Keywords
Lava Ultimate - IPS e-max - Celtra
Introduction
The ceramic veneers are requested by many patients as they are used for esthetic purposes
with minimal tooth preparation that could be only in enamel, slight preparation is
done to give thickness to the veneer to assure its strength and color.[1] The ceramic veneers have advantages over direct veneers of much less possibility
of discoloration or development of recurrent caries under them.[2] Indirect veneers have limitations and concerns because of the material used in fabrication
and the bonding system. In addition, they are affected by parafunctional habits such
as bruxism. The main failures of these types of restorations include fractures and
dislodgement. The mechanical states and behavior of fixed prostheses can be assessed
using finite element analysis.[3] Three common types for laminate veneer preparations are as follows: (1) window or
contact lens, (2) classic or conventional, and (3) wrap around or 3/4th type. In the
window type, there is no preparation of the incisal edge, it is indicated when teeth
have enough length. The wrap around is indicated mainly when there is a need to modify
the incisal length or translucency.[4] Some studies investigated the stress distribution of veneers with different designs;
finite element analysis showed that joint or classic preparation tolerates stress
better, but overlap wrap around distributes stresses more uniformly. The window type
causes stress concentration at the incisal area.[5] In meta-analysis of these studies, it was suggested that the overlap type is more
subjected to fracture than the window type.[6] Few studies investigated the effect of incisal overlapping on prognosis. Survival
rates between covering the incisal edge or not showed no difference in a 2.5 years
study.[7] Resin matrix ceramics are a group of prosthetic materials combining the properties
of polymers having a modulus of elasticity of dentin but reinforced with ceramics.[8] One type of these materials is the resin nanoceramic containing zirconia nanoparticles,
with zirconia silica nanoclusters linked with high-cured resin matrix (bisphenol A
glycol dimethacrylate, urethane dimethacrylate, ethoxylated bisphenol A glycol dimethacrylate,
and triethylene glycol dimethacrylate).[9] Resin nanoceramic has 12 GPa dentin-like modulus of elasticity and high flexural
strength (approximately 150 Mpa) and fracture toughness (approximately 1.2 Mpa m1/2).[10]
[11]
[12] The use of different materials (resin nanoceramic “Lava Ultimate,” lithium disilicate
glass ceramic “IPS e-max” and zirconia reinforced Lithium silicate “Celtra Duo”) is
investigated in this study under two loading conditions using finite element analysis.
The null hypothesis is that the type of preparation which has an effect on the stresses
affecting the veneer and underlying tooth. It is a new study idea to evaluate the
effects of different preparation types with three different ceramic materials.
Materials and Methods
Two three-dimensional (3D) models for central incisor were prepared for this study.
Tooth “central incisor” geometry was acquired using a laser scanner (Geomagic Capture,
3D Systems, Cary, NC, United States) of a sample plastic tooth. The scanner produced
data file containing a cloud of point coordinates, as presented in [Fig. 1]. An intermediate software was required (Rhino 3.0, McNeel Inc., Seattle, WA, United
States) to trim a newly created surface by the acquired points. Then, the solid (closed)
tooth geometry was exported to the finite element program in the STEP file format.
As presented in [Fig. 1], the two types of preparations (window and wrap around) were removed using Boolean
operations. The geometric configurations of the laminate veneer preparation designs
and their dimensions simulating a clinical preparation protocol were introduced into
the ANSYS software program (0.5 mm buccal and proximal reduction plus 0.1 mm for cement
layer, cervical margin placed 1.0 mm away from the cemento-enamel junction, and 0.5 mm
chamfer made for all finish lines).
Fig. 1 Laser scanner and scanned tooth as cloud of points and after creating its surface.
Bone geometry was simplified and simulated as two coaxial cylinders, with the inner
one representing the spongy bone with 14 mm diameter and 22 mm height, filling the
internal cylindrical space of the other cylinder (shell of 1 mm thickness) that represented
cortical bone (outer diameter of 16 mm and its height of 24 mm). A set of Boolean
operations was used to create a root cavity in bone and create a periodontal ligament
(PDL) layer.
Three restoration materials were tested in this study: Lava Ultimate, IPS e-max, and
Celtra. All materials used in this study were assumed isotropic, homogeneous, and
linearly elastic (their properties are listed in [Table 1]). Each of the model components (bone, tooth structure, etc.) was assigned to material
properties on the finite element package.
