Universal resin cements: In vitro studies as a predictor of their bonding potential

Because of the trend towards less invasive preparations that preserve larger amounts of healthy tooth structure, non-retentive designs (like veneer and tabletop preparations) or partial designs (such as inlays and onlays) are increasingly preferred over retentive (crown and bridge) preparations. The success of indirect restorations with a non-retentive design, however, depends on a strong chemical adhesion between the tooth structure and the restoration – which can be obtained with the right resin cement system.

The first commercially available resin cements offered the necessary adhesion, but came at the cost of a complex, technique-sensitive clinical procedure and difficult removal of excess material. Fortunately, continuous improvements made by leading resin cement manufacturers have solved most of these issues. However, until recently, many dental professionals still needed to work with a conventional cement plus multiple dual-cure resin cements for specific indications. Whenever possible, dentists prefer to use a self-adhesive resin cement because of its simpler, more efficient workflow, but keep an adhesive resin cement on hand for cases that require maximum bond strength.

The problem of needing multiple resin cements has been overcome by a new category of universal resin cements. They work in both self-adhesive and adhesive mode (combined with a system adhesive). The system adhesive is typically used to maximize the self-adhesive bond strength in particularly challenging situations.

IDENTIFYING THE PERFECT ALTERNATIVE

To simplify indirect restorative procedures, it might be worth switching to a newer resin cement system that consists of fewer components, offers universality, and requires a less complex clinical procedure. Before doing so, however, you want to be sure that the selected system will offer reliable, lasting performance. Although long-term clinical studies are not available for the latest products, in vitro study results can predict the clinical behavior of a cementation system quite well. Look for results that show immediate bond strength to enamel, dentin and indirect substrates (the restorative materials), as well as bond durability through artificial aging of the test samples.

BONDING PERFORMANCE

Establishing a reliable bond to enamel, dentin and the restoration is a crucial requirement for any resin cement system. In the case of a universal dual-cure resin cement, the bond should be reliable regardless of the adhesive mode selected, the presence of moisture, the curing mode (light-cure versus self-cure), and the selected restorative material (zirconia, silicate ceramics, composite, or metal).

Many different tests are used to evaluate the bonding performance of adhesives and self-adhesive resin cements, including testing shear and tensile bond strength before and after artificial aging (fig 1a, 1b). However, there are no universally accepted testing standards. This means that testing procedures can vary with respect to the type of surface treatment, sample preparation, storage time and conditions, thermal and mechanical stress, and test geometry. While it’s not possible to directly compare results from different studies, it is important to look at results from multiple studies. This is true for both
immediate bond strength testing and for bond strength testing following artificial aging, a procedure that involves even more variables.

Figure 1a: Set up for a tensile bond strength test. The test sample is embedded in resin and the bonded material is pulled off with a calibrated test device

Figure 1b: Set up for a shear bond strength test. The test sample is embedded in resin and the bonded material is sheared off with a calibrated test device

Adequately assessing the bonding performance of a universal dual-cure resin cement across all combinations of mode and indication requires numerous in vitro tests. They need to cover:
• Self-adhesive and adhesive modes
• Different substrates like enamel, dentin, zirconia and glass ceramics
• Light-cure and self-cure modes
• Bonding performance after artificial aging
• Different test methods

AN EXCELLENT EXAMPLE

All of the in vitro tests listed above were used to evaluate the performance of 3M™ RelyX™ Universal Resin Cement before its commercial launch. The results confirm that when using the product according to the manufacturer’s instructions in self-adhesive mode, or adhesive mode in combination with 3M™ Scotchbond™ Universal Plus Adhesive, you can be confident that the bond will be reliable and long-lasting.

When used alone, RelyX Universal Resin Cement delivers reliable self-adhesion to zirconia, metal and enamel. You can expect a strong self-adhesive bond strength to dentin whether the resin cement is used in light-cure or self-cure mode, even after artificial aging (fig. 2).^1-6

Figure 2: Shear bond strength to dentin after artificial aging by thermocycling (5.000 cycles, 5°C-55°C).¹

When combined with Scotchbond Universal Plus Adhesive, the resin cement allows bonding to glass ceramics and enhanced bond strength to all substrates for cases that demand maximum adhesion. The values obtained with this two-component system are comparable to those achieved with a three-component gold-standard system.^{4,5, 7-9}

Some of these results are summarized in the following graph (fig. 3).

Figure 3: Self-cure shear bond strengths to dentin, unetched enamel, sandblasted 3M™ Lava™ Esthetic Fluorescent Full-Contour Zirconia, and HF etched IPS e.max® CAD glass ceramic after 24-hour storage in 37°C/98.6°F deionized water.⁵

In Summary

Comprehensive in vitro data about a new product’s bonding capabilities under varying conditions allows dental professionals to predict its clinical performance. A resin cement system that can bond to tooth structure and to a great number of restorative materials, independent of the application and curing mode, can be expected to be reliable under many different clinical conditions. Additional information on handling properties in the clinical environment is offered by practice-based clinical evaluations and field tests. For RelyX Universal Resin Cement, a field test and two handling evaluations from independent research groups validate the exceptional performance suggested by laboratory testing.^{10, 11}

As we all know, clinical long-term studies are the true test for any resin cement system. For RelyX Universal Resin Cement used with or without Scotchbond Universal Plus Adhesive, several clinical evaluations are currently being conducted. Initial reports will be available in 2022.