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We live in the esthetic age, and dentistry is no exception. The pursuit of the perfect smile is a constant in today’s society, and ceramic veneers are among the most commonly performed treatments. With the trend toward minimally invasive dentistry, these veneers are becoming more conservative, with thicknesses less than 0.5 mm. To perform such treatments, the ceramic restorative material must adhere to a dental substrate using a bonding agent. Each of these individual factors, which together form a restorative system, has different optical characteristics that will influence the final color of the restoration. For ceramics, the type, microstructure, thickness, translucency, and color need to be analyzed beforehand. For the dental substrate, saturation and tissue type (dentin or enamel) will have an impact; and for the bonding agent, its color, translucency/opacity, brand, thickness, internal composition, and color changes with aging will be important considerations. This chapter is aimed at clinicians who seek to make esthetic restorative planning for ceramic veneers more precise, meet patient expectations, and successfully navigate the challenges of complex cases. Here, we present the variables involving the optical and color effects that influence conservative ceramic restorations and their behavior in terms of longevity and esthetic maintenance. Enjoy!
Esthetic dentistry is increasingly gaining ground in society. This is linked to its relationship with patients’ quality of life, since it has implications for psychological factors, such as greater self-esteem, security in interpersonal relationships and satisfaction with each individual’s self-image [1, 2, 3, 4, 5]. More recently, the “minimally invasive” approach has gained ground in dentistry, which seeks to refund esthetics and function, along with restoring the patient’s self-esteem through less wear of the dental hard tissues, mimicking the dental structure through restorations adhered preferably to enamel, with tiny volumes/thickness [6, 7, 8, 9].
Ceramics are the materials of choice for esthetic restorations, as they can mimic the natural tooth and promote great longevity, even in thin layers, rehabilitating the smile [7]. Among ceramics, lithium disilicate-based glass ceramics are an excellent option for creating conservative laminates on anterior teeth, as in addition to favorable esthetics, they have good mechanical properties (Figure 1) [10, 11].
In restoration with ceramic laminates, due to their thinness and greater translucency, the effect of the luting material and substrate on the final color will be more impactful, which will require care and planning from the clinician to perform the case accurately [12, 13, 14, 15]. Since resin cement is more influential in these cases, it can be used to help mask the saturated substrate or neutralize the color difference between substrates, when in high opacity, as it will serve as a background blocker, similar to opacifiers in composite resin restorations [13, 14, 16, 17, 18, 19]. In this way, the masking effect associated with opaque material is achieved, but the natural esthetics are maintained, considering the use of a ceramic that is not completely opaque, but rather allows translucency and a depth effect of light, such as dental enamel. This will reduce the effect of the ceramic on the final color of the restoration, which will reduce the thickness of the preparation and, consequently, of the ceramic piece, promoting the preservation of healthy tooth structure, long-lasting and stable adhesion because it is made of enamel, and not compromising pulp and gingival health [20].
In this sense, it is important to evaluate the change in optical properties to which the cementation line is subject, since it is the least mechanically resistant material among the restorative set [21, 22, 23, 24, 25]. Because it is formed by a low molecular weight polymer network, resin cements are susceptible to hydrolytic degradation, which will lead to opacification and yellowing of the material, especially noticeable in high-value chromatic resin cements and when associated with translucent ceramics [26, 27]. This fact may compromise the esthetic result in the long term, leading to the replacement of the restoration [28].
Esthetics is deeply rooted in our society, often dictating our behavior, ways of thinking and how we relate to other individuals. Since ancient times, it has been sought after as a way of acquiring advantages for those considered beautiful. However, the esthetic standard is not unique, and has changed throughout history and in different societies and respective cultures. Present in all social segments, dentistry is no different. In recent years, esthetics has been gaining more space and notoriety within dentistry, due to the great search by patients for harmonious smiles that meet their esthetic expectations. There is no doubt that the smile plays a central role in facial esthetics, and it is a challenge for the clinician to deliver highly esthetic results, but that value correct function and that harmoniously relate the dental elements to each other and to the facial elements [1, 2, 3, 29].
