Open Journal of Stomatology, 2013, 3, 411-418 OJST Published Online November 2013 (
Porcelain fracture of metal-ceramic tooth-supported and
implant-supported restorations: A review
Rola M. Shadid1, Nasrin R. Sadaqah1, Layla Abu-Naba’a2, Wael M. Al-Omari3
1Department of Prosthodontics, Faculty of Dentistry, Arab American University, Jenin, Palestine
2Department of Substitutive Dental Sciences, Faculty of Dentistry, Taibah University, KSA-Formerly Jordan University of Science
and Technology, Irbid, Jordan
3Department of Prosthodontics, Faculty of Dentistry, Jordan University of Science and Technology, Irbid, Jordan
Received 31 May 2013; revised 2 July 2013; accepted 21 October 2013
Copyright © 2013 Rola M. Shadid et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Metal-ceramic restorations are widely used in den-
tistry with a high degree of general success. However,
fracture of these restorations does occur and usually
frustrates both the dentist and the patient. Objective:
This literature review discusses the factors that may
lead to the fracture of these restorations whether they
are tooth-supported or implant-supported with the
aim of making dentists and technicians aware of these
factors to avoid them. Factors reviewed include: tech-
nical factors, dentist-related factors, inherent mate-
rial properties, direction, magnitude and frequency of
applied loads, environmental factors, screw-retained
implant-supported restorations, and posterior canti-
levered prostheses. Material and Methods: A net-
based search in “Pubmed” was performed and com-
bined with a manual search. The search was limited
to articles written in English. Conclusions: the pub-
lished literature revealed that the factors predispos-
ing to fracture of metal-ceramic restorations may be
related to the technician, dentist, patient, environ-
ment, design of the restoration, or to inherent struc-
ture of ceramics and others. However, if the dentist
and technician understand these factors and respect
the physical characteristics of the materials, most of
those are avoidable.
Keywords: Metal-Ceramic; Fracture; Implant-Supported
Restoration; Screw-Retained
Although most of the concentration today is on all-ce-
ramic restorations, metal-ceramic restorations, whether
they are tooth-supported or implant-supported, are still
considered as the gold standard because of their excellent
biocompatibility, consistent esthetics, superior strength,
and marginal adaptation. Also, metal-ceramic restora-
tions are durable and long-lasting [1]; however, several
investigators [2-5] demonstrated that the fracture of ce-
ramic veneers is not an uncommon problem in clinical
practice and may cause the premature failure of fixed
partial dentures. Bragger et al. [6] found that there is an
interrelation between porcelain fracture and the long-
term survival of the fixed partial denture. The event of
porcelain fracture increased the risk for the suprastruc-
ture to become a failure at 10 years compared to a supra-
structure with no porcelain fracture [6].
Numerous studies [4,5,7] have reported on the out-
come of MC restorations supported by natural teeth
In a survey of crown and fixed partial denture failures,
Walton et al. [5] found that the incidence of porcelain
fracture was the second most common cause of MC fixed
partial denture replacement, accounting for 72 (16%) of
451 failed restorations. Also, they found that porcelain
fracture was the most common cause of failure with sin-
gle crown restorations. This is in agreement with another
7-year follow-up study from Strub et al. [4] who found
that porcelain fracture was the most common cause of
MC prosthesis failure.
A systematic review [7] of 8 papers and 1,192 pros-
theses supported by natural teeth abutments showed that
veneer fracture was a common complication of metal-
ceramic prostheses, with a mean incidence of up to 3%
reported for single crowns and FPDs.
Regarding implant-supported MC restorations, it has
been shown that porcelain fracture is also a common
complication in implant-supported restorations [8].
A follow-up study [3] of 92 cement-retained metal-
ceramic implant-supported prostheses, including single-
R. M. Shadid et al. / Open Journal of Stomatology 3 (2013) 411-418
tooth restorations, showed that the porcelain fracture
cumulative failure rate was 2.34%.
