In vitro performance of full-contour zirconia single crowns

Abstract

Objectives

Zirconia based restorations exhibited high failure rates due to veneering–porcelain fractures. Milling to full-contour might be an alternative approach for zirconia restorations. The aim of this study was to evaluate full-contour zirconia crowns in terms of light-transmission, contact wear (restoration and antagonist) and load-bearing capacity. Powder build-up veneered zirconia substructures and CAD/CAM-veneered zirconia substructures served as controls.

Methods

Four different kinds of crowns were fabricated on 12 metal dies: zirconia substructure with powder build-up porcelain (veneering technique), zirconia substructure with CAD/CAM generated veneering (sintering technique), full-contour zirconia glazed (glazed full-contour) and full-contour zirconia polished (polished full-contour). All crowns had the same dimensions. After light-transmission was measured the crowns were cemented on the corresponding metal dies. The specimens were loaded according to a special wear method in the chewing simulator (120,000 mechanical cycles, 5 kg load, 0.7 mm sliding movement, 320 thermocycles). Wear of the restoration and the antagonist were measured. All specimens were loaded until failure. One-way ANOVA and a LSD post-hoc test were used to compare data at a level of 5%.

Results

Polished full-contour showed significantly higher light transmission than the other groups ( p = 0.003; ANOVA). Polished full-contour exhibited significantly less contact wear at the restoration ( p = 0.01; ANOVA) and higher contact wear at the antagonist ( p = 0.016; ANOVA) compared to the other groups. Glazed full-contour zirconia showed similar contact wear at the antagonist compared to veneering technique ( p = 0.513, post-hoc LSD). Crowns with conventional veneering showed significantly lower load-bearing capacity ( p < 0.001; ANOVA).

Significance

Milling zirconia to full-contour with glazed surface might be an alternative to traditionally veneered restorations.

Introduction

Metal-free, all-ceramic restorations have become more widely distributed due to their high esthetic potential and their excellent biocompatibility . Today, many framework structures for prosthetic restorations are fabricated in computer aided design (CAD)/computer aided manufacturing (CAM) procedures, which means that a major part in the working sequence is carried out by industrial machines . On the one hand, frameworks can be fabricated more efficiently. On the other hand, it is possible to achieve industrial quality standards, which are particularly important for ceramic materials. Every pore and imperfection is a potential starting point for cracks and thus for clinical failure of ceramic restorations. The veneering material, however, has been layered according to the well-known fabrication process of the metal–ceramic technique to date. Failure rates between 0 and 25% have been reported for fixed dental prostheses (FDPs) after 3 years of clinical service . The typical failure pattern of a veneering material in the daily clinical practice is known as ceramic chipping . This fracture pattern is associated with a thin layer of glass ceramic that remains on the zirconia framework .

From the economical point of view, the esthetic and functional completion of crown and FDP frameworks involving traditional methods, such as the powder layering technique, appears to be inefficient. Applying veneering porcelain by brush in several bakes is time consuming and costly. Sintering a CAD/CAM fabricated veneer cap made from lithium-disilicate to zirconia was reported to offer high mechanical stability in vitro . Another possibility for increasing the cost-effectiveness involves the industrial fabrication of mono-blocks and machine the entire restoration by means of CAD/CAM technology . These mono-block restorations, however, are fabricated from glass ceramics, which are less stable in comparison to zirconia-based restorations. Therefore, the indication range is clearly limited to single crowns and small FDPs . Fabricating mono-block restorations from pure zirconia could increase the mechanical stability and expand the range of indications. However, zirconia is known as a whitish, opaque core material and its wear behavior has not been understood completely . Prior to clinical application full-contour zirconia restorations have to proof their suitability in vitro.

The aim of this study was to evaluate full-contour zirconia single crowns to established veneered zirconia substructures with identical dimensions in terms of esthetics, wear and load-bearing capacity. The working hypothesis is that full-contour zirconia crowns will show superior load-bearing capacity and less light-translucency compared to traditional zirconia-based crowns.

