Reliability and failure behavior of CAD-on fixed partial dentures

Highlights

  • This study evaluates the reliability of ceramic fixed partial dentures prepared using the CAD-on technology.

  • This study tested the hypothesis that cyclic fatigue influences the reliability and failure behavior of the FPDs when compared to fast fracture testing.

  • Porcelain chipping was the predominant (60%) mode of failure for FPDs tested in fast fracture and connector failure was predominant (67%) under fatigue.

  • When chipping and connector fracture data were analyzed separately, the β values were 7.9 and 2.9.

  • The test method (fast fracture or fatigue) significantly influenced the reliability of FPDs fabricated using the CAD-on technique, but it did not influence their failure behavior.

Abstract

Objectives

To estimate the reliability and failure behavior of fixed partial dentures (FPDs) fabricated using the CAD-on technique.

Methods

FPDs ( n = 25) were fabricated using a CAD/CAM system: IPS e.max ZirCAD – Crystall./Connect and IPS e.max CAD (Ivoclar). The restoration type (“three-unit bridge”) and design method (“multilayer”) based on Biogenerics were used. Framework and porcelain structures were united using a fusion ceramic (Crystall./Connect, Ivoclar). Mechanical fatigue was tested in a servohydraulic load frame machine at a cyclic loading (frequency: 2 Hz; load ratio: 0.1). Based on previous data from specimens tested in fast fracture, three different stress profiles were used. The lifetime data were analyzed using an inverse power law-Weibull cumulative damage model (ALTA PRO, Reliasoft). All failed specimens were examined under a field emission scanning electron microscope.

Results

Porcelain chipping was the predominant (60%) mode of failure for FPDs tested in fast fracture and connector failure was predominant (67%) under fatigue. For fast fracture data, the Weibull modulus ( β ) of FPDs was 7.8 combining the two failure modes. When chipping and connector fracture data were analyzed separately, β values were 7.9 and 2.9. For the step stress fatigue test, β values were lower than estimated using fast fracture, being 1.6 for connector fracture and 1.3 for porcelain chipping.

Significance

The test method (fast fracture or fatigue) significantly influenced the reliability of FPDs fabricated using the CAD-on technique, but it did not influence their failure behavior.

Introduction

Recent improvements in dental ceramics have resulted in extensive use of all-ceramic fixed partial dentures (FPDs). For the anterior teeth, all-ceramic FPDs show clinical performance similar to metal-ceramic restorations . For the posterior teeth, all-ceramic FPDs have also been used, particularly after the introduction of yttrium-oxide tetragonal partially-stabilized zirconia (Y-TZP) and the improvement in manufacturing technique provided by CAD/CAM (computer-aided design/computer-aided machining) technology . Yet, the clinical success of posterior all-ceramic FPDs has been compromised by high rates of porcelain chipping .

Y-TZP shows high fracture toughness when compared to other ceramics but its optical properties do not suit the esthetic needs of most patients. Therefore, most restorations are veneered with an esthetic ceramic (e.g. glass-ceramic, feldspathic porcelain). Veneering can be performed by different methods, such as: conventional layering, slip casting and hot-pressing .

However, the multilayered configuration of all-ceramic Y-TZP-based FPDs has been associated to clinical failures such as chipping, cracking, and delamination of the porcelain veneer. These failures might be influenced by differences in elastic moduli between the ceramic layers, tooth preparation and infrastructure design, and veneering thickness . Failures have been also attributed to thermally induced residual stresses produced by coefficient of thermal expansion mismatches , cooling rates, and differences in the materials’ thermal diffusivity .

In order to reduce chipping, it has been suggested that a more resistant veneering material, such as the lithium disilicate ceramic, may be used as an alternative to the conventional feldspathic porcelain . In addition, in 2011, a new technique known as “CAD-on” was introduced to produce multilayered all-ceramic restorations using only the CAD/CAM system. Through this technique, both the Y-TZP infrastructure and the lithium disilicate veneer are fabricated using CAD/CAM technology and subsequently bonded together using a fusion ceramic. Being a relatively new technique, there is scarce information regarding the mechanical reliability of these restorations .

The mechanical behavior of a dental restorative material is usually evaluated under controlled laboratory conditions. To produce clinically relevant data, these in vitro studies should consider the influence of the geometry and configuration of multilayered restorations on the stress distribution and the effect of cyclic fatigue on the fracture strength and failure modes. Ceramic materials are especially susceptible to a phenomenon called subcritical crack growth (SCG), which is the stable growth of pre-existing flaws under sub-critical conditions. SCG is influenced by humidity, pH, temperature fluctuations, and stresses induced by cyclic loading, which are present in the oral environment .