In this sense, dentistry has a strong impact on people’s quality of life, interfering with their self-esteem, the way they see themselves and their security in relationships with other individuals [1, 4, 5]. Therefore, a patient who is dissatisfied with the appearance of their smile and, therefore, stops smiling, is not only repressing a feeling, but is also impacting their health, since this is a combination of physical, mental and social factors and not just the absence of disease. This observation is supported by the definition of health by the World Health Organization (WHO), which, in 1946, defined health as a state of complete physical, mental and social well-being, not just as the absence of disease or illness. The search for a perfect smile and the solution to personal dissatisfaction makes many patients willing to undergo invasive, irreversible and painful procedures in the name of beauty. This is because solving a problem, changing the smile, considered by the patient can restore their self-esteem and personal confidence again, and the appreciation of their own smile [30].
3. Minimally invasive or sufficiently invasive esthetic dentistry: Advantages and challenges
No restorative material can surpass natural dental tissues. In this regard, a conservative approach has gained traction in dentistry, advocating for the maximum preservation of hard dental tissues, which cannot be regenerated once lost [6, 9, 31]. Viewing the dental element as an organ with a unique and complex internal system supports this minimally invasive ideology, where restorative dental procedures aim to remove only the necessary amount of mineralized tissue, ensuring that preparations remain in enamel. This approach promotes the conservation of dental tissue and preserves the vitality and health of the dentin/pulp complex [9]. Another advantage is improved adhesion and longevity of the restoration, which is a growing concern for dentists [7]. Adhesion to dentin, being a more heterogeneous tissue constantly moist due to the presence of dentinal tubules and associated tubular fluid, poses challenges for adhesion, making it more critical and less durable than adhesion to enamel [8].
However, this makes the planning and execution of esthetic restorative work more challenging, as the substrate and the cementing material in indirect restorations have a greater influence on the final color result, given the thinner ceramic layers. This thickness makes it more difficult to mask unfavorable, saturated, and darkened substrates. The concern for conserving as much healthy dental structure as possible does not preclude the need for prosthetic preparations when indicated, such as in cases of masking substrates and achieving the necessary thickness to accommodate the material, restoring both function and esthetics. Failure to perform preparations in cases like ceramic veneers, for example, can lead to overcontours, cervical steps, and periodontal issues that compromise patient health and case esthetics due to an inadvertent interpretation of this approach [32]. A study [33] analyzed conventional preparations and no-prep/minimally invasive veneers over a 9-year period and found higher survival rates for no-prep/minimally invasive veneers.
Another major concern for clinicians is achieving a natural appearance in their restorations, which should mimic the healthy dental element in terms of anatomical shape, color, and translucency. The choice of material is crucial in restorative esthetics to restore function and appearance in harmony with the biological aspects of teeth. Therefore, the selection of restorative material must be based on its optical and mechanical properties [31]. One such material for esthetic restorations with excellent mechanical properties is dental ceramics.
Fully ceramic restorations first appeared in the dental field around 1960. However, it was not until the 1980s, with the advent of acid-etchable porcelains, that they could be bonded to hard dental tissues, particularly enamel, due to the improved longevity of adhesion [11].
Initially, fully ceramic crowns were made with significant thickness to compensate for the absence of a metal coping, which enhanced the mechanical properties of the restoration. By increasing the thickness of the piece, the intrinsic strength of the ceramic material was improved. It was only with advances in adhesion knowledge and the development of acid-etchable ceramics, combined with high-strength ceramics such as lithium disilicate and zirconia, that it became possible to create more conservative ceramic restorations with thinner layers [34]. This reduces the likelihood of endodontic treatments and hypersensitivity in extensively prepared teeth compared to minimally invasive preparations, while preserving healthy dental structure [9].
The emergence of mechanically more resistant and adhesive all-ceramic restorations has reduced the use of metal-ceramic restorations. This is due to the advantage of using more translucent ceramics that better mimic natural teeth by allowing more light to pass through. Unlike metal materials, which require an opaque ceramic layer to mask their presence and necessitate greater preparation thickness to accommodate different layers of feldspathic ceramics (translucent, opaque, and effect) for esthetic purposes, modern ceramics provide a more effective solution [11, 12, 35]. Furthermore, with the development of ceramic restoration techniques, it is now possible to combine different types of ceramics in multi-layered pieces. This approach leverages the virtues of each ceramic material, typically with better mechanical properties in the infrastructure and superior esthetics in the final layer, unlike monolithic pieces [36].