In another retrospective study, Ekfeldt et al [9] re-
ported that 1 out of 39 cement-retained MC implant-sup-
ported restorations failed due to porcelain fracture.
Although different repair techniques are currently
available, these techniques are still costly and time con-
suming. Therefore, the clinicians should be aware of the
reasons that cause fracture of these restorations to avoid
The purpose of this article was to discuss the factors
that lead to the fracture of metal-ceramic restorations
whether they are tooth-supported or implant-supported,
under the following headings: technical factors, den-
tist-related factors, inherent material properties, direction,
magnitude and frequency of applied loads, environ-
mental factors, screw-retained implant-supported resto-
rations, and posterior cantilevered prostheses.
2.1. Surface Treatment and Design of the Metal
Warpeha and Goodkind [10] found that the fracture
strength of porcelain was severely reduced when porce-
lain was fused to an un-oxidized metal surface and when
an improper thickness of the coating agent was applied.
Anthony and associates [11] indicated that when the al-
loy surface was depleted of oxide, a thirty percent (30%)
reduction in the bond strength was noted. However,
thicker oxides have been shown to increase the risk of
metal-porcelain bonding failure [12].
There is controversy concerning whether bond
strength is affected by increasing the roughness of the
metal surface. Kelly and colleagues [13] stated that very
rough surfaces may increase stress concentration at the
bond. Nonetheless, Shell and Nielsen [14] believe that
the metal-ceramic bond is two-thirds chemical and
one-third van der Wall’s force. Hence, the effect of sur-
face roughness on bond strength is minimal as the au-
thors minimize the importance of mechanical bonding. A
finely roughened surface, however, may be wetted more
easily, thereby possibly increasing the bond strength
It has been shown that a design with a definite acute-
ness in the metal substructure has a lower ultimate frac-
ture strength [10].
In addition, the cross-sectional dimensions and con-
tours of connectors have a significant effect on frame-
work strength and stability [15]. The connector must be
thick enough to provide adequate resistance to occlusal
loads; however, occlusal and gingival embrasures must
be formed such as to ensure esthetics of restoration
[16,17]. Furthermore, the occluso-gingival thickness of
the pontic has an effect on deflection of framework.
Bending or deflection varies directly with the cube of the
occluso-gingival thickness or the pontic, making the
pontic one half as thick will also make it bend eight
times as much [16,18].
2.2. Compatibility between the Coefficient of
Thermal Expansion of the Metal and
It has been reported that stress concentration at the
metal-porcelain interface is due to the disparity between
the coefficient of thermal expansion of the metal and
porcelain. [19]
A slightly lower coefficient of thermal expansion of
porcelain compared with metal is considered beneficial.
Such a relationship places the porcelain under compres-
sion after firing [20]. Generally, a 0.5 × 106˚C difference
in the coefficients is desirable [21].
2.3. Ceramic Build-Up and Firing Technique
Evans et al. [22] highly recommended minimizing air
entrapment between the ceramic particles because poros-
ity does occur during ceramic application and can ac-
count for eventual ceramic fracture.
Cracks within ceramics may form due to incomplete
densification which leaves behind angular pores. Flaws
(cracks) also form in the surface of ceramics through
abrasion (by dust); such cracks, coupled with low frac-
ture toughness, impair the strength of ceramics [23].
The rate of cooling and heating during porcelain firing
may also affect the stress concentration at the metal-ce-
ramic interface [24].
Repeated firings or excessively high oven tempera-
tures have been regarded as causes of superficial and
deep imperfections or porcelain blistering [25].
2.4. Thickness of Porcelain
It has been stated that “the thicker the porcelain the
weaker the restoration.” The reasons behind this state-
ment are: (1) the direct relationship between the thick-
ness of the porcelain and the stress concentration at the
metal-porcelain interface; and (2) the inherent weakness
of the porcelain under tension [19]. The porcelain adja-
cent to the interface is generally under compression be-
cause the metal contracts more than the porcelain; how-
ever, the further the surface of the porcelain is from the
interface, the greater the tension [14]. Therefore, in order
to minimize the formation of microcracks, a fairly uni-
form thickness of porcelain is recommended [2].
