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Introduction
Non-carious cervical lesions (NCCLs) are characterized
by the loss of mineralized dental tissue without bacterial
involvement. They are typically located at the cemen-
toenamel junction (CEJ) and extend along the tooth sur-
face near the gingival margin.1 A systematic review
reported that NCCLs occur in 40%–60% of adults,
depending on age and geographical location.2 These
lesions are frequently observed on the facial surfaces of
Effect of composite resins with
and without fiber-reinforcement
on the fracture resistance of teeth
with non-carious cervical lesions
Ahmet Hazar1 and Ecehan Hazar2
Abstract
Objective: Non-carious cervical lesions (NCCLs) are commonly observed in clinical dentistry, leading to tooth fractures,
sensitivity, and compromised pulp vitality. Therefore, their restoration is essential for both the aesthetic and structural
integrity of teeth. This study aimed to compare the fracture resistance of NCCLs restored using different materials: an
injectable universal composite, flowable bulk-fill composites with or without fiber-reinforcement.
Methods: Seventy-five double-rooted maxillary premolars were selected for the study. Fifteen teeth were left intact
as a control. A wedge-shaped cavity was prepared in the cervical region of the remaining sixty teeth, which were then
divided into four groups (n = 15): unrestored, restored with an injectable composite, restored with a flowable bulk-fill
composite (SDR® flow+), and restored with a flowable short-fiber-reinforced composite (everX Flow™). All teeth
underwent fracture testing under oblique static loading at a 30° angle using a universal testing machine. Fracture patterns
were classified as repairable, possibly repairable, or unrepairable. Data were analyzed using one-way analysis of variance,
Pearson chi-square, and Tukey HSD post hoc tests (p = 0.05).
Results: Intact teeth exhibited the highest fracture resistance (743.481 N), while unrestored teeth showed the lowest
(371.49 N) (p < 0.001). There was no significant difference in fracture resistance between the injectable composite
(553.289 N) and SDR® flow+ (497.368 N) (p = 0.055). The everX Flow™ group displayed significantly higher fracture
resistance (673.787 N) (p < 0.001) and a repairability rate of 60% within the restored groups. Unrestored (60%),
injectable composite (53.3%), and SDR® flow+ (53.3%) groups were mostly unrepairable.
Conclusion: The everX Flow™ demonstrated improved fracture resistance and favorable fracture pattern for maxillary
premolars with wedge-shaped NCCLs.
Keywords
Injectable composite, short-fiber-reinforced composite, smart dentin replacement, non-carious cervical lesions,
fracture resistance
Date received: 6 September 2024; revised: 22 October 2024; accepted: 11 November 2024
1 Department of Restorative Dentistry, Faculty of Dentistry, Zonguldak
Bülent Ecevit University, Zonguldak, Turkey
2 Department of Endodontics, Faculty of Dentistry, Zonguldak Bülent
Ecevit University, Zonguldak, Turkey
Corresponding author:
Ahmet Hazar, Department of Restorative Dentistry, Faculty of
Dentistry, Zonguldak Bülent Ecevit University, Zonguldak 67600,
Turkey.
Email: dt.ahmethazar@yahoo.com.tr
1303327JBF0010.1177/22808000241303327Journal of Applied Biomaterials & Functional MaterialsHazar and Hazar
research-article2024
Original Research Article
2 Journal of Applied Biomaterials & Functional Materials 00(0)
teeth, particularly maxillary premolars.3 The development
of NCCLs is generally attributed to a combination of fac-
tors, such as biocorrosion, abfraction, attrition, and the
effects of saliva.4 NCCLs will continue to progress if left
untreated, despite their non-carious nature.5 These lesions,
characterized by the loss of the enamel and dentin struc-
ture can lead to tooth sensitivity while lesions can compro-
mise pulp vitality.6 Restoring deep NCCLs is necessary
not only for aesthetic reasons, but also to preserve the
tooth’s structural integrity and prevent fractures.7
NCCLs are classified into categories by their shape –
that is, wedge, notched, shallow, concave, and irregular.8
Wedge-shaped defects form similar to a wedge with its
base located in the enamel and its pointed end apex located
in the dentin. Research by Jakupović et al.9 showed that
V-shaped wedge defects lead to a four-fold increase in
mechanical stress concentration at the cavity floor com-
pared with other shapes under the same load conditions.
