Chai SY, Bennani V, Aarts JM, Lyons K, Das R. Stress distribution within the ceramic veneer-tooth system with butt joint and feathered edge incisal preparation designs. J Esthet Restor Dent. 2021 ;33(3):496-502

1. Stress distribution within the ceramic veneer-tooth
system with butt joint and feathered edge incisal
preparation designs
Chai SY, Bennani V, Aarts JM, Lyons K, Das R. Stress distribution within the ceramic veneer-
tooth system with butt joint and feathered edge incisal preparation designs. J Esthet Restor
Dent. 2021 ;33(3):496-502.
Presented by
Dr Nadeem Aashiq
MDS 1st year

2. INTRODUCTION
 Photo elastic analyses and finite element analyses (FEA) are the two most
common ways to evaluate the stress distribution within the veneer-tooth
system with various incisal preparation designs.
 FEA is a numerical method that can precisely analyze and predict stress
distribution within a given structure by utilizing modeling data.
 Photoelasticity is a technique which utilizes the birefringence properties of
certain transparent media to measure the direction of principal stress and
the magnitude of principal stress difference of the photoelastic model.

3.  Despite the development of FEA, photoelastic analysis is easily conducted
and has been proven to be a valid methodology to evaluate stress
distribution.
 Only two photoelastic stress analyses on incisal preparation designs of
ceramic veneers have been reported in the literature.
 Highton et al compared the feathered edge (FE) and the butt joint (BJ)
groups under axial and inclined loads. This study found better stress
distribution in the BJ group and greater stress intensity within the FE
group.
 Hui et al compared window, FE and BJ incisal preparation designs and
found the least stress concentration within the window group, while the FE
group demonstrated more favorable stress distribution compared with the
BJ group.

4.  Clinically, ceramic chipping and fracture is the most common reason for
failure of the veneer restorations.
 Incisal preparation design is regarded as one of the predisposing factors for
failure of these restorations.
 However, heterogeneity in these studies has prevented the direct comparison of
incisal preparation designs. Hence, various in-vitro studies have been
performed to evaluate the incisal preparation designs of ceramic veneers,
whether it is through mechanical testing or stress analysis.
 According to the literature review, there are no stress analyses which have
compared and correlated their results to other in-vitro studies such as load-
to-failure test, fatigue testing, and/or fractographic analysis. The
development of stress patterns under increasing load increments in in-vitro
settings has not been reported in the literature. These stress analyses have
also demonstrated significant variabilities in terms of model fabrication,
mounting apparatus, plunger type and dimension, and loading conditions.
No correlation between the stress distribution and failure mode of these
restorations has been found.

5. AIM
 The purpose of this investigation was to study the stress distribution within
the veneer-tooth system with FE and BJ incisal preparation designs at two
loading angulations using a photoelastic stress analysis technique.

6. METHODS
 Three typodont teeth representing maxillary left central incisors, of which
one with no veneer preparation (control), one with FE veneer preparation
design and the other had a BJ veneer preparation design, were obtained
from Nissin Dental Products Inc. (Kyoto, Japan).
 The FE veneer preparation design incorporated a buccal reduction of 0.5-
mm thickness which does not extend past the proximal contact zone. The
BJ typodont tooth has an additional 1.5 mm incisal reduction. (Figure 1).

7. FIGURE 1 Veneers (A) BJ incisal preparation design; (B) FE incisal preparation design) were
designed on scanned typodont teeth using Ceramill Mind CAD design software

8.  Six photoelastic models of various incisal preparation configurations and
loading angulations were fabricated with an epoxy resin material and were
divided into six groups:
 (a) Control 0˚
 (b) Control 20˚
 (c) BJ 0˚
 (d) BJ 20˚
 (e) FE 0˚
 (f) FE 20˚
 Lithium disilicate ceramic veneers were bonded to the BJ and FE
photoelastic models using resin cement.
 The control, FE, and BJ typodont teeth were embedded in modeling wax at
0˚ angulation.

