Success of dental veneers according to preparation design and material type

Abstract

Abstract

Background: Due to their high aesthetic outcome and long term predictability, laminate veneers have become a common restorative procedure for anterior teeth. However, because of the variety in the preparation designs and the material types, the clinician faces a dilemma of which approach to use.

Aim: to compare the survival rate of dental veneers according to different preparation designs and different material types. The sub aim is to reach to a favourable preparation design and material based on scientific evidence.

Methodology: comprehensive electronic search of the dental literature via PUBMED, MEDLINE and Scopus databases was performed using the following keywords: “porcelain veneers”, “composite veneers”, “all ceramic veneers”, “success of porcelain veneers”, “preparation design”, “preparation geometry”, “patient’s satisfaction”. Additionally, references from the selected studies and reviews were searched for more information.

Conclusion: under the limitations of the available literature, the clinician preference is the decisive factor for choosing the preparation design. Nonetheless, incisal overlap preparation seems to have the most predictable outcome from all the preparation designs. Porcelain veneers show excellent aesthetic results and predictable longevity of the treatment, while composite veneers can be considered as a good conservative option, but with less durability.

Full text

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ID Design Press, Skopje, Republic of Macedonia
Open Access Macedonian Journal of Medical Sciences. 2018 Dec 20; 6(12):2402-2408.
https://doi.org/10.3889/oamjms.2018.353
eISSN: 1857-9655
Dental Science - Review
The Success of Dental Veneers According To Preparation Design
and Material Type
Yousef Alothman1, Maryam Saleh Bamasoud2*
1AlFarabi Colleges of Medicine, Dentistry, and Nursing, Riyadh, Saudi Arabia; 2Clinical Dentistry, Cardiff University, Wales,
United Kingdom
Citation: Alothman Y, Bamasoud MS. The Success of
Dental Veneers According To Preparation Design and
Material Type. Open Access Maced J Med Sci. 2018 Dec
20; 6(12):2402-2408.
https://doi.org/10.3889/oamjms.2018.353
Keywords: laminate veneers; anterior teeth; restorative
procedure; survival rate of dental veneers
*Correspondence: Maryam Saleh Bamasoud. Clinical
Dentistry, Cardiff University, Wales, United Kingdom. E-
mail: maryam.sbsb@hotmail.com
Received: 01-Aug-2018; Revised: 03-Nov-2018;
Accepted: 04-Nov-2018; Online first: 14-Dec-2018
Copyright: © 2018 Yousef Alothman, Maryam Saleh
Bamasoud. This is an open-access article distributed
under the terms of the Creative Commons Attribution-
NonCommercial 4.0 International License (CC BY-NC 4.0)
Funding: This research did not receive any financial
support
Competing Interests: The authors have declared that no
competing interests exist
Abstract
BACKGROUND: Due to their high aesthetic outcome and long-term predictability, laminate veneers have become
