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CAM-materials in implant-supported molar crowns
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Available online at www.sciencedirect.com
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In-vitro fatigue and fracture testing of CAD/CAM-materials in implant-supported molar crowns Verena Preis ∗ , Sebastian Hahnel, Michael Behr, Laila Bein, Martin Rosentritt Department of Prosthetic Dentistry, University Medical Center Regensburg, 93042 Regensburg, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To investigate the fatigue and fracture resistance of different CAD/CAM-materials
Received 4 November 2016
as implant- or tooth-supported molar crowns with respect to the clinical procedure
Received in revised form
(screwed/bonded restoration).
21 December 2016
Methods. 168 crowns were fabricated from different CAD/CAM-materials (n = 8/material):
Accepted 18 January 2017
ZLS (zirconia-reinforced lithium silicate ceramic; Suprinity, Vita-Zahnfabrik), COB (compos-
Available online xxx
ite; Brilliant Crios, Coltene), COL (composite; Lava Ultimate, 3M Espe), PMV/PPV (polyether ether ketone (PEEK) + milled composite veneer/composite paste veneer; BioHPP + HIPC
Keywords:
veneer/Crealign veneer, Bredent), COH (composite; Block HC, Shofu), and ZIR (zirconia; IPS
CAD/CAM
e.max ZirCAD, Ivoclar-Vivadent) as reference. Three groups were designed simulating the
PEEK
following clinical procedures: (a) chairside procedure ([CHAIR] implant crown bonded to
Composite
abutment), (b) labside procedure ([LAB] abutment and implant crown bonded in laboratory,
Zirconia
screwed chairside), and (c) reference ([TOOTH] crowns bonded on human teeth). Combined
Zirconia-reinforced lithium silicate
thermal cycling and mechanical loading (TCML) were performed simulating a 5-year clinical
Chewing simulation
situation. Fracture force was determined and failures were documented. Data were statisti-
Implant crown
cally analyzed (Kolmogorov–Smirnov-test, one-way-ANOVA; post-hoc-Bonferroni, ˛ = 0.05).
Abutment
Results. All crowns of group LAB-PPV showed cracks after TCML. The other groups sur-
Fracture resistance
vived fatigue testing without failures. Fracture forces varied between 921.3 N (PPV) and 4817.8 N (ZIR) [CHAIR], 978.0 N (COH) and 5081.4 N (ZIR) [LAB], 746.7 N (PPV) and 3313.5 N (ZIR) [TOOTH]. Significantly (p < 0.05) different fracture values were found between materials in all three groups. Only ZLS crowns provided no significant (p > 0.05) differences between the individual groups. Significance. Different ceramic and resin-based materials partly performed differently in implant or tooth situations. Individual resin-based materials (PPV, COB, COH) were weakened by inserting a screw channel. Most CAD/CAM-materials may be clinically applied in implant-supported crowns without restrictions. © 2017 The Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗ Corresponding author at: UKR University Medical Center Regensburg, Department of Prosthetic Dentistry, 93042 Regensburg, Germany. Fax: +49 941 944 6171. E-mail address: [email protected] (V. Preis). http://dx.doi.org/10.1016/j.dental.2017.01.003 0109-5641/© 2017 The Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Preis V, et al. In-vitro fatigue and fracture testing of CAD/CAM-materials in implant-supported molar crowns. Dent Mater (2017), http://dx.doi.org/10.1016/j.dental.2017.01.003
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1.
Introduction
Advanced digital techniques and an increasing number of CAD/CAM (computer-aided design/computer-aidedmanufacturing) machinable materials enable continuous innovations in implant prosthetics. The benefits of a digital workflow including intra-oral scanning and CAD/CAM in combination with choosing an appropriate dental material may contribute to the success of implant-supported crowns. While implants with preformed or custom abutments are state of the art in implant dentistry, the success of chairside cemented or bonded abutments and crowns might be limited by gingival and per-implant inflammation caused by residual cement remaining in areas difficult to access [1]. To resolve this problem screwed titanium bases with bonded abutments and crowns are available that enable bonding areas distant from the sulcus. Synchronization of the titanium base and the implant platform guarantees perfect fit and force-fit connections, avoiding fitting inaccuracies like observed for custom CAD/CAM fabricated ceramic abutments [2,3]. Bonding implant crowns to the titanium base in the laboratory in advance and leaving a screw channel may have further advantages: a screw-retained chairside fixation of the abutment–crown combination allows easy and reversible access to the screw for retightening as well as an uncomplicated maintenance of the implant restoration if necessary. Superior bonding quality (dry conditions, surface activation, optimized polymerization) may be achieved under laboratory conditions, improving bonding durability and reducing inflammatory reactions. Nevertheless, the strength of the crown might be affected by the presence of the screw channel [4–8]. Besides an optimized fabrication process and chairside/labside procedure, the selection of the appropriate crown material may essentially contribute to enduring success. A broad range of CAM machinable blocks is available for resin-based materials (composites, PEEK, PMMA), ceramics (feldspar, zirconia, lithium disilicate, zirconia-reinforced lithium silicate), and resin-infiltrated ceramics, which may be applied as monolithic restorations or with subsequent veneering. As implant crowns are more prone to occlusal overloading than tooth-supported crowns due to the missing of the physiological semi-elastic connection (periodontal ligament) and the tactile sensitivity, the application of brittle materials may cause numerous in vivo complications like fracture or chipping [9,10]. To overcome or minimize the risk of fracture, resin-based materials with improved shock absorbing capacity or monolithic ceramics of high strength might by preferred. However, despite of promising results of resin-based materials in implant-supported restorations [11,12], their mechanical resistance may be inferior to ceramics [13]. Up to date only limited scientific information and even less clinical data are available that show the performance of different currently available CAD/CAM materials used in implant-supported crowns with respect to the labside and chairside procedure. To give a first predication of their clinical survival, in vitro fatigue and fracture testing of CAD/CAMfabricated crowns may be helpful.
Fig. 1 – Designs of groups: LAB, CHAIR, and TOOTH (shaded area: artificial periodontium).