2.5. Thickness and Elastic Modulus of the Metal
It has been demonstrated that the support of the veneer-
ing porcelain is directly related to the modulus of elastic-
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R. M. Shadid et al. / Open Journal of Stomatology 3 (2013) 411-418 413
ity, not to the strength of the substructure material [26].
Alloys with an elevated elastic modulus resist deforma-
tion better [27]. Also, a frequent reason for porcelain
fracture is the lack of rigidity and the distortion of the
metal substructure [25].
2.6. Location of Porcelain-Metal Finish Lines
When partial porcelain coverage is decided upon, the
position of the lingual or occlusal finish lines is impor-
tant. The junction of porcelain and metal should not be
located at centric occlusion contacts in order not to ex-
pose the porcelain-metal bonding to extra load. The por-
celain-metal occlusal junction should also have a 90-
degree or a greater angle to avoid thin “lips” of metal
that may distort during function [25].
3.1. Anterio-Posterior Length of Pontic Span
Long anterio-posterior metal substructures flex under
heavy or complex loads leading to porcelain fracture
[28]. .A fixed partial denture with two-tooth pontic span
will bend eight times as much as a single-tooth pontic
fixed partial denture will, if everything else remains un-
changed [16,18]. Replacing three posterior teeth with a
fixed partial denture rarely has a favorable prognosis,
especially in the mandibular arch. Under such circum-
stances, it is better to go for implant-supported prosthesis
or removable partial denture [29].
3.2. Adequacy and Design of Tooth Preparation
Inadequate tooth preparation, which results in too little
inter-occlusal space for the metal substructure and the
overlying porcelain, is also a reason for porcelain frac-
ture [2]. Further, acute line-angled preparations encour-
age the formation of microcracks within porcelain during
firing procedures [30].
3.3. Incorrect Registration of Occlusion and
Articulation often Causes Destructive
Premature Contacts
As a result, poor diagnosis and an improper design are
important factors affecting the long-term success of fixed
partial dentures, and the clinical skill of the dentist is
extremely important for increasing the longevity of
metal-ceramic restorations.
Ban and Anusavice [31] indicated that the mechanical
fatigue of ceramics is probably controlled by several
factors including microstructure, crack length and frac-
ture toughness. It has been shown that amorphous mate-
rials like glasses or glassy materials do not have an or-
dered crystalline structure as do metals, and dislocations
of crystalline lattice do not exist in glassy materials; thus,
they have no mechanism for yielding without fracture.
Llobell et al. [32] stated that mastication, parafunction
and intraoral occlusal forces create repetitive dynamic
loading; they considered impact load and fatigue load as
reasons for intraoral ceramic fracture. Anusavice and
Zhang [33] also reported that high biting forces could
cause glass-containing dental restorations to break down.
Stress direction is another contributory factor for fail-
ure, as sometimes failure occurs at sites of relatively low
local stress just because there is a large flaw oriented in
the stress field and this is ideal for causing fracture [2].
White and Li [34] stated that the possible sites from
which failure may start are highly unpredictable since
this depends on flaw size and is related to the stress dis-
tribution. These observations support the need for pro-
tective splints for MC suprastructures to prevent fracture
due to bruxism or parafunctional habits [35].
Since 1958, it has been found that water can act chemi-
cally at crack tips, decreasing the strength of glasses and
ceramics. This phenomenon is termed “chemically as-
sisted crack growth” or “static fatigue” [36].