Currently, due to the advent of ceramics suitable for conditioning, it is possible to create restorations that rely exclusively on adhesion without the need for geometric preparation and mechanical retention. This makes the fabrication of ceramic restorations, including crowns or onlays, more conservative compared to metallic and metal-ceramic restorations. This is particularly true when using resin cement for the cementation of ceramic pieces, which contributes to increased mechanical strength of the restoration, making the bonded piece more durable than the pure ceramic before cementation [37]. This perspective encourages clinicians to pursue increasingly minimally invasive preparations, minimizing damage to the tooth.
Lithium disilicate-based glass ceramics have intermediate properties between feldspathic porcelains and zirconias, offering excellent mechanical and esthetic properties, making them suitable for both posterior and anterior restorations. They are composed of an amorphous phase of crystalline quartz and lithium disilicate crystals in varying proportions, which determines different levels of translucency, ranging from high translucency to high opacity. The higher the proportion of crystals, the opaquer the material becomes [38]. This allows lithium disilicate to be used in highly esthetic conservative restorations, as it maintains good mechanical and optical properties even in thin layers [10]. Furthermore, its suitability for anterior esthetics is largely based on its various translucency levels, which can ensure natural-looking restorations that mimic healthy teeth. Literature indicates that one of the key aspects of naturalness in restoration is the translucency of the restorative material [39, 40]. Since dental enamel is highly translucent, the opaquer the dental material, the more noticeable the restoration will be. Conversely, materials with high translucency can more effectively mimic natural dental tissue and achieve an “invisible restoration” appearance. It is important to note that translucency is affected by the thickness and type of ceramic material, with greater thickness resulting in more light scattering, thus creating an opaque effect [41, 42, 43]. The reduction in translucency of lithium disilicate veneers is a crucial factor in masking saturated substrates and a key element in the final color of the restoration [16, 17].
5. Color effects in conservative indirect restorations
The optical properties of materials directly influence their esthetic indication and, consequently, the success of a harmonious and natural smile rehabilitation, particularly when biological limits are respected and natural tooth characteristics are used as guiding parameters. Mimicking the natural characteristics of a tooth to achieve esthetically satisfactory and imperceptible restorations, while also being functionally adequate, is not a simple task for the dentist. This is increasingly demanded by patients and requires not only technical skill from the professional but also a good understanding of the optical characteristics of teeth, such as morphology, texture, and color, as well as the properties of restorative materials suitable for each case and their ability to mimic natural dental structures [29, 31, 44, 45, 46].
These properties result from the interaction between incident light and the physical-chemical structure of the material. One such property is translucency, which reveals the relative amount of light that is transmitted and how much is diffusely reflected through a turbid medium [47, 48]. This optical phenomenon of light transmission and reflection is considered an intermediate state between transparent (total transmission) and opaque (total reflection), described as partial opacity [49, 50].
Reproducing the natural characteristics of dental elements, such as shape, color, and texture, in restorative work to achieve satisfactory esthetic results and mimic adjacent natural teeth is the clinician’s goal and undoubtedly one of their greatest challenges [44, 45, 46]. This is primarily due to the complex color of a dental element, which is not monochromatic, having a more translucent enamel and more saturated dentin, variable according to the location and histo-anatomical arrangement of these tissues. This variability causes differences in saturation and luminosity in different regions of the same dental element [44]. Therefore, understanding the factors involved in an esthetic restoration is essential for better predictability of restorative work.
Many variables affect the final color of an indirect restoration, such as the color and translucency of the ceramic, substrate saturation, and the color and translucency of the resin cement [13, 14, 17, 19, 44, 51, 52]. Among these variables, the ceramic has the greatest influence on the restoration color, with thickness and translucency being key factors. Greater thickness and lower translucency reduce light transmission and reflection of the substrate and resin cement color, highlighting the characteristics of the ceramic material [12, 13, 14, 15].
The choice of ceramic material type and translucency remains the most influential factor for thinner ceramic pieces, such as veneers. However, the color of the resin cement also significantly impacts these cases, especially as the ceramic veneer becomes thinner and more translucent [13, 14, 15, 53]. Therefore, careful attention is needed when choosing the resin cement for bonding conservative ceramic veneers, focusing on translucency and value, which affect the restoration more than chroma and hue [53]. In this regard, resin cement can aid in masking unfavorable substrates, as shown in studies [16, 17, 19], which observed that combining less translucent veneers with more opaque resin cements positively contributes to this process and maintains the restoration’s esthetics.
