When Should Adhesive Cementation Be Used?
What Are the Properties of Adhesive Cements?
Which Materials Can Be Adhesively Luted?
How Should Different Materials Be Conditioned for Adhesive Cementation?
How Should Dental Tissues Be Conditioned for Adhesive Cementation?
How Important Is the Thickness of the Cement?
Indications and Clinical Procedures for Indirect Restorations of Adhesively Luted Front Teeth
Adhesive cementation is the final step of restorative treatment. In some cases its sole function is bonding and retention of the restoration to the tooth, whereas in other types of rehabilitation adhesive cementation also helps restore crown stiffness and improve the mechanical and physical properties of the tooth-restoration complex.
Good cementation relies on properly programmed design and operative phases, in order to achieve the best possible therapeutic outcome. It is important to emphasize that cementation should not be used to compensate for errors made during the previous phases, such as faulty restoration or inadequate tooth preparation.
This chapter is devoted to the work of the clinician, who—depending on the materials used for the restoration and the retentive features of the tooth preparation—can choose between different luting materials and clinical protocols.
All of the sections are based on questions that the clinician may have regarding choices and operative steps and that we will try to answer.
In the dental field, adhesion is undoubtedly one of most widely used terms of the last decade.
Daily practice has changed with the use of adhesive techniques and composites, which are used not only for direct restorations in the anterior and posterior but also for indirect restorations and prosthetic dentistry, owing to improvements in luting materials (Figures 8-1 to 8-4).
To avoid confusion, it is important to note that adhesive cements are composites (methacrylate resins). Composites form a large family. Indeed, by changing the percentage of constituent elements, the compound derived from monomer, silane, and fillers can be turned into materials with different characteristics for optimal adaptation to different clinical situations.
The goal of modern dentistry is a minimally invasive treatment plan devised specifically for the individual patient. Prosthetic dentistry has changed in light of this consideration; by binding to dental structures, adhesive cements permit the complete bonding not only of restorations such as all-ceramic crowns but also of partial restorations such as inlays and veneers, without interfering, preserving healthy dental tissues.
In this chapter we will explain the features, limitations, and advantages of adhesive cementation in the anterior areas, which are crucial not only functionally but also because of their esthetic impact, a need increasingly felt by patients.
In the field of adhesive techniques, materials that can be bonded to the tooth are classified into two main groups—adhesive materials and cemented materials—according to the substrate’s ability to be conditioned. The former exhibit strong adhesion because of their ability to be treated, whereas the structural properties of the latter do not permit conditioning and their intrinsic properties generally do not require strong adhesion to withstand mechanical stresses.
Adhesive cementation is not mandatory for all types of restoration, but it unquestionably represents a great advantage. Therefore it is imperative that we understand the details, difficulties, and care required when applying it. We will examine several clinical cases in order to explain the crucial steps for long-term success.
It is also important to emphasize that collaboration between clinicians and the industry has made it possible to market materials that are increasingly better suited to professional needs. Part of the dentist’s job is thus devoted to personal research into the thickness of the materials, with the hope of contributing to the development of composites.
Figure 8-1 Initial status. The patient requires replacement of the metal-ceramic crowns on teeth 9 and 10, together with the esthetic improvement of the whole front tooth group, while maintaining the interincisal diastema. The image shows clearly the presence of large composite restorations on the two right incisors, which have signs of wear and have lost their original morphology.
Figure 8-2 A, Ceramic feldspathic veneers, which permit better preservation of the tooth structure during preparation but have poor primary retention and must thus be adhesively luted. B, Zirconia copings for full-coverage crowns, which offer good support to the ceramic coating, improving the response to mechanical and physical stress. C, Finished zirconia-ceramic crowns before cementation, which can be adhesive or applied with a more traditional cement, such as a glass-ionomer, without employing any adhesive technique.
Figure 8-3 A, Initial radiograph of teeth 9 and 10 showing the metal-ceramic crowns joined together. B, Final radiograph of the same teeth after endodontic retreatment, restorations with fiberglass posts and composite, and individual zirconia ceramic crowns. (Endodontic treatments performed by Luca Ivaldi, Acqui Terme, Italy.)
Figure 8-4 Finished case with four different direct restorations: glass-ceramic (feldspathic) veneers on teeth 7 and 8 and crystalline ceramic crowns (zirconia) on teeth 9 and 10. Different clinical protocols were used, in this case always with adhesive resin cements. (Dental lab work performed by Mauro Ferraris, Alessandria, Italy.)
