Dentistry and Medical Research

ORIGINAL ARTICLE
Year
: 2021  |  Volume : 9  |  Issue : 2  |  Page : 68--72

Effect of different manufacturing techniques on color in zirconia-based all ceramics


Mehmet Ugur, idris Kavut 
 Department of Prosthodontics, Faculty of Dentistry, Van Yuzuncu Yil University, Tuşba, Van, Turkey

Correspondence Address:
Mehmet Ugur
Dentistry Faculty, Van Yuzuncu Yıl University, Tuşba 65080, Van
Turkey

Abstract

Aim: The aim of this in vitro study was to investigate the effect of different veneering techniques on the color, translucency, of zirconia-based systems. Materials and Methods: Specimens were randomly divided into three groups (n = 12): layering (L), pressing (P), and computed aided design (CAD)-on (CO). Group L was veneered with nano-fluorapatite veneering ceramic. Group P was heat pressed with fluorapatite glass-ceramic ingots. Group CO the veneering ceramics were designed with and milled from lithium disilicate glass-ceramic blocks in CAD/computed aided manufacturing unit. Color parameters (L* a* b*) were measured with a spectrophotometer and color difference (ΔE), and translucency parameter (TP) properties were calculated. The results were analyzed with one-way ANOVA (P < 0.05). Results: There were significant differences between the ΔE of the groups (P < 0.05). The highest value was observed in the L group (ΔE = 4.17 ± 0.98); the lowest value was observed in the P group (ΔE = 2.08 ± 0.54). There were significant differences between the TP values of the groups (P < 0.05). Conclusion: Optical properties of the zirconia-based systems are greatly affected by the fabrication techniques.



How to cite this article:
Ugur M, Kavut i. Effect of different manufacturing techniques on color in zirconia-based all ceramics.Dent Med Res 2021;9:68-72


How to cite this URL:
Ugur M, Kavut i. Effect of different manufacturing techniques on color in zirconia-based all ceramics. Dent Med Res [serial online] 2021 [cited 2022 Jan 20 ];9:68-72
Available from: https://www.dmrjournal.org/text.asp?2021/9/2/68/331400


Full Text



 Introduction



Full ceramic restorations have developed rapidly with the increasing esthetic expectations in current dentistry. Full ceramic restorations have thorough-paced esthetic properties. Full ceramic restorations have thorough-paced esthetic properties because of allowing light passage. They also scatter and emit light. They create a translucent depth by this means and imitate the color of the natural tooth.[1],[2] In recent years, restorations demonstrating sufficient clinical performance have been produced with computed-aided design/computed-aided manufacturing (CAD/CAM).[3]

Dentists and dental technicians are constantly making efforts to produce ceramic restorations esthetically suitable to patients' natural teeth. However, it is not sufficient to provide only structural esthetics, and restorations should have the same color and optical properties when compared to adjacent teeth. Scientific understanding of color and optical properties and working in compliance with the technician are critical factors.[4],[5]

It is important to correctly design elements that comply with the patient's natural dentition such as the opacity, color, and thickness of the ceramic layer to produce natural-looking ceramic restorations. The elements that determine the esthetic results of the restorations are the combination of veneer ceramic used, thickness, color, and opacity. Furthermore, the manufacturer of ceramic powder, the number of baking of ceramics, and the applied condensation technique affect esthetic result.[5],[6]

Some of the materials previously used in full ceramic systems are no longer used due to their weak mechanical properties. Therefore, the usability of many materials has been tried. Recently, restorations with zirconia substructure have attracted attention.[7] Compared to other materials used in full ceramic systems, zirconia is a highly superior material with its high rupture and bending strength properties, wearing resistance, and biocompatibility. However, zirconia substructure should be covered with a ceramic layer due to the disadvantage of its optical properties.[7],[8]

A successful color match is an important expectation for any esthetic restoration. Natural enamel has a translucent structure. Therefore, it is important that the ceramic restorations are equally translucent and can mimic the color of the natural tooth. However, ensuring color harmony between porcelain crowns and natural dentition is a bit problematic.[9]

There are many veneering methods on zirconia substructures. These are the traditional layering, method of pressing with heat, and the recent multilayer method, where zirconia substructure and veneer ceramics are produced with CAD/CAM and combined with glass fusion ceramics. It is reported that this technique can eliminate veneer disintegration and technician errors, which is a problem that is commonly seen between zirconia substructure and veneer ceramic.[10],[11],[12]

The purpose of this study is to examine the effect of different production patterns on the final color of the full ceramics with the zircon substructure. The hypothesis is that the color of restoration is affected by the production patterns.

