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ORIGINAL ARTICLE
Effect of gamma radiation on bonding to human enamel
and dentin
Lucas Z. Naves & Veridiana R. Novais &
Steven R. Armstrong & Lourenço Correr-Sobrinho &
Carlos J. Soares
Received: 30 August 2011 /Accepted: 14 February 2012 /Published online: 14 March 2012
# Springer-Verlag 2012
Abstract
Purposes This study evaluated the effect of gamma radia-
tion on the microtensile bond strength of resin-based com-
posite restoration to human enamel and dentin performed
either before or after radiotherapy.
Methods Thirty sound human third molars were sectioned
mesio-distally into buccal and lingual halves and then ran-
domly divided into enamel or dentin groups. Enamel and
dentin substrates were randomly divided into three sub-
groups (n010): Control, which received no irradiation;
specimens were irradiated before restoration protocol; and
specimens were irradiated after restoration protocol. Radia-
tion therapy was defined by application of 60-Gy dose
fractionally with daily exposures of 2 Gy, 5 days a week,
over 6 weeks. Restorations were carried out using Adper
Single Bond adhesive system and Filtek Z250 resin com-
posite. The specimens were sectioned producing 4 sticks per
specimen and submitted to microtensile on a testing ma-
chine. Data were submitted to two-way ANOVA followed
by Tukey test (p<0.05). Failure modes were examined
under optical microscopy and SEM.
Results Bond strength to enamel was significantly higher
than to dentin irrespective radiation therapy. Radiotherapy
applied before restoration significantly reduced the bond
strength to both substrates. A predominance of adhesive fail-
ures was detected for control groups and groups restored
before radiotherapy. Cohesive failures in dentin and enamel
increased when the specimens were restored after irradiation.
Conclusions The gamma radiation had a significant detri-
mental effect on bond strength to human enamel and dentin
when the adhesive restorative procedure was carried out
after radiotherapy.
Keywords Bond strength .Microtensile . Dentin . Enamel .
Gamma radiation . Radiotherapy
Introduction
Radiotherapy is a vital therapy for many patients with ma-
lignant neoplasm, aiming to exterminate abnormal cells to
decrease or eliminate the tumor. Although modern treat-
ments advocates the use of linear accelerators and other
high-energy machines, appropriately fitted 60Cobalt units
could be fully acceptable in the treatment of a large majority
of the patients undergoing radiation treatment for carcinoma
of the head–neck region, breast, and sarcomas of soft tissues
of the extremities. However, this procedure may cause side-
effects to normal cells neighboring affected tissues [1]. The
irradiation dosage in the head and neck region usually
ranges between 40 and 70 Gy [1], but even lower doses
may also indirectly or directly injure dental tissues, poten-
tially leading to problems such as salivary gland alteration
[2–4], changes of oral flora [5] or hypo-vascularization [6].
L. Z. Naves : L. Correr-Sobrinho
Department of Restorative Dentistry, Dental Materials Division,
Piracicaba Dental School, University of Campinas—UNICAMP,
P.O. Box 52, Av. Limeira, 901,
13414-903, Piracicaba, São Paulo, Brazil
L. Z. Naves :V. R. Novais :C. J. Soares (*)
Operative Dentistry and Dental Materials Department,
School of Dentistry, Biomechanics Research Group,
Federal University of Uberlândia,
Av. Pará, 1720,
38405-902, Uberlândia, Minas Gerais, Brazil
e-mail: carlosjsoares@umurama.ufu.br
S. R. Armstrong
Department of Operative Dentistry, College of Dentistry,
University of Iowa,
Iowa City, IA 52242-1001, USA
Support Care Cancer (2012) 20:2873–2878
DOI 10.1007/s00520-012-1414-y
All these features may pose a risk to caries-producing
microorganisms to act in abnormal levels [1, 7].
In addition to predisposing radiation caries, the radiother-
apy can cause effects on dental tissues [8]. The severity and
extent of radiation effects are dependent on the mineral and
organic content of tooth structures. The mineralized portion is
susceptible to damage in its crystalline phase [9–12], decreas-
ing its mechanical properties [11] and wear resistance [13],
and reducing its resistance to acid attack [10]. In enamel,
radiotherapy may produce physical and chemical alterations
[9, 13–15], reducing the ultimate tensile strength due to alter-
ations into the protein component of this substrate [8]. De-
mineralization depth and solubility of enamel is controversial,
some studies demonstrated no difference between irradiated
and non-irradiated enamel [15, 16], others described increased
solubility after irradiation [17, 18], or even a slight decrease in
solubility [12]. In addition, it has been reported that irradiated
dental tissues might accommodate free radicals within their
structure for long periods of time [14]. These radicals could
impair the bonding process, acting in a similar way of hydro-
gen peroxide (O– highly reactive radicals interfering with
polymerization) [19], sodium hypochlorite (collagen denatur-
ation by free radical activity) [20], or blood contamination
(hemoglobin iron-dependent free radicals) [21, 22].