The hypothesis tested in this in vitro study was that molar crowns show different in vitro performance and fracture resistance when a) bonded to abutments chairside, bonded in the laboratory and screwed on implants chairside, or bonded to human teeth, or b) different CAD/CAM materials were used.
2.
Materials and methods
A total of 168 identically shaped molar crowns (tooth 46) were fabricated from different CAD/CAM materials (n = 8 per material and group), representing three resin-based composites, one polyether ether ketone (PEEK) combined with two different types of composite veneers, one zirconia-reinforced lithium silicate ceramic, and one zirconia ceramic (reference material). Details on the materials and their manufacturers are given in Table 1. For each material, three groups were designed to simulate the following clinical procedures (Fig. 1): a Group ‘CHAIR’ (chairside procedure): the crown was directly bonded onto the implant-abutment analog and the excess luting material was removed. b Group ‘LAB’ (labside procedure): a screw channel was manually drilled into the central fossa of the crown with a diamond bur (red/fine, diameter: 1.5 mm, water cooling). The crown was bonded onto the implant-abutment analog, the excess luting material was removed, and the screw channel was restored with composite (Filtek Supreme; Elipar Trilight 40 s, 3M Espe, D). c Group ‘TOOTH’ (reference group): crowns were luted on prepared human molar teeth. In the groups ‘CHAIR’ and ‘LAB’, the implant-abutment analogs (n = 112; Straumann, D, titanium grade IV, implant diameter 4.1 mm, implant length 12 mm, abutment length 6 mm, 6◦ ) were vertically positioned in resin blocks (Palapress Vario, Heraeus-Kulzer, D) in order to simulate the posterior implant situation replacing tooth 46. For the group ‘TOOTH’ extracted caries-free human molars (n = 56) were collected at
Please cite this article in press as: Preis V, et al. In-vitro fatigue and fracture testing of CAD/CAM-materials in implant-supported molar crowns. Dent Mater (2017), http://dx.doi.org/10.1016/j.dental.2017.01.003
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Table 1 – Materials, manufacturers and crown treatment. Code
Material, manufacturer
Comment
ZLS COB COL PMV PPV COH ZIR
Suprinity, Vita Zahnfabrik, D Brilliant Crios, Coltene, CH Lava Ultimate, 3M Espe, USA BioHPP + HIPC veneer, Bredent, D BioHPP + Crealign veneer, Bredent, D Block HC, Shofu, J IPS e.max ZirCAD, IvoclarVivadent, FL
Zirconia-reinforced lithium silicate ceramic, crystallized Composite Composite Polyether ether ketone + milled composite veneer Polyether ether ketone + composite paste veneer Composite Zirconia
the University Medical Center Regensburg and stored in 0.5% chloramine solution for no longer than 4 weeks. The variability of human molars was respected by preselecting teeth with comparable size and shape and by statistically randomly dividing the teeth to the subgroups. Preselecting also guaranteed comparable preparation conditions. The teeth were prepared according to ceramic guidelines with a 1.5 mm axial and occlusal anatomical reduction and a 1 mm circumferential deep shoulder with rounded inner angles at an isogingival height of the tooth cervix and a convergence angle of 4◦ . All teeth were prepared by one person with identical preparation equipment. Standardized preparation was performed on basis of an original model, and preparation design was controlled with a gage. For simulating the resilience of the human periodontium the roots of the teeth were coated with a 1 mm polyether layer (Impregum, 3M Espe, D) and were vertically positioned in resin blocks (Palapress Vario, Heraeus Kulzer, D). The procedure of fabricating an artificial periodontium was described previously [14,15]. Abutments and prepared teeth were digitalized (Cerec Omnicam, Sirona, D) and full-contour molar crowns were milled (Cerec, MCXL, Sirona, D) from all materials except for polyether ether ketone. The circular and occlusal wall thickness of the full-contour crowns depended on the abutment, but in all cases was >1.5 mm. For the polyether ether ketone material substructures for subsequent veneering were milled in a first fabrication step. Then, the crowns were completed with either milled composite veneers (PMV) or conventional composite paste veneers (PPV) by using a composite primer (visio.link, Bredent, D). Abutments were sandblasted (110 m Al2 O3 , 1.5 bar) and teeth were conditioned (ED Primer II; Panavia F2.0, Kuraray, J; Elipar Trilight, 3M Espe, US; 3 × 60 s). Inner sides of the crowns were pretreated as recommended by the individual manufacturers (Table 1). All bonding was done adhesively (Clearfil Ceramic Primer: 60 s, Panavia F 2.0, Kuraray, J; Elipar Trilight, 3M Espe, USA; 3 × 60 s). The specimens were subjected to simultaneous thermal cycling (TC) and mechanical loading (ML) in order to simulate fatigue failures. Crowns were loaded pneumatically with a three-point contact situation in the central fossa by applying a load of 50 N for 1.2 × 106 cycles at a frequency of 1.6 Hz (simulated mouth opening: 2 mm). Steatite balls (CeramTec, Plochingen, D) with a diameter of 12 mm served as standardized antagonists. During mechanical loading specimens were thermally aged for 2 × 3000 cycles in distilled water at changing temperatures of 5 and 55 ◦ C, with a duration of 2 min for each cycle. These parameters are based on literature data on zirconia and ceramic restorations and might simulate a maximum of five years of oral service [16,17]. Online
Treatment crown 20 s 5% HF 50 m Al2 O3 , 1.5 bar 50 m Al2 O3 , 1.5 bar 110 m Al2 O3 , 2.0 bar 110 m Al2 O3 , 2.0 bar 50 m Al2 O3 , 1.5 bar 50 m Al2 O3 , 1.5 bar
failure control was performed and obviously damaged specimens were excluded from the further simulation. Crowns that failed during TCML were investigated in detail (light-microscope) for failure analysis. Fracture force of surviving restorations was determined by mechanically loading the specimens to failure in a universal testing machine (1446, Zwick, v = 1 mm/min). In analogy to chewing simulation the force was applied on the center of the crowns using a steel sphere (d = 12 mm) with a 0.25 mm tin foil (Dentaurum, D) inserted between crown and sphere to prevent force peaks. All systems were optically examined after fracture testing and the failure mode was documented. Calculations and statistical analysis were carried out using SPSS 22.0 for Windows (SPSS Inc., Chicago, IL, USA). Power calculation (G*Power 3.1.3, Kiel, G) provided an estimated power of >90% using eight specimens per group. Distribution of the data was controlled with Kolmogorov–Smirnov-test. Means and standard deviations were calculated, and analyzed using one-way analysis of variance (ANOVA) and the Bonferroni-test for post-hoc analysis. The level of significance was set to ˛ = 0.05.
