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Original Contributions
Incisal preparation design for ceramic veneers
A critical review
Sy Yin Chai, BDSc; Vincent Bennani, DDS, PhD; John M. Aarts, BEd, MHealSci;
Karl Lyons, BDS, MDS, PhD
ABSTRACT
Background. The authors reviewed and identified the evidence for the various incisal preparation
designs for ceramic veneers.
Types of Studies Reviewed. The authors searched MEDLINE with PubMed and Ovid to
identify any articles in the English language related to the topic up through March 2017 using a
combination of key words: “porcelain veneer or ceramic veneer or dental veneer or labial veneer”
AND “preparation,”NOT “composite veneer,”NOT “crown,”NOT “implant,”NOT “fixed partial
denture or bridge or denture,”NOT “porcelain-fused-to-metal,”NOT “marginal gap or fit.”
Results. In vitro studies showed that the palatal chamfer preparation design increases the risk of
developing ceramic fractures. The butt joint preparation design had the least effect on the strength
of the tooth.
Conclusions. Surveys show the 2 most common incisal preparation designs provided are butt joint
and feathered-edge. Clinical studies have identified that incisal ceramic is the most common
location of ceramic fracture. In addition, there is a lack in standardization of the modeling structures
and type of finite element analysis.
Practical Implications. The evidence seems to support the use of butt joint over palatal chamfer
incisal preparation design. Fracture or chipping is the most frequent complication and the risk
increases with time. Incisal ceramic is the most common location of ceramic fracture.
Key Words. Veneer preparation; porcelain laminate; incisal edge; teeth.
JADA 2018:149(1):25-37
https://doi.org/10.1016/j.adaj.2017.08.031
The dental literature has long reported various descriptions of different preparation designs for
ceramic veneers.
1-7
In general, the preparation for ceramic veneers can be divided into buccal
surface preparation (no preparation, minimal preparation, conservative, or conventional
preparation); proximal finish (slice or chamfer margin); incisal preparation (overlap or nonoverlap);
and cervical preparation (chamfer or knife edge).
8-10
Although the incisal preparation design for ceramic veneers has been widely discussed, there is no
consensus on whether incisal reduction is necessary and how much of the incisal overlap should be
provided when an increase in incisal length is not required.
10-14
Not only that but the amount of
incisal reduction varies widely from 0.5 millimeter
1
to 2 mm.
13
In retrospect, many recommenda-
tions for the incisal preparation design are likely based on either clinical experience or anecdotal
reports.
1-7,15-19
Incisal preparation can be divided into 2 broad categories: overlap and nonoverlap. Four common
incisal preparation designs that have been described are the window (or intraenamel), the feathered
edge, the palatal chamfer (or overlapped), and the butt joint (or incisal bevel) (Figure 1). The
window and the feathered-edge preparation designs belong to the nonoverlap category, and the butt
joint and the palatal chamfer designs belong to the overlap category.
4,14
The demand for ceramic veneers has increased drastically in both general and specialist dental
practice from an increase in esthetically driven patients, and from veneers’clinical success and
conservative nature.
12,20
The evolution of bonding systems, ceramic materials, and fabrication
methods, particularly pressed and computer-aided design and computer-aided-manufacturing
(CAD/CAM) technology, have changed the way we approach these restorations.
Copyright ª2018
American Dental
Association. All rights
reserved.
JADA 149(1) nhttp://jada.ada.org nJanuary 2018 25
The aim of this critical review of the literature on the various incisal preparation designs for
ceramic veneers was to summarize the evidence for incisal preparation designs of ceramic veneers,
based on clinical trials and laboratory studies published in the peer-reviewed literature. Studies on
maxillary anterior teeth were considered only in terms of differences in biomechanics between
maxillary and mandibular teeth. Early reports on ceramic veneers were included to provide an
understanding of the evolution of preparation designs for these restorations.
METHODS
We adapted the review methodology outlined in the Preferred Reporting Items for Systematic
Reviews and Meta-Analyses statement’s item checklist and flowchart.
21
Search strategy
We conducted a comprehensive literature search for studies on ceramic veneers and incisal prep-
aration designs. We searched MEDLINE (PubMed) and Ovid databases from 1980 up through
ABCD
Figure 1. The window (A), feathered-edge (B), palatal chamfer (C), and butt joint incisal (D) preparation designs.
Key words search
"porcelain veneer or ceramic veneer or
dental veneer or labial veneer,"
"preparation," NOT "composite veneer,"
NOT "crown," NOT "implant," NOT "xed
partial denture or bridge or denture,"
NOT "porcelain-fused-to-metal," NOT
"marginal gap or t"
Abstract screening process
Abstracts screened after removal of duplicates
n = 322
Full-text screening process
Full-text articles screened to identify
potentially relevant studies
n = 182
Studies included in review
n = 40
Studies excluded after
reading title and abstract
n = 140
Full-text article studies
excluded
n = 142
Initial result from key words search
n = 342
Initial screening process
Figure 2. Search strategy diagram.