The mechanical stress that occurs is a crucial factor in res-
toration failure. In addition, the stiffness tensor of the
materials used to restore these lesions considerably influ-
ences stress distribution and mechanical integration with
the tooth.10 Due to the complex stress patterns to which
NCCL restorations are subjected, there is no consensus on
the best material to use for restoring these defects.11
Therefore, it is still an important issue to select the optimal
restorative material for NCCL restorations. There are sev-
eral options for restoring NCCLS, including direct restora-
tions with glass ionomer cement (GIC), resin-modified
glass ionomer cement (RGIC), polyacid-modified resin
composites, and conventional resin composites while indi-
rect inlay restorations can be performed.11,12 The indirect
technique is more expensive and requires a longer treat-
ment time than direct restorations. Therefore, direct resto-
rations are still the most common technique for restoring
NCCLs in clinical practice. Although conventional GICs
and RGICs are frequently used materials for direct restora-
tions of NCCLs, these materials have limitations, such as
mechanical strength, color stability, and retention.13
The continuous flexion movement in the cervical region
that occurs during chewing can adversely affect conven-
tional resin-based materials.14 Studies have compared
flowable and packable resin composites regarding the effi-
ciency of restoring NCCLs and reported that flowable res-
ins generally offer better marginal adaptation but have
issues with polishability and surface irregularities due to
lower filler content.15,16 To overcome these disadvantages,
a new category of flowable resin composite materials,
known as injectable composites, has been developed with
improved mechanical properties as well as good marginal
adaptability.17 One of these materials, G-aenial Universal
Injectable composite, has shown promising biomechanical
properties for dental applications.18
SDR® flow+ (Dentsply Sirona, Charlotte, NC, USA) is
marketed as a low stress flowable base material that can be
placed in layers up to 4 mm thick. It contains a proprietary
patented modified urethane dimethacrylate (UDMA) mon-
omer of high molecular weight and with embedded photo-
active groups, which contributes to reduced shrinkage and
a better degree of conversion.19 SDR® flow+ is reported
for its superior cavity wall adaptation, polymerization effi-
ciency, and reduced porosity compared with packable
bulk-fill materials, and can be used in Class I, III, and V
restorations by the manufacturer.20
Fiber-reinforced resin composites have been developed
for restorations in high-stress bearing areas and for enhanc-
ing the fracture resistance of the teeth.21,22 In addition,
these materials have good adhesion properties due to their
semi-interpenetrating polymer network contents.23 A flow-
able bulk-fill resin composite (everX Flow™) that was
introduced in 2019, is a flowable short fiber-reinforced
composite (SFRC) that can be used as a dentin replace-
ment material. To improve its predecessor’s (everX
Posterior) handling issues, everX Flow™ has been devel-
oped with shorter and thinner fibers than everX Posterior.24
The incidence of NCCLs has increased due to the
longer life expectancy of the population and the longevity
of permanent teeth, therefore, the prevention and treatment
of these lesions has gained importance. To the best of our
knowledge, no study has compared the effect of G-aenial
Universal Injectable resin composite and flowable dentin
replacement resin composites with or without fiber rein-
forcement on the fracture resistance of maxillary premo-
lars with NCCLs. This study aimed to compare the fracture
resistance of maxillary premolars with wedge-shaped
NCCLs restored with the G-aenial Universal Injectable,
everX Flow™ bulk shade, or SDR® flow+ bulk-fill flow-
able composites. The null hypothesis of this study was that
the restorations made with flowable SFRC would increase
the fracture resistance and improve the fracture pattern of
maxillary premolars with NCCLs.
Methods
The materials used in this study are presented in Table 1.
