Investigating the Effect of Different Implant Abutments on the Optical Properties of Monolithic Zirconia Crowns with High and Low Translucency and Different Thicknesses after Cementing with Temporary Cement

Abstract

Objective

Various factors affect the esthetics, color, and optical properties of implants. Among these factors, we can refer to the type of abutment, the thickness of the crown, and translucency.

Materials and Methods

Two types of zirconia and titanium abutments with the same shape were placed on two regular-sized Bego bone-level fixtures. A total of 48 crowns including 12 monolithic zirconia crowns with high translucency and 12 with low translucency in two thicknesses of 0.4 and 0.8 mm for the titanium abutment and 12 monolithic zirconia crowns with high translucency and 12 with low translucency in two thicknesses of 0.4 and 0.8 mm for the zirconia abutment were made and then were bonded by temporary cement based on zinc oxide (Kerr temp-bond) on the existing abutments and then subjected to visual inspection.

Results

The color differences of the crowns in the titanium abutment group in both white and black backgrounds were significantly higher than in the zirconia abutment group (p <0.0001). After the intervention, the average translucency in the zirconia abutment group with high translucency at a thickness of 0.8 mm was the highest compared to other groups (p <0.05). In the zirconia abutment group on a black background with a thickness of 0.8 mm, the color difference of the crown with an average of 0.84 ± 0.46 was significantly lower than that of the 0.4 mm thickness with an average of 1.22 ± 0.22 (p = 0.018), but this difference was not significant in the titanium group.

Conclusion

The translucency of titanium abutments, regardless of the thickness and color, was higher than that of zirconia before cement. However, after cement, the translucency of zirconia generally increased. Titanium abutments before and after cementing the crowns showed greater color differences than zirconia abutments. An increase in thickness in both groups reduced the color difference.

Keywords

implant crown - monolithic zirconia - color - translucency - cement

Introduction

Currently, there is a growing demand for dental implants worldwide. The increase in the need and use of dental implants in the treatment of patients is due to the combined effect of several factors such as an aging population, the consequences of failure of fixed dental prostheses, anatomical complications, poor function of removable dentures, complications of partial dentures, the prediction of long-term results of implant-supported prostheses and their benefits, and increasing public awareness.[1] Dental abutment mostly refers to the intermediate structure between the implant and the prosthesis. It plays the role of support and fixation for the prosthesis. It is involved in preventing the rotation of the prosthesis and its position. The abutment can also affect the soft and hard tissues around the implant and the visual aspects of the prosthesis.[2]

Various types of abutments are currently available. Among them, titanium and ceramic abutments are the most clinically used.[3] Titanium abutments are highly durable. However, they may cause discoloration of the peri-implant mucosa, which can affect the esthetics of the final restoration.[4] Ceramic abutments have better light transmission and a color similar to natural teeth, but are less durable than titanium abutments.[5] Selecting the right dental abutment is essential to improve the prognosis and esthetics of implant crowns.[6] [7] All-ceramic fixed dental prostheses (FDPs) are an acceptable alternative to metal-ceramic FDPs. The primary reason for using all-ceramic prostheses is their higher esthetics.[8] All-ceramic restorations naturally mimic the visual properties of teeth. The tendency to use all-ceramic crowns has increased due to the rising cost of manufacturing PFM crowns with noble alloys.[9]

The primary drawback of the first introduced ceramics was their poor mechanical properties, limiting the indication of all-ceramic crowns to anterior and single-unit areas.[10] In recent years, several new dental ceramic materials have been developed to increase their stability while maintaining esthetic advantages. Among them, leucite or lithium-disilicate leucite and oxide ceramics such as alumina and zirconia seem to be very promising for various applications. The applications of these ceramics have recently been extended to the posterior areas of the mouth and even include multiunit FDPs.[11] Newly introduced ceramic systems have improved color matching and stability.

