The application of desensitizing agents for occluding the dentin

The main objective of this in vitro
study was to compare the performance between two different glass ionomer based
desensitizing materials with regard to their 
obliteration and penetration abilities inside the dentinal tubules.

 

Dentin hypersensitivity is caused by
fluid movement within the patent dentinal tubules. One of the strategies of
treatment of dentin hypersensitivity is based on application of desensitizing
agents for occluding the dentin tubules.

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I.The Materials

I.1) Resin modified
glass ionomer

RMGI  was
discussed by Wilson et al. (1990) and Mitra (2009), Their chemistry and
properties are examined and compared with CGI. RMGI have the advantage of a
long working time combined with a fast set and higher early strength. They are
also easily bonded to resins. They have the strength properties comparable to
CGI rather than composite resins, but they share with composite resins the
disadvantage of  having free monomers and
so, may not be as biocompatible as CGI.(29,33) 

 

There are two different
desensitizing agents that were used, the first material was Vanish XT which is
a light cured RMGI. It is claimed to provide the major benefits of glass
ionomer materials including adhesion to tooth structure and sustained fluoride
release. In addition, Vanish XT varnish provides the added benefit of calcium,
and phosphate release. It also offers a combination of a prolonged working time
with a short set time achieved by light curing. While the other material used
was Fuji Triage which is a chemical cured conventional glass ionomer. Self-bonding,
high fluoride releasing GC Fuji triage creates a strong, acid-resistant layer.

 

I.2) Conventional glass
ionomer

Glass ionomers chemistry was explained
as it consist of interpenetrating networks of inorganic and organic components
forming a matrix in which particles of unreacted glass are embedded. The
chemistry of  RMGI  is also discussed and shown to be more
complex than that of the CGI. The presence of the resin component slows down
the ionic cure reaction of the conventional cement, and leads to both a
significant exotherm and a set material capable of absorbing water reversibly.
It was found that the microstructure of the set cement depends totally on
chemical composition and the kinetics of the setting process.(32) 

 

The two desensitizing agents were
evaluated together with the techniques used to address their effects on the
prepared dentin surface and the ability of these agents to decrease
permeability through tubule occlusion. It can be concluded that the use of this
model to determine surface characteristics and reductions in dentin
permeability through tubule narrowing or occlusion, provides a useful screening
method for evaluating potential desensitizing agents.(24) 

 

After the removal of
enamel layer and exposing the dentinal tubules using the isometric saw, it was
important to use a suitable chelating agent that would help in the removal of
the smear layer and widens the dentinal tubules before the application of the
desensitizing agents and thus an effective simulation of a hypersensitive
condition, so EDTA was choosen with concentration of 17% and it was applied for
threemins on each surface of the controlled and tested specimens .

 

II.Application of EDTA

Many
materials were used to remove the smear layer(50), sodium hypochlorite was said to
be used as it has a good ability to dissolute organic tissues.(51,52) However,
its capabilty to remove all smear layer from the abraded dentin walls has been
found to be inefficient(53) and it also alters the properties
of dentin.(54,55)  In the current study, EDTA was
chosen because the tested material was glass ionomer, so potent acids such as
phosphoric or hydrochloric acids that could be used in acid etching for composite
resins are not indicated for bonding of glass ionomer since they have acidic pH
that do demineralization and not chelation, also it has been reported that pre-surface
treatment of dentin with different agents rather than EDTA may cause
alterations in the chemical and structural nature of dentin, which as a result
may change its permeability and solubility characteristics.(56,57) EDTA
effeciency is related to its ability to withdrawl the inorganic portion of the
smear layer by reacting with the calcium ions in dentin to form soluble calcium
chelates.(58) 

 

The
time of application of EDTA ranges from 30 sec. to 10 min.(59) 
Using EDTA for 30 sec. has shown efficient smear layer removal.(60) It
was observed that the most amount of calcium ions were removed during the first
three min. when using EDTA.(61) It was stated that EDTA exposure
for 10 min. causes excessive erosion of dentin around and inside the tubule
which could result in an increase of the tubule diameter, therefore, it was not
suggested to use it for periods longer than three min.(62) 
Hence
three min. exposure was chosen.

 

It
was reported that concentration of EDTA should be between 15 to 24% in order to
get an effective smear removal.(63) Another study reported a similar
cleaning power for 24% EDTA gel and 17% EDTA solution during removal of smear
layer.(64) In our study, a concentration of
17% EDTA was used, which was in agreement with many other studies.

 

Although
the action of gel and solution forms of EDTA is the same, the benefit of
lubricant in gel form is that it allows better mastering of the operator on its
application. That is reason why gel should be favoured over liquid EDTA.(65) 

 

In order to minimize the
variables that are found in the study, each sample was set to be its own
control, the reason for that was to standardize the dentin surface as much as
possible regarding the conditions, parameters, dentinal tubules diameter
between both tooth halves the treated and untreated ones, by having a common
reference so as to be able to measure the percent of  dentinal tubules obliteration in comparison
with the pretreated dentin surface patency.