However, maintaining the color obtained immediately after restoration over time is a challenging factor for long-term esthetic success [27, 54]. Additionally, literature indicates that restorations with ceramic veneers cemented with resin cement show some pigmentation with aging, which is important for the long-term esthetics of the restoration [55, 56, 57, 58, 59].
5.1 Ceramics
Understanding the optical characteristics of indirect restorative materials used in clinical practice is essential for the dentist. This knowledge enables the precise selection of the most suitable material for each clinical situation, leading to more accurate color outcomes [13, 14, 15, 53].
Glass ceramics are highly recommended for anterior esthetic work, such as veneers, due to their adequate translucency, though they have a lower masking ability [48]. The translucency of an object directly influences the material’s masking capability, as it affects the amount of light absorption, reflection, and transmission, and consequently, how much of the underlying structure is visible [47, 48]. Small variations in thickness in more translucent ceramics are more noticeable compared to opaque ceramics, which also makes the substrate more easily perceived and influences the color of the restoration [48, 53, 60].
Translucency in dental ceramics is significantly dependent on the ceramic composition, including the size and number of crystals, pigments, and the size, quantity, and distribution of internal defects such as porosities and microcracks [60, 61]. The greater the difference in refractive indices between the phases of a ceramic, the lighter scattering occurs between particles, resulting in lower light transmission and higher opacity [47, 60, 61, 62]. When the crystals are smaller than the visible light spectrum (400 to 740 nm), the material is transparent. However, larger crystal diameters increase light scattering, making the restoration opaquer. For example, the translucency difference between the high translucency (HT) and Low Translucency (LT) ceramics from the IPS e.max CAD line is due to HT having fewer large-diameter crystals, while LT has more of these larger crystals, resulting in HT having a more apparent glassy phase and less light scattering, making it more translucent than LT [47, 61, 62].
In addition to composition, translucency in ceramics is inversely affected by the thickness of the piece, for the same type and brand of ceramic [62, 63]. The type of ceramic and its thickness (intrinsic and extrinsic translucencies) affect the amount of light absorption, reflection, and transmission, and consequently, how much of the underlying structure is perceived [13, 47, 48, 60]. Furthermore, surface alterations caused by abrasive procedures can change the surface texture of the ceramic material, affecting its translucency [47, 61, 62].
The stability of these optical characteristics in restorations is crucial for maintaining the esthetics of the smile. Ceramics are more resistant to the chemical challenges of the oral environment compared to resin composites, suffering less from color and translucency changes [21, 28, 64, 65]. According to the study by Subasi et al. [64], which evaluated different thicknesses of ceramic veneers (0.5, 0.7, 1 mm) and types of ceramics subjected to aging, the type and thickness of veneers were more significant in altering color and translucency than aging itself, remaining stable over time. This contrasts with resin materials and even the dental substrate, which undergoes mineral content changes, increasing translucency [47, 57, 58, 66, 67, 68]. The composition of the ceramic material is more significant in determining its optical properties than thickness and the technique used to fabricate the ceramic piece (milled, pressed, and layered) [65].
Thus, a significant portion of color alteration in a ceramic restoration set comes from the dental substrate and the underlying resin cement, which undergoes water absorption and pigmentation during the restoration’s aging process, potentially affecting the esthetic outcome [58, 64, 69].
5.2 Substrate
The substrate will also significantly influence the final color of a conservative ceramic veneer restoration and will be one of the most challenging factors for the clinician, as it is intrinsic to the case and cannot be completely altered. For cases involving preparations for ceramic veneers on discolored teeth from trauma, endodontically treated teeth, or teeth with intrinsically altered formation processes, masking these substrates to achieve satisfactory esthetics can be challenging. Additionally, when covering multiple adjacent teeth with varying saturation levels with ceramic veneers, the dentist faces a major challenge in neutralizing the effect of these substrates conservatively while maintaining the requested esthetics with thin veneers and without using highly opaque materials. In this regard, esthetic restorative planning is crucial, and understanding the masking potential of the chosen ceramic and the influence of the resin cement is also fundamental.