9.  A silicone mold was made of each typodont tooth embedded in wax using
vinyl polysiloxane impression material and lab putty.
 The embedded typodont tooth was then removed and the epoxy resin
material was mixed and poured into the silicone mold and allowed to set
under pressure according to the manufacturer's instructions.
 This epoxy resin was selected for its flexural modulus (0.32 GPa), minimal
shrinkage and availability.
 The base of the photoelastic model was trimmed to specific dimensions to
fit into the mounting jig of the Instron Universal Testing Machine.
 The same procedures were applied to fabricate photoelastic models at 20˚
angulation.

10.  The typodont teeth with FE and BJ veneer preparations were scanned
using Ceramill Map 400+.
 The FE and BJ incisal preparation designs were then designed using the
Ceramill Mind CAD design software.
 Each design has a designated cement gap of 50 μm. Four ceramic veneers
(two of FE design, two of BJ design) were milled using IPS e. max CAD
blocks (HT A1/C14, Ivoclar Vivadent), fired in a Programat P500 furnace
(Ivoclar Vivadent), and finished according to parameters and guidelines
recommended by the manufacturer.
 The surface preparation (veneer and photoelastic model) and cementation
procedures were tested and confirmed in pilot study. The fit of the ceramic
veneers was checked on the photoelastic model under a light microscope
prior to cementation.

11.  About 5% HF acid (IPS Ceramic Etching Gel, Ivoclar Vivadent) was
applied to the bonding surface of ceramic veneers for 20 seconds, rinsed
thoroughly with water and dried with oil-free compressed air.
 The surface was then primed using Monobond Plus and allowed to react
for 60 seconds before air dispersion.
 After lightly sandblasting the cementation surface of photoelastic model
with 50 μm aluminum oxide particles at 0.2 MPa (2 bar) pressure for 10
seconds, the ceramic veneers were cemented onto the photoelastic models
using a dual-cure cement to ensure consistency.
 The veneers were seated on the photoelastic models by one operator using
sustained finger pressure and any excess cement was removed.

12.  A small area of the veneer was tack-cured for 3 seconds (Bluephase, 650
mW/cm2, LOW mode, Ivoclar Vivadent) to ensure consistency.
 Excess cement was removed carefully. The restoration margins were
covered with glycerin gel/air block (Liquid Strip, Ivoclar Vivadent)
immediately and each surface (mesiobuccal, distobuccal, buccal,
mesiopalatal, distopalatal, and palatal surfaces) was light cured for further
20 seconds.
 The photoelastic model was secured onto the holder of the Instron 1185
Universal Testing Machine (Instron Corp). The orientation was
standardized using a protractor.
 The customized plunger was attached to the Instron load cell (1000 N).

13.  The plunger was designed to simulate anterior contacts of 0.5 mm in
diameter, with a 100 N loading force and 509 MPa contact pressures.
 The Instron 1185 Universal Testing Machine (Instron Corp) was calibrated
using a standard calibration weight before the start of the test.
 A webcam was set up to record the change in stress development within
the photoelastic models and transfer the information to the computer
software.
 The stress pattern resulting under load was viewed and recorded with a
circular transmission polariscope. The circular polariscope consisted of a
light source, light diffuser, polarizing filter mounted model and the second
polarizing filter (FIG 2)

14. FIGURE 2 The set-up consisted of a polariscope and a digital camera to record
the stress development within the photoelastic model

15.  All photoelastic models were loaded at the incisal edge at crosshead speed
of 0.25 mm/min up to a maximum of 100 N to replicate the lower range of
the maximum anterior bite force, which can range between 100 and 200 N.
 A Microsoft Excel (Version 15.22) spreadsheet containing the raw data
including time (s), compressive load (N), and compressive extension (mm)
was exported after each test. The time where the load was at 0, 10, 50, and
100 N, respectively, was identified via the Microsoft Excel spreadsheet
and images of the photoelastic model were exported from the webcam.
The isochromatic fringe patterns were recorded. The isochromatic fringes
revealed the location and the intensity of stress concentrations, and the
distance of isochromatic fringes from each other represents stress
concentration.

16. RESULTS
 At 0˚ loading angulation, stress patterns were noted at the incisal edge of
Control photoelastic model directly under the point of loading when initial
load was applied. As the load was increased, the stress distribution of the
Control photoelastic model seemed to spread axially (Figure 3).
 Under the same load, the BJ photoelastic model showed better stress
distribution compared with the control and FE photoelastic models.
Minimal stress at the cement line was visible and at the incisal edge under
initial load application. As the load was increased, the stress distributed
along the cement line toward the cervical area of the tooth was confined to
the cement layer (Figure 3).