Tooth selection
Ethical approval of this study was obtained from the Non-
Interventional Clinic Research Ethics Committee of
Zonguldak Bülent Ecevit University (protocol number:
2024/10). The sample size was calculated using G*Power
(version 3.1.9.7, Kiel University, Kiel, Germany) and the
one-way analysis of variance (ANOVA) for five groups
based on the findings of a previous study.25 The effect size
was 0.4, the type I error was α = 0.05, and the statistical
power was 0.8. The required total sample size was 70, and
the required sample size for each group was 14.
In this study, 75 maxillary first premolar teeth extracted
for orthodontic treatment were obtained from patients with
Hazar and Hazar 3
age 18–30 years. The teeth were selected according to cor-
onal and root dimensions (Figure 1(a)). The dimensions
were measured using a digital caliper (SC- 6 digital cali-
per, Mitutoyo Corporation, Tokyo, Japan) and the meas-
urements were statistically analyzed (one-way ANOVA,
α = 0.05) to establish five experimental groups (n = 15) that
were similar to each other.
Radiographs were taken from the teeth and those with a
buccal enamel and dentin thicknesses of 2.5–3 mm at the CEJ
were included. After the removal of debris and soft tissue rem-
nants, teeth were examined under a dental operating micro-
scope (EZ4W, Leica Microsystems, Milton Keynes, UK) to
exclude those with cracks or resorptive lesions. The selected
teeth were stored in a 0.1% thymol solution at 37°C until use.
Table 1. Materials used in the study.
Material Manufacturer Fillers Matrix
G-aenial Universal
Injectable composite
GC Corporation, Tokyo,
Japan (Lot number: 2309201)
Silicon dioxide (SiO2), barium glass, 69%
wt, 50% vol
UDMA, Bis-MEPP,
TEGDMA
everX Flow™, short-fiber-
reinforced-composite
GC Corporation, Tokyo,
Japan (Lot number: 2106221)
Micrometer scale glass fiber filler, barium
glass, 70% wt, 46% vol
Bis-EMA, TEGDMA,
UDMA
SDR® flow+, smart dentin
replacement
Caulk Dentsply, York,
PA, USA (Lot number:
2310000021)
Barium and strontium aluminofluoro-
silicate glasses, CQ photo initiator, photo
accelerator, BHT, Iron oxide pigments
and Fluorescing agent, 68% wt, 47% vol
Modified UDMA,
EBPADMA, TEGDMA
Scotchbond Universal Plus
Adhesive
3M Deutschland GmbH,
Neuss, Germany (Lot
number: 10665120)
Bis-GMA, 10-MDP, 2-HEMA, Vitrebond copolymer, ethanol, water,
initiators, fillers methacrylate, water
Figure 1. Representative preparation images: (a) Maxillary first premolar teeth extracted for orthodontic treatment were
obtained from patients, (b) a wax layer was created by immersing the roots in wax up to 3 mm below the CEJ to create a biological
space, (c) teeth were embedded in methacrylate resin up to 3 mm below the CEJ to simulate the alveolar bone limit, (d) the teeth
were removed from the methacrylate resin, and the remaining wax removed from the acrylic resin and the root surface using hot
water, (e) an elastomeric impression material was placed into the acrylic resin alveolus space, and (f) the teeth were embedded into
the space created to simulate the periodontal ligament.
4 Journal of Applied Biomaterials & Functional Materials 00(0)
Periodontal ligament simulation
The roots of 75 teeth were coated with a base plate wax
(Cavex Set Up Regular, Cavex Holland BV, Haarlem,
Holland) liquefied at 60°C. A wax layer at approximately
0.3 mm thickness was created by immersing the roots in
wax up to 3 mm below the CEJ for 2 s to create a biological
space, as reported in previous study (Figure 1(b)).26 The
teeth were then embedded in methacrylate resin (Technovit
4004, Heraeus-Kulzer, Hanau, Germany) up to 3 mm
below the CEJ to simulate the alveolar bone limit (Figure
1(c)). Once the polymerization process was complete, the
teeth covered in wax were removed from the methacrylate
resin, and the remaining wax was removed from the acrylic
resin and the root surface using hot water (Figure 1(d)). An
elastomeric impression material (Oranwash L, Zhermack,
Badia Polesine, Italy) that was mixed with an activator
(Indurent gel, Zher-mack) was placed into the acrylic resin
alveolus space (Figure 1(e)), then the teeth were embedded
into this space to simulate the periodontal ligament (Figure
1(f)). The prepared specimens were stored in distilled
water at 37°C until being subjected to the fracture resist-
ance test.
