Additionally, the Grandio (nanohybrid; Voco) composite
restoration was selected for this study due to its excellent mechanical
properties (compressive strength, hardness, and flexural strength), as well as
the low polymerization shrinkage that results from the high filler
concentration. It was also chosen because it has a high Young's modulus, as the
stress concentration in the restored area was discovered to be inversely
related to the value of the restorative material's Young's modulus [22]. Due to
the extremely small size of the filler particles, it also has outstanding
aesthetic qualities [23]. The adhesive used in this investigation, however, is
Adhese Universal (AU, Ivoclar Vivadent AG, Liechtenstein), which has a high
bond strength that can reach 37 and 35 MPa in enamel and dentin, respectively
[17]. In the current investigation, the plain chamfer type of class IV
restoration, followed by 2 mm bevel, then butt joint types, caused the stresses
to reach their peak values in the cervical region on both the labial and
palatal sides. These findings are consistent with the study's hypothesis and
the idea that increased stressors result in less tooth structure surviving.
According to the current findings, class V restorations with all varieties of
class IV restoration exhibited only slight variations in values of total
deformation. This also applies to Von Mises stress on class V restorations with
butt joints and 2 mm bevel types, however with plain chamfer types of class IV
restorations, there were a considerable number of modifications reaching 100%
in the labial side when compared with the control cases. Von Mises values were
higher at the bottom and occlusal surface of all restorations on all models,
with the exception of the palatal side with a plain chamfer class IV
restoration, where the total deformation showed higher values at the edges of
the gingival surface of the restoration on all models. The results of the investigation
are consistent with this pattern of deformation and stress concentration [17].
Apart from the plain chamfer type, which showed the least values due to the
failure at the enamel-restoration interface in this type of preparation and
will be discussed later, the stresses were also higher in the palatal class V
restoration. However, in the labial class V restoration, the stress reached its
maximum values with the plain chamfer type of class IV restoration, which is
the more destructive preparation type that leaves less residual dental
structure. According to the load
circumstances, the first loading condition revealed total deformation and Von
Mises stress to be about 80 microns and 20 MPa, respectively. Increased overall
deformation and Von Mises stress to 90 microns/30 MPa during the third loading
condition. Second loading circumstances, on the other hand, revealed increased
total deformation and Von Mises stress as 100 microns / 40 MPa, but everything
was still within acceptable limits. Additionally, the labial side experienced
higher tensile and compressive stress values under the third loading condition,
whereas the palatal side experienced the opposite, with the lowest tensile and
compressive stress values shown under the third loading condition due to high
stress developed at the impact site [24]. In the adhesive layer, according to
the load circumstances, the first loading condition revealed total deformation
and Von Mises stress to be about 80 microns and 20 MPa, respectively. Increased
overall deformation and Von Mises stress to 90 microns/ 25 MPa during the third
loading condition. Second loading circumstances, on the other hand, revealed
increased total deformation and Von Mises stress as 100 microns / 30 MPa, but
everything was still within acceptable limits. The first and third loading
circumstances are virtually identical but applied to the teeth (palatal/labial)
in opposing orientations, which caused the same amount of bending stress
(dominant factor), which is why these findings are expected. The bending stress
level is higher during the second loading condition due to the longer arm to
fixation site (cortical bone crest) [25]. Furthermore, tensile and compressive
stresses are distributed according to the same class V restoration pattern as
was previously explained. Because of the significant geometric changes brought
about by various class IV restoration procedures, enamel findings in the
current investigation revealed wide diversity. Enamel results showed minor
deformation differences regardless of the kind of class IV restoration, however
Von Mises showed a small to moderate rise with class IV butt join and 2mm bevel
types of class IV preparation. On the other hand, the plain chamfer type
significantly enhanced enamel stresses. This increase reached 43%–46% and 68%
on the labial side under the first, second, and third loading conditions,
respectively, while it reached 53%–75% and 62% on the palatal side under the
first, second, and third loading conditions. From these results, we can realize
that the percentage of the increase of stresses was higher on the palatal side
because of the sudden increase in the tensile stress value under the second
loading condition and increasing of tensile and compressive stresses under the
first and third loading conditions in comparison to the control cases. These
high values of stresses that were generated around class V exceeded the bond
strength, so that led to failure in the enamel-restoration interface. These
results agreed with previous studies on stress distribution of class V
restoration and Restorative interface [26,27]. Furthermore, The generated
tensile stress value under the second loading condition reached 110 MPa and
according to Rees et al, this high value can be considered as a reasonable failure
value for enamel tissue because of the inability of it to resist more than 80
MPa of tensile stress and because of the least enamel thickness of the enamel
in the palatal side [28]. Generally, this distribution profile of stresses
agreed with previous studies performed [29]. Moreover, under the second loading
condition, the enamel showed higher values of Von Mises stress than that
reported on the palatal side because this condition of loading produces dental
flexion in the labial direction that leads to more stress concentration in the
enamel of the labial side. In the current study, dentin's results showed minor
or negligible change in total deformation with different types of class IV
restorations, while Von Mises stress results showed that the values under the
second loading condition were the highest in both labial and palatal sides.
Furthermore, in comparing the results of both sides the values of Von Mises
stresses were too close to each other in the first and second loading
conditions, while it showed more increase in the palatal side than that
reported in the labial side under the third loading condition this increasing
reach to 93% in the plain chamfer type. In general, dentin showed a fixed
pattern that the differences between types of class IV restorations were
negligible in both types of class V restorations because of dissipation of the
stresses by the enamel tissue. This profile of the stress distribution can be
found in previous studies on bruxism and traumatic cases [30]. Figure 3 Shows the
complete model deformations and stresses under first load condition, that
locations of extreme values (maximum and minimum) were pointed by red and blue
arrows respectively. In the first run, the restoration edges received the
highest amount of deformation and compressive stress, while Von Mises and
tensile stresses were more concentrated in the bottom (Figure 4). 18 Mega
Pascal as maximum Von Mises stress appeared on the adhesive layer (Figure 5).
The occlusal Periphery of adhesive layer received the extreme values of
stresses, while the gingival periphery showed the highest value of total
deformation. In Figure 6 enamel shows high compressive stress around class V
restoration (Figure 6). While it shows low to moderate changes in the total
deformation, Von Mises and tensile stresses. Figures 7. Demonstrate that the
dent in connection with cortical bone received extreme values of principal
stresses that is the supporting site against the applied load.