3.
Results
In all groups crowns showed comparable wear traces on the occlusal contact areas after TCML. All PPV crowns in the LAB group showed cracks after TCML and were not further loaded to failure. Specimens of the other groups survived fatigue testing without failures. After TCML, fracture values (Table 2) in the three groups varied between 921.3 N (PPV) and 4817.8 N (ZIR) [CHAIR], 978.0 N (COH) and 5081.4 N (ZIR) [LAB], 746.7 N (PPV) and 3313.5 N (ZIR) [TOOTH]. Fracture forces differed significantly (p < 0.05) between the materials in the three groups. In all groups the highest fracture values were found for the reference zirconia material ZIR, and the lowest values for polyether ether ketone material with composite paste veneer. The materials COB, COL, PMV, PPV, COH, and ZIR showed significantly (p < 0.05) different fracture values for the individual groups. However, the materials COL and ZIR did not differ significantly (p > 0.05) between the groups CHAIR and LAB. ZLS crowns provided no significant (p > 0.05) differences between all individual groups. Failure analysis after loading to fracture identified different fracture patterns (Table 3). The majority of failure patterns were characterized either by crown factures in mesial–distal, buccal–oral, or mixed (mesial–distal/buccal–oral) directions. Only for the composite materials COB and COL fractures of the cusps were found, primarily in the TOOTH group (each 2×) and once in group LAB-COB. Crowns of the zirconia material ZIR
Please cite this article in press as: Preis V, et al. In-vitro fatigue and fracture testing of CAD/CAM-materials in implant-supported molar crowns. Dent Mater (2017), http://dx.doi.org/10.1016/j.dental.2017.01.003
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Table 2 – Fracture force (N) (mean and standard deviation, identical letters indicate significant differences (p < 0.05), probability (p), small letters: row, capital letters: line). CHAIR Mean ZLS COB COL PMV PPV COH ZIR p (material)
LAB Std
abc
2717.3 1667.6ABd 1490.3Ae 1920.9f 921.3ABag 1329.8Ach 4817.8Abdefgh 0.000
Mean
941.6 316.6 465.1 503.1 145.7 204.9 1600.7
abcde
2772.3 1090.8ACafgh 1526.8Bbij 1964.3fklm 0ACcgikno 978.0ABmoep 5081.4Bdhjlnp 0.000
Table 3 – Number and pattern of failure after fracture test (A: fracture mesial–distal; B: fracture buccal–oral; AB: mixed mesial–distal and buccal–oral; C: fracture cusp, D: no fracture of crown, bending of implant (group CHAIR)/fracture of embedding resin (group TOOTH); E: fracture of the veneer).
ZLS COB COL PMV PPV COH ZIR
CHAIR
LAB
1xA, 1xB, 6xAB 4xA, 1xB, 3xAB 8xA 7xA, 1xAB 7xA, 1AB 8xA 2xAB, 6xD
4xA, 5xAB 3xA, 4xB, 1xC 4xA, 2xB, 2xAB 3xA, 5xAB Cracks during TCML 5xA, 3xAB 4xA, 2xB, 2xAB
TOOTH 3xA, 5xAB 2xA, 4xAB, 2xC 2xA, 1xB, 3xAB, 2xC 8xE 8xE 5xA, 3xB 1xB, 7xD
stayed intact in most specimens of groups CHAIR-ZIR (6×) and TOOTH-ZIR (7×), but all crowns fractured in group LAB-ZIR. In specimens without crown fracture bending of the implants (group CHAIR) or fractures of the embedding resin (group TOOTH) were found instead. Crowns made of PEEK substructures combined with composite veneers (PMV, PPV) showed exclusively fractures of the veneer in the TOOTH group. In contrast, in the CHAIR group the prevalent failure mode was a mesial–distal fracture both for PMV and PPV (each 7×).
4.
Discussion
The first part of the hypothesis that molar crowns show different in vitro performance and fracture resistance when bonded to abutments chairside, bonded to abutments labside and screwed on implants chairside, or bonded to human teeth was widely confirmed. The results indicated a material-dependent performance and fracture force, supporting the second part of the hypothesis. Only one material (ZLS) provided no differences in fracture resistance between the three different groups. Assuming an altered loading situation with increased masticatory forces by rigid implant bearing, a higher fatigue and fracture resistance are commonly required for implant-supported restorations [9,18,19]. The tooth- or implant-supported loading situation in the present study influenced the fracture forces of most crown materials, and was further influenced by the labside/chairside procedure in the implant situation. For the composite materials COB, COL, and COH significantly higher fracture forces were found in the TOOTH group compared to the CHAIR or LAB groups, while
TOOTH Std 745.5 251.8 368.1 508.5 0 158.7 979.1
Mean a
2531.5 2143.5BCb 2565.8ABc 2588.4d 746.7BCe 1402.3Bacdf 3313.5ABbef 0.000
p (group) Std 528.5 448.3 419.0 601.0 746.7 177.0 210.3
0.799 0.000 0.000 0.038 0.000 0.000 0.011
only the zirconia material showed the significantly highest fracture resistance for the both implant groups. For PMV the TOOTH group showed also higher fracture forces but differences to CHAIR or LAB were not significant. Comparing groups CHAIR and LAB, the LAB group had inferior fracture resistance for COB and COH, while the clinical procedure had no significant influences for the other materials. All crowns of group LAB-PPV had already failed during aging, indicating low resistance against fatigue. The presence of the screw channel in the LAB group may therefore be a weak point and a critical factor for the success of implant-supported crowns if composite materials are applied. According to previous studies, the screw channel was expected to reduce the strength of screw-retained restorations [8,20,21]. In contrast, another study found no significant influence of the screw channel on the failure load of monolithic zirconia, lithium disilicate and veneered zirconia ceramic crowns [22], which is in agreement with the present results for the ceramic materials (ZLS, ZIR). Therefore, our results suggest that a weakening effect of a screw channel is closely related to the type of crown material, especially if materials of lower strength like composites are applied. Although promising in vitro results of resin-based materials used for implant-supported restorations have been reported [4,11,12], their mechanical properties differ from those of ceramics or metals [13,23]. Nevertheless, the present results have shown that the mechanical strength of the majority of crown materials investigated was high enough to withstand force peaks applied in the posterior region, which are reported to reach up to 900 N [24]. Only the material PPV may be applied with limitations. Here, the strength of the crowns was further decreased by the presence of a screw channel in the LAB group. Despite of the identical PEEK substructure in groups PMV and PPV, largely different in vitro performance and fracture resistance were observed, indicating a high influence of the type of the veneer in bi-layered structures. Superior mechanical resistance of the CAD/CAM milled composite veneer was shown, which may be attributed to a more homogenous structure and better material quality. Polymer-based CAD/CAM blocks are polymerized and monitored under standardized industrial conditions at high pressure and temperature, resulting in a higher conversion rate and a lower residual monomer content [25]. Compared to conventionally layered composite paste veneers, physical material properties [26,27] and wear resistance [28,29] are improved.
