Adaptation of all-ceramic fixed partial dentures

Abstract

Objectives

To measure the marginal and internal fit of three-unit fixed partial dentures (FPDs) using the micro-CT technique, testing the null hypothesis that there is no difference in the adaptation between the ceramic systems studied.

Methods

Stainless steel models of prepared abutments were fabricated to design the FPDs. Ten FPDs were produced from each framework ceramic (YZ – Vita In-Ceram YZ and IZ – Vita In-Ceram Zirconia) using CEREC inLab according to the manufacturer instructions. All FPDs were veneered using the recommended porcelain. Each FPD was seated on the original model and scanned using micro-CT. Files were processed using NRecon and CTAn software. Adobe Photoshop and Image J software were used to analyze the cross-sections images. Five measuring locations were used as follows: MG – marginal gap; CA – chamfer area; AW – axial wall; AOT – axio-occlusal transition area; OA – occlusal area. The horizontal marginal discrepancy (HMD) was evaluated in another set of images. Results were statistically analyzed using ANOVA and Tukey tests ( α = 0.05).

Results

The mean values for MG, CA, AW, OA and HMD were significantly different for all tested groups ( p < 0.05). IZ exhibited greater mean values than YZ for all measuring locations except for AW and AOT. OA showed the greatest mean gap values for both ceramic systems. MG and AW mean gap values were low for both systems.

Significance

The ceramic systems evaluated showed different levels of marginal and internal fit, rejecting the study hypothesis. Yet, both ceramic systems showed clinically acceptable marginal and internal fit.

Introduction

Ceramic systems with high crystalline content were introduced in dentistry with the objective of replacing the metal frameworks used for metal–ceramic crowns and fixed partial dentures (FPDs). All-ceramic structures are more translucent than the metallic ones, resulting in a natural-looking restoration . The finish line of the prepared tooth can be placed at the free gingival margin without compromising the esthetic appearance and avoiding the violation of the biological width, which reduces the risk of iatrogenic periodontal disease . Also, ceramic materials have low thermal conductivity and are highly biocompatible .

Alumina and zirconia-based systems were developed to provide materials with superior mechanical properties to produce larger restorations, such as FPDs. The use of these materials was advanced by the introduction of the CAD/CAM technology, which made the processing method easier. Ceramic systems such as the yttria partially stabilized tetragonal zirconia polycrystals (Y-TZP), e.g. In-Ceram YZ, and the glass-infiltrated zirconia-reinforced alumina-based ceramic, e.g. In-Ceram Zirconia (IZ), are available as prefabricated blocks for CAD/CAM processing .

In addition to esthetic and strength, all-ceramic restorations promise precision of fit. Poor marginal adaptation can result in damage to the tooth, to the periodontal tissues and to the restoration . Large marginal discrepancies result in dissolution of the luting agent and favor microleakage of bacteria and their byproducts . As a consequence, the tooth becomes more susceptible to inflammation of the vital pulp (post-operative sensitivity), secondary caries, and marginal discoloration . Felton et al. investigated the in vivo the relationship between marginal adaptation and periodontal tissue health and noticed that an increase in the marginal discrepancy resulted in an increase in gingival inflammation. In addition, it has already been demonstrated that periodontal disease can be induced by changes in the subgingival microflora and plaque retention caused by an inadequate marginal adaptation .

The width of the internal adaptation (gap) or cement space is also an important aspect to be considered. Tuntiprawon and Wilson observed that increasing the cement thickness of all-ceramic crowns reduced their fracture strength. In addition, there is evidence that excessive cement space could be related to failures on the veneering material . The cement space should be uniform and facilitate seating without compromising retention or resistance forms .

The fitting accuracy of a restoration produced using the CAD/CAM technique is influenced by the scanning process, software design, milling and shrinkage effects . Y-TZP ceramic is available as: (1) fully sintered blocks fabricated by a process known as hot isostatic pressing (HIP), or (2) partially sintered pre-fabricated blocks, which are used to produce enlarged restorations to compensate the final sintering shrinkage. Milling of fully sintered blocks may produce more accurate fitting restorations but is associated to high wear rates of the milling burs and is time-consuming. Conversely, the increased milling efficiency obtained using partially sintered blocks has the disadvantage of a potentially lower precision of fit due to the sintering shrinkage. Although a compensatory software design is used to guarantee an accurate fit, it is not sure that the shrinkage can be controlled in FPDs with long spans .

IZ is a ceramic core material used for CAD/CAM restorations. It consists of an alumina-based porous structure with addition of about 33% ceria partially stabilized zirconia infiltrated with a lanthanum oxide-based glass . The system is available as dry-pressed blocks that can be milled into the final dimension and subsequently glass infiltrated .