 

The abrasion resistance
testing was to mimic the intra oral situation of regularly tooth brushing and
test the durability of the material in resisting the abrasion of tooth brush,
thus testing the survival of the material after abrasion that might cause
relapse of the condition in case of using a material lacking the effective abrasion
resistance. While the thermocycling challenge was used to test the effect of
difference in coefficient of thermal expansion and contraction in comparison
with dentin. Also to evaluate the effect of the resinous component of RMGI when
exposed to moisture degradation and solubility in comparison to CGI, therefore
simulating the intraoral conditions, since the temperature fluctuation
intraorally might provoke hypersensitivity easily once again.(41) 

 

It
was found in some studies that thermocycling increase leakage, and that 10,000
cycles is equivalent to one year of clinical life.(38) 
Two mechanisms occur in thermocycling, where the hot water can speed up
hydrolysis and decomposition of the components and the repeated thermal
expansion and contraction stress can be generated. It was observed
thermocycling generate thermal expansion and contraction stresses with an increase
in chemical degredation, also the same study listed that RMGI undergo
hygroscopic expansion when stored for one week in water which reduce gap
formation.(45) 

 

ESEM
was used to view the
patency of dentinal tubules, digital analysis was done by Semaphore software and
results were tabulated .
ESEM
is one of the well-known and most widely used tools for the examination of
dentin surfaces. Not only qualitatively through the analysis of smear layer
deposits on dentinal tubules, but also quantitively through determining the number,
diameter and surface area of dentinal tubules.(13) 

 

 

 

 

III.
Challenges performed

III.1)Abrasion

Regarding the effect of abrasion on
the percent of obliteration of dentinal tubules by the materials, it was shown
that CGI (Fuji Triage) is more resistant to abrasion than
RMGI (Vanish) which maybe due to the lower abrasion resistance and surface
hardness of RMGI than CGI where both had different rate of abrasive wear and
changing in surface roughness when subjected to tooth brush abrasion, which
agrees with Momoi et al. (1997) & Forss et al. (1991)(37,31) , this could be attributed to the defeciency in maturation of
superficial layer of the RMGI as a result of the air inhibition due to the
presence of an oxygen inhibited layer.(66) It was found that the percent of obliteration of dentinal
tubules was higher in CGI (Fuji) than RMGI (Vanish) which is similar to what
was found by Tantbirojn et al. (2006)(67) where it was stated that
the sensitivity score for CGI was significantly lower than RMGI. The abrasive wear strength of RMGI was found to be lower than that
of the CGI when the results were evaluated in the present study through tooth
brushing machine. Polderman et al. (2007)
compared the effectiveness of CGI and Resin based glutraldehyde containing
primer (Gluma desensitizer) in treating dentin hypersensitivity. It was
concluded that CGI is more effective in treating hypersensitive teeth than
Gluma desensitizer after three
months.(17) This agrees to some extent with this current study. Also, there  are 
some controversy  regarding the
abrasion resistance of  RMGI as hydrogel
salt and poly HEMA would be unlikely to interpenetrate thus forming separate
phases as Wilson et al. (1990) mentioned.(29) Furthermore, Nicklson et al. (2008) explained
that the presence of HEMA alters acid base reaction in RMGI so that it becomes
slower and significantly weaker material with compressive strength equals or
less than that of the CGI(32),  and that agrees with our study which
concluded that GIC has higher abrasion resistance than RMGI .

 

On the other hand, there were some studies that disagreed, a study concluded that RMGI has superior physical
properties and wear strength than CGI, this opposes our current study that
concluded CGI (Fuji) has higher resistance to abrasion than RMGI (Vanish) and
it could be due to the different is materials used between the two studies
where a restorative RMGI was used in the other study which has high tensile
bond strength compared to CGI used and RMGI hardens initially by free radicals
photo-polymerization of resin component that decrease initial hardening time
and increase abrasion resistance.(40) Where our study used a varnish
desensitizing glass ionomer that differs from the restorative glass ionomer.

 

The results concluded that CGI (Fuji)
occluded the tubules more effectively than RMGI (Vanish) which opposes Mielczarek et al. (2013) who compared the effect of a RMGI varnish with a sodium fluoride
containing varnish on dentin tubules occlusion
and demonstrated that RMGI varnish was more effective in dentin tubules
occlusion than the other varnish(68),  this might be due to that a different
material which was used (Colgate Duraphat varnish) is more effective in caries
prevention than hypersensitivity treatment, applied in thin layer, suspension
and has low resistance to tooth brushing unlike the Fuji Triage (CGI) that was
used which has better chemical bonding and sealing abilities together with
different consistency, application and technology.