Therefore, the objective of this study was to estimate the reliability and failure behavior of FPDs fabricated using the CAD-on technique, testing the hypotheses that (1) the reliability of FPDs tested in cyclic fatigue is lower than in fast fracture, and (2) the failure behavior is influenced by the test mode (fast fracture and cyclic fatigue).

Materials and methods

Preparation of FPDs

The ceramic materials used to produce the FPDs are described in Table 1 . Twenty-five FPDs were produced by the CAD/CAM system (CEREC, Sirona, NY, EUA), using the CAD-on method (Ivoclar Vivadent). Simulated abutment structures were constructed using fiberglass-reinforced epoxy polymer (NEMA G10; Piedmont Plastics, Charlotte, NC, USA), which has similar elastic modulus of hydrated dentin and allow for adhesive bonding of resin-based restorative materials . The abutments base had 8 mm in diameter and 6 mm in height, prepared with a rounded shoulder cervical finish line (radius 0.5 mm) and a 12° total occlusal convergence . These abutments were scanned using the laboratory scanner (inEos, Sirona) to fabricate the FPD frameworks. The restoration type (“three-unit bridge”) and design method (“multilayer”) were input in the CAD/CAM system and calculated based on the Biogenerics software. Y-TZP blocks (IPS e.max ZirCAD, Ivoclar Vivadent) were milled and the resulting infrastructures were sintered at 1500 °C for 7 h in a ceramic furnace (Fire HTC; Sirona Dental Services, Bensheim, Germany). The Y-TZP infrastructure were scanned, the veneer layer was designed and milled using lithium disilicate ceramic blocks (IPS e.max CAD, Ivoclar Vivadent). The dimensions of the FPDs were as follows: framework thickness of 0.7 mm, veneer thickness of 1 mm, and 9 mm 2 of connector cross-section area.

Table 1
Description of the materials used in the present study.
Material a Description Clinical indication
IPS e.max ZirCAD Yttria partially stabilized tetragonal zirconia polycrystal (Y-TZP) Framework
IPS e.max Crystall./Connect Fusion glass-ceramic Bonding the framework and veneer structures
IPS e.max CAD Lithium disilicate glass-ceramic Veneer

a Manufactured by Ivoclar Vivadent, Schaan, Liechtenstein.

The infrastructure and veneer structures were cleaned in a sonic water bath and fused together using a glass-ceramic material (IPS e.max CAD Crystall./Connect, Ivoclar Vivadent). The pre-dosed capsule containing the powder and liquid of the fusion glass-ceramic was mixed in vibration (Ivomix, Ivoclar Vivadent) for 10 s. The capsule was opened, and the material was applied to the intaglio surface of the lithium disilicate veneer structure, briefly pressing its occlusal surface against the vibrating plate to evenly dispense the fusion glass-ceramic. The Y-TZP infrastructure was adapted onto the veneer structure, and the two structures were lightly pressed against each other. Excess fusion glass was removed with a microbrush before sintering. The fusion process and crystallization of the veneer ceramic were conducted simultaneously (Multimat Touch & Press; Dentsply Int. York, PA, USA) following manufacturer’s instructions ( Table 2 ).

Table 2
Firing parameters a used for framework-veneer fusion and veneer crystallization.
Working temperature 403 °C
Closing time 2 min
Heating rate t 1 30 °C/min
Temperature T 1 820 °C
Heating rate t 2 30 °C/min
Temperature T 2 840 °C
Exposure time H 1 2 min
Exposure time H 2 7 min
Vacuum V1 1 /V1 2 550/820 °C
Vacuum V2 1 /V2 2 820/840 °C
Cooling 600 °C

a The sintering protocol was carried out according to the manufacturer’s recommendations.

Cementation into the abutments was performed using dual-cure resin cement (Multilink Automix, Ivoclar Vivadent). Preparations were cleaned with isopropyl alcohol, the bonding area was etched with 10% hydrofluoric acid for 1 min, washed in water, dried using oil free air, silanated (Monobond Plus, Ivoclar Vivadent), and the adhesive system (Multilink, primer A and B; Ivoclar Vivadent) was applied according to the manufacturer’s instructions. The FPDs were cleaned with isopropyl alcohol, the intaglio surface was airborne-particle abraded with 50-μm alumina particles at 25 psi pressure for 20 s from a distance of 10 mm. The FPDs were sonically cleaned in a deionized water bath for 5 min, dried with oil-free air and silanated. The cement was dispensed from the auto mix syringe, applied onto the intaglio surface of the FPDs that was sit on the preparations. The excess of cement was removed with a microbrush, the FPDs were light-cured for 40 s, and the specimens were stored in distilled water.