a common restorative procedure for anterior teeth. However, because of the variety in the preparation designs
and the material types, the clinician faces a dilemma of which approach to use.
AIM: To compare the survival rate of dental veneers according to different preparation designs and different
material types. The sub-aim is to reach a favourable preparation design and material based on scientific evidence.
METHODS: Comprehensive electronic search of the dental literature via PUBMED, MEDLINE and Scopus
databases was performed using the following keywords: “porcelain veneers”, “composite veneers”, “all-ceramic
veneers”, “success of porcelain veneers”, “preparation design”, “preparation geometry”, “patient’s satisfaction”.
Additionally, references from the selected studies and reviews were searched for more information.
RESULTS: Under the limitations of the available literature, the clinician preference is the decisive factor for
choosing the preparation design. Nonetheless, incisal overlap preparation seems to have the most predictable
outcome from all the preparation designs.
CONCLUSION: Porcelain veneers show excellent aesthetic results and predictable longevity of the treatment,
while composite veneers can be considered as a good conservative option, but with less durability.
Introduction
Since 1930s dental veneers have been used
to improve the aesthetic and protection of teeth
(Calamia, 1988) [1], the indications of dental veneers
include: 1) discoloured teeth due to many factors such
as tetracycline staining, fluorosis, amelogenesis
imperfect, age and others 2) restoring fractured and
worn teeth 3) abnormal tooth morphology 4)
correction of minor malposition 5) Intra-oral repair of
fractured crown and bridge facings [2], [3], [4].
Unfavourable conditions of dental veneers include 1)
patients with parafunctional habits such as bruxism 2)
edge to edge relation 3) poor oral hygiene 4)
insufficient enamel [5], [6]. Many studies reported
positive clinical outcomes veneers, with a survival rate
of 91% in 20 years [7] dental veneers are considered
a predictable aesthetic correction of anterior teeth.
The materials of dental veneers have evolved
remarkably, early materials that had been used had
many disadvantages such as the materials needed to
be too thick to cover any discolouration, difficulty to
polish which can cause abrasion of the opposing
dentition and easy to stain [8], [9]. Researchers and
dental material manufacturers have aimed to develop
new materials with better aesthetic characteristics
through the years. In 1975 laminate veneers were
introduced as a better material of choice to mask the
dentition, the restorations were 1 mm in thickness and
were made from a cross-linked polymeric veneer [10].
The use of laminate veneers resulted in a better
aesthetic outcome and less chair time [11]. The
progress of developing new materials reached
porcelain in the 1980s when enamel was etched, and
the porcelain surface was treated to improve the
bonding [12], [13].
The desire for more durable aesthetic
outcomes did not confine to improve the material type
only; new preparation designs were introduced to the