Pre-restorative whitening in such cases can be advantageous as it reduces the color difference between elements and the chosen shade, minimizing the necessary preparation thickness [20]. This is particularly important when using translucent pieces. Moreover, the chromatic effect of the substrate will vary depending on the dental tissue where the veneer is bonded. More invasive preparations, such as those for full crowns, will highlight the saturation and hue of the tooth, also increasing the restoration’s opacity due to the greater visible dentin content. Therefore, the color difference between the tooth before and after veneer cementation will be more noticeable [52]. Conversely, if the veneer is bonded to enamel, there will be less impact from substrate saturation and hue, although there will be a greater effect of intrinsic translucency to natural dental enamel, which can lead to greater reduction in the restoration’s value, requiring careful attention to avoid graying. When analyzing these hard tissues separately, it is evident that enamel has greater translucency than dentin of the same thickness, with dentin being the primary contributor to the visible color of the tooth [63]. The translucency of enamel and dentin is directly related to the orientation of enamel prisms and dentinal tubules, impacting how light spreads through these structures [47, 70]. This contributes to the notion that the final color of a ceramic restoration will be affected not only by the substrate color/saturation but also by the opacity/translucency of the tissue to which the piece is bonded. Errors in selecting material translucency/opacity and, consequently, the reflection/value of a restoration are considered the most critical and noticeable since human eyes are more sensitive to brightness differences, making accurate selection in restorative work crucial for the restoration’s esthetics, more so than color and hue [61].
The substrate will also undergo optical changes with the aging of the dental structure, with a tendency for increased translucency. Aging will increase the translucency of enamel due to significant loss of surface structure over the dental lifespan, making this structure thinner, as well as greater alignment and organization of enamel prisms transversely [70]. Meanwhile, in dentin, there will be a reduction in the pulp chamber volume and decreased blood supply, with dentin deposition in dentinal tubules (reducing their diameter), increasing its chroma and translucency due to increased mineral content [47, 71].
5.3 Cement line
In conservative veneer restorations, the cement line will affect the final color of the restoration and should be considered in esthetic restorative planning, as it can negatively impact the color result if its color/opacity is not planned. On the other hand, it can help neutralize the effect of unfavorable substrates (Figure 2), as the ceramic material’s masking ability and its translucency play a significant role, but the resin cement also affects the final color of the restoration, contributing to masking discolored substrates when used with high opacity [13, 16, 17, 18].
Figure 2.
Contrast between favorable substrates (bottom) and unfavorable substrates (top).
In a resin cement, its color, opacity, composition, and brand will directly affect its optical properties and influence the final color result of a restoration with ceramic laminates. The study by Silva et al., which evaluated the influence of different translucency levels of lithium disilicate (IPS e.max CAD – HT, MT, and LT) and the color/opacity of resin cements from the Variolink Esthetic light-cure (LC) line (light+, high opacity, and Neutral, high translucency) (Figure 3), aimed at masking the difference between substrate shades A1 and A4, showed that all groups with light+ resin cement (opaque) had smaller color differences compared to the groups with Neutral cement, regardless of the translucency level of the disilicate [16].
Figure 3.
Lithium disilicate ceramic laminates with Neutral resin cement (Ivoclar) on the left and Light+ resin cement (Ivoclar) on the right.
This demonstrates not only that the opaque resin cement contributed to substrate masking but also that it reduced the influence of the ceramic on the final color of the restoration. This supports the use of this cement as an adjunct in neutralizing unfavorable substrates in a more conservative manner while maintaining natural esthetics. The opaque resin cement acts as an opacifier, blocking the substrate and allowing it to be paired with a less opaque ceramic laminate, resulting in partial light transmission through the restoration (Table 1). Additionally, when comparing color differences between opaque and translucent cements, all groups showed significant and clinically impactful differences, according to the parameters developed by Paravina et al., reinforcing the statement that resin cement affects the esthetic color result [72].
Laminate translucency
Resin cement shade
Substrate saturation
Mean ± SD
ΔE00
HT
Light+
A1 vs. A4
(3.6 ± 0.7)B
Neutral
A1 vs. A4
(5.7 ± 0.5)A
MT
Light+
A1 vs. A4
(4.1 ± 0.4)B
Neutral
A1 vs. A4
(5.2 ± 0.2)A
LT
Light+
A1 vs. A4
(2.7 ± 0.2)C
Neutral
A1 vs. A4
(4.3 ± 0.4)B
Table 1.