ABBREVIATION KEY
BJ: Butt joint.
C: Edge chipping cracks.
CAD/
CAM:
Computer-aided design
and computer-aided
manufacturing.
FE: Feathered edge.
I: Asymmetric inner cone
crack.
M: Median crack.
O: Asymmetric outer cone
crack.
P: Partial cone crack.
PC: Palatal chamfer.
PS: Prospective study.
R: Radial cracks at
cementation surface.
RS: Retrospective study.
26 JADA 149(1) nhttp://jada.ada.org nJanuary 2018
March 2017 following a strategy (Figure 2) similar to that described by Moher and colleagues
21
using a combination of key words.
Study selection criteria
We selected the studies according to multiple inclusion and exclusion criteria (Box 1); such limits
included full-text English-language articles only.
Review methods and categories. We removed duplicates from the search. We checked the titles
and abstracts for relevance. We identified 40 studies that addressed the aim of our critical review. Of
the 40 studies included, 13 were anecdotal reports, 3 were surveys, 8 were in vivo studies, and 16
were in vitro studies.
RESULTS
When ceramic veneers were popularized in the 1980s, various anecdotal reports on preparation
designs of ceramic veneers were published (Box 2).
1,3-7,15-19,22,23
Early in vivo studies were
mainly short-term, and the type and amount of incisal preparations were not well described.
4,24-29
It is beyond the scope of this critical review to discuss the findings of all in vivo studies. The
focus of this critical review, therefore, is to review the in vivo studies that have compared 2 or
more incisal preparation designs (Table 1).
30-37
In general, in vitro studies have focussed on 2
main areas: fracture strength (Table 2)
38-49
and stress distribution (Table 3).
38,49-54
Stress ana-
lyses of ceramic veneers has been conducted via photoelastic analysis
38,50
and finite element
analysis.
49,51-54
Our review results are organized according to the different types of incisal preparation designs and
the supporting evidence for each preparation design.
Box 1. Inclusion and exclusion selection criteria.
INCLUSION CRITERIA
nOriginal articles written in the English language
nFull-text articles only
nArticles published in peer-reviewed journals
nAnecdotal reports on incisal preparation design
nSurveys and reviews
nIn vitro and in vivo studies comparing 2 or more incisal preparation designs
nRandomized controlled trials comparing 2 or more incisal preparation designs
nMeta-analysis or systematic reviews on incisal preparation designs
nStudies on veneers on maxillary anterior teeth only
EXCLUSION CRITERIA
nNo clear description of incisal preparation design
nAcrylic or composite veneers
Box 2. Anecdotal reports or reviews supporting a particular incisal
preparation design.
nWindow preparation: Ben-Amar,
5
1989
nFeathered-edge preparation: Boksman and colleagues,
3
1985; Garber,
6,7
1991, 1993
nPalatal chamfer preparation: Sheets and Taniguchi,
23
1990; Garber,
6,7
1991, 1993
nButt joint preparation: Quinn and colleagues,
15
1986; Calamia,
1,22
1988, 1985; Clyde
and Gilmour,
4
1988; Weinberg,
16
1989; Christensen,
17,18
1991, 1999; Gilmour and
Stone,
19
1993
JADA 149(1) nhttp://jada.ada.org nJanuary 2018 27
Nonoverlap incisal preparation design
Window preparation. Ben-Amar
5
suggested the use of window incisal preparation design as this
will result in acceptable thickness of ceramic of 0.4 to 0.7 mm near the incisal edge, decrease the risk
of experiencing porcelain fracture and wear of opposing teeth, and will not interfere with incisal
guidance. However, it had not been widely adopted
55
for various reasons such as the difficulty in
masking the ceramic finish line,
14
and the risk of experiencing chipping of the unsupported enamel
on the incisal edges.
4
An in vitro study by Hui and colleagues
38
showed the least stresses were found in ceramic veneers
with window incisal preparation design, followed by feathered-edge and overlapped (or palatal
chamfer) design. The authors also demonstrated strong correlation between the photoelastic anal-
yses with the load-to-failure testing. In addition, a finite element analysis by Seymour and col-
leagues
54
reported lower maximum stress at the labial margin porcelain and labial composite lute for
veneers with window incisal preparation design compared with palatal chamfer preparation when
the veneers had either the chamfer or shoulder margin preparation.
Feathered-edge preparation. Boksman and colleagues
3
and Garber
6,7
recommended the
feathered-edge incisal preparation design. Reduction of the unsupported incisal edge is only required
if the remaining incisal enamel is too thin. This nonoverlap incisal preparation design has been
recommended for patients with normal overbite
3,26
and to avoid direct contact of ceramic veneers
with their antagonistic tooth structure.
26
On the other hand, other authors have suggested that the
feathered-edge incisal preparation design may result in a weak veneer, high risk of experiencing
ceramic chipping, and difficulty with seating of the veneers.