 Materials and Methods



The substructure, veneer materials used in the study, its chemical content, and mechanical properties are given in [Table 1].{Table 1}

Trial samples were divided into three groups according to full ceramic construction techniques. A total of 45 samples were prepared, to be 15 samples in each group. All ceramic materials were selected in A2 color. For the zirconia substructures, IPS e.max ZirCAD MO1 (Ivoclar Vivadent, Schaan, Liechtenstein) blocks, which were precolored and compatible with A2 color after sintering, were used. IPS e.max Ceram Dentin A2 (Ivoclar Vivadent) veneer ceramic recommended by the manufacturer was used in the layering technique. Blocks of IPS e.max CAD A2 High Translucent HT (Ivoclar Vivadent) color recommended by the manufacturer were used in the CAD-on technique. Besides, IPS e.max ZirPress A2 HT (Ivoclar Vivadent) ingots were preferred for pressing ceramics.

Samples were sintered in a sintering furnace (Sirona inFire HTC speed; Dentsply Sirona) according to the manufacturer's instructions. Sintered zirconia substructures were randomly divided into three groups (L, P, and CO). IPS e.max ZirLiner Clear (Ivoclar Vivadent) powder and IPS e.max ZirLiner Build-up Liquid (Ivoclar Vivadent), which are recommended to be used in substructures of T and P groups with A2 color, were used per the manufacturer's instructions. The liner was not applied to the substructures to be used in the CO group. All zirconia substructures were cleaned in distilled water for 10 min in an ultrasonic cleaner (Mercury, Istanbul, Turkey) before veneering.

A mold of metal and teflon with a 0.5 mm depth and 10 mm diameter inner surface where the zirconia substructure fit and a total depth of 1.5 mm were used for standardization in the veneer ceramic application for the layering group. PS e.max Ceram Dentin (Ivoclar Vivadent) powder in A2 color and liquid (IPS e.max Ceram Build-up Liquid, Ivoclar Vivadent) were mixed on a cement glass according to the manufacturer's instructions and baked in Programat EP 3000 (Ivoclar Vivadent) furnace.

15 zirconia substructures, where a zirliner was applied for the pressing group, were placed in a previously prepared metal mold. After the waxes of the samples were removed using the lost wax technique method, IPS e.max ZirPress HT A2 (Ivoclar Vivadent) ingots were pressed under heat and pressure in the EP Empress 600 (Ivoclar Vivadent) furnace according to the manufacturer's instructions.

A 0.9 mm thick 10 mm diameter wax sample was prepared using a metal mold prepared for other groups to prepare the veneer ceramic of the CO group. Digital measurement of wax samples was taken by CEREC, Bluecam camera (Sirona). The designed samples were milled through IPS e.max CAD HT A2 (Ivoclar Vivadent) lithium disilicate blocks in the CEREC MC XL (Sirona) milling unit. After obtaining Veneer ceramics, the process of joining with zirconia substructure was made. IPS e.max Crystall/Connect 4 (Ivoclar Vivadent) glass fusion ceramic in A2 color was used. The CO group prepared was cooked in the Programat EP 3000 furnace per the manufacturer's instructions for crystallization of the lithium disilicate veneer ceramics and fusion with zirconia substructure.

Veneer surfaces of the samples and TK group samples were sanded under water using 400, 600, 800, and 1000 grit silicon carbide abrasive papers (Metkon Gripo 2V, Bursa, Turkey), respectively. Subsequently, they were polished using a low-speed micromotor and an angle drive (KaVo Dental GmbH, Biberach, Germany) with porcelain polishing tires (3M Espe, Seefeld, Germany) under quench cooling at 10,000 rpm.