Mean and Standard Deviation (SD) of color differences (ΔE00) between A1 and A4 substrates (Influence of substrate on restoration final color). Subscript equal letters indicate lack of statistical difference among groups evaluated from Tukey test with 95% confidence intervals (ρ < 0.0001).
The brand of resin cement will also affect the chromatic characteristics of a restoration, such as its ability to mask substrates [73]. This is because the intrinsic composition of each manufacturer will vary, leading to different optical properties, with different refractive indices/opacities between brands, which was significant in the study.
Moreover, [13] a study demonstrated that the thickness of the cement line will also affect the final color of the restoration, but in a less significant and clinically visible manner than the variation in ceramic thickness. For ceramic pieces thicker than 2 mm, such as traditional crowns, the resin cement’s influence is not clinically visible, indicating that, although opaque resin cement positively affects masking, the main factor determining the final color result of the restoration is the ceramic material itself. However, for ceramic pieces less than 1.5 mm thick, the color of the cement has a greater influence, necessitating careful selection to avoid errors in the final restoration color. This can be mitigated by using try-in pastes. Similar results were obtained by Cubas et al. [14], who evaluated the masking ability of ceramic systems in groups with 1, 1.5, and 2 mm thicknesses cemented with opaque, intermediate, and no resin cements. The conclusions were consistent: groups cemented with opaque resin cement showed a significant improvement in masking ability, especially in thinner ceramics, with the 2 mm groups achieving the best masking results due to the considerable ceramic thickness.
This effect is intensified with thinner thicknesses, such as in ceramic laminates. According to Begum et al., in their study comparing ceramic pieces with 0.5, 1, and 1.5 mm thicknesses cemented with opaque and translucent resin cements over a darkened substrate (C3), it was shown that the color difference between the 1 mm lithium disilicate group with translucent cement and the 0.5 mm group with opaque cement is small [19]. This indicates that the masking ability of a 0.5 mm laminate with opaque resin cement is equivalent to that of a 1 mm thick piece with translucent cement, but with the advantage of greater preservation of healthy dental structure and a more natural final appearance. However, the color of the substrate will be decisive for achieving optimal esthetic results. The study by Kandil et al. not only compared the influence of resin cement opacity on the masking ability of a darkened substrate but also evaluated the role of the translucency of the restorative ceramic material in this process [17]. They cemented 0.5 mm thick ceramic pieces with opaque and translucent cements in two translucency levels and concluded that the group cemented with opaque cement was more effective in masking the darkened substrate compared to the translucent cement, although all groups showed detectable color differences. This demonstrates the advantages of opaque cement and its contribution to substrate masking compared to translucent cement. However, the degree of translucency of the ceramic had a greater impact on this process, with opaque ceramics providing the best masking of the substrate.
Thus, using thin lithium disilicate-based ceramic veneers with high-opacity resin cement, as opposed to high-translucency cements which only promote cementation, ensures various benefits such as masking saturated substrates, achieving a natural restoration appearance, better mechanical properties, and longevity. This approach promotes a conservative esthetic, providing good esthetic outcomes and improved quality of life for the patient [14, 16, 17, 19]. However, even though this material can positively influence masking, it is crucial to properly choose the translucency level and thickness of the ceramic material, which is primarily responsible for the final color of the restoration, to effectively mask the substrate and perform a more conservative preparation on the remaining dental structure.
However, even if this material plays a beneficial chromatic role, the thickness of the cement line should be as thin as possible, as it is the weakest link in the restoration system. There is still a gap regarding the cement line in ceramic laminates, as ISO 4049:200 [74] standards require a resin cement thickness not exceeding 50 μm, but the majority of studies use 100 μm as a standard thickness [75, 76, 77]. It is indicated [78] that the ideal thickness for cement in laminates is around 90 μm. At this thickness, the maximum stress transferred to the ceramic is minimized compared to other thicknesses, reducing stress levels in the restoration, which aligns with [79], who found 100 μm as an acceptable cement thickness.