What is Adhesive Cementation?
The two main types of adhesion are chemical adhesion and micromechanical adhesion. Chemical adhesion involves different types of bonds (mainly hydrogen and covalent). In dental practice these bonds occur when different materials are applied to the dental hard tissues.
For example, when using a glass-ionomer cement or a two-step self-etch adhesive resin system (in which the first phase consists of an acidic primer and the second phase consists of a resin bonding agent), the dentist performs a procedure based on chemical adhesion because it does not remove the smear layer and it makes use of the hydroxyapatite in the residual collagen for adhesion. The use of a three-step etch-and-rinse resin adhesive (orthophosphoric acid etching, primer, and a resin bonding agent) instead removes the smear layer, thus creating the condition for resin tags, which are the main element responsible for micromechanical adhesion.
This chapter will not discuss glass-ionomer cements, given their unsatisfactory adhesion compared with resin cements (Rosenstiel, Land, and Crispin, 1998) and the fact that this type of material is used in dentistry for traditional cementation in lieu of the most widespread and traditional techniques employing zinc oxyphosphate cement. The chapter will instead focus on cementations with resin products and conditioning systems for adhesive substrates, which are also resin based.
It should be noted, however, that even glass-ionomer cements (from the carboxylate family, like polycarboxylates and resin-modified glass-ionomer cements) still have the ability to bond to dental hard tissues.
Another key distinction must be made regarding resin cements, depending on the type of activation of the polymerization reaction. In fact, there are self-curing cements, and their setting reaction depends on the mixing of a base and an accelerator; light-curing cements (Figures 8-5 to 8-7), which are not mixed with an activator but whose base paste contains photoactivators that can react to light stimulation at a particular wavelength; and dual cements, which have both light-curing and self-curing characteristics.
Figure 8-5 Glass-ceramic veneer during placement of the luting material (microhybrid restorative composite) heated to 50° C in a special oven.
Figure 8-6 Glass-ceramic veneers during setting of the luting material, which in this case is exclusively light curing. Proper cementation requires a high-power curing lamp and a translucent restoration that is not too thick.
Figure 8-7 Glass-ceramic veneer with glycerin gel applied to the edges before the last curing cycle.
The setting reaction of these cements begins after the base and catalyst have been mixed and is completed by light irradiation.
When Should Adhesive Cementation Be Used?
Materials that have good color characteristics but poor response to mechanical and physical stress (such as feldspathic and die-cast ceramics) require very strong cementation—in terms of adhesive bonding—to restore the crown stiffness lost after partial or total removal of the enamel of the natural tooth and thus attain acceptable mechanical features. Therefore in these cases it is essential to choose an adhesive protocol with resin cement and conditioning of the prepared tooth and the inner surface of the restoration (Figure 8-8).
Figure 8-8 Glass-ceramic veneer in place before removal of excess light-curing cement. Complete removal of excess composite before light application is crucial.
Otherwise, restorations such as full crowns that use an internal structure with good intrinsic mechanical and physical characteristics (such as metal-ceramic or metal-free prosthetic restorations with a core of infiltrated or polycrystalline oxides) do not need to have these features improved through strong adhesion. In any event, for retention to be enhanced—especially in the case of preparations with a nonretentive shape—it is important to choose procedures that promote the best adhesion for the specific type of material and interface, in order to ensure a good prognosis for the restorative treatment.
What are The Properties of Adhesive Cements?
The luting material plays an important role in the life and functional performance of the restoration, and incorrect use or mishandling can definitely affect the accuracy and prognosis of the rehabilitation.
The ideal properties of cements are analyzed according to different aspects (Massironi and Ferraris, 2004): mechanical and physical, biologic, functional, and esthetic.
Mechanical and Physical Properties
Low Solubility in Oral Fluids
Low solubility is crucial to avoid dissolution in oral fluids, which is potentially very dangerous for the prognosis of the restoration because loss of the cement film at the margin would leave space for the infiltration of oral bacteria at the tooth-restoration interface. This means a high risk of secondary caries, which is a very frequent cause of prosthetic failure.