Various ceramics have been extensively employed in implant dentistry for dental implants, implant abutments, and implant-supported crowns. With the development of computer-aided design and computer-aided manufacturing (CAD/CAM) technology, zirconia has become increasingly popular due to its excellent mechanical properties and tooth-like color. Zirconia-reinforced lithium silicate glass ceramic (ZLS; VITA Suprinity PC), introduced in the past few years, includes about 10% zirconia by weight. The appearance of ceramic restorations has been investigated regarding their thickness, the optical effect of the underlying surface, and the color of the foundation restoration. Other factors such as the abutment color, translucency, thickness, and ceramic restoration type also affect the optical properties. Thus, the substrate material and ceramic thickness affect the final color of a ceramic restoration.[12]

Monolithic dental crowns fabricated via CAD/CAM technology have been extensively employed in prosthetics in recent years due to their high level of dimensional accuracy, marginal compliance, attractive mechanical properties, and optical properties. Additionally, they are associated with a lower risk of laboratory processing errors and cohesive failure compared to classical crown frameworks. Ceramic crowns are potentially prone to fracture despite their many advantages. The compositional and microstructural properties and the presence of defects and impurity phases affect the mechanical performance of ceramic materials. Thus, new CAD/CAM materials with enhanced mechanical properties have been continuously developed through composition modification and reduction of structural defects.

Various materials such as glass ceramics, zirconia, hybrid resin ceramics, and polymer infiltration ceramics are commercially available currently for the fabrication of monolithic crowns in the clinic. The most recently developed hybrid materials take advantage of the structural composition and properties of high-strength ceramics and more compatible polymers. Particle-filled ceramic resin nanocomposites are among these hybrid materials that are polymerized under high-temperature (180–200°C) and high-pressure (300 MPa) conditions to achieve a higher degree of conversion and polymerization, and improved mechanical properties. The clinical performance and durability of ceramic dental crowns depend on the physical and mechanical properties of the prosthetic raw materials and various clinical design parameters such as thickness, preparation height, termination line geometry, cusp slope, the presence of surface irregularities, and sharp points of the occlusal anatomy.

Researchers have examined the mechanical properties of various CAD/CAM crown materials and the effects of design and preparation factors on the fracture behavior of monolithic ceramic crowns in vitro.[13] A 2019 study used 70 semitranslucent monolithic zirconia crowns were designed in two thicknesses, 0.4 and 0.8 mm. The study results showed that thin-walled semitranslucent monolithic zirconia crowns were more affected by aging and wear processes than thick-walled crowns.[14] Monolithic zirconia restorations, produced exclusively with CAD/CAM technology, have considerable advantages such as high flexural strength, the need for more conservative tooth preparation, minimal wear of antagonist teeth, satisfactory esthetics, the need for less laboratory time, and reduced dental appointments.[15] [16] Their major disadvantage over the past years was their poor esthetics due to the inability to achieve acceptable translucency.[16] However, recent changes in composition, structure, and manufacturing methods have led to the creation of monolithic zirconia ceramics with better translucency but with reduced strength.[17] However, some laboratory studies have reported good performance in their mechanical strength.[18] [19]

Using ceramic abutments instead of titanium produces restorations with better color and translucency.[20] Surface texture, translucency, fluorescence, opalescence, brand, firing number, and thickness of the porcelain and its sintering technique affect the color match of ceramic restorations to natural teeth.[21] [22] Increasing the crystalline content of the substructure material improves strength, but generally reduces translucency.[23]

Cemented implant restorations are more popular than screw-retained crowns among clinicians. Elimination of loosening of prosthetic screws, improved esthetics of the crowns, and simpler laboratory and clinical procedures are among the advantages of these restorations.

The selection of cement used is crucial in achieving sufficient minimum adhesion and achieving the desired esthetics in the final restoration. Nowadays, temporary cements are known as the most common treatment option for bonding implant restorations. These cements include temporary resin cements and zinc oxide–based temporary cements. In clinical situations, these cements should be such that they provide sufficient retention of the restoration during function, and if necessary, allow the existing crowns to be removed from the abutment. The effect of these cements on the visual properties and esthetic factors of the restoration after cementation has been considered in recent years. A common clinical challenge in this regard is the color matching between zirconia crowns and natural teeth. It is essential to investigate and select the most appropriate materials available to achieve this goal due to the increasing use of dental implants and the increasing expectations of patients regarding esthetics. The present study investigates the use of zirconia abutments and crowns with different translucencies and thicknesses, and the application of the desired temporary cement to determine their visual properties and help select their appropriate composition.

