2.1 Introduction
The resin cements are the newest type of cements for indirect restorations, and they have the ability to bond to the tooth structure and the internal surface of the restora- tion. Resin cements are composed of the same basic component as the composite restorative material but with lower concentration of filler particles (Simon and Darnell 2012). These cements have higher compressive, flexural, and tensile strength than the conventional cements and can be used for almost any type of res- toration and restoration material. These cements however are more complex than the conventional cements and are highly technique sensitive.
The resin cements are the newest type of cements for indirect restorations, and they have the ability to bond to the tooth structure and the internal surface of the restora- tion. Resin cements are composed of the same basic component as the composite restorative material but with lower concentration of filler particles (Simon and Darnell 2012). These cements have higher compressive, flexural, and tensile strength than the conventional cements and can be used for almost any type of res- toration and restoration material. These cements however are more complex than the conventional cements and are highly technique sensitive.
To maximize the properties of resin cements, a clear understanding of the fac- tors that affect its clinical performance is of paramount importance. These factors are interrelated. The most important factor affecting the success of resin cements is the bond strength of the resin cement. Bond strength in turn is affected by pre- treatment procedures, the depth of cure and degree of polymerization of the resin cement, and incompatibilities between the adhesive resin and the resin cement. Factors that may affect polymerization include cement film thickness, opacity, and translucency of both the cement and restoration and shade of the restoration. A properly cured resin cement will exhibit high compressive and flexural strengths, properties that enhance bond strength. Properly cured resin cements are also vir- tually insoluble to oral fluids. The mode of delivery and method of mixing the resin cement are also factors that may affect the overall clinical performance of the resin cement.
Understanding how all these factors are interrelated will minimize errors and enhance the longevity of bonded indirect restorations. This is intelligent cementation.
2.2 Pretreatments Prior to the Cementation Procedure
The resin cement bonds the underlying tooth structure to the internal surface of the restoration. Regardless of the type of resin cement, a bond should exist between the dentin and the cement (tooth-cement interface) and between the cement and the internal surface of the restoration (cement-restoration interface) (Fig. 2.1). For these bonds to form, the tooth and the internal surface of the resto