Please cite this article in press as: Preis V, et al. In-vitro fatigue and fracture testing of CAD/CAM-materials in implant-supported molar crowns. Dent Mater (2017), http://dx.doi.org/10.1016/j.dental.2017.01.003
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In accordance to the present results, strong differences in the fatigue and fracture resistance between ceramic and composite CAD/CAM crowns as well as between different types of ceramics in implant-supported crowns were reported previously [4,23,30]. However, failure analysis indicated contact-induced cracks and fractures in most materials. Force absorption, as naturally provided by the periodontal ligament of the tooth, is restricted to damping effects of the crown materials in implant-supported restorations. Therefore, resinbased crown materials that compensate for the absence of the natural periodontal damping effect might be advantageous under biomechanical considerations and minimize the risk of mechanical complications of the implant-abutmentcrown-combination and their connecting mechanics [13,31]. Previous studies have shown that resin-based materials and composites have higher shock absorbing capacity than ceramics [32,33]. Materials of a well-balanced ratio of sufficient strength and adequate shock absorbing capacity might be preferred for implant-supported restorations. According to our results, all resin-based crown materials except for PPV might meet these requirements. Though, no protective effect due to higher damping was obvious in this study as all materials (except for PPV) provided good TCML performance. During subsequent fracture testing, low shock-absorbing capacity and high-strength of zirconia may have resulted in bending of the implants in group CHAIR-ZIR and fracture of the embedding resin in group TOOTH-ZIR instead of damage to the crown, which may be classified as catastrophic failure in vivo as the whole implant would have to be removed. However, fracture testing does not reflect any clinically observable failure modes and the found fracture forces widely exceed physiological chewing forces and force peaks in vivo. As all crowns were bonded adhesively in this study, the influence of the cementation and luting has to be considered, too. None of the crowns debonded during TCML, indicating a good individual surface activation of the crown material by sandblasting or etching, respectively. Conventional cementation of high-strength zirconia implant-supported crowns might have improved shock absorbing capacity and buffered chewing forces without decreasing fracture strength. A severe loading situation for implant-supported crowns was also observed for PMV and PPV. Lower fracture resistance and more severe fracture patterns were found for CHAIR-PMV, LAB-PMV, and CHAIR-PPV compared to the TOOTH groups. For tooth-supported crowns made of veneered PEEK, only the veneer and not the substructure failed. Fracture forces of the ZLS material were comparable or even higher than reported in previous in vitro studies and were similar to values reported for lithium disilicate crowns [4,34]. Remarkably, a recent microstructural analysis of the material Suprinity has shown that the advertised material classification “zirconia-reinforced lithium silicate” has to be discussed critically, as no crystalline zirconia has been detected by Raman spectroscopy, suggesting that zirconia is dissolved in a glassy phase instead of being present as reinforcing particle [35]. Thus, a minor reinforcement mechanism as well as similar mechanical properties between ZLS and other glass ceramics like lithium disilicate might be explained. In contrast to a previous study [4], the presence of a screw channel did not decrease fracture resistance of ZLS, which
5
might draw attention to the drilling process. If done carefully, no pre-damaging of the brittle material structure might occur and no negative effects on strength might be expected for ZLS. The present study design focused on investigating implantsupported CAD/CAM crowns with regard to the material and clinical procedure. Using an one-piece implant dummy design without any further screwing joint, a critical discussion of the clinical significance is necessary. While screwed connections are often identified as the weakest part in implant-supported restorations [36,37], any failures related to the screw were eliminated in this study. Nevertheless, it could be shown that the presence of a screw channel weakened low-strength materials. A further limiting factor of the significance of this study may be the use of steatite antagonists instead of tooth antagonists for fatigue testing. Although guaranteeing antagonistic standardization, a different loading and wear situation may occur. Nevertheless, steatite spheres have proven their suitability for testing implant-supported restorations in previous studies [4,36,38]. In contrast to TCML where fatigue and wear phenomena of the antagonist might be wishful, fracture results may not be influenced by damage or deformation of the antagonist. Therefore, steel spheres of identical diameter were chosen for loading to fracture. Besides fatigue and fracture resistance of new CAD/CAM materials, biological aspects (e.g. plaque accumulation, soft tissue response) of new materials are considered important for clinical success, too. Commonly, higher plaque affinity and inflammatory effects have been attributed to resin-based materials and composites than to ceramics, further influenced by the surface roughness [39,40]. Further investigations may also consider the biological impact of new CAD/CAM materials for implant-supported restorations in order to enable a long-term clinical estimation of these materials.