Most authors agree that marginal openings below 120 μm are clinically acceptable . There is a large range of marginal fit values related to the location of a crown and type of restoration . In vitro investigations with YZ an IZ all-ceramic FPDs reported mean marginal gap values in a range of 9–110 μm . Reich et al. evaluated the clinical fit of all-ceramic and metal–ceramic FPDs and showed mean marginal gap values from 54 to 95 μm. An in vivo study reported a mean gap value of 190 μm in the cervical area for YZ FPDs .

Different methods have been used to evaluate internal and marginal gaps. The clinical marginal fit can be roughly estimated directly with a mirror and a probe, or indirectly by taking an impression of the tooth and producing an epoxy replica that can be taken to an optical or scanning electron microscope . The internal fit can be assessed by a replica technique, in which a silicon impression of the cement space is used . In laboratory studies, it is possible to section the tooth-restoration sample for direct evaluation under a microscope . A new method that uses micro-computed tomography (micro-CT) has been applied for non-destructive analysis of the restorations . This technique allows 2D and 3D investigation of the marginal and internal gaps within the range of a few micrometers at multiples sites and directions .

The purpose of this study was to measure the marginal and internal fit of three-unit all-ceramic FPDs using the micro-CT technique. Two ceramic framework materials (Vita In-Ceram YZ and Vita In-Ceram Zirconia) were evaluated, testing two hypotheses: (a) that there is no difference in the internal and marginal gaps between these two systems and (b) that there is no difference among the gaps measured in different locations for both ceramic systems.

Materials and methods

The experimental groups and materials used in this study are described in Table 1 .

Table 1
Experimental groups and materials used in this study.
Groups Framework material a and basic composition Veneering material a
YZ Vita In-Ceram YZ – yttria partially stabilized tetragonal zirconia polycrystal Vita VM9
IZ Vita In-Ceram Zirconia – glass infiltrated zirconia-reinforced alumina-based ceramic Vita VM7

a Vita Zahnfabrik, Bad Sackingen, Germany.

A stainless steel model simulating prepared abutment teeth was constructed. The prepared die has 4.5 mm height, 6° of taper and 120° chamfer as finish line, as proposed by Sundh et al. ( Fig. 1 ). The distance between the centers of the dies was 16 mm, corresponding to the distance between a lower second premolar and a lower second molar (span of 10 mm). An artificial gingiva was produced with acrylic resin (JET, Classico, Sao Paulo, SP, Brazil). A polyvinyl siloxane impression of the model was taken (Aquasil™, Dentsply, Petropolis, RJ, Brazil). A working cast was made using type IV special CAD/CAM stone (CAM-base, Dentona AG, Dortmund, Germany).

Fig. 1
(A) Stainless steel models simulating prepared abutments; (B) dimensions of the dies: 4.5 mm height, 6° of taper and 120° chamfer as finish line, 6.0 mm diameter in the marginal area and 3.1 mm diameter in the occlusal area.

The stone cast was digitized by the internal laser scanner component of CEREC inLab unit (Sirona Dental Systems, Charlote, NC, EUA) to generate a tridimensional image that was used to design the FPDs frameworks for both ceramic systems (YZ and IZ). After the milling process, YZ frameworks were sintered using the Zyrcomat furnace (Vita Zahnfabrik, Bad Sackingen, Germany) and IZ frameworks were glass infiltrated (Z21N Zirconia Glass Powder, Vita Zahnfabrik, Germany) using the Inceramat 3 furnace (Vita Zahnfabrik, Germany). The glass infiltration cycle was performed at 1110 °C for 6 h, according to the manufacturer’s instruction. The excess glass was removed with airborne particle abrasion using 50-μm aluminum oxide particles. Only the external surfaces of the FPDs were air abraded.

Vita In-Ceram YZ are porously pre-sintered zirconium dioxide blocks (Y-TZP), in which enlarged restorations are produced to compensate the final sintering shrinkage (around 20%). On the other hand, Vita In-Ceram Zirconia are dry-pressed blocks that can be milled into the final dimension, since these blocks are not subjected to a further sintering process, only to a glass infiltration process. Both types of ceramic blocks display a printed bar code, which can be read by the CEREC InLab scanner. This enables the shrinkage factor or absence of shrinkage of the batch used to be automatically read and taken into account for the grinding process in order to achieve an accurate final result. CEREC InLab software does not allow extensive customization. Thus, the manufacturer default recommendations were followed.

The frameworks were veneered with the manufacturer recommended porcelain, i.e., VM9 for YZ and VM7 for IZ. The porcelains were sintered according to the following cycle: pre-drying at 500 °C for 6 min, heating to 910 °C at a rate of 55 °C/min under vacuum, heating at 960 °C for 1 min and slow cooling (∼6 min). Before veneering, a bonding agent (Effect Bonder, Vita Zahnfabrik, Bad Sackingen, Germany) was applied on the YZ framework in order to improve bonding between the core and veneering material. The bonding agent was sintered according to the cycle recommended by the manufacturer. Ten three-unit FPDs were fabricated for each group.

