There is a considerable variation of the internal occlusal fit of Y-TZP copings made by CAD/CAM technology.
A thicker cement layer can lead to greater risk of porcelain fractures in Y-TZP based ceramic crowns.
Manufacturing procedures might exert some influence on the internal fit and, as consequence, on the stress distribution in Y-TZP base ceramic crowns.
To evaluate the stress distribution in bi-layered Y-TZP based crowns, according to the occlusal internal spacing between coping and abutment.
Twelve premolar shaped Y-TZP copings were made by a CAD/CAM system and seated on an abutment to evaluate the internal fit at the occlusal third using micro-CT images. Considering the fitting range obtained experimentally, two 3D finite element models, consisting on bone tissue, a titanium implant, a zirconia abutment, cement layer and a bi-layered Y-TZP ceramic crown were constructed based on the micro-CT images, one corresponding to the thinnest cement space and other representing the specimen with the thickest cement space obtained experimentally. A 250 N axial load was applied at the center of the occlusal surface of the crown (≅0,8 mm 2 area) and the first principal stress distribution was plotted and analyzed.
The greatest maximum principal stress occurred within the veneer ceramic right below the site of loading. The thickest cement model showed higher stress concentration at the center of occlusal surface of veneer and the center of occlusal internal surface of coping.
Knowledge of stress distribution in ceramic crowns with different cement thicknesses will help clinicians to properly adjust crown fit, in seeking to avoid porcelain fractures.
Y-TZP is the most used polycrystalline ceramic to fabricate prosthetic frameworks from CAD/CAM, and it has the highest fracture resistance among dental ceramics . However, clinical trials and systematic review studies have reported mechanical failures of Y-TZP based fixed prostheses, which might be related to the prosthesis design, the material properties, the intensity of load applied to the prosthesis and to its internal fit.
Y-TZP frameworks are milled oversized to compensate for the subsequent sintering process, which leads to a linear shrinkage of approximately 20–25% . There are some in vitro studies showing a lack of uniform sintering shrinkage for Y-TZP with distortions occurring to geometrical specimens . These distortions could lead to unpredictable internal fit in Y-TZP based fixed prostheses. In addition, the scanning and milling processes could have an important role in the marginal and internal fit, since sharp edges could not be accurately read by the scanner, and small details cannot be reproduced by the milling unit .
The cement thickness between the Y-TZP coping and the abutment is dependent on the resultant internal fit after Y-TZP milling and sintering, which must also be considered as a determining factor for long-term success of ceramic restorations, as it can influence the strength of this type of restoration. A larger space for cementation may increase the risk of fracture of the ceramic crown . On the other hand, an insufficient space for the luting agent may hinder the seating of the crown causing a higher marginal leakage .
How the stresses are dissipated on the ceramic restoration is another important factor that can predict their clinical success . Among the methods available for measurement of force dissipation of dental prosthesis systems is finite element analysis, which is based on three-dimensional virtual models of restorations, allowing the simulation of the forces applied to the restoration in order to enable the visualization of high stress regions . Some studies have shown the effectiveness of this method in assessing the dissipation of forces and expectation of longevity of all-ceramic crowns and prostheses .
The effect of the resultant cement space on the stress distribution in Y-TZP crowns is still unclear. Studies considering the stress distribution in Y-TZP based bi-layered crowns in function of the cement space range were not found. The purpose of this study was to evaluate the stress distribution in bi-layered Y-TZP based crowns, according to the occlusal internal spacing between coping and abutment. The null hypothesis tested was: the resultant cement space from Y-TZP copings and abutment does not influence the stress distribution in bi-layered Y-TZP based ceramic crowns.
Material and methods
Twelve pre-molar shaped copings were fabricated from Y-TZP blocks (ZirCAD—Ivoclar Vivadent; Liechtenstein) using the CAD/CAM technology (CEREC inLab 4.0—Sirona Dental Systems; Germany) ( Fig. 1 ) from the scanning of a Y-TZP implant abutment (InEOS Blue Scanner, Sirona Dental Systems; Germany). The individual restoration was selected and the restoration type used was “infrastructure”. The default parameters of CEREC 4.0 software for Y-TZP infra-structures were used, as follows: chamfer finish line; minimal thickness of 500 μm for axial walls and of 700 μm for the occlusal wall, marginal thickness of 300 μm and cement space of 80 μm. After the milling process in the MCXL milling unit (Sirona Dental Systems; Germany) the copings were taken to a high temperature furnace for the sintering process, as described in Table 1 .
