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Flexural resistance of Cerec CAD/CAM system ceramic blocks. Part 1: Chairside
materials
Article  in  American journal of dentistry · October 2013
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_______________________________________________________________________________________________________________________________________________________________ 
Research Article
_______________________________________________________________________________________________________________________________________________________________ 
Flexural resistance of Cerec CAD/CAM system ceramic blocks.
Part 1: Chairside materials 
 
ALESSANDRO VICHI, DDS, PHD, MAURIZIO SEDDA, DDS, FRANCESCO DEL SIENA, DDS, CHRIS LOUCA, BSC, BDS, PHD 
& MARCO FERRARI, MD, DDS, PHD 
 
ABSTRACT: Purpose: This study tested the materials available on the market for Cerec CAD/CAM, comparing the 
mean flexural strength in an ISO standardized set-up, since the ISO standard for testing such materials was issued later 
than the marketing of the materials tested. Methods: Following the recent Standard ISO 6872:2008, eight types of 
ceramic blocks were tested: Paradigm C, IPS Empress CAD LT, IPS Empress CAD Multi, Cerec Blocs, Cerec Blocs 
PC, Triluxe, Triluxe Forte, Mark II. Specimens were cut out from ceramic blocks, finished, polished, and tested in a 
three-point bending test apparatus until failure. Flexural strength, Weibull characteristic strength, and Weibull modulus, 
were calculated. Results: The results obtained from the materials for flexural strength were IPS Empress CAD 
(125.10±13.05), Cerec Blocs (112.68±7.97), Paradigm C (109.14±10.10), Cerec Blocs PC (105.40±5.39), Triluxe Forte 
(105.06±4.93), Mark II (102.77±3.60), Triluxe (101.95±7.28) and IPS Empress CAD Multi (100.86±15.82). All the 
tested materials had a flexural strength greater than 100 MPa, thereby satisfying the requirementsof the ISO standard 
for the clinical indications of the materials tested. In all tested materials the Weibull characteristic strength was greater 
than 100 MPa. (Am J Dent 2013;26:255-259). 
CLINICAL SIGNIFICANCE: Although a statistically significant difference in flexural strength was found, all tested 
materials fulfilled the requirements of 100 MPa as indicated in the ISO standards for Class 2 ceramics. 
 
�: Dr. Alessandro Vichi, Via Derna 4, 58100 Grosseto, Italy. E-�: alessandrovichi1@gmail.com 
 
