Dimensional changes from the sintering process and fit of Y-TZP copings: Micro-CT analysis

Highlights

  • The sintering shrinkage of Y-TZP might depend on the specimen shape.

  • Dimensional changes from sintering of Y-TZP do not occur uniformly in pre-molar shaped single copings.

  • Dimensional changes from sintering process of Y-TZP can lead to non-uniform cement space in single copings.

  • Manufacturing process of Y-TZP blanks may have an influence on sintering shrinkage pattern and, consequently, on the prostheses fit.

Abstract

Objective

To evaluate the dimensional changes from the sintering process of Y-TZP and relate them to the fit of zirconia copings.

Methods

The sintering shrinkage rate (SSR) was obtained from the measurement of geometric specimens (4 × 4 × 2 mm). Thirty-six zirconia copings made using CAD/CAM were equally divided into three groups (n = 12): ZMAX — IPS e.max ZirCAD (Ivoclar Vivadent, Liechtenstein); ZYZ — InCeram YZ (Vita Zahnfabrik, Germany); and ZK — Zirklein (Zirklein, Brazil). The copings were scanned in micro-CT before and after sintering so that SSR was obtained. The SSR of geometrical specimens and copings was compared to each other and those the manufacturers reported (ANOVA-2 and Tukey, p ≤ .05). The copings were settled on an abutment and taken to the micro-CT to evaluate their marginal and internal fit. The data enabled the statistical comparison (ANOVA-2 and Tukey, p ≤ .05) between groups and measurement sites and between the fit obtained with that stipulated by the CAD/CAM software (80 μm) (Dunnett test, p ≤ .05).

Results

All groups showed statistical differences between the SSR the manufacturer reported and those obtained experimentally and between the SSR of the geometric specimens and copings. In general, the SSR of the copings showed no uniformity. There was no statistical difference among the groups for marginal fit, with differences only for internal fit and between the different regions measured. The fit obtained experimentally differed from the internal space determined in the CAD/CAM software.

Significance

The lack of uniformity of sintering shrinkage might lead to a non-uniform internal fit of Y-TZP copings.

Introduction

Yttria-stabilized tetragonal zirconia (Y-TZP) is the most used polycrystalline ceramic for the purposes of prosthetic dentistry . Y-TZP frameworks are made using CAD/CAM technology from the milling of pre-manufactured blocks. Commercially, these blocks are available in two different sintering stages: the pre-sintered and fully sintered stages . In comparison with fully sintered blocks, pre-sintered blocks present the advantage of an easier and faster milling procedure and less wear of the machining tools .

However, after the milling process by the CAD/CAM system, the material must be sintered at a high temperature . During the sintering, the material shrinks, making the framework denser and stronger. Thus, shrinkage in the sintering process has to be compensated by the CAD/CAM system to avoid prosthesis framework misfit .

Previous studies in the literature show the influence of the Y-TZP block stage and the size of the prosthetic infrastructure on the prosthesis fit . Larger frameworks lead to higher sintering shrinkage and might result in higher prosthesis misfit . Furthermore, one study has reported a higher misfit for frameworks milled from pre-sintered blocks , whereas other authors report no difference in prosthesis fitting when made from pre-sintered and fully sintered blocks . From these findings, the authors emphasize the possible influence of the sintering process on prosthesis fit .

Higher prosthesis misfits might lead to marginal leakage, causing gingival inflammation and caries . Furthermore, larger internal misfit can make ceramic crowns more fragile once the thicker cement layer has lower stiffness in the prosthetic system. This can cause the framework to suffer increased deformation, leading to high stress concentration in the ceramic crown and facilitating its fracture .

A lack of studies addresses the dimensional changes resulting from the sintering process and the influence of such changes on the fit of ceramic prostheses. Thus, the null hypotheses of this study were as follows: (1) there would be no difference between the sintering shrinkage rate reported by the manufacturers and that obtained experimentally; (2) there would be no difference between sintering shrinkage from geometric and anatomic shapes; (3) there would be no difference regarding the internal and marginal fit of the Y-TZP copings made from different trademarks; and (4) there would be no difference regarding the space between the coping and abutment obtained experimentally compared to that determined to be a parameter in the CAD/CAM software.

