JCDA Express Issue 6 2012

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Making sense of dentin bonding

In this edition of JCDA Express, Dr. Stephen Ferrier, assistant professor in restorative dentistry at the Schulich School of Medicine and Dentistry, brings forward 4 articles that examine dentin bonding: the advantages of different systems, laboratory results versus clinical practice, and a comparison of bonding systems in clinical studies.

Introduction

• Dentin bonding is vital to the success of resin composite (and other types) of restorations. However, it is a complicated and difficult process. Failure of dentin bonds contributes to the relatively short survival time of composite restorations—5.7 years on average.¹
• Enamel and dentin are very different bonding substrates. Bonding to acid-etched enamel creates a reliable hybrid layer of apatite prisms and crystallites surrounded by resin. In dentin, the presence of water and collagen makes the hybridization bonding process much more complex. Acid etching dentin denudes the collagen of hydroxyapatite mineral producing a hybrid layer consisting of collagen fibrils surrounded by resin.
• Dentin bonding systems contain 3 basic components: conditioner (acid etch), primer and adhesive resin. However, commercial products vary in their component formulations and combinations, leading to a variety of presentations and a wide range of products—with different application procedures.
• It is now well understood how dentin bonds degrade over time. New materials are being developed to address this problem.

What are the advantages of multi-step-etch-and rinse systems?

Pashley DH, Tay FR, Breschi L, Tjaderhane L, Carvalho RM, Carrilho M, et al. State of the art etch-and-rinse adhesives. Dent Mater. 2011;27(1):1-16.

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JCDA Clinical Pearl: Three-step etch-and-rinse systems are the oldest on the market. They require meticulous clinical technique but offer the highest dentin bond strengths (often ≥ enamel bond strengths) and retention rates of resin-based bonding systems. Each separate stage can potentially be modified to address the problem of bond degradation.

Key Points:

• This article summarizes our current understanding of the hybridization process that allows two completely incompatible materials—dentin and composite resin—to bond to one another.
• One of the key advances is the total-etch technique, whereby enamel and dentin are treated with phosphoric acid. Adverse pulpal reactions arise because of failed bonds and microleakage—not because of acid treatment.
• Another key concept is wet-bonding: collagen must be kept hydrated to allow the primer to infuse between the fibrils. Drying the dentin causes the collagen network to collapse and results in a poor bond. Happily, if accidentally over dried, the collapsed collagen network can be restored if it is simply rehydrated.
• Two factors appear to cause dentin bond degradation:

1. The dentin bonding process is imperfect. Dentin may be demineralized too deeply, too much moisture remains, and the primer does not completely diffuse around the demineralized collagen fibres. This leaves voids and channels filled with water where collagen is not surrounded by resin.
2. Acid demineralization of dentin not only exposes the collagen that is vital for the dentin bond, but also releases other proteins, including hydrolytic enzymes, that were laid down or entrapped during tooth formation. Over time, these proteins diffuse through the voids and channels and break down exposed collagen fibres.

Reasons for Recommending this Article:

This article reviews the theoretical, scientific and clinical research behind dentin bonding.  It provides a comprehensive overview of how our understanding of this process has evolved over the last 3 decades.

What are the advantages of self-etching systems?

Van Meerbeek B, Yoshihara K, Yoshida Y, Mine A, De Munck J, Van Landuyt KL. State of the art of self-etch adhesives. Dent Mater. 2011;27(1):17-28.

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JCDA Clinical Pearl: Self-etching systems considered as mildly acidic appear to offer the best results. However, their mild acidity results in inferior enamel bonding and increased propensity for marginal breakdown. Therefore, it is recommended to selectively etch (and rinse) the enamel only with phosphoric acid, then apply the mild self-etching adhesive to both the un-etched dentin and pre-etched enamel.

Key Points:

• Self-etching adhesives may come in many different presentations:
• Two stage systems with a self-etching primer stage and a separate adhesive resin stage
• Single stage systems that require two components to be mixed immediately before application
• Single stage systems that are supplied premixed (true one bottle adhesives)
• Self-etching adhesive systems contain functional (acidic) monomers that react with hydroxyapatite. The key to the success of self-etch systems appears to be the stability of the compounds formed when these functional monomers react with hydroxyapatite in tooth structure, since the reaction products are not washed away as they are with etch-and-rinse systems.
• Clinical application is considered less time-consuming and more user-friendly, as a separate etch-and-rinse stage and, in most cases, the tricky wet bonding procedure, are not required.
• These agents simultaneously demineralize and infiltrate tooth structure so denuded collagen is not left exposed to hydrolytic enzymes. 
• Less post-operative sensitivity has been reported with self-etching adhesives—probably due to their milder acidity, which tends not to obliterate smear-layer plugs and open dentinal tubules as aggressively as conventional etching with phosphoric acid.
• Acidity is critical to success. The best results are from systems described as ‘mildly acidic’ with a pH of around 2, that use a compound known as 10-MDP.
• In general, two stage self-etching systems are more successful than one stage. In one stage systems, mixing of hydrophobic and hydrophilic substances results in lower bond strength, reduced bond durability, increased water sorption and permeability of the hybrid layer.

