Optical properties and light irradiance of monolithic zirconia at variable thicknesses

Highlights

  • Polishing of monolithic zirconia increases its surface gloss. The degree of surface gloss is brand dependent, and thickness shows no effect on the gloss.

  • Different brands of zirconia have different translucencies, which is highly influenced by the thickness of the material. FSZ is relatively more translucent than PSZ.

  • Total irradiant energy and irradiance through zirconia is brand and thickness dependent. Certain thickness of zirconia may have an effect on the polymerization of resin-based cements.

Abstract

Objectives

The aims of this study were to: (1) estimate the effect of polishing on the surface gloss of monolithic zirconia, (2) measure and compare the translucency of monolithic zirconia at variable thicknesses, and (3) determine the effect of zirconia thickness on irradiance and total irradiant energy.

Methods

Four monolithic partially stabilized zirconia (PSZ) brands; Prettau® (PRT, Zirkonzahn), Bruxzir® (BRX, Glidewell), Zenostar® (ZEN, Wieland), Katana® (KAT, Noritake), and one fully stabilized zirconia (FSZ); Prettau Anterior® (PRTA, Zirkonzahn) were used to fabricate specimens ( n = 5/subgroup) with different thicknesses (0.5, 0.7, 1.0, 1.2, 1.5, and 2.0 mm). Zirconia core material ICE® Zircon (ICE, Zirkonzahn) was used as a control. Surface gloss and translucency were evaluated using a reflection spectrophotometer. Irradiance and total irradiant energy transmitted through each specimen was quantified using MARC® Resin Calibrator. All specimens were then subjected to a standardized polishing method and the surface gloss, translucency, irradiance, and total irradiant energy measurements were repeated. Statistical analysis was performed using two-way ANOVA and post-hoc Tukey’s tests ( p < 0.05).

Results

Surface gloss was significantly affected by polishing ( p < 0.05), regardless of brand and thickness. Translucency values ranged from 5.65 to 20.40 before polishing and 5.10 to 19.95 after polishing. The ranking from least to highest translucent (after polish) was: BRX = ICE = PRT < ZEN < KAT < PRTA ( p < 0.05). The ranking from least to highest total irradiant energy was: BRX < PRT < ICE = ZEN < KAT = PRTA ( p < 0.05). There was an inverse relationship between translucency, irradiant energy, and thickness of zirconia and the amount was brand dependent ( p < 0.05).

Significance

Brand selection, thickness, and polishing of monolithic zirconia can affect the ultimate clinical outcome of the optical properties of zirconia restorations. FSZ is relatively more polishable and translucent than PSZ.

Introduction

Monolithic zirconia restorations have been increasingly used in restorative dentistry given their favorable properties, reasonable esthetics, simple clinical technique, and relative low cost compared to cast gold restorations. One notable disadvantage of full contour zirconia restorations, compared to other all-ceramic systems, is their lack of translucency owing to zirconia’s high refractive index mismatch between its particles and the matrix and due to the dispersed particles (slightly greater in size to the wavelength of the incident light) . While improvements in zirconia’s translucency have been claimed by many manufacturers, a better understanding of zirconia’s optical properties, including surface gloss and translucency, can help inform material development and selection for improved esthetic outcomes.

Surface gloss may be defined as the degree a surface approaches that of a mirror, and is used primarily as a measure of surface shine . Specular reflection is defined as light reflected from a smooth surface at a defined angle; whereas diffuse reflection is produced by rough surfaces that tend to reflect light in all directions. The rougher the surface the lower the gloss, meaning the specular component is weak and the diffused light is stronger. A spectrophotometer measures the spectral reflectance of the specimen. It can operate two different measuring geometries of specular component excluded (SCE) where it excludes the specular reflectance of light, and specular component included (SCI) where it includes the specular reflectance of light . Surface gloss can be estimated by a spectrophotometer. By lighting two xenon lamps in quick succession, the system can provide virtually simultaneous SCE/SCI measurements and also enable the calculation of 8° gloss. The difference between the SCE and SCI reflectance (Δ E *SCE − SCI) gives estimation about the surface gloss . Another method has been reported in the literature for measuring the surface gloss performed by using a small-area glossmeter, and the gloss measurements were expressed in Gloss units (GU) .