4,19
Other problems reported also
include marginal discoloration and poor marginal adaptation.
24
Walls and colleagues
14
also sug-
gested that the ceramic veneers with feathered-edge preparation may be subjected to peel and shear
forces during protrusive guidance.
An in vitro study by Bergoli and colleagues
49
showed that ceramic veneers with a feathered-edge
preparation design had significantly higher fracture load compared with a palatal chamfer prepa-
ration design. Bergoli and colleagues
49
confirmed the results with the finite element analysis, which
Table 1. Clinical studies of ceramic veneers involving at least 2 types of incisal preparation designs.
STUDIES TYPE OF STUDY COMPARISON TYPE OF INCISAL PREPARATION FOLLOW-UP PERIOD
Meijering and Colleagues,
30
1998 RS
†
Overlap versus nonoverlap Not stated 2.5 years
Dumfahrt and Schäffer,
31
2000 RS Overlap versus nonoverlap
‡
FE and
§
PC 2-10.5 years
Smales and Etemadi,
32
2004 RS Overlap versus nonoverlap Not stated 5-7 years
Cötert and Colleagues,
33
2009 RS Overlap BJ
{
and PC 3 months-1.5 years
Granell-Ruiz and Colleagues,
34
2010 RS Overlap versus nonoverlap FE and PC 3-11 years
Beier and Colleagues,
35
2012 RS Overlap versus nonoverlap FE and PC 2-20 years
Gurel and Colleagues,
36
2013 RS Overlap versus nonoverlap Not stated Up to 12 years
Guess and Colleagues,
37
2014 PS
#
Overlap BJ and PC 7 years
*Assessmentof risk of bias using Cochrane Collaboration’s tool
85
;†RS:Retrospective study; ‡FE:Feathered edge; §PC:Palatal chamfer; {BJ:Butt joint; #PS:Prospective study.
28 JADA 149(1) nhttp://jada.ada.org nJanuary 2018
also demonstrated that veneer with feathered-edge incisal preparation design generated less tensile
stress values in the ceramic compared with palatal chamfer design.
Clinical studies by Meijering and colleagues,
30
Granell-Ruiz and colleagues,
34
Beier and col-
leagues,
35
and Gurel and colleagues
36
showed slight advantage in survival rate for nonoverlap
incisal design compared with overlap incisal design, although Smales and Etemadi
32
reported the
opposite. The results from these clinical studies failed to show statistically significant difference in
failure and complication rates of ceramic veneers with different incisal preparation designs.
30-32,34-36
Dumfahrt and Schäffer
31
found no difference in survival rate of ceramic veneers between non-
overlap and overlap groups. A 2016 meta-analysis and systematic review by Albanesi and col-
leagues
56
concluded that there is a lack of evidence to show the effect of incisal preparation on
clinical survival of ceramic veneers.
Overlap incisal preparation designs
Butt joint preparation. Most of the early anecdotal reports advocated the incisal bevel preparation
(butt joint) with 0.5- to 1-mm incisal reduction.
1,4,15-19,23
The advantages of incisal overlap include
masking of the otherwise noticeable incisal finish line, thicker ceramic and reinforcement of incisal
edge, and positive seating of ceramic veneers.
1
Calamia
1
cited the incisal overlap design (butt joint)
as the primary reason for the low fracture rates observed. Calamia
1
proposed that restorations with
extended coverage should be considered in cases of anterior crossbite or deep overbite (Class II
Division 2 malocclusion), and veneers should be placed on both maxillary and mandibular arches
for patients with edge-to-edge occlusion. For different reasons, Seebach
56
suggested that the incisal
edges should be overlapped to allow translucency of incisal edges and a more natural appearance.
57
An in vitro study by Castelnuovo and colleagues
40
showed that the feathered-edge and the butt
joint groups had the greatest fracture resistance, comparable with the control teeth. The authors
further elaborated that the butt joint incisal preparation design was favorable compared with the
palatal chamfer preparation design for a number of reasons which included its simpler preparation,
faciopalatal path of insertion, increased fracture strength, low risk of developing a fracture of thin
unsupported palatal ceramic ledges, improved esthetics at incisal one-third of veneers, favorable
bonding to exposed enamel prisms, easier impression, and easier identification of the finish line
on the model.
40
The results of that study are also supported by Stappert and colleagues.
