Clinical Significance of Bis-GMA and HEMA Orthodontic Resins Bonding to Enamel and Ceramic Materials

A recent study has shown that the rates of adults receiving orthodontic care is increasing.¹¹ Many of these patients present with new ceramic crowns, which have been developed to replace decayed or missing teeth. The ceramics include zirconia and lithium disilicate (ie, IPS e.max, Ivoclar Vivadent, ivoclarvivadent.com). The advantages of these new ceramics are biocompatibility, esthetics, cost-effectiveness, fracture resistance, and fabrication accuracy.¹² With the advancement of ceramic crowns for dental treatment, however, new methods must be developed to bond to these denser materials. When bonding to lithium disilicate, which is used largely for anterior crowns, and zirconia, employed primarily for posterior crowns, conventional bonding mechanisms have proven inadequate.^13,14 Zirconia is a nonpolar ceramic and is relatively inert¹⁵; thus, it does not react with chemical adhesives as readily as enamel does. Zirconia is a type of porcelain, and many clinicians attempt to use the same methods to bond to both zirconia and lithium disilicate.¹³ Bracket failure on these surfaces can lead to increased treatment time and cost and additional patient visits.⁵ Hydrofluoric acid-etch with silane has been shown to increase the bond strength when bonding to porcelain.^14,16 Therefore, when a practitioner uses Assure Universal Bonding Resin for bonding to ceramics, the manufacturer recommends using a hydrofluoric acid-etch. This is done instead of the standard 37% phosphoric acid-etch used on enamel. Hydrofluoric acid, however, is a strong chemical and is potentially dangerous to both the clinician and the patient.^13,17

Assure® PLUS All Surface Bonding Resin (Reliance Orthodontic Products, Inc.) is a recently released product that can bond to ceramic materials and eliminate the need to use acid as an etchant. It is a bisphenol A glycidylmethacrylate (bis-GMA) compound that is light cured and bonds to enamel through micromechanical retention after polymerization. Bis-GMA’s mechanism of action involves the carbon-carbon interaction of the methacrylate groups in the polymer to attach the substance to the workable surface.¹⁸ The large size of the difunctional monomer allows for lower volatility, a lesser degree of polymerization shrinkage, rapid hardening, and development of stronger resins, making it an ideal substance for use as a resin bonding adhesive.¹⁸

Objective

The purpose of this study was to compare bond strengths between Assure Universal Bonding Resin (and its recommended application) and Assure PLUS All Surface Bonding Resin (and its recommended application) as well as between enamel, zirconia, and lithium disilicate. The hypothesis was that Assure PLUS (without acid-etching) would have a clinically significant 30% greater bond strength when compared to Assure (etching with a 9% hydrofluoric acid solution) when used on enamel, zirconia, and lithium disilicate.

Materials and Methods

Enamel

With institutional review board approval, 40 freshly extracted human maxillary premolars without caries or restorations were collected and stored in a 10% formalin solution until the start of the study. The teeth were cleaned of soft tissue, and their surfaces were polished with rubber prophylactic cups for 10 seconds. They were then randomly divided into two groups, one to test Assure and one to test Assure PLUS.

For each group, the teeth were etched with 37% phosphoric acid-etch for 30 seconds. They were then washed for 10 seconds to neutralize the acid. The bonding resins were then applied in thin coats with a brush. Light Bond™ cement without fluoride (Reliance Orthodontic Products, Inc.) was then placed on each tooth followed by a standard edgewise universal premolar bracket (American Orthodontics, americanortho.com), and the bracket was light cured for 20 seconds. Bonded brackets were allowed to sit at room temperature overnight, and after 24 hours their shear bond strength was tested using an Instron® universal testing machine (Instron, instron.us).

Zirconia

To emulate zirconia crowns, Jensen XT Zirconia rods (1 cm x 1 cm x 4 cm) (Jensen Dental, jensendental.com) were used. Brackets were spaced vertically on the rods 4 mm apart. Twenty brackets were tested per group.

For Assure, the zirconia rods were sandblasted using 50-µm particle size alumina oxide. (Author’s note: Sandblasting zirconia can reduce the shine of a crown and, thus, caution should be used when sandblasting anterior teeth.) Porc Etch™ (9% hydrofluoric acid solution) (Reliance Orthodontic Products, Inc.) was then applied for 4 minutes. Afterward, the solution was wiped off, and the surface was rinsed for 45 seconds and air dried. A thin coat of Reliance Porcelain Conditioner (Reliance Orthodontic Products, Inc.) was placed on the surface with a brush, followed by Assure Universal Bonding Resin. Light Bond cement without fluoride was then applied, along with a standard edgewise universal premolar bracket, which was light-cured for 40 seconds (20 seconds each from opposite directions). The bond strength was tested after 24 hours using an Instron universal testing machine.

