Effect of sintering cycle on the strength and translucency of multilayered zirconia

Abstract

Purpose

A newly introduced sintering protocol promises to offer higher translucency while not significantly compromising the flexural strength of the material. However, the effect of the novel sintering protocol has not been thoroughly validated. The purpose of this study was to measure and compare the effect of two sintering protocols on the translucency and flexural strength of two multilayered zirconia materials.

Materials and Methods

Two types of multilayered zirconia materials (ZirCAD Prime and Prime esthetic) were selected. Presintered disk specimens were obtained from Translucent, Gradient, and Dentin layers (n = 20). The disks were allocated to 2 groups: standard sintering protocol (peak temperature 1500°C) and high translucency sintering protocol (peak temperature 1600°C). After the sintering process, 10 specimens from each group were randomly selected. The optical values (L^*, a^*, b^*) were measured and used to assess translucency using the relative translucency parameter (RTP₀₀) and translucency differences (ΔRTP₀₀). Then, all 20 specimens were tested for biaxial flexural strength. The outcomes were analyzed. The analysis of variance is used to analyze any significant effects on translucency and flexural strength. Then, any significant difference in the translucency and flexural strength between all pairs of materials was analyzed using Bonferroni‐corrected Student's t‐test (α = 0.05).

Results

The high translucency sintering protocol significantly decreased biaxial strength in the Prime translucent and dentine layer, Prime esthetic translucent, and gradient layer. RTP₀₀ was significantly reduced in the Prime gradient and Prime esthetic gradient layer when sintered with a high translucency protocol. The lowest ΔRTP₀₀ was observed in the Prime dentine layer, while the highest ΔRTP₀₀ was observed in the Prime esthetic dentin layer.

Conclusions

High translucency protocol significantly lowers the biaxial flexural strength of both multilayered materials, but the alteration in translucency is within clinically acceptable thresholds (TAT₀₀ = 2.62).

Zirconia‐based materials have gained popularity over the past decades due to their superior physical properties, esthetic appearance, transformation toughening, and biocompatibility. Yttria (Y₂O₃) is added to resist phase transformation during cooling. Retaining cubic and tetragonal phases in zirconia improves the optical and physical properties of the material, respectively. ¹

When first introduced, 3 mol% yttria‐stabilized tetragonal zirconia polycrystal (3Y‐TZP) captivated the dental market due to its high hardness (13 GPa), flexural strength (800–1000 MPa), transformation toughening characteristics, and wear and corrosion resistance. ² However, due to the low cubic phase proportion of less than 15 percent, 3Y‐TZP has an opaque appearance and must be veneered with translucent feldspathic porcelain to improve its esthetics. ³

In the 2000s, various strategies were developed to enhance zirconia's translucency, such as reducing alumina, increasing yttria content, and increasing the sintering temperature. ⁴ Increasing Yttria dopant to 4–6 mol% has been one strategy widely used to improve translucency by increasing the cubic phase proportion in the material to more than 25 percent. High‐yttria zirconia, often regarded as partially stabilized zirconia (PSZ), ⁵ has an improved light transmission characteristic; however, its drawback is reduced mechanical properties due to a lack of transformation toughening behavior of cubic zirconia (biaxial strength < 400 MPa for 6Y‐PSZ).

A recent trend in dental zirconia is multilayered zirconia, which is often a combination of high translucency (5Y‐PSZ) and high strength (3Y‐TZP) zirconia to enhance esthetics while maintaining clinically acceptable strength. These multilayered zirconia blocks are often divided into three zones: incisal or translucent, gradient or transition, and dentin or body zone. The translucent zone primarily consists of 5Y‐PSZ, offering high translucency yet low strength due to high cubic phase content. The dentine zone mainly consists of 3Y‐TZP or 4Y‐PSZ, which offers high strength but low translucency. The gradient zone is a mixture between high translucency and high strength zirconia where a high proportion of the 5Y‐PSZ congregates near the translucent zone and decreases toward the dentin zone. In contrast, the proportion of 3Y‐TZP or 4Y‐PSZ increases from the translucent to the dentin zone. The gradient zone enhances the esthetic outcome by mimicking the natural translucency of the tooth. Due to the intricated composition of multilayered zirconia, its physical properties are not only influenced by yttria content but also by the proportion of constituent phases, stabilizers, manufacturing process, differential coefficient of thermal expansion, and grain size. ² , ⁶

