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 requirements of 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- : [email protected]
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 performance 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 “outsourcing”, 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 outsourcing.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 manufacturing 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. esthetic 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, particularly 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.
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256 Vichi et al Table 1. Recommended clinical indications (ISO 6872:2008).
Flexural strength minimum (mean) MPa
Recommended clinical indications
Esthetic ceramic for coverage of a metal or a ceramic substructure. Esthetic-ceramic: single-unit anterior prostheses, veneers, inlays, or onlays
Esthetic-ceramic: adhesively cemented, single-unit, anterior or posterior prostheses. Adhesively cemented, substructure ceramic for single-unit anterior or posterior prostheses.
Esthetic-ceramic: non-adhesively cemented, single-unit, anterior or posterior prostheses.
Substructure ceramic for non-adhesively cemented, single-unit, anterior or posterior prostheses. Substructure ceramic for three-unit prostheses not involving molar restoration.
Substructure ceramic for three-unit prostheses involving molar restoration.
Substructure ceramic for prostheses involving four or more units.
Table 2. Results of tested materials ordered by flexural strength. _______________________________________________________________________________________________________________________________________________________________________________________________________________
125.10 ±13.05 112.68 ± 7.97 109.14 ±10.10 105.40 ± 5.39 105.06 ± 4.93 102.77 ± 3.60 101.95 ± 7.28 100.86 ±15.82
a b b, c b, c b, c b, c c c
2 4 1 5 8 6 7 3
Ivoclar IPS Empress CAD LT Sirona Cerec Blocsa 3M Paradigm Cc Sirona Cerec Blocs PC Vita Triluxe Forted Vita Mark IId Vita Triluxed Ivoclar IPS Empress CAD Multib
Leucite-reinforced glass-ceramic Feldspathic ceramic Leucite-reinforced glass-ceramic Feldspathic ceramic Feldspathic ceramic Feldspathic ceramic Feldspathic ceramic Leucite-reinforced glass-ceramic
11.55 16.68 12.69 22.45 25.55 33.80 16.35 7.52
130.64 116.24 113.58 107.93 107.27 104.42 105.25 107.38
0 = Flexural strength (mean and standard deviation); Sig = Significance; m = Weibull modulus; 0 = Weibull characteristic strength. The same letter of significance 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). Specimens 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: V
3Pl 2 wb 2
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: Pf
ª §V · 1 exp « ¨¨ ¸¸ «¬ © V 0 ¹
º » »¼
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
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Flexural resistance of Cerec CAD/CAM 257
Fig. 1. Flexural strength of the tested materials.
Fig. 2. Two-parameter Weibull cumulative distribution function.
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 distribution function is shown in Fig. 2.
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 mechanical property for brittle materials that are much weaker in tension 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, sometimes reported as BFS or ‘piston-on-three-ball’ test).16,20 In all such tests, a static load is applied until failure. Ceramic specimens 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 specimen edges as a revision of the specimen preparation standard
Discussion This study aimed to compare in the same experimental setup the ceramic materials available on the market for the CEREC CAD/CAM system. Most of the data currently available are derived from studies performed with different experimental 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 differences in: (1) the selection of the materials tested, (2) the investi-
American Journal of Dentistry, Vol. 26, No. 5, October, 2013
258 Vichi et al published in 1995.26 Recently it was reported that surface finishing influences the biaxial flexural strength test as well.27 Before September 2008, neither a specimen preparation method, nor a flexural strength 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 barshaped 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 characteristic strength are generally used to obtain a more accurate representation of the structural reliability of dental ceramics.2931 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 “leucitereinforced” 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
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. b. c. d. e. f.
Sirona, Bernsheim, Germany. Ivoclar, Schaan, Liechtenstein. 3M ESPE, St. Paul, MN, USA. VITA Zahnfabrik, Bad Sackingen, Germany. Buehler, Lake Bluff, IL, USA. 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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