Introduction 
 Computer-aided design and computer-aided manufacturing 
(CAD/CAM) was first introduced in dentistry in the 1980s.1-4 
This technology has a growing interest both for clinicians and 
manufacturers.5-7 Both hardware and software developments 
have improved accuracy, ease of use and the clinical per-
formance of the restorations.8,9 CAD/CAM systems are 
commonly categorized as “insourcing” (or “chairside”), where 
the restoration is fabricated in the clinician’s office, or “out-
sourcing”, when the manufacturing process is partly or entirely 
carried out by a dental laboratory with or without the support of 
a milling center. For the chairside approach the aim is to 
produce a prosthetic restoration in a single appointment, with 
the entire manufacturing process carried out within the dental 
office. The CERECa system was initially developed more than 
25 years ago with the aim of manufacturing a dental ceramic 
restoration within the same day.10 Ongoing improvements of 
this system and particularly recent enhancements have lead to a 
wider acceptance in dental practice.11-13 Along with hardware 
and software improvements, materials have been improved 
and/or newly developed. For the CEREC system, several 
materials are available and the selection criteria are related to 
clinical use,13,14 with mechanical and optical properties as 
pivotal. Among these materials, metals and high-strength 
ceramic materials are generally used to obtain a framework that 
requires ceramic veneering, thus needing access to out-
sourcing.14 Some materials require additional manufacturing 
processes such as sintering or glass infiltration that have to be 
performed in dedicated furnaces. Other materials such as 
lithium disilicate require an additional time consuming manu-
facturing process (crystallization) that has to be performed in a 
furnace, so they can hardly be defined as chairside materials. 
Conversely, feldspathic and leucite-reinforced ceramics require 
only a finishing and polishing procedure that can be performed 
manually, so they are suitable for chairside use. These materials 
combine the advantages of all-ceramic restorations (e.g. es-
thetic appearance, biocompatibility, and durability15) with the 
advantages of being manufactured by a CAD/CAM system 
(e.g. time savings, cost effectiveness, and quality control11). 
Although these materials have been the subject of several 
investigations, it was only in late 2008 that the International 
Organization for Standardization released the specification for 
testing some of the properties of CAD/CAM materials, particu-
larly the flexural strength of CAD/CAM ceramic materials.16 In 
these ISO specifications, the minimum mean flexural strength 
values for the various clinical indications were indicated (Table 
1), as well as the specification required to perform the Weibull 
statistics for dental ceramic CAD/CAM materials. Esthetic 
ceramics, used for veneers, inlays, and onlays, are classified as 
Class 1 ceramics, and should have a minimum mean flexural 
strength of 50 MPa; esthetic ceramic, used for adhesively 
cemented, single-unit, anterior or posterior prostheses, are 
classified as Class 2 ceramics, and should have a minimum 
mean flexural strength of 100 MPa. 
 Since most of these materials were marketed before the 
publication of the reported ISO standards, it is of interest to test 
the ceramic materials available on the market for chairside use 
with the CEREC system. This will verify whether these 
ceramics fulfill the ISO standard for the clinical indications 
given by the manufacturers and allow a comparison of the 
mean flexural strengths. The null hypotheses tested were: (1) 
the selected materials did not meet the minimum mean flexural 
strength indicated as they were introduced on the market 
before the release of the ISO standards, and (2) there were no 
statistically significant differences between the various 
materials. 
256 Vichi et al American Journal of Dentistry, Vol. 26, No. 5, October, 2013 
Table 1. Recommended clinical indications (ISO 6872:2008). 
_______________________________________________________________________________________________________________________________________________________________________________________________________________ 
 Flexural strength minimum 
 Class Recommended clinical indications (mean) MPa 
_______________________________________________________________________________________________________________________________________________________________________________________________________________ 
 1 (a) Esthetic ceramic for coverage of a metal or a ceramic substructure. 
 (b) Esthetic-ceramic: single-unit anterior prostheses, veneers, inlays, or onlays 50 
 2 (a) Esthetic-ceramic: adhesively cemented, single-unit, anterior or posterior prostheses. 
 (b) Adhesively cemented, substructure ceramic for single-unit anterior or posterior prostheses. 100 
 3 Esthetic-ceramic: non-adhesively cemented, single-unit, anterior or posterior prostheses. 300 
 4 (a) Substructure ceramic for non-adhesively cemented, single-unit, anterior or posterior prostheses. 
 (b) Substructure ceramic for three-unit prostheses not involving molar restoration. 300 
 5 Substructure ceramic for three-unit prostheses involving molar restoration. 500 
 6 Substructure ceramic for prostheses involving four or more units. 800 
_______________________________________________________________________________________________________________________________________________________________________________________________________________ 
 
Table 2. Results of tested materials ordered by flexural strength. 
_______________________________________________________________________________________________________________________________________________________________________________________________________________ 
 Flexural strength Weibull statistics 
 _________________________________ ___________________________________________ 
Group Material Type � (MPa) Sig m �0 (MPa) 
_______________________________________________________________________________________________________________________________________________________________________________________________________________ 
 2 Ivoclar IPS Empress CAD LTb Leucite-reinforced glass-ceramic 125.10 ±13.05 a 11.55 130.64 
 4 Sirona Cerec Blocsa Feldspathic ceramic 112.68 ± 7.97 b 16.68 116.24 
 1 3M Paradigm Cc Leucite-reinforced glass-ceramic 109.14 ±10.10 b, c 12.69 113.58 
 5 Sirona Cerec Blocs PC Feldspathic ceramic 105.40 ± 5.39 b, c 22.45 107.93 
 8 Vita Triluxe Forted Feldspathic ceramic 105.06 ± 4.93 b, c 25.55 107.27 
 6 Vita Mark IId Feldspathic ceramic 102.77 ± 3.60 b, c 33.80 104.42 
 7 Vita Triluxed Feldspathic ceramic 101.95 ± 7.28 c 16.35 105.25 
 3 Ivoclar IPS Empress CAD Multib Leucite-reinforced glass-ceramic 100.86 ±15.82 c 7.52 107.38 
_______________________________________________________________________________________________________________________________________________________________________________________________________________ 
�0 = Flexural strength (mean and standard deviation); Sig = Significance; m = Weibull modulus; �0 = Weibull characteristic strength. The same letter of signifi-
cance indicates no statistically significant differences. 
Materials and Methods 
 
