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A Comparative Study on Zirconia Bridge Shrinkage Depending on Zirconia Block
Int J Clin Prev Dent 2022;18(1):22-24
Published online March 31, 2022;  https://doi.org/10.15236/ijcpd.2022.18.1.22
© 2022 International Journal of Clinical Preventive Dentistry.

Sang Ha Kim, Chung-jae Lee

Department of Dental Technology & Science, Shinhan University, Uijeongbu, Korea
Correspondence to: Chung-jae Lee
E-mail: best8463@hanmail.net
https://orcid.org/0000-0001-5648-0830
Received March 7, 2022; Revised March 24, 2022; Accepted March 30, 2022.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
Objective: The purpose of this study was to evaluate how much the shrinkage of a zirconia specimen differs from the design.
Methods: A specimen model was produced and tested using ready-made abutments. Two zirconia block manufacturers were manufactured and processed a total of 30 specimens (15 specimens each) in the shape of a cuboid for ease of length measurement.
Results: When the average length was compared to the original design, the block of company K showed a difference in length of 0.003 mm, 0.0013 mm, 0.0123 mm, and 0.0037 mm by location, whereas the block of company C showed a length difference of 0.009 mm, 0.002 mm, 0.363 mm, 0.1123 mm, and 0.025 mm.
Conclusion: In the case of company K, the result was smaller than the original, whereas in the case of company C, the result was larger than the original, and the block of company K produced a result that was closer to the original design. In addition, when the actual specimen was mounted on the test piece, 1 out of 15 for company K and 8 out of 15 for company C showed an insufficient fit.
Keywords : zirconia, sintering, shrinkage
Introduction

Recently, zirconia grafted with CAD/CAM technology has been widely used to overcome the limitations of conventional prosthetic materials, such as gold and porcelain [1-4]. From the viewpoint of prosthetic materials, zirconia has properties such as high heat resistance, low thermal conductivity, chemical stability to acid and alkali, low thermal distension, high strength and hardness, and aesthetics, making it suitable as a core for all-ceramic crowns [5,6]. If the partially sintered zirconia block is cut, it goes through a complete sintering process at a high temperature of 1200 degrees or higher. In our case, approximately 20% shrinkage occurred. As a result of shrinkage, an enlarged printout is manufactured during the CAM calculation process. Therefore, in order to obtain a precise prosthesis, it was considered important to investigate the shrinkage rate [7].

There have been numerous studies on the fitness of abutment teeth in relation to zirconia shrinkage, and the currently used zirconia block shows fitness within the clinically allowable range due to improvements in manufacturing technology and continuous research and development. However, in the case of a long-length zirconia bridge prosthesis, the shrinkage deformity is more visible because there is a difference in shrinkage due to the deformity of each abutment and the difference in thickness between the abutment connector and the crown. Therefore, the purpose of the study is to measure the degree of shrinkage deformity during plasticity that occurs in the zirconia bridge prosthesis. Rather than confirming the degree of individual fit this time, a virtual bridge bar was designed based on the average measured value of natural teeth to measure the overall length and positional deformity. In addition, to compare the degree of shrinkage by manufacturers, zirconia blocks from two manufacturers, K and C, were used. To determine the consistency of the contracted prosthesis, 15 specimens were prepared for each block, and the length of the final designed file and the actual prosthesis was measured and compared using digital vernier calipers. We intend to use this information for clinical performance prediction data.

Materials and Methods

1. Experimental materials

Two zirconia blocks from different manufacturing countries were selected among the zirconia blocks usually used in domestic factories.

2. Experimental methods

In the case of experimental specimens, 15 specimens were prepared per block to determine the consistency of contracted prostheses, and measurements were taken with digital vernier calipers (Shane, China) (measurement range: 0-150 mm, resolution: 0.01 mm, error ±0.02 mm). In the case of the measurement part, the distance between the holes located in the specimen and the sides of the entire block was immediately determined and compared in order to understand the positional relationship of each abutment. In the case of the original design file, it was measured using cross-sectional views and ruler functions in the program.

Results

When the average length was compared with the original design, the block of company K showed a length difference of 0.003 mm, 0.0013 mm, 0.0123 mm, and 0.0037 mm by location, whereas the block of company C showed a length difference of 0.009 mm, 0.002 mm, 0.363 mm, 0.1123 mm, and 0.025 mm by location (Table 1, 2).

Table 1 . Block K measurement (unit: mm)

 ABDWH
N15
M7.08806.63278.445327.09277.7853
Original M0.0030.00130.01370.01230.0037
pp<0.05


Table 2 . Block C measurement (unit: mm)

 ABDWH
N15
M7.08806.63278.445327.09277.7853
Original M0.0030.00130.01370.01230.0037
pp<0.05


Accordingly, in the case of company K, the length of the test piece was shorter than the original, whereas in the case of company C, the length of the test piece was longer than the original, and the block of company K showed a result that was closer to the original design. In addition, when the actual specimen was mounted on the test piece, 1 out of 15 for company K and 8 out of 15 for company C showed insufficient fit. Significant differences were observed, but both products were within the allowable range for clinical application.

Discussion

As a result of conducting this study, compared to the actual CAD program, the K block was sintered generally smaller than the original, and the C block was sintered generally larger than the original. For the length of each measurement element. Company C’s W showed a difference of 0.1123 mm and the inner fit of each measurement was less than 120 micrometers, so the position of each measurement would not affect the fit.

Conclusion

In this experiment, the zirconia bridge specimens milled from blocks by two different manufacturers were obtained and sintered using a CAD program. As a result of evaluating how far the actual shrinkage differs from the design through length comparison of sintered specimens, the following conclusions were obtained:

1. In the case of company K, the length of the test piece was shorter than the original. In the case of company C, the length of the test piece was longer than the original, and the block of company K showed a closer result to the original design.

2. When the actual specimen was mounted on the test piece, 1 out of 15 for company K and 8 out of 15 for company C showed an insufficient fit.

3. Both products were within the allowable range for clinical application.

Conflict of Interest

No potential conflict of interest relevant to this article was reported.

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March 2022, 18 (1)