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field of dental veneers. There are four different main
designs of teeth preparation commonly mentioned in
the literature (Figure 1): 1) window preparation: in
which the incisal edge of the tooth is preserved 2)
feather preparation: in which the incisal edge of the
tooth is prepared Bucco-palatable, but the incisal
length is not reduced 3) bevel preparation: in which
the incisal edge of the tooth is prepared Bucco-
palatable, and the length of the incisal edge is
reduced slightly (0.5-1 mm) 4) incisal overlap
preparation: in which the incisal edge of the tooth is
prepared Bucco-palatable, and the length is reduced
(about 2 mm), so the veneer is extended to the palatal
aspect of the tooth [14], [15], [16], [17].
Figure 1: Showing common veneer preparations a) window b)
feather c) bevel d) incisal overlap [17]
Influence of preparation design on the
survival of dental veneers
Different opinions have been reported about
superior preparation design over the others. In fact,
due to the great variety in the materials, preparations
designs and luting cement, favourable approaches to
restore teeth with veneers have been controversial.
This review aims to compare the survival rate
of dental veneers according to different preparation
designs and different material types. The sub-aim is to
reach a favourable preparation design and material
based on scientific evidence.
One important aspect to investigate is the
tooth preparation of dental veneers and how it might
affect the fracture resistant of the material and
reinforcement of the abutment tooth. Unfortunately,
clinical trials that investigate the survival rate of dental
veneers according to preparation designs are few, the
criteria of investigation would include more than one
factor which can affect the outcome of the treatment
[16], [18]. In contrast, many in vitro studies have been
conducted to evaluate the influence of different
preparations design. Although such studies do not
mimic the actual clinical environments and factors,
they can provide criteria and guidelines for the
clinician and further clinical investigations [5]. Table 1
illustrates the results of multiple in vitro studies
regarding the influence of preparation design.
Table 1: In vitro studies that investigated the influence of
preparation design on dental veneers
Study
Preparation
design
Method of
loading
Number
of
samples
Survival
probability
Conclusion
Remarks
(Highton &
Caputo 1987)
[26]
Incisal overlap-
chamfer FL
Window
preparation
Slight labial
preparation only
Unprepared
Four
directions:
Central
vertical
Distal vertical
Central
inclined
Distal inclined
4 (one of
each)
High
Moderate
Low
Lowest
Labial, proximal,
incisal and
gingival
reduction is
recommended.
Samples were
photoelastic
teeth
(Castelnuovo
et al. 2000)
[14]
Incisal overlap
(1mm)-chamfer
finish line
Butt joint incisal
reduction (1mm)
Feather edge
preparation
Deep incisal
overlap(4mm)
Unprepared
Static loading
at a 90-
degree angle
to the
palatal
surface of the
sample
50 (10
each)
Moderate
High
High
Low
Control
Butt joint incisal
reduction and
feather edge
prep. Provide the
best retention to
the restoration.
Deep incisal
overlap is not
recommended
-
(Stappert et al.
2005) [16]
Incisal overlap
(2mm) butt joint
Deep incisal
overlap (3mm)-
butt joint
Window
preparation
Unprepared
Dynamic
loading and
thermal
cycling 135-
degree angle
in the
masticatory
stimulator
64 (16
each)
High
Low
Low
Control
Incisal overlap
provides the best
support. Deep
preparation is
not necessary.
-
(Zarone et al.
2005) [28]
Incisal overlap-
chamfer FL
Window
preparation
Static loading
at the long
axis of the
tooth
4
High
Low
Incisal overlap is
a better design
than window
prep.
Samples were
3D
computerised
models
(Schmidt et al.
2011) [31]
Incisal reduction
– chamfer FL
Incisal reduction
– butt joint
Static loading
at a 90-
degree angle
to the palatal
surface of the
sample
32 (8
each)
Low
High
Having a
chamfer FL
increase the
failure rate of the
veneer
Amount of
existing tooth
structure was
considered in
the study
(Lin et al.
2012) [23]
Incisal reduction
– butt joint
Three quarter
preparation
Static loading
at a 125-
degree angle
of the palatal
surface of the
sample
48 (12
each)
High
Moderate
Three-quarter
prep. Requires
stronger material
for support
Influence of
restorative
materials was
included in the
study
(Alghazzawi et
al. 2012) [32]
Incisal reduction
– butt joint
Three quarter
preparation
Dynamic
loading at a
135-degree
angle of the
palatal
surface of the
sample
60 (30
each)
High
High
No significant
difference
between the two
preparations
-
General concepts
Some features of the preparation design are
highly recommended in the majority of the literature
and lab studies. For example, restricting the
preparation to enamel is considered to be a critical
factor for a favourable bonding strength, thus more
durable outcome [6], [18], [19], [20]. Additionally,
preserving the interproximal contact is recommended
in most of the literature and studies, this is due to
preserving more enamel and tooth structure, allowing
a positive seat for cementation in a conservative
approach [16], [21], [22], [23]. However, the clinician
might face certain situations where removing the
interproximal contact can provide better aesthetic
results such as malaligned teeth or diastema [24],
[25]. Moreover, the amount of labial reduction
concurrent at 0.4-0.7 mm for ceramic veneers [1], [3],
[15]. This is due to the enamel thickness in the
anterior teeth, according to Ferrari et al., (1991) [3],