Materials and Methods

In the present laboratory study, 24 crowns in each abutment group (a total of 48 samples) were examined. Two regular-sized Bego bone-level system fixtures were selected to implement this research process. A polymethyl methacrylate generator made according to the spectrophotometer device in black and white with standard dimensions of 4 mm × 4 mm and a height of 10 mm was used to increase the accuracy of measurement and stability of the samples. Then, two types of abutments made of titanium and zirconia with the same shape were placed on these fixtures, and a torque of 30 N/cm³ was applied using the corresponding wrench and ratchet. The abutments were scanned in the laboratory and four groups of central crowns were placed so 12 monolithic zirconia crowns with high translucency (highly translucent ZircoStar, Kerox Dental Ltd.) in 2 thicknesses of 0.4 and 0.8 mm for titanium abutments (6 thicknesses of 0.4 mm and 6 thicknesses of 0.8 mm) and 12 monolithic zirconia crowns with high translucency (highly translucent ZircoStar, Kerox Dental Ltd.) in 2 thicknesses of 0.4 and 0.8 mm for zirconia abutments (6 thicknesses of 0.4 mm and 6 thicknesses of 0.8 mm) (Several studies have demonstrated that thicker zirconia layers, specifically at or above 1.0 mm, contribute to better color masking, shade consistency, and overall esthetic outcomes. The use of thinner layers in our experiment was primarily due to methodological constraints and aims to evaluate the color differences at reduced thicknesses; however, this does limit the direct clinical applicability of our findings) and 12 monolithic zirconia crowns with low translucency (highly translucent ZircoStar, Kerox Dental Ltd.) in 2 thicknesses of 0.4 and 0.8 mm for titanium abutments (6 thicknesses of 0.4 mm and 6 thicknesses of 0.8 mm), and 12 monolithic zirconia crowns with low translucency (highly translucent ZircoStar, Kerox Dental Ltd.) in two thicknesses of 0.4 and 0.8 mm for zirconia abutments were prepared (6 pieces of thickness 0.4 mm and 6 pieces of thickness 0.8 mm).

Zirconia crowns will be manufactured by CAD/CAM technology and with color A2 based on the VITA Classic shade guide. The crowns of each group will be randomly numbered from 1 to 12. In the last stage, the crowns will be bonded to the existing abutments with zinc oxide-based temporary cement (Kerr temp-bond) and then subjected to visual examinations. The translucency parameter (TP) is the most common color property examined for monolithic zirconia ceramics. To calculate this parameter, the values of L (lightness), a (red/green coordinate), and b (yellow/blue coordinate) are determined using a spectrophotometer. Then, translucency is calculated using the following equation, where b is the color coordinate against a black background and w is the color coordinate against a white background. The color coordinates are according to the CIE laboratory color system:

Equation [(1)]

The color parameters of each crown were determined by a spectrophotometer before cementation. Then, the crowns were placed on the abutments, and cemented, and the color parameters were determined again by spectrophotometer. Then, possible differences in these parameters between the study groups were examined. To increase the accuracy of the color scanning of the samples by the device, a base was made that completely matched the spectrophotometer head, and the samples were placed in the middle of this base. The base color of the samples L*, a*, and b* was determined and recorded by the Shade pilot spectrophotometer (DeguDent, Hanau, Germany). The color quality was determined by the CIE Lab system (ΔL*, Δa*, Δb*). The device was calibrated using a reference sample before measuring the color in each group:

Equation [(2)]

The color of the crowns was measured three times from the middle third (3 mm apical to the incisal edge) of the facial surfaces of the crowns using a spectrophotometer. All color measurements were performed by the same physician in a room with controlled temperature, humidity, and daylight. Finally, the collected data were entered into SPSS (Version 26) software. At the descriptive statistics level, indices such as mean and standard deviation were used. At the descriptive statistics level, based on the result of the Kolmogorov–Smirnov's test indicating the normality of the data distribution, the analysis of variance test was used to compare the means of the variables among the eight groups. For pairwise comparison of the mean of the variables in the groups, the independent t-test or the univariate analysis test was used by adjusting the value of the variable before the intervention. The paired t-test was also used to compare the mean of the variables after the intervention, compared to before in each of the groups. Pearson's correlation coefficient was used to examine the relationship between translucency and the color of the crowns. The significance level was considered to be less than 0.05 in all analyses.