High Mechanical Compressive and Tensile Strength
Masticatory loads on natural teeth—and therefore also on prosthetically restored teeth—can have different intensity and direction. The prognosis of restorative treatment and dental rehabilitation can improve greatly when the bonding material is able to withstand compression and oppose displacement forces.
Reduced Film Thickness
Obtaining a thin film of cured luting material is a key requisite for cementing restorations with good margin accuracy. Moreover, knowing the thickness of the luting material to be used is important in order to establish the amount of spacer required by the dental laboratory to fabricate indirect restorations.
Film thickness is also affected by handling and mixing.
American Dental Association (ADA) specification No. 8 and ADA/ANSI specification No. 96 call for a maximum thickness of 25 microns for luting material.
Good Adhesion to the Mineralized Tissues of the Tooth and Prosthetic Restoration
Today adhesion to the hard tissues of the tooth plays a key role in direct restorations, and more and more techniques are being developed to achieve a bond between the luting material and dental hard tissues, the restorative materials used to fabricate the abutments, and the inner substrate of the prosthetic restoration.
Conditioning steps are often required to optimize adhesion. Therefore one of the objectives of research is to obtain—if possible—stronger adhesive bonds with simplified steps. Achieving these results permits more conservative and, in general, less retentive dental preparations.
Low Water Absorption
Dimensional stability is a key feature for preserving the cement’s tooth-restoration seal. Water absorption is a very negative factor and can occur when the marginal cement comes into contact with oral fluids. Therefore it is important that dental cements exhibit almost no water and fluid absorption.
Nontoxicity and Insulation of the Pulp
Indirect prosthetic restorations on vital teeth are becoming increasingly frequent, in keeping with an approach of minimally invasive dentistry along with functional recovery and a good prognosis.
Consequently, nontoxicity for the pulp—both during the setting reaction of the cement and after polymerization—is becoming more and more important.
Caries Resistance and Low Solubility
Secondary carious lesions at the tooth-restoration interface are among the most common causes of failure (Karlsson, 1986; Glantz and colleagues, 1993). In order to avoid this occurrence it is essential that the cement be unable to dissolve in the oral environment, especially at the prosthetic margin where this would leave room for bacterial contamination and the possible onset of carious lesions. Moreover, the ability to release substances that can remineralize dental hard tissues, such as fluoride, is an excellent way to confer caries-resistant properties to adhesive cements. In addition to materials such as glass-ionomer cements, which release fluoride naturally (Wilson, Groffman, and Kuhn, 1985), this property has also been added to certain resin cements.
Adequate Setting Time
The setting reaction of cements—which leads to complete hardening of the material—includes a first working phase, in which the cement is handled to create the proper conditions for it to be activated and react. This is followed by a setting reaction, in which complete hardening of the material occurs. For resin cements the setting reaction corresponds to polymerization.
It is important that cements have an adequate working time to allow handling before placement in the oral cavity for bonding. Manual mixing requires a minimum time that varies depending on the material, but, given the many variables involved with manual procedures, it is generally preferable to use mechanical mixing with applicators that contain premeasured amounts of material and provide proper and predictable mixing. Mechanical mixing also seems to optimize the characteristics of the material (Kern, Schaller, and Strub, 1993).
Good Esthetic Integration
The luting material also plays a major role in the final appearance of the restoration: the more translucent the restoration, the more the color of the cement will be visible.
In these cases the approach to choosing cement color may differ. One can use a neutral material, without any major color interference with the restoration, or cements in different colors that during the testing phase enable the use of specific devices, called try-ins, that make it possible to simulate the final shade of restorations before cementation.
The luting material may undergo color changes owing to curing and aging. In the case of a translucent restoration made of glass-ceramic or composite resin, this can affect the result to some extent. Therefore the material must have excellent color stability (Asmussen, 1983; Berrong, Weed, and Schwartez, 1993; Imaz and colleagues, 1995).
Which Materials Can Be Adhesively Luted?
Adhesive cementation can be performed on different substrates, depending on the material used for cementation and the type of conditioning of the adherent surfaces.
It must be noted, however, that for many types of restorations adhesive cementation is secondary. Consider the long clinical and scientific experience with metal-ceramic crowns cemented with a zinc phosphate material, which offers low but adequate adhesion, to which the preparation criteria for abutment shaping also contribute.
