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Fig. 1) "Super-Adhesion"
in the mouth. This tooth suffered vertical root fracture and was
bonded together with C&B-Metabond. Ten and a half years later
the original Richmond Crown was removed prior to replacement with
a more esthetic post/core and crown. Notice the long-term seal provided
by the adhesive. No discoloration in the tooth or cement. No evidence
of leakage into the canal.
A few researchers have had the guts to admit that conventional bond
tests don't have much noticeable relationship to clinical success,
but nobody has paid much attention to them.1,2
Bond strength data for a particular bonding agent can vary wildly
from study to study. As a result, it's not too difficult for any
manufacture to dig up some research somewhere that "proves"
its adhesive is the best.
(As a side note, the sole exceptions to this rule are Amalgambond-Plus
amalgam-to-dentin bond strength and C&B- Metabond's anything-to-dentin
strength. In virtually every independent comparative study conducted
over the past 10 years, Amalgambond-Plus or C&B-Metabond has
placed either first or second in strength. However, we cite this
data in our propaganda, not because we believe the numbers have
direct clinical significance, but to demonstrate that the system
is extremely forgiving. (When everybody gets good results with a
particular system, it's logical to suggest that small differences
in technique won't affect performance.)
If you're selecting your resin bonding agent based primarily on
published bond strength numbers, you're operating under a delusion.
Manufacturers have been relatively successful in creating bonding
agents that will retain a resin restoration (assuming, of course,
they're used properly.) So most bonding agents now on the market
create an adequate clinical bond.
Unfortunately manufacturers haven't been that successful in preventing
sensitivity and leakage.
Bond vs Seal
In Japan there's a new concept that roughly translates (somewhat
awkwardly) as "Super-Adhesion." This philosophy holds
that the first function of a bonding agent isn't retention of the
restoration. It's "super-adhesion" to the dentin surface.
That's because the quality of the bond to the dentin (not the bond
strength) determines how much protection the adhesive will offer
the dentin/pulp complex.
By a "good quality bond" they mean the adhesive resin
has penetrated all the way through the collagen on the surface of
the preparation and encapsulated crystals of hydroxyapatite below.
If done properly, the interaction of the adhesive and dentin will
create an acid-resistant hybrid layer that functions almost like
synthetic enamel to create a biological seal. When this occurs,
there's no post-op sensitivity. More important, the bonding agent
will provide long-term protection from potential irritants. Fewer
pulpal complications. Less chance of secondary caries.
Japanese dentists who use the "super-adhesion" philosophy,
always apply a bonding agent to cut dentin. ALWAYS! Even when they
intend to cement the crown with non-adhesive zinc-phophate or glass
ionomer.
The peculiar perils of deep preparations
In 1993, the University of Michigan conducted a huge retrospective
study to help dentists reduce the number of needless periapical
radiographs. They carefully entered the records of 2369 teeth into
a computer and analyzed the data to determine which specific conditions
increase a restored tooth's chance of developing periapical pathosis.3
They asked the computer if a restored tooth is more likely than
a non-restored tooth to eventually develop pathosis.
The answer was "yes and no." "No" for teeth
with shallow preps. "Yes" for teeth with deep preps. If
your bur removes less than half the dentin, the restored tooth isn't
much more vulerable than a virgin tooth. Go deeper than that, however,
and there's a significant correlation between depth and subesequent
endo.
Almost anything from the oral environment that reaches the pulp
can cause problems. There's bacteria, of course. Then there are
the toxins that the bacteria produce. Even if the bacteria themselves
can't squeeze through to the pulp, those toxins can cause complications.
And if you're placing a resin composite, there's the restorative
material itself. (Remember, light-cured restorative materials are
rarely more than 70% polymerized ... That leaves 30% free monomer
to irritate the pulp.)
Now here's the dilemma:
1.) The deeper the dentin ... the greater the need for a protective
seal because you're so close to the pulp.
2.) But as I'll show in the next section, the deeper the dentin,
the harder it is to create an effective seal.
Figure 2
"Super-Adhesion" in the laboratory. Here composite resin
(R) was bonded to dentin using the 4-META- MMA/TBB system that forms
the heart of Amalgambond and C&B-Metabond. A bond failure was
simulated by mechanically stressing the restoration until it sheared
off. The tooth was then dissolved in acid and the interface microscopically
examined.