Materials and methods

Three commercial brands of Y-TZP blocks were used to fabricate the specimens. The composition of materials and the distribution of the groups are described in Table 1 .

Table 1
Group distribution and material specification (n = 12).
Group Product Manufacturer Composition
ZK Zirklein Zirklein (Marília, Brazil) Zr(Hf)O 2 = 94.70%
Y 2 O 3 = 5.25%
SiO 2 = 0.02%
Fe 2 O 3 = 0.01%
Na 2 O = 0.01%
Cl — 0.02%
TiO 2 = 0.01%
ZMAX IPS e.max Ivoclar Vivadent (Schaan, Liechtenstein) ZrO 2 = 95.00%
Zircad HfO 2 + Al 2 O 3 + Y 2 O 3 + others = 5.00%
ZYZ In Ceram YZ Vita Zahnfabrik (Bad Säckingen, Germany) ZrO 2 = 91.00%
Y 2 O 3 = 5.00%
HfO 2 < 3.00%
Al 2 O 3 + SiO 2 < 1.00%

Sintering shrinkage measurement

Specimen preparation

To evaluate the influence of specimen shape on the dimensional changes from the sintering process, the sintering shrinkage rate (SSR) was measured using geometrical and anatomical specimens. The geometrical specimens were obtained from CAD/CAM blocks, which were cut by an automatic cutting machine (Struers, Inc., Cleveland, USA) and a diamond saw, resulting in pre-sintered specimens of 5 × 5 × 2.5 mm. After the sintering process, the specimens reached a final dimension of 4 × 4 × 2 mm approximately ( Fig. 1 ). For the sintering process, a high-temperature furnace was used (Sintramat High Temperature Furnace; Ivoclar Vivadent; Liechtenstein) with a default cycle of 7 h and 52 min and a maximum temperature of 1500 °C.

Fig. 1
Geometrical specimen before (left) and after (right) the sintering process.

The anatomical specimens corresponded to copings made by a CAD/CAM system (Cerec In Lab 4.0, Sirona Dental Systems GmbH — Bensheim, Germany) from the scanning of a maxillary premolar-shaped zirconia abutment. The abutment features were as follows: chamfer finish line; 6° taper; 2-mm-height cervical margin on the lingual and proximal area; and a 1-mm-height cervical margin on the buccal aspect. The copings were designed with a minimal thickness of 0.3 mm at the margin, 0.5 mm at the axial walls, 0.7 mm at the occlusal aspect, and a uniform internal space of 80 μm. After the design procedure, the copings were obtained using a milling procedure (MC XL milling machine; Sirona Dental Systems GmbH — Bensheim, Germany) from three different brands of Y-TZP blanks, as described in Table 1 . The copings were sintered in the high-temperature furnace (Sintramat High Temperature Furnace; Ivoclar Vivadent; Liechtenstein) with a default cycle of 7 h and 52 min and a maximum temperature of 1500 °C.

SSR measurement

The geometrical specimens were measured at 15 different points with the aid of a dial caliper with 0.05-mm accuracy (Yates & Bird Inc.; Chicago, USA) before and after the sintering process, allowing for the percentage of sintering shrinkage at three points, including height (H1–H3), three points for width (W1–W3), and nine points for thickness (T1–T9) ( Fig. 2 ). The mean value of the SSR was obtained for each specimen.

Fig. 2
Measurement of a geometrical specimen using a dial caliper (left) and the locations measured for the same specimen (right).

Regarding the SSR of anatomical specimens, the measurement was made from images taken using a micro-CT scanner (Skyscan 1172 — Microphotonics; Belgium). For this, the copings were scanned before and after the sintering process with a resolution of 34 μm.

The micro-CT scanning allowed for the creation of image files in TIFF format, corresponding to different specimen cuts. These image files were then grouped in Skyscan NRecon software (Microphotonics; Belgium) and converted into a bitmap (.bmp) file extension, which allowed for the three-dimensional reconstruction of the specimen. Using the Skyscan DataViewer software (Microphotonics; Belgium), two slices per specimen were selected: one corresponding to the center of the specimen in the buccal-palatal direction and the other in the mesial–distal direction.

The SSR of the copings was determined from a measurement of the dimensional change of their wall thicknesses at predetermined points ( Fig. 3 ) and from the distance between the opposite walls of the coping using ImageJ software (NIH, USA). The distances between the buccal and palatal walls were measured from the buccal–palatal section for the cervical and medium third ( Fig. 4 a).