For Assure PLUS, the zirconia surface was sandblasted using 50-µm particle size alumina oxide, and the bonding resin was then applied to the surface and light-cured for 10 seconds. Light Bond cement without fluoride was placed, followed by a standard edgewise universal premolar bracket, which was light cured for 20 seconds. The bond strength was tested after 24 hours using an Instron universal testing machine (Figure 1).

Lithium Disilicate

To represent lithium-disilicate crowns, IPS e.max CAD rods (1 cm x 1 cm x 2 cm) (Ivoclar Vivadent) were created. Brackets were spaced vertically on the rods 2 mm apart. Twenty brackets were tested per group.

For Assure, the lithium-disilicate rods were sandblasted using 50-µm particle size alumina oxide. Porc Etch (9% hydrofluoric acid solution) was then applied for 4 minutes. Afterward, the solution was wiped off, and the surface was rinsed for 45 seconds and air dried. A thin coat of silane porcelain conditioner was placed on the surface with a brush, followed by Assure Universal Bonding Resin. Light Bond cement without fluoride was then applied, along with a standard edgewise universal premolar bracket, which was light-cured for 40 seconds (20 seconds each from opposite directions). The bond strength was tested after 24 hours using an Instron universal testing machine.

For Assure PLUS, the surface was sandblasted using 50-µm particle size alumina oxide. One coat of silane porcelain conditioner was applied with a brush and dried. Assure PLUS was then applied to the lithium disilicate and lightly air dried. Light Bond cement without fluoride was then placed along with a standard edgewise universal premolar bracket, which was light cured for 20 seconds. The bond strength was tested after 24 hours using an Instron universal testing machine (Figure 2).

The manufacturer’s instructions were used for all applications. Directions for application of Assure to porcelain and zirconia were obtained from the Reliance Orthodontic Products, Inc. (The Bonding Bulletin, Winter 2013).

Measurements

During shear bond strength measurement with the Instron universal testing machine, both teeth and rods were aligned so that the tip of the machine was completely parallel to the top surface of the bracket. The roots of the teeth were set in a chemically cured acrylic and held in a vice to prevent any movement. The bases of the rods were also held securely in a vice. The computer electronically recorded all measurements and the units were represented in megapascals (MPa).

Statistical Calculations

A power calculation was conducted using nQuery Advisor (Version 7.0) (Statistical Solutions Ltd., statsols.com). The calculation assumed a mean shear bond strength of 16.4 MPa for Assure on enamel and a within-group standard deviation of 3.5 MPa based on previous literature.⁸ It was further assumed, based on previous literature using Transbond™ XT Primer (3M Unitek, 3M.com) (a product statistically similar to Assure), that the mean shear bond strength for lithium disilicate would be 24% lower than for enamel.^8,19 Finally, it was assumed that zirconia and lithium disilicate would have equal mean shear bond strengths to each other (because these two surface types are in the same family¹³) and that Assure PLUS would exhibit a 30% increase in mean shear bond strength compared to Assure. Based on these assumptions, a sample size of at least n = 19 per group was adequate to obtain a type I error rate of 5% and a power greater than 99% for detecting differences between both the bonding resins and the surface types. As will be explained below in the Results section, at least 19 samples were obtained per group.

Descriptive statistics (means and standard deviations) were calculated for each group. Two-way ANOVA was used to assess statistical significance, with Tukey’s HSD used for post-hoc tests. P values less than .05 were considered significant. SPSS Version 22 (IBM Corp., ibm.com) was used in all analyses.

Results

Table 1 presents the mean, standard deviation, and sample size for Assure and Assure PLUS tested on enamel, lithium disilicate, and zirconia. Twenty samples were tested for each group; however, only 19 samples were obtained for Assure applied to enamel due to misalignment of the shearing apparatus on the respective bracket. The highest mean shear bond strengths were found for enamel, then lithium disilicate, then zirconia. The difference between Assure and Assure PLUS within each surface type was 1.40 MPa or less.

The two-way ANOVA was used to determine whether there was a statistically significant difference between Assure and Assure PLUS, as well as whether there was a significant difference between the three surface types. The comparison between Assure and Assure PLUS was not statistically significant (P = .564). The comparison between surface types was statistically significant (P = .011).

Due to the statistically significant difference between surface types, Tukey’s HSD was used in post-hoc testing. The post-hoc tests showed a significantly higher mean shear bond strength for enamel than for zirconia (P = .007). The difference between enamel and lithium disilicate was not statistically significant (P = .201), nor was the difference between lithium disilicate and zirconia (P = .365).