Moreover, it has been reported that the apatite crystals of
dental hard tissues incorporate some sodium, carbonate, and
magnesium by entrapment during their formation [23].
When irradiated, these defects could be mobilized from the
surface layer of the crystals, removing the entrapped ions
and modifying the structure of the crystals, thus potentially
interfering with adhesion. The above-mentioned alterations
may occur in higher frequency in enamel, since it is formed
by 92–96 wt% of inorganic matter [24], as compared to dentin
with 70 wt% [25]. On the other hand, other features such as
morphologic and compositional alteration in intra- and inter-
tubular collagen [26], or metabolic alterations [27], could also
have an effect on bond strength to dentin. However, major
radiotherapy alterations in dentin are due to damage to colla-
gen fibrils [26], resulting in a significant reduction in; hard-
ness [11], wear resistance [28], ultimate tensile strength [8],
and stability of the amelo–dentin junction [29].
Recommendations have been proposed that adhesive re-
storative techniques are more indicated for patients under-
going radiotherapy treatment, since metallic restorations
could increase the incidence of radio-mucositis due to the
backscattering of secondary-radiation caused by the high
atomic mass number of metal compounds [30]. However,
there is no consensus on whether the restorative procedure
should be carried out before or after irradiation. Therefore,
the aim of this study was to evaluate the effect of gamma
irradiation on the microtensile bond strength (μTBS) of
adhesive restorations to enamel and dentin performed either
before or after the radiation treatment. The null hypothesis
tested in this study is that the adhesion of resin bond com-
posites (RBC) to dentin or enamel is unaffected by gamma
irradiation either before or after restoration placement.
Materials and methods
Specimen preparation
Thirty sound human third molars were collected and stored
in 0.2 % thymol solution for no longer than 2 months after
extraction (approved by the Ethical Committee in Research
of Federal University of Uberlândia, Brazil, protocol No.
013/05). The teeth were cleaned and the roots were removed
approximately 5 mm apically to the cementum–enamel
junction using a water-cooled diamond disc (KG Sorensen,
Barueri, São Paulo, Brazil). The tooth was vertically serially
sectioned into the mesio-distal direction, each half randomly
designated for either enamel or dentin substrate group (n0
30). The surfaces were ground to produce flat enamel or
superficial dentin surfaces. Specimens for each group were
randomly divided into three sub-groups (n010): Cont—
control group, which received no irradiation, IrB—speci-
mens were irradiated before restoration placement, and IrA
—specimens were irradiated after restoration placement.
Radiotherapy wasdefined by a total application of 60 Gy
of gamma radiation in a 60Cobalt irradiation unit (Theratron
Phoenix 60Cobalt Radiotherapy Treatment Unit—Thera-
tronics International, Ltd., Atomic Energy of Canada, Ltd.,
AECL Medical, Ottawa, ON, Canada), with 2 Gy exposure
5 days a week. The placement of the beam and the radiation
dose was precisely calculated based on a mean of the most
applied doses on head and neck patients reported in the
specific literature [1] and used as radiotherapy routine at
Cancer Hospital of Federal University of Uberlandia, Minas
Gerais, Brazil. Specimens were rotated at different angles
aiming to simulate the protocol tumor’s attack from different
sides avoiding overexposing healthy tissue. All groups of
specimens were stored in distilled water changed daily.
Restoration placement was completed by cleaning the
enamel and dentin with a pumice-water slurry, followed by
etching with 35% phosphoric acid (Scotch Bond etchant, 3 M
ESPE, Batch No. 7BT, St. Paul, MN, USA) for 15 s, then
rinsing with water-air spray for 15 s. Immediately followed by
two consecutive coats of a one bottle adhesive system (Adper
Single Bond 2, 3M ESPE, Batch/Lot No. 4YE, St, Paul, MN,
USA) applied to etched enamel and dentin for 15 s using a
fully saturated applicator and gentle agitation. Gentle air thin-
ning for 5 s was performed to evaporate solvents while being
careful to avoid excess adhesive on surfaces. Light activation
was performed with a halogen light curing unit (XL3000; 3M
ESPE) with a 600 mW/cm2 output for 20 s. An RBC block
was incrementally built-up in three increments using Filtek
2874 Support Care Cancer (2012) 20:2873–2878
Z250 composite (3M ESPE, batch/lot no. 4LU, 4ME, and
4MG) and individually light-cured for 20 s.