 Eight types of ceramic blocks marketed for the CEREC 
CAD/CAM system were used in this study (Table 2). Speci-mens were prepared according to ISO 6872:2008. The blocks 
were fixed to a low speed water-cooled diamond saw (Isomete) 
by the use of a proprietary device. The device was able to 
initially cut the blocks longitudinally, and then by turning them 
90° clockwise, was able to produce 3 to 4 bar-shaped samples, 
depending on the block size. To minimize the stress of the 
materials, the speed was maintained below 250 rpm and no 
extra weight was put on the blocks. For each tested material, 
four different blocks were used to produce 15 samples, which 
were wet-finished with 600 grit paper until dimensions of 15 ± 
0.2 mm length, 4 ± 0.2 mm width and 2 ± 0.2 mm height were 
achieved. Samples were then wet-polished with 600 and 1,200 
grit paper. According to the standard, a 45° edge chamfer was 
prepared at each of the major edges, by keeping the specimens 
at 45° on the 1,200 grit paper disc with a metal rig. 
 A three-point bending test appliance was used. The tip and 
the supports were made in Cobalt-HSS (high speed steel), using 
polished rollers 2.0 mm in diameter. The remaining part of the 
rig was milled from a stainless steel block (A.I.S.I. type 316). 
The span was set at 13.0 mm. Tests were performed in a 
universal testing machine (Triax 50f), with a cross-head speed 
of 1 mm/minute. The fracture load was recorded in N and the 
flexural strength (�) was calculated in MPa using the following 
equation: 
22
3
wb
Pl
�� 
 
where: P is the fracture load in N, l is the distance between the center 
of the supports in mm, w is the width in mm, and b is the height in 
mm. 
 
 Data were tested to fit a normal distribution with a 
Kolmogorov-Smirnov test and the homogeneity of variances 
was verified with a Levene’s test. A one-way ANOVA was 
then performed, followed by a Games-Howell test for post hoc. 
The significance level was set at P< 0.05. The Weibull 
characteristic strength (�0) and the Weibull modulus (m) were 
calculated, according to ISO Standard 6872:2008, with the 
following equation: 
�
�
�
�
�
�
�
	
�
�

�
�
���
m
fP
0
exp1
�
� 
 
where: Pf is the probability of failure between 0 and 1, � is the flexural 
strength in MPa, �0 is the Weibull characteristic strength in MPa (to 
which the 63.2% of the specimens fail), and m is the Weibull modulus. 
Results 
 The mean flexural strength (�), the Weibull modulus (m) 
and the Weibull characteristic strength (�0) of the materials 
tested are reported in Table 2 and Fig. 1. All the tested 
materials achieved a flexural strength greater than 100 MPa, 
required for the clinical indications of the materials tested. The 
Kolmogorov-Smirnov test performed confirmed the normal 
distribution of data (P= 0.975). The one way ANOVA showed 
a statistically significant difference among groups (P< 0.001). 
IPS Empress CAD LT (Ivoclar) obtained a statistically 
significantly higher flexural strength than the other tested 
materials. No statistically significant difference was found 
between Cerec Blocs, Paradigm C, Cerec Blocs PC, Triluxe 
Forte, and Mark II. No statistically significant differences were 
found between Paradigm C, Cerec Blocs PC, Triluxe Forte, 
Mark II, Triluxe, and IPS Empress CAD Multi. In all the tested 
American Journal of Dentistry, Vol. 26, No. 5, October, 2013 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1. Flexural strength of the tested materials. 
 
materials the Weibull characteristic strength was greater than 
100 MPa. The extreme values of the Weibull modulus were 
33.80, obtained by Mark II, and 7.52, obtained by IPS Empress 
CAD Multi. The two-parameter Weibull cumulative distribu-
tion function is shown in Fig. 2. 
 