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the enamel thickness of 114 extracted anterior teeth
was 1.0 to 2.1 mm at the incisal third, 0.6 to 1.0 mm at
the middle third and 0.3 to 0.5 at the gingival third,
therefore, minimal preparation is advisable.
Preparation designs
Although there are different opinions and
different results in studies that investigate the
influence of preparation design on the survival of the
restoration. It seems that incisal overlap preparation
provides the best support for the restoration and
distributes occlusal forces over a larger surface area.
In the window preparation, the occlusal stress is
highly concentrated on the incisal third which may
lead to fracture of the restoration. Also, incisal
translucency can be better achieved when the incisal
edge is reduced [14], [16], [23], [26]. However, it is
controversial whether it is favourable to add a chamfer
finish line palatable or have a shoulder finish line (butt
joint). Troedson and Dérand (1999) [27] and Zarone et
al., (2005) [28] reported that it is required to have a
chamfer finish line palatable for the restoration to
tolerate the occlusal stress.
In contrast, Castelnuovo et al., (2000) [14]
suggested that having a chamfer finish line doesn’t
add to the longevity of the restoration.
Additionally, they reported that veneers with
butt-joint finish line could provide more than one path
of insertion (Figure 2). However, having a single path
of insertion can be considered as an advantage
because it prevents any displacement of the veneer
during cementation. Eventually, the study stated that
an overlap preparation with chamfer finish line does
not decrease the longevity and predictability of the
treatment.
Figure 2: Incisal overlap with shoulder finish line (A) provide more
than one path of insertion while incisal overlap with chamfer finish
line (B) provide only one path of insertion (Castelnuovo et al., 2000)
[14]
Ultimately, the biting force of the anterior teeth
is considered to be low (100 – 200 N) (Carlsson 1973)
[29] and with the absence of a strong well-conducted
clinical study, the decision of preparation design is the
clinician preference mainly, while incisal overlap can
always be chosen to re-establish anterior guidance
(Hahn et al., 2000) [30].
Influence of material type on the survival
of dental veneers
A range of materials are available in the
market to restore aesthetic/functional complications by
the mean of veneering teeth; the most common
material is porcelain, resin composite. Each material
type has its unique composition, optical characteristics
and fabrication process. Thus, it can be expected that
the treatment outcome and longevity will differ
according to the material used (Font et al. 2006) [33].
Table 2 shows multiple clinical studies that
investigated the survival rate of dental veneers with a
variety of material types.
Table 2: Clinical studies are illustrating the survival rate of
dental veneers. Adapted from Peumans et al., (2000) [18]
Study
Type of study
Number of
veneers
(number of
patients)
Observation
period
Survival rate
Remarks
Porcelain laminate veneers (PLVs)
(Peumans et
al., 1998)
[43]
Prospective
87 (25
patients)
5 years
93%
-
(Meijering et
al., 1998)
[61]
Prospective
263 (112
patients)
2.5 years
100%
-
(Dumfahrt &
Schäffer
2000) [62]
Retrospective
191 (72
patients)
1 – 10 years
91% in 10
years
Failure increase
when PLVs are
bonded to
dentin
(Magne et
al,. 2000)
[63]
Prospective
48 (16
patients)
4.5 years
100%
-
(Smales &
Etemadi
2003) [48]
Retrospective
110 (50
patients)
Up to 7
years
95%
Compared two
different
preparation
designs as well
(Chen et al.,
2005) [64]
Retrospective
546 ( not
mentioned)
2.5 years
99%
All patients had
tetracycline
staining
(Granell et
al., 2010)
[65]
Prospective
323 (70
patients)
3 – 11 years
87% over 11
years
Failure
increased with
the presence of
composites and
bruxism
(Beier et al.,
2011) [47]
Retrospective
318 (84
patients)
Up to 20
years
94% in 5 y. –
93% in 10 y. -
82% in 20 y.
50% of the
patient were
diagnosed with
bruxism
(Layton &
Walton
2012) [7]
Prospective
499 (155
patients)
Up to 21
years
96% in 10 y.
91% in 20 y.
Bonding to
enamel is a
critical factor for
survival
Resin composites- direct and indirect (DC –IC)
(Peumans et
al., 1997)
[59]
Prospective
87 (23
patients)
5 years
89%
DC-Main failure
due to wear
(Meijering et
al., 1998)
[61]
Prospective
263 (112
patients)
2.5 years
90% for IC -
74% for DC
Results for DC
and IC
(Wolff et al.,
2010) [54]
Retrospective
327 (101
patients)
5 years
79%
Result for DC
(Gresnigt et
al., 2012)
[60]
Prospective
96 (23
patients)
3.4 years
87%
Split mouth
design- no
difference
between
composite type-
all DC
Porcelain veneers
One of the most common materials that are
used to fabricate laminate veneers is feldspathic
porcelain (Figure 3).