5.
Conclusion
Based on the present in vitro results, most CAD/CAM materials, except for polyether ether ketone with composite paste veneers, may be applied in implant-supported crowns without restrictions. Different ceramic and resin-based materials partly performed differently in the implant or tooth situation. The insertion of a screw channel resulted in a total failure rate of polyether ether ketone crowns with composite paste veneers, and reduced fracture resistance for two composite materials (COB, COH).
Acknowledgment We would like to thank Bredent for providing the materials.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
In-vitro fatigue and fracture testing of CAD/CAM-materials in implant-supported molar crowns Verena Preis ∗ , Sebastian Hahnel, Michael Behr, Laila Bein, Martin Rosentritt Department of Prosthetic Dentistry, University Medical Center Regensburg, 93042 Regensburg, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To investigate the fatigue and fracture resistance of different CAD/CAM-materials
Received 4 November 2016
as implant- or tooth-supported molar crowns with respect to the clinical procedure
Received in revised form
(screwed/bonded restoration).
21 December 2016
Methods. 168 crowns were fabricated from different CAD/CAM-materials (n = 8/material):
Accepted 18 January 2017
ZLS (zirconia-reinforced lithium silicate ceramic; Suprinity, Vita-Zahnfabrik), COB (compos-
Available online xxx
ite; Brilliant Crios, Coltene), COL (composite; Lava Ultimate, 3M Espe), PMV/PPV (polyether ether ketone (PEEK) + milled composite veneer/composite paste veneer; BioHPP + HIPC
Keywords:
veneer/Crealign veneer, Bredent), COH (composite; Block HC, Shofu), and ZIR (zirconia; IPS
CAD/CAM
e.max ZirCAD, Ivoclar-Vivadent) as reference. Three groups were designed simulating the
PEEK
following clinical procedures: (a) chairside procedure ([CHAIR] implant crown bonded to
Composite
abutment), (b) labside procedure ([LAB] abutment and implant crown bonded in laboratory,
Zirconia
screwed chairside), and (c) reference ([TOOTH] crowns bonded on human teeth). Combined
Zirconia-reinforced lithium silicate
thermal cycling and mechanical loading (TCML) were performed simulating a 5-year clinical
Chewing simulation
situation. Fracture force was determined and failures were documented. Data were statisti-
Implant crown
cally analyzed (Kolmogorov–Smirnov-test, one-way-ANOVA; post-hoc-Bonferroni, ˛ = 0.05).
Abutment
Results. All crowns of group LAB-PPV showed cracks after TCML. The other groups sur-
Fracture resistance
vived fatigue testing without failures. Fracture forces varied between 921.3 N (PPV) and 4817.8 N (ZIR) [CHAIR], 978.0 N (COH) and 5081.4 N (ZIR) [LAB], 746.7 N (PPV) and 3313.5 N (ZIR) [TOOTH]. Significantly (p < 0.05) different fracture values were found between materials in all three groups. Only ZLS crowns provided no significant (p > 0.05) differences between the individual groups. Significance. Different ceramic and resin-based materials partly performed differently in implant or tooth situations. Individual resin-based materials (PPV, COB, COH) were weakened by inserting a screw channel. Most CAD/CAM-materials may be clinically applied in implant-supported crowns without restrictions. © 2017 The Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗ Corresponding author at: UKR University Medical Center Regensburg, Department of Prosthetic Dentistry, 93042 Regensburg, Germany. Fax: +49 941 944 6171. E-mail address: [email protected] (V. Preis). http://dx.doi.org/10.1016/j.dental.2017.01.003 0109-5641/© 2017 The Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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1.
Introduction
Advanced digital techniques and an increasing number of CAD/CAM (computer-aided design/computer-aidedmanufacturing) machinable materials enable continuous innovations in implant prosthetics. The benefits of a digital workflow including intra-oral scanning and CAD/CAM in combination with choosing an appropriate dental material may contribute to the success of implant-supported crowns. While implants with preformed or custom abutments are state of the art in implant dentistry, the success of chairside cemented or bonded abutments and crowns might be limited by gingival and per-implant inflammation caused by residual cement remaining in areas difficult to access [1]. To resolve this problem screwed titanium bases with bonded abutments and crowns are available that enable bonding areas distant from the sulcus. Synchronization of the titanium base and the implant platform guarantees perfect fit and force-fit connections, avoiding fitting inaccuracies like observed for custom CAD/CAM fabricated ceramic abutments [2,3]. Bonding implant crowns to the titanium base in the laboratory in advance and leaving a screw channel may have further advantages: a screw-retained chairside fixation of the abutment–crown combination allows easy and reversible access to the screw for retightening as well as an uncomplicated maintenance of the implant restoration if necessary. Superior bonding quality (dry conditions, surface activation, optimized polymerization) may be achieved under laboratory conditions, improving bonding durability and reducing inflammatory reactions. Nevertheless, the strength of the crown might be affected by the presence of the screw channel [4–8]. Besides an optimized fabrication process and chairside/labside procedure, the selection of the appropriate crown material may essentially contribute to enduring success. A broad range of CAM machinable blocks is available for resin-based materials (composites, PEEK, PMMA), ceramics (feldspar, zirconia, lithium disilicate, zirconia-reinforced lithium silicate), and resin-infiltrated ceramics, which may be applied as monolithic restorations or with subsequent veneering. As implant crowns are more prone to occlusal overloading than tooth-supported crowns due to the missing of the physiological semi-elastic connection (periodontal ligament) and the tactile sensitivity, the application of brittle materials may cause numerous in vivo complications like fracture or chipping [9,10]. To overcome or minimize the risk of fracture, resin-based materials with improved shock absorbing capacity or monolithic ceramics of high strength might by preferred. However, despite of promising results of resin-based materials in implant-supported restorations [11,12], their mechanical resistance may be inferior to ceramics [13]. Up to date only limited scientific information and even less clinical data are available that show the performance of different currently available CAD/CAM materials used in implant-supported crowns with respect to the labside and chairside procedure. To give a first predication of their clinical survival, in vitro fatigue and fracture testing of CAD/CAMfabricated crowns may be helpful.
