In-vitro fatigue and fracture testing of CAD/CAM-materials in implant-supported molar crowns

Abstract

Objective

To investigate the fatigue and fracture resistance of different CAD/CAM-materials as implant- or tooth-supported molar crowns with respect to the clinical procedure (screwed/bonded restoration).

Methods

168 crowns were fabricated from different CAD/CAM-materials (n = 8/material): ZLS (zirconia-reinforced lithium silicate ceramic; Suprinity, Vita-Zahnfabrik), COB (composite; Brilliant Crios, Coltene), COL (composite; Lava Ultimate, 3M Espe), PMV/PPV (polyether ether ketone (PEEK) + milled composite veneer/composite paste veneer; BioHPP + HIPC veneer/Crealign veneer, Bredent), COH (composite; Block HC, Shofu), and ZIR (zirconia; IPS e.max ZirCAD, Ivoclar-Vivadent) as reference. Three groups were designed simulating the following clinical procedures: (a) chairside procedure ([CHAIR] implant crown bonded to abutment), (b) labside procedure ([LAB] abutment and implant crown bonded in laboratory, screwed chairside), and (c) reference ([TOOTH] crowns bonded on human teeth). Combined thermal cycling and mechanical loading (TCML) were performed simulating a 5-year clinical situation. Fracture force was determined and failures were documented. Data were statistically analyzed (Kolmogorov–Smirnov-test, one-way-ANOVA; post-hoc-Bonferroni, α = 0.05).

Results

All crowns of group LAB-PPV showed cracks after TCML. The other groups survived 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. ​

Introduction

Advanced digital techniques and an increasing number of CAD/CAM (computer-aided design/computer-aided-manufacturing) 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 . 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 .

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 .

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 . 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 , their mechanical resistance may be inferior to ceramics .

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/CAM-fabricated crowns may be helpful.

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.

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.

Table 1
Materials, manufacturers and crown treatment.
Code Material, manufacturer Comment Treatment crown
ZLS Suprinity, Vita Zahnfabrik, D Zirconia-reinforced lithium silicate ceramic, crystallized 20 s 5% HF
COB Brilliant Crios, Coltene, CH Composite 50 μm Al 2 O 3 , 1.5 bar
COL Lava Ultimate, 3M Espe, USA Composite 50 μm Al 2 O 3 , 1.5 bar
PMV BioHPP + HIPC veneer, Bredent, D Polyether ether ketone + milled composite veneer 110 μm Al 2 O 3 , 2.0 bar
PPV BioHPP + Crealign veneer, Bredent, D Polyether ether ketone + composite paste veneer 110 μm Al 2 O 3 , 2.0 bar
COH Block HC, Shofu, J Composite 50 μm Al 2 O 3 , 1.5 bar
ZIR IPS e.max ZirCAD, IvoclarVivadent, FL Zirconia 50 μm Al 2 O 3 , 1.5 bar

Fig. 1
Designs of groups: LAB, CHAIR, and TOOTH (shaded area: artificial periodontium).

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 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 .

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 μm Al 2 O 3 , 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 × 10 6 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 . Online 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 α = 0.05.

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.

Table 1
Materials, manufacturers and crown treatment.
Code Material, manufacturer Comment Treatment crown
ZLS Suprinity, Vita Zahnfabrik, D Zirconia-reinforced lithium silicate ceramic, crystallized 20 s 5% HF
COB Brilliant Crios, Coltene, CH Composite 50 μm Al 2 O 3 , 1.5 bar
COL Lava Ultimate, 3M Espe, USA Composite 50 μm Al 2 O 3 , 1.5 bar
PMV BioHPP + HIPC veneer, Bredent, D Polyether ether ketone + milled composite veneer 110 μm Al 2 O 3 , 2.0 bar
PPV BioHPP + Crealign veneer, Bredent, D Polyether ether ketone + composite paste veneer 110 μm Al 2 O 3 , 2.0 bar
COH Block HC, Shofu, J Composite 50 μm Al 2 O 3 , 1.5 bar
ZIR IPS e.max ZirCAD, IvoclarVivadent, FL Zirconia 50 μm Al 2 O 3 , 1.5 bar

Nov 22, 2017 | Posted by in Dental Materials | Comments Off on In-vitro fatigue and fracture testing of CAD/CAM-materials in implant-supported molar crowns
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