Increasing the sintering temperature to 1530°C–1600°C has been shown to improve zirconia's translucency by increasing the material's grain size and density while reducing its porosity. ² , ⁷ , ⁸ However, its impact on zirconia's mechanical properties is still controversial. Stawarczyk et al. ⁷ reported that sintering temperatures above 1600°C led to a significant decrease in flexural strength, while several studies reported that zirconia sintered at 1600°C did not demonstrate a statistically significant difference in flexural strength. ⁹ , ¹⁰

Recently, a new high translucency sintering protocol ¹¹ was introduced with the aim of enhancing the translucency of multilayered I.P.S. e.max ZirCAD Prime and Prime esthetics while maintaining mechanical properties. This protocol achieves higher translucency by using a more rapid heating‐cooling cycle and higher sintering temperature of 1600°C for 120 min completing the sintering cycle in 6 h 10 min, while the manufacturer's recommended standard sintering protocol employs a slow heating‐cooling cycle and a lower sintering temperature of 1500°C for 120 min. The total sintering time is 9 h 50 min (Table 1).

TABLE 1.

Sintering parameters of standard and high translucency protocol.

Sintering protocol	Phase	Rate (°C/min)	Holding temperature (°C)	Holding time (min)	Rate (°C/min)	Holding temperature (°C)	Holding time (min)	Heating rate (°C/min)	Holding temperature (°C)	Holding time (min)	Opening (min)	Total time
Standard	Heating	10	900	30	3.3	1500	120	10	900	‐		9 h. 50 min
	Cooling	8.3	300	‐	0						15
High translucency	Heating	90	900	15	3	1350	0	5	1600	120		6 h. 10 min
	Cooling	50	1100	0	1	0					15

Esthetically, translucency is one of the most significant indicators of the color and shade of restorative materials that mimic natural tooth appearance. Recent studies have suggested using the CIELab and CIEDE2000 formula to calculate the relative translucency parameter (RTP₀₀) ¹² , ¹³ , ¹⁴ to measure dental material translucency and translucency difference (RTP₀₀) ¹⁵ , ¹⁶ to determine clinical perceptibility and acceptability. On the other hand, one of the most effective ways to evaluate the mechanical properties of dental ceramic material is biaxial flexural tests recommended by ISO 6872 guidelines. Not only is this method less sensitive to flaws near specimen edges, but also, the disk‐shaped specimen has a surface‐to‐volume ratio closer to that of anatomically contoured dental crowns. Consequently, the effect of specimen geometry on flexural strength values is reduced. ¹⁷

This study aims to measure and compare the effect of two sintering protocols on the flexural strength and translucency of two multilayered zirconia. The null hypothesis is that after being sintered with the standard protocol and the high translucency protocol, there is no statistically significant difference in flexural strength and RTP00 of each layer of I.P.S. e.max ZirCAD Prime, and Prime esthetic.

MATERIALS AND METHODS

Two types of multilayered zirconia materials were included: I.P.S. e.max ZirCAD Prime (Ivoclar AG, Schaan, Liechtenstein) described by the manufacturer as containing 3 and 5 mol% yttria, and I.P.S. e.max Prime esthetic (Ivoclar AG, Schaan, Liechtenstein) described by the manufacturer as containing 4 and 5 mol% yttria (Table 2). Presintered cylinder‐shaped specimens (14 mm diameter x 16 mm height) were designed (Meshmixer, Autodesk, CA) and nested (PrograMill CAM software, Ivoclar AG, Schaan, Liechtenstein) to the topmost layer of the zirconia disks for uniformity in specimen preparation. All cylindrical specimens were machined using a dry milling protocol (PrograMill PM7, Ivoclar AG, Schaan, Liechtenstein), before being sectioned with a high‐precision electrical diamond saw (Isomet 1000, Buehler Ltd., Lake Bluff, IL) into three different specimens according to layer: Translucent layer (Tr), Gradient layer (Gr), and Dentine layer (De) resulting in disk‐shaped specimens from each layer (n = 20) (Figure 1). All the specimens were in shade A2 with no additional staining or characterization. The specimens were hand dry‐polished until 1200‐grit silicon carbide paper (MetLab, Niagara Falls, NY). The specimens from each type of zirconia were randomly assigned into two groups according to sintering cycles: (1) the manufacturer's recommended standard sintering protocol and (2) the high translucency sintering protocol. The comparison of the sintering protocol is displayed in Table 1. The specimens were sintered using a sintering furnace (Programat S1 1600, Ivoclar AG, Schaan, Liechtenstein) according to group assignment where the manufacturer's recommended standard sintering protocol, as described, has a peak temperature: of 1500°C, dwelling time: of 120 min, total time: 9 h 50 min, and high translucency sintering protocol with a peak temperature: 1600°C, dwelling time: 120 min, total time: 6 h 10 min. The final dimensions were measured using a digital caliper to dimensions of 14 ±2 mm in diameter and 1.2 ±0.2 mm in thickness, as recommended by ISO 6872 guidelines.

