Key words:
Bond strength - Er - Cr:YSGG laser - gamma radiation - nanoglass ionomer - nanocomposite
- nanoleakage
Introduction
The vital line of treatment for oral cancer patients is the radiotherapy with a dose
ranging between 40 and 70 Gy causing side effects to the adjacent healthy tissues.[1] This irradiation may affect their restored teeth. Even with the great advancements
in the field of dental restorations, no enough data are available on the effect of
radiotherapy on them.
The high filler content and reduced particle size of nanofilled restorative materials
improve their mechanical and esthetic behavior.[2] Simultaneously, as an efficient alternative for conservative cavity preparation,
Er, Cr:YSGG laser has been introduced. Its high water and hydroxyapatite absorption
lead to selective removal of carious tissue.[3]
The degree of success of the dental restoration is mainly determined by the bond strength
and the marginal adaptation.[4] The primacy of microshear bond strength test is obtaining more than one specimen
from a single tooth.[5]
Nanoleakage occurs at the bottom of the hybrid layer where dentinal and oral fluid
can slowly invade the interface causing degradation of the bonding system.[6] Numerous researchers have been performed to determine the marginal quality of resin
composite fillings in cavities prepared by erbium laser.[7],[8] However, evaluating gamma radiation effect on laser prepared cavities restored with
nanorestorative materials is deficient.
Thus, this study is aimed at determining gamma radiation effect on bond strength and
nanoleakage of esthetic restorations in laser prepared cavities. The null hypothesis
established for this research is that there is no variation between the control and
gamma irradiated groups in terms of microshear bond strength and nanoleakage.
Materials and Methods
The materials’ name, composition, and manufacturer used in this study are presented
in [Table 1].
Teeth selection and preparation of specimens
Fifty six freshly extracted human third molars, free of caries and cracks, were collected
from 20 to 36 years old patients. They were randomly distributed among eight groups
using an excel sheet. Teeth were cleaned with ultrasonic cleaner, stored in weekly
changed distilled water, and used in 3 months.
Microshear bond strength specimen’s preparation
Twenty eight molars (n = 7), except the buccal surface, were mounted in self curing acrylic resin (Acrostone,
Egypt). A high speed superfine diamond bur was used to remove enamel from the buccal
surfaces and to reveal flat dentin surfaces.[10] The surfaces were checked by a magnifying lens to confirm complete removal of enamel
and then polished according to Adebayo et al. in 2012.[11] Er, Cr:YSGG laser was used in a focused mode to irradiate the dentin surfaces for
10 s (Biolase),[12] with parameters of 4 W power, 20 Hz repetition rate, and 50/30% air/water coolant.[13]
The prepared teeth were classified into four groups. In the first and second groups,
application of Single Bond Universal Adhesive was performed according to the manufacturer’s
instructions and then light cured using Dr’s Light LED light curing unit RF America
IDS (1600 mW/cm2). A radiometer was used after curing every 10 specimens to ensure adequate curing
intensity for all specimens. Before the bond light curing, two vinyl Tygon tubes with
diameter ±0.8 mm and height 2 mm were sited on each dentin surface. The resin composite
(Filtek Z350) was packed into the tubes using dental pluggers. Over it, a transparent
matrix strip was positioned and light cured for 20 s at zero distance. The mean microshear
bond strength of each tooth was calculated from these two readings to get a total
number of 28 mean values in MPa.
In the third and fourth groups, Ketac Nano Primer was applied according to the manufacturer’s
instruction and then light cured. The nanoglass ionomer (Ketac N100) was packed into
the Tygon tubes and light cured for 20 s.
Table 1:
Materials and methods
|
Material and trade name
|
Composition
|
Manufacturer
|
|
VBCP: VitreBond™ copolymer, MDP: Methacryloxydecyl dihydrogen phosphate
|
|
Nanocomposite (Filtek™ Z350) Shade A2
|
Bis-GMA, UDMA, TEGDMA, and Bis-EMA resins, nonagglomerated fillers 4-11 nm zirconia,
20 nm silica, and an aggregated zirconia/silica cluster fillers (0.6-10 µ). The filler
loading is 78.5% by weight
|
3M ESPE, Dental Products, St. Paul, MN, USA
|
|
Self-etch adhesive (Single Bond Universal)
|
HEMA, monomer MDP, Dimethacrylate resins, VBCP, filler, ethanol, water, initiators,
and silane
|
|
|
Nanoglass ionomer (Ketac™ N100) Shade A3
|
Polyalkenoic acid VBCP, HEMA, Deionized water, fluroaluminosilicate glass (1 µ), surface.treated
silica/zirconia nanofillers (5-25 nm), and nanoclusters (1-1.6 µ). The filler loading
is 69% by weight
|
|
|
Self.etch primer (Ketac Nano Primer)
|
VitreBond− copolymer, HEMA, water, and photoinitiator
|
|
|
Artificial saliva
|
Na3 PO4 - 3.90 mM, NaCl2 - 4.29 mM, KCl - 17.98 mM, CaCl2 - 1.10 mM, MgCl2 - 0.08 mM, H2SO4 - 0.50 mM, NaHCO3 - 3.27 mM, and distilled water. The pH was set at a level of 7. 2.[9]
|
|
Application of gamma radiation
The second and fourth groups were irradiated by fractionated gamma radiation at a
dose of 60 Gy, three times a week (day after day) for 1 week (20 Gy/3 fractions/week).[14] The radiation was carried out using 137 Cesium Gamma Cell 40 at the Atomic Energy
Authority, with a dose rate 0.708 rad/sec at the time of experiment. A solution of
artificial saliva was used to store all groups for 24 h.
