Effects of thickness and polishing treatment on the translucency and opalescence of six dental CAD-CAM monolithic restorative materials: an in vitro study

Dental computer-aided design and computer-aided manufacturing (CAD-CAM) restorative materials have gained popularity in dentistry for indirect restorations [1]. The optical properties of CAD-CAM materials play a crucial role in restorative dentistry, aiming to recreate natural dental structures from esthetic perspective. To achieve excellent esthetics, it’s essential for the restorative team to possess a thorough understanding of the basic principles and optical characteristics of CAD-CAM materials to replicate the complex optical appearance of affected teeth [2].

Translucency and opalescence are key factors in achieving natural-looking results, which should dental restorations exhibit comparable to adjacent teeth [3, 4]. Translucency refers to the amount of light transmitted or diffused from the substrate, representing the material's state between complete opacity and transparency [3, 5]. The translucency parameter (TP) is commonly used in esthetic dentistry and calculated as the color difference from a white and black background using the Commission Internationale de l’Éclairage (CIE) color space, allowing quantitative evaluation of translucency [6]. Higher TP values indicate higher translucency. Opalescence is the optical characteristic of dental materials that exhibit a bluish-white appearance in reflected light and an orange-brown appearance in transmitted light, which is evaluated as opalescence parameter (OP) [3, 7]. This characteristic arises from the light scattering phenomenon caused by shorter or equal wavelengths of the visible spectrum in translucent materials [3, 8]. The opalescence of materials contributes to the masking of background color along with translucency, particularly when translucency is within a similar range [9].

The translucency and opalescence of CAD-CAM restorations, utilizing monolithic blocks, can be influenced by various factors, including material type, thickness, and surface treatments [3, 8, 10‐16]. Firstly, fabricating esthetic dental restorations poses significant challenges for dental technicians due to the varying thickness requirements for each restoration, greatly impacting translucency and opalescence. The esthetic success of tooth-colored restorations often relies on the experience and skill of laboratory technicians in handling translucent materials [17]. As translucency and opalescence prediction is a rapidly growing research area in dentistry [16, 17], comprehensive knowledge of expected changes in translucency and opalescence based on material thickness is crucial for successful dental restorations. Several studies have reported a correlation between translucency and thickness, demonstrating a decrease in translucency values with increasing thickness [8, 12‐16]. However, a precise mathematical formula for this correlation remains elusive due to significant variations among different studies [16, 17]. Consequently, obtaining color information at different thicknesses and accurately understanding the quantitative relationship are essential initial steps towards achieving predictable and highly aesthetic CAD-CAM restorations [8, 16, 17].

Meanwhile, there is a need for quantitative studies to determine whether variations in translucency are perceptible or clinically acceptable. Errors in translucency are particularly noticeable as they are closely tied to the lightness of a material, and the human eye is more sensitive to differences in lightness than hue or chroma [18]. Visual translucency thresholds have been widely employed as quality control tools and guides for evaluating translucency differences in dental materials, as well as in the analysis of clinical and in vitro research findings [6, 19]. Translucency thresholds for restorative dental materials using TP₀₀ have been studied by Salas et al. [6], who assessed the basis of 50:50% translucency acceptability thresholds at 2.62 units and perceptibility thresholds at 0.62 units.

Secondly, the optical properties of CAD-CAM materials may undergo changes during prosthesis repair or adjustments such as grinding or polishing [20‐22]. Meanwhile, wear, aging, and acid etching occur naturally to the restoration [23‐25]. These factors could alter the topography and roughness of CAD-CAM materials, consequently influencing light transmittance and altering translucency and opalescence [23‐30]. Previous studies have primarily focused on comparing translucency and opalescence results between different surface treatments, such as glazing or aging. However, there is a need to quantitatively evaluate the degree of color change after multiple roughening treatments to simulate the daily wear of CAD-CAM restorations.