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The main component of feldspathic porcelain
is feldspar; a naturally occurring glass which contains
silicon oxide, aluminium oxide, potassium oxide and
sodium oxide (Layton & Walton 2012) [7]. Feldspathic
porcelain has many advantages; the material is very
thin so it can be almost translucent which result in an
appearing natural restoration. Also, it requires minimal
tooth preparation. Therefore enamel can be
preserved. Moreover, it is possible to etch feldspathic
porcelain with hydrofluoric acid which gives a great
bonding strength to the remaining enamel (Calamia
1982, Nicholls 1988, Stacey 1993, Layton & Walton
2012) [7], [12], [34], [35]. Nevertheless, feldspathic
porcelain has some disadvantages. The fabrication of
feldspathic porcelain can be done by two methods:
the refractory die technique and the platinum foil
technique (Horn 1983, Plant & Thomas 1987, Clyde &
Gilmour 1988) [13], [15], [36], these methods are
technique sensitive and the fabricated veneer requires
good care prior to bonding (Layton & Walton 2012)
[7]. Additionally, masking heavy discoloured teeth can
be difficult because the porcelain is very thin.
Moreover, it was reported that etching the inner
surface of the porcelain can cause micro-cracks which
can lead to decrease the flexural strength of the
porcelain and eventually fracture the veneer (Yen et
al., 1993) [37].
Figure 3: A case showing before and after the treatment with
porcelain veneers (Nalbandian & Millar 2009) [38]
New ceramic systems have been developed
recently such as IPS e.max press from Ivoclar
Vivadent ©, leucite is added to the glass matrix in order
to increase the strength of the ceramic (Rasetto et al.,
2001) [39], however, such new systems lack well-
conducted clinical studies that investigate the success
of using them as laminate veneers. Thus, future
studies in this field are required.
Adhesion complex
The adhesion complex between porcelain,
luting composite and enamel is considered to be a
great advantage of porcelain veneers. It has been
reported that the bonding strength of that complex is
around 63 MPa while the bond between composite
and enamel is about 31 MPa and between composite
and porcelain alone is 33 MPa (Stacey 1993) [35].
Also, some in vitro studies suggest that extracted
teeth that are restored with porcelain veneers have
regained their original strength (Andreasen et al.,
1992, Stokes & Hood 1993) [40], [41]. This can
explain the low failure rate (0 – 5%) in clinical studies
due to debonding of the porcelain veneer especially
when parafunctional habits are missing, (Rucker et al.
1990, Kihn & Barnes 1998, Peumans et al., 1998)
[42], [43], [44]. Respectively, some authors reported
that when porcelain veneers are bonded to composite
rather than enamel, porcelain veneers tend to have a
higher failure rate (Dunne & Millar 1993, Shaini et al.,
1997) [45], [46].
Longevity of porcelain veneers
Many studies investigated the longevity of
porcelain veneers. Beier et al., (2011) [47] reported in
a retrospective clinical study a survival rate of 94.4%
after five years and 93.5% after ten years; they found
the main reason for failure is a ceramic fracture. A
randomised clinical trial done by Layton and Walton
(2012) [7] showed similar results, with a survival rate
of 96% after ten years and 91% after 20 years. Also,
Smales and Etemadi (2003) [48] reported a survival
rate of 95% for porcelain veneers throughout 7 years.
It is essential to stress that these studies and others
that reported high survival rate of porcelain veneers
had a strict assessment of remaining enamel and
bonding systems. As a result, careful, conservative
preparation and optimum isolation during cementation
are required to ensure predictable outcomes.
There are other studies which reported a
lower survival rate for porcelain veneers. A
retrospective study of 2,563 veneers in 1,177 patients
done by Burke and Lucarotti (2009) [49] reported a
survival rate of 53% over 10 years. The material type
of the veneers was not reported. Moreover, the study
evaluated veneers that were done by the general
dental service, and thus, it is possible that
preparations of teeth did not meet the criteria of
specialists’ level. Another retrospective study was
done by Shaini et al., (1997) [46] reported a survival
rate of 47% in 7 years. The veneers were done by
undergraduate students and staff member at
Birmingham University in the United Kingdom. The
study reported that over 90% of veneers were placed
on unprepared teeth, this can be a reason for high
failure rate as it is suggested that the bond to
aprismatic enamel is much weaker than prepared
enamel (Perdigão & Geraldeli 2003, Layton & Walton
2012) [7], [50].
The high survival rates that are reported by
well-designed clinical studies suggest that feldspathic
porcelain can act as a reliable and effective material
to restore anterior teeth.
Resin composite
Resin-based composites are restorative
materials that have mainly the following three
compositions: 1) resin matrix 2) inorganic filler 3)
coupling agent. The most commonly used monomer in
the resin is Bis-GMA which has a higher molecular

