A Guide to Dental Adhesives

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A Guide to Dental Adhesives

Dental Bonding Adhesives

The development of dental adhesives has played a critical role in the revolution of restorative and preventative dentistry, in addition to helping pave the way for minimal invasive dentistry. Through the use of adhesives clinicians are able to make a more conservative cavity design, preserving healthy tissue as mechanical retention is no longer required. Dovetails, grooves and sharp angle undercuts are a thing of the past. Microleakage is a major issue, being the most probable cause of secondary caries. Through the use of adhesives, restorations are not only more aesthetically pleasing, but also create a tighter seal between healthy tissue and the restoration, reducing or eliminating the risk of secondary caries.

Dental adhesives were first introduced in 1955 by Dr. Michael Buonocore, who discovered the benefits of acid etching to improve adhesion to enamel. Over time technologies advanced and dental adhesives evolved, from no etch to total etch (4th and 5th generation) to self etch (6th, 7th and 8th generation). These days adhesives are classified by three strategies: total-etch, self-etch and resin-modified glass-ionomer approach which possesses the unique properties of self-adherence to the tooth tissue. The latest innovations in adhesives include universal (or multi-mode) adhesives, which can be used as either self-etch or total-etch.

In order to create a complete understanding of adhesives we must run through the development of adhesives, which is subsequently broken down into 8 separate generations.

1st generation

The 1st generation of adhesives were introduced by Dr. Michael Buonocore in 1956, who demonstrated that use of a glycerophosphoric acid dimethacrylate-containing resin would bond to acid-etched enamel. It was believed that this bond was due to the interaction of the bifunctional resin molecule with the calcium ions of hydroxyapatite. However this bond was greatly reduced by water immersion. Buonocore worked on this issue for many years, after 9 years the issue was partially resolved through the use of a coupling agent. Made up of N-phenylglycine and glycidyl methacrylate (NPG-GMA), the coupling agent bonds one side of the molecule to the dentin whilst the other bonds (polymerises) with the composite resin. This method of bonding produced very low clinical results and produced a bond strength of 1-3MPa. This generation of bond is no longer used in practice.

2nd generation

The 2nd generation was developed in the late 1970s and sought after evolving the coupling agents utilised in the 1st generation. The 2nd generation of coupling agents primarily used polymerisable phosphates to promote the bonding to calcium in mineralised tooth structure. This bond mechanism created an ionic bond between the both the phosphates and the calcium that unfortunately would rapidly degrade in water. Saliva produced and even water in the dentin would cause debonding and inflict microleakage. At this point in time it was considered unethical amongst the profession to remove the smear layer (inorganic matter remaining on the tooth surface after preparation with burs). It is believed that the smear layer was responsible for the weak bonds made with this generation of bonding agents, which are no longer used in today's dentistry. The bond strengths produced ranged from 4-6MPa.

3rd generation

During the late 1970s and early 1980s a significant change was made marking a dramatic step forward for adhesives: the acid etching of the dentin in an effort to modify or partially remove the smear layer. This opened the dental tubules to allow the primer to penetrate once the etch had been rinsed away. This bond proved to be much stronger but was very controversial at this time as it was believed that dentin should not be etched. Once the primer was added an unfilled resin would be placed on both the dentin and enamel. Regrettably the unfilled resin was not very effective at penetrating the smear layer making the chances for microleakage an underlying issue. The bond strengths in 3rd generation adhesives ranged from 12-15 MPa.

Optibond FL Bottle Kit

4th generation

It was in the 1980s and 1990s that the 4th generation of adhesives was established, and it was the first of its kind to achieve complete removal of the smear layer. By using 30-40% phosphoric acid on both the dentin and enamel the smear layer was removed and the total-etch technique was introduced. Three primary components are used in this generation: etchant, primer and bonding, each packaged individually and applied sequentially. Once the etch has been applied (15-20 seconds) it is then rinsed and dried, but left moist enough to avoid collagen collapse. This is called wet bonding and is critical to forming a strong bond.  Next the hydrophilic primer solution is applied to infiltrate the exposed collagen network to form the hybrid layer, a structure formed in the hard tissues (enamel, dentine and cementum) by demineralisation of the surface and subsurface followed by infiltration by monomers and subsequent polymerisation. The hybrid layer is formed by the resin infiltrated surface layer on dentin and enamel, providing high bond strengths and a dentin seal with significantly reduced margin leakage in comparison to previous generations. The 4th generation of adhesives give an impressive (when compared to previous generations) average of 25 MPa, however the procedure is very technique sensitive and time consuming. Due to the complexity of multiple bottles dentists were requesting for a simplified system.