The amount of light that is absorbed, reflected, and transmitted depends on the amount of crystals within the core matrix, their chemical nature, and their size compared to the incident light wavelength . The translucency of dental porcelain is largely dependent on light scattering and thickness . If the majority of light passing through a ceramic is intensely scattered and diffusely reflected, the material will appear opaque. If only part of the light is scattered and most is diffusely transmitted, the material will appear translucent . Porcelain translucency is usually expressed by contrast ratio (CR) and/or translucency parameter (TP) . The CR is defined as the ratio of illuminance ( Y ) of the test material when placed over a black background ( Y b ) to the illuminance of the same material when placed over a white background ( Y w ), where 0 is most translucent and 1 is most opaque . The TP is defined as the color difference (Δ E ) between a uniform thickness of a material over a white and a black backing . The higher the TP value the more opaque the material. In most studies, the translucency of dental ceramics was mainly studied at a certain thickness, generally, the thinnest recommended by the manufacturers . In clinical situations, ceramic restorations with various thicknesses are required, depending on the different conditions of the tooth to be restored. Therefore, an accurate knowledge of the relationship between the translucency and thickness of restorative materials is fundamental to improving the esthetic outcome of restorations .

Zirconia and its opacity at variable thicknesses can have an immense effect on light cure irradiation. Light transmission and curing efficiency of light activated resin luting agents is influenced by the shade and the thickness of the restorative material. Generally, the thicker the restoration or the darker its shade, the more critical the irradiance of the incident light is to achieve optimal photopolymerization of the material . Other factors can also affect the degree of cure such as ceramic translucency, resin cement composition, and polymerization type as well as the curing light’s output power, curing duration, and distance . The International Organization for Standardization (ISO) recommends irradiance for polymerization lights of 300 mW/cm 2 , and the standard depth-of-polymerization requirement is 1.5 mm . Optimal cure is always critical because inadequately polymerized resin cements are prone to have altered mechanical properties and dimensional stability with decreased bonding to tooth structures resulting in microleakage, decreased biocompatibility, discoloration, and postoperative sensitivity .

The aims of this study were to (1) estimate and compare the effect of polishing on the surface gloss of monolithic zirconia, (2) measure and compare the translucency of zirconia at variable thicknesses, and (3) determine the effect of thickness on irradiance and total irradiant energy.

The null hypotheses tested for this study were: (1) there is no difference in surface gloss before and after polishing; (2) translucency of the zirconia material is not influenced by the thickness, and (3) light transmittance though zirconia is not affected by its thickness.

Materials and methods

Sample preparation

Four brands of monolithic partially stabilized zirconia (PSZ) and a fully stabilized zirconia (FSZ) were studied in this study, and a zirconia core (ICE Zircon) was used as a control ( Table 1 ). Square shaped specimens (10 mm × 10 mm, n = 5/per subgroup) were cut into different thicknesses (0.5, 0.7, 1.0, 1.2, 1.5, and 2.0 mm) using a cutting device (Struers Secotom-50, Copenhagen, Denmark). In green stage, each specimen was sequentially ground to the specific thickness using silicon carbide grinding paper (FEPA #1200, 2400, 4000) (Struers LaboPol 21, Struers A/S, Rodovre, Denmark). The final thickness (±0.1 mm) was measured using a digital caliper (Mitutoyo Corporation, Kanagawa, Japan). The specimens were sintered according to the manufacturers’ instructions using the manufacturers’ furnace for each brand. The specimens were cleaned ultrasonically in distilled water for 10 min before testing (Quantrex 90, L&R Ultrasonics Manufacturing Co., Kearny, NJ, USA) then air-dried individually for 20 s. Optical, light cure irradiance and total irradiant energy measurements described following were conducted (baseline measurements). Then one side of each specimen was polished by a single experienced operator (TS) using a straight lab handpiece (K5plus, Kavo, Germany) connected to an electrical control unit (K-control 4960, Kavo, Germany) with diamond polishers (Zircpol Plus and Zircoshine Plus, Diatech, Switzerland) followed by a polishing paste (Zircon-Brite; Dental Ventures of America Inc., Corona, CA, USA) at a constant speed of 10,000 rpm under constant pressure and standard time in a single directed motion, following manufacturers’ instructions. The specimens were not glazed in this experiment. The specimens were ultrasonically cleaned and dried as previously explained. Then the optical, light cure irradiance and total irradiant energy measurements were conducted after polishing.