TABLE 2.

Zirconia specimens and allocation.

Material	Layer	Y2O3	Standard sintering protocol (St)	High translucency sintering program (Ht)
IPS e.max ZirCAD Prime (Pr)	Translucent (Tr)	5 mol% (5Y‐PSZ)	PrTrSt (n = 20)	PrTrHt (n = 20)
	Gradient (Gr)	3–5 mol% (3Y‐TZP/5Y‐PSZ)	PrGrSt (n = 20)	PrGrHt (n = 20)
	Dentin (De)	3 mol% 3Y‐TZP	PrDeSt (n = 20)	PrDeHt (n = 20)
IPS e.max ZirCAD Prime Esthetic (Pe)	Translucent (Tr)	5 mol% (5Y‐PSZ)	PeTrSt (n = 20)	PeTrHt (n = 20)
	Gradient (Gr)	4–5 mol% (4Y‐PSZ/5Y‐PSZ)	PeGrSt (n = 20)	PeGrHt (n = 20)
	Dentin (De)	4 mol% 4Y‐PSZ	PeDeSt (n = 20)	PeDeHt (n = 20)

FIGURE 1.

Schematic diagram of I.P.S. e.max ZirCAD Prime and Prime esthetic showing layers indicated by manufacturer and location of test specimens.

After sintering, 10 specimens from each group were randomly selected. The optical properties were determined using a spectrophotometer (Ultrascan VIS, HunterLab, Reston, VA) with an Ø8‐mm aperture in reflectance mode, with C.I.E. 2‐degree standard observation and D65 standard illumination. Measurements were performed against white (L^* = 90.42, a^* = 0.54, b^* = 2.96) and black backgrounds (L^* = 26.81, a^* = 0.14; b^* = 1.33). Three records against each background were obtained per specimen, with no repositioning. Glycerin gel (Liquid strip, Ivoclar AG, Schaan, Liechtenstein) was applied between the specimens' and background surfaces. Based on the CIELab color system, the brightness value (L^*) indicates the darkness to lightness, a^* indicates the greenness or redness, and b^* indicates the blueness or yellowness. The L^*, a^*, b^* values were measured, and the RTP₀₀ were calculated using the following equation: ¹⁵

where L', C', and H' refer to lightness, chroma, and hue against black (B) and white (W) backgrounds, respectively. R_T refers to the rotation function that accounts for the interaction between C’ and H’ differences in the blue region. The weighting functions, S_L, S_C, and S_H modify the overall color difference for variation in the location of the color difference pair in L’, a’, and b’ coordinates. The parametric factors (k_L, k_C, and k_H) used in the current study were k_L = k_C = k_H = 1 (RTP00 (1:1:1)).

The relative translucency parameter difference (ΔRTP00) ¹⁵ , ¹⁶ was calculated by using the differences between the values of RTP00 obtained from St protocol (RTPSt) and Ht protocol (RTPHt):

$$\left|\mathrm{\Delta }\mathrm{RTP}00\right|=\mathrm{RT}{\mathrm{P}}_{\mathrm{St}}-\mathrm{RT}{\mathrm{P}}_{\mathrm{Ht}},$$

ΔRTP₀₀ was evaluated using previously published data on the 50%:50% translucency perceptibility (TPT₀₀ = 0.62) and acceptability (TAT00 = 2.62) thresholds. ¹⁵

After translucency evaluations, all 20 specimens from each group were biaxial flexural tested using a universal testing machine. The disk was positioned in the center on an 11 mm circle diameter. A crosshead speed of 0.5 mm/min along with a load cell of 10 kN and load radius of 1.4 mm were used until specimen fracture occurred. The biaxial flexural strength was calculated using the following equation ¹⁸ :

$$\sigma =-0.2387\mathrm{P}\left(\mathrm{X}-\mathrm{Y}\right)/{\mathrm{b}}^2,$$

where $X=(1+\nu )\ln {({\mathrm{r}}_2/{\mathrm{r}}_3)}^2+[(1-\nu )/2]{({\mathrm{r}}_2/{\mathrm{r}}_3)}^2$ and $Y=(1+\nu )[1+\ln {({\mathrm{r}}_1/{\mathrm{r}}_3)}^2]+(1-\nu ){({\mathrm{r}}_1/{\mathrm{r}}_3)}^2$, where σ is biaxial flexural strength in MegaPascal, P is the load at fracture in Newtons, ν is Poisson's ratio, r1 = radius of support circle in millimeters, r2 = radius of piston tip in millimeters, r3 = radius of specimen in millimeters, b = specimen thickness at fracture origin in millimeters.

The power analysis of the study was calculated to determine the required sample size with 80% power and 0.05 level of significance. ¹⁹ Statistical analyses were performed by using a statistical software program (PROC GLIMMIX, SAS Proprietary Software 9.4; SAS Institute Inc., Cary, NC). RTP₀₀ and biaxial strength were analyzed by the ANOVA, where between‐subjects were the material and the layer, and within‐subject effects were the protocol, the RTP₀₀ or biaxial strength, and all interactions in the model. The AIC and BIC values were determined for each of the lognormal error distributions at each covariance structure. The restricted maximum likelihood was used for variance component estimation. Then, any significant difference in the translucency and flexural strength between all pairs of materials was found by Bonferroni‐corrected Student's t‐tests.

RESULTS

The RTP₀₀ means and standard deviations of tested material sintered with different protocols are presented in Table 4. A two‐way ANOVA identified that the sintering protocol and layer significantly affected the translucency of both zirconia (p = 0.014) (Table 3). Bonferroni‐corrected student's t‐test showed a statistically significant difference in RTP₀₀ of the Prime gradient and Prime esthetic gradient layer (p = 0.013). Amongst all materials tested, Prime dentin sintered with Ht protocol displayed the lowest RTP00 value (11.50 ±0.42) while Prime esthetic sintered with St protocol displayed the highest RTP₀₀ value (16.00 ± 2.53) (Table 4, Figure 2).

TABLE 4.

Mean, standard deviation of the RTP₀₀, and the │ΔRTP00│.

Material	Layer	Sintering protocol	Mean RTP00	Std Dev	│ΔRTP00│
ZirCAD prime	Translucent	Standard	13.78	1.37	1.07
		High translucent	14.85	1.20
	Gradient	Standard	13.12	0.29	0.91
		High translucent	12.21	1.12
	Dentin	Standard	11.53	0.49	0.03
		High translucent	11.50	0.42
ZirCAD prime esthetic	Translucent	Standard	16.00	2.53	0.62
		High translucent	15.38	0.33
	Gradient	Standard	13.70	0.82	0.73
		High translucent	12.97	0.88
	Dentin	Standard	12.96	0.20	0.16
		High translucent	13.12	0.33

Abbreviations: RTP₀₀ relative transparency parameter; │ΔRTP00│, transparency differences.

TABLE 3.

Tests of the Fixed effect of RTP₀₀.

	Degree of freedom
Effect	Numerator	Denominator	F value	p‐value
Material	1	115	50.15	<0.001
Layer	2	115	72.92	<0.001
Material × Layer	2	115	3.54	0.032
Protocol	1	115	0.68	0.410
Material × Protocol	1	115	0.63	0.428
Layer × Protocol	2	115	4.40	0.014
Material × Layer × Protocol	2	115	2.49	0.088

Abbreviation: RTP₀₀ relative transparency parameter.

FIGURE 2.

The RTP₀₀ and the biaxial flexural strength value by layer location and sintering protocol. ^* represents significant differences p < 0.05, ^** represents significant differences p < 0.01. RTP, relative translucency parameter.

The relative translucency parameter differences for tested materials sintering with 2 different protocols are presented in Table 4. The │ΔRTP₀₀│ values of all layers tested were above the 50%:50% clinical translucency perceptibility threshold (TPT00 = 0.62), except for Prime dentin and Prime esthetic dentin, and within the clinically acceptable threshold (TPT₀₀ = 0.62).

Biaxial flexural strength means, standard deviations, and effect of the sintering protocols among the tested zirconia are presented in Table 5 and Figure 2. A two‐way ANOVA identified that the sintering protocol significantly affected the biaxial strength of the tested material (p<0.001) (Table 6). Bonferroni‐corrected student's t‐test showed a statistically significant difference in biaxial strength of Prime translucent (p<0.001) and dentin (p = 0.042), Prime esthetic translucent (p = 0.020), and gradient (p<0.001). A range of mean ± standard deviation biaxial strength was 1.7‐fold from 675.93 ± 116.86 MPa for ZirCAD Prime translucent layer sintered with Ht protocol to 1175.11 ±161.37 MPa for ZirCAD Prime dentin layer sintered with St protocol.

TABLE 5.

Mean and standard deviation of the flexural strength and the effect of the sintering cycle.

Material	Layer	Sintering protocol	Mean (MPa)	Std Dev	p‐value
ZirCAD prime	Translucent	Standard	844.09	112.99	<0.001
		High translucent	675.93 **	116.86
	Gradient	Standard	858.38	146.04	0.324
		High translucent	802.17	80.65
	Dentin	Standard	1175.11	161.37	0.042
		High translucent	1054.37 *	213.64
ZirCAD prime esthetic	Translucent	Standard	848.78	129.67	0.020
		High translucent	735.57 *	85.57
	Gradient	Standard	922.77	128.55	<0.001
		High translucent	721.96 **	118.41
	Dentin	Standard	1009.90	152.86	0.325
		High translucent	1074.15	96.68

^*represents significant differences p < 0.05.

^**represents significant differences p < 0.01.

TABLE 6.

Tests of the fixed effect of biaxial flexural strength.

	Degree of freedom
Effect	Numerator	Denominator	F value	p‐value
Material	1	228	0.23	0.629
Layer	2	228	103.33	<0.001
Material × Layer	2	228	2.43	0.090
Protocol	1	228	37.01	<0.001
Material × Protocol	1	228	0.54	0.461
Layer × Protocol	2	228	5.86	0.003
Material × Layer × Protocol	2	228	8.17	0.004

DISCUSSION

The null hypothesis that the sintering protocol does not affect the translucency and flexural strength of multilayer zirconia (ZirCAD Prime and Prime Esthetic) was rejected.

In the present study, the high translucency sintering protocol was shown to significantly affect the biaxial strength of both materials. However, the reduction in biaxial strength is still within the range of the manufacturer's specification. The range of biaxial strength of Prime and Prime esthetic reported by the manufacturer are 650–1200 MPa and 650–850 MPa, respectively, ¹¹ where the resultant biaxial strengths of Ht protocol are 675–1054 MPa and 721–1074 MPa, respectively. In this study, high sintering temperature seemed to affect high strength (3Y‐TZP) and high translucent (5Y‐PSZ) zirconia but not 4Y‐PSZ. Vult von Steyern et al. ²⁰ demonstrated that high sintering temperature significantly affected the mechanical properties of 3Y‐TZP and 4Y‐PSZ zirconia due to an increase in the proportion of cubic phase, changes in crystal formation, and grain size. Mayinger et al. ²¹ reported a contradictory result that high sintering temperature did not significantly affect the mechanical properties of 4Y‐PSZ. They showed that a higher sintering temperature of 4Y‐PSZ led to less evenly distributed grains, which may help deflect microcracks and dissipate crack energy. Previous studies ²¹ , ²² showed a similar trend on 5Y‐PSZ. They found that the high sintering temperature did not affect the biaxial strength of the tested 5Y‐PSZ. The crystal structure analysis showed that a higher sintering temperature led to an increase in the proportion of the nontransformable tetragonal phase (t’), but the increase was less than 5%, and thus did not impact the biaxial strength. ²⁰ However, the studies above investigated homogenous zirconia material, not multilayered zirconia. A recent phase analysis of various multilayered zirconia materials showed that different multilayered zirconia materials have different cubic‐tetragonal proportions, although labeled as the same Yttia content. ²³ , ²⁴ , ²⁵ Thus, the flexural strength is significantly affected. ²³ Only a limited number of studies investigated the effect of sintering parameters on multilayered zirconia; hence, more studies are necessary to understand these trends further.

The tensile surfaces of the test specimens in this study were polished prior to the sintering process. This prepolishing step aimed to achieve several benefits. First, a smoother presintered surface can improve the surface quality of the final sample after sintering. ²⁶ Second, by minimizing surface irregularities, prepolishing reduces the impact of these features on light interaction, leading to more accurate translucency and strength measurements. ²¹ Finally, polishing prior to sintering ensures the samples remain unaltered after the sintering process, allowing for more translucency measurements standardized for the sintering. The authors opted against postsintering polishing for this investigation for several reasons. Polishing after sintering can potentially alter the final microstructure, which could influence the measured flexural strength and translucency. ²⁴ , ²⁶ To isolate the effect of the sintering protocol on the material's inherent properties (translucency and strength), prepolishing creates a more controlled and consistent surface for both groups undergoing standard and high translucency sintering protocols. ²⁶ Additionally, some zirconia materials exhibit increased susceptibility to microfracture during postsintering polishing, which we aimed to avoid in this study. Therefore, comparisons should be made with caution.

Recent studies have suggested using the CIEDE2000 RTP₀₀ in dental research and applications. ²⁷ , ²⁸ , ²⁹ It was reported that the CIEDE2000 formula improves the correlation between the visual color and calculated differences. ¹⁵ , ³⁰ In this study, the significant effect of high translucency sintering protocol on the RTP00 has been observed in the gradient layer of both materials. A similar trend was found where only a slight change in translucency was observed in homogenous materials represented by translucent and dentin layers in this present study. ³¹ , ³² The study by Kaizer et al. ³¹ on the effects of different sintering parameters (sintering temperature 1580°C ‐dwelling time 10 min, sintering temperature 1510°C ‐dwelling time 25 min, and sintering temperature 1510°C‐dwelling time 120 min) on translucency parameter of translucent zirconia found only a slight improvement in translucency when a higher sintering temperature was used. Similarly, Atachoo et al. ³² reported the effect of sintering temperature on monolithic zirconia (3Y‐TZP). The authors found no significant difference in translucency when the zirconia was sintered at temperatures of 1450°C and 1550°C. On the contrary, Vult von Steryen et al. ²⁰ demonstrated a significant effect of sintering temperature on light transmission of zirconia specimens with different Yttria contents. A significantly higher transmittance was found when a high sintering temperature (1575°C) was used to sintered 3Y‐TZP and 5Y‐PSZ specimens compared to a regular sintering temperature of 1500°C. However, the heterogeneous material, represented by the gradient layer in this study, seems to be negatively affected by sintering temperature. Mayinger et al. ²¹ demonstrated a similar trend where there was a significant decrease in the translucency of the heterogenous zirconia layer (3Y/4Y‐PSZ) when sintered at high firing temperature (1600°C) while a significant increase in translucency was observed in homologous 3Y‐TZP and 4Y‐PSZ layer. A few studies have reported the effect of sintering time and temperature on the translucency of heterogenous zirconia material, therefore, further investigation of the matter is required.

The ΔRTP₀₀ were calculated on specimens sintered with St and Ht protocol. The interpretation following the 50%:50% translucency perceptibility (TPT₀₀) and acceptability (TAT₀₀) thresholds ¹⁵ showed that perceivable differences (ΔTP₀₀>0.6) in translucency were observed in the translucent and gradient layers of both materials. This phenomenon can be attributed to the higher susceptibility of 5Y‐PSZ to high sintering temperature in changing optical properties than 3Y‐TZP and 4Y‐PSZ. ³² , ³³

To obtain a more clinically relevant understanding of the material's performance, future studies could consider fabricating anatomically shaped crowns (both anterior and posterior) and evaluating their flexural/fracture resistance alongside color and translucency. Such an approach would provide a more comprehensive picture of how these zirconia materials behave under conditions that mimic actual clinical use. Existing studies ²¹ , ³⁴ , ³⁵ have adopted this methodology, successfully evaluating color, translucency, and fracture resistance of crowns fabricated from various zirconia materials. These investigations offer valuable benchmarks for future research in this area.

Moreover, this study has investigated only two multilayered zirconia materials from a single manufacturer. Miura et al. demonstrated that different multilayered zirconia reacted differently to a change in the sintering parameter. ³⁶ Therefore, further investigation of different multilayered zirconia is needed to improve the understanding of the effect of the sintering parameter on the microstructure and mechanical and optical properties.

CONCLUSIONS

Within the limitations of this in vitro study, the following conclusions were drawn. The High translucent sintering protocol significantly lowered the biaxial flexural strength of Prime and Prime esthetic. Though the Ht sintering protocol significantly affected the translucency, the alteration was within clinically acceptable thresholds (TAT₀₀ = 2.62).

CONFLICT OF INTEREST STATEMENT

The authors report no conflicts of interest related to this study.