Though the bond failed, the 4-META hybrid layer(H) remained part
of the tooth. It didn't break off with the restoration. The only
part of the tooth that didn't dissolve in acid was the 5 micron-thick
hybrid layer where resin had penetrated into the intertubular and
pertubular dentin.
Unless you use the right adhesive, pinkish dentin near the pulp
is extremely difficult to bond to. That's one reason why some dentists
do automatic endo whenever they see pink (fig 5).
As the tubules approach the pulp they converge like spokes of a
wheel nearing the hub. This convergence means each square centimeter
in a deep prep exposes many times the number of tubules in a shallow
prep. Incidentally, a crown preparation can open as many as 12 million
tubules! 4
And it gets worse. Because each tubule widens as it approaches the
pulp, a tubule in deep dentin presents nine times the area it did
near the DEJ.
When you're bonding to deep dentin, much of what you're bonding
to are big, fat, fluid-filled holes.
Why Amalgambond and C&B-Metabond
are so effective at sealing vital dentin
Over the years our adhesive advertising has emphasized the 4-META
molecule. As a result, 4-META has become sort of the "hero"
of the Amalgambond, C&B-Metabond and now the Touch&Bond
system. The 4-META molecule is smallest of all adhesive molecules
(about half the size of the BPDM molecule in All-Bond and One-Step¨),
so it penetrates the tooth surface more easily.
But, the fact is, two other components in the Amalgambond and C&B-Metbond
systems (the proprietary TBB catalyst and the unique dentin etchant)
are at least as important as the 4-META molecule in determining
these systems' extraordinary performance on vital deep dentin.
A catalyst that loves water
Amalgambond and C&B-Metabond are the only adhesives in the world
that employ partially oxidized tri-N-butyl-borane as a catalyst
(nickname "TBB.")
TBB is extremely expensive to manufacture and involves huge amounts
of waste. That makes it expensive. However, it is simply the most
effective polymerization initiator we've discovered for bonding
to vital dentin.
In the strictest sense, TBB itself isn't the catalyst. Oxygen and
water trigger a complex reaction in the TBB, and this reaction is
what initiates polymerization. In fact, if you mix a small batch
of Amalgambond or C&B- Metabond and very carefully float a drop
in a container of water, it will set first where it touches the
water.
Virtually all other self-cure bonding agents use a peroxide-based
catalyst. The fluid and oxygen in the dentin and pulp can inhibit
traditional peroxide-initiated reactions. As a result when placed
on vital dentin, they can leave a thin monomer-rich layer smack
against the tubules.
Figure 3.
No matter how long they are, the resin tags above (fig 3) can't
protect the pulp because they aren't in intimate contact with the
tubule walls.
This poorly cured layer can trigger a nasty short-term pulpal response
and then allow long-term leakage. Since TBB actually uses water
and oxygen to trigger the curing reaction, polymerization initiates
first within the deep oxygen-rich dentin itself and then continues
outward. So you get thorough polymerization near the pulp for a
tight seal.
A gentle activator that protects collagen
Many adhesives now on the market rely on phosphoric acid to decalcify
the dentin.
10%-40% phosphoric acid is fairly aggressive. Much more aggressive
than the Dentin Activator (10% citric acid, 3% ferric chloride)
included in C&B-Metabond and Amalgambond kits.
When you put strong phosphoric acid on dentin, several things happen
that are not helpful near the pulp.
First of all, you remove a lot of calcium...about three times as
much as you do when you use 4-META's green activator.5 The most
obvious result is that phosphoric acid makes those already-large
tubules near the pulp even larger. (Not a terrific way to start
the sealing process.)
And second, the phosphoric acid attacks the collagen...so the fibrils
collapse into a gooey mess that can be hard for the adhesive resin
to penetrate.
Figure 4
Long Tags Don't Necessarily Seal Tubules -- It used to be fashionable
for manufacturers to show micrographs of long resin tags extending
into the dentin tubules to imply strong bonds and good seal of the
dentin. In fact the length of tags into the tubules has relatively
little to do with the bond ... and nothing to do with the seal.
