THE JOURNAL OF - Dentsply

Söderholm
resulted in a new generation of adhesives, the so-called 3rd
generation adhesives. Manufacturers of adhesives were still
somewhat reluctant to suggest an aggressive acid etching of
the dentin. Instead, they tried to remove or modify the smear
layer with a conditioner (often a weaker acidic solution) that
was rinsed away before a hydrophilic primer was placed.
These primers often consisted of 4-META and BPDM, and after
primer application, an unfilled resin was placed. Other
primers contained PENTA, HEMA and ethanol. One product
used EDTA instead of acid to remove the smear layer and expose
the collagen, and treated the exposed collagen fibers
first with an aldehyde and then HEMA. 34 With that treatment,
it was believed that the aldehyde would chemically
bond to the collagen fiber and that the HEMA would then
bond to the attached aldehyde molecules via a condensation
reaction. The methacrylate groups of the HEMA molecule
would then react with the composite.
Interesting to mention is that Bowen and Cobb published
a paper in 1983 entitled “A method for bonding to dentin
and enamel”, 6 in which they claimed they could achieve an
in vitro tensile bond strength corresponding to one ton per
square inch. Translated into metric units, that would be 15.5
MPa. The bonding procedure was rather complex, but in that
article, Bowen regarded this approach as a major breakthrough
despite its complexity.
Even though the 3rd generation adhesives were improvements,
they did not result in long-term success. In
1985, Hansen and Asmussen correlated gap sizes around
standardized cavities with the shear bond strength and
found that gap-free restorations would be possible at shear
bond strength values of around 23 MPa. 25 The best adhesives
then had shear strength values of 18 MPa.
Toward the end of the 1980s, some interesting research
was published. The first paper came in 1985, when Bowen
presented an abstract at an IADR meeting that contained information
on the NTG-GMA he had been so enthusiastic
about in his and Cobb’s 1983 publication. 5 In the 1985 presentation,
he revealed that the adhesive he had used in
1983 contained an impurity. That impurity was nitric acid,
and after purifying, the bond strength declined substantially.
What he inadvertantly showed was how important it was
to etch the dentin surface.
Another important finding came in 1989, when Chigira et
al treated an EDTA-conditioned dentin surface with aldehyde
plus HEMA or HEMA only and found no difference in
bond strength. 17 What they showed with that study was that
collagen exposure and resin infiltration was the key behind
dentin bonding, thereby supporting Nakabayashi’s hybridization
theory.
Some of the most important information came in 1994,
when Van Herle’s group at the Catholic University of Leuven
showed in a clinical study that the success rate of Scotchbond
2, a third generation dentin adhesive, exceeded 95% after 3
years. 43 Because of poor storage stability, Scotchbond 2 disappeared
from the market almost immediately after it had
been accepted and was replaced by Scotchbond MP, a 4th
generation adhesive. At that time, it had started to become
clear that by using more aggressive dentin treatments, the
bond strength improved and exceeded the levels predicted by
Hanssen and Asmusen for gap-free restorations. 25
Even though Fusayama and later on Nakabayashi had
done extensive research on dentin etching and dentin bonding
during the 1980s, it was during the early 1990s that
dentin etching and dentin bonding first became widely accepted.
The acceptance coincided with a symposium sponsored
by 3M and later on published in Operative Dentistry. 4
Another important discovery was when Kanka 28 and Gwinnett
24 in two independent papers showed that bonding to
dentin could be enhanced by using so-called moist bonding.
Knowledge transfer is rarely an easy task, and when it
came to introducing the 4th generation adhesives to the
dental profession, it was soon clear that dentists did not really
know how to do dentin bonding. Sometimes different
components were used in the wrong order, and when it came
to moist dentin, the definition differed quite widely. Because
of these factors, there was a need for products that were
easier to use, a demand to which the manufacturers soon
responded.
In an attempt to simplify dentin bonding, the primer and
adhesive resin were combined. Unfortunately, the general
perception among clinicians was often that by using a 5th
rather than a 4th generation adhesive, the time needed for
the bonding procedure could be decreased. In reality,
though, such a belief was incorrect, because these systems
required more time for the primer to diffuse into the collagen
structure. Clinical studies that came out comparing the 4th
and the 5th generation adhesives often suggested that the
4th generation adhesives performed somewhat better than
the 5th generation adhesives. 1,14,31,42 Whether that difference
was due to 5th generation adhesives curing too quickly
has not been proven. However, in vitro results of different
generation adhesives can be similar, independent of generation.
19
Simultaneously with the introduction of the 5th generation
adhesives, new adhesives consisting of an acidic primer
and an adhesive were also introduced. These systems often
came from Japanese manufacturers of dental adhesives,
and did not include the etching gel. Because of the generation
terminology, these systems are referred to as the 6th
generation of adhesives.
In 1998, a new adhesive named Prompt-L-Pop was introduced,
consisting of a delivery system that mixed the primer
and the adhesive before it was applied. The popularity of this
system grew quickly and it soon became quite clear that the
market wanted a system that was self-etching and available
in one single container. During the past few years, the trend
has been to move the adhesive one step further by combining
the acidic primer with the adhesive resin in an attempt
to develop an all-in-one system, often marketed as 7th generation
adhesives.
The interest in the 7th generation products has been
quite significant, despite the fact that these systems often
perform less satisfactorily than many of the previous generation
adhesives. 39 However, it is important to emphasize
that all products, including the 7th generation adhesives,
have learning curves. The knowledge generated during
these trial periods is used to develop and improve products
that do not perform as well as originally believed. Unfortunately,
these development periods can be quite frustrating
for the clinicians when faced with patients who have re-
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