41
A
meta-analysis of in vitro studies by da Costa and colleagues
58
concluded that although there was no
Table 1. (Continued)
NUMBER OF VENEERS SETTING SURVIVAL RATE RISK OF BIAS*COMMENTS
180 Mixed private and
university
98% (nonoverlap);
95% (overlap)
Selection bias
Performance bias
Detection bias
Attrition bias
Reporting bias
Biased allocation to interventions; form of preparation and type
of restoration determined by operator
No blinding of participants and operators
No blinding of outcome assessment
Incomplete outcome data
Amount, nature, and handling of attrition or exclusion not described
Selective outcome reporting
191 (54 FE; 137 PC) University 91% (all veneers) As above As above
110 (46 overlap;
64 nonoverlap)
Private specialist
practice
95.8% (overlap);
85.5% (non-overlap)
As above As above
200 University 97.8% (PC)
84.7% (BJ)
As above As above
323 (124 FE; 199 PC) University 94% (FE)
84.7% (PC)
As above As above
292 (245 overlap;
47 nonoverlap)
University 100% (nonoverlap);
82.5% (overlap)
As above As above
580 (261 overlap;
319 nonoverlap)
General practice 94.4% (nonoverlap);
90.8% (overlap)
As above As above
66 (42 BJ; 24 PC) University 97.6% (BJ); 100% (PC) Selection bias
Performance bias
Attrition bias
Form of preparation and type of restoration determined by operator
No blinding of participants and operators
High patient drop-out rate resulting in incomplete outcome data
Handling of attrition not described
JADA 149(1) nhttp://jada.ada.org nJanuary 2018 29
statistical difference in ceramic fractures between a butt joint and a palatal chamfer preparation
design, the butt joint design had the least effect on the strength of the tooth.
A photoelastic analysis comparing the butt joint incisal preparation design with the feathered-
edge incisal preparation design demonstrated that the butt joint preparation design exhibited
more favorable stress distribution for the 4 loading conditions compared with the feathered-edge
design.
50
Magne and Douglas,
51
through finite element analysis, showed that a long palatal
chamfer margin places the area of maximum stress concentration on the thin unsupported ceramic
at the palatal concavity of maxillary anterior teeth. They recommended a butt joint incisal prep-
aration design with a slight bevel.
51
Guess and colleagues
37
showed a slightly better survival rate for veneers with butt joint incisal
preparation design compared with the palatal chamfer design in a prospective clinical study whereas
Cötert and colleagues
33
reported the opposite results. The retrospective study by Cötert and
Table 2. Laboratory tests on fracture strength and frequency of failure of ceramic veneers with different incisal
preparation designs.
STUDY
TYPE OF INCISAL PREPARATION
AND AMOUNT OF REDUCTION VENEER MATERIALS
LOADING TESTS (CYCLIC
LOADING VERSUS STATIC
LOAD-TO-FAILURE)
Hui and
Colleagues,
38
1991
Window
Feathered edge
Butt joint (not specified)
Feldspathic porcelain
(Vitadur, VITA)
Static load-to-failure
Wall and
Colleagues,
39
1992
Feathered edge
Butt joint (0.5-millimeter,
1-mm, 2-mm incisal reduction)
Feldspathic porcelain
(Optec VP, Jeneric/Penton)
Static load-to-failure
Castelnuovo and
Colleagues,
40
2000
Feathered edge
Butt joint (2-mm incisal reduction)
Palatal chamfer (1-mm,
4-mm incisal reduction)
Leucite-reinforced ceramic
(IPS Empress, Ivoclar
Vivadent)
Static load-to-failure
Stappert and
Colleagues,
41
2005
Window
Butt joint (2-mm incisal reduction)
Palatal chamfer (2-mm,
3-mm incisal reduction)
Leucite-reinforced ceramic
(IPS Empress I, Ivoclar
Vivadent)
Wet fatigue test; surviving
specimens then loaded
to fracture
Zarone and
Colleagues,
42
2006
Window
Palatal chamfer (2-mm incisal
reduction)
Feldspathic porcelain
(IPS d.Sign [low-fusing],
Ivoclar Vivadent)
Static load-to-failure
Chun and
Colleagues,
43
2010
Palatal chamfer (1-mm incisal
reduction)
Palatal chamfer (1-mm incisal
reduction) with incorporatin of
interproximal restorations
Leucite-reinforced ceramic
(IPS Empress)
Static load-to-failure
Chaiyabutr and
Colleagues,
44
2009
Butt joint (4-mm incisal reduction)
Palatal chamfer (4-mm incisal
reduction)
Leucite-reinforced ceramic
(IPS Empress)
Wet fatigue test
D’Arcangelo and
Colleagues,
45
2010
Butt joint (2-mm incisal reduction) Feldspathic porcelain
(Omega 900, VITA
Zahnfabrik)
Static load-to-failure
Ako
glu and
Gemalmaz,
46
2011
Butt joint (2-mm, 4-mm incisal
reduction)
Leucite-reinforced ceramic
(IPS Empress)
Static load-to-failure
Schmidt and
Colleagues,
47
2011
Butt joint (2-mm incisal reduction)
Palatal chamfer (2-mm incisal
reduction)
Leucite-reinforced ceramic
(IPS Empress)
Static load-to-failure
Jankar and
Colleagues,
48
2014
Feathered edge
Butt joint (1-mm incisal reduction)
Palatal chamfer (1-mm incisal
reduction)
Feldspathic porcelain
(Vitadur-alpha, VITA)
Static load-to-failure
Bergoli and
Colleagues,
49
2014
Feathered edge
Palatal chamfer (2-mm incisal
reduction)
Lithium disilicate ceramic
(IPS e.max Press, Ivoclar
Vivadent)
Wet fatigue, surviving
specimens loaded to
fracture
30 JADA 149(1) nhttp://jada.ada.org nJanuary 2018
colleagues
33
reported on 200 ceramic veneers, but their follow-up was up to 1.5 years. Guess and
colleagues’
37
prospective study of up to 7 years suffered from low sample size and high patient
dropout rate.