In contrast the C&B-Metabond tags above (fig 4) formed an acid-resistant
hybrid layer within the tubule walls. (See the arrow pointing to
the hybrid layer on the side of the resin tag.) So there was no
gap to allow leakage ...and no interface that might degrade with
time.9
(There is a way to bond to denatured collagen. If you put water
on the gooey mass, it will generally fluff up again. To use a "collagen/hair"
analogy: Imagine a woman with long hair diving into a swimming pool.
While she's swimming under water, her hair is fluffy. But when she
climbs out of the pool, what happens? Right, her hair mats down
on her head.)
If an acid is particularly aggressive, it may decalcify the dentin
more deeply than the adhesive resin can penetrate. That's not good,
because it means there will be a thin band of naked collagen below
the hybrid layer.
For example, one study found that when "weak" 10% phosphoric
acid was placed on dentin for just 15 seconds, it removed calcium
to a depth of 7.5 microns. However, when the adhesive was applied,
it penetrated this decalcified surface to a depth of just 2.5 microns.5
In other words, there were 5 microns of naked collagen under the
adhesive!
When this happens, the reseachers may still report pretty good initial
bond strength numbers because collagen (like sinew) can be fairly
strong. But remember Dental School Biochemistry? Collagen is nothing
but protein, and protein degrades over time in water. The tooth's
natural hydroxyapatite or the resin in the hybrid layer protect
the collagen fibrils, otherwise teeth would fall apart in their
wet, oral environment.
Figure 5. Endo? Or "super-adhesion?"
Deep pinkish dentin is so difficult to seal and the pulp so perilously
close that some dentists automatically perform endo. Others use
a water-loving adhesive like Amalgambond or C&B-Metabond.
More insidious than phosphoric acid's tendency to widen tubules
and denature the collagen, is its greater depth of decalcification.
Adhesive resins (even 4-META) can only penetrate so far into the
dentin.If the acid treatment is too aggressive, naked collagen will
remain exposed below the hybrid layer. This will eventually disintegrate,
and oral fluid may seep into the gap, putting the pulp in jeopardy.
The seal and bond strength will degrade over a 1-5 year period.
The 10% citric acid in Amalgambond's and C&B-Metabond's Dentin
Activator creates a shallower decalcified zone, so the 4-META resin
has less depth to penetrate in order to get to those hydroxyapatite
crystals at the bottom. And the 3% ferric chloride content makes
remaining collagen more permeable. So the reason these 4-META adhesives
seal so well is multifaceted
- A smaller penetrator molecule (4-META) slips
through collagen network more easily.
- A weaker acid removes less calcium, so the
penetrator molecule has less distance to travel.
- A weaker acid removes less calcium, so the
penetrator molecule has less distance to travel.
- A special permeability component (ferric chloride)
in the Activator makes the collagen easier to penetrate.
- A water-loving initiator that assures efficient
polymerization within the damp tooth surface.
A study presented at the latest meeting of the American Association
for Dental Research strongly suggests that C&B-Metabond's ability
to create a long-term seal is due to the unique ability of this
4-META/MMA-TBB system to penetrate through the collagen and encapsulate
the hydroxyapatite crystals below.6
The hybrid layer in enamel... In this case resin (R) was
bonded to enamel (E) and the sample then placed in acid. As you
can see the resin infiltrated the interstitial material and encapsulated
the enamel prisms. It is speculated that this is the mechanism that
results in such low incidents of caries and enamel decalcification
when C&B-Metabond is used to bond orthodontic brackets.
The Touch&Bond seal
Parkell's Touch&Bond no-etch bonding agent uses a slightly different
sealing strategy. The 4-META monomer is itself acidic, so Touch&Bond
uses the monomer as both the penetrator and the decalcifier. That
is, the 4-META dissolves the hydroxapatite and simultaneously replaces
it, so there's no possiblility of etching beyond the depth of penetration
and leaving naked collagen. It just can't happen.
That's one reason Touch&Bond has shown such amazingly low levels
of post-op sensitvity. In one test (admittedly in-house) cervical
margins of Class V restorations bonded with Touch&Bond showed
less than half the leakage of those bonded with Prime&Bond¨.
The other reason is that unlike most bonding agents, Touch&Bond
doesn't blast the tubules wide open and then try to seal them again
with resin. In fact, Touch&Bond leaves about half the tubules
blocked with smear plugs that haven't been dissolved. Instead they
are penetrated by the monomers and then "fixed" during
polymerization, so the plugs become part of the resin-reinforced
hybrid layer.