Wedge-shaped lesion preparations
Standardized artificial wedge-shaped NCCLs were pre-
pared on the facial surface of 60 teeth using a fissure
diamond bur (G837/018, Dia Tessin, Vanetti, Gordevio,
Switzer-land) under water cooling by a single trained oper-
ator (EH), while 15 were left unprepared as the intact teeth
(positive control group). Diamond burs were not used for
more than three cavity preparations. The artificial lesion
dimensions were prepared with a height of 4 mm (2 mm
above and 2 mm below the CEJ; Figure 2(a)), a 2 mm depth
(in the midline of the lesion and at the CEJ level; Figure
2(b)), and with a full mesiodistal width (extending from
the mesial line angle to the distal line angle; Figure 2(c))
and checked with a periodontal probe (Hu-Friedy Mfg.
Co., Chicago, IL, USA).
Adhesive application
Adhesive application and restoration procedures were per-
formed by a single operator (AH) on 45 teeth, while 15
teeth were left untreated as the negative control group. The
enamel was selectively etched with 37% phosphoric acid
(RubyEtch, rubydent, Istanbul, Türkiye) for 15 s, rinsed
with water, and dried with an air–water spray. The adhe-
sive (Scotchbond Universal Plus Adhesive, 3M
Deutschland GmbH, Neuss, Germany) was applied to the
dentin, cementum, and enamel walls of the cavity and
rubbed in for 20 s. The adhesive was then gently air-dried
for 5 s, and light-cured for 10 s using a light-emitting diode
(LED) dental curing unit (Elipar S10, 3M ESPE, USA) at
1200 mW/cm². After adhesive application, specimens were
Figure 2. Representative preparation images: (a, b, c) The measurements of the artificial NCCL cavity dimensions were made with
a periodontal probe, (d) the cavities were restored with universal injectable composite (G-aenial), (e) the cavities were restored
with universal injectable a flowable bulk-fill composite (SDR® flow+), and (f) the cavities were restored with a flowable SFRC
(everX Flow™).
Hazar and Hazar 5
divided into three groups based on the materials used for
the restoration of the cavities (n = 15).
Restorative procedures
The prepared NCCLs were restored with three different
materials: universal injectable composite (G-aenial) on 15
teeth (Figure 2(d)), a flowable bulk-fill composite (SDR®
flow+ on 15 teeth (Figure 2(e)), and a flowable SFRC
(everX Flow™) on 15 teeth (Figure 2(f)). The restorations
were light-cured from the facial surface of the teeth for
20 s using an LED dental curing light. The finishing and
polishing procedures were performed using alumina disks
(OptiDisc, Kerr, Bioggio, Switzerland).
Fracture resistance test
The specimens were subjected to a fracture resistance test
using a universal testing machine (Lloyd LRX; Lloyd
Instruments Ltd., Fareham, UK) with a 4-mm-diameter
crosshead. A 30° oblique compressive load was applied to
the long axis of the tooth and at the inner incline center of
the buccal cusp at a constant speed of 0.5 mm/min (Figure
3(a)). The fracture patterns were inspected visually with a
dental operating microscope (EZ4W, Leica Microsystems,
Milton Keynes, UK) at ×40 magnification by a single
operator (AH) to categorize them as repairable (fractures
not extending below the CEJ; Figure 3(b)), possibly repair-
able (fractures extending below the CEJ, but not below the
acrylic line; Figure 3(c)), or unrepairable (fractures extend-
ing below the acrylic line; Figure 3(d)). The frequency per-
centages of the identified fracture patterns were recorded
as described.
Statistical analyses
IBM SPSS (V23, Chicago, IL, USA) software was used to
analyze the data. The Shapiro–Wilk test was used to confirm
if the data had a normal distribution according to the groups.