Surveys related to the incisal preparation designs of ceramic veneers show a wide disparity in the
incisal preparation designs provided by clinicians.
17,55,59
Christensen
17
conducted a clinical survey
involving 200 clinicians on their routinely provided incisal preparation designs for ceramic veneers.
He reported that 78% of clinicians provided incisal coverage most of the time, whereas only 22%
provided incisal coverage consistently. A survey of laboratory models from United Kingdom general
practitioners showed that the incisal bevel (or butt joint) preparation design was the most common
incisal preparation design provided (36%), followed closely by the feathered-edge incisal prepara-
tion design (34%).
55
Hooper and colleagues
59
reported that the 2 most commonly taught incisal
preparation designs by the 12 dental schools in the United Kingdom are the butt joint and the
feathered-edge preparation design.
Table 2. (Continued)
ABUTMENT TOOTH
(EXTRACTED OR
TYPODONT TEETH)
LOADING ANGLE
AND LOCATION
TYPE OF PREPARATION
WITH HIGHEST FRACTURE
RESISTANCE
Extracted teeth (not
specified) and acrylic
replicas
Along long axis of tooth;
at incisal edge
Window
Typodont teeth 130and 137to long axis
of tooth; at incisal edge
No significant
difference
Extracted maxillary
central incisors
90to lingual surface of tooth;
2.5 mm from incisal edge
Butt joint and
feathered edge
Extracted maxillary
central incisors
Dynamic load at 135to long
axis of tooth; 2.5 mm from
incisal edge
Static load long axis of tooth;
at incisal edge
No significant difference
Extracted maxillary
central incisors
90to lingual surface of
tooth; 2 mm from incisal edge
No significant difference
Extracted maxillary
central incisors
140to long axis of tooth;
at incisal edge
No significant difference
Extracted maxillary
central incisors
135to long axis of tooth;
at incisal edge
Palatal chamfer
Extracted maxillary
central incisors
135to long axis of tooth;
between middle and cervical
thirds of crown
Only 1 preparation design
Extracted maxillary
central incisors
90to lingual surface of
tooth; 1 mm from incisal edge
Only 1 preparation design
Extracted maxillary
central incisors
90to lingual surface of tooth;
1 mm from incisal edge
Palatal chamfer
Extracted maxillary
central incisors
135to long axis of tooth;
2.5 mm from incisal edge
Palatal chamfer
Extracted maxillary
central incisors and
canines
135to long axis of tooth;
at incisal edge
No significant difference
JADA 149(1) nhttp://jada.ada.org nJanuary 2018 31
Palatal chamfer preparation. Garber
6,7
advocated the palatal chamfer preparation design if the
incisal edges are thin buccolingually or when an increase in crown length is desired. Garber pro-
posed that the palatal chamfer design increases the surface area for bonding and avoids a sharp angle
which may propagate cracks. Sheets and Taniguchi
22
believed that the palatal chamfer design
provides adequate ceramic thickness at the incisal edge.
Schmidt and colleagues
47
reported that palatal chamfer incisal preparation group had signifi-
cantly higher failure load than the butt joint incisal preparation group in both nonworn and worn
tooth samples. Jankar and colleagues
48
reported the highest fracture load with palatal chamfer
incisal preparation design, followed by butt joint and feathered-edge preparation design. The results
of these studies are also supported by Chaiyabutr and colleagues.
44
However, the meta-analysis by da
Costa and colleagues
58
concluded that a palatal chamfer incisal preparation design increased the
risk of developing ceramic fractures.
Afinite element analysis by Zarone and colleagues
52
comparing the window and the palatal
chamfer incisal preparation designs showed that ceramic veneer with palatal chamfer incisal
preparation design had the highest stress tolerance under functional loading, and the incisal
preparation helped distribute the stress throughout the surface of preparation without overloading
the incisal edge. Li and colleagues
53
reported similar results when comparing ceramic veneers with
butt joint and palatal chamfer incisal preparation designs.
Amount of incisal preparation. An incisal reduction of 0.5 mm with slight overlap was recom-
mended by Calamia.
1
Others have recommended 0.5 mm to 1 mm,
4,15,16
1.5 mm,
7
and 1.5 to 2
mm.
13
More than 2-mm incisal reduction is rarely required for material thickness or esthetic rea-
sons.
