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 Table of Contents  
ORIGINAL ARTICLE
Year : 2020  |  Volume : 15  |  Issue : 1  |  Page : 94-97

Effect of multiple firing on flexural strength and color stability of pressable all ceramic material: An In-vitro study


1 Department of Prosthodontics, Sharad Pawar Dental College, Datta Meghe Institute of Medical Sciences (Deemed to be University), Sawangi (M), Wardha, Maharashtra, India
2 Department of Oral and Maxillofacial Surgery, Sharad Pawar Dental College, Datta Meghe Institute of Medical Sciences (Deemed to be University), Sawangi (M), Wardha, Maharashtra, India

Date of Submission25-Sep-2019
Date of Decision27-Sep-2019
Date of Acceptance29-Sep-2019
Date of Web Publication13-Oct-2020

Correspondence Address:
Dr. Seema Kambala
Department of Prosthodontics, Sharad Pawar Dental College, Sawangi (M), Wardha - 442 001, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jdmimsu.jdmimsu_145_19

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  Abstract 


Introduction: Pressable all-ceramic materials are widely used in dentistry. Determining the effect of multiple firing on color stability and flexural strength will help to improve these materials so that they can remain resistant to fracture in restorative work. Aim: The aim of this study was to evaluate the change in flexural strength and color stability of pressable all ceramic material after repeated firing, which may be unavoidable when color and shape corrections are necessary for use in dental restorations. Materials and Methods: Thirty standardized bar specimens measuring 25-mm length, 10-mm width, and 2-mm thickness were fabricated from VITA PM 9 pressable all ceramic material according to the manufacturer's instructions. Each group of specimen was tested for color change using the spectrophotometer and flexural strength (three-point flexural strength test) using the universal testing machine after third (n = 10), fourth (n = 10), and fifth (n = 10) firings. The data were analyzed using a one-way analysis of variance. The Tukey honestly significant difference test was used to perform multiple comparisons. Results: L*a*b* values of the ceramic specimens were affected by the number of firings (3, 4, and 5 firings) (P < 0.05). A statistically significant difference in color change (ΔE) was found between Group 1 – Group 2 versus Group 1 – Group 3 (P < 0.05), Group 1 – Group 3 versus Group 2 – Group 3 (P < 0.05), whereas it was nonsignificant between Group 1 – Group 2 versus Group 2 – Group 3 (P > 0.05). There were statistically significant differences (P < 0.05) in the mean flexural strength values of the three groups. Conclusion: Repeated firing of leucite-based heat-pressed ceramic exhibited color changes that are clinically acceptable, but the strength of the ceramic decreases significantly. Thus, repeated firing is an important factor with respect to color and strength of definitive restoration and should be considered during shade selection and fabrication.

Keywords: Color stability, flexural strength, leucite, repeated firing


How to cite this article:
Sancheti Y, Kambala S, Godbole S, Kambala R, Dhamande M, Pisulkar S. Effect of multiple firing on flexural strength and color stability of pressable all ceramic material: An In-vitro study. J Datta Meghe Inst Med Sci Univ 2020;15:94-7

How to cite this URL:
Sancheti Y, Kambala S, Godbole S, Kambala R, Dhamande M, Pisulkar S. Effect of multiple firing on flexural strength and color stability of pressable all ceramic material: An In-vitro study. J Datta Meghe Inst Med Sci Univ [serial online] 2020 [cited 2020 Oct 23];15:94-7. Available from: http://www.journaldmims.com/text.asp?2020/15/1/94/297967




  Introduction Top


The need for esthetically acceptable restoration is continuously increasing due to the increase in expectations and esthetic sense of the population. Dental ceramics have attained a special place in dentistry. Its color, translucency, and esthetic quality cannot as yet be matched by any other restorative material. The greatest advantage of ceramic is the ability to match natural dentition. In spite of their excellent esthetic qualities and their good biological compatibility, dental ceramics are brittle which may cause the failure of the restoration. They are susceptible to fracture at the time of placement or during function. New ceramic materials for restorative dentistry have been developed and introduced in recent years. Several methods for fabrication of dental ceramic are available which includes powder condensation, slip casting, heat pressing, and CAD/CAM technology. The long-term survival of all-ceramic restorations is related to the fabrication method. Restorations fabricated using the hot pressing technique has the highest long-term survival.[1]

Pressable glass-ceramics are one of the most popular dental restorative systems due to several factors: ease of fabrication, occlusal accuracy, better marginal integrity, translucency, good mechanical properties, net shaped forming by pressing, and decreased porosity.[2]

Dentists sometimes have their dental technicians fire a pressable all-ceramic restoration several times to produce a natural appearance by correcting its form and color, but the effect of repeated firing on the flexural strength and color stability of a ceramic material is still unknown. The aim of this study was to investigate the flexural strength and color stability of pressable all-ceramic materials after repeated firings. The null hypothesis was that the flexural strength and color of pressable all-ceramic materials would be affected by repeated firings.