One-way ANOVA was used to compare the fracture resist-
ance values of the groups and the difference between them
was analyzed with Tukey’s honest significant difference
Figure 3. (a) Fracture resistance test, (b) representative image of the repairable fracture pattern (fractures not extending below
the CEJ), (c) representative image of the possibly repairable fracture pattern (fractures extending below the CEJ but not below the
acrylic line), and (d) representative image of the unrepairable fracture pattern (fractures extending below the acrylic line).
6 Journal of Applied Biomaterials & Functional Materials 00(0)
post-hoc test. The frequency percentages of the fracture pat-
terns between the groups were compared using Pearson’s
chi-squared test. The level of statistical significance was
0.05.
Results
Figure 4 presents the mean and standard deviation values of
the groups tested in this study. Considering the one-way
ANOVA results, there was a significant difference between
the fracture resistance values of the groups (p < 0.001).
According to Tukey’s honest significant difference, the
highest fracture resistance values were found in the positive
control/intact teeth group (743.481 N), followed by the
everX Flow™ (673.787 N), G-aenial Universal Injectable
composite (553.289 N), SDR® flow+ (497.368 N), and the
negative control (unrestored cavities; 371.49 N) groups
(p < 0.001). The mean fracture resistance values of the
SDR® flow+ and the G-aenial Universal Injectable com-
posite groups were not significantly different (p = 0.055).
Figure 5 illustrates the fracture pattern distributions of
the groups. The type of restorative material used signifi-
cantly affected the repairability of the fractures (p < 0.001).
The positive control/intact teeth group had the highest
repairability rate at 100%, followed by the everX Flow™
group with 60%. In contrast, the negative control group
had the lowest repairability rate at 13.3% and the highest
percentage of unrepairable (60%) and possibly repairable
Figure 5. The fracture pattern distributions of the groups.
Figure 4. The mean and standard deviation fracture resistance values of the groups (one-way ANOVA/Test statistic:
135.383/p < 0.001).
a–d: Different lowercase letters indicate significant differences between groups (Tukey HSD).
Hazar and Hazar 7
(26%) fractures. The fracture pattern in G-aenial Universal
Injectable composite (53.3%) and SDR® flow+ (53.3%)
groups was mostly unrepairable.
Discussion
The fracture resistance of maxillary first premolars with
artificial NCCLs restored using different materials was
evaluated in this study. NCCLs are dental defects with a
prevalence of 53%–72%, most commonly affecting maxil-
lary premolars. These teeth have a thin dentin volume in
the cervical region and sharp cusp inclinations that make
them vulnerable to compressive and shear bite forces dur-
ing chewing, thus, are susceptible to NCCLs and tooth
fractures.6,27 Therefore, maxillary first premolars were
subjected to oblique compressive load for fracture resist-
ance tests in this study.
It is reported that the standardization of artificially pre-
pared NCCLs can be challenging for extracted teeth due to
morphological variations.28 In this study, maxillary first
premolars that were extracted due to orthodontic reasons
from patients between age 18–30 years were used to mini-
mize the dimensional variation in teeth selection. In addi-
tion, for standardization, the mesiodistal and buccopalatal
dimensions in both the coronal and cervical parts of the
teeth were measured and statistically analyzed to establish
whether the teeth dimensions were similar or not.
Periodontal ligaments establish a functional connection
between the alveolar bone and the cementum structure of
teeth, enabling minor teeth movements in the alveolar
space. Thus, bone and the periodontal ligaments can influ-
ence the fracture pattern by affecting the force transmis-
sion to the tooth.26 In the experimental model, the absence
of simulating periodontal ligament results in the tooth root
being fully attached to the model, thereby replicates the
characteristics of an ankylotic tooth. It can be predicted
that this situation will create a difference in force transmis-
sion from the teeth to the alveolar bone and reduce clinical
mimicking. In this study, elastomeric impression material
was used to simulate the periodontal ligament and acrylic
resin was used to simulate the alveolar bone to obtain more
accurate results.
There are several morphological shapes of NCCLs.8 In
maxillary premolars, wedge-shaped lesions are more prev-
alent on the facial surface of the cervical area due to the
tensile stress of occlusal forces. Therefore, artificial
wedge-shaped NCCLs were prepared on the facial sur-
faces of the cervical area for this study.