8
Friedman
25
suggested that the incisal extension of ceramic veneer should be limited to
0.5 mm or less. If incisal reduction is more than 1 mm, the ceramic is not supported by tooth
structure and is at risk of experiencing fracture. Other authors recommend placement of palatal
finish lines according to the morphologic and functional requirements.
15,26,60,61
Placement of
palatal finish lines at centric stops increases the risk of experiencing incisal edge chipping.
60
Other
authors concur with the former in which contact relationships between incisors and canines in
centric occlusion and during excursive movements were found to affect the fracture resistance of the
restoration.
15,60,61
The placement of a ceramic finish line also depends on the location of the tooth
(whether maxillary or mandibular) and the type of restoration on the antagonist.
61
Wall and colleagues
39
evaluated the influence of ceramic thickness on the fracture strength of
ceramic veneers in an in vitro study, finding no significant difference in the strength of ceramic
veneers with different thicknesses between 0 mm to 2 mm. Their study also highlighted that the
loading angulation affected the fracture strength of ceramic veneers.
39
Table 3. Finite element analysis or photoelastic studies on ceramic veneers with different incisal preparation designs.
STUDY TYPE OF STUDY
TYPE OF INCISAL PREPARATION
AND AMOUNT OF INCISAL REDUCTION
TYPE OF INCISAL PREPARATION
WITH MOST FAVORABLE
STRESS DISTRIBUTION
Highton and Colleagues,
50
1987 Photoelastic Feathered edge
Butt joint (1.25-millimeter incisal reduction)
Butt joint
Hui and Colleagues,
38
1991 Photoelastic Window
Feathered edge
Butt joint (not specified)
Window
Magne and Douglas,
51
1999 2-dimensional finite
element analysis
Feathered edge
Butt joint (1.5-mm incisal reduction)
Short palatal chamfer (1.5-mm incisal reduction)
Long palatal chamfer (margin at palatal concavity)
Butt joint and short palatal chamfer
Seymour and Colleagues,
54
2-dimensional finite
element analysis
Window
Palatal chamfer (not specified)
Window
Zarone and Colleagues,
52
2005 3-dimensional finite
element analysis
Window
Palatal chamfer (not specified)
Palatal chamfer
Li and Colleagues,
53
2014 3-dimensional finite
element analysis
Butt joint (1-mm incisal reduction)
Palatal chamfer (1-mm incisal reduction)
Palatal chamfer
Bergoli and Colleagues,
49
2014 2-dimensional finite
element analysis
Feathered edge
Palatal chamfer (2-mm incisal reduction)
Feathered edge
32 JADA 149(1) nhttp://jada.ada.org nJanuary 2018
DISCUSSION
Feldspathic porcelain was the single most prevalent material used for ceramic veneers in the
1980s because of its high translucency and ability to provide the “contact lens effect.”
62
All of
the anecdotal reports and early clinical studies used feldspathic porcelain, fabricated with
platinum foil or a refractory die technique.
1,3-7,15-19,23
Pressed leuciteereinforced or lithium
disilicate dental ceramics were not introduced until the 1990s and the CAD/CAM system for
these in the 2000s.
63,64
Most of these studies were published before these dental ceramics were
available.
1,3-7,15-19,23
Nordbø and colleagues
24
commented that feldspathic porcelain is brittle and
is the “weakest link in the chain.”However, with better bonding systems, stronger dental ce-
ramics, and understanding of biomimetics,
9
ceramic material is no longer the limitation of
ceramic veneers. The anecdotal reports or reviews have limited scientific evidence, and should
therefore be interpreted with caution.
The systematic review and meta-analysis of clinical studies comparing the incisal preparation
designs of ceramic veneers did not show any statistically significant difference between the incisal
coverage and nonincisal coverage groups with regard to their survival rates.
56
Some authors proposed
that the type of incisal reduction (overlap or nonoverlap) depends on the buccolingual width of the
incisal edge, esthetic requirements, and the patient’s occlusion.
7,14,26,31
This may explain the het-
erogeneity of incisal preparation designs of ceramic veneers in clinical practice. Most of the clinical
studies were short to medium term, and were retrospective in nature.
30-33,35,36,65
Clinical evidence
does not show superiority of 1 incisal preparation design over the other. Randomized longitudinal
controlled trials with large study samples are required to show any difference in clinical survival rates
and complication rates of ceramic veneers with different incisal preparation designs.
28
This has
proved to be a challenge in most prosthodontic studies with the difficulty of standardizing patients
(age, health, diet), occlusion, the dynamicity of intraoral environment, quality of bonding, bonded
tooth substrate (enamel versus dentin), tooth vitality, and conducting a long-term clinical trial.