The fact remains that adhesive luting is often synonymous with low solubility in oral fluids because of the characteristics of the resin cements used for this type of cementation. Therefore trends in the market and among dentists could go toward employing adhesive cementation to achieve a better prognosis, even in cases of less retentive abutments and especially to prevent secondary caries. Indeed, the latter continues to be the most frequent cause of failure in fixed prosthetic dentistry (Karlsson, 1986; Glantz and colleagues, 1993), essentially because of inaccurate margins of the restoration and excessive dissolution of the cement in contact with oral fluids.
In order to further prevent the possibility of secondary caries, certain resin luting materials provide a gradual release of fluoride (Rosenstiel, Land, and Crispin, 1998), although according to some studies (Wiegand, Buchalla, and Attin, 2007) there is no apparent direct correlation between fluoride release and the significant reduction of secondary caries.
Other types of restoration, such as feldspathic ceramic veneers, need strong adhesive cementation not only to minimize the risk of detachment from the prepared tooth—which otherwise would be very high—but also to improve the physical and mechanical characteristics of the tooth-restoration complex as well as prognosis. Therefore, having clarified that certain types of restorations unquestionably require adhesive luting, it should be noted that all materials containing silica can be treated very effectively with surface conditioning and resin cements containing methacrylate monomers.
All glass ceramics such as those based on feldspar or lithium silicate can be treated to increase their surface roughness (micromechanical adhesion, usually with strong acid etching) with a silane-based promoter (chemical bonding) and usually with a resin bonding agent (depending on the type of cement to be used; it is highly indicated with light-curing restorative cements).
Composites can also be considered silica-based materials, hence their ability to be luted adhesively in the case of indirect restorations. Composite conditioning is similar to the previously described procedure, but here blasting conditioning is preferable over acid etching.
Ceramics that are crystalline rather than glass based, such as zirconia and alumina, can increase their adhesion properties with other types of treatment (see the following section). The fact remains that the absence of silica oxide within their structure does permit the same approach as glass ceramics, which are referred to as adhesive ceramics, whereas crystalline (or polycrystalline) ceramics are called cemented ceramics.
In fixed prosthodontics the metal component—mainly represented by noble alloys—does not have a good natural predisposition to adhesive bonding with most resin systems. Some, such as the phosphate monomer–based type, may adhere to noble alloys after the use of a special conditioner. Alternatively, they can be conditioned to modify the nonvitreous surface into one with silica on the surface and then treated as a glass surface with subsequent adhesive cementation, as we will discuss in the following section.
How Should Different Materials Be Conditioned for Adhesive Cementation?
The aim of adhesive cements is to unite—with a strong bond—two substrates, usually represented by at least two different materials and tissues. Before defining the cement to use and the type of conditioning procedures that are required, we should define the materials of the inner surface of the restoration to be cemented, the prepared dental hard tissues (dentin, enamel, or both), the need for prerestorative restorations, and their constituent materials.
Noble and non-noble alloys have a wide variety of adhesion values to different types of cement, including resin-based ones (Aboush and Yenkins, 1989; Atta, Smith, and Brown, 1990; Fayyad, 1990; Kohli and colleagues, 1990; Cotert and Ozturk, 1996). The final adhesion is micromechanical, and it usually requires some type of surface treatment to increase the adhesive bond.
Noble alloys have lower adhesive ability than other materials such as base-metal alloys (Sen, Nayir, and Pamuk, 2000). Nevertheless, for many types of restoration this is not an absolute limitation to their clinical use. For example, metal-ceramic crowns can carry an excellent prognosis with nonadhesive cementation, because retention is mainly ensured by the shape of the prosthetic abutment regardless of the type of definitive cement that is used. Moreover, by having a rigid substrate below the ceramic coating, this type of restoration does not need to adhere strongly to the substrate to improve the response to mechanical and physical stress.
The improvement of adhesion is an interesting issue, especially on multiple crowns (where debonding of a single crown can quickly cause secondary caries), on partial coverage restorations, on adhesive bridges (e.g., Maryland bridges), or in the presence of poorly retentive abutments resulting from limited occlusal height or other reasons. Therefore alloys of noble metals such as gold, silver, copper, and palladium are the type most commonly used in fixed prosthodontics, although non-noble alloys have been employed extensively for adhesive bridges.
Metal alloys are highly biocompatible and do not corrode easily. This stability is a challenge for the adhesive bond because metal alloys do not lend themselves to good spontaneous adhesion without proper treatment and conditioning before their contact with the luting material.