Fast fracture and cyclic fatigue tests

Ten FPDs were tested in rapid monotonic loading (fast fracture), and 15 FPDs were used for the step-stress cyclic fatigue test. A larger number of specimens were used for fatigue to increase statistical power because this test method results in highly scattered data. The fast fracture test was performed under compressive load applied by a 6-mm diameter tungsten carbide spherical piston in the center of the pontic using a servo-hydraulic load frame machine (MTS, Flextest 60, Eden Prairie, MN, USA) in a 37 °C deionized water bath at 26 MPa/s constant load rate. A polyethylene film was placed between the piston and the specimen during loading.

Mechanical fatigue was performed using the same configuration of the fast fracture test. A servo-hydraulic load frame machine was used and FPDs were immersed in 37 °C deionized water during testing. Load was applied at 2 Hz frequency and with 0.1 load ratio. The step-stress method was used to perform the fatigue test. Based on data from specimens previously tested in fast fracture and their individual lifetimes, three different stress profiles were used: mild ( n = 2), moderate ( n = 6), and aggressive ( n = 7), and the number of cycles to failure was recorded. Fatigue testing always started with 200 N peak waveform load; then the peak load was increased by 1 N per 1000 cycles for the mild stress profile, 1 N per 600 cycles for the moderate stress profile, and 1 N per 313 cycles for the aggressive stress profile. Failure was detected by an acoustic system (Song Meter SM2+; Wildlife Acoustics, Concord, MA, USA) using a hydrophone placed in the water bath. Data were recorded in memory cards and analyzed using a software (Audacity Sound Editor; Free Software Foundation, Boston, MA, USA). Files were searched for the first evident acoustic emission that should correspond to the first supercritical crack.

Lifetime data were analyzed using an inverse power law-Weibull cumulative damage model (ALTA PRO; Reliasoft, Tucson, AZ, USA). A cumulative damage model with an inverse power law (IPL) lifetime-stress relation and a Weibull lifetime distribution were used to fit the fatigue data (ALTA PRO 7, Reliasoft).

Fractographic analysis

After testing, all specimens were sonically cleaned in an anionic detergent bath for 5 min followed by a second bath with isopropyl alcohol for 5 min and dried. Fractographic analysis of the fracture surfaces was initially performed using a stereomicroscope. Then the fracture surfaces were gold coated for scanning electron microscopy (SEM–SUPRA 40, Carl Zeiss Microimaging, Thornwood, NY, USA). All fracture surfaces were examined to identify fracture markings and to determine the critical flaw and failure modes. Fischer exact test ( α = 0.05) was used to investigate the association between failure mode and test method (fast fracture or fatigue).

Materials and methods

Preparation of FPDs

The ceramic materials used to produce the FPDs are described in Table 1 . Twenty-five FPDs were produced by the CAD/CAM system (CEREC, Sirona, NY, EUA), using the CAD-on method (Ivoclar Vivadent). Simulated abutment structures were constructed using fiberglass-reinforced epoxy polymer (NEMA G10; Piedmont Plastics, Charlotte, NC, USA), which has similar elastic modulus of hydrated dentin and allow for adhesive bonding of resin-based restorative materials . The abutments base had 8 mm in diameter and 6 mm in height, prepared with a rounded shoulder cervical finish line (radius 0.5 mm) and a 12° total occlusal convergence . These abutments were scanned using the laboratory scanner (inEos, Sirona) to fabricate the FPD frameworks. The restoration type (“three-unit bridge”) and design method (“multilayer”) were input in the CAD/CAM system and calculated based on the Biogenerics software. Y-TZP blocks (IPS e.max ZirCAD, Ivoclar Vivadent) were milled and the resulting infrastructures were sintered at 1500 °C for 7 h in a ceramic furnace (Fire HTC; Sirona Dental Services, Bensheim, Germany). The Y-TZP infrastructure were scanned, the veneer layer was designed and milled using lithium disilicate ceramic blocks (IPS e.max CAD, Ivoclar Vivadent). The dimensions of the FPDs were as follows: framework thickness of 0.7 mm, veneer thickness of 1 mm, and 9 mm 2 of connector cross-section area.

Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Reliability and failure behavior of CAD-on fixed partial dentures
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