Morimoto and colleagues
66
reported high overall cumulative survival rates for glass ceramic and
feldspathic porcelain veneers (89%) in a median follow-up period of 9 years. The most frequently
reported complications were fracture or chipping (4%), followed by debonding (2%), marginal
discoloration (2%), and endodontic problem (2%). In a 15-year clinical observation of 3,500
ceramic veneers, Friedman
25
found that most clinical failures (more than 67%) were from veneer
fracture. A dramatic increase in the number of fractures from 5 years (4%) to 10 years (34%) was
observed in a prospective 10-year clinical trial by Peumans and colleagues.
67
Clinical studies also
showed that incisal ceramic is the most common location of ceramic fracture.
25,26,28,67
Several in vivo studies have analyzed and related the survival and complications of ceramic
veneers to the preparation design of the veneer and other clinical factors. Nordbø and col-
leagues
26
and Christensen and Christensen
28
reported that the feathered-edge incisal preparation
design was the main reason for ceramic chipping. Peumans and colleagues
67
reported that 21% of
all the ceramic veneers had fracture lines, seen more commonly on the palatal aspect from the
long and thin palatal extension (palatal chamfer) and on the facial and cervical aspect of the
veneers, and 2% showed minor chipping of incisal ceramic. The authors proposed placing the butt
joint incisal preparation to reduce the occurrence of crack lines and fractures on the palatal side.
67
Although there is no direct clinical evidence to show that incisal coverage is the superior design,
many studies have suggested the butt joint preparation design for the aforementioned reasons.
28,67
The meta-analysis of in vitro studies by da Costa and colleagues
58
showed that the butt joint
incisal preparation design may be more favorable compared with the palatal chamfer design in
terms of ceramic fracture and frequency of tooth failure. The meta-analysis highlighted the diffi-
culties in comparing studies because of all the variables, such as veneer preparation designs,
amount of tooth preparations, veneer fabrication techniques, type of ceramic materials, cement
type and thickness, cementation procedures, mounting apparatus, simulation of periodontal liga-
ments, loading angulations and points, dimension and shape of plunger, type of load (static or
cyclic), and thermocycling procedure. Others have also reported the lack of standardization of the
tooth specimens.
40,42,43,47
Someauthorshavealsofailedtoobtainasufficient sample size for
appropriate statistical power analysis.
54
Storage conditions of the natural teeth vary in terms of the
storage medium, length, and temperature
41,45,46
; some storage conditions were unknown.
43
More
in vitro investigations that focus on applying clinically relevant loading conditions and
JADA 149(1) nhttp://jada.ada.org nJanuary 2018 33
standardizing abutment teeth are needed. Use of abutment replicas may be considered to eliminate
the inconsistencies in the quality and dimension of extracted human teeth, and the difficulty in
obtaining a large sample size.
One of the major debates of in vitro studies is the standardization and application of appropriate
testing conditions. Various authors have expressed concerns regarding the load-to-failure testing
methodology because the fracture mode of ceramic restorations in vitro does not correlate to the
clinical failure mechanism.
68-70
Despite its limitations, static load-to-failure testing allows ease of
test standardization, overview of fracture behavior of a tooth restoration complex, comparisons of
material strength, estimation of failure risk, data collection, and comparison among
studies.
40,46,58,68,71,72
Under appropriate conditions, the fracture modes seen in clinically failed
restorations can be reproduced in in vitro studies
68,73
with thermocycling, which best reproduces
the intraoral environment.
70
The difficulty in applying cyclic mechanical loading on ceramic ve-
neers on maxillary teeth has been recognized and further attributed to the sliding of the loading
device along the palatal contour of natural teeth, resulting in fracture or cracks in abutment teeth
rather than ceramic veneers.
40,46
Stappert and colleagues,
41
Chaiyabutr and colleagues,
44
and
Bergoli and colleagues
49
applied fatigue testing to their specimens. Bergoli and colleagues
49
and
Stappert and colleagues
41
reported no veneer fractures during initial cyclic loading simulating 4 and
5 years of clinical service, respectively, whereas the study by Chaiyabutr and colleagues
44
reported
failure of all veneers under 100,000 cycles (equivalent to less than one-half a year of clinical ser-
vice). These contrasting results indicate that fatigue testing for ceramic veneers is highly variable
according to the loading frequency, magnitude, distance and direction, and the quality of sample
teeth. It is noteworthy to mention that the simulation protocols intended to replicate clinical
fractures of ceramic veneers are not standardized.
70
The photoelastic analyses by Highton and colleagues
50
and Hui and colleagues
38
showed different
results. This is due to the fact that both studies compared different incisal preparation designs and
used different photoelastic material, model construction, model dimension, loading angulations, and
magnitude. Photoelastic analysis does not produce fine details or quantitative measurements of
R
R
R
M
P
I
O
C
CIP
M
O
Figure 3. Schematic diagram depicting various fracture modes in ceramic veneers: axisymmetric outer (O) and inner (I)
cone cracks, median (M) cracks, partial (P) cone cracks, edge chipping (C) cracks, and radial (R) cracks at cementation
surface. Source: Adapted from Zhang and colleagues.