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weight than methyl methacrylate resins. Therefore,
the polymerisation shrinkage of Bis-GMA (7.5%) is
significantly less than that of methyl methacrylate
resins (22%). Wide range of fillers such as quartz
have been added to composites through the years,
the addition of fillers offers many advantages like: 1)
reduction of the polymerisation shrinkage 2) reduction
of coefficient thermal expansion of the monomer 3)
improve mechanical characteristics 4) some metallic
fillers such as barium provide better radiopacity. The
bonding between the resin and the filler is achieved by
the use of coupling agents i.e. salines, the most
commonly one that is used in resin composite is γ-
MPTS. Dental composites can be categorised
according to the particle size of the filler traditional
composites have a mean particle size of 10-20 µm, on
the other hand, micro filled composites have a mean
particle size of 0.02 µm. new generations of
composites are introduced by the dental company
through the years, aiming for better aesthetic and
physical properties (Bonsor & Pearson 2012, Van
Noort 2013) [51], [52].
It was thought once that composites in the
anterior area would be replaced with porcelain
veneers due to their success (Garber 1989) [53].
However, the aesthetic and physical properties of
resin composite have improved remarkably lately.
Thus, it has been used extensively in clinical practice
(Wolff et al. 2010) [54]. The main advantage of
composite veneer is that it can be used directly,
resulting in less chair time with good initial aesthetic.
Nonetheless, composite veneers are more prone to
discolouration and wear (Wakiaga et al. 2004) [55].
Additionally, the clinician skill in placing, finishing and
polishing the composite plays a major factor in the
aesthetic outcome.
Composite veneers do not require heavy
preparations. Therefore enamel can be preserved for
good adhesion. It is documented that the bonding
strength between etching porcelain and enamel is
greater than resin composite and enamel (Lacy et al.,
1988, Nicholls 1988, Lu et al., 1992) [34], [56], [57].
Correspondingly, it has been reported that composite
veneers do not significantly restore the stiffness of the
prepared tooth (Reeh & Ross 1994) [58]. Although
composite veneers can be made indirectly in dental
laboratories, the used composite is essentially the
same one that is applied directly. Thus, it shares the
same physical properties and limitations of direct
composite restorations such as polymerisation
shrinkage (Van Noort 2013) [52].
Longevity of composite veneers
The survival rate of composite veneers in
many clinical studies is constant. Peumans et al.
(1997) [59] placed 87 direct composite veneers for 23
patients; they reported a survival rate of 89% after 5
years. Wolff et al., (2010) [54] did a retrospective
study on 327 direct composite veneers for 101
patients; the estimated survival rate was 80% after 5
years. A recent randomised control trial to compare
two different types of composites reported a survival
rate of 87% in over 3 years (Gresnigt et al., 2012)
[60]. The use of resin composite to veneer the anterior
teeth is justifiable; it is a fast procedure with the good
aesthetic outcome and reasonable longevity (Figure
4).
Figure 4: A case showing before and after treatment with direct
composite veneers (Nalbandian & Millar 2009) [38]
Patients’ satisfaction
Generally, aesthetic satisfaction is a complex
process as it is considered subjective [38], [61].
However, some factors may play an important role in
patients’ satisfaction such as the durability of the final
aesthetic outcome, the required amount of teeth
preparation for the material type and the cost of the
treatment.
Many clinical studies that evaluated the
longevity of porcelain veneers have also considered
patients’ satisfaction of the treatment, the range of
satisfaction in these studies is 80-100 % [43], [44],
[46]. Other studies have been conducted to evaluate
patients’ satisfaction with different material types for
veneers. Meijering et al., (1997) [67] compared
patients’ response to three different types of veneers
restorations after two years: feldspathic porcelain,
direct composite and indirect composite. Porcelain
veneers had the best response from patients (93%)
followed by indirect composite veneers (82%) and
lastly direct composite veneers (67%). In contrast,
Nalbandian and Millar (2009) [38] found no statistical
difference between patients’ response to composite
veneers and porcelain veneers. These two studies
might be subjected to bias, the degree of preoperative
discolouration or malposition can affect the grade of
transformation postoperatively, and thus, affect the
response of the patient.
From the result of the previous studies, it can
be concluded that porcelain veneers can provide a
predictable aesthetic acceptance, while composite
veneers can be the treatment of choice for patients
who appreciate minimally invasive approaches.

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Conclusion
The influence of preparation design and
material type on the success of dental veneers is
controversial. Usually, the clinician preference decides
the preparation geometry. Nevertheless, veneers with
incisal coverage seem to have better aesthetic and
more predictable outcomes, while having a chamfer
finish line palatable seems to be unnecessary and
limiting the preparation to a butt-join finish line is more
sensible. According to multiple clinical studies,
porcelain veneers have excellent aesthetic results, the
longevity of the treatment and patient’s satisfaction;
the most critical factors to ensure a successful
treatment are to obtain bonding to enamel and
absence of parafunctional habits. Respectively,
composite veneers provide good aesthetic outcome
and patient’s satisfaction; however, due to its physical
properties and to the bonding strength when
compared to porcelain veneers, composite veneers
tend to fail significantly faster than porcelain veneers.
Further clinical trials are needed to evaluate different
types of composites and new ceramic systems for
longer observation time.
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