Fig. 3
Measurement points of the wall thickness of the copings in the buccal–palatal (left) and mesial–distal (right.) cuts: P — 0.6 mm away from the palatal margin of the coping; AP — the center of the palatal axial wall; OP — the tip of the palatal cusp; OB — the tip of the buccal cusp; AB — the center of the buccal axial wall; B — 0.6 mm away from the buccal margin; M — 0.6 mm away from the mesial margin of the coping; OC — the center of the occlusal surface; D — 0.6 mm away from the distal margin of the coping.

Fig. 4
(A) The distances D1 (buccal margin to palatal margin) and D2 (the medium point of the buccal wall to the medium point of the palatal wall) were measured before and after sintering to determine the shrinkage rate; (B) Measurement of angles between the internal axial walls (buccal and palatal) and the occlusal walls of the coping; the angle formed between the internal aspects of the buccal and palatal cusps was also measured; PO = the angle between the palatal and occlusal walls; CO = the angle formed by the occlusal aspect of palatal and buccal cusps; BO = the angle between the palatal and occlusal walls.

The same image from the buccal–palatal direction was used to evaluate changes in the internal angles between the axial and occlusal walls and between the internal walls of the buccal and palatal aspects of the coping’s occlusal surface ( Fig. 4 b). The measurement of these angles was performed with AutoCAD 2012 software (AutoDesk, San Rafael/CA, USA) before and after sintering. The values of the angular change of the copings allowed for evaluation of the uniformity of the dimensional changes in the occlusal third and comparison between the different groups.

Copings Fit

The same copings used for the sintering shrinkage experiment were used for fit evaluation (n = 12). The specimens were seated on the same zirconia abutment, and an axial load of 5 kgf was applied to the center of the occlusal surface for 5 s (as reported by Martins et al. ). Before the coping seat, the abutment was screwed to an external hexagon implant (Osseotite, Biomet 3i, Palm Beach, USA), and the hole to access the abutment screw was restored with universal composite resin (Tetric N-Ceram; Ivoclar Vivadent, Liechtenstein).

The set composed of the implant/abutment/coping was taken to the micro-CT scanner. The implant/abutment set position was standardized with a silicone matrix to guarantee the same position for all copings, and the scanning process was performed with 17-μm resolution.

The scanning process generated approximately 500 axial sections for each specimen in TIFF format files. These files were imported into NRecon software for three-dimensional model reconstruction. After the reconstruction, the files in TIFF format were converted to .bmp format, allowing for the exportation of such files to DataViewer software, which was used for the virtual segmentation of the specimen and selection of the cuts to be measured.

Four standard cuts were selected for each specimen, one from the center of the specimen in the buccal-palatal direction and three in the mesial-distal direction (one from the center of the specimen and the others corresponding to the center of the buccal and palatal half of the specimen, respectively). A total of 33 points was measured per specimen, distributed in the marginal , chamfer , axial and occlusal regions, as described in Fig. 5 .

Fig. 5
Distribution of measuring points for coping fit: (A) buccal-palatal cut at the center of the specimen; (B) mesial-distal cut at the center of the specimen; (C) center of the palatal half of the specimen; and (D) center of the buccal half of the specimen. The measurements in the marginal region corresponding to the points beginning with the letter “M”; the measurements in the chamfer area (places with names beginning with the letter “C”) were performed in the center of the notch area in a path perpendicular to the inner surface of the coping. The measurements in the middle third region of the coping were conducted at the midpoint of the axial walls (points beginning with letter “A”). The measurements of the occlusal surfaces were made at peak regions of the cusps (OB and OP) in the center of the occlusal surface (OC), in the axio-occlusal limits of the copings on the buccal (OMB and ODB) and palatal halves (ODP and OMP) and at the center of the buccal (OCB) and palatal (OCP) cusps.

The fit was measured based on Holmes et al.’s study for absolute marginal discrepancy and internal fit using Image J software (NIH, USA). Each point was measured three times by the same operator, and a mean value was obtained for each point.