Fig. 1 – Designs of groups: LAB, CHAIR, and TOOTH (shaded area: artificial periodontium).
The hypothesis tested in this in vitro study was that molar crowns show different in vitro performance and fracture resistance when a) bonded to abutments chairside, bonded in the laboratory and screwed on implants chairside, or bonded to human teeth, or b) different CAD/CAM materials were used.
2.
Materials and methods
A total of 168 identically shaped molar crowns (tooth 46) were fabricated from different CAD/CAM materials (n = 8 per material and group), representing three resin-based composites, one polyether ether ketone (PEEK) combined with two different types of composite veneers, one zirconia-reinforced lithium silicate ceramic, and one zirconia ceramic (reference material). Details on the materials and their manufacturers are given in Table 1. For each material, three groups were designed to simulate the following clinical procedures (Fig. 1): a Group ‘CHAIR’ (chairside procedure): the crown was directly bonded onto the implant-abutment analog and the excess luting material was removed. b Group ‘LAB’ (labside procedure): a screw channel was manually drilled into the central fossa of the crown with a diamond bur (red/fine, diameter: 1.5 mm, water cooling). The crown was bonded onto the implant-abutment analog, the excess luting material was removed, and the screw channel was restored with composite (Filtek Supreme; Elipar Trilight 40 s, 3M Espe, D). c Group ‘TOOTH’ (reference group): crowns were luted on prepared human molar teeth. In the groups ‘CHAIR’ and ‘LAB’, the implant-abutment analogs (n = 112; Straumann, D, titanium grade IV, implant diameter 4.1 mm, implant length 12 mm, abutment length 6 mm, 6◦ ) were vertically positioned in resin blocks (Palapress Vario, Heraeus-Kulzer, D) in order to simulate the posterior implant situation replacing tooth 46. For the group ‘TOOTH’ extracted caries-free human molars (n = 56) were collected at
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Table 1 – Materials, manufacturers and crown treatment. Code
Material, manufacturer
Comment
ZLS COB COL PMV PPV COH ZIR
Suprinity, Vita Zahnfabrik, D Brilliant Crios, Coltene, CH Lava Ultimate, 3M Espe, USA BioHPP + HIPC veneer, Bredent, D BioHPP + Crealign veneer, Bredent, D Block HC, Shofu, J IPS e.max ZirCAD, IvoclarVivadent, FL
Zirconia-reinforced lithium silicate ceramic, crystallized Composite Composite Polyether ether ketone + milled composite veneer Polyether ether ketone + composite paste veneer Composite Zirconia
the University Medical Center Regensburg and stored in 0.5% chloramine solution for no longer than 4 weeks. The variability of human molars was respected by preselecting teeth with comparable size and shape and by statistically randomly dividing the teeth to the subgroups. Preselecting also guaranteed comparable preparation conditions. The teeth were prepared according to ceramic guidelines with a 1.5 mm axial and occlusal anatomical reduction and a 1 mm circumferential deep shoulder with rounded inner angles at an isogingival height of the tooth cervix and a convergence angle of 4◦ . All teeth were prepared by one person with identical preparation equipment. Standardized preparation was performed on basis of an original model, and preparation design was controlled with a gage. For simulating the resilience of the human periodontium the roots of the teeth were coated with a 1 mm polyether layer (Impregum, 3M Espe, D) and were vertically positioned in resin blocks (Palapress Vario, Heraeus Kulzer, D). The procedure of fabricating an artificial periodontium was described previously [14,15]. Abutments and prepared teeth were digitalized (Cerec Omnicam, Sirona, D) and full-contour molar crowns were milled (Cerec, MCXL, Sirona, D) from all materials except for polyether ether ketone. The circular and occlusal wall thickness of the full-contour crowns depended on the abutment, but in all cases was >1.5 mm. For the polyether ether ketone material substructures for subsequent veneering were milled in a first fabrication step. Then, the crowns were completed with either milled composite veneers (PMV) or conventional composite paste veneers (PPV) by using a composite primer (visio.link, Bredent, D). Abutments were sandblasted (110 m Al2 O3 , 1.5 bar) and teeth were conditioned (ED Primer II; Panavia F2.0, Kuraray, J; Elipar Trilight, 3M Espe, US; 3 × 60 s). Inner sides of the crowns were pretreated as recommended by the individual manufacturers (Table 1). All bonding was done adhesively (Clearfil Ceramic Primer: 60 s, Panavia F 2.0, Kuraray, J; Elipar Trilight, 3M Espe, USA; 3 × 60 s). The specimens were subjected to simultaneous thermal cycling (TC) and mechanical loading (ML) in order to simulate fatigue failures. Crowns were loaded pneumatically with a three-point contact situation in the central fossa by applying a load of 50 N for 1.2 × 106 cycles at a frequency of 1.6 Hz (simulated mouth opening: 2 mm). Steatite balls (CeramTec, Plochingen, D) with a diameter of 12 mm served as standardized antagonists. During mechanical loading specimens were thermally aged for 2 × 3000 cycles in distilled water at changing temperatures of 5 and 55 ◦ C, with a duration of 2 min for each cycle. These parameters are based on literature data on zirconia and ceramic restorations and might simulate a maximum of five years of oral service [16,17]. Online
Treatment crown 20 s 5% HF 50 m Al2 O3 , 1.5 bar 50 m Al2 O3 , 1.5 bar 110 m Al2 O3 , 2.0 bar 110 m Al2 O3 , 2.0 bar 50 m Al2 O3 , 1.5 bar 50 m Al2 O3 , 1.5 bar
failure control was performed and obviously damaged specimens were excluded from the further simulation. Crowns that failed during TCML were investigated in detail (light-microscope) for failure analysis. Fracture force of surviving restorations was determined by mechanically loading the specimens to failure in a universal testing machine (1446, Zwick, v = 1 mm/min). In analogy to chewing simulation the force was applied on the center of the crowns using a steel sphere (d = 12 mm) with a 0.25 mm tin foil (Dentaurum, D) inserted between crown and sphere to prevent force peaks. All systems were optically examined after fracture testing and the failure mode was documented. Calculations and statistical analysis were carried out using SPSS 22.0 for Windows (SPSS Inc., Chicago, IL, USA). Power calculation (G*Power 3.1.3, Kiel, G) provided an estimated power of >90% using eight specimens per group. Distribution of the data was controlled with Kolmogorov–Smirnov-test. Means and standard deviations were calculated, and analyzed using one-way analysis of variance (ANOVA) and the Bonferroni-test for post-hoc analysis. The level of significance was set to ˛ = 0.05.
