Key words:
Carbonated drink - dental erosion - gastric juice - microhardness
INTRODUCTION
Dental erosion is a new risk for oral health, introduced by today’s lifestyles and
dietary habits. Erosion is a tooth wear attributed to dissolution of the dental-hard
tissues by acids, without the involvement of bacteria and may be classified as extrinsic
or intrinsic.[1] Extrinsic factors include frequent consumption of acidic foodstuffs or beverages
and some medications,[2] while intrinsic factors are related to eating disorders and gastric reflux.[3]
Once in contact with enamel, the acid with hydrogen ion or its chelating capacity
begins to dissolve the hydroxyapatite crystal. The nonionized form spreads throughout
the enamel’s interprismatic areas, dissolving the mineral in the subsurface region.[4] Frequent contact between acids and tooth surfaces causes loss of this structure
resulting in a surface susceptible to mechanical abrasion.[5] Furthermore, in severe situations, as in gastroesophageal reflux disease (GERD),
a significant loss of tooth structure, vertical dimension, and/or function, hypersensitivity,
esthetically unacceptable defects, and pulp exposure could occur.[1]
In the past, patients were left untreated or the rehabilitation was performed with
extensive crown and bridge work.[6]
[7] However, as a result of the improvements in adhesive materials, it has become possible
to rehabilitate eroded teeth in a less invasive manner using direct restorative material
(RM) such as composite resins and glass ionomer cement (GIC).[6]
[7] These materials are capable of reestablishing the function and esthetics of tooth
structure as well as controlling the hypersensitivity.[1]
It is known that the longevity of dental restorations depend on the durability of
the material and its properties such as wear resistance, integrity of the tooth/restoration
interface, surface roughness (Ra), and microhardness.[7] To preserve or improve the properties of direct RM, surface sealants were developed.
According to the literature,[8]
[9] this is a dental biomaterial where nanotechnology was applied that is able to improve
the properties of existing materials, by filling the cracks, decreasing the porosity,
increasing the wear resistance, and maintaining the marginal integrity of restorations.
This study aimed to evaluate in situ the effect of different acidic solution (AS) on Ra and microhardness of RM, including
a sealed composite. The null hypothesis was that AS cause any effect on the Ra and
microhardness of RM.
MATERIALS AND METHODS
Screening of participants
After receiving all relevant information to conduct the research, the volunteers underwent
an anamnesis and clinical examination for the inclusion and exclusion criteria checking.
The inclusion criteria applied for selection of research volunteers were good general
health, normal salivary flow, absence of active caries lesions, no noncarious injuries
(e.g., cervical lesions), absence of periodontal disease, absence of prosthesis, absence
of orthodontic appliances, and availability to attend the research site. Furthermore,
the exclusion criteria were as follows: medical treatment, digestive disorder, use
of any drug, pregnancy, and smoking habit. Eight volunteers (four men and four women)
with a mean age of 22 years were selected for this research.
Specimen preparation, microhardness, and profilometric measurements
A total of 48 samples of each material (Ketac Nano [KN], Esthet-X [EX] and Supreme
XT [SXT]) measuring 4 mm in diameter and 1.5 mm in thickness were made, using a metal
die. The die cavity was completely filled with the materials. A polyester strip and
a thin glass plate were placed on the material surface to remove the excess and standardize
the finishing of the samples. The materials were light polymerized for 40s (Ultralux,
Dabi Atlante, Ribeirão Preto, SP, and Brazil) and then stored in distilled water at
37°C for 24 h. After that, the sample’s surfaces were ground flat with water-cooled
carborundum discs (# 600, 800, and 1200 Grades of Al2O3 papers; Extec, Corp., Enfield, CT, USA) and polished with felt paper wet by diamond
spray (1 μm; Extec). The samples were cleaned in an ultrasonic cleaning device (Cristófoli,
Campo Mourão, SP, Brazil) for 10 min after each disc and after the final polishing.
Half of the samples were etched with 32% phosphoric acid (Bisco Inc., Schaumburg,
IL, USA) for 15 s, washed with distilled water, and dried with air spray. On the conditioned
surfaces, the surface sealant B was applied (Bisco Inc., Schaumburg, IL, USA) and
light polymerized for 30 s.