Several treatments can be performed in order to prepare a metal surface for good adhesion with resin-based systems.
It is believed that one of the most effective systems for metal conditioning entails the use of a promoter solution (usually fluid) before application of a resin cement (Turner and Sinclair, 1990; Nabadalung, Powers, and Connelly, 1998) such as phosphate monomer–based materials.
Another very effective treatment for obtaining adhesion with a resin adhesive system on metal dental materials is tribochemical silicatization (which will be described in the section on composite and crystalline ceramic bonding) and subsequent treatment with a silane (Matinlinna and Vallittu, 2007).
This section will not examine all the options known in the literature to increase adhesion between cement and metal alloys but will focus on the protocol that can yield the best clinical results, according to the authors and with the support of scientific studies.
Likewise, metal can be conditioned with macroretentions and specific primers for metal alloys, thus allowing better adhesion between the metal and coating material, especially with composite resins (Figure 8-9). In recent years composite-metal prostheses won the favor of many clinicians, and the improved adhesion between these two materials may have contributed to the growing use of this technique.
The use of microhybrid composites for indirect restorations, usually partial ones—such as in the anterior or for inlays or veneers—is increasingly common.
The bond between the restoration composite (direct restorative materials are often used in the dental office for indirect restorations) and resin cements is quite favorable and creates a good bond. Therefore from the standpoint of the dental substrate adhesion with resin cements, this type of material can also be considered quite suitable for indirect restorations.
The aspect of indirect restorations that is most prone to problems—even in the case of cementation of composite restorations—is cements exposed to the oral environment (Kawai, Isenberg, and Leinfelder, 1994; Shinkai and colleagues, 1995). Nevertheless, the low water solubility of resin cements (Rosenstiel, Land, and Crispin, 1998) contributes to a good prognosis.
There are factors that can benefit indirect ceramic restorations, such as improved reproduction of the stiffness of natural tooth enamel (Magne and Douglas, 1999a, 1999b, and 2000) and certain color characteristics, even if the esthetic performance of modern composites has improved markedly in recent years.
Nevertheless, composite restorations have certain favorable clinical aspects: a single biomechanical behavior with cement and the possibility of basic restoration, and easier management of any repairs or modifications.
With respect to older composite materials, modern ones offer better wear resistance (Ferracane and Condon, 1992; Park, 1996), physical properties (Uçta li, Wilson, and Zaimo lu, 1993; Reinhardt, Boyer, and Stephens, 1994), and color stability (Wendt and Leinfelder, 1992; Donly and colleagues, 1999). Indirect composite restorations can be placed even in the anterior with excellent results.
Various studies have investigated the possible treatment of composite surfaces for best adhesion (Swift and colleagues, 1992; Stokes, Tay, and Pereira, 1993; Hummel and colleagues, 1997) and good durability. Several treatments can be performed before application of an adhesive bonding agent: slight surface roughening with a bur, the application of hydrofluoric acid, the application of orthophosphoric acid, and blasting with aluminum oxide or silica-coated aluminum oxide (silicatization). In addition to these treatments, a silanizing agent commonly known as silane can also be applied. Silane is effective when the substrate surface contains silica particles, which composites contain naturally.
Recent studies have shown that orthophosphoric acid etching is not effective in increasing the adhesion between two composite surfaces (Valandro and colleagues, 2007), although dentists sometimes use this treatment to clean the surface further after conditioning. In reality, the latter aspect requires further scientific investigation.
Some studies show that silicatization (a chemical tribologic silica-coating process) and subsequent silanizations confer higher adhesion values compared with other treatments (Ozcan and colleagues, 2007).
Other authors believe that sandblasting and the subsequent application of silane are the most effective treatments, although, of the two treatments, blasting with 50-micron aluminum oxide can be considered more important than silane treatment for the purpose of better adhesion (D’Arcangelo and Vanini, 2007). The same study notes that hydrofluoric acid and silane on the composite do not significantly increase adhesion.
In conclusion, adhesion attained with sandblasting performed both with 30-micron silica-coated (silicatization) alumina particles and 50-micron alumina particles exhibits similar and very high adhesion values, followed in both cases by application of silane (Valandro and colleagues, 2007).