Results

[Table 1] presents the results of comparing the mean translucency of monolithic zirconia crowns between two types of titanium and zirconia abutments (without distinction of translucency and thickness). Based on this table, before the intervention, the translucency in the titanium abutment group, with a mean of 2.69 ± 1.05, was significantly higher than that in the zirconia abutment with a mean of 1.82 ± 0.72 (p = 0.002). However, after the intervention, the translucency in the titanium abutment group, with a mean of 3.05 ± 1.71, was significantly lower than that in the zirconia abutment group with a mean of 4.07 ± 2.62 (p = 0.010). Moreover, the increase in translucency after the intervention compared to before was not significant in the titanium abutment (p = 0.218). However, in the zirconia abutment group, the translucency after the intervention with a mean of 4.07 ± 2.62 was significantly higher than before it with a mean of 1.82 ± 0.72 (p <0.001).

Table 1 The mean translucency of monolithic zirconia crowns between the two types of titanium and zirconia abutments

Translucency	Titanium abutment	Zirconia abutment	Significance
Before intervention	1.05 ± 2.69	1.82 ± 0.72	0.002[a]
After intervention[c]	3.05 ± 1.71	4.07 ± 2.62	0.01[a]
Significance[b]	0.218	<0.001

^a Significance level from an independent t-test to compare mean translucency between the two abutments.

^b Significance level from univariate analysis by adjusting for preintervention translucency to compare mean translucency between the two abutments.

^c Significance level from paired t-test to compare mean translucency after the intervention compared to before in each abutment.

[Fig. 1] presents the mean translucency of monolithic zirconia crowns between two types of titanium and zirconia abutments (without distinction of translucency and thickness).

Fig. 1 Bar chart of mean translucency of monolithic zirconia crowns between two types of titanium and zirconia abutments.

[Table 2] and [Fig. 2] present the results of comparing the mean color of monolithic zirconia crowns between two types of titanium and zirconia abutments (without distinction of translucency and thickness) in each of the white and black backgrounds. Based on this table, the difference in crown colors in the titanium abutment group in both white and black backgrounds, with a mean of 2.29 ± 0.58 and 1.82 ± 0.33, respectively, was significantly higher than that in the zirconia abutment group with a mean of 1.26 ± 0.30 and 1.03 ± 0.39, respectively (p <0.001).

Table 2 Determination and mean color of monolithic zirconia crowns between the two types of titanium and zirconia abutments

Color	Titanium abutment	Zirconia abutment	Significance[a]
In the white background	2.29 ± 0.58	1.26 ± 0.3	<0.001
In the black background	1.82 ± 0.33	1.03 ± 0.39	<0.001

^a Significance level from an independent t-test to compare mean translucency between the two abutments on each of the white and black backgrounds.

Fig. 2 Bar chart of the color difference of monolithic zirconia crowns between two types of titanium and zirconia abutments on two white and black backgrounds.

[Table 3] presents the results of comparing the mean translucency of monolithic zirconia crowns between the two high and low translucencies in each of the abutments. Based on this table, in the titanium abutment group with the high translucency before and after the intervention, the translucency of the crown was significantly higher, with means of 3.63 ± 0.45 and 4.25 ± 1.18, respectively, than in the low translucency group with means of 1.76 ± 0.47 and 1.86 ± 0.88, respectively (p <0.05). In the zirconia abutment group, in the high translucency before and after the intervention, the crown translucency was significantly higher with means of 2.40 ± 0.48 and 5.09 ± 32.3, respectively, than the low translucency with means of 1.24 ± 0.04 and 3.06 ± 1.05, respectively (p <0.05). In other words, the monolithic zirconia crown with high translucency had higher translucency than the low translucency. This higher translucency did not depend on the abutment type. Additionally, investigating the changes in the translucency of the crown after the intervention compared to before revealed that in the titanium abutment group, the increase in translucency in both the higher and lower translucencies was not significant compared to before (p >0.05). However, in the zirconia abutment group, a significant increase was observed in the crown translucency after the intervention compared to before (p <0.05).