Dynamic loading is designed to simulate the functional
mastication forces and is informative in determining how
materials behave under normal occlusal forces. Static
loading mimics the parafunctional forces that occur when
biting hard foods (i.e. nuts), or bruxism for which approxi-
mately one of ten adults is likely to suffer as reported in an
epidemiological study.29 Psychological factors, such as
depression and anxiety, are associated with bruxism and
with the increase in cases worldwide, the simulation of
traumatic forces is becoming increasingly important.30 The
fracture resistance of the restorative material when exposed
to occlusal forces and its ability to strengthen the remain-
ing tooth structures are important factors for a successful
restoration. It is possible that NCLLs, which often occur
because of exposure to parafunctional forces, can still be
exposed to higher-than-normal occlusal forces after restor-
ative treatment. Therefore, static force was applied in this
study to establish the maximum resistance of different
materials used to treat these lesions.
The negative control/unrestored group had significantly
lower fracture resistance values while the intact teeth/posi-
tive control group had significantly higher values than the
restored groups, which is consistent with previous stud-
ies.31,32 Zeola et al.32 reported that NCCLs evaluated with
Finite Element Analysis increased stress concentration
around the cervical region even if as shallow as 0.5 mm,
and that the stress magnitude increased with the lesion
size. This data shows that restoring the NCCL with resin-
based materials adds strength by providing a more homo-
geneous distribution of functional loads within the tooth
structure, among other requirements (i.e. prevention of
sensitivity and plaque retention or aesthetic requirements).
This data indicates the importance of NCCL restorations
as they can replace lost dental tissues and restore strength.
In addition to the findings mentioned earlier, Zeola et al.32
reported that the fracture resistance of composite-restored
NCCLs increased almost to the fracture resistance level of
intact teeth. However, none of the restored groups showed
as much fracture resistance as the intact tooth in the cur-
rent study. Discordance in the findings might be due to
these methodological differences. It is stated that the depth
of NCCLs affects the strain distribution.33 Natural dentin is
reinforced by collagen fibers that can stop and deflect
cracks initiated in enamel thus larger NCCLs make the
teeth more prone to fracture. The reason why the fracture
resistance of the restored groups was not as high as the
fracture resistance of the intact tooth may be due to the
larger NCCLs prepared in this study. In the study by Zeola
et al.,32 a packable composite was used and the cavity
depth was prepared at 1.5 mm at most. In our study, both
flowable restorative materials were used and the prepared
cavities were deeper (2 mm) and higher (4 mm). Therefore,
further studies are needed for extended NCCL restorations
made with different materials with properties similar to
healthy teeth and resist physiological or parafunctional
forces.
In this study, there was no significant difference in the
fracture resistance of the teeth restored with the G-aenial
Universal Injectable composite (553.289 N) and the SDR®
flow+ bulk-fill composite (497.368 N). There are several
studies contrary to our results showing that bulk-fill resin
composites have better fracture resistance compared with
8 Journal of Applied Biomaterials & Functional Materials 00(0)
conventional resin composites in different cavity types.34,35
The polymerization shrinkage and depth cure of bulk-fill
resin composites play a crucial role in the fracture resist-
ance of restorations.36 The reduced polymerization shrink-
age and increased depth cure properties of these materials
give them an advantage over conventional resin compos-
ites when restoring cavities with high C-factor by improv-
ing the overall quality and durability of the restorations.37
NCCL cavities have a smaller C-factor that minimizes the
effect of bulk-fill materials’ advantages, such as polymeri-
zation shrinkage due to the cavity shape. Therefore, the
success of NCCL restorations is mostly dependent on the
material’s bonding capability and strength properties.38
Thus, the reason that there were no differences in fracture
resistance between the conventional composite and bulk-
fill composite without fiber-reinforcement could depend
on the cavity shape that we tested in this study. de Abreu
et al.31 compared the effect of nanohybrid and bulk-fill
resin composites on the fracture resistance of artificial
NCCLs with similar dimensions to those prepared in this
study and found no significant difference between the frac-
ture resistance of materials tested, which was consistent
with our study results.