Therefore one of the most effective protocols is the following:
• Sandblasting of the inner surface of the restoration with aluminum oxide for about 10 seconds (Figure 8-10)
Figure 8-10 Sandblasting with 50-µm alumina oxide particles is one of the best pretreatments to enhance adhesion to composite, followed by silane and bonding.
• Silanization (if necessary with activation by a hot air jet or in special ovens for silane drying)
• Drying after about 30 seconds
• Application of a layer of resin bonding agent and luting with a dual-cure resin or a light-curing restoration composite (which is usually heated first in a special oven) inside the restoration or on the dental preparation
Glass ceramics, or silica-based ceramics (silica oxide), can be conditioned very effectively for adhesion with resin materials.
This type of ceramic can be feldspathic, commonly used as a coating in metal-ceramic prostheses (Weinstein, Katz, and Weinstein, 1962) (see Figures 8-1 and 8-2, A) or as a coating for metal-free restorations with a core with high mechanical resistance (Andersson and Oden, 1993). Other glass ceramics are die-cast, such as materials containing lithium silicate, which respond better than feldspathic ceramics to mechanical and physical stress (Raigrodski, 2004).
Because of their high translucency and esthetic characteristics, glass ceramics are ideal materials for porcelain veneers (McLaughlin, 1998), and they are also indicated for partial restoration in the posterior, such as ceramic inlays (Blatz, 2002).
In any event, it should also be considered that, if not properly supported by a rigid structure, all glass ceramics unfortunately offer limited performance when they are subjected to masticatory loads—for example, flexural strength and mechanical stress in general (Raigrodski, 2004). However, when final cementation is performed according to the appropriate procedures for adhesive bonding with composite material, this can increase the resistance to fracture of the ceramic restoration (Jensen, Sheth, and Tolliver, 1989) and the tooth-restoration complex, which can regain the crown stiffness lost when the natural enamel was removed (Magne and Douglas, 1999a, 1999b, 2000).
This is why it is so important to implement proper procedures for this type of adhesive material, not only to increase the retention of the restoration on the tooth but also to improve the mechanical and physical behavior of the restoration and thus the prognosis of the rehabilitation.
Good surface roughness and a clean surface are essential conditions in order to give the resin interdigitation and ensure a good chemical bond to the ceramic surface (Semmelmann and Kulp, 1968; Jochen and Caputo, 1977; Ferrando and colleagues, 1983; Sorensen and colleagues, 1991; Bailey and Bennett, 1992; Chen, Matsumura, and Atsuta, 1998a, 1998b). The type of surface conditioning may vary: surface roughening (Semmelmann and Kulp, 1968), preparation of ceramics with rotary instruments to create a rough surface (Semmelmann and Kulp, 1968; Jochen and Caputo, 1977; Ferrando and colleagues, 1983), acid etching of the surface (Bailey and Bennett, 1992), and sandblasting (Calamia, 1985; Lacy and colleagues, 1988). More than one treatment can be performed on the same artifact, and they can be combined in an attempt to achieve even better results in terms of adhesion. The application of hydrofluoric acid is an effective way to create a suitable surface before the application of various adhesive conditioning agents, increase the roughness of the ceramic material, and attain micromechanical retention between adhesive and substrate.
The application of 2.5% to 10% hydrofluoric acid on the surface for about 2 to 3 minutes seems to be the most effective way to roughen the surface of glass ceramics and prepare it for a strong adhesive bond (Sorensen and colleagues, 1991; Chen, Matsumura, and Atsuta, 1998a, 1998b) (Figure 8-11, A). After acid etching the etching gel should always be rinsed completely with water (Figure 8-11, B). The residue of ceramic salts derived from hydrofluoric acid conditioning can be noted on the surface of the restoration and must be removed, because these salts can adversely affect adhesion. Etching with orthophosphoric acid or solvents such as acetone or alcohol can be used for this purpose. Usually the next step is immersion in pure, clear alcohol in an ultrasonic cleaner for a few minutes (2 to 4 minutes).
Figure 8-11 A, Glass-ceramic veneers during etching with hydrofluoric acid, which creates a rough surface that allows better micromechanical adhesion of cement. B, Acid is then washed away with plenty of water, but ceramic salts might leave a residue on the surface; the latter will be removed by immersion of the veneers in pure alcohol and ultrasonic vibration for a few minutes.