Statistical analysis

Geometric specimens

The data were analyzed using the Shapiro–Wilk normality test. For the analysis of the variability between the different measured locations and among different groups, two-way ANOVA was performed ( p ≤ .05). To compare the different regions measured, the Friedman repeated ANOVA test ( p = .05) was used. The comparison between the sintering rates for different groups was evaluated using the Kruskal–Wallis test ( p ≤ .05).

The average SSR obtained for geometrical specimens of each group was compared to the sintering shrinkage values reported by the respective manufacturers after an independent t test ( p ≤ .05). The value of the sintering shrinkage was directly informed by the manufacturer of the ZK group, whereas for the other groups, the manufacturer’s rate was obtained from the code bar of the Y-TZP CAD/CAM block used in this study.

Anatomical Specimens

The data were analyzed using the Kolmogorov–Smirnov normality test. The variability between the measurement sites and between the groups was analyzed using 2-way ANOVA ( p ≤ .05). For the analysis of changes in the walls’ thickness, ANOVA-2 for repeated measures and Tukey’s test ( p ≤ .05) were used to compare the different measured areas and groups and to verify a possible interaction between both factors.

Tukey’s test ( p ≤ .05) was conducted for the evaluation of dimensional changes related to the distance between the walls of the cervical and middle thirds of the copings and to evaluate the angular changes in the occlusal third, allowing for a comparison between the groups.

The Pearson correlation test was performed to verify the correspondence between the results obtained for the geometric specimens and those obtained for the anatomical specimens.

Copings Fit

The data of the coping-fit evaluation were grouped according to location and were divided into the following categories: marginal adaptation, Chamfer area, middle third, and occlusal third of the vertical plane ( Fig. 6 ). The data were also grouped according to the different sides of the crown, that is, mesial, distal, buccal, and palatal ( Fig. 7 ). The grouped data were used for a comparison between groups and between locations to verify the uniformity of the spacing between the coping and abutment.

Fig. 6
Data grouping based on the measurements’ location on the horizontal plane.

Fig. 7
Data grouping based on the measurements’ location on the vertical plane.

The comparison between groups and between locations was performed using 2-way ANOVA and Tukey’s test ( p ≤ .05). The experimentally obtained spacing between the coping and abutment was compared to those used as a parameter in the Cerec 4.0 software, which corresponded to 80 μm, using the Dunnett test ( p ≤ .05).

Materials and methods

Three commercial brands of Y-TZP blocks were used to fabricate the specimens. The composition of materials and the distribution of the groups are described in Table 1 .

Table 1
Group distribution and material specification (n = 12).
Group Product Manufacturer Composition
ZK Zirklein Zirklein (Marília, Brazil) Zr(Hf)O 2 = 94.70%
Y 2 O 3 = 5.25%
SiO 2 = 0.02%
Fe 2 O 3 = 0.01%
Na 2 O = 0.01%
Cl — 0.02%
TiO 2 = 0.01%
ZMAX IPS e.max Ivoclar Vivadent (Schaan, Liechtenstein) ZrO 2 = 95.00%
Zircad HfO 2 + Al 2 O 3 + Y 2 O 3 + others = 5.00%
ZYZ In Ceram YZ Vita Zahnfabrik (Bad Säckingen, Germany) ZrO 2 = 91.00%
Y 2 O 3 = 5.00%
HfO 2 < 3.00%
Al 2 O 3 + SiO 2 < 1.00%

Sintering shrinkage measurement

Specimen preparation

To evaluate the influence of specimen shape on the dimensional changes from the sintering process, the sintering shrinkage rate (SSR) was measured using geometrical and anatomical specimens. The geometrical specimens were obtained from CAD/CAM blocks, which were cut by an automatic cutting machine (Struers, Inc., Cleveland, USA) and a diamond saw, resulting in pre-sintered specimens of 5 × 5 × 2.5 mm. After the sintering process, the specimens reached a final dimension of 4 × 4 × 2 mm approximately ( Fig. 1 ). For the sintering process, a high-temperature furnace was used (Sintramat High Temperature Furnace; Ivoclar Vivadent; Liechtenstein) with a default cycle of 7 h and 52 min and a maximum temperature of 1500 °C.

Nov 22, 2017 | Posted by in Dental Materials | Comments Off on Dimensional changes from the sintering process and fit of Y-TZP copings: Micro-CT analysis
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