Microshear bond strength test assessment
The specimens were fixed in an Instron Machine (Model 3345; Instron Universal Testing
Machine, England Instruments, with a load cell of 5 KN, and data record was done using
computer software BlueHill 3 Instron). An orthodontic wire loop (diameter = 0.14 mm)
was wrapped around the base of the bonded microcylinder assembly and aligned with
the loading axis of the movable upper compartment of the machine. The force was loaded
to failure, at a crosshead speed of 0.5 mm/min. Calculations of the microshear bond
strength values were performed and expressed in MPa. The obtained results did not
include resin cylinders with premature failure.
Nanoleakage specimen’s preparation
Class V cavities, with the dimensions according to Marotti et al. in 2010, were prepared in the buccal surfaces of the other 28 selected teeth.[15] To standardize the cavity outline, a window with the selected width and length was
cut on a stainless steel matrix band and the depth was measured by a periodontal probe.
Hence, three readings were obtained from each cavity to get a total number of 84 readings
from all the teeth, and the mean nanoleakage value was obtained for each tooth. At
the cervical third of the teeth, all cavities were prepared, 2 mm occlusal to the
cementoenamel junction using Er, Cr:YSGG laser with parameters 6 W in enamel and 4
W in dentin.[16] The nanocomposite was packed in a bulk using a plastic instrument. Ketac Nano Primer
was applied and the nanoglass ionomer was packed and light cured as in the case of
microshear bond strength specimens. Half of the specimens were exposed to gamma radiation
and stored as previously mentioned.
Sticky wax was used to seal the root apices and two layers of nail varnish were used
to coat the entire tooth, except for 1 mm apart from the margins of the restoration.[17] Then, the specimens were submerged in a solution of 50% ammoniacal silver nitrate
(pH 9.5) for 24 h in a dark chamber.[18] Teeth were then thoroughly rinsed in distilled water and immersed in a photodeveloping
solution for 8 h under a fluorescent light.[19]
Nanoleakage test assessment
The selected specimens were divided buccolingually across the restoration center with
a diamond saw in a cutting machine (IsoMet 4000; Buehler, Lake Bluff, IL, USA), under
a water coolant. They were polished using a graded series of Soflex discs (3M Co.)
in the descending order from the course to fine one and then ultrasonically cleaned
to remove the smear layer. Finally, one section of each preparation was examined by
a X ray microanalyzer (Module Oxford 6587 INCA X sight) attached to JEOL JM 5500 LV
scanning electron microscopy using high vacuum mode at 20 KV. Electron dispersive
analytical X ray (EDAX) analysis was also performed to identify the presence of metallic
silver particles. Three points at the interfaces between the teeth and the restorations
(occlusal and gingival) were selected for scanning and EDAX quantification. The mean
percentage of the silver ion deposition was calculated.[20]
Statistical analysis
Data were collected and analyzed using IBM SPSS Statistics for Windows, Version 23.0
(Armonk, NY: IBM Corp., USA). All normally distributed continuous data are presented.
Two way ANOVA was done to examine the main effects and interactions relating to types
of filling and tested groups on microshear bond strength (MPa) or nanoleakage (Ag
%), respectively. Independent sample t-test was used to examine if there were any differences found between groups.
RESULTS
Microshear bond strength values are presented in [Table 2] and [Figure 1].
Table 2:
The mean and descriptive statistics for microshear bond strength results of all tested
groups
|
Nanocomposite
|
Nanoglass ionomer
|
P
|
|
Mean
|
SD
|
Mean
|
SD
|
|
*** Significant at P ≤ 0.05. NS: Not significant, SD: Standard deviation
|
|
Control group
|
11.7
|
1.8
|
0.86
|
0.074
|
0.0001***
|
|
Gamma-radiated group
|
12.5
|
1.04
|
0.37
|
0.059
|
0.0001***
|
|
P
|
0.197 (NS)
|
0.0001***
|
|
Figure 1: Column chart of microshear bond strength mean values for the tested groups
Nanoleakage values are shown in [Table 3] and [Figure 2].
Figure 2: A column chart of nanoleakage mean values for the tested groups
Representative photomicrographs were taken at magnification ranges from ×200 to ×300,
as presented in [Figures 3]
[4]
[5]
[6]. For the nanocomposite groups, either control or gamma irradiated, the gap size
corresponds to the low Ag% as shown in [Figures 3] and [4]. However, both the nanoglass ionomer groups revealed wider gap size that corresponds
to the higher Ag% in relation to the nanocomposite groups as shown in [Figures 5] and [6].