Another type of conditioning is blasting with aluminum oxide particles (average grain particle 50 microns). However, according to several studies that examined repair after chipping, this treatment has not proven to be as effective when used alone without etching (Lacy and colleagues, 1988). Thorough sandblasting of the ceramic surface can often lead to an excessive loss of material and cause fractures (Calamia, 1985; Kern and Thompson, 1994). Therefore sandblasting is not advisable for metal-free restorations consisting of a fragile material with a low resistance to fracture, such as glass ceramic. A study (Kato, Matsumura, and Atsuta, 2000) that compared sandblasting with different types of etching concluded that hydrofluoric acid provided the best and most durable results in terms of adhesive bonding.
Pretreatment with hydrofluoric acid (which is the treatment recommended by the authors) is followed by chemical conditioning with the application of silane (Figure 8-12, A), which predisposes to covalent bonds, and hydrogen (Horn, 1983; Bailey, 1989).
Figure 8-12 A, Glass-ceramic veneers after etching and alcohol immersion, during application of the silane that will be blown with an air jet. B, The last step before cement placement is the application of an adhesive resin or a bonding agent that will be cured later during cement polymerization.
Silane is considered the factor that most contributes to creating adequate adhesion between resin cements and glass ceramics (Paffenbarger, Sweeney, and Bowen, 1967; Horn, 1983; Calamia, 1985; Lacy and colleagues, 1988; Stokes, Hod, and Tidmarsh, 1988; Tjan and Nemetz, 1988; Bailey, 1989; Bertolotti, Lacy, and Watanabe, 1989; Diaz-Arnold and Aquilino, 1989; Diaz-Arnold, Schneider, and Aquilino, 1989; Diaz-Arnold and colleagues, 1993; Matsumura and colleagues, 1989; Cooley, Tseng, and Evans, 1991; Llobell and colleagues, 1992; Pratt and colleagues, 1992; Suliman, Swift, and Perdigao, 1993; Kern and Thompson, 1994; Thurmond, Barkmeier, and Wilwerding, 1994; Aida, Hayakawa, and Mizukawa, 1995; Kupiek and colleagues, 1996; Pameijer, Louw, and Fischer, 1996; Chen, Matsumura, and Atsuta, 1998a, 1998b; Kamada, Yoshida, and Atsuta, 1998; Shahverdi and colleagues, 1998; Braga and colleagues, 1999; Barghi, 2000; Della Bona, Anusavice, and Shen, 2000; Estafan and colleagues, 2000; Frankenberger, Kramer, and Sindel, 2000; Kato, Matsumura, and Atsuta, 2000; Rosentritt and colleagues, 2000; Szep and colleagues, 2000). Moreover, one of the aims of silane is to increase the wettability of ceramics, allowing better contact with the adhesive resin material. One study showed that without pretreatment with hydrofluoric acid, silane does not yield the same results, while the two procedures combined led to a decrease in marginal microleakage (Sorensen and colleagues, 1991).
Two-component silane ensures better stability by avoiding hydrolysis or dissolution of the solvent, as can happen when the silanization agent is in a single, ready-to-use solution. It is advisable to note the appearance of the silane before use. The solution should be clear. If it looks cloudy, it should not be used (Blatz, Sadan, and Kern, 2003b).
The application of silane may be followed by a warming phase in special ovens or using a jet of hot air, which helps maximize the effect. Thirty seconds after the silane has been applied to the surface, it is dried with an air jet.
Before coming in contact with the cement, a light-curing resin bonding agent (see Figure 8-12, B), which can be filled to varying extents, is usually applied to the inner surface of the artifact. It is not generally cured until the restoration has been placed with the luting cement, and the excess material has been removed completely. The adhesive is applied very thinly with an applicator or a small brush and is left to settle for about 30 seconds. Then it is carefully blown with an air jet so that it will be spread only over the inner surface of the restoration.
It is preferable that only light-curing cement be used with glass-ceramic restorations, especially if it is applied thinly, because the applied light has a high power (at least 1000 mW/cm2) and can completely convert the composite material through the ceramic layer. The main advantage of this system is that it permits complete removal of excess cement before polymerization, which will occur only if all the excess material is removed while the material is still in a plastic state (see Figure 8-8). Dual-cure composites not only leave the clinician very little time to remove the material that overflows during cementation, but they can also be too fluid, making/>