Color measurements were made three times in the exact middle of the samples with the help of a specially calibrated spectrophotometer (VITA Easyshade, Vident, Germany). The data were obtained according to the Commission International de l'Eclairage (CIE) L* a* b* color system.

To find the differences of the samples with A2 color on the color scale, the calculation was made using the formula ΔE.

ΔE = [(L2 − L1) 2+ (a2 − a1) 2+ (b2 − b1) 2]1/2

Statistical analyses were performed using the SPSS 22.0 program (SPSS Inc., Chicago, IL, USA) program. The statistical analysis of the data obtained was performed by one-way ANOVA test method in the data providing parametric distribution (P < 0.05). Tukey's HSD test was applied for multiple comparisons.

 Results



L* a* b*, color differences (ΔE), and translucence values of the samples are shown in [Table 2]. The group with the lowest L* average is CO and is statistically significantly different from other groups. There is no difference between the other two groups. The lowest a* value was observed in the P group (P < 0.05). No statistical difference was observed between CO and L groups (P > 0.05). The group with the highest a* value mean was found to be the T group. While the lowest b* value was seen in the P group, the highest b* value was seen in the L group.{Table 2}

The results of one-way ANOVA are shown in [Table 3]. There is a statistical difference between ΔE values according to construction techniques (P < 0.05). The highest ΔE value was observed in the T group. The lowest ΔE value was obtained in the P group.{Table 3}

There is a difference between the translucency parameters (TP) of the groups according to the construction techniques (P < 0.05). The CO group showed the highest mean TP values with 9.95 ± 0.3. The lowest TP value was observed in the L group. No statistically significant difference was found in TP values between P and CO groups according to the multiple comparison test (P > 0.05).

 Discussion



The effect of different production techniques on color and translucency in full ceramics with zirconia substructure has been investigated in this study. The hypothesis was accepted as a result of the study.

Much effort is made to produce restorations similar to natural teeth and other biological features, due to the increased esthetic needs. However, there are still problems waiting to be solved, which represent the dental porcelains' optical properties including the color of natural teeth.[5] The color and elements of color such as brightness of color, the intensity of color, translucency, opacity, color, light transmission, reflection, metamerism, and fluorescence affect the esthetic properties of a restoration. Besides, natural teeth and ceramics have different light absorption and reflection properties.[4]

Full ceramic restorations consist of the core material and veneer ceramic to provide excellent esthetic and physical properties.[13] Although Yttria-stabilized zirconia ceramics exhibit high bending and rupture strength and superior fracture strength within all ceramic restorations, the white color of Yttria-stabilized tetragonalzirconia polycrystalline (Y-TZP) affects the esthetics of the restoration.[14] Therefore, veneering is necessary to provide an esthetic effect in restoration. Restorations are thus obtained that can be barely distinguished from natural dentition by providing individual optical properties.[15]

Different methods are used to veneer the zirconia. These are traditional layering techniques, pressing techniques. In addition to these techniques, in CAD-on technique introduced as a new technique, zirconia substructure, and durable superstructure are produced by CAD/CAM and combined with fusion ceramics.[10],[11],[16]

To use the construction techniques effectively for full ceramic restorations, clinicians and technicians need to know if the esthetic properties are affected by the veneering technique.[13] In this study, the effects of production techniques on full ceramic color and brightness were examined.

It was reported that the optical properties of Y-TZP core material did not change as a result of additional baking after Y-TZP core material was fully sintered. Therefore, although the same substructure was used, the difference in optical properties might be due to the veneering technique.

Accurate and reliable evaluation of color is a prerequisite for a successful esthetic result. Color selection with color scales and photographic methods, which are among the subjective methods, can cause unpredictable results to be obtained. For this reason, electronic color measuring instruments such as spectrophotometers and colorimeters can measure color differences objectively.[17] CIE can be used to achieve consistent results using the L* a* b* system with these tools.[18],[19] Color measurement was made by spectrophotometer in this study.

