Abstract
Objectives
The aim of this study was to evaluate the marginal and internal fit of single crowns, compared to 14-unit frameworks made of translucent yttria-stabilized zirconia. We hypothesized that there is an influence of the type of restoration on the marginal and internal fit.
Methods
Eight teeth (FDI locations 17, 15, 13, 11, 21, 23, 25 and 27) of a typodont maxillary model were provided with a chamfer preparation to accommodate a 14-unit prosthesis or four single crowns (SCs). Ten 14-unit fixed dental prostheses (FDPs) and 40 single crowns were fabricated using a computer aided design (CAD)/computer aided manufacturing (CAM) system with pre-sintered translucent yttria-stabilized zirconia blanks. The restorations were cemented onto twenty master dies, which were sectioned into four pieces each. Then, the marginal and internal fits were examined using a binocular microscope. In order to detect the differences between the two types of restorations a non-parameteric test (Mann–Whitney- U ) was carried out; to detect differences between the abutment teeth and the abutment surfaces non-parametric tests (Kruskal–Wallis) and pairwise post hoc analyses (Mann–Whitney- U ) were performed after testing data for normal distribution (method according to Shapiro–Wilk). Level of significance was set at 5%.
Results
The mean (SD) marginal opening gap dimensions were 18 μm (14) for the single crowns and 29 μm (27) for the 14-unit FDPs ( p < 0.001). Abutment 21 of the FDPs showed statistical differences concerning the location of the teeth in both marginal and internal fit ( p < 0.001). The measured gaps (types I–IV) revealed statistical differences between all types, when comparing SCs to the FDPs ( p < 0.001).
Significance
Single crowns showed significantly better accuracy of fit, compared to the 14-unit FDPs. However, both restorations showed clinically acceptable marginal and internal fit.
1
Introduction
All-ceramic restorations are becoming increasingly popular because of their high esthetic potential and outstanding biocompatibility. Zirconia unites all of the positive characteristics of ceramics, although it has limited esthetics, due to its high opacity. Recent research by Stawarczyk et al. reveals that varying the sintering temperature can influence the translucency of yttria-stabilized zirconia. When using higher sintering temperatures, the material showed higher translucency, which led to better esthetic results. In a pre-sintered state, yttria-stabilized zirconia—which showed a flexural strength from 31 to 50 MPa—can be processed in a time-saving manner and with little wear on tools using most computer-aided design (CAD)/computer aided manufacturing (CAM) machines . Predictable sinter shrinkage and avoiding sintering distortion are of paramount importance and depend on the uniform density of the blank.
Uniaxial, isostatic and biaxial molding procedures are commonly used to manufacture yttria-stabilized zirconia blanks . The uniaxial pressed blanks showed high differences in density between the central and peripheral zones, which limits their application to single crowns and fixed dental prostheses (FDPs) of up to four units .
In the biaxial molding procedure, pressure is applied on the zirconia powder by an upper and lower rigid punch, which also rotate around their own axes . The benefits of both biaxial pressing and isostatic pressing include minimum sintering distortion, achieving high flexural strength after sintering (over 1300 MPa), and better accuracy of fit . The mechanical behavior of all-ceramic crowns, in terms of strength, resistance, and retention, is essentially influenced by the accuracy of a framework .
In clinical practice, the marginal and internal fit determines the durability of a restoration. There is a consensus between various authors that marginal openings below 120 μm are clinically acceptable . Although gaps of less than 80 μm are difficult to detect in clinical practice, some authors postulate marginal gaps of 50–75 μm and 70 μm (10) as clinically acceptable limits. However, poor marginal adaptation of restorations increases plaque retention and changes the composition of the microflora, which can lead to the onset of periodontal disease . Furthermore, the risk of secondary caries increases with the marginal gap width . Concerning the influence of the internal fit on the durability of the restorations, cement layers above 70 μm tend to reduce the fracture strength significantly .