It has been demonstrated that silicate bonds in the
glassy ceramic matrix are susceptible to hydrolysis by
environmental moisture in the presence of mechanical
stress. Reductions of 20% to 30% in metal-ceramic bond
strength were found in moist environments [37]. As a
result, this static fatigue leads to the propagation of frac-
tures along the microcracks causing failure in the resto-
ration [2].
Additionally, Anusavice and Zhang [33] showed that
common beverages with low pH ranges could also cause
fractures in glass-containing dental restorations.
It has been demonstrated that implant-supported pros-
theses are more susceptible to fracture than prostheses
supported by natural teeth since it was found that more
porcelain fractures occur in implant-supported restora-
tions compared with restorations supported by natural
abutments [38]. Also, implant supported metal ceramic
fixed partial dentures were found to have significantly
higher risk of porcelain fracture in patients with bruxism
habits when a protective occlusal device was not used,
[39] and when the restoration opposed another implant
Copyright © 2013 SciRes. OPEN ACCESS
R. M. Shadid et al. / Open Journal of Stomatology 3 (2013) 411-418
supported metal ceramic restorations [40].
This is because the natural teeth and their periodontal
ligaments provide proprioception and early detection of
occlusal loads and interferences, while the implants lack
this proprioceptive mechanism [41-43]. In addition to
loss of shock absorbing feature in the ankylosed implant
bone interface, both sensitivity and mobility of natural
teeth cannot be duplicated in endosseous implants [44-46].
Therefore this different behavior to masticatory forces
may lead to excessive load on the restoration especially
if the prosthesis is supported by an implant at one end
and a tooth at the other end. Complications of the supra-
structure such as fracture of veneering porcelain, and
others were observed in 5% - 90% of cases of tooth-im-
plant connection [47]. However, it was demonstrated that
the use of non-rigid connector decreases the forces on the
suprastructure [47].
Screw-Retained Implant-Supported
Since retrievability is an important factor for implant-
supported restorations to allow for their easy and safe
removal; therefore, the prosthodontic components can be
adjusted, the screws can be refastened, and the fractured
components can be repaired [48], screw-retained restora-
tions are preferred by some clinicians and are recom-
mended in some clinical situations [49].
However, it has been shown that the presence of
screw-access opening in the occlusal surface of the res-
torations significantly decreased porcelain fracture strength
This is because the centric contact of the screw-access
hole, which is usually developed with the head of the
screw or with composite restorative material, may oc-
cupy 50% to 66% of the intercuspal occlusal table [57].
Hence, a minimal width of porcelain remains around the
screw-access opening and thus, becomes more suscepti-
ble to fracture [53]. In addition, it has been shown that
the screw-access hole of the screw-retained restoration
disrupts the structural continuity of porcelain, thereby
modifying the position of the center of mass of the ce-
ramic bulk toward which the ceramic shrinks during the
sintering process. This will affect the behavior of porce-
lain in these restorations compared with their cemented
counterparts [58].
Relatively few studies have been published on the long
term efficacy of cantilever bridge work supported by
natural teeth. Randow et al. [59] reported a higher fre-
quency of failure with cantilevered units than bounded
units; the same was stated by Strub et al. [60] who found
out that the technical failure rate for cantilever bridges
was 12.7% in patients with low-grade periodontitis.
Similarly, posterior cantilevered prostheses supported
by a relatively small number of implants, seem to be par-
ticularly susceptible to fracture[61]. Technical complica-
tions (including porcelain fracture) were found to be
more frequent for cantilever implant -supported prosthesis
than for end implant abutment-supported one [62-64].
Therefore, to increase the success rate of cantilever
fixed partial dentures, the leverage effect must be mini-
mized by decreasing the pontic size to as small as possi-
ble representing a premolar [15,61]. Also, the pontic
should possess maximum occluso-gingival height to en-
sure rigidity [15]. In addition, considering the different
biomechanical demands for cantilever fixed partial den-
tures, various occlusal schemes have been advocated;
such as freedom in the retrusion/protrusion range on can-
tilever, anterior guided lateral movements, and the ab-
sence of non-working side contacts on the cantilever [65].