79
34 JADA 149(1) nhttp://jada.ada.org nJanuary 2018
stress. The photoelastic material is homogenous and isotropic, unlike anisotropic or orthotropic
structures such as dentine, periodontal attachment apparatus, and bone.
74
Despite some of the
limitations and the development of the finite element method, this method is still widely used in
dentistry as well as in other fields to provide a good understanding of stress distribution.
75-77
The
lack of standardization in finite element analyses and variations in modeling structures and processes
have led to inconsistent results and conclusions in the dental literature. There are no guidelines in
the literature regarding selection of a finite element model that best represents the natural teeth and
the most appropriate experimental design. Magne and Douglas,
51
Seymour and colleagues,
54
and
Bergoli and colleagues
49
considered forces within the buccolingual plane only, whereas others
applied a 3-dimensional finite element analysis.
53,78
Magne and Douglas
51
used the modified von
Mises criterion to evaluate failure, which may be more appropriate for porcelain material owing to its
brittleness. The other studies investigated the maximum principal stress, without taking the relative
compressive versus tensile stresses into account.
52,53
None of the in vitro studies can reproduce
all the variables within the oral cavity. All in vitro studies have different limitations. Two studies in
this critical review have implemented the hybrid method, combining both experimental and nu-
merical methods to evaluate failure of a restoration or prosthesis.
38,49
The finite element analysis by Zarone and colleagues
42
reported that stress concentration
occurred mainly at the incisal edge and cervical region of the tooth whereas Li and colleagues
53
reported stress concentration on the labial cervical third. The results from the finite element ana-
lyses seem to correlate with a study by Zhang and colleagues.
79
A schematic diagram, which was
adapted from Zhang and colleagues,
79
depicts various fracture modes in ceramic veneers (Figure 3).
Outer and inner cone cracks are formed within the contact circle during normal loading whereas
asymmetric partial cone cracks form during a sliding loading.
80
The slow crack growth of inner cone
cracks and partial cone cracks are the result of pumping effect of fluid during multicycle loading.
81
With increasing load or loading cycles, chipping cracks may also extend downward.
82
The use of
small spheres or sharp indenters will result in median cracks. Subsurface radial cracks are a result of
low concentration of tensile stresses at the cementation surface between the ceramic and the un-
derlying core. The tensile stresses then spread sideways and upward along the interface.
79
Various
investigations have been conducted on all-ceramic restorations to understand the mechanism of
crack initiation, crack propagation, cohesive and adhesive fracture, and the fracture modes of the
restorations.
79-81,83,84
To date, there is no fractographic study of clinically failed ceramic veneers in
the literature. To better understand the fracture behavior of ceramic veneers in vivo, fractographic
analysis of clinically failed restorations is necessary. The analysis will also aid in the implementation
of proper loading protocol and strategy in laboratory studies, which will provide more clinically
relevant results.
CONCLUSION
We learned from surveys that the 2 most common incisal preparation designs provided by clinicians
were butt joint and feathered edge. Furthermore, from our literature review we concluded that
anecdotal reports and reviews provide limited scientific evidence regarding the best incisal prepa-
ration designs for ceramic veneers. There was limited clinical evidence to support a particular incisal
preparation design. There was no statistically significant difference in survival rates of ceramic
veneers between the incisal coverage and nonincisal coverage groups. Fracture or chipping was the
most frequent complication and incisal ceramic was the most common location of ceramic fracture.
In addition, butt joint incisal preparation may provide better esthetics and reduce the incidence of
incisal ceramic fracture.
In vitro studies showed that the palatal chamfer preparation design increased the risk of devel-
oping ceramic fractures. The butt joint preparation design had the least effect on the strength of the
tooth. The evidence, therefore, supported the use of butt joint over palatal chamfer incisal prep-
aration design.
Although finite element analysis studies were useful in identifying stress concentration and dis-
tribution, there was a lack in standardization of the modeling structures and type of finite element
analysis (2-dimensional or 3-dimensional).
Authors of finite element analysis studies recommended using a hybrid method, combining both
experimental and numerical methods to evaluate failure of a restoration or prosthesis. n
JADA 149(1) nhttp://jada.ada.org nJanuary 2018 35
Dr. Chai is a postgraduate student, Department of Oral Rehabilitation,
University of Otago School of Dentistry, Dunedin, New Zealand.
Dr. Bennani is an associate professor, Department of Oral Rehabilitation,
Faculty of Dentistry, School of Dentistry, University of Otago, PO Box 647,
Dunedin 9054, New Zealand, e-mail vincent.bennani@otago.ac.nz.
Address correspondence to Dr. Bennani.
Mr. Aarts is a senior lecturer, Department of Oral Rehabilitation,
University of Otago School of Dentistry, Dunedin, New Zealand.
Dr. Lyons is a professor, Department of Oral Rehabilitation, University of
Otago School of Dentistry, Dunedin, New Zealand.
Disclosure. None of the authors reported any disclosures.
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