It was observed that the CO group had the lowest L* value. Although the lithium disilicate ceramic is translucent in nature, the glass fusion ceramic used may be a cause of low L* values.

The L* value, which expresses the brightness of the samples, decreases with the increase in thickness after veneering. This phenomenon can be explained by the increase of absorption of incoming light in thick samples and the decrease of reflected light, and a decrease of L* value as a result.[20],[21] The color difference value (ΔE) refers to the distance between the two colors L* a* b* coordinates. The clinically acceptable color difference limit is 3.7. In many studies, the color difference limit was accepted as 3.7.[22],[23],[24] Therefore, the acceptable color difference limit was accepted as 3.7 in this study, too.

The researchers thought that, although the ingots used in the pressing technique and the porcelain powder used in the layering technique have the same A2 color, there may be differences in the amount of pigment. Besides, it was stated that color tone and color intensity may differ depending on different internal structures, homogeneity, and pore volume in pressing and layering techniques.[25]

In this study, ΔE values of the groups have been listed as T > CO > P. Since the ΔE value in the T group was greater than 3.7, a clinically unacceptable color difference occurred. The lowest ΔE value was observed in the P group. The best color match with the color scale was in the P group. The homogeneity of the ingots used in heat pressed veneers, and the disintegration of crystals without proportionally obvious porosity may be shown as a reason.[13]

In the study, in which Bagis and Turgut examined the optical properties of laminae, they reported that baking, pressing, or milling operations have no effect on the color of the ceramics and that different crystalline compositions of materials may be more effective on color than production technique.[26]

In a study conducted by Kim et al., they compared the clinical acceptability of the colors of full ceramic restorations produced by the digital veneering technique and layering technique. Color differences of the samples produced in the same color tone and the same core thickness were measured according to the different production techniques. It was emphasized that the color differences (ΔE ≤3.7) between the two production techniques were within clinically acceptable limits. Furthermore, color differences depending on zirconia core thickness and veneering technique were observed in samples with the same color.[6]

The final translucency of the core-veneer system is important for optimal esthetic results. The luminous transmittance of ceramics is affected by many factors.[13] These crystal microstructure (crystal volume and refractive index of crystal, particle size), number of bakes, the physical structure of the particles in the matrix, and their refractive index also affect the amount of light scattering.[27]

Luo and Zhang calculated the light transmittance of veneering techniques in restorations with zirconia substructure using an integral equation and stated that the most translucent technique was the pressing technique and the least translucent technique was the cut-back technique. Porcelain/powder ratio, vibration, condensation technique, and baking degree which are the limitations of the layering technique cause undesired particle growths. This leads to inhomogeneous distribution and asymmetric growth of the crystals. The content and distribution of crystals may differ in layering and pressing techniques. Therefore, the light is reflected between the interface, and as a result, the light transmittance decreases in core-veneer systems produced with different techniques.[13]

Full ceramic systems have a different composition, microstructure, crystalline content, and phases. This affects the optical properties of full ceramic systems. Increasing the crystal content for obtaining better strength often results in increased opacity.[28],[29] Ceramics containing less crystalline content are generally thought to be more translucent.[26] This information supports the result of the study.

The limitations of this study are that this study was performed in in vitro conditions and the oral conditions could not be fully reflected in the study. Furthermore, to standardize the samples, preparing the samples geometrically instead of crowns is another limiting factor. Further studies are needed to examine the effect of resin cement, surface color on which restoration will be cemented, and the effects of different surface treatments on the optical properties of zirconia-based ceramics.

 Conclusion



No complete color match has been achieved between the color scale proposed by the manufacturer companies and the zirconia-based full ceramic system. The color difference was found at a level noticeable by the observers in all groups. The technique that shows the most color incompatibility with the color scale was the layering technique. The technique with the best color match is the pressing technique. The zirconia-based full ceramic systems used in the study all have translucency properties. The technique with the highest translucency property is the CAD-on technique.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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