Materials and methods

A 1.2 mm, 360° chamfer preparation was made on a mandibulary right first molar (Frasaco, Tettnang, Germany) and the occlusal surface was reduced by 1.5 mm. To control volumetric reduction, a silicon impression (Optosil, Heraeus Kulzer, Hanau, Germany) was made prior to tooth preparation and used as a guideline for preparation. Additionally, the provisional crown (Protemp Garant, 3M ESPE, Seefeld, Germany) was used to verify the thickness, thus the circumferential and occlusal reductions could be quantified (Dial Caliper, Kori Seiki, Tokyo, Japan). The preparation was completed with a surveyor (F1, DeguDent) using a carbide bur (Komet H 356 RGE 103.031, Gebr. Brasseler, Lemgo, Germany) to ensure that the preparation had an 8° tapered angle. Twelve specimens were tested in each group ( n = 12). Therefore twelve silicone impressions were made (Adisil blau, Siladent Dr. Boehme und Schoeps GmbH, Goslar, Germany) with a custom impression tray (U3 # 141163 Orbilock, Orbis Dental, Munster, Germany) in order to duplicate the prepared tooth into metal-dies. The impressions were filled with wax (Nawax compact, Yeti Dental Products, Engen, Germany). After cooling the wax patterns were removed and invested (rema dynamic S, Dentaurum, Ispringen, Germany). The wax was burnt out and metal-alloy (Remanium 2000, Dentaurum) was casted into the mold. The metal dies were finished and capped into polymethylmetacrylat-resin (Paladur, Heraeus Kulzer). To ensure the correct preparation angle all 12 master casts were finished with the surveyor using a carbide bur. These 12 master casts functioned as testing models. From each testing model an impression (Impregum, 3M ESPE, Seefeld, Germany) was taken using a plastic impression tray (Inlay, Heko, Berlin, Germany). After 24 h a resin modified die material (Resin Rock, Whipmix, Dortmund, Germany) was poured into the impressions. Twelve master dies derived from 12 impressions. Each master die was scanned (Everest Scan, KaVo, Biberach, Germany) and two zirconia copings were manufactured by a CAD/CAM-system (Zeno 4820 Premium, IMES, Eiterfeld, Germany) using pre-sintered Zirconia (ZirLuna, ACF, Amberg, Germany). A wall thickness of 0.5 mm and a virtual spacer layer of 10 μm had been chosen. After the milling procedure the enlarged copings were removed from the CAM-machine and sintered (Zirluna Fire, ACF).

The frameworks were examined for deformation and debris, adapted if necessary and cleaned with steam. Each framework was seated on a definitive die. The frameworks were evaluated on the dies by visual inspection under a microscope (magnification 8×, Stemi DV 4, Zeiss) for marginal discrepancy. The inspection was performed by three previously calibrated evaluators (two dental technicians and one dentist) .

Therefore two copings for each of the 12 testing models were available after framework fabrication.

Veneering technique (VT)

One coping was randomly chosen from each testing model and veneered using the powder build-up technique. The fabrication of specimens from group VT was reported .

Sintering technique (ST)

One major goal of the study was to fabricate exact duplicates of the veneered crowns using alternative fabrication techniques. The objective to received an exact copy of the veneered restorations from group VT by CAD/CAM-technologies was easily reached. The outer surfaces of crowns from group VT were dimmed by applying a contrast spray (Dentaco, Bad Homburg, Germany) and scanned by a white-light-scanner (Everest Scan, KaVo, Biberach, Germany). The used CAD/CAM-system (Everest, KaVo) provides the function of a double scan. So the outer and the inner shape with the seated coping can be scanned, matched together and the CAD/CAM-system allows manufacturing the space between both scans from a material of choice.

The fabrication process was performed as described in an earlier publication .

Twenty-four full-contour crowns were manufactured using the same presintered zirconia and CAD/CAM-system (Zeno 4820 Premium, IMES, Eiterfeld, Germany) according to the outer surface of group VT. The restorations were sintered to full density at a temperature of 1450 °C for 4 h (Zirluna Fire, ACF). After sintering all crowns were cleaned and adapted as described above. As the result of this manufacturing process two full-contour crowns for each of the 12 testing models were available.

Glazed full-contour (GF)

One full-contour crown was randomly chosen from each testing model for group GF. After the sintering process the outer surface of each crown of group GF was covered with glaze and stain liquid (IPS e.max Ceram Glaze and Stain allround, Ivoclar Vivadent) and fired at 725 °C.

