Dental Asia March/April 2019

Clinical Feature
7 TH GENERATION
ADHESIVE SYSTEMS
By George Freedman DDS, FAACD, FIADFE, Dipl.ABAD
Over the past five decades,
the evolution of adhesive
techniques has transformed
the scope of dental practice.
The vast majority of direct
and indirect restorations are bonded
to natural tooth structure rather than
cemented or mechanically retained.
Extensive research and product
development have improved adhesives,
concurrently with patient demands for
improved oral appearance.
The widespread demand and use of
dental adhesives has fueled an intensive
development of better and easier dental
adhesives in rapid succession; inundating
dentists with successive “generations”
of adhesive materials. While the term
“generation” has no scientific basis in the
realm of dental adhesives and is to a great
extent arbitrary, it has served a useful
purpose in the organisation of the myriad
of commercially available materials into
more comprehensible categories.
The “generational” definitions assist in
identifying the chemistries involved, the
strengths of the dentinal bond, and the
ease of clinical use. Ultimately, this type
of classification benefits both dentist
and patient by simplifying the clinician’s
chairside choices.
The beginning of the generational
adhesive systems
The 1 st generation adhesives in the late
1970s were rather unsuccessful. While
their bond strength to enamel was high,
their dentinal adhesion was pitifully low
at typically less than 2 MPa. Generally, all
the adhesive generations bond well to the
microcrystalline structure of enamel; it is
their bond strength to the semi-organic
dentin that is the greater concern.
“Bonding” was achieved through chelation
of the bonding agent to the calcium
component of the dentin. While tubular
penetration did occur, it contributed little
to the retention of the restoration. It was
common to see debonding at the dentinal
interface within several months. 1 These
bonding agents were recommended for
small, retentive Class III and Class V
cavities. 2 Post-operative sensitivity was
common when these bonding agents were
used for posterior occlusal restorations. 3
In the early 1980s, a distinct 2 nd
generation of adhesives was developed,
attempting to use the smear layer
as a bonding substrate. 4 This layer
bonded to the underlying dentin at a
negligible level of 2-3 MPa. The weak
2-8 MPa dentinal bonding strength of
2 nd generation adhesives meant that
mechanical retention form was still
required. Restorations with margins in
dentin saw extensive microleakage, and
posterior occlusal restorations were
likely to exhibit significant post-operative
sensitivity. The long-term stability of 2 nd
generation adhesives was problematic;
one-year retention rates were as low as
70%. 5,6
In the late 1980s, revolutionary twocomponent
primary/adhesive systems
were introduced. Their marked
clinical improvement warranted their
classification as 3 rd generation adhesives.
Significant increases in dentin bonding
strength (8-15 MPa) diminished the
need for cavity retention form. Erosion,
abrasion, and abfraction lesions were
treatable with minimal tooth preparation,
hence the beginning of ultraconservative
dentistry. A noticeable decrease in
postoperative sensitivity with posterior
occlusal restorations was very welcome.
3 rd generation adhesives were the first
“generation” that bonded not only to
tooth structure, but (weakly) to dental
metals and ceramics as well. The downside
of 3 rd generation bonding agents was
their lack of longevity; intraoral adhesive
retention started to decrease after three
years. Despite significant levels of postoperative
sensitivity, patient demands for
tooth-coloured restorations convinced
some dentists to begin providing posterior
composite fillings routinely. 7,8,9
The transformation and popular
generations of the adhesive systems
In the early 1990s, 4 th generation bonding
agents transformed dentistry. Their high
bond strength to dentin (17-25 MPa)
and limited post-operative sensitivity
in posterior occlusal restorations
encouraged many dentists to switch from
amalgam to direct posterior composite
fillings. 4 th generation is characterised
by the process of hybridisation at the
dentin-composite interface.
Hybridisation is the replacement of the
hydroxyapatite and the water in the
surface dentin by resin. This resin, in
combination with the remaining collagen
fibres, constitutes the hybrid layer.
Hybridisation involves both the dentinal
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DENTAL ASIA MARCH / APRIL 2019