  Materials and Methods Top


Fabrication of samples

Thirty standardized bar specimens measuring 25-mm length, 10-mm width, and 2-mm thickness were fabricated from VITA PM 9 pressable all ceramic material according to manufacturer's instructions [Figure 1]. A specially designed metal mold was used in this study to fabricate standardized wax patterns. They were then invested using VITA PM investment material in a silicone ring and burnt out in a furnace at 850°C. The cold VITA PM 9 press pellets were then placed into the investment ring and heat pressed at 1000°C. After completion of the press program, the ring was immediately removed from the furnace and left to cool down to room temperature. The specimens were devested using airborne-particle abrasion with aluminum oxide particles (40 μm). Finishing was done using fine grit and sharp diamond tools. The samples were subjected to ADD ON firing according to the manufacturer's recommendations using VITA VM 9 ADD-ON powder and liquid. Similar procedure was carried out for glaze firing.
Figure 1: Final sample

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Ethical clearance

Ethical clearance was obtained from the Institutional Ethical Committee of SPDC, Sawangi (Meghe), Wardha, on 5th June 2019. With ethical clearance no DMIMS(DU)/IEC/2019-20/342.

Grouping of samples

Samples were divided into three groups based on the number of firings. Samples in the Group 1 (control group) were subjected to 1 ADD ON firing and 1 Glaze firing, Group 2 were subjected to 2 ADD ON firing and 1 Glaze firing, and Group 3 were subjected to 2 ADD ON firing and 2 Glaze firing.

Evaluation of color change and flexural strength

The samples were subjected to spectrophotometric evaluation for measurement of color change [Figure 2] and to the three-point bend test for flexural strength values [Figure 3]. In the spectrophotometric system, the sample was exposed to an emission of light, and reflected light was analyzed. All measurements were recorded in CIELAB coordinates and transferred to a personal computer for the analysis. The CIELAB measurements make it possible to evaluate the quantity of perceptible color changes in each sample. Total color differences were calculated with the following equation:
Figure 2: Spectrophotometer

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Figure 3: Three point bend test

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ΔE = [(ΔL*)2+ (Δa*)2+ (Δb*)2]1/2

After the spectrophotometric evaluation, samples were then subjected to a 3-point bending test on Universal testing machine for estimation of flexural strength. In the three-point flexure test, samples were placed on two parallel supporting pins. The loading force was applied in the middle by means of a loading pin. As a result of the loading, the specimen bends, causing the formation of in its convex side, and compression stress in the concave side. The crosshead speed was 1 mm/min. The maximum stress and corresponding maximum strain were calculated for every load value. The results were then plotted in the stress-strain diagram.

The results of testing were analyzed statistically. In addition to calculation of the mean value and standard deviation, a one-way analysis of variance (ANOVA) was used to analyze the data for significant differences and Tukey honestly significant difference test to perform multiple comparisons.


  Results Top


The results of the ANOVA of L * a* b* color parameters after multiple firing are listed in table. The L * a* b* values of the ceramic system were affected by the number of firings (3, 4, and 5 firings) (P < 0.05). An increase in the number of firings resulted in an appreciable increase in L* values (P < 0.05), which created lighter specimens. The a* and b* color values also increased after repeated firings, which resulted in specimens that were redder and more yellow (P < 0.05). The ΔE values were calculated to determine the mean color difference (ΔE), depending on the number of firings [Table 1]. Statistically significant difference in color change (ΔE) is found between Group 1 and Group 2 versus Group 1 – Group 3 (P < 0.05), Group 1 – Group 3 versus Group 2 – Group 3 (P < 0.05) whereas it was nonsignificant between Group 1 and Group 2 versus Group 2 – Group 3 (P > 0.05) [Table 2]. The flexural strength values after repeated firings are listed in [Table 3]. There were statistically significant differences (P < 0.05) in the mean flexural strength values of the three groups [Table 4]. Repeated firing was found to be associated with a decrease in the flexural strength.
Table 1: ΔE values depending on the number of firings

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Table 2: Mean difference and confidence interval between all three groups

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Table 3: Flexural strength values after the number of firings

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Table 4: Statistically significant difference in mean flexural strength values of three groups

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  Discussion Top


All ceramic is majorly a choice of restoration when esthetic is of primary concern.[3] Provisional restoration plays a vital role in the treatment modality of fixed partial denture (FPD) till the time the final restoration is cemented.[4]

In this in vitro study, flexural strength and color stability of heat-pressed ceramic was evaluated after repeated firing (3, 4, and 5 firings). Within the limitations of this study, the results support the hypothesis regarding the effect of repeated firing on color difference and flexural strength of pressable all ceramic material. There were significant differences in color change within groups, and the strength values were decreased by repeated firing.

Dental ceramics have attained a special place in dentistry. Its color, translucency, and esthetic quality cannot as yet be matched by any other restorative material. The greatest advantage of ceramic is the ability to match natural dentition. Their durable physical and optical properties make them the material of choice to restore demanding esthetic cases.

The leucite-reinforced glass-ceramic (VITA PM 9) is translucent and has a lower crystal content within the matrix, and the relative amount of light transmission from the surface is higher.[5] These differences in optical properties affected the results of the study.