Twenty-four enamel samples (4 mm × 4 mm × 1.5 mm) were prepared from extracted bovine
incisors, which were previously stored in 2% formaldehyde solution (pH 7.0) for 30
days at room temperature. One sample was cut from each crown, using an APL-4 cutting
machine (Arotec Ind. Com., Cotia, SP, Brazil). The enamel surface was also ground
flat with water-cooled carborundum discs (# 320, 600, 800, and 1200 Grades of Al2O3 papers; Extec) and polished with felt paper wet by diamond spray (1 μm; Extec). The
enamel samples were also cleaned in an ultrasonic cleaning device (Cristófoli) for
10 min after each disc and after the final polishing. Bovine enamel was used as an
intrinsic control of each group to confirm that the erosive period adopted in this
research could lead to the erosion of the specimens.
The surface microhardness was determined by performing five indentations in different
regions of the samples (Knoop diamond, 25g, 10 s, HMV-2000; Shimadzu Corporation,
Tokyo, Japan). The Ra was determined using a profilometer (SJ-401, Mitutoyo, Kanagawa,
Japan). The Ra value was used because it represents the arithmetical mean of roughness
of a surface, and it is the most used parameter for this purpose. Each measurement
was obtained after turning the specimen 120°, totaling three measurements using a
cutoff of 0.25 mm.
Intraoral phase: mouth appliances, tested products, and intervention.
Alginate impressions (hydrogun) were made directly from eight volunteers producing
a negative mold, in which dental stone was poured. The intraoral palatal devices (IPD)
were manufactured with acrylic resin. Seven holes measuring 4 mm × 4 mm × 2 mm were
prepared to fix the specimens. Each one received one sample of each material as well
as one enamel sample. The samples were fixed with wax into cavities (4 mm × 4 mm ×
2 mm) located at each left and right sides of the IPD. The position of each sample
was defined by drawing. The experimental design of the crossover study is showed in
[Figure 1].
Figure 1: Experimental design used in the study
In the first phase of this study, volunteers were instructed to immerse the IPD in
30 mL of HCl at a concentration of 0.01 M (Dinâmica®, Química Contemporânea Ltda,
Diadema, SP, Brazil), pH 2.0 for 10 min[7] and three times a day during 14 days. The erosive solution was renewed for each
erosive challenge. After the immersion, the intraoral devices were washed with water
for 30 s and then replaced into the mouth. In the first 12 h of the intraoral phase,
samples were not subjected to erosive treatment to allow the formation of a salivary
pellicle.[5] After this first phase, the samples were removed and new Ra and microhardness measurements
were done in the same manner as described above.
In the second phase, new samples of each material were positioned in the same holes
of the intraoral device and the same volunteers were instructed to immerse intraoral
devices in 30 mL of soft drink (SD) (Sprite®, Coca-Cola Co, Ribeirão Preto, SP, Brazil),
pH 3.6 for 10 min and three times a day during 14 days. The erosive solution was renewed
for each erosive challenge. After this second phase, the samples were removed and
new Ra and microhardness measurements were done in the same manner as described above.
In the third phase, new samples of each material were positioned in the same holes
of the intraoral device and the same volunteers were instructed to keep the samples
only in saliva, without submitting them to any extraoral treatment during 14 days.
The samples were removed and new Ra and microhardness measurements were done in the
same manner as described above.
A washout period of 14 days was performed among the phases. Volunteers received instructions
to wear the appliances continuously, but to remove them during meals. After meals,
they had to brush their teeth using a soft toothbrush (Colgate Classic Clean, Colgate-Palmolive
industrial LTDA, São Bernardo do Campo, SP, Brazil) with a small portion of the same
fluoridated toothpaste (Colgate-Palmolive industrial LTDA, São Bernardo do Campo,
SP, Brazil). During this period, the appliance was stored in wet gauze. After the
brushing process was complete, the appliances were replaced into the mouth. Volunteers
received oral and written information to refrain from using any fluoridated mouthwashes.
