1

Introduction

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.

2

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 .

Premolar shaped coping designed by CEREC 4.0 software.

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 .

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).

Measured locations (represented by yellow lines); A) coronal slice with two locations (OB1 and OP1); B) sagittal cut in the center of the specimen with one location measured (OC); C) buccal slice—three locations (OM2, OB2 and OD2); D) palatal slice—three locations (OM1, OP2 and OD1). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)