A study by Lund and Piotrowski[6] suggested that certain metal oxides are not color stable after being subjected to firing temperatures.

In a study conducted by Ozturk et al.,[7] they found increase in the number of firings resulted in an appreciable increase in L*, a* and b* values which are similar to the results obtained by the present study.

Although studies have demonstrated the minimal effect of repeated firings on the color of body ceramic, O'Brien[8] reported that firing ceramic specimens up to six times resulted in perceptual color changes. He attributed color change after repeated firings to the color stability of metal oxides during firing.

In the present study, the highest mean ΔE value was 2.7 between the control Group (2 firings) and Group 3 (4 firings). When △E values are >2 △E units, all observers can apparently detect the color difference between two colors. The clinically acceptable limit of the color difference value is considered 3.7 △E units.[9] Therefore, though the results show a significant difference in color after repeated firing, it is clinically acceptable.

In spite of their excellent esthetic qualities and their good biological compatibility, dental ceramics are brittle. The long-term survival of all-ceramic restorations is related to the fabrication method. Restorations fabricated using the hot pressing technique has the highest long-term survival.[1] Brittle materials including ceramics are tested by flexure test. Statistically significant difference was found in flexural strength between Group 1 – Group 2, Group 1 – Group 3, and Group 2 – Group 3 (P < 0.05). The positive value of difference indicates that the flexural strength decreases after multiple firing.

The results of the present study are similar to the results obtained in a study conducted by Gozneli et al.,[2] in which they concluded that repeated firing was associated with a slight decrease in the flexural strength values. The optimum strength of ceramics is related to the fabrication procedure and is achieved by minimization of flaws in the content.

In addition, Claus[2] reported that the firing cycle and temperature affect the distribution of the sintering, glass, and crystal phases in the microstructure of the porcelain, which may affect the optimum strength of all-ceramic restorations.

It is known that leucite ceramics exhibit an increase in leucite content after repeated firing.[2] Although these kinds of changes in the microstructure of ceramics containing leucite have been reported, in this study, the flexural strength of these ceramics was not positively affected by repeated firing. Thus, it may be considered that the changes in the microstructure of a leucite crystal do not always affect its flexural strength values positively.

The present study demonstrated that repeated firing of leucite-based heat-pressed ceramic exhibited color changes that are clinically acceptable, but the strength of the ceramic decreases significantly. Thus, repeated firing is an important factor with respect to color and strength of definitive restoration and should be considered during shade selection and fabrication.


  Conclusion Top


Within the limitations of this in vitro study, the following conclusions were drawn:

  1. Flexural strength decreases after multiple firing
  2. The L* a* b* values were affected by the number of firings (2, 3, and 4 firings). An increase in the number of firings resulted in an appreciable increase in L*, a* and b* values which resulted in samples that were lighter, redder, and more yellow
  3. The mean color difference caused by repeated firings was perceptible for all the specimens fired for more than 2 firings (ΔE >1). The highest ΔE value was observed for specimens between second and fourth firing.


Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Griggs JA. Recent advances in materials for all-ceramic restorations. Dent Clin North Am 2007;51:713-27, viii.  Back to cited text no. 1
    
2.
Gozneli R, Kazazoglu E, Ozkan Y. Flexural properties of leucite and lithium disilicate ceramic materials after repeated firings. J Dent Sci. 2013;9:144-50. DOI: 10.1016/j. jds.2013.02.019.  Back to cited text no. 2
    
3.
Dudhekar A, Nimonkar S, Belkhode V, Borle A, Bhola R. Enhancing the esthetics with all-ceramic prosthesis. J Datta Meghe Inst Med Sci Univ 2018;13:155-7.  Back to cited text no. 3
  [Full text]  
4.
Sathe S, Karva S, Borle A, Dhamande M, Jaiswal T, Nimonkar S. Comparative Evaluation of the effect of three polishing agents on staining characteristics of provisional restorative material: An in vitro study. J Int Soc Prev Community Dent 2019;9:250-5.  Back to cited text no. 4
    
5.
Uludag B, Usumez A, Sahin V, Eser K, Ercoban E. The effect of ceramic thickness and number of firings on the color of ceramic systems: An in vitro study. J Prosthet Dent 2007;97:25-31.  Back to cited text no. 5
    
6.
Lund PS, Piotrowski TJ. Color changes of porcelain surface colorants resulting from firing. Int J Prosthodont 1992;5:22-7.  Back to cited text no. 6
    
7.
Ozturk O, Uludag B, Usumez A, Sahin V, Celik G. The effect of ceramic thickness and number of firings on the color of two all-ceramic systems. J Prosthet Dent 2008;100:99-106.  Back to cited text no. 7
    
8.
O'Brien WJ, Fan PL, Groh CL. Color differences coefficients of body-opaque double layers. Int J Prosthodont 1994;7:56-61.  Back to cited text no. 8
    
9.
Celik G, Uludag B, Usumez A, Sahin V, Ozturk O, Goktug G. The effect of repeated firings on the color of an all-ceramic system with two different veneering porcelain shades. J Prosthet Dent 2008;99:203-8.  Back to cited text no. 9
    


    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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