Thirdly, various materials for CAD-CAM restorations, including glass–ceramics, zirconia, and composites, are available in dentistry currently [31]. Although manufacturers claim good translucency for these CAD-CAM materials, independent data comparing the materials on the market are limited. The quantitative relationship between translucency, opalescence, and thickness, as well as the differences in translucency and opalescence among different CAD-CAM materials, remains unclear, posing challenges in material selection and replicating tooth color.

Therefore, the aim of this study was to quantitatively evaluate and compare differences in translucency and opalescence among six different contemporary CAD-CAM materials, considering clinically relevant thicknesses and roughening treatments. The null hypothesis posited that material type, material thickness, and roughening treatment would not affect translucency and opalescence.

Table 2 summarize the results of MANOVA on the effects of material type, thickness, and roughening treatment on TP₀₀ and OP. The analysis revealed significant influences of material type, thickness, and roughening treatment on both translucency and opalescence (P < 0.05).

Figures 1 and 2 display the mean and standard deviation values of TP₀₀ and OP. A general decrease in TP₀₀ (average from 30.08 to 10.97) was observed as the thickness increased. TP₀₀ ranged from 37.80 (observed in 0.5mm LS) to 5.66 (observed in 2.0mm VS). The OP of most materials increased firstly and then decreased with increasing thickness, with the exception of LU showed continuous increase and VS showed continuous decrease. OP ranged from 5.66 (observed in 0.5mm LU) to 9.55 (observed in 0.5mm VS).

The variations in TP₀₀ (ΔTP₀₀) between adjacent thicknesses for the same material (Fig. 3) showed a decline as the thickness increased, ranging from 9.85 (between 0.5mm and 1.0mm) to 3.64 (between 1.5mm and 2.0mm). The highest variations in TP₀₀ were observed in LU between 1.5mm and 2.0mm (ΔTP₀₀ = 2.10) and lowest were observed in LS between 0.5mm and 1.0mm (ΔTP₀₀ = 15.29). All variations were higher than the TAT, except for LU between 1.5 and 2.0mm. The variations in OP ranged from 0.20 (CD between 0.5mm and 1.0mm) to 2.77 (VE between 1.0mm and 1.5mm).

Significant correlations between TP₀₀, OP, and roughening treatments were observed in all materials except for LS (P < 0.05). Figure 4 illustrates the surface roughness of materials after different treatments. Rougher specimens exhibited lower TP₀₀ and higher OP (P < 0.001). Roughening by P300-grit decreased TP₀₀ and OP by an average of 2.59 (close to TAT) and 0.43 for 0.5mm specimens, while 1.26 (higher than TPT but lower than TAT) and 0.25 for 2.0mm specimens compared to the polished ones. The variations in TP₀₀ between roughening treatments ranged from 0.21 (2.0mm LS between P800-grit and P300-grit roughened) to 3.91 (0.5mm TE between polished and P300-grit roughened), while the variations in OP between roughening treatments ranged from 0.03 (2.0mm LS between P300-grit and P800-grit roughened) to 0.85 (0.5mm VS between polished and P300-grit roughened).

The analysis of the regression curves (linear, exponential, logarithmic, and quadratic) for the tested materials indicated that the quadratic regression curves provided the best fit (R² closer to 1.0) for VE, LU, TE, and SU, while logarithmic regression curves provided the best fit for LS and CD (Table 3).

The results of this study rejected the null hypothesis, indicating that material type, thickness, and roughening treatment all had significant effects on translucency and opalescence.

Translucency and opalescence of dental materials are essential factors in achieving natural-looking dental restorations [3]. Dentists and technicians commonly evaluate these characteristics visually or using digital techniques. However, visual assessment is subjective and can be influenced by external factors such as ambient light and individual observers [35, 36]. To obtain a more objective analysis, spectrophotometers, like the Vita Easyshade V used in this study, offer clinically accurate and acceptable measurements of translucency and opalescence [34, 37].