Also, optimal retention from abutments were also rec-
ommended [65].
However, in implant-supported cantilever fixed partial
dentures, further precautions must be taken into consid-
eration. Clinical experiences suggest that the distal can-
tilever should not extend more than 2.5 times the anterior
posterior spread of the implants under ideal conditions
(e.g. no parafunction, no bruxism) [66]. Several biome-
chanical studies using an analytical mathematical models
[67] and finite element analysis [68] demonstrated that a
spread out arrangement of implants in the arch is more
significant than the number of implants per se for the
distribution of masticatory forces especially if these im-
plants will support cantilever prosthesis. In addition to
this, the inclination of distal implants reduces the axial
force and bending moments independently from the
number of abutments when cantilever is needed. This
inclination allows simultaneous reduction of the cantile-
ver length at the connection abutment-framework and
increases the prosthesis support area [69]. Bevilacqua et
al. stated that tilted distal implants with consequent re-
duction of the posterior cantilevers resulted in decreased
stress values for the metal frameworks by 11.5% for
15-degree configuration, 31.3% for 30-degree configure-
tion, and 85.6% for the 45-degree configuration [70].
Figures 1 and 2 summarize all of the factors affecting
the fracture resistance of metal-ceramic restorations in
marco-level and micro-level.
The published literature revealed that many different
factors may cause fracture of metal-ceramic tooth-sup-
ported and implant-supported restorations. These factors
may be related to the technician, dentist, environment,
design of the restoration, or to inherent structure of ce-
Copyright © 2013 SciRes. OPEN ACCESS
R. M. Shadid et al. / Open Journal of Stomatology 3 (2013) 411-418 415
Figure 1. Macro-level variables affecting fracture resistance of
metal-ceramic restorations. These include: Tooth preparations
A (adequate tooth preparation, sufficient inter-occlusal space,
rounded line-angles of preparations); Support type B (Tooth vs.
implant); Surface integrity of crown C (screw vs. cement-ret-
ained); Occlusion factors D (avoidance of premature contacts,
design cusps to guide occlusal forces in favorable directions,
avoidance of parafunctions and bruxism/clenching habits);
Connector variables E (the cross-sectional shape, dimensions,
contours of connections within framework); Pontic variables
(the occluso-gingival height, occlusal table area, length of pon-
tic span); Partial coverage variables (location of porcelain-
metal finish lines away from occlusal loads); Cantilever vari-
ables F (posterior length of cantilever in relation to support
span, occlusal engagement vs freedom of movement, direction
of forces on cantilever area, width of occlusal table area of
cantilevered section, occluso-gingival height); Diet variables G
(beverages with low pH ranges, biting hard food or structures
accidently or habitually).
Figure 2. Micro-level variables affecting fracture resistance of
metal-ceramic Restorations. These include: Metal variables
(thickness A, roughness B); Oxide layer variables (presence,
thickness, uniformity C, surface roughness D, wetting ability);
Metal-porcelain junction variables (rounded internal angles E,
90-degree or a greater external angles at porcelain-metal junc-
tion F); Porcelain layer variables (various thickness at different
locations not exceed 2 mm G; Smoothness and polishing of
surface H); Physical properties (compatibility between coeffi-
cient of thermal expansion of metal and porcelain CTE, elastic
modulus of metal EM, mechanical fatigue resistance of porce-
lain, static fatigue resistance of porcelain under humidity);
Porcelain impurities (air voids J, cracks internal K, or external
L, dust impurity M); Porcelain firing procedures (rate of cool-
ing and heating N, Number of repeated firings O, excessively
high oven temperatures P).
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the optimum results inherent in porcelain-fused-to-gold
restorations, the dentist must understand and respect the
physical characteristics of the materials and guide the
design and fabrication of the restoration so as to exploit
their strengths and compensate for their weaknesses.”
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