The Touch&Bond "no-etch" strategy for preventing
sensitivity.
The acid treatment in Amalgambond and C&B-Metabond removes all
trace of tubule debris (or "smear plugs") left by your
bur. These systems then reseal the tubules with resin tags bonded
to the peritubular dentin on the sides of the tubules. (See fig
4) In contrast, Touch&Bond's weak acidity leaves plugs (P) in
about half the tubules. The 4-META resin penetrates these plugs
and then polymerizes, to seal them without ever opening them. (The
other half are sealed with tags, just like Amalgambond and C&B-Metabond.)
Ignore what you've been
told.
Despite what most dentists think, a "good quality bond"
isn't synonymous with "high bond strengths." You can get
humongous tensile and shear strengths from agents that leak a lot.
Think about it a minute. When you replace a bonded restoration,
isn't it more likely to be due to secondary caries or marginal discoloration,
or even post-op sensitivity than due to loss of the restoration?
In these cases the bonding agent has failed ... even though it's
demonstrated adequate bond strength.
The challenge with today isn't retention. It's seal.
You can actually see the enamel-like seal created
by the 4-META/MMA-TBB system.
These micrographs show a hybrid layer created in a vital tooth.
The tooth was bonded intraorally (live pulp, tubules filled with
fluid) ... then extracted and analyzed.
(Figure 6) The tubules in this particular sample happened to
run parallel to the cut surface. This was fortunate, because it
demonstrates a very interesting phenomenon.
After the surface was bonded with C&B-Metabond, the sample was
treated in order to visually emphasize the hybrid layer. Notice
that the adhesive not only penetrated into the tubules from the
side, but also created a hybrid layer within the peritubular dentin.
This means that the resin tags that formed in the tubules actually
extend into the walls of the tubules. They don't just "fill"
the tubules ... they "seal" them. 7
(Figure 7) A TEM scan of another bonded sample at much higher magnification.
(This too was a living tooth bonded in the mouth.) The tiny dark
specs in the picture are hydroxapatite crystals (calcium). The pure
dentin (D) is very dark, because it contains a lot of hydroyapatite.
You can see how the adhesive resin penetrated through the collagen
fibrils, to encapsulate the hydroxapatite crystals at their base.
With a hybrid layer like this, there is no clear interface between
resin and tooth. Instead there's a zone of gradual transition from
pure resin at the top to pure dentin at the bottom. This is the
key to creating a long-term seal.8
A = Resin
B = Resin-Impregnated Collagen
C = Resin Encapsulated Hydroxyapetite Crystals
D = Dentin
1 Sudsangiam S, van Nort R. Do dentin bond
strength tests serve a useful purpose. Jour Adhsv Dent. Vol1:No1,
p57-67. 99
2 Dehof PH, et al. Three-dimensional finite element analysis of
the shear bond test. Dental Materials. 2:126- 31, 95 3 Brooks SL.
et al. Validation of a specific selection of criterion for for dental
periapical radiography. Oral Surg Oral Med Oral Pathol 75(3):383-6,
93)
4 Pashley DH, et al. Clinical considerations of microleakage. Jour
Endodontics, 16:70-77, 1990
5 Van Meerbeek B. et al. Morphological aspects of the resin-dentin
interdiffusion zone with different dentin adhesive systems. J Dent
Res, 71.'8, p1530-1540, Aug 92
6 Tuntiprawon M, et al. Prevention of microleakage in fixed restorations.
Jour Dent Res. 80:Spec. Abstr #1284, Jan 01
7 Nakabayashi N, et al. Intra-oral bonding of 4-META/MMA- TBB resin
to vital human dentin. Amer Jour Dent. 8:1, p37- 42, Feb 95
8 Nakabayashi N, et al. Identification of a resin-dentin hybrid
layer in vital human dentin created in vivo: durable bonding to
vital dentin. Quint Int. 23(2) 135-141 92
9 Nakabayashi N, et al. Intra-oral bonding of 4-META/MMA- TBB resin
to vital human dentin. Amer Jour Dent. Vol 8:1, p37-42, Frb 95
For more information on C&B Metabond, please click
here.
For more information on Touch&Bond, please click
here.
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