However, regarding current popular manufacturing processes, according to Aboushelib et al. [80], CAD/CAM-milled ceramics result in an average cement thickness of 340.35 and 106.74 μm for the injected technique, and this thick cement under the cemented laminate can interfere with the mechanical integrity of the restoration, increase polymerization stress, or influence the color and translucency of the restoration [35, 81]. Additionally, there is an increased chance of iatrogenic gingivitis caused by the increased porosity in the thick resin cement layer, as the cementing agent layer is the most critical and prone to degradation when dealing with enamel-cement-laminate systems.
5.3.1 Alteration of optical properties of the cement line due to aging
Thin, translucent ceramic veneers will have their color influenced by the color of the cement. Therefore, the stability of the optical properties of the cement line is crucial for the long-term esthetic success of restorations, as it is affected by the aging of the restoration [54]. Changes in color and translucency that ceramic restorations undergo over time, along with challenges in the oral environment, can significantly impact the esthetic outcome, especially in the anterior region where there is a high demand for color and shine maintenance, potentially leading to restoration replacements [28].
During the aging process, the cement line will undergo more significant alterations in physical and chemical characteristics than the ceramic, which has stable color and shine and does not degrade in the oral environment like resin composites do. These composites represent instability in their optical properties, with resin cement being the main contributor to changes in color and translucency in the indirect restoration system [21, 56, 67, 68]. Over time and with exposure to oral challenges, there is a tendency for the cement line to become more opaque, which clinicians must consider, particularly for already opaque resin cements, as it can reduce the natural effect of the restoration [23, 67, 82, 83].
The optical properties of resin cements, such as translucency and color, are influenced by the type and brand of the material. This can be explained by differences in composition, the amount of polymer matrix, the quantity, type, and distribution of filler particles, and the degree of monomer conversion due to differences in refractive index between the filler and matrix of these cements [21, 54, 56, 84, 85]. Additionally, the addition of pigments also impacts translucency, with more chromatic composites having greater opacity than less saturated and translucent ones, where generally fewer pigments are added. There is a significant correlation between translucency and chroma [26, 49].
The average particle size of the resin cement will directly affect the material’s degree of translucency, as it relates to the amount of light transmission/scattering. Smaller particles, around 0.1 μm, allow greater light transmission and higher translucency, while particles of 10 μm in diameter cause greater light scattering, making the material more opaque [22, 65]. Thus, the quantity and size of filler particles and the type of monomers present will also affect the translucency changes a material undergoes with aging in the mouth [22, 56, 65, 66, 67, 84, 86, 87]. Therefore, the cement line will be more resistant to degradation and staining and will have more stable color and translucency if it has a higher proportion of filler and a lower organic portion in its composition, as the organic portion is more susceptible to hydrolysis in the oral environment [54, 82, 88].
However, it will also be affected by extrinsic factors such as polymerization, food, beverages, and medications [85, 89, 90, 91]. In this sense, a parallel can be drawn between the color stability of composite resins and resin cements, as both are materials with a polymeric network subject to degradation in a moist environment and have characteristics similar to flowable resins, with lower filler particle proportions to achieve lower viscosity and higher flow, making them more prone to degradation in the oral environment [52, 55, 67].
Hydrolytic degradation, which alters the color of resin composites, may be related to the distribution of filler particles, with composites having smaller particles and better distribution being more resistant to hydrolysis, and to the composition of the polymer matrix network, related to the hydrophilicity of the monomers present, such as TEGDMA and UDMA, with polar functional groups more susceptible to water absorption [56, 67, 86, 87]. Additionally, as studied by [82], resin composites with different average particle sizes, with higher packing of particles, and with a higher volume percentage of filler showed greater color stability when exposed to pigmented beverages. This study also demonstrates that this degradation leads to changes in opacity/translucency of the composites after aging, with increased reflectance for most groups.
With water absorption and the breakdown of the polymer network of resin cement, there is a rearrangement of the non-organic particles in the material, causing a change in the refractive index of the cement line after aging compared to the immediate moment. This is because changes in the physical and chemical characteristics of the polymer network will affect the refractive index, altering the amount of light reflected/transmitted by the restoration, which will affect its translucency, with lower translucency as the difference in refractive index increases [22, 23, 24]. The greater the difference between the refractive indices of the filler particles and the matrix of a resin material, the greater its opacity, according to [23]. Additionally, the breakdown of monomers can lead to changes in the pigment distribution of resin cements, with lighter cements being more prone to greater color changes after aging, as suggested by [26]. After aging, there is a change in the color of the cement line, with a tendency for yellowing of resin cements (increased values on the b* coordinate, blue-yellow axis), according to studies [26, 27]. Besides the breakdown of resin monomer bonds, water absorption leads to the expansion and plasticization of the polymer network, producing microcracks that allow greater penetration of pigmented substances into the cement line [83].