Discussion 
 This study aimed to compare in the same experimental set-
up the ceramic materials available on the market for the 
CEREC CAD/CAM system. Most of the data currently avail-
able are derived from studies performed with different experi-
mental set-ups, are company produced and were performed 
before ISO Standard 6872:2008 became available for CAD/ 
CAM ceramic material testing. Due to the extensive number of 
materials included, with different indications and with a broad 
variation of mechanical behavior, the data from the study were 
presented in two papers, based on the clinical use of the 
materials, both “chairside” (Part 1, the current article) or 
“outsourcing” (Part 2, in press17). 
 All the materials tested in the present study are indicated by 
the manufacturers for inlays, onlays, veneers, partial crowns, 
anterior crowns, and posterior crowns. The exception is for 
Triluxe, Triluxe Forte, and Cerec Blocs PC, where there is no 
reported indication for inlays. For Cerec Blocs PC, there is also 
no indication of use for onlay. On the basis of these indications, 
as reported by the manufacturers, all the materials tested have 
to fulfill the requirements for Class 2 of the ISO standard 
classification. Since all the materials tested in this study 
obtained a mean flexural strength greater than the requirement 
of 100 MPa, the first null hypothesis was rejected. 
 A statistically significant difference (P< 0.001) was found 
in the mean flexural strength of the tested materials, also 
leading to a rejection of the second null hypothesis. 
 Obtaining a clear picture of the mechanical properties of 
CEREC chairside ceramic materials is difficult due to differ-
ences in: (1) the selection of the materials tested, (2) the investi- 
Flexural resistance of Cerec CAD/CAM 257 
 
 
Fig. 2. Two-parameter Weibull cumulative distribution function. 
gated properties, (3) the test methods used and (4) how the 
results are expressed. Furthermore, despite the marketing of 
several CAD/CAM materials there are few independent studies 
published. Among the materials tested in the present study, the 
Mark II was previously investigated in other papers. Tinschert 
et al18 reported the Mark II to have a flexural strength of 86.3 ± 
4.3 MPa when measured with a four-point bending test, 
(4BPT), while Buso et al19 reported a mean biaxial flexural 
strength (BFS) of 102.1 ± 13.65 MPa. These findings are in 
agreement with the present study; the results of 4PBT generally 
are 20-25% lower than those of 3BPT.20,21 It must be taken into 
account that feldsphatic and leucite-reinforced ceramics require 
an adhesive cementation step, as specified in the ceramics 
classification reported in ISO 6872:2008. If an esthetic ceramic 
is used for the fabrication of non-adhesively cemented crowns, 
the requirement of the minimum mean flexural strength is 300 
MPa (Class 3 ceramics). May et al22 demonstrated that the 
failure loads of a CAD/CAM ceramic crown depend on the 
bonding condition and the cement thickness. The authors 
performed both finite element analysis (FEA) and physical 
testing; in that study, pre-cementation spaces around 50-100 
μm were recommended; in addition, bonding benefits were lost 
at thicknesses approaching 450-500 μm due to polymerization 
shrinkage stresses. 
 The flexural strength can be considered a relevant mechani-
cal property for brittle materials that are much weaker in ten-
sion than in compression.23 Common ways to assess this 
property are the three-point bending test (3PBT), the four-point 
bending test (4PBT), and the biaxial flexure test (BFT, some-
times reported as BFS or ‘piston-on-three-ball’ test).16,20 In all 
such tests, a static load is applied until failure. Ceramic speci-
mens are very sensitive to edge or surface machining damage,24 
so the BFT, where a disk is loaded in the center, is believed to 
reduce the probability of edge failure.25 However, in 2003, 
Della Bona et al23 suggested the rounding of bar-shaped speci-
men edges as a revision of the specimen preparation standard 
258 Vichi et al 
 