Reasons for Recommending this Article:

This article considers the principles of self-etching adhesives and reviews the effect of different self-etching formulations on tooth structure and the resultant bond quality. It offers clinical insights into the different techniques and outcomes that depend on which product is used.

How does the science relate to clinical performance of dentin bonding agents?

Carvalho RM, Manso AP, Geraldeli S, Tay FR, Pashley DH. Durability of bonds and clinical success of adhesive restorations. Dent Mater. 2012; 28(1):72-86.

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JCDA Clinical Pearl: In general, laboratory and clinical studies show that dentin bonding systems that fare well in the laboratory show improved results in clinical applications. However, important disparities between laboratory results and clinical success mean any individual parameter in isolation, e.g. bond strength, must be interpreted with caution.

Key Points:

• Laboratory dentin bond testing is usually done on pristine dentin from third molars, carefully prepared with fine grit sandpaper. In contrast, the bonding substrate in a prepared cavity is bur cut, possibly caries-affected dentin.
• Clinical evaluations of dentin bonding agents were carried out on non-carious cervical lesions (NCCL), according to American Dental Association recommendations. NCCLs are good models for testing adhesive systems because they offer easy access for placement and visual examination and easy/minimal preparation, reducing many of the potentially confounding clinical variables. However, this dentin is a difficult bonding substrate for any system; therefore, it is no surprise that bond strengths from clinical evaluations of NCCLs are significantly lower than those from lab tests.
• Dentin bond degradation can be produced rapidly in the lab but occurs more slowly in the clinical setting. Lab studies show that dentin bond degradation may be prevented in cases where enamel bonding around the entire periphery is able to completely seal the cavity preparation, whereas in vivo studies of class I restorations still show evidence of dentin bond degradation.
• Dentin bond failure clearly contributes to restoration failure but it is important to keep this in perspective.  Secondary caries are a major cause of composite resin restoration failure and caries risk is a better predictor of restoration failure than the type of bonding agent used.

Reasons for Recommending this Article:

The authors consider bond durability and its relationship to clinical outcomes, highlighting several important differences between clinical and laboratory studies.

How do these bonding systems compare clinically?

Peumans M, Kanumilli P, De Munck J, Van Landuyt K, Lambrechts P, Van Meerbeek B. Clinical effectiveness of contemporary adhesives: A systematic review of current clinical trials. Dent Mater. 2005; 21(9):864-81.

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JCDA Clinical Pearl: Glass ionomer cements and their derivatives (GICs) outperformed all classes and types of resin-based dentin bonding agents in this comprehensive review of clinical retention data. To say that GICs are ‘better’ dentin bonding agents is a misleading overgeneralization; however, they certainly offer the most reliable way to restore NCCLs.

Key Points:

• The authors reviewed clinical studies, including conferences abstracts and peer-reviewed journal articles, from January 1998 to May 2004 that related to retention rates of facial class V restorations in NCCLs.
• The results were presented as mean annual failure rate (in %), with standard deviation and range for five classes of adhesives as follows:
• Three step etch-and-rinse systems: 4.8 ± 4.2% (range 0-16% )
• Two step etch-and-rinse systems: 6.2 ± 5.5% (range 0-19.5%)
• Two stage self-etching systems: 4.7 ± 5.0% (range 0-19.3%)
• One stage self-etching systems: 8.1 ± 11.3% (range 0-48%)
• Glass ionomer and resin-modified glass ionomer cements: 1.9 ± 1.8% (range 0-7.6%)
• The authors conclude that bonding systems based on both etch-and-rinse and self-etch were capable of producing acceptable clinical retention rates. However, attempts to simplify the application procedure led to poorer performance in both classes of bonding agents. GICs and their derivatives outperformed all resin-based bonding systems.
• A review of this study in 2010⁴ identified several limitations, such as the study inclusion dates and the high degree of variability between studies of the same type of bonding agent. Also, restoration loss was the only failure criterion and GICs have different mechanical properties than composite resin.  Again, while the GICs clearly came out on top in a review of clinical studies aimed at testing clinical adhesion, it does not mean that they are the best dentin bonding agents.

Reasons for Recommending this Article:

The original data for this article are approximately 8 years old; however, this report is still considered the most authoritative and complete to date—it is cited in all 3 articles in this JCDA Express and is included in many standard dental textbooks.^2,3

References

1. National Institutes of Health: National Institute for Dental and Craniofacial Research. National Institute of Dental and Craniofacial research: Strategic Plan 2009-2013. 2009.
2. Van Noort R. Introduction to Dental Materials. 3rd ed. Edinburg (NY): Mosby/Elsevier; 2007.
3. Summitt JB, Robbins JW, Hilton TJ, Schwartz RS, editors. Fundamentals of Operative Dentistry: A Contemporary Approach. 3rd ed. Chicago (IL): Quintessence Publishing; 2006.
4. Heintze SD, Roulet JF. Glass ionomer derivates have better retention rates in cervical restorations compared to self-etching adhesive systems. J Evid Based Dent Pract. 2010; 10(1):18-20.