What Factors Affect Long-Term Bond Durability, and How Can Bond Strength Be Improved?

Salvatore Sauro, BSc, PhD, is a dental research coordinator and a professor in the Department of Dental Biomaterials and Minimally Invasive Dentistry at CEU Cardenal Herrera University.

Gary Alex, DMD, maintains a private practice specializing in prosthetic and cosmetic dentistry in Huntington, New York, and is a member of the International Association of Dental Research and an accredited member of the American Academy of Cosmetic Dentistry.

Richard Price, BDS, DDS, MS, PhD: As a patient, how long would you expect a restoration to last in your mouth and where would you expect it to fail? Most Class II resin composite restorations that fail do not do so at the occlusal surface, but instead, at the bottom of the proximal box. This region is often in dentin and is the furthest away from the tip of the curing light. Light travels in straight lines, and unless the curing light tip is perfectly positioned above the tooth, it is difficult to deliver sufficient light to the bottom of the proximal box and thus, both the bonding agent and the resin composite in this region may be undercured.

Although manufacturers often use small increases in bond strength values as a selling point for their adhesives, are these differences really that important? Unfortunately, the answer is no.¹ Astute dentists should recognize that many shear bond strength studies are basically useless when it comes to clinical relevance because the correlation between a high bond strength and the long-term clinical success of a restoration is weak. Almost all of the bond strength studies have been conducted using unerupted third molars, and these teeth do not even come close to representing the clinical realities affecting teeth in the mouth where there is caries-affected dentin, deep dentin, sclerotic dentin, and other issues. In addition, the majority of laboratory bond strength studies only report early 24-hour bond strength test results,² use a composite resin that has a high modulus of elasticity,³ bond to a flat surface using a low configuration factor,^4,5 and deliver far more light energy to the bonding system and the composite resin than they would ever receive in the mouth, especially at the bottom of the proximal box.^6-9

These four factors will all artificially inflate the results and contribute to a study's lack of clinical relevance. Therefore, instead of relying on inflated and often irrelevant data, the dentist should instead ask the manufacturer for 5-year or, even better, 10-year data on the clinical success of restorations placed, not by "experts," but by average dentists working in average dental offices. This can be a scary question because many manufacturers simply cannot provide such information. They either have not collected the long-term clinical data, or their product has been discontinued and replaced by an "improved" version of the product.^2,10

So, as a practicing dentist, what do you do? In 2002, a study concluded that if a dentist had sufficient experience and received sufficient education, any of the bonding systems evaluated at that time could produce reasonable results.¹¹ Almost 16 years later, I believe that this conclusion is still valid. When used correctly, posterior resin-based composites should last for 15 or more years,^12,13instead of the current 6- to 8-year survival rate that most achieve in "real world" dental offices.^14-16 It is imperative for dentists to carefully follow all of a manufacturer's instructions, especially when using a new product, and learn how to properly light-cure resin restorations in all areas of their patients' mouths.

Salvatore Sauro, BSc, PhD: Resin-dentin bonding has progressed significantly since the beginning of adhesive dentistry. However, problems still remain regarding long-term bond durability. The current consensus among the research community is that the degradation of the hybrid layer over time is one of the primary reasons for the failure of adhesively bonded restorations. Two factors that contribute to this are the proteolytic degradation of exposed collagen fibers that make up the hybrid layer¹⁷ and hydrolysis due to water sorption.^18,19 As the profession's knowledge and understanding of hybrid layer degradation has increased, many new materials and techniques have been tested to prevent this degradation from occurring. For example, there are a number of studies that have shown that chlorhexidine has the potential to inhibit, at least for a limited time, some of the proteolytic enzymes responsible for the degradation of collagen fibers in the hybrid layer.^20,21

In addition, bioactive materials that release calcium, phosphates, and other therapeutic ions that can provide a favorable environment for the remineralization of acid demineralized dentin may also play an important role in the future of adhesive and restorative dentistry. As previously mentioned, water sorption by resinous materials normally has negative connotations because it can cause hydrolytic degradation of the resin matrix, but with the use of ion-releasing materials, it may actually have a positive effect.²² The presence of water could promote the infiltration of calcium and phosphate ions into spaces within the dentin collagen matrix and initiate remineralization, which in turn would decrease the availability of free water molecules and thus prevent the activation of the endogenous dentinal proteolytic enzymes.²³ The substitution of water by hydroxyapatite with the use of bioactive ion-releasing agents may be an appropriate strategy to extend the lifespan of resin-dentin bonds.²⁴

In summary, resin-dentin bonding has progressed significantly; however, problems still remain regarding long-term bond durability. Various strategies have been tested to prevent or minimize both proteolytic and hydrolytic degradation of the hybrid layer. Although additional long-term research is needed, bioactive materials and substrate pretreatment strategies represent an innovative approach to remineralize and protect the resin-dentin interface, and they will likely have a significant influence on the future of adhesive dentistry.²⁵

Gary Alex, DMD: In his response, Price touches on two very important and often overlooked points regarding bonding. The first is that bond strength values, which are often reported and used by manufacturers for marketing purposes, may have little correlation to actual clinical performance. Although in vitro bond strength testing can provide some empirical and comparative data, as Price notes, flaws in the testing methodology reveal a disconnect between in vitro testing and clinical reality.

The second point that Price mentions, which I will elaborate on, could potentially be be the biggest variable of all when it comes to adhesive dentistry-the dentist. In my opinion, the profession now has a number of excellent and chemically sound adhesive systems. However, even good chemistry will not compensate for poor clinical technique. If you want a better adhesive system, then become a better dentist. Meticulous attention to details, such as control and isolation of the working area, proper conditioning and priming, solvent evaporation, and using a good curing light with proper technique, are critical to success in adhesive dentistry. Price also recommends something very basic: ensuring that you thoroughly read the directions. All adhesive systems tend to have their own specific idiosyncrasies when it comes to placement and handling. These must be followed precisely for optimal results. What works well for one system may not be applicable for another.

In Sauro's response, he highlights how breakdown of the collagen in the hybrid layer by proteolytic enzymes, along with hydrolysis, contributes to degradation of the adhesive interface over time. He discusses ways to attenuate this breakdown through the use of antimicrobials, such as chlorhexidine, and through the use of ion-releasing bioactive materials that actually increase in activity in the presence of water. Indeed, much research is being conducted on materials and chemical formulations that will improve the durability of the hybrid layer and adhesive interface by inhibiting the enzymes responsible for proteolysis as well as materials with active ion exchange mechanisms that have the potential to remineralize and strengthen tooth tissues.

In conclusion, proper management of the adhesive interface is crucial for the predictable placement of adhesively bonded restorations. This requires an understanding of the materials being utilized and the substrate being bonded to as well as a correct and precise clinical protocol. It is incumbent upon all dentists to learn the specifics about the adhesive systems they are using, including their idiosyncrasies, strengths, weaknesses, and how to maximize their performance.