Accurately predicting translucency and opalescence that closely resemble natural teeth in CAD-CAM restorations remains a challenge. The aesthetic success of prostheses often relies on the expertise of laboratory technicians working with translucent materials. As the prediction of translucency and opalescence continues to advance, gaining precise knowledge of how these characteristics change with material thickness based on mathematical functions can greatly contribute to the success of dental restorations [16, 17]. The current study analyzed the translucency and opalescence of CAD-CAM materials across a range of thicknesses (0.5mm to 2.0mm), which are commonly encountered in clinical restorations such as veneers, inlays, onlays, overlays, full crowns, and monolithic crowns [3, 12‐15].

The findings of this study demonstrated that translucency and opalescence varied with different thicknesses. TP₀₀ exhibited a continuous decline and curvilinear relationship with increasing thickness, consistent with previous studies [8, 14, 15, 38, 39]. While, the variations in opalescence (OP) were material-dependent, indicating differences among the materials. Thinner specimens exhibited greater differences in TP₀₀ and OP between adjacent thicknesses compared to thicker specimens. We observed the highest average variations in TP₀₀ (TP₀₀ = 9.72) between 0.5mm and 1.0mm and the lowest (TP₀₀ = 3.41) between 1.5mm and 2.0mm, aligning with findings by Bayindir et al. [38]. Similarly, Kang et al. found that TP decreased as the thickness of resin-based composites and glass–ceramics increased, particularly at lower thicknesses [14]. However, this observation may be attributed to the limitations of clinical spectrophotometer, as variations in accuracy have been reported between clinical spectrophotometer like Vita Easyshade V and laboratory spectrophotometer [40]. The observed range in OP was from 5.66 (0.5 mm LU) to 9.55 (0.5 mm VS), consistent with results reported by Shirani et al. [3]. However, none of the tested groups in this study exhibited opalescence comparable to that of enamel [9].

Studies on the correlation between translucency and thicknesses have been reported [16, 17, 41‐44]. However, obtaining a precise relationship, particularly at low thicknesses, and reaching a conclusive mathematical formula have proven challenging due to significant variations among different studies. The study on monolithic zirconia stained with a coloring liquid by Kim et al. [41] reported a linear correlation between translucency and thickness. While an exponential relationship between translucency and thickness of glass ceramics and zirconia ceramics was described by Wang et al. [42] and Sulaiman et al. [43]. A logarithmic relationship of translucency and thickness was described by Brodbelt et al. [44], Erdelt et al. [16] and Schweiger et al. [17] for ceramic materials and zirconia, respectively. In this study, four regression curves (linear, exponential, logarithmic, and quadratic) of the tested materials were analyzed. The results revealed that the quadratic regression curves provided the best fit for TP₀₀ in most materials, except for LS and CD, which exhibited a logarithmic regression curve. However, due to limitations in thickness variation, drawing a unified conclusion about the correlation was challenging.

Translucency change in CAD-CAM materials is particularly noticeable to patients and clinicians, as it is closely related to lightness, which is more perceptible to human eyes than hue or chroma [18]. Visual translucency difference thresholds have been widely used as a quality control tool to guide the selection of esthetic dental materials, assess clinical performance, standardize procedures, and interpret findings in clinical dentistry and dental research [19]. In our study, we observed average TP₀₀ variations between adjacent thicknesses ranging from 3.64 (between 1.5mm and 2.0mm) to 9.85 (between 0.5mm and 1.0mm). Except for LU specimens, the variations of all groups exceeded the translucency acceptability threshold. These findings indicated that changes in translucency due to thickness were visually apparent. Therefore, careful attention should be given to the adjustment of restoration thickness, as variations of 0.5mm or more can lead to clinically noticeable and potentially unacceptable differences in translucency, particularly for restorations less than 2.0mm thick [14].

In the present study, the six tested CAD-CAM materials were evaluated based on their typical material types and common use in dentistry. Our findings revealed that translucency was primarily influenced by material type, whereas opalescence was more affected by thickness, contradicting the findings of Barizon et al. [39], who stated that translucency was primarily influenced by thickness. We observed significant differences in translucency and opalescence among the tested materials, with the VS specimens exhibiting significantly lower translucency and higher opalescence compared to the other groups. The LS and LU specimens showed the highest translucency and lowest opalescence, respectively. These results indicate that these materials cannot be used interchangeably in clinical situations, particularly for veneers, considering their differences in translucency and opalescence.