Postoral phase: microhardness, profilometric, and microscopic analysis.
After Ra and microhardness analysis, representative samples of each group were selected
for microscopic examination using scanning electron microscope (SEM) (JSM-5410, JEOL,
Tokyo, Japan).
Statistical analysis
The Ra and microhardness data were submitted to repeated-measures ANOVA at 5% significance
level and pairwise comparisons were performed using the Fisher’s post-hoc test (software Statview 5.0.1).
RESULTS
Microhardness results are in [Table 1]. The aging of the materials in HCl caused decrease in microhardness, with significant
difference to other solutions and initial values. Storage in SD caused decrease in
microhardness only to SXT and Esthet-X + Biscover LV (EX-B) with significant difference
to initial values. For other materials, the SD did not cause significant changes in
microhardness. There was no difference between microhardness values, before and after
in situ test performed only in saliva, for all materials studied.
Table 1:
Initial and final microhardness values of the studied materials (KHN)
|
Baseline
|
Saliva
|
Sprite
|
HCl
|
|
*Distinct capital letter in columns and lower case letter in rows are statistically
different (P<0.05)
|
|
SXT
|
82.7 (3.9) Aa
|
81.5 (6.1) Aa
|
76.2 (4.9) Ba
|
60.7 (13.6) Ca
|
|
SXT-B
|
29.4 (1.6) Ad
|
27.7 (2.9) ABe
|
28.4 (3.2) Ade
|
25.6 (1.7) Bd
|
|
EX
|
59.6 (3.3) Ab
|
55.5 (2.9) Ab
|
57.3 (7.6) Ab
|
40.5 (9.7) Bb
|
|
EX-B
|
30.5 (2.7) Ad
|
31.4 (2.3) Ad
|
26.9 (2.8) Be
|
25.4 (2.0) Bd
|
|
KN
|
45.2 (3.1) ABc
|
49.6 (2.7) Ac
|
46.3 (9.6) Ac
|
30.0 (4.6) Cc
|
|
KN-B
|
32.1 (2.8) Ad
|
33.7 (3.0) Ad
|
31.2 (3.4) Ad
|
26.8 (3.4) Bd
|
Comparing the materials, the SXT composite resin (CR) showed highest microhardness
average in all periods, followed by EX CR and KN GIC, with significant difference
between them. The materials sealed with Biscover LV (B) presented the lowest microhardness
compared to materials without sealing, in all periods.
Roughness results are in [Table 2]. The aging of the materials in HCl caused increase in Ra compared to initial values
for all materials studied, except for the sealed ones. In the sealed materials, the
difference from baseline and after immersion in saliva, SD, and HCl was not statistically
significant. The SD caused increase in the Ra of GIC without sealing. The saliva caused
increase in Ra for the composites SXT and EX with significant difference to initial
values.
Table 2:
Initial and final Ra values of the studied materials μm)
|
Baseline
|
Saliva
|
Sprite
|
HCl
|
|
*Distinct capital letter in columns and lower case letter in rows are statistically
different (P<0.05)
|
|
SXT
|
0.024 (0.005) Cc
|
0.028 (0.003) Bc
|
0.027 (0.005) Bb
|
0.035 (0.005) Ac
|
|
SXT-B
|
0.037 (0.012) Aab
|
0.043 (0.003) Aab
|
0.040 (0.016) Ab
|
0.041 (0.014) Abc
|
|
EX
|
0.028 (0.007) Bc
|
0.040 (0.015) Aab
|
0.027 (0.005) Bb
|
0.051 (0.028) Ab
|
|
EX-B
|
0.033 (0.013) Ab
|
0.043 (0.003) Aab
|
0.036 (0.010) Ab
|
0.032 (0.010) Ac
|
|
KN
|
0.040 (0.008) Ba
|
0.048 (0.015) Ba
|
0.076 (0.043) Aa
|
0.078 (0.016) Aa
|
|
KN-B
|
0.036 (0.008) Aab
|
0.033 (0.007) Abc
|
0.038 (0.008) Ab
|
0.031 (0.007) Ac
|
Comparing the materials, the KN GIC had the highest Ra after EC. For this material,
sealing with B possibly caused minor changes in Ra after aging in saliva, SD, and
HCl. The application of the sealant did not reduce Ra values for the two composites
studied, except for EX after challenge in HCl.