In this study, the everX Flow™ group had significantly
higher fracture resistance values among the restored
groups. It is known that inorganic filler ratios and resin
types affect the mechanical properties of materials.39 In
this study, the inorganic filler ratios and resin types of the
materials tested were similar, therefore, it is possible that
the difference between the fracture resistance of the mate-
rials was due to the fiber content of everX Flow™.
The flexural modulus influences stress distribution;
higher stiffness materials increase stress in cervical
regions, while lower stiffness materials reduce stress effec-
tively in the lesion area.40 In a current study, Szczesio-
Wlodarczyk et al.41 reported that there was no difference
between the flexural modulus of the materials tested in this
study. Therefore, the difference in fracture resistance of
restorations made with these materials may be due to
whether they contain fibers or not.
It is reported that everX Flow™ has increased fracture
toughness and fatigue strength than resin composites,
since the fibers within are oriented randomly and are three-
dimensionally isotropic in all directions.42 In addition, the
inclusion of short fibers in a semi-IPN resin matrix led to
substantial improvements in this mechanical property. The
behavior of E-glass fiber’s behavior in resin matrix pro-
vides a crack-stopping therefore, a small crack propagat-
ing through the material cannot grow any further when it
encounters these fibers.22 Conversely, dentin replacement
material containing modified UDMA or injectable nano-
hybrid universal composite consists of small filling parti-
cles and does not contain particles that can prevent crack
formation.43 Furthermore, randomly orientated E-glass
fibers can dissipate energy, which improves their
mechanical performance. Due to the different materials
and preparation techniques used, no study can be directly
compared with the results of this study. However, these
results are consistent with previous studies evaluating
fracture resistance in different cavity types, such as mesio-
occlusal-distal cavities.21,24,44
In terms of fracture pattern, in teeth restored with everX
Flow™, more repairable fractures were observed when
compared with G-aenial Universal Injectable composite,
SDR® flow+ bulk-fill composite, and unrestored teeth. It
is reported that short-glass fibers in fiber-reinforced resin
composites break gradually to reduce the stress energy at
the crack tip by upscaling the crack deflection mechanism
to macrometric events under the stress load.45 Therefore,
consistent with previous studies, the observation of domi-
nantly repairable fracture patterns in everX Flow™ group
was not unexpected.21,42 The null hypothesis that the
NCCL restorations made using a flowable SFRC in maxil-
lary premolars would increase the fracture resistance and
improve the fracture pattern of the restorations was
accepted.
One of the limitations of this study was the inability to
create in the laboratory, cervical lesions occurring under
natural process. Another limitation is static load-to-frac-
ture testing was used without fatigue testing and multiple
directions of actual biting force. Furthermore, clinically
these types of restorations are frequently exposed to abra-
sive wear or acidic erosion. The limitation of SFRC may
be the degradation of its surface and exposure of fibers due
to these factors. Further studies are needed to include the
wear resistance, surface roughness, and discoloration of
these materials, and the effect of thermal aging should also
be investigated.
Consclusion
Within the limitations of this study:
- Wedge-shaped NCCLs reduced the fracture resistance
of maxillary first premolar teeth, while direct restora-
tion of these lesions increased the fracture resistance of
teeth.
- everX Flow™ provided teeth with NCCL higher frac-
ture resistance than SDR® flow+ and G-aenial
Universal Injectable.
- Flowable short-fiber-reinforced composite restora-
tions demonstrated a more favorable fracture pattern on
teeth with wedge-shaped NCCLs than restorations
without fiber reinforcement.
Author contributions
AH and EH researched the literature and conceived the study.
AH was involved in protocol development, prepared the speci-
mens, and performed the experiments. AH analyzed the data. AH
Hazar and Hazar 9
wrote the first draft of the manuscript. AH and EH reviewed and
edited the manuscript and approved the final version of the
manuscript.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
article.
Funding
The author(s) received no financial support for the research,
authorship, and/or publication of this article.
ORCID iD
Ahmet Hazar https://orcid.org/0000-0002-3931-5179
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