Polished full-contour (PF)

Specimens from group PF were polished with a special polishing kit for all-ceramic restorations available for dental laboratories (Polishing Kit for Ceramic Materials, Nr. 4326A.104, Komet, Gebr. Brasseler). A diamond polishing paste (Grain size D3, Nr. 54000140, Bredent, Senden, Germany) with a brush was used for finalizing the surfaces. Polishing was carried out by a master ceramist (20 years of experience fabricating ceramic restorations) under the stereomicroscope. The surface quality was evaluated by two calibrated investigators (one dentist and one dental technician) to be clinically acceptable.

Light translucency

The translucency was determined by direct transmission in an industrial spectrophotometer (LabScan XE, Hunterlab, Murnau, Germany). Light generated by a xenon flash served as source. The edges of the crowns were sealed by black modeling clay to exclude penetration of any light other than through the ceramic material. Light translucency through the buccal surface of the crown was determined. The sensor of the digital spectrophotometer with a spectrum of 400–700 nm detected the quantity of light transmitted through the crowns. The spectrophotometer illuminated the specimens in a 0° angle while detection was performed in a 45° angle. Reflected light was collected and averaged by a fiber-optic ring. A sample illumination size of 5 mm was chosen. First a negative control test was performed using a metal crown with identical dimensions pressed to the modeling clay and the instrument was evaluated for zero light reading. Next, three crowns ( n = 3) were randomly chosen from each group and measurements were performed. The results were expressed in translucency related to the control group. Measurement was repeated three times for each specimen and the mean was calculated.

Three-dimensional measurement of the occlusal surface and the antagonist

To simulate wear in the artificial mouth a stainless steel antagonist with a diameter of 6 mm was used according to earlier publications . All surfaces of the crowns and the antagonists were digitized before and after contact wear testing. Scan powder (Met-L-Chek Developer D 70, Helling, Heidgraben, Germany) was applied on the surfaces to make them scanable. The surfaces were scanned with a triangulation sensor (Laserscan 3D Pro, Willytec, Graefelfing, Germany). To minimize false-positive results the surfaces were scanned from two different directions as described in earlier publications . The matched dataset of each surface before wear testing served as baseline.

Aging

The abutments and crowns were mounted in the chewing simulator (Willytec, Graefelfing, Germany) in a resin mold with cold setting denture material (PalaXpress, HeraeusKulzer). The stainless steel antagonists were positioned on the molar crowns to achieve contact to both buccal cusps and one lingual cusp. The contact points were checked with 12-μm occlusion foil (Hanel, Coltene/Whaledent, Langenau, Germany). The chewing simulator with integrated thermocycling performed a special program that had proven its suitability for evaluating wear of ceramic surfaces . For this method the weight was set at 5 kg and the sliding movement at 0.7 mm . Sliding was guided by the inner surfaces of the buccal cusps. A total of 120,000 cycles of unidirectional antagonist movements with a frequency of 1.6 Hz were carried out. Thermocycling with a frequency of 320/120,000 cycles and a temperature difference between 5 °C and 55 °C was included in the wear testing process.

After the aging process the occlusal surfaces of all crowns and the antagonists were scanned again. The surfaces were prepared and scanned as described above. Finally the baseline datasets and the datasets after contact wear were superimposed and images showing the differences were generated. The negative changes were displayed as shades of red, whereas positive changes were visible in gray shades ( Fig. 1a–c and Fig. 2a–c ). Data were evaluated and scattered as described in previous studies . Then the loss of height of each crown and antagonist was imported for analysis in a statistical program (SPSS 16.0, SPSS Software, Munich, Germany).

Fig. 1
(a): Three-dimensional surface scan of a crown veneered with veneering porcelain before chewing simulation. (b): Three-dimensional surface scan of a crown veneered with veneering porcelain after chewing simulation. (c): Visualization of matched datasets before and after chewing simulation of a crown veneered with veneering porcelain. Loss of volume is highlighted in red color (for interpretation of the references to color in this figure legend, the reader is referred to the web version of the article).

Fig. 2
(a): Three-dimensional surface scan of a stainless steel antagonist opposing a crown veneered with veneering porcelain before chewing simulation. (b): Three-dimensional surface scan of a stainless steel antagonist opposing a crown veneered with veneering porcelain after chewing simulation. (c): Visualization of matched datasets before and after chewing simulation of a stainless steel antagonist opposing a crown veneered with veneering porcelain. Loss of volume is highlighted in red color. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Load-bearing capacity

All specimens were loaded until failure in a universal testing machine (Zwick, Ulm Germany) at a crosshead-speed of 0.5 mm min −1 . The force was transferred to the central fossa of the occlusal surface via a tungsten ball (10.0 mm diameter) on an interposed polyethylene foil (1.0 mm thickness) . A sudden decrease in force of more than 15 N was regarded as an indication of failure, and the maximum force up to this point was recorded as force at fracture. Using visual examination, crack location and fragmentation of core and veneering material were assessed.