DISCUSSION
The design and conduct of this study were based on the evidence that after repeated
exposure of dental enamel to gastric contents or acidic foods and beverages, a dental
erosion typically occurs.[8] Concurrently, with this approach, it is assumed that this acidic exposure could
also have effects on RM. Once direct RM as CR and GIC are constantly used to repair
eroded tooth surfaces, it was pertinent to investigate the effects of acidity on these
materials as well as new techniques to increase the restorations longevity.[7]
[10] Data analysis revealed that the AS altered the Ra and microhardness of the materials
[Tables 1]
[2], rejecting the null hypothesis of the study.
Several studies that evaluated the effect of acids on enamel[3]
[4]
[7]
[8]
[11]
[12] and RM[12] under in vitro methodologies. However, in situ studies simulate a more realistic condition to that found clinically since they are
conducted, in part, in the oral environment and suffer the environmental interference,
such as saliva,[13] temperatures, microorganisms, and enzymes. In this study, the action of saliva may
have contributed to these results since it helps in the neutralization of the acids
used in this research.[5]
In this study, the SD was chosen because of its large consumption, high erosive potential,
and buffering capacity. According to Larsen and Nyvad,[14] the capability of an SD to erode dental enamel depends not only of the pH of the
drink but also of its buffering effect, the ability of the drink to resist a change
of pH. The solution of HCl (0.01M, pH 2.0)[7] would simulate a severe condition with high risk for the formation and progression
of erosive lesions. In addition, despite that pure acids have a pH between 0.9 and
1.5,[11] the pH in the oral cavity after episodes of vomiting rarely or never is <1.5 because
of the buffering effect in the esophagus and dilution with food and drinks.[11] Thus, the use of HCl solution in this concentration simulates a more real condition,
making the situation clinically relevant.
The evaluation methods used in this research are very useful to quantify the changes
of enamel and RM subjected to erosive challenges.[15] The profilometer was used to measure the Ra of materials since previous studies[16] have established that the critical threshold for the Ra is 0.2 μm, above which could
occur a simultaneous increase in plaque accumulation, thereby increasing the risk
for both caries and periodontal inflammation. The evaluation of the surface microhardness
is sensitive in determining the erosive lesions at early stages[15] and is considered an important indicator of materials’ mechanical properties.[17]
The results of this study showed a significant change in Ra and microhardness after
immersion in AS of SD and HCl [Tables 1]
[2]. In this context, Badra et al.[12] also reported a decrease in the microhardness of CR and glass ionomer resin modified
when immersed in SDs that have low pH. It is believed that a reduction in the surface
microhardness of CR soaked in organic acids has been attributed to the softening of
bisphenol A glycidyl methacrylate (bis-GMA)-based polymers, which could be caused
by leaching of the diluent agents such as triethylene glycol dimethacrylate (TEGDMA).[5] The softening of the resin matrix could promote displacement of the filler particles,
contributing to the formation of a rough surface,[12] as observed and illustrated [Figure 2c]
[f]
[i] in this study.
Figure 2: Microscopic images of samples (×1000). (a) Supreme XT after aging in saliva. (b)
Supreme XT after aging in HCl. (c) Supreme XT after aging in soft drink (d) Esthet.X
after aging in saliva. (e) Esthet.X after aging in HCl. (f) Esthet.X after aging in
soft drink. (g) Ketac Nano after aging in saliva. (h) Ketac Nano after aging in HCl.