17.  There was greater stress concentration in the FE photoelastic model in
contrast to the BJ photoelastic model under the same load when load was
applied axially at the incisal edge. At initial load, great stress concentration
was noted at the interface between the ceramic veneer and the photoelastic
model directly under load application. As load was increased, stress
distributed axially along the long axis of the photoelastic model and along
the cement line (Figure 3).
 When load was applied at a 20˚ angulation, the control photoelastic model
demonstrated stress concentration which developed initially at the incisal
edge. When load was increased, stress concentration was observed at the
palatal concavity and the mid-body of the photoelastic model along the
long axis of the tooth.

18. FIGURE 3 Photoelastic models loaded on the incisal edge at a 0˚ angulation
(Red arrow = fringe patterns)

19.  Similar stress patterns and distribution were also observed for theBJ
photoelastic model. When low load was applied, stress developed at the
interface between the ceramic veneer and the incisal edge of the photoelastic
model. As the load was increased, the stress concentration along the cement
line and at the mid-body of the photoelastic model also increased (Figure 4).
 When load was applied at a 20˚ angulation to the FE photoelastic model, stress
concentration is consistently demonstrated at interface between the ceramic
veneer and the photoelastic model close to the incisal edge where initial load
was applied. However, compared with the control and BJ photoelastic models,
the FE photoelastic model showed significantly higher stress development at
the interface between the ceramic veneer and the photoelastic model close to
the incisal edge. When load was increased, stress concentration also increased
significantly and appeared to spread from the palatal concavity and cement line
toward the cervical area of model and around the mid-body of the photoelastic
model (Figure 4).

20. FIGURE 4 Photoelastic models loaded on the incisal edge at a 20 angulation (Red
arrow = fringe patterns)

21. DISCUSSION
 This photoelastic study showed that stress was better distributed in the BJ
group, during all loading strategies.
 The BJ 0˚ photoelastic model has the least stress concentration when
loaded. This finding is consistent with other photoelastic stress analyses
studies which showed that stress distribution over a wider area was found
in the BJ (or overlap) design compared with the non-incisal overlap
design.
 These findings may partly explain the results of a recent in-vitro study
which reported the greatest load-to-failure mean value in group BJ 0˚,
followed by groups BJ 20˚, FE 0˚, and FE 20˚. The higher load-to-failure
mean values in the BJ groups can be attributed to their ability to distribute
stress more evenly and favorably to avoid a single stress concentration
area.

22.  In the same study,the SEM analysis showed that fracture origin of the BJ
20˚ and BJ 0˚ samples was at the ceramic-resin interface layer under the
loading zone.
 The crack propagated externally from the interface layer in incisal and
buccal directions. Compression curls were found exclusively at the
superficial surface of the ceramic veneers.
 These findings support the current photoelastic study which showed initial
stress patterns at the interface between the ceramic veneer and the incisal
edge of the photoelastic model. As the load was increased, stress
concentration was observed along the cement line and toward the mid-
body of the photoelastic model.

23.  Significant stress concentration was observed for the FE photoelastic
model at 20˚ loading angulation, typically at the interface between the
ceramic veneer and the photoelastic model close to the incisal edge as well
as the cement line toward the cervical area. In the load-to-failure testing,
group FE 20˚ showed the lowest load-to failure mean value
 The majority of the samples in group FE 20˚ exhibited cohesive failure
involving predominantly the incisal ceramic. The stress patterns
demonstrated in this study are in line with the SEM analysis which
demonstrated the origin of failure of this group at the ceramic-resin
interface under the loading zone. The high stress concentration could
increase the risk of ceramic fracture for this preparation design.
 To allow for consistency and reproducibility, only one homogenous
photoelastic material with a flexural modulus of 0.3 GPa was used to
represent the natural tooth structure within the photoelastic model and the
ceramic veneer was bonded to the photoelastic models using resin cement.