 

The outcome revealed that the abrasion resistance of
CGI (Fuji) was higher than that of RMGI (Vanish), this outcome contradicted
Daniela et al. (2002) who concluded that the
flowable consistency glass ionomers (diluted Vitremer and Fuji Plus) were less
resistant to tooth brushing abrasion and had the greatest increase in
superficial roughness when compared to resin based sealant and restorative
ionomers. This contradiction with our study could possibly be due to the
restorative RMGI that was used and not a varnish form, that could probably
underwent finishing process thus the oxygen inhibited layer was removed which
might have lead to increase in the wear resistance of the RMGI(69),
also difference in the materials and consistency of the CGI used. It was
believed that the greater the size and ratio of filler particles the greater
the abrasion resistance and less solubility, where smaller filler particles sizes increase the
susceptibility to erosion, causing displacement of inorganic particles &
greater exposure of air bubbles incorporation during mixing.(38)

 

Carvalho et al. (2012) compared the
wear resistance of nanofilled GI with that of CGI, RMGI, and a nanofilled resin
composite after tooth brush abrasion. Both glass ionomer and composite behaved
differently after abrasion in surface roughness and morphology, this difference
could be due to major consitituents as filler size and polymer matrix, setting
reaction and adhesion between filler and matrix. Roughness after abrasion was
similar between CGI and RMGI. Incorporation of nano fillers improve abrasion
resistance by increasing filler loading and decreasing particles size resulting
in reducing interparticle spacing.(41) 

 

 

 

III.2)Thermocycling

While regarding the thermocycling
effect on both RMGI and CGI, Some studies were not on the same page with the
present study, Sampaio et al. (2011) who found that Specimens with CGI liners
had gap size higher than groups with RMGI when subjected to thermocycling. this
opposes our result in the current study where CGI (Fuji Triage)  had 
higher obliteration  percent than
RMGI (Vanish) after thermocycling, this could be due to different  different dwell time, temperature, different
experimental conditions and different numbers of cycles performed.(45)  

 

The result when thermocycling only was done, showed that the
percent of obliteration was higher in CGI than RMGI which means CGI was more
resistant to thermocycling than vanish. Arici et al. (2003) disagreed with this result as the effects
of thermocycling on the shear bond strength of RMGI  and CGI was discussed. This could be due to
thermocycling decrease shear bond strengths as a result of absorption of water
and alternating stressing of the system resulting in mismatch in thermal coefficient
of expansion between adhesives that in turn would affect negatively the
adhesion of resin to other parts, and since  RMGI 
consist of GI and resin, so the extra interface between two might make
this cement more prone to this adverse effect. The results suggest strongly
that RMGI provide a viable alternative to composite resins, with satisfactory
in vitro shear bond strength even after 20,000 cycles.(44) 

 

Thermocycling was shown to have a significant effect on the
desensitizing material and this disagreed with Doer et al. (1996) who compared
the effect of thermocycling on the microleakage of CGI and RMGI and revealed
through statistical analysis that neither thermocycling or type of material had
a significant effect on dye penetration. This disagreement with the result of
the current study might be because of the difference in number of cycles,
laboratory  conditions  and  materials.(43) 

 

IV) Depth of penetration

Abdulla (2000) evaluated the micromorphological interface between
hybrid ionomers and dentin. He found that RMGI produced a narrow hybrid layer
as well as formation of very short resin tags inside the tubules, he refered to
the amount of resinous content (HEMA) and its effect on the depth of
penetration inside the tubules(46) which is in agreement with this
study. Also the mode of surface conditioning with organic or inorganic acid to
remove smear layer was discussed(26) unike our study where EDTA was
used for smear layer removal.

 

Berg and Croll (2015) discussed the uses of CGI through surface
sealing and adhesion to the dentinal tubules by ionic bond which might be the
result of polyacrylate ions replacing phosphate ions in the surface of
hydroxyapatite with neither hybrid layer nor resin tags formation which agrees
with our study.(28)

 

Also in our study it was found that RMGI even though the formation
of a hybrid layer and the shallow penetration of resin tags inside the tubules
were obvious, yet its inferior surface properties (hardness 12.4 Hv) and the
value of coefficient of thermal expansion failed to withstand the challenges
received (abrasion and thermocycling), while CGI performed better against
thermocycling and abrasion which could be due to its higher surface properties
(hardness 18.2 Hv), that’s probably why its indicated for pits and fissure
protection according to manufacturer instructions. This agrees with Madruga et
al. (2017)(24) and momoi et al. (1997).(37)

 

Unlike Freitas et al. (2011)(40) and Daniela et al.
(2002)(38) who disagreed as the physical properties of RMGI and CGI were
compared and showed that RMGI has superior physical properties than CGI.