After 24-h water storage at 37°C the teeth were vertically
serially sectioned into several 1.0-mm slabs, then gently
trimmed with an ultra-fine diamond bur under magnification
to an hourglass shape with a square-shaped cross-sectional
area of approximately 1.0 mm2. Four hourglass-shaped
specimens were obtained from each tooth.
Microtensile bond strength test
All faces of each specimen were covered with cyanoacrylate
glue (Loctite Super Bonder, Henkel Loctite Co., Munich,
Germany) followed by the specimen attachment to a Bencor
Multi-T metallic grip and then submitted to μTBS on a
mechanical testing machine (DL2000; EMIC, São José dos
Pinhais, Paraná, Brazil) at crosshead speed of 0.5 mm/min
until failure. After testing, the specimens were removed from
the grips with a scalpel blade, and the cross-sectional area at
the site of the fracture was measured to the nearest 0.01 mm
using a digital caliper (Starret, Itu, São Paulo, Brazil). μTBS
values were expressed inMPa, and data submitted to two-way
ANOVA followed by Tukey’s test. Statistical significance was
set in advance at α00.05, considering tooth the statistical unit
by averaging specimens obtained from the same tooth.
Failure mode analysis
After bond strength testing, fractographic analysis was per-
formed using stereomicroscopy (LeicaMicrosystems, Wetzler
GmbH Germany) at ×30 to ×100 magnification. For those
questionable fractured surfaces, the failure modes were con-
firmed using a scanning electric microscopy (SEM; LEO 435
VP; LEO Electron Microscopy Ltd., Cambridge, UK).The
work distances ranged between 22 and 18 mm, according to
specimens height. Then each specimen was classified accord-
ing to the predominant remaining structure upon its surfaces
following the described failure mode: adhesive failure (mode
1); cohesive failure within dental substrate, enamel, or dentin
(mode 2); mixed failure involving bonding agent, RBC, and/
or tooth structure (mode 3); or cohesive failure within the
RBC (mode 4). The results of failure mode classification were
submitted to Fisher’s exact test (p<0.05).
Results
Means and standard deviation of μTBS are shown in Table 1.
Two-way ANOVA revealed significance for substrate (p0
0.01), and radiotherapy study factors (p00.001); however no
significant interaction was found between the two factors. The
bond strength to enamel was significantly higher than to
dentin (p00.03). Compared with the control group, the
radiotherapy resulted in a significant reduction in bond
strength to both enamel and dentin only when the restorative
procedure was carried out after the radiotherapy (p00.01),
with a reduction in μTBS of 31.3 % for dentin and 20.1 %
for enamel.
Failure mode results are shown in Table 2. The predom-
inance of the adhesive failures was detected for control
groups and groups restored before radiotherapy. Otherwise,
cohesive failures in dentin and enamel increased when the
specimens were restored after irradiation. The Fisher’s Exact
Test of the failure modes within each substrate showed a
significant association between substrate and restoration
timing relative to radiotherapy (p00.00000075 for enamel;
p00.000036 for dentin). The most representative micro-
graphs are shown in Fig. 1.
Discussion
When compared with the control group, radiotherapy sig-
nificantly reduced μTBS to both human enamel and dentin,
but only when the restorative procedure was carried out after
irradiation. The null hypothesis was rejected. In addition,
the results showed an increase in occurrence of cohesive
failures within the dental substrate for groups restored after
radiotherapy. When the restorative procedure is carried out
before irradiation, the μTBS was similar to non-irradiated
group (control). In contrast, when restored after irradiation,
the dental substrate might have experienced radiation effects
that could compromise bonding ability possible by impair-
ing hybrid layer formation.
In addition to the reduction in bond strength, the in-
creased number of cohesive failures within the substrates
(Fig. 1b, c, f, g) after irradiation provides further informa-
tion to corroborate previous studies on changes in dental
tissues due to irradiation [8, 28, 29, 31], and reinforces the
need for additional studies on adhesion to irradiated teeth.