Statistical analysis

The data of light translucency, data of wear (restoration and antagonist) and force at fracture data were imported into a statistical program (SPSS 16.0). To compare the results of the different groups, a one-way analysis of variance (ANOVA) and a post-hoc test (LSD-test) were performed. The level of significance was set at 5%.

Materials and methods

A 1.2 mm, 360° chamfer preparation was made on a mandibulary right first molar (Frasaco, Tettnang, Germany) and the occlusal surface was reduced by 1.5 mm. To control volumetric reduction, a silicon impression (Optosil, Heraeus Kulzer, Hanau, Germany) was made prior to tooth preparation and used as a guideline for preparation. Additionally, the provisional crown (Protemp Garant, 3M ESPE, Seefeld, Germany) was used to verify the thickness, thus the circumferential and occlusal reductions could be quantified (Dial Caliper, Kori Seiki, Tokyo, Japan). The preparation was completed with a surveyor (F1, DeguDent) using a carbide bur (Komet H 356 RGE 103.031, Gebr. Brasseler, Lemgo, Germany) to ensure that the preparation had an 8° tapered angle. Twelve specimens were tested in each group ( n = 12). Therefore twelve silicone impressions were made (Adisil blau, Siladent Dr. Boehme und Schoeps GmbH, Goslar, Germany) with a custom impression tray (U3 # 141163 Orbilock, Orbis Dental, Munster, Germany) in order to duplicate the prepared tooth into metal-dies. The impressions were filled with wax (Nawax compact, Yeti Dental Products, Engen, Germany). After cooling the wax patterns were removed and invested (rema dynamic S, Dentaurum, Ispringen, Germany). The wax was burnt out and metal-alloy (Remanium 2000, Dentaurum) was casted into the mold. The metal dies were finished and capped into polymethylmetacrylat-resin (Paladur, Heraeus Kulzer). To ensure the correct preparation angle all 12 master casts were finished with the surveyor using a carbide bur. These 12 master casts functioned as testing models. From each testing model an impression (Impregum, 3M ESPE, Seefeld, Germany) was taken using a plastic impression tray (Inlay, Heko, Berlin, Germany). After 24 h a resin modified die material (Resin Rock, Whipmix, Dortmund, Germany) was poured into the impressions. Twelve master dies derived from 12 impressions. Each master die was scanned (Everest Scan, KaVo, Biberach, Germany) and two zirconia copings were manufactured by a CAD/CAM-system (Zeno 4820 Premium, IMES, Eiterfeld, Germany) using pre-sintered Zirconia (ZirLuna, ACF, Amberg, Germany). A wall thickness of 0.5 mm and a virtual spacer layer of 10 μm had been chosen. After the milling procedure the enlarged copings were removed from the CAM-machine and sintered (Zirluna Fire, ACF).

The frameworks were examined for deformation and debris, adapted if necessary and cleaned with steam. Each framework was seated on a definitive die. The frameworks were evaluated on the dies by visual inspection under a microscope (magnification 8×, Stemi DV 4, Zeiss) for marginal discrepancy. The inspection was performed by three previously calibrated evaluators (two dental technicians and one dentist) .

Therefore two copings for each of the 12 testing models were available after framework fabrication.

Veneering technique (VT)

One coping was randomly chosen from each testing model and veneered using the powder build-up technique. The fabrication of specimens from group VT was reported .

Sintering technique (ST)

One major goal of the study was to fabricate exact duplicates of the veneered crowns using alternative fabrication techniques. The objective to received an exact copy of the veneered restorations from group VT by CAD/CAM-technologies was easily reached. The outer surfaces of crowns from group VT were dimmed by applying a contrast spray (Dentaco, Bad Homburg, Germany) and scanned by a white-light-scanner (Everest Scan, KaVo, Biberach, Germany). The used CAD/CAM-system (Everest, KaVo) provides the function of a double scan. So the outer and the inner shape with the seated coping can be scanned, matched together and the CAD/CAM-system allows manufacturing the space between both scans from a material of choice.

The fabrication process was performed as described in an earlier publication .