Table 3 Determination and comparison of the mean translucency of monolithic zirconia crowns between the higher and lower translucencies in each of the abutments

Translucency	Titanium abutment			Zirconia abutment
	Higher translucency	Lower translucency	Significance	Higher translucency	Lower translucency	Significance
Before intervention	3.63 ± 0.45	1.76 ± 0.47	>0.001	2.4 ± 0.48	1.24 ± 0.34	<0.001[a]
After intervention	4.25 ± 1.18	1.86 ± 0.88	0.006	5.09 ± 3.32	3.06 ± 1.05	0.043[b]
Significance[c]	0.245	0.716		0.022	<0.001

^a Significance level from an independent t-test to compare mean translucency between the higher and lower translucencies in each of the two abutments.

^b Significance level from univariate analysis by adjusting the level of translucency before intervention to compare the mean translucency between higher and lower translucencies in each of the two abutments.

^c Significance level from a paired t-test to compare mean translucency after the intervention compared to before in each of the higher and lower translucencies separately for the two abutments.

[Fig. 3] illustrates the mean translucency of monolithic zirconia crowns between the higher and lower translucencies in each of the titanium and zirconia abutments. Although the mean translucency in the titanium abutment group was higher than the zirconia abutment group in both higher and lower translucencies before the intervention, the mean translucency in the zirconia abutment group was higher than that in the titanium abutment group after the intervention.

Fig. 3 Bar chart of mean translucency of monolithic zirconia crowns between the two higher and lower translucencies in each of the abutments.

[Table 4] and [Fig. 4] present the results of comparing the mean color of monolithic zirconia crowns between the two higher and lower translucencies in each of the two white and black backgrounds, separately for the titanium and zirconia abutments. Based on this table, the difference in color of the crown in the titanium abutment group in the white background was not significant between the two higher and lower translucencies (p = 0.351). However, in the black background, in the high translucency with a mean of 1.65 ± 0.22, it was significantly lower than that in the low translucency with a mean of 1.97 ± 0.35 (p = 0.014).

Table 4 The mean color of monolithic zirconia crowns between the two types of high and low translucencies, separately for the titanium and zirconia abutments in each of the two white and black backgrounds

Color	Titanium abutment			Zirconia abutment
	Higher translucency	Lower translucency	Significance	Higher translucency	Lower translucency	Significance
In a white background	2.41 ± 0.69	2.18 ± 0.43	0.351	1.48 ± 0.22	1.03 ± 0.18	<0.001
In a black background	1.65 ± 0.22	1.97 ± 0.35	0.014	1.22 ± 0.34	0.84 ± 0.37	0.018

^a Significance level obtained from an independent t-test to compare the mean translucency between high and low translucencies, separately for each of the two types of abutments in each of the white and black backgrounds.

Fig. 4 Bar chart of the mean color of monolithic zirconia crowns between the two types of high and low translucency, separately for titanium and zirconia abutments in each of the two white and black backgrounds.

In the zirconia abutment group, the difference in crown color in high translucency with a mean of 1.48 ± 0.22 and 1.22 ± 0.34, respectively, was significantly higher than that in low translucency with a mean of 1.03 ± 0.18 and 0.84 ± 0.37, respectively (p <0.05).

[Table 5] presents the results of comparing the mean translucency of monolithic zirconia crowns between the two thicknesses of 0.4 and 0.8 mm, separately for the two titanium and zirconia abutments. Based on this table, both before and after the intervention, the mean translucency at the thickness of 0.4 mm was not significantly different from the thickness of 0.8 mm in both titanium and zirconia abutment groups (p >0.05). However, examining the translucency changes after the intervention compared to before revealed that in the titanium abutment group in the thickness of 0.4 mm, the mean translucency increased significantly after the intervention compared to before (p = 0.015), and a very slight and insignificant decrease was observed (p = 0.480) in the thickness of 0.8 mm. In contrast, a significant increase in the mean translucency was observed after the intervention compared to before in the zirconia abutment group in both thicknesses of 0.4 and 0.8 mm (p <0.05).