Figure 3: Images of tooth–restoration interface for the nanocomposite control group
Figure 4: Images of tooth–restoration interface for the nanocomposite gamma–radiated group
Figure 5: Images of tooth–restoration interface for the nanoglass ionomer control group
Figure 6: Figure 6: Images of tooth–restoration interface for the nanoglass ionomer gamma–radiated
group
Discussion
In the minimally invasive dentistry field, Er, Cr:YSGG laser was approved to be an
effective and conservative tool. Among its numerous advantages, it induces less vibration
and noise, preserves more tooth structure, eliminates the need for anesthesia, and
has an antibacterial effect.[3] As the restorative procedures are extremely stressful for patients receiving head
and neck radiotherapy, laser became the most beneficial tool for the treatment of
such patients who seek for a comfortable and painless procedure while restoring their
teeth.[21] Bonded restorations tend to be the most efficient methods to restore irradiated
teeth. However, by literature reviewing, many controversies were revealed regarding
the success of such restorations.
In the current study, microshear bond strength showed significant higher bond strength
values of the nanocomposite than those of nanoglass ionomer [Table 2] and [Figure 1]. This could be referred to the self.etching adhesives used with nanocomposite that
allows resin penetration into the dentinal tubules and infiltration to the underlying
demineralized dentin forming a hybrid layer. Another factor that enhances wetting
of dentin is the hydrophilicity of HEMA adhesive group.[22] Application of nanoglass ionomer following the primer, without any intermediary
bonding material, lowers the values of microshear bond strength.[23] Moreover, the increased viscosity of glass ionomer restorations decreases the penetration
of the material through the full depth of the available irregularities of the prepared
surfaces.[24]
Table 3:
The mean and descriptive statistics for nanoleakage values in Ag% of all tested groups
|
Nanocomposite
|
Nanoglass ionomer
|
P
|
|
Mean
|
SD
|
Mean
|
SD
|
|
**, *** Significant at P ≤ 0.05. NS: Not significant, SD: Standard deviation
|
|
Control group
|
1.18
|
063
|
2.07
|
0.35
|
0.001**
|
|
Gamma-radiated group
|
0.9
|
0.43
|
2.07
|
0.28
|
0.0001***
|
|
P
|
0.355 (NS)
|
0.168 (NS)
|
|
In this study, the results revealed insignificant increase in the values of the bond
strength for the investigated nanocomposite after gamma radiation. This was in relative
agreement with Seif et al. in 2013,[14] who reported an increase in the bond strength of nanocomposite significantly after
gamma radiation due to the continued polymerization arising from the incident gamma
radiation beam increasing the degree of polymerization. The nanoglass ionomer restored
specimens showed extremely statistical significant decrease in the microshear bond
strength values after gamma radiation. That was in accordance with Yesilyurt et al. in 2008, who stated that the setting reaction of glass ionomer and its bonding to
dentin was directly affected by irradiation.[25]
[Table 3] and [Figures 2]
[3]
[4]
[5]
[6] show that none of the tested restorative materials completely eliminated nanoleakage
due to the high C factor of Class V cavities, which accentuate the effect of the polymerization
shrinkage stresses. This was supported by Price et al. in 2003, who found that pathways become available for dye penetration in cavities
with high C factor which decreased the bond strength.[26]
Our study showed high significant increase in nanoleakage of nanoglass ionomer restorations
than nanocomposite restorations in both groups due to the micromechanical bonding
in the nanocomposite restored cavities. Furthermore, mild self etch adhesives, which
are less sensitive to moisture, have characteristic property that lies in incomplete
elimination of hydroxyapatite from the interaction zone which protects the collagen
against hydrolysis, as well the available calcium has a chemical interaction with
specific adhesive monomers which provide stronger adhesion.[27]
Toledano et al. in 2003 reported that nanoglass ionomer had higher penetration due to its great
susceptibility to water sorption and solubility than resin composite.[28] The bonding mechanism of nanoglass ionomer depends mainly on chemical bonding rather
than micromechanical bonding, which is an important factor in resisting polymerization
shrinkage stresses in high C factor cavities Class V cavities.[29]
Our results showed nonsignificant decrease of the nanoleakage values in the gamma
irradiated groups in both nanocomposite and nanoglass ionomer restorations than the
control group. This was in harmony with Bulucu et al. in 2009[30] and Seif et al. in 2013,[14] who stated that irradiation did not influence the microleakage in Class V cavities.
In this study, there is no relationship between the bond strength results and the
nanoleakage values because the cavity margins were totally located in dentin in the
bond strength test. However, in nanoleakage test, the margins were located in enamel
leading to difference in the polymerization shrinkage stresses.[31] The null hypothesis for this study was partially accepted.
Conclusion
-
Therapeutic dose of gamma radiation has minimal effect on the microshear bond strength
and nanoleakage of nanocomposite
-
The microshear bond strength of nanoglass ionomer is adversely affected by gamma radiation
while nanoleakage is not affected
-
Nanocomposite is more suitable as a restorative material for cancer patients than
nanoglass ionomer.
Financial support and sponsorship
Nil.