(i) Ketac Nano after aging in soft drink. (j) Sealed materials after aging in saliva;
(k) Sealed materials after aging in HCl. (l) Sealed materials after aging in soft
drink
The pH, the titratable acidity (the volume of alkali needed to neutralize an acid),
and dissociation constant (the ease with which H+ are released from an acid) are factors
in the chemistry of acids.[18] It is probable that the titratable acidity or the dissociation constant are more
important than the pH in assessing the erosive potential of an acidic solution.[19] In our study, the hydrochloric acidic solution (HCl 0.01M, pH 2.0) caused the largest
changes in Ra and microhardness of RM when compared with the SD (Sprite®) as observed
in [Figure 2b]
[e]
[h]. Our results are consistent with studies of Bartlett et al.[18] that compared the erosive potential of gastric juice and carbonated drinks, demonstrating
the sovereignty of the first one. These findings are a reflection of low pH and acidity
of HCl when compared to the SD and confirm the assumptions that the gastric juice
has the potential to produce severe erosion patterns in patients with eating disorders
and GERD.[4]
It is important to consider the role of saliva in neutralizing acids. It is known
that during the intake of acidic drinks, the pH of the oral environment falls below
the critical value, the rate of salivary flow increases and the drinks are so diluted
by saliva.[5]
This is because esophageal acid clearance occurs in a two-step procedure: peristalsis
clearing the volume, followed by saliva neutralizing the acid.[13] However, in this study, the interaction of saliva with the acidic solution may have
been impaired because the samples were washed with water after the exposure to the
solutions.
Comparing the materials, it was found that the KN glass ionomer was more unstable
than the CR. These findings could be explained by matrix dissolution in the periphery
of glass particles of glass ionomer, which could result from dissolution of the siliceous
hydrogel layer [Figure 2h]
[i].[5]
[20] Other factors that could also have contributed to these results are the manipulation
and composition of these materials. The presence of glass particles in the composition
of resin-modified glass ionomers, such as KN, may reduces its homogeneity and increases
its surface roughness. Furthermore, the components have different microhardness, and
they are manually handled, which can generate porosity because of the inclusion of
visually imperceptible air bubbles [Figure 2g].[21] SEM studies of Abu-bakr et al.[22]
have shown images of rough surfaces with the presence of voids and protruding glass
particles, which clinically add up to a rough and dull surface, which could explain
the higher Ra values of resin-modified glass ionomer materials [Table 2].
On the other hand, under acidic conditions, the composites were more stable due to
the formulation of the material and morphology of the filler particles, which are
nanosized and regular, allowing the incorporation of a large inorganic volume.[23] This statement explains the higher microhardness values found for the SXT. According
to Dos Santos et al.,[6] composites with small filler particles are more wear-resistant since they are more
homogeneous and their particles are less prominent on the surface, resulting in a
lower roughness as showed in [Figure 2a]. Whereas the type of filler, size, and quantity of the particles influence the properties
and quality of polishing of composite resins (for example [Figure 2d]), the reduction in space between the inorganic nanoclusters[23] is possibly responsible for the superior physical properties of SXT.
In this study, the composite resin sealed with B showed significantly lower microhardness
values when compared with the values of unsealed resins [Table 1]. These differences could be attributed to resin monomers and the ethanol solvent
present in the sealant surface. Bertrand et al.
[24] demonstrated that when examined by SEM, the sealant was shown to reduce the surface
microhardness, improving the surface quality of composites because of the disappearance
of microcracks and minor surface irregularities. However, according to the results
of the present study, the maintenance of lower Ra and microhardness values for the
sealed composite after immersion in AS as well as the nonexposure of the particles
as illustrated in microscopy images [Figure 2j]
[k]
[l] could be indicative of the sealing material remaining on the composite surface,
showing that the material was able to withstand the acidic challenges.
CONCLUSIONS
Data from this study demonstrated some of the changes caused by low pH solutions in
direct RM concluding that: (1) The acids used in this study were able to change the
Ra and microhardness of RM, except of the sealed materials; (2) The resin modified
GIC showed the most significant changes after immersion in AS; and (3) The composites
sealed with B, even after immersion in acidic solutions (AS), showed the lowest Ra
values and the least degradation in microhardness, especially when subjected to low
pH solutions. However, further research is needed to assess simultaneously the effect
of other degradation processes such as mechanical abrasion. In addition, to complement
the results of this research, a long time erosion could be used to assess differences
in the behavior of RM as well as clinical and epidemiological studies.
Financial support and sponsorship
Nil.