 
published in 1995.26 Recently it was reported that surface fin-
ishing influences the biaxial flexural strength test as well.27 
 Before September 2008, neither a specimen preparation 
method, nor a flexuralstrength test were available specifically 
for dental CAD/CAM ceramic materials, so published papers 
referred to general mechanical testing standards.20 Moreover, 
CAD/CAM materials are commonly commercialized in blocks, 
and the preparation of the rectangular-section bars necessary for 
3PBT and 4PBT is simplified in respect to the one required for 
BFT, where a disc has to be produced. Furthermore, in order to 
obtain accurate disc-specimens with the required dimensions of 
12-16 mm in diameter and 1.2 ± 0.2 mm in thickness, a milling 
apparatus is necessary, whereas for the preparation of bar-
shaped specimens it is not. The results of 3PBT and 4PBT are 
however related, with the 4PBT providing generally lower 
values.20,21 It is recognized that the physical properties of dental 
ceramics should not be characterized only by the flexural 
strength.24,28 The Weibull modulus and the Weibull charac-
teristic strength are generally used to obtain a more accurate 
representation of the structural reliability of dental ceramics.29-
31 The Weibull characteristic strength (�0), or scale parameter, 
indicates the 63.21 percentile of the strength distribution; the 
Weibull modulus (m), or shape parameter, is a measure of the 
distribution of flaws, generally for brittle materials. Being 
associated with crack size distribution, it is often preferable to 
obtain a higher m, even if associated with slightly lower mean 
fracture strength, than a lower m associated with a higher mean 
fracture strength.28 Materials with high Weibull moduli are 
more predictable and less likely to break at a stress much lower 
than a mean value. In particular, a m greater than 20 indicates a 
higher level of structural integrity of the material and greater 
reliability. A typical m for ceramics is reported to be 5-15.32 In 
this study, only Mark II, Triluxe Forte and Cerec Blocs PC 
obtained a m > 20 (33.80, 25.55 and 22.45 respectively). In 
general, the leucite-based glass-ceramics (Groups 1-3) obtained 
a lower m (7.52-12.69) when compared with that of feldspathic 
ceramics (16.35-33.80). Leucite is added to porcelains by 
manufacturers to improve the resistance to crack propagation, 
due to the phenomenon of crack deflection around leucite, and 
so obtaining a higher fracture toughness.33,34 It would have been 
expected that leucite-reinforced ceramic materials would have 
obtained higher values of flexural strength, characteristic 
strength and Weibull modulus when compared to feldspathic 
materials. In this study however, ceramics indicated as “leucite-
reinforced” by the manufacturers did not obtain better results 
than the feldspathic ceramics, except for flexural strength and 
characteristic strength of IPS Empress CAD LT. This is in 
agreement with the study performed by Cesar et al35 which 
found that the leucite content did not affect resistance to slow 
crack growth regardless of the test environment (air or artificial 
saliva). Optimization of the microstructure of leucite reinforced 
glass ceramic can improve the biaxial flexural strength.36 This 
finding was recently con-firmed by Chen et al,33 who found a 
higher flexural strength and Weibull modulus, both measured 
with a BFT, by optimizing the microstructure of a fine-grained 
leucite glass-ceramic. 
 The 3PBT performed in the present study has the limitation 
of a monotonic test, in which the load is applied until specimen 
failure. This is not completely representative of the clinical 
American Journal of Dentistry, Vol. 26, No. 5, October, 2013 
 
 
situation in which the restoration is subjected to cyclical load 
and thermal variations. The test method is a very important 
parameter for brittle materials and it has been demonstrated that 
a change in the test method can result in significantly different 
flexural strength values.37 Since no systematic reviews on 
flexural strength are currently available, in the present study the 
standard ISO 6872:2008 was strictly followed, for specimen 
preparation and storage, for test apparatus set-up, and for 
expression of the results. This approach allowed for a report of 
the mean flexural strength of the ceramic materials for CEREC 
CAD/CAM in chairside use to be presented. 
 
a. Sirona, Bernsheim, Germany. 
b. Ivoclar, Schaan, Liechtenstein. 
c. 3M ESPE, St. Paul, MN, USA. 
d. VITA Zahnfabrik, Bad Sackingen, Germany. 
e. Buehler, Lake Bluff, IL, USA. 
f. Controls, Milan, Italy. 
 
Disclosure statement: The authors declared no conflict of interest. 
 
Dr. Vichi is Assistant Research Professor, Dr. Sedda is a PhD student, Dr. Del 
Siena is Clinical Instructor, and Dr. Ferrari is Professor, Department of Fixed 
Prosthodontics and Dental Materials, Tuscan School of Dental Medicine, 
University of Siena, Italy Siena, Italy. Dr. Louca is Senior Clinical Lecturer, 
Eastman Continuing Professional Development, University College London, 
Eastman Dental Institute, London, United Kingdom. 
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