1

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.

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.

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.

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.

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

1. Fatigue testing of biomaterials and their interfaces
2. Response to antiseptic agents of periodontal pathogens in in vitrobiofilms on titanium and zirconium surfaces
3. Effects of antioxidants on DNA double-strand breaks in human gingival fibroblasts exposed to dental resin co-monomer epoxy metabolites
4. Direct resin composite restorations for fractured maxillary teeth and diastema closure: A 7 years retrospective evaluation of survival and influencing factors
5. Effects of two grading techniques of zirconia material on the fatigue limit of full-contour 3-unit fixed dental prostheses
6. Strain development in bulk-filled cavities of different depths characterized using a non-destructive acoustic emission approach

Stay updated, free dental videos. Join our Telegram channel

Join