Chemical composition, and consequently the type of cement (light-cured, dual-cured, self-adhesive), will imply different responses to aging, with a tendency for the cement line to suffer from both opacification and marginal pigmentation. Cements with chemical activation are more prone to changes in optical properties due to lower chemical stability and higher susceptibility to water absorption [22, 54, 67, 83, 84, 88, 92]. Resin composites, such as resin cements, with higher proportions of TEGDMA compared to UDMA, for example, have a greater tendency to suffer from water absorption, as TEGDMA is a low-viscosity monomer with high hydrophilicity, while a cement with higher proportions of UDMA will be more stable, as the polymer network of the cement line will be less degraded and less prone to pigmentation [56, 67, 84, 86, 87]. For ceramic piece cementation, light-cure (LC) or dual-cure (DC) resin cements are the first choice. Light-activated cements tend to be more stable against oral challenges due to their composition of aliphatic amines, which are more resistant to oxidation than tertiary amines present in dual-cure and self-adhesive (chemically activated) resin cements. This results in lower resistance to water absorption and greater marginal pigmentation of restorations, as the oxidation of residual monomers forms yellowish compounds, which is more likely to occur in dual-cure and self-adhesive cements [24, 57, 58, 66, 67, 68, 83, 88, 92]. This process can be intensified in cases of thicker and more opaque ceramic pieces, due to the tendency for subpolymerization of the cement line, impacting the degree of monomer conversion in both light-cured and dual-cured cements [21, 56, 68, 93, 94].
According to a systematic review [21], light-cure cement showed a higher degree of monomer conversion compared to dual-cure cement when used for cementing ceramic pieces up to 2 mm thick, providing greater color stability to the restoration. Additionally, due to higher degrees of conversion, light-cure cements have improved mechanical properties, making them less prone to water absorption in areas such as the gingival sulcus where the cement line is more exposed. However, even dual-cure cements are affected by reduced light exposure, as chemical polymerization alone is not sufficient to compensate for inadequate light polymerization. Both physical and chemical stimuli are necessary for proper polymerization of dual-cure cements, as noted [21]. The less effective the light polymerization of the cements, with lower irradiance received, the lower the degree of monomer conversion, resulting in a mechanically weakened cement line with poorer long-term esthetic properties [93]. According to Janda et al. [89], the type, curing time, and photopolymerizer influence the photopolymerization and, consequently, the color stability of a resin composite over time.
Dual-cure resins generally show less color stability due to the possibility of degradation of the tertiary amines used for the chemical polymerization reaction of monomers and oxidation of residual monomers. This modification in the chemical structure of the cement line contributes to the formation of yellowish compounds and changes the color of the cement [24, 25, 27, 57, 58, 67, 68, 86]. Unlike dual-cure cements, light-cure cements are more resistant to staining, as they contain aliphatic amines, which are more resistant to oxidation than tertiary amines, co-initiators less resistant to degradation in the oral environment [83, 86].
Knowledge of the color, translucency/opacity of both the natural tooth and the restorative materials is directly related to the success of esthetic dental treatments. Mastery of each of these factors is essential. In a conservative indirect restoration using ceramic veneers, many factors will influence the chromatic outcome. The ceramic will have a significant impact, but so will the resin cement and the substrate, considering the thin thickness of the piece. Therefore, understanding these factors not only at the time of cementation but also regarding how the optical behavior of the restoration will evolve over time is important.
This research was funded by the Institutional Scientific Initiation Scholarship Program of the National Council for Scientific and Technological Development (PIBIC – CNPq), number: 143597/2021-3, and the Coordination of superior Level Staff Improvement (CAPES), number: 88882.376604/2019-2101.
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Written By
Carlos Eduardo Francci, Samara Silva and Mylena Régis
Submitted: 21 August 2024Reviewed: 13 September 2024Published: 21 October 2024
Open access peer-reviewed chapter