Table 5 Determination and comparison of the mean translucency of monolithic zirconia crowns between the two thicknesses of 0.4 and 0.8 mm in each of the abutments

Translucency	Titanium abutment			Zirconia abutment
	Thickness 0.4	Thickness 0.8	Significance	Thickness 0.4	Thickness 0.8	Significance
Before intervention	2.44 ± 1.11	2.94 ± 0.97	0.242	1.87 ± 0.59	1.77 ± 0.85	0.737[a]
After intervention	3.44 ± 1.92	2.67 ± 1.44	0.282	3.38 ± 1.12	4.76 ± 3.48	0.204[b]
Significance[c]	0.015	0.480		0.002	0.009

^a Significance level from an independent t-test to compare mean translucency between two thicknesses of 0.4 and 0.8 mm in each of the two abutments.

^b Significance level from univariate analysis by adjusting the level of translucency before the intervention to compare mean translucency between two thicknesses of 0.4 and 0.8 mm in each of the two abutments.

^c Significance level from paired t-test to compare mean translucency after the intervention compared to before in each of the thicknesses of 0.4 and 0.8 mm separately for the two abutments.

[Fig. 5] illustrates the mean translucency of the monolithic zirconia crown between the two thicknesses of 0.4 and 0.8 mm in each of the titanium and zirconia abutments. Before the intervention, the mean translucency in both thicknesses was higher in the titanium abutment group than in the zirconia abutment group. However, after the intervention, the mean translucency in the zirconia abutment group was higher in the 0.8-mm thickness than in the titanium abutment group.

Fig. 5 Bar chart of mean translucency of monolithic zirconia crowns between two thicknesses of 0.4 and 0.8 mm in each of the abutments.

[Table 6] and [Fig. 6] present the results of comparing the color difference of monolithic zirconia crowns between two thicknesses of 0.4 and 0.8 mm in each of the two white and black backgrounds, separately based on the type of titanium and zirconia abutments. Based on this table, the color of the crowns in the titanium abutment group in both the white and black backgrounds between the two thicknesses of 0.4 and 0.8 mm did not differ significantly (p >0.05). However, the color difference of the crowns with a mean of 0.84 ± 0.46 was significantly less than that of the thickness 0.4 mm with a mean of 22 ± 0.22 in the zirconia abutment group in the black background at thickness 0.8 mm (p = 0.018). However, no significant difference was found in the mean color of the crown between the two thicknesses in the white background in this abutment (p = 0.358).

Table 6 The mean color of monolithic zirconia crown between the two thicknesses of 0.4 and 0.8 mm, separately for titanium and zirconia abutments in each of the two white and black backgrounds

Color	Titanium abutment			Zirconia abutment
	Thickness 0.4	Thickness 0.8	Significance	Thickness 0.4	Thickness 0.8	Significance
In a white background	2.42 ± 0.63	2.17 ± 0.51	0.316	1.20 ± 0.39	1.32 ± 0.18	0.385[a]
In a black background	1.87 ± 0.42	1.75 ± 0.19	0.395	0.84 ± 0.46	1.22 ± 0.22	0.018

^a Significance level obtained from an independent t-test to compare the mean translucency between two thicknesses of 0.4 and 0.8 mm, separately for each of the two abutment types in each of the white and black backgrounds.

Fig. 6 Bar chart of the mean color of monolithic zirconia crowns between two thicknesses of 0.4 and 0.8 mm, separately for each titanium and zirconia abutment in each of the two white and black backgrounds.

[Table 7] presents the results of comparing the mean translucency of monolithic zirconia crowns in the eight study groups (with different abutments, in high and low translucencies, and two thicknesses of 0.4 and 0.8 mm). As shown in this table, there was a significant difference in the mean translucency among the eight study groups both before and after the intervention ([Fig. 7]).