Due to the growing demand for long-span FDPs, more scientific studies concerning the fit of full arch restorations compared to single crowns are needed. Therefore, the aim of this study was to examine the marginal and internal fit of single crowns, compared to 14-unit frameworks made of yttria-stabilized zirconia (ICE Zirkon Translucent, Zirkonzahn S.r.l., Gais, Italy). The working hypothesis of this approach is that the type of restoration (single crowns or 14-unit FDPs) will show an influence on the internal and marginal fit.
2
Materials and methods
A maxillary typodont model (standard working model AG-3, Frasaco GmbH, Tettnang, Germany) with eight abutment teeth was used. Therefore, the maxillary central incisors, canines, second pre-molars and second molars were provided with a 360° 1.0 mm chamfer preparation. The occlusal and incisal reductions were 1.5–2.0 mm. Twenty polyether impressions (Impregum, 3 M ESPE, Seefeld, Germany) of the typodont model were made with metal impression trays (U3, Orbilock, Orbis Dental Handelsgesellschaft mbH, Münster, Germany). Subsequently, twenty models of a class IV special scan die stone (Rocky Mountain orange label, Klasse 4 Dental GmbH, Augsburg, Germany) were fabricated.
The models were digitized ( Fig. 1 ) with a white light projector scanner (S 600, Zirkonzahn S.r.l.) showing details of less than 10 μm (unpublished data Zirkonzahn S.r.l.).
The single crowns and 14-unit zirconia FDPs were designed with a CAD software (Zirkonzahn.Modellier, Zirkonzahn S.r.l.): the full-contour design was reduced by 0.8 mm for the veneering porcelain, with a minimum thickness of 0.5 mm for the framework. For the 14-unit zirconia framework design, distances between the pontics and the gingiva of 0.1 mm and a connector with a round cross-section and a minimum area of 9 mm 2 were used. Further milling parameters for the crown foundation were the following: cement-gap thickness of 0.035 mm starting 0.3 mm of the preparation margin. At the transition from the axial wall to the occlusal surface 0.45 mm cement gap thickness were used while 0.4 mm were used for the occlusal surface. All parameters were chosen according to the manufacturer’s recommendations . The design was sent to CAM software (Zirkonzahn.Nesting, Zirkonzahn S.r.l.) to position it in a virtual zirconia blank. In this case, the software automatically shrank the volume of the sintering foot, to fit the 14-unit framework. The aim was to avoid deformation of the curved arch in the anterior during the following sintering process.
All restorations were milled with a 5 + 1 axes milling unit (M5, Zirkonzahn S.r.l.), using three different burr diameters (2.0 mm, 1.0 mm and 0.5 mm) and pre-sintered zirconia blanks (Ice Zirkon Translucent 95H18, Zirkonzahn S.r.l.) that were characterized by a Weibull modulus of 17.1, a flexural strength of 1198.53 MPa (81.84) and a Vickers hardness of 1368 (28) . Both the single crowns and the 14-unit frameworks were sintered to full density in a special furnace (600 V −2 , Zirkonzahn S.r.l.) at 1500 °C for 12 h. Next, the single crowns and 14-unit FDPs were checked for fit using a standardized protocol from the literature . Color (Bite-X Articulating Paste, Asami Tanaka Dental Enterprises Europe GmbH, Friedrichsdorf, Germany) was applied onto the abutment teeth, and both the single crowns and 14-unit frameworks were set back onto the model without force. If red spots occurred on the inner surfaces, they were removed using a red ring diamond burr (Diamond bur 201 Round end taper, Zirkonzahn S.r.l.) with water-cooling spray. This procedure was repeated until the restoration had a good clinical seat on the model, without imperfections, and provided a correct marginal closure. The time needed for adaptation was recorded.