3.
Results
In all groups crowns showed comparable wear traces on the occlusal contact areas after TCML. All PPV crowns in the LAB group showed cracks after TCML and were not further loaded to failure. Specimens of the other groups survived fatigue testing without failures. After TCML, fracture values (Table 2) in the three groups varied between 921.3 N (PPV) and 4817.8 N (ZIR) [CHAIR], 978.0 N (COH) and 5081.4 N (ZIR) [LAB], 746.7 N (PPV) and 3313.5 N (ZIR) [TOOTH]. Fracture forces differed significantly (p < 0.05) between the materials in the three groups. In all groups the highest fracture values were found for the reference zirconia material ZIR, and the lowest values for polyether ether ketone material with composite paste veneer. The materials COB, COL, PMV, PPV, COH, and ZIR showed significantly (p < 0.05) different fracture values for the individual groups. However, the materials COL and ZIR did not differ significantly (p > 0.05) between the groups CHAIR and LAB. ZLS crowns provided no significant (p > 0.05) differences between all individual groups. Failure analysis after loading to fracture identified different fracture patterns (Table 3). The majority of failure patterns were characterized either by crown factures in mesial–distal, buccal–oral, or mixed (mesial–distal/buccal–oral) directions. Only for the composite materials COB and COL fractures of the cusps were found, primarily in the TOOTH group (each 2×) and once in group LAB-COB. Crowns of the zirconia material ZIR
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Table 2 – Fracture force (N) (mean and standard deviation, identical letters indicate significant differences (p < 0.05), probability (p), small letters: row, capital letters: line). CHAIR Mean ZLS COB COL PMV PPV COH ZIR p (material)
LAB Std
abc
2717.3 1667.6ABd 1490.3Ae 1920.9f 921.3ABag 1329.8Ach 4817.8Abdefgh 0.000
Mean
941.6 316.6 465.1 503.1 145.7 204.9 1600.7
abcde
2772.3 1090.8ACafgh 1526.8Bbij 1964.3fklm 0ACcgikno 978.0ABmoep 5081.4Bdhjlnp 0.000
Table 3 – Number and pattern of failure after fracture test (A: fracture mesial–distal; B: fracture buccal–oral; AB: mixed mesial–distal and buccal–oral; C: fracture cusp, D: no fracture of crown, bending of implant (group CHAIR)/fracture of embedding resin (group TOOTH); E: fracture of the veneer).
ZLS COB COL PMV PPV COH ZIR
CHAIR
LAB
1xA, 1xB, 6xAB 4xA, 1xB, 3xAB 8xA 7xA, 1xAB 7xA, 1AB 8xA 2xAB, 6xD
4xA, 5xAB 3xA, 4xB, 1xC 4xA, 2xB, 2xAB 3xA, 5xAB Cracks during TCML 5xA, 3xAB 4xA, 2xB, 2xAB
TOOTH 3xA, 5xAB 2xA, 4xAB, 2xC 2xA, 1xB, 3xAB, 2xC 8xE 8xE 5xA, 3xB 1xB, 7xD
stayed intact in most specimens of groups CHAIR-ZIR (6×) and TOOTH-ZIR (7×), but all crowns fractured in group LAB-ZIR. In specimens without crown fracture bending of the implants (group CHAIR) or fractures of the embedding resin (group TOOTH) were found instead. Crowns made of PEEK substructures combined with composite veneers (PMV, PPV) showed exclusively fractures of the veneer in the TOOTH group. In contrast, in the CHAIR group the prevalent failure mode was a mesial–distal fracture both for PMV and PPV (each 7×).
4.
Discussion
The first part of the hypothesis that molar crowns show different in vitro performance and fracture resistance when bonded to abutments chairside, bonded to abutments labside and screwed on implants chairside, or bonded to human teeth was widely confirmed. The results indicated a material-dependent performance and fracture force, supporting the second part of the hypothesis. Only one material (ZLS) provided no differences in fracture resistance between the three different groups. Assuming an altered loading situation with increased masticatory forces by rigid implant bearing, a higher fatigue and fracture resistance are commonly required for implant-supported restorations [9,18,19]. The tooth- or implant-supported loading situation in the present study influenced the fracture forces of most crown materials, and was further influenced by the labside/chairside procedure in the implant situation. For the composite materials COB, COL, and COH significantly higher fracture forces were found in the TOOTH group compared to the CHAIR or LAB groups, while
TOOTH Std 745.5 251.8 368.1 508.5 0 158.7 979.1
Mean a
2531.5 2143.5BCb 2565.8ABc 2588.4d 746.7BCe 1402.3Bacdf 3313.5ABbef 0.000
p (group) Std 528.5 448.3 419.0 601.0 746.7 177.0 210.3
0.799 0.000 0.000 0.038 0.000 0.000 0.011
only the zirconia material showed the significantly highest fracture resistance for the both implant groups. For PMV the TOOTH group showed also higher fracture forces but differences to CHAIR or LAB were not significant. Comparing groups CHAIR and LAB, the LAB group had inferior fracture resistance for COB and COH, while the clinical procedure had no significant influences for the other materials. All crowns of group LAB-PPV had already failed during aging, indicating low resistance against fatigue. The presence of the screw channel in the LAB group may therefore be a weak point and a critical factor for the success of implant-supported crowns if composite materials are applied. According to previous studies, the screw channel was expected to reduce the strength of screw-retained restorations [8,20,21]. In contrast, another study found no significant influence of the screw channel on the failure load of monolithic zirconia, lithium disilicate and veneered zirconia ceramic crowns [22], which is in agreement with the present results for the ceramic materials (ZLS, ZIR). Therefore, our results suggest that a weakening effect of a screw channel is closely related to the type of crown material, especially if materials of lower strength like composites are applied. Although promising in vitro results of resin-based materials used for implant-supported restorations have been reported [4,11,12], their mechanical properties differ from those of ceramics or metals [13,23]. Nevertheless, the present results have shown that the mechanical strength of the majority of crown materials investigated was high enough to withstand force peaks applied in the posterior region, which are reported to reach up to 900 N [24]. Only the material PPV may be applied with limitations. Here, the strength of the crowns was further decreased by the presence of a screw channel in the LAB group. Despite of the identical PEEK substructure in groups PMV and PPV, largely different in vitro performance and fracture resistance were observed, indicating a high influence of the type of the veneer in bi-layered structures. Superior mechanical resistance of the CAD/CAM milled composite veneer was shown, which may be attributed to a more homogenous structure and better material quality. Polymer-based CAD/CAM blocks are polymerized and monitored under standardized industrial conditions at high pressure and temperature, resulting in a higher conversion rate and a lower residual monomer content [25]. Compared to conventionally layered composite paste veneers, physical material properties [26,27] and wear resistance [28,29] are improved.