Discussion

With the advent of new dental materials in the formation of crowns, the efficacy of advanced methods to bond to these restorations must be assessed. Previous studies have shown that resin adhesion boosters have yielded greater bond strength when applying brackets to enamel.^2-8 However, difficulty occurs when clinicians try bonding to inert ceramics. This study was aimed at testing the differences between a 2-hydroxyethyl methacrylate (HEMA) orthodontic bonding resin (Assure) and a bis-GMA orthodontic bonding resin (Assure PLUS).

Previous work has suggested that Assure bonds equally as well when used in moisture-contaminated settings as a dry application procedure.²⁰ When compared to other studies of both dry and wet bond strengths of Assure, the authors’ values were slightly above⁸ and below^7,21 other trials and, thus, offered similar results. No studies were found analyzing the shear bond strength of Assure PLUS on any surface.

Based on the conducted ANOVA test, no statistically significant difference was found between the two bonding resin products and their respective protocols. Previous work has shown that a bond strength between 5.8 MPa and 7.8 MPa is of adequate force for retaining orthodontic brackets.⁵ Thus, either of these options offers ample strength to properly accommodate movement of lithium-disilicate or zirconia crowns during orthodontic treatment.

A statistically significant difference was found between surfaces. Tukey’s HSD test determined no statistically significant difference between enamel and lithium disilicate or between lithium-disilicate and zirconia. A significantly higher shear bond strength was found for enamel as compared to zirconia. This could prove problematic for clinicians trying to use these crowns and confirms previous research expressing difficulty bonding to zirconia.¹³

Research has been done analyzing different surface treatments for preparing lithium-disilicate and zirconia crowns for optimal bonding.^13-16,22,23 The general consensus is that hydrofluoric acid-etch with silane greatly increases the bond strength to lithium disilicate but is not as effective when conditioning zirconia. Hydrofluoric acid-etch acts to create a rougher surface for the polymers to bind to.¹⁶ A very strong chemical that can be potentially harmful to both the patient and practitioner, hydrofluoric acid can produce toxic vapors and burn skin and mucous membranes.¹³ Sandblasting is another method used in the preparation of ceramic surfaces for bonding and has been shown in previous studies to increase the tensile strength when applying brackets, especially to zirconia.^13,15 Care should be taken when sandblasting zirconia, however, because it is difficult to polish the surface to its original shine after treatment.

During the clinical application of Assure to porcelain, the manufacturer recommends using hydrofluoric acid with silane. Special care must be taken when applying this 4-minute etch, including placing a rubber dam and a barrier gel on the gingiva to avoid contact with soft tissue. Assure PLUS does not require this potentially dangerous step; only sandblasting is needed to roughen the ceramic materials for this product. Therefore, although this study found no statistically significant difference between the two bonding resins, the authors believe the relative ease of use of Assure PLUS, including no need for rubber dam, barrier gel, 4-minute caustic hydrofluoric acid-etch, or 45-second rinse, makes it a superior product when bonding to zirconia or lithium-disilicate materials.

This study explored assessing and testing new ways of bonding orthodontic appliances to ceramic crowns. The study could be expanded by trying similar tests with other products and ranking their clinical usefulness based on shear bond strength, ease of use, and time of application in an attempt to provide practitioners a thorough understanding of which products may be most useful in the clinical setting.

Conclusion

With an aging population that is living longer, older individuals are opting for orthodontic treatment, with general dentists often performing these procedures. Many of these older patients have lithium-disilicate and zirconia crowns, which are difficult to bond to, with many bonding products recommending a hydrofluoric acid-etch to enhance bracket retention.

In this study, a conventional HEMA resin bonding adhesive (Assure) and a new bis-GMA product (Assure PLUS) that is made to bond to these ceramic surfaces without the caustic etch were compared to test the efficacy of this new system. The results showed that there was no clinically significant difference between the two products on any surface tested, but that bonding to zirconia is more difficult than bonding to enamel. Clinically, however, the authors found the Assure PLUS was easier and more convenient to work with, and they conclude that because it does not require the hydrofluoric acid-etch, it is a superior product for the practicing dentist.

About the Authors

Timothy Reichheld, BA
Third-Year Dental Student
Tufts University School of Dental Medicine
Boston, Massachusetts

Gregory Monfette, BA
Third-Year Dental Student
Tufts University School of Dental Medicine
Boston, Massachusetts

Ronald D. Perry, DMD, MS
Professor of Comprehensive Care
Director of International Students
Tufts University School of Dental Medicine
Boston, Massachusetts

Matthew Finkelman, PhD
Associate Professor
Director of Biostatistics and Experimental Design
Tufts University School of Dental Medicine
Boston, Massachusetts

Eric Gheewalla, DMD
The Medford Center for Orthodontics and Pediatric Dentistry
Medford, Massachusetts

Gerard Kugel, DMD, MS, PhD
Professor of Comprehensive Care
Associate Dean for Research
Tufts University School of Dental Medicine
Boston, Massachusetts