24.  This represents one of the limitations of this study as the tooth substitute
used has a quite low elastic modulus in comparison with the elastic
modulus of the ceramic veneer since the type of stress within the
photoelastic models is interpreted according to the fundamental basics of
photoelastic applications. Using different photoelastic materials, may
influence the development of fringe patterns.Therefore, the quasi-
threedimensional technique employed in this study does not provide
detailed three-dimensional stress distribution within the model. This
photoelastic stress analysis mainly highlights location and intensity of
stress concentrations within the photoelastic model.
 Three main locations of stress concentration were identified in this study:
a) veneer-tooth interface at the incisal edge below then area of load
application
b) cement line,
c) the palatal concavity of the photoelastic tooth model.

25.  In this in-vitro study the ceramic veneer was cemented on the photoelastic
model instead of a photoelastic material, to allow better visualization of stress
patterns typically at the cement line or interface between the veneer and the
photoelastic model.
 A dual-cure cement was used to ensure consistency in the methodology of our
earlier load-to-failure published study to enable a correlation of the
findings.The combination of a photoelastic material nonphotoelastic material
to simulate prosthesis-oral structure have also been validated in other studies in
the dental field involving implants post and core systems, and inlay and onlay
restorations.
 Different loading angulations may result in different type of stress within
the photoelastic model. Based on the photoelastic theory,inclined loads
(20˚) resulted in bending and compressive-type stresses, while axial (0˚)
loading generally resulted in mainly compressive stress within the
photoelastic model. When loaded along the long axis the BJ and FE
photoelastic models stress concentration appeared initially at the veneer-
tooth interface at the incisal edge, below the area of load application

26.  When load is increased, stress concentration was seen along the cement line
typically in the FE photoelastic model, while stress distribution within the
body was observed. This in-vitro study showed that areas of stress
concentration vary according to the type of incisal preparation design and
loading angulations. When subjected to inclined loads (20˚), Control and FE
photoelastic models showed stress concentration in all three areas: veneer-
tooth interface at the incisal edge below the area of load application, cement
line, and palatal concavity of the photoelastic tooth models. With one
exception being the BJ photoelastic model which appeared relatively quiescent
in the area of the palatal concavity.
 The stress concentration and distribution in this photoelastic study also explain
various forms of clinical fractures involving ceramic veneers in clinical
practice.
 These stress concentration areas include the incisal edge directly under
load application, cement interface at buccal half of the tooth, and at the
buccocervical region of the tooth.

27.  In general, the BJ photoelastic models showed favorable stress distribution and
much lower stress concentration compared with the FE photoelastic models,
especially under the axial (0˚) load. Clinical studies have shown that the incisal
edge is the most common location of ceramic fracture.
 Nordbø et al also reported that other locations of ceramic failure such as
buccoincisal chipping, failures at composite-cement interface and buccal
surface of veneer, which are consistent with the significant stress concentration
areas found in this study.
 FEA studies also reported a better stress distribution pattern within the
ceramic veneers (BJ) with a lower loading angulation (60˚ vs 125˚)
although the opposite was reported for the palatal chamfer group.
 FEA study by Magne and Douglas showed that the tooth behaves like a
cantilever beam during function as a result of the bending forces, resulting
in mainly tensile stress at the palatal half of the tooth and compressive
stress at the buccal half of the tooth. This further reiterates the conclusion
that long palatal chamfer incisal preparation design should be avoided due
to brittle nature of dental ceramics

28.  Highton et al showed lower stress intensity at load-bearing area of the BJ
photoelastic model compared with the FE photoelastic model at axial and
inclined loading angulation (25˚).
 They concluded that the BJ incisal preparation design is favorable for
patients with Class I incisal relationship (overbite and overjet between 2
and 4 mm).
 Hui et al reported the greatest force transmission was on the BJ veneer
compared with the FE veneer. Their photoelastic study concluded that the
more conservative incisal preparation design (window) is preferred to the
BJ preparation design.
 However, the photoelastic models were constructed in the form of
rectangular bars, and the stress patterns for the inclined loading conditions
were not illustrated in the article.

29.  In addition, there was no veneer-to-plunger contact for the window design.
The different conclusions drawn in these two studies can be attributed to
the variations in incisal preparation designs, photoelastic materials,
dimensions of the photoelastic models, model construction techniques and
loading parameters (eg, loading angulations, load points, and the
magnitude of the load).
 Only one FEA study has compared the FE and the BJ incisal preparation
designs for ceramic veneers. The authors reported slightly better stress
distribution for the BJ group compared with the FE group.