Concerning the full-arch prostheses in the upper jaw, a slight distortion between the central incisors and second molars became obvious during the adaption process. The next step was to cement the single crowns and the 14-unit FDPs onto the models with glass ionomer cement (Ketac Cem Maxicap, 3M ESPE) according to the manufacturer’s instructions. After applying the glass ionomer cement into the crowns with microbrushes, the single crowns and the 14-unit frameworks were placed onto the models with finger pressure, and the excess was removed with foam pellets. For the remaining setting time, which was 7 min, starting from the activation of the capsule, the copings and FDPs were loaded evenly with 50 N in a cementation device . The four dies on the left side of the arch were chosen for measurement, which led to a total number of 80 specimens (40 of the single crowns and 40 of the FDPs). The sectioning lines (mesial–distal, facial–lingual) were marked centrally on the die, in order to have comparable cross-sections. To avoid fracture of the cemented specimens during sectioning, the substructures were embedded into gypsum (Resin Rock, Whip Mix, Louisville, USA). Twenty-four hours later, they were segmented into smaller units, and each die was sectioned into four pieces by a cutting machine (Secotom-50, Struers GmbH, Willich, Germany), according to the marked lines. Due to the brittleness and Vickers hardness of yttrium-stabilized zirconia, a cut-off wheel (M1D13, Struers GmbH) was used under permanent water-cooling with a speed of 0.1 mm/s.
After cleaning, the frameworks were examined at original magnification 50× (Axioskop 2 MAT, Carl-Zeiss AG, Oberkochen, Germany). One image of a calibration slide was made at the same magnification and used as a reference for calibration at each imaging session. In detail, three to eight digital images of each aspect of the die (mesial, facial, distal, lingual) were made with a digital single-lens reflex camera (Nikon D 100, Nikon Corporation, Tokyo, Japan), mounted on the microscope (Axioskop 2 MAT). An imaging program (Adobe Photoshop CS5, Adobe Systems Incorporation, San Jose, USA) was used to combine the single photos to a complete cross-section of the die. The images were transferred to an imaging data program (Optimas 6.5, Media Cybernetics Incorporation, Rockville, USA) and measured according to the protocols of prior studies ( Fig. 2 ) .
The marginal opening (type IV) was defined as the closest convergence of the zirconia framework and the plaster die. Furthermore, the values were measured of the cement gaps for the chamfer preparation (type I), the axial wall (type II), and the occlusal space (type III). Concerning the marginal opening, the smallest value was chosen, while for all other types of cement gaps, a mean value was generated. The obtained data were imported into a statistical program (SPSS 20.0, SPSS Incorporation, Chicago, USA) and evaluated. Mean values were calculated. In order to detect the differences between the two types of restorations a non-parameteric test (Mann–Whitney- U ) was carried out; to detect differences between the abutment teeth, the measurement locations (types I–IV) and the different abutment surfaces (mesial, lingual, distal, buccal) non-parametric tests (Kruskal–Wallis) and a pairwise post hoc analyses (Mann–Whitney- U ) were performed after data failed the testing for normal distribution (method according to Shapiro–Wilk). Level of significance was set at 5%.
2
Materials and methods
A maxillary typodont model (standard working model AG-3, Frasaco GmbH, Tettnang, Germany) with eight abutment teeth was used. Therefore, the maxillary central incisors, canines, second pre-molars and second molars were provided with a 360° 1.0 mm chamfer preparation. The occlusal and incisal reductions were 1.5–2.0 mm. Twenty polyether impressions (Impregum, 3 M ESPE, Seefeld, Germany) of the typodont model were made with metal impression trays (U3, Orbilock, Orbis Dental Handelsgesellschaft mbH, Münster, Germany). Subsequently, twenty models of a class IV special scan die stone (Rocky Mountain orange label, Klasse 4 Dental GmbH, Augsburg, Germany) were fabricated.
The models were digitized ( Fig. 1 ) with a white light projector scanner (S 600, Zirkonzahn S.r.l.) showing details of less than 10 μm (unpublished data Zirkonzahn S.r.l.).