The influence of inner structures and compositions on translucency and opalescence has been reported in previous studies [45, 46]. Materials with higher mechanical properties tend to have lower translucency [47, 48]. Differences in light transmission characteristics among monolithic materials can be attributed to factors such as monomer and filler type and content, filler size, polymerization, defect distribution, porosity, and inorganic content [12, 46, 49]. The manufacturers of LS reported that this glass ceramic exhibits variations in translucency and opalescence due to the presence of large and small lithium meta-silicate crystals in the pre-crystallized state [45]. Differences in inorganic filler content may explain the variation in translucency between these materials [49]. Additionally, the presence of fillers with radio-opacifying properties can affect material translucency [12]. These factors contribute to the differences in translucency between resin-nano ceramic (LU) and polymer-infiltrated ceramic materials (VE). Zirconia-reinforced lithium silicate ceramics, such as SU and CD, have gained popularity in CAD-CAM systems due to their combination of esthetic properties from glass ceramics and strength from ZrO₂ particles [50]. Consistent with previous studies, our results showed that CD, LS, and LU exhibited higher TP₀₀ compared to other groups [14]. The nano size of ZrO₂-SiO₂ ceramic particles contributes to the translucency of the materials [51]. VS exhibited lower TP₀₀ than CD and showed significantly higher opalescence, in line with the findings of Shirani et al. [3]. The sintering process after milling for VS may result in alterations in crystal size and structure, such as more compact interlocking of microstructures in crystals, thus leading to lower translucency and higher opalescence [52].

We also investigated the effect of different roughening treatments on translucency and opalescence [12]. Increasing surface roughness caused a reduction in TP₀₀ and an increase in OP. As thickness decreased, the variations in TP₀₀ and OP among the different roughening treatments increased. The influence of surface treatments on the translucency of restorative materials has been previously studied, demonstrating that roughness and topographical alterations affect light transmittance [29, 30]. This may be because light direction and incidence are altered when light transmits through a roughened surface, which may alter optical characteristics, especially material opacity [25, 29]. We observed that the difference in TP₀₀ between the P300-grit roughened and the polished specimens in 0.5mm was 2.59 on average, exceeding the perceptible threshold for translucency and approaching the acceptability threshold [6]. The average TP₀₀ difference decreased to 1.39 for 2.0mm thick specimens, still surpassing the perceptible threshold but falling below the acceptability threshold. These findings indicate that the translucency difference caused by roughening is perceptible and potentially clinically unacceptable. Moreover, the effect of roughening treatments on translucency and opalescence appeared to be material-specific. LS showed less variation in translucency and opalescence with different roughening treatments compared to other materials, while TE and VS exhibited the highest variation respectively. This phenomenon may be attributed to the greater hardness and dense internal molecular structure of lithium disilicate glass ceramics [45]. The same roughening treatments led to fewer changes in surface roughness, and, consequently, less variation in translucency and opalescence. Therefore, when selecting restorations, the surface condition of the material should be given equal consideration alongside translucency and opalescence. Posterior processing treatments, such as high-gloss polishing, play a crucial role in restoring the appearance of dental restorations based on the results of this study.

It is important to note some limitations of our study. Firstly, it should be noted that clinical spectrophotometers like Vita Easyshade V may not be as accurate as laboratory measuring instruments. Therefore, the results obtained from clinical spectrophotometers should be interpreted with caution, as the translucency and opalescence were not obtained using a laboratory spectrophotometer [40]. Secondly, the findings may not directly apply to clinical situations since the effects of underlying structures like abutments and luting agents were not considered. Thirdly, some materials used in our study can undergo glazing, which can influence their translucency and opalescence.

Based on the limitations of our study, we draw the following conclusions: