr. Liang Chen received his PhD degree from Tulane University Department of Chemistry in 2005. In 2006 he became a research associate at Louisiana State University Dental School, where he and his co-workers invented novel fluoride-releasing/recharging dental monomers/materials (metal-fluoride chelating dental monomers). Since 2008 he has been working with BISCO, Inc. as a Senior Research Scientist.

Dr. Douglas Brown is the Senior Manager of Clinical Affairs at BISCO, Inc.

Dr. Brown has been involved in the creation and implementation of numerous dental products, including composites, glass ionomers, resin cements, and adhesives, and their incorporation into minimally invasive dentistry.

Figures 4-17 Courtesy of Michael J. Morgan, DDS, Private Practice, Hinsdale, IL.

Oral Health welcomes this original article.

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