The single crowns and 14-unit zirconia FDPs were designed with a CAD software (Zirkonzahn.Modellier, Zirkonzahn S.r.l.): the full-contour design was reduced by 0.8 mm for the veneering porcelain, with a minimum thickness of 0.5 mm for the framework. For the 14-unit zirconia framework design, distances between the pontics and the gingiva of 0.1 mm and a connector with a round cross-section and a minimum area of 9 mm 2 were used. Further milling parameters for the crown foundation were the following: cement-gap thickness of 0.035 mm starting 0.3 mm of the preparation margin. At the transition from the axial wall to the occlusal surface 0.45 mm cement gap thickness were used while 0.4 mm were used for the occlusal surface. All parameters were chosen according to the manufacturer’s recommendations . The design was sent to CAM software (Zirkonzahn.Nesting, Zirkonzahn S.r.l.) to position it in a virtual zirconia blank. In this case, the software automatically shrank the volume of the sintering foot, to fit the 14-unit framework. The aim was to avoid deformation of the curved arch in the anterior during the following sintering process.
All restorations were milled with a 5 + 1 axes milling unit (M5, Zirkonzahn S.r.l.), using three different burr diameters (2.0 mm, 1.0 mm and 0.5 mm) and pre-sintered zirconia blanks (Ice Zirkon Translucent 95H18, Zirkonzahn S.r.l.) that were characterized by a Weibull modulus of 17.1, a flexural strength of 1198.53 MPa (81.84) and a Vickers hardness of 1368 (28) . Both the single crowns and the 14-unit frameworks were sintered to full density in a special furnace (600 V −2 , Zirkonzahn S.r.l.) at 1500 °C for 12 h. Next, the single crowns and 14-unit FDPs were checked for fit using a standardized protocol from the literature . Color (Bite-X Articulating Paste, Asami Tanaka Dental Enterprises Europe GmbH, Friedrichsdorf, Germany) was applied onto the abutment teeth, and both the single crowns and 14-unit frameworks were set back onto the model without force. If red spots occurred on the inner surfaces, they were removed using a red ring diamond burr (Diamond bur 201 Round end taper, Zirkonzahn S.r.l.) with water-cooling spray. This procedure was repeated until the restoration had a good clinical seat on the model, without imperfections, and provided a correct marginal closure. The time needed for adaptation was recorded.
Concerning the full-arch prostheses in the upper jaw, a slight distortion between the central incisors and second molars became obvious during the adaption process. The next step was to cement the single crowns and the 14-unit FDPs onto the models with glass ionomer cement (Ketac Cem Maxicap, 3M ESPE) according to the manufacturer’s instructions. After applying the glass ionomer cement into the crowns with microbrushes, the single crowns and the 14-unit frameworks were placed onto the models with finger pressure, and the excess was removed with foam pellets. For the remaining setting time, which was 7 min, starting from the activation of the capsule, the copings and FDPs were loaded evenly with 50 N in a cementation device . The four dies on the left side of the arch were chosen for measurement, which led to a total number of 80 specimens (40 of the single crowns and 40 of the FDPs). The sectioning lines (mesial–distal, facial–lingual) were marked centrally on the die, in order to have comparable cross-sections. To avoid fracture of the cemented specimens during sectioning, the substructures were embedded into gypsum (Resin Rock, Whip Mix, Louisville, USA). Twenty-four hours later, they were segmented into smaller units, and each die was sectioned into four pieces by a cutting machine (Secotom-50, Struers GmbH, Willich, Germany), according to the marked lines. Due to the brittleness and Vickers hardness of yttrium-stabilized zirconia, a cut-off wheel (M1D13, Struers GmbH) was used under permanent water-cooling with a speed of 0.1 mm/s.
After cleaning, the frameworks were examined at original magnification 50× (Axioskop 2 MAT, Carl-Zeiss AG, Oberkochen, Germany). One image of a calibration slide was made at the same magnification and used as a reference for calibration at each imaging session. In detail, three to eight digital images of each aspect of the die (mesial, facial, distal, lingual) were made with a digital single-lens reflex camera (Nikon D 100, Nikon Corporation, Tokyo, Japan), mounted on the microscope (Axioskop 2 MAT). An imaging program (Adobe Photoshop CS5, Adobe Systems Incorporation, San Jose, USA) was used to combine the single photos to a complete cross-section of the die. The images were transferred to an imaging data program (Optimas 6.5, Media Cybernetics Incorporation, Rockville, USA) and measured according to the protocols of prior studies ( Fig. 2 ) .