Notwithstanding, the results also suggest that when hybrid-
ization is obtained previously to irradiation, the alterations
in the substrate might not be great enough to yield signifi-
cant differences in bond strength. At least theoretically, the
Table 1 Mean uTBS relative to timing of radiotherapy (means ± SD;
MPa; n040 per group)
Groups Enamel Dentin
Control (no radiotherapy) 39.1±3.0aA 29.1±3.4aB
Restored before radiotherapy 35.9±2.4abA 27.3±3.8abB
Restored after radiotherapy 31.2±3.9bA 20.0±2.4bB
Groups identified with different upper case letter superscripts (analysis
in columns) and lower case letters (analysis in rows) represent statis-
tically significant differences (p<0.05)
Support Care Cancer (2012) 20:2873–2878 2875
hybridization of the substrate may enhance its structural
stability during irradiation, also increasing the stability of
the bonding assembly.
Despite the fact that cohesive failures within the restor-
ative material (Fig. 1d, h) were not frequently observed,
properties of the resin composite might also be influenced
by gamma radiation [32, 33]. There are reports describing
chain scission, brittle fracture, and color alteration in matrix
polymers [34]. Damage in RBC would also possibly include
polymer cracking or fracture, delamination, interphase
cracking, and filler dislodgement and debonding. The dam-
age mechanisms operating and the rate of damage accumu-
lation would probably be dose related. In situations where
the damage mechanisms are not completely clarified, all
possibilities have to be considered, because this kind of
alteration could result in reduced restoration service-life.
The results of the present study may have clinical implica-
tions, as it was shown that the restoration timing relative to
radiotherapy affected bonding ability to enamel and dentin.
Therefore, it might be suggested, given that cancer therapy is
not unduly delayed, that dental restorative procedures in
patients with head and neckcancer should be conducted
before irradiation. In addition, since the radiotherapy effects
are cumulative and dose dependent to the dentition [1], resto-
rations placed during the time frame of radiotherapy might
have improved clinical service compared to restorations
placed after completion of radiotherapy.
This was an in vitro study, limiting the ability to predict
clinical outcomes due to constraints of study design. For
example, the absence of saliva, limits ion exchange within
the immersion media, altering the remineralization process
[11]. However, two studies carried out by our research group
Table 2 Failure mode distribution for all groups
Group Modes of failure
Adhesive Cohesive within substrate Cohesive within RBC Mixed
Dentin Control (no radiotherapy) 85 (34) 2.5 (1) 10 (4) 2.5 (1)
Fisher’s exact test Restored before radiotherapy 80 (32) 10 (4) (0) 10(4)
(p00.000036) Restored after radiotherapy 52.5 (21) 35 (14) (0) 12.5 (5)
Enamel Control (no radiotherapy) 82.5 (33) 5 (2) 7.5 (3) 5 (2)
Fisher’s exact test Restored before radiotherapy 82.5 (33) 7.5 (3) 5 (2) 5 (2)
(p00.00000075) Restored after radiotherapy 37.5 (15) 42.5 (17) (0) 20 (8)
[%, (n out of 40)]
Fig. 1 a and e Specimen surface showing adhesive failure between dentin and RBC (mode 1); b and f cohesive failure within dentin (mode 2); c
and g cohesive failure within enamel (Mode 2); d and h cohesive failure within RBC (Mode 4). Mode 3—mixed failure (not shown)
2876 Support Care Cancer (2012) 20:2873–2878
have used different media to store teeth during simulated
irradiation protocols, distilled water [8] and artificial saliva
[35], both studies found similar results regarding SEM analy-
sis and UTS values. One of these studies [35] found that
rinsing with chlorhexidine 0.12% partially limits mechanical
property alterations of irradiated coronal dentin possibly due
to inhibition of radiotherapy-activated proteolytic enzymes.
Therefore, further studies should be conducted to evaluate the
effect of proteolytic inhibitors during the restorative procedure
in an attempt to stabilize the bonding interface formed be-
tween adhesive systems and dentin.
Conclusions
Gamma radiation had a significant detrimental effect on
bond strength to human enamel and dentin when the adhe-
sive restorative procedure was carried out after radiotherapy.
When the restoration was carried out before irradiation, no
significant alteration in bond strength was detected.
Acknowledgments The authors are grateful to FAPEMIG for financial
support. Authors are indebted to Dr. E.W. Kitajima, Dr. F.A.O. Tanaka
and R.B. Salaroli (NAP/MEPA-ESALQ/USP, Brazil) for SEM equip-
ment support.
Conflict of interest None.
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2878 Support Care Cancer (2012) 20:2873–2878
	Effect of gamma radiation on bonding to human enamel and dentin
	Abstract
	Abstract
	Abstract
	Abstract
	Abstract
	Introduction
	Materials and methods
	Specimen preparation
	Microtensile bond strength test
	Failure mode analysis
	Results
	Discussion
	Conclusions
	References