Prime&Bond NT

5th generation

In effort to reduce the number of steps necessary to form a strong bond, and to reduce the risk of collagen collapse, throughout the 1990s the 5th generation of adhesives were implemented. Distinguished as a one-step (or one bottle) system, the 5th generation adhesive combined both primer and adhesive in one bottle.  When the one step etch-and-rinse system was introduced many clinicians flocked to the new simplified system, however it was noticed that the rate of post-operative sensitivity was rising. It is believed that this may be due to the lack of manipulative control of the primer, as with the 4th generation adhesives clinicians were able to readily influence the rate of diffusion of the primer. It is also known that many of the 5th generation adhesives are more susceptible to water degradation over time than 4th generation adhesives. The polymerised primer tends to be hydrophilic in nature, whereas the 4th generation hydrophilic resin is covered with a hydrophobic resin, making it less susceptible to water sorption. 5th generation adhesives have bond strength range of 20-25 MPa, similar to that of the previous generation.

Adper Prompt L-Pop

6th generation

It was in the late 1990s to 2000s that adhesives took a huge leap in technology; self-etching primers were developed and the etching step was eliminated. The innovation of 6th generation adhesives meant that there was less dependency on the hydration state of the dentin than compared to total etch systems. The first evaluation of these systems gave a strong bond to conditioned dentin but unfortunately were less effective bonding to enamel. To overcome this issue clinicians often use traditional phosphoric acid to etch the enamel prior to applying the self-etching primer. Although if the initial etch reaches the dentin this can cause an over-etch by which the demineralisation zone is too deep for the primer to penetrate.

Many clinicians utilise 6th generation adhesives after first selectively etching the enamel. Bonds to the dentin are extremely strong but bonds to enamel are 25% weaker than both 4th and 5th generation adhesives. Bond strengths range from 18-25MPa

Scotchbond Universal

7th generation

The 7th generation adhesives were introduced in the early 2000's and were known as one bottle self-etching systems, and are the latest simplification of adhesive systems. They hold all the ingredients for bonding in one bottle, eliminating errors that could be made when mixing the components used in alternative systems. However holding the formulation for a strong adhesive system in one bottle, that can remain stable over a long period of time can prove quite the challenge. Many of these systems hold a significant amount of water, making them prone to hydrolysis and chemical breakdown. Once the system is placed and polymerised they tend to be more hydrophilic than most two-step self-etching systems; this condition makes them more prone to water sorption, limits the depth of resin infiltration into the tooth and creates some voids. The main advantage of 7th generation systems is that there is no mixing required and bond strengths are consistent, bond strengths range from 18-25 MPa

Futurabond DC

8th generation

In 2010 Voco released Futurabond DC as an 8th generation adhesive system containing nanosized fillers, with an average particle size of 12nm. The use of these nanosized fillers increases the penetration of resin monomers and hybrid layer thickness.  Nano-bonding agents are solutions of nano-fillers that have proven to produce a better enamel and dentin bond strength, stress absorption and a longer shelf life. These new agents from self-etch generations have acidic hydrophilic monomers and can be easily used on the etched enamel after contamination with saliva or moisture. 8th generation adhesive systems are capable of producing bond strengths of over 30 MPa

Classification by mechanism of adhesive

Although the development of adhesives is constantly moving forwards, there are still traits from previous generations that are favoured by clinicians. It was brought forward that adhesives are to be classified by their clinical steps and mode of use rather than their historical development.

3 steps

  • Etch > prime > bond
  • Requires most technique
  • Great bond strength and durability
  • Total-etch technique
  • Examples include: OptiBond FL

2 steps (type I)

2 steps (type II)

  • Etch + prime > bond
  • Less effective at bonding to enamel - may need to selectively etch enamel first
  • Less dependant on hydration state of dentin
  • Self-etch technique
  • Examples include: Clearfil SE Bond 2,  and Adper Prompt L-Pop.

1 step

Universal Adhesives

Universal adhesives are the latest development in bonding technology. Also known as multi-mode or multi-purpose adhesives, they are able to be used in either total-etch, selective-etch (selectively etching to enamel) or self etch technique when adhering to the tooth. This one bottle solution provides a simplified 2-step system for total-etch and selective etch-technique and a 1 step system for self-etch technique. A solution that offers the use of a range of techniques, simplified, to suit each clinical situation or simply the preferred technique by the clinician.

Conclusion

In conclusion there is no definitive technique or adhesive system that outshines the rest, each having their own pros and cons. Clinicians may have their preferred technique or bonding system or may simply use each according to the clinical situation.

The advancement of adhesives over the past 30 years has helped create more aesthetic restorations, improved the preservation of healthy tissue and greatly reduced the need for extraction.

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Dr. Bharat Katarmal
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Thank you. Very clear information about generation. classification of bonding agent is very confusing. You have done it in very simple way. Thank you for sharing this valuable information.
brigitte kunjumon
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Excellent information in a concrete manner. Thanks a lot.
mister ad
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Hello excellent descriptions ... for the restorative materials.