|–||Room –1100 °C—1 h 12 min|
|–||1100°–1350 °C—1 h 11 min|
|–||1350°–1500 °C—30 min|
|–||1500 °C for 30 min|
|–||Cooling to room temperature|
|–||Total: 7 h 52 min|
After the sintering process, the copings were cleaned in an ultrasonic bath submerged in deionized water and detergent solution for 2 min and dried with compressed air. After that, the specimens were seated on a Y-TZP abutment, which was screwed to an external hexagon implant (Osseotite—Biomet 3i; Palm Beach, Florida; USA). This set was taken to a high resolution micro-CT scanner (Skyscan 1172—Microphotonics; Belgium) where the scanning process allowed for the acquisition of a series of radiograph projections taken from different viewing angles. The implant position was standardized by a silicone matrix. The parameters used for the scanning are described in Table 2 .
|Image pixel size||17 μm|
|Filter||Cu + Al|
|Flat field correction||On|
The images obtained were imported by NRecon software for the virtual specimen reconstruction using the Volume of Interest (VOI) tool, which allowed the determination of a standardized virtual cube for all specimens. This software saves the specimen as axial slices in .bmp files. Due to use of reference points on the occlusal surface, it was possible to ensure the same positioning of the specimens in the virtual cube, facilitating locating the proper image section for the measuring procedure.
DataViewer software (Skyscan, Belgium) was used for virtual segmentation of the specimen and selection of the slices to be measured, in seeking to obtain standardized slices from mesial–distal and buccal–palatal aspects of the specimen. Four slices were obtained for each specimen, one in the buccal–palatal direction and three in the mesial–distal direction. One slice was obtained from a coronal cut in the center of the specimen, while the others were taken from sagittal cuts in the center of the specimen, and in the middle of the buccal and palatal portion of the specimen, respectively. A total of nine points were measured in the occlusal aspect of each specimen ( Fig. 2 A–D).
Image J software was used to measure the copings fit from 2D images, corresponding to virtual slices of the specimens. To the dimensional accuracy was assessed by measuring the distance from the internal surface of the coping to the external surface of the abutment, being the measurement done perpendicularly to the internal wall of the coping. Each location was measured three times by the same operator. A mean occlusal gap size was obtained for each specimen.
Finite element model
Two finite element models (FEM) were constructed, based on the mean cement space obtained experimentally, one corresponding to the thickest cement space and the other one corresponding to the thinnest cement on the occlusal aspect out of all of the specimens. The .bmp files of the thinnest and the thickest gap size specimens were imported by the Mimics software (Materialise; Leuven, Belgium) for the 3D models construction. The occlusal internal space obtained for the measured points of each model is presented in Table 3 , where the model with the thicker cement showed a mean of 239.97 μm, while the thinner cement space showed a mean cement thickness of 95.93 μm. The thresholding tool and region growing tool were used to generate and segment all the parts of the specimen, in this case coping, abutment and the gap corresponding to the cement space. Due to artifacts resulting from the high density of the zirconia and the reflection-ray absorption, the cement space filling had to be refined (painted) manually in all the slices.
The implant was scanned separately in the micro-CT scanner, and the thresholding and region growing tools were used to generate the 3D model of this part. A U-shaped acrylic resin block was also scanned separately to simulate the peri-implant bone. A Boolean operation was used to subtract the 3D implant model from the bone block, generating the implant site.
For the modeling of the veneer part, a bi-layered Y-TZP based crown was scanned in the micro-CT using the same VOI of the specimens. A Boolean operation was used to subtract the copings of the thinner and thicker models from the complete crown, obtaining the veneer part. The model was composed of six parts: bone tissue; implant; abutment; cement; coping and veneer; all of these parts were assembled together in Mimics software ( Fig. 3 ). The 3D model containing all the parts was saved as .STL files and exported to the 3-matic software (Materialise; Leuven, Belgium) to generate the volumetric mesh for both the thick gap and thin gap models.
An allowable geometrical error of 0.04% was used for the mesh generation to preserve the original geometry. For the mesh refinement a filter tool was used to avoid high aspect ratio elements which could result in calculation error by the FEA software. The meshed parts were composed of tetrahedral elements and were exported to the Abaqus software (V6.13; Simulia, USA) for the FE analysis. In the Abaqus software the material properties of each part were inserted. For the ceramic materials it was obtained experimentally, using the Archimedes method for density and ultrasonic pulse method to determine the Poisson’s ratio (Y) and Young’s elastic modulus (E) ( Table 4 ). All of the parts and materials were considered isotropic.
|Y-TZP||217.45||0.31||6.01||Abutment and coping||Experimentally|
|Titanium alloy||110.00||0.35||4.50||Implant||Lewinstein et al.|
|Resin cement||8.30||0.30||2.05||Cement||Gurgel-Juarez et al.|
|Bone type III||1.60||0.30||1.30||Bone||Yokoyama et al.|