Table 7 Determining and comparing the mean translucency of monolithic zirconia crowns in eight case groups

	Translucency	After intervention	Before intervention	Significance[††]
Titanium abutment	Low translucency in the thickness of 0.4 mm	c1.46 ± 0.48	c1.74 ± 0.81	0.520
	High translucency in the thickness of 0.4 mm	a3.42 ± 0.44	ab0.75 ± 5.14	0.008
	Low translucency in the thickness of 0.8 mm	b2.06 ± 1.95	c1.98 ± 1.01	0.828
	High translucency in the thickness of 0.8 mm	a3.84 ± 0.37	bc3.36 ± 1.54	0.530
Zirconia abutment	Low translucency in the thickness of 0.4 mm	c1.35 ± 0.32	bc3.39 ± 1.09	0.011
	High translucency in the thickness of 0.4 mm	b2.39 ± 0.19	bc3.38 ± 1.24	0.113
	Low translucency in the thickness of 0.8 mm	c1.13 ± 0.34	c2.73 ± 0.98	0.005
	High translucency in the thickness of 0.8 mm	b2.41 ± 0.69	a6.8 ± 3.96	0.045
Significance[†]		<0.001	<0.001

Note: Identical letters in each column indicate no significant difference in comparing the mean translucency between the two groups, and nonidentical letters indicate a significant difference in comparing the mean translucency between the two groups.

^† Significance level from one-way ANOVA test to compare the mean translucency among the eight study groups.

^†† Significance level from paired t-test to compare the mean translucency after the intervention compared to before in each of the eight study groups.

Fig. 7 Bar chart of mean translucency of monolithic zirconia crowns among the eight study groups before and after the intervention.

Investigating before intervention revealed that the mean translucency in the high translucency titanium abutment group in the thickness of 0.4 and 0.8 mm had the highest values, while the mean translucency in the low translucency titanium abutment group in the thickness of 0.4 and 0.8 mm had the lowest values and in the low translucency zirconia abutment group in the thickness of 0.4 and 0.8 mm. After the intervention, the mean translucency in the titanium abutment group with high translucency in the thickness of 0.4 mm and in the zirconia abutment group with high translucency in the thickness of 0.8 mm had the highest values compared to the other groups (p <0.05).

Investigating the changes in translucency after the intervention compared to before in the titanium abutment group with high translucency in thickness 0.4 mm showed that in the zirconia abutment group with low translucency with two thicknesses 0.4 and 0.8 mm, and high translucency with thickness 0.8 mm, there was a significant increase in crown translucency after the intervention compared to before (p <0.05). However, no significant increase or decrease in translucency was observed in other groups (p >0.05).

[Table 8] presents the results of the correlation between translucency and color (on white and black backgrounds) of monolithic zirconia crowns before and after the intervention. Based on this table, the crown color on a white background had a direct and weak (nonsignificant) correlation with translucency before and after the intervention in both titanium and zirconia abutments (p >0.05). However, the color on a black background had an inverse and nonsignificant correlation with translucency before and after the intervention in titanium abutments (p >0.05). In other words, crown color and its translucency did not have a significant correlation with each other in any of the abutments.

Table 8 Correlation between the crown color and translucency (on white and black backgrounds) of monolithic zirconia before and after the intervention

Translucency color		Titanium abutment		Zirconia abutment
		Translucency before intervention	Translucency after intervention	Translucency before intervention	Translucency after intervention
On a white background	Correlation coefficient	0.145	0.206	0.535	0.211
	Significance	0.498	0.335	0.007	0.322
On a black background	Correlation coefficient	−0.312	−0.380	0.333	0.317
	Significance	0.138	0.067	0.111	0.131

Discussion

Based on the results of this study, it can be stated that the translucency of titanium abutments before cementation was higher than that of zirconia. However, the translucency of zirconia increased after cementation. Regarding the color difference, titanium abutments showed a higher color difference before and after cementation, and increasing the thickness reduced this color difference. Dede et al examined the effect of different titanium, gold-palladium, and zirconia abutments and different universal white opaque and translucent cements on the final color of all-ceramic implant crowns. They concluded that one-way analysis of variance of DL, Da, Db, and DE values of the control and test groups was significant and that the use of titanium or gold-palladium abutments for implants is under question esthetically, and only zirconia abutments are acceptable in color. Opaque cement may also be useful for masking the dark color of titanium abutments, which is same as the results of this study.[24]