Each FPD was seated on the original stainless steel model and scanned by the micro-CT equipment SkyScan 1172 with 10 megapixel camera (Skyscan, Aartselaar, Belgium). The scanning parameters were: accelerating voltage of 100 kV, current of 100 μA, exposure time of 2950 ms per frame, Al + Cu filter, and rotation step at 0.4° (180° rotation). The X-ray beam was irradiated perpendicularly to the preparation long axis, and the image pixel size was 17 μm. The X-ray projections were reconstructed using SkyScan’s volumetric reconstruction software (Nrecon) that uses the set of acquired angular projections to create a set of cross section slices through the object. This program uses a modified Feldkamp algorithm with automatic adaptation to the scan geometry in each micro-CT scanner. Reconstructed slices were saved as a stack of bmp-type files. Beam hardening correction of 80% and ring artifact correction of 7 were used for the reconstruction. No internal adjustment was made in the crowns and the scanning procedure was performed before cementation.

The CTAn software (Skyscan, Aartselaar, Belgium) was used to obtain cross-section images through the center of the die ( x -axis), in the mesio-distal ( Fig. 2 ) and bucco-lingual directions ( Fig. 4 A). Using this software it was possible to choose a region of interest (ROI) and the number of slices for the selected region. Therefore, the number of slices was standardized for all specimens, and the same slice, corresponding to the center of the crown, in both bucco-lingual and mesio-distal directions, was analyzed for each crown. These images were transferred to Adobe Photoshop software to delimit the internal space between the die and the crown, and Image J software was used to perform the measurements. All measurements were performed by a single examiner. The presence of small radiographic artifacts did not allow the use of any automatic tool. Therefore, all measurements were manually taken, and the measuring locations were standardized to minimize errors.

Fig. 2
Cross-section image of an IZ FPD in the mesio-distal direction.

Five measuring locations were used ( Fig. 3 ): MG – marginal gap: perpendicular measurement from the internal surface of the crown to the margin of the die ; CA – chamfer area: 800 μm occlusal to the margin of the die; AW – axial wall: internal adaptation at the midpoint of the axial wall; AOT – axio-occlusal transition area: transition from the occlusal plateau to the axial wall, this location was determined by the dotted lines in Fig. 3 ; (5) OA – occlusal area: 500 μm from the axio-occlusal angle in the direction of the center of the occlusal plateau.

Fig. 3
Representation of the measuring locations: MG; CA; AW; AOT; OA.

In addition, cross-section images from the marginal area of the FPDs, taken at the horizontal plane ( y -axis), were analyzed to measure the horizontal marginal discrepancy (HMD) ( Fig. 4 ). For this measurement, the selection of the slice that would be evaluated using the CTAn software was also standardized, as previously described. One slice from each crown was analyzed, and the mean horizontal marginal discrepancy was calculated using twelve measurement points.

Fig. 4
(A) Cross-section image of a crown in the bucco-ligual direction ( x -axis). The white line indicates where the HMD was recorded. (B) Cross-section image in the y -axis used to measure the HMD.

For each material, results from different measurement points were statistically analyzed using one way analysis of variance (ANOVA) and Tukey tests at a significance level of 5%. To compare the values at each measurement location for the two ceramic systems t -test was used ( p < 0.05). The correlation between the measurement locations was tested using Pearson correlation test.

Materials and methods

The experimental groups and materials used in this study are described in Table 1 .

Table 1
Experimental groups and materials used in this study.
Groups Framework material a and basic composition Veneering material a
YZ Vita In-Ceram YZ – yttria partially stabilized tetragonal zirconia polycrystal Vita VM9
IZ Vita In-Ceram Zirconia – glass infiltrated zirconia-reinforced alumina-based ceramic Vita VM7

a Vita Zahnfabrik, Bad Sackingen, Germany.

A stainless steel model simulating prepared abutment teeth was constructed. The prepared die has 4.5 mm height, 6° of taper and 120° chamfer as finish line, as proposed by Sundh et al. ( Fig. 1 ). The distance between the centers of the dies was 16 mm, corresponding to the distance between a lower second premolar and a lower second molar (span of 10 mm). An artificial gingiva was produced with acrylic resin (JET, Classico, Sao Paulo, SP, Brazil). A polyvinyl siloxane impression of the model was taken (Aquasil™, Dentsply, Petropolis, RJ, Brazil). A working cast was made using type IV special CAD/CAM stone (CAM-base, Dentona AG, Dortmund, Germany).

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Adaptation of all-ceramic fixed partial dentures
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