30. CONCLUSIONS
Within the limitations of this study, this photoelastic study indicated the
following:
1. Parallel to the results of our earlier load-to-failure published study, both
incisal preparation designs affect stress distribution within the ceramic
veneer-tooth system.
2. BJ photoelastic model demonstrated a more uniform distribution
compared with FE photoelastic model under both axial (0˚) and inclined
loads.
3. The FE photoelastic model showed that the stress was directed toward
the incisal edge region and along the cement interface under any of
loading conditions tested.
4. Loading angulation has an effect on stress direction within the
photoelastic models.

31. Clinical Significance:
 BJ incisal preparation design has more uniform stress distribution than FE
preparation design within the ceramic veneer-tooth system.

32. Effect of incisal preparation design on load-to-
failure of ceramic veneers
Chai SY, Bennani V, Aarts JM, Lyons K, Lowe B J Esthet Restor Dent. 2020 Jun;32(4):424-432.
 Objective: This in vitro study aims to evaluate load-to-failure of ceramic veneers
with butt joint (BJ) and feathered edge (FE) incisal preparation designs, and to
correlate these results to the failure mode of the restorations.
 Materials and methods: One hundred and forty-eight typodont teeth (customized
Nissin A25A-UL19B) were divided into two different preparation configurations
BJ and FE and two different loading angulations, 0° and 20°. Lithium disilicate
ceramic veneers (IPS e.max CAD, Ivoclar Vivadent) were milled using computer-
aided-design-and-computer-aided-manufacturing (CAD/CAM) techniques. Veneers
were bonded to typodont teeth with resin cement (IPS Variolink Esthetic, Ivoclar
Vivadent). Each group was loaded at the incisal edge using an Instron Universal
Testing Machine at a crosshead speed of 0.01 mm/s till failure.

33.  Results: Pairwise comparison showed veneers from the BJ groups had a
significantly higher load-to-failure value compared to the FE groups.
Veneers with a FE preparation design loaded at 20° angulation had the
lowest load-to-failure value.
 Conclusions: Within the limitations of the present study, both incisal
preparation designs and loading angulations have significant effects on the
load-to-failure values of ceramic veneers. BJ group exhibits a significantly
higher load-to-failure value compared to the FE group.
 Clinical significance: BJ incisal preparation is preferred over FE
preparation design.

34. Clinical performance of porcelain laminate
veneers for up to 20 years
Beier US, Kapferer I, Burtscher D, Dumfahrt H. Int J Prosthodont. 2012 Jan-Feb;25(1):79-85.
 Purpose: The aim of this clinical retrospective study was to evaluate the clinical
quality, success rate, and estimated survival rate of anterior veneers made of
silicate glass-ceramic in a long-term analysis of up to 20 years.
 Materials and methods: Anterior teeth in the maxillae and mandibles of 84
patients (38 men, 46 women) were restored with 318 porcelain veneer restorations
between 1987 and 2009 at the Medical University Innsbruck, Innsbruck, Austria.
Clinical examination was performed during patients' regularly scheduled
maintenance appointments. Esthetic match, porcelain surface, marginal
discoloration, and integrity were evaluated following modified California Dental
Association/Ryge criteria. Veneer failures and reasons for failure were recorded.
The study population included 42 (50.0%) patients diagnosed with bruxism and 23
(27.38%) smokers. The success rate was determined using Kaplan-Meier survival
analysis.

35.  Results: The mean observation time was 118 ± 63 months. Twenty-nine
failures (absolute: 82.76%, relative: 17.24%) were recorded. The main
reason for failure was fracture of the ceramic (44.83%). The estimated
survival rate was 94.4% after 5 years, 93.5% at 10 years, and 82.93% at 20
years. Nonvital teeth showed a significantly higher failure risk (P = .0012).
There was a 7.7-times greater risk of failure associated with existing
parafunction (bruxism, P = .0004). Marginal discoloration was
significantly greater in smokers (P ⋜ .01).
 Conclusion: Porcelain laminate veneers offer a predictable and successful
restoration with an estimated survival probability of 93.5% over 10 years.
Significantly increased failure rates were associated with bruxism and
nonvital teeth, and marginal discoloration was worse in patients who
smoked.

36. THANK YOU