Table 1
Monolithic zirconia brands used in the study.
Brand name Code Manufacturer Composition
Partially stabilized zirconia (PSZ)
Prettau Zirconia PRT Zirkonzahn, Taufers, Italy 4–6% Y 2 O 3 , <1% Al 2 O 3 , max. 0.02% SiO 2 , max. 0.01% Fe 2 O 3 , max. 0.04% Na 2 O
Bruxzir Zirconia BRX Glidewell Laboratories, Irvine, USA Unknown
Wieland Zenostar Translucent ZEN Ivoclar Vivadent, Principality of Liechtenstein Unknown
Katana High Translucent KAT Kurary Noritake INC, Noritake, Japan (ZrO 2 + HfO 2 + Y 2 O 3 ) > 99.0%, yttrium oxide (Y 2 O 3 ) > 4.5–≤6.0%, hafnium oxide (HfO 2 ) ≤5.0%, other oxides ≤1.0%
Fully stabilized zirconia (FSZ)
Prettau Anterior PRTA Zirkonzahn, Taufers, Italy <12% Y 2 O 3 , <1% Al 2 O 3 , max. 0.02% SiO 2 , max. 0.01% Fe 2 O 3 , max. 0.04% Na 2 O
Control (PSZ)
ICE Zircon ICE Zirkonzahn, Taufers, Italy 4%–6% Y 2 O 3 , <1% Al 2 O 3 , max. 0.02% SiO 2 , max. 0.01% Fe 2 O 3 , max. 0.04% Na 2 O

Optical measurements

To estimate the surface gloss (Δ E *SCE − SCI), TP, and CR values of each specimen, a reflection spectrophotometer (CM-700d, Konica Minolta Sensing Inc., Tokyo, Japan) was used according to the CIE 1976 L * a * b * color scale relative to the CIE standard illuminant D65 (as defined by the International Commission on Illumination) which corresponds to “average” daylight (including ultraviolet wavelength region with a correlated color temperature of 6504 K). The SCE and SCI geometries were determined according to the CIE L * a * b * color scale using standard illuminant D65 over white (CIE L * = 98.1, a * = −0.5 and b * = 2.8) and black (CIE L * = 4.7, a * = −0.1 and b * = 0.0) background.

Differences in surface gloss (Δ E *SCE − SCI) values were calculated by the following equation:

ΔE*SCESCI=[(ΔL*SCESCI)2+(Δa*SCESCI)2+(Δb*SCESCI)2]1/2
Δ E * SCE − SCI = [ ( Δ L * SCE − SCI ) 2 + ( Δ a * SCE − SCI ) 2 + ( Δ b * SCE − SCI ) 2 ] 1 / 2

Calibration of the spectrophotometer was executed before measurement of each specimen. Then the TP of each specimen was obtained by calculating the color difference between the specimen against the white background and against the black background using the following equation:

TP=[(Lb*Lw*)2+(ab*aw*)2+(bb*bw*)2]1/2
TP = [ ( L b * − L w * ) 2 + ( a b * − a w * ) 2 + ( b b * − b w * ) 2 ] 1 / 2
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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Optical properties and light irradiance of monolithic zirconia at variable thicknesses
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