Twenty-four full-contour crowns were manufactured using the same presintered zirconia and CAD/CAM-system (Zeno 4820 Premium, IMES, Eiterfeld, Germany) according to the outer surface of group VT. The restorations were sintered to full density at a temperature of 1450 °C for 4 h (Zirluna Fire, ACF). After sintering all crowns were cleaned and adapted as described above. As the result of this manufacturing process two full-contour crowns for each of the 12 testing models were available.

Glazed full-contour (GF)

One full-contour crown was randomly chosen from each testing model for group GF. After the sintering process the outer surface of each crown of group GF was covered with glaze and stain liquid (IPS e.max Ceram Glaze and Stain allround, Ivoclar Vivadent) and fired at 725 °C.

Polished full-contour (PF)

Specimens from group PF were polished with a special polishing kit for all-ceramic restorations available for dental laboratories (Polishing Kit for Ceramic Materials, Nr. 4326A.104, Komet, Gebr. Brasseler). A diamond polishing paste (Grain size D3, Nr. 54000140, Bredent, Senden, Germany) with a brush was used for finalizing the surfaces. Polishing was carried out by a master ceramist (20 years of experience fabricating ceramic restorations) under the stereomicroscope. The surface quality was evaluated by two calibrated investigators (one dentist and one dental technician) to be clinically acceptable.

Light translucency

The translucency was determined by direct transmission in an industrial spectrophotometer (LabScan XE, Hunterlab, Murnau, Germany). Light generated by a xenon flash served as source. The edges of the crowns were sealed by black modeling clay to exclude penetration of any light other than through the ceramic material. Light translucency through the buccal surface of the crown was determined. The sensor of the digital spectrophotometer with a spectrum of 400–700 nm detected the quantity of light transmitted through the crowns. The spectrophotometer illuminated the specimens in a 0° angle while detection was performed in a 45° angle. Reflected light was collected and averaged by a fiber-optic ring. A sample illumination size of 5 mm was chosen. First a negative control test was performed using a metal crown with identical dimensions pressed to the modeling clay and the instrument was evaluated for zero light reading. Next, three crowns ( n = 3) were randomly chosen from each group and measurements were performed. The results were expressed in translucency related to the control group. Measurement was repeated three times for each specimen and the mean was calculated.

Three-dimensional measurement of the occlusal surface and the antagonist

To simulate wear in the artificial mouth a stainless steel antagonist with a diameter of 6 mm was used according to earlier publications . All surfaces of the crowns and the antagonists were digitized before and after contact wear testing. Scan powder (Met-L-Chek Developer D 70, Helling, Heidgraben, Germany) was applied on the surfaces to make them scanable. The surfaces were scanned with a triangulation sensor (Laserscan 3D Pro, Willytec, Graefelfing, Germany). To minimize false-positive results the surfaces were scanned from two different directions as described in earlier publications . The matched dataset of each surface before wear testing served as baseline.

Aging

The abutments and crowns were mounted in the chewing simulator (Willytec, Graefelfing, Germany) in a resin mold with cold setting denture material (PalaXpress, HeraeusKulzer). The stainless steel antagonists were positioned on the molar crowns to achieve contact to both buccal cusps and one lingual cusp. The contact points were checked with 12-μm occlusion foil (Hanel, Coltene/Whaledent, Langenau, Germany). The chewing simulator with integrated thermocycling performed a special program that had proven its suitability for evaluating wear of ceramic surfaces . For this method the weight was set at 5 kg and the sliding movement at 0.7 mm . Sliding was guided by the inner surfaces of the buccal cusps. A total of 120,000 cycles of unidirectional antagonist movements with a frequency of 1.6 Hz were carried out. Thermocycling with a frequency of 320/120,000 cycles and a temperature difference between 5 °C and 55 °C was included in the wear testing process.

After the aging process the occlusal surfaces of all crowns and the antagonists were scanned again. The surfaces were prepared and scanned as described above. Finally the baseline datasets and the datasets after contact wear were superimposed and images showing the differences were generated. The negative changes were displayed as shades of red, whereas positive changes were visible in gray shades ( Fig. 1a–c and Fig. 2a–c ). Data were evaluated and scattered as described in previous studies . Then the loss of height of each crown and antagonist was imported for analysis in a statistical program (SPSS 16.0, SPSS Software, Munich, Germany).

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on In vitro performance of full-contour zirconia single crowns
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