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In accordance to the present results, strong differences in the fatigue and fracture resistance between ceramic and composite CAD/CAM crowns as well as between different types of ceramics in implant-supported crowns were reported previously [4,23,30]. However, failure analysis indicated contact-induced cracks and fractures in most materials. Force absorption, as naturally provided by the periodontal ligament of the tooth, is restricted to damping effects of the crown materials in implant-supported restorations. Therefore, resinbased crown materials that compensate for the absence of the natural periodontal damping effect might be advantageous under biomechanical considerations and minimize the risk of mechanical complications of the implant-abutmentcrown-combination and their connecting mechanics [13,31]. Previous studies have shown that resin-based materials and composites have higher shock absorbing capacity than ceramics [32,33]. Materials of a well-balanced ratio of sufficient strength and adequate shock absorbing capacity might be preferred for implant-supported restorations. According to our results, all resin-based crown materials except for PPV might meet these requirements. Though, no protective effect due to higher damping was obvious in this study as all materials (except for PPV) provided good TCML performance. During subsequent fracture testing, low shock-absorbing capacity and high-strength of zirconia may have resulted in bending of the implants in group CHAIR-ZIR and fracture of the embedding resin in group TOOTH-ZIR instead of damage to the crown, which may be classified as catastrophic failure in vivo as the whole implant would have to be removed. However, fracture testing does not reflect any clinically observable failure modes and the found fracture forces widely exceed physiological chewing forces and force peaks in vivo. As all crowns were bonded adhesively in this study, the influence of the cementation and luting has to be considered, too. None of the crowns debonded during TCML, indicating a good individual surface activation of the crown material by sandblasting or etching, respectively. Conventional cementation of high-strength zirconia implant-supported crowns might have improved shock absorbing capacity and buffered chewing forces without decreasing fracture strength. A severe loading situation for implant-supported crowns was also observed for PMV and PPV. Lower fracture resistance and more severe fracture patterns were found for CHAIR-PMV, LAB-PMV, and CHAIR-PPV compared to the TOOTH groups. For tooth-supported crowns made of veneered PEEK, only the veneer and not the substructure failed. Fracture forces of the ZLS material were comparable or even higher than reported in previous in vitro studies and were similar to values reported for lithium disilicate crowns [4,34]. Remarkably, a recent microstructural analysis of the material Suprinity has shown that the advertised material classification “zirconia-reinforced lithium silicate” has to be discussed critically, as no crystalline zirconia has been detected by Raman spectroscopy, suggesting that zirconia is dissolved in a glassy phase instead of being present as reinforcing particle [35]. Thus, a minor reinforcement mechanism as well as similar mechanical properties between ZLS and other glass ceramics like lithium disilicate might be explained. In contrast to a previous study [4], the presence of a screw channel did not decrease fracture resistance of ZLS, which
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might draw attention to the drilling process. If done carefully, no pre-damaging of the brittle material structure might occur and no negative effects on strength might be expected for ZLS. The present study design focused on investigating implantsupported CAD/CAM crowns with regard to the material and clinical procedure. Using an one-piece implant dummy design without any further screwing joint, a critical discussion of the clinical significance is necessary. While screwed connections are often identified as the weakest part in implant-supported restorations [36,37], any failures related to the screw were eliminated in this study. Nevertheless, it could be shown that the presence of a screw channel weakened low-strength materials. A further limiting factor of the significance of this study may be the use of steatite antagonists instead of tooth antagonists for fatigue testing. Although guaranteeing antagonistic standardization, a different loading and wear situation may occur. Nevertheless, steatite spheres have proven their suitability for testing implant-supported restorations in previous studies [4,36,38]. In contrast to TCML where fatigue and wear phenomena of the antagonist might be wishful, fracture results may not be influenced by damage or deformation of the antagonist. Therefore, steel spheres of identical diameter were chosen for loading to fracture. Besides fatigue and fracture resistance of new CAD/CAM materials, biological aspects (e.g. plaque accumulation, soft tissue response) of new materials are considered important for clinical success, too. Commonly, higher plaque affinity and inflammatory effects have been attributed to resin-based materials and composites than to ceramics, further influenced by the surface roughness [39,40]. Further investigations may also consider the biological impact of new CAD/CAM materials for implant-supported restorations in order to enable a long-term clinical estimation of these materials.
5.
Conclusion
Based on the present in vitro results, most CAD/CAM materials, except for polyether ether ketone with composite paste veneers, may be applied in implant-supported crowns without restrictions. Different ceramic and resin-based materials partly performed differently in the implant or tooth situation. The insertion of a screw channel resulted in a total failure rate of polyether ether ketone crowns with composite paste veneers, and reduced fracture resistance for two composite materials (COB, COH).
Acknowledgment We would like to thank Bredent for providing the materials.
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