Capa et al also examined the impact of different colors of resin cement and zirconia cores on the TP of ceramic crowns that simulated an implant-supported fixed prosthesis. Consistent with our study, they concluded that adding a zirconia core under the resin cement significantly reduced the translucency (TP) values, and the presence of a titanium layer reduced the translucency (TP) value and gave a darker appearance.[25] In another study, Capa et al examined the effect of different types of cement and different zirconia colors on the final color of the restoration and observed that the cement type, the zirconia color, and the presence of titanium were among the important factors in the final color of the restoration.[26] Çeken et al examined the optical properties of the new generation of 3Y-TZP (MZ) monolithic zirconia with different abutment types and resin cement shades. They concluded that the least color change with or without cementation was observed in the crowns with zirconia abutments, which was in line with our results. Zirconia and hybrid abutments showed significantly lower ΔE002* and ΔE003* values in combination with yellow cement. The use of opaque cement in titanium/anodized titanium groups may have caused the clinically unacceptable ΔE00* value to reach an acceptable level.[27] Li et al examined the color of high-translucency zirconia crowns on abutments with different colors and surface variations similar to our study. They concluded that the darker the color of the underlying abutment, the greater the color difference of the crown.[28]

In line with our study, Jirajariyavej et al concluded that the thicker the zirconia crown, the less the color difference.[12] Baş and Çakan examined the effect of anodized titanium and zirconia thickness on the final color of zirconia restorations and observed that the zirconia thickness and the titanium abutment color affected the color of zirconia restorations. With increasing the thickness, the color difference will be lower.[29] Consistent with our study, Totou et al examined the difference between titanium and zirconia abutments and concluded that titanium abutments gradually caused a darker shade of color in the surrounding gingival tissue, and zirconia abutments showed a better final color in the restoration.[30]

Naveau et al examined the appearance and mechanical properties of zirconia abutments in the anterior region. They concluded that zirconia abutments were superior in appearance and esthetic properties to titanium. However, further studies are needed to examine mechanical properties.[31] Bidra and Rungruanganunt examined abutments in the anterior region and observed that anterior abutment fractures were limited to ceramic abutments. Studies using spectrophotometry showed less gingival discoloration with zirconia abutments. However, no evidence was reported of a difference between ceramic and metal abutments in patient esthetic satisfaction.[32] Soares et al concluded that two-layer systems (Zir + Pc) were the most predictable approach to adequately cover titanium abutments. Increasing the thickness of the restoration provides a higher coverage capability.[33] After examining the various properties of zirconia abutments, Nakamura et al concluded that zirconia abutments are preferable for anterior regions regarding visual and mechanical properties.[34]

Limitations

Several studies have demonstrated that thicker zirconia layers, specifically at or above 1.0 mm, contribute to better color masking, shade consistency, and overall esthetic outcomes. The use of thinner layers in our experiment was primarily due to methodological constraints. Other limitations were that the limited number of samples was due to cost constraints.

Only one implant system was used because of the availability of zirconia abutments; only one type of zirconia was used in the current area of the crowns for the measurement.

Future research incorporating the recommended minimum thicknesses could provide further insights into the color performance of monolithic zirconia in more typical restorative scenarios.

Conclusion

The present study revealed that the translucency of titanium abutments was higher than that of zirconia before cementation, regardless of thickness and color. However, zirconia translucency generally increased after cementation. Regarding the color difference, titanium abutments showed a greater color difference than zirconia abutments before and after cementation of the crowns, and increasing the thickness of the titanium abutments did not cause a significant change in the color difference. However, increasing the thickness of the zirconia abutments significantly reduced the color difference.

Conflict of Interest

None declared.

Authors' Contribution

A.F. contributed to conceptualization, methodology, software, and writing the original draft; F.N. performed validation, formal analysis, and investigation; S.N. collected resources, performed data curation, and also contributed to writing review and editing; R.M. was responsible for project administration, visualization, and R.K. was responsible for supervision.

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Publication History

Article published online:
05 September 2025