Table 1
Material properties used in analysis
|
Young's modulus [MPa]
|
Poisson's ratio
|
|
Cortical bone
|
18,800
|
0.30
|
|
Cancellous bone
|
10,700
|
0.30[14]
|
|
Periodontal ligament
|
69
|
0.45[15]
|
|
Enamel
|
84,100
|
0.33[14]
|
|
Dentin
|
14,700
|
0.31[14]
|
|
Pulp
|
2
|
0.45[13]
|
|
Resin cement
|
6,000
|
0.30[1]
|
|
Lava Ultimate
|
12,770
|
0.47
|
|
IPS e-max
|
95,000
|
0.22[3]
|
|
Celtra
|
107,000
|
0.22
|
The final models were meshed by brick element that has three degrees of freedom as
translations in the global directions on the finite element package ANSYS version
16 (ANSYS Inc., Canonsburg, PA, United States). Adequate mesh density was selected
to ensure results' accuracy for the discrete model. Mesh density is another relevant
parameter. As the geometries are complex, increasing the mesh density improves the
results' accuracy for the discrete model. Another effect of increasing the number
of elements is the reduction in sharp angles created artificially by the process of
substituting the geometric model by the mesh, reducing artificial peak stresses by
improving the representation of the actual geometry. The used mesh density (number
of nodes and elements) in each component is given in [Table 2].
Table 2
Mesh density
|
Model #1: window
|
Model #2: wrap around
|
|
Number of nodes
|
Number of elements
|
Number of nodes
|
Number of elements
|
|
Cortical bone
|
63,460
|
37,503
|
63,460
|
37,503
|
|
Cancellous bone
|
113,632
|
78,929
|
113,632
|
78,929
|
|
Periodontal ligament
|
22,960
|
11,556
|
22,960
|
11,556
|
|
Tooth structure
|
164,641
|
114,943
|
166,534
|
116,526
|
|
Resin cement
|
27,941
|
13,620
|
43,736
|
21,494
|
|
Veneer layer
|
27,349
|
13,383
|
42,105
|
20,801
|
The highest plane of the model was considered fixed in the three directions as a boundary
condition. The applied loads were set as 50N, directed with 135° oblique angle from
the vertical plane to the following points:
Total 1212 linear static analyses were performed on a personal computer (Intel Core
i7 processor, 2.4 GHz, 6.0 GB RAM), using commercial multipurpose finite element software
package (ANSYS version 16.0).
Results
Minor or negligible differences of total deformation were recorded by changing loading
position and/or the preparation type on bone and PDL. Total deformation and von Mises
stresses on bone (cortical and cancellous) and PDL showed nearly the same values according
to the preparation type and applied loading condition (see [Fig. 2]).
Fig. 2 Comparison of total deformation appeared on each model component.
The two models' total deformations comparison in [Fig. 2] showed a slight increase in tooth structure, cement layer, and veneer layer with
Lava Ultimate in comparison to the other two materials. Equivalent values of total
deformation were recorded with IPS e-max and Celtra in all cases.
In model 1 ([Fig. 3]), finish line resists veneer layer movement under tip loading; thus, it received
maximum values of von Mises stress with relatively high values. On the contrary, junction
loading creates much lower values of stresses (approximately 30%) in comparison to
tip loading.
Fig. 3 Von Mises stress comparison on Model #1, and example of IPS e-max veneer layer under
tip loading.
The general trends of total deformation and von Mises stress in all model components
did not change even with changing loading positions. More stiff (or rigid) veneer
layer caused and received more stresses on tooth structure, cement, and veneer layers.
Although the increase of finish line contour in model 2 (wrap around) in comparison
to model 1 (window) showed higher stresses, the increase of finish line moved the
extreme stress to the new location such that extreme von Mises stress values appeared
under the applied loads. The wrap around preparation directly received the applied
load at the junction between incisal and middle thirds; thus, it received higher stress
in comparison to the window type that did not receive this load ([Fig. 4]).
Fig. 4 Von Mises stress comparison on Model #2, and sample of cement layer von Mises stress
under Celtra veneer layer.
Cement layer received higher deformations and stresses under veneer layer prepared
as wrap around, transferring the load to the tooth structure, where it received loading
via veneer in the two load cases.
Discussion
Veneers are considered a successful type of restorations as stated by the systematic
review by Aljazairy in 2020, showing their high success rate.[16] The perfect way to test a type of dental restoration is in the oral cavity but clinical
studies are time-consuming and are not cost-effective.[17] So finite element analysis is used to evaluate the effect of different veneer preparation
designs with different materials for fabrication. The preparation of veneers is an
important factor in their success as stated by Linhares et al in 2020 especially with
premolars.[18] Other recent case reports showed the satisfaction of the patients with tooth preparation
of veneers or even without tooth preparation as shown by Sá et al in 2018.[19] There are different preparation designs for laminate veneers, window, feathered
edge, palatal chamfer, and butt joint.[20] Increasing the extension of preparation increases the bonded surface area, which
is one of the important points in the veneer survival. The decision was made here
to compare the cases with incisal coverage and without incisal coverage. A distinct
difference between the four types was not always possible.[21] Also names of the preparation designs were not always the same in all studies. Generally,
the failures of cases with incisal coverage were less compared to cases without incisal
coverage.[2] Other studies as that by Beier et al in 2012 showed that the failures with the overlap
design were more compared to no overlap preparation designs.[22]
Bone (cortical and cancellous) and PDL were insensitive to veneer material, while
minor or negligible changes were noticed by changing the preparation type or loading
position. All structures were included in the study to have more realistic results.[23] Our results regarding the bone were in accordance with Tsouknidas et al in 2020
who found that stresses with veneers were the same as with the natural tooth regarding
the supporting structures.[14]
Tooth structure, cement, and veneer layers showed equivalent values of total deformation
under IPS e-max and Celtra in all loading cases that may be referred to close values
of elasticity modulus, while the slight increase was also observed in values under
Lava Ultimate that may also be referred to the same reason. This was in accordance
with Fernandes et al in 2021 who found that the stresses are nearly the same at the
tooth structure regardless of the material of fabrication or the preparation design.
However, on the contrary, they found higher stresses in the palatal chamfer on the
veneer itself.[13]
Window preparation type showed a gradual increase in stresses with increasing veneer
layer elasticity (correlated to rigidity) that may be caused by the higher material
resistance to deflect under load and transfer load to underneath structures at the
veneer layer finish line. This was in accordance with Chai et al in 2021 who found
in a photoelastic study that stresses were better when covering the incisal edge with
the veneer due to distribution over a wider surface area.[15]
Although the increase of finish line contour in model 2 (wrap around) in comparison
to model 1 (window) showed higher stresses, this may be referred to the loading transfer
mechanism, where the veneer layer is floating on (supported by) weaker material (cement
layer) that allows veneer layer micro-movement. In addition to having direct contact
with loading at junction between incisal and middle thirds, this was in accordance
with Fernandes et al in 2021who found that in the case of wrap around the veneers
are more susceptible to fracture as stress concentration occurs at the tooth restoration
interface at the junction between incisal and middle thirds.
As the cement layer covers, the tooth under veneer works like a cushion to reduce
deformation and stresses on the tooth structure. This is why it receives more stresses
and deformations under wrap around veneer that it was loaded via veneer layer in both
loading cases. This also supported by Li et al in 2014 finite element study who found
the same with incisal coverage but under loading with little load angulation 60°.[5]
More studies are recommended to evaluate the effect of other materials used for the
construction of veneers. In our study, the null hypothesis was proved.
Conclusions
Bone (cortical and cancellous) and PDL are insensitive to veneer material, while minor
or negligible changes may be noticed by changing preparation type or loading position.
The veneer layer finish line and its contact with cement layer and tooth structure
play a crucial role in the loading transfer mechanism. Thus, the preparation type
alters the values of stresses on tooth structure, cement, and veneer layers.
With window preparation type, extreme stresses appear at finish line, while they appeared
under the loading site with wrap around preparation. Veneer and cement layer withstand
the majority of load energy with wrap around preparation and reduce tooth structure
stresses. As deformations and stresses are within physiological limits, the lifetime
of veneer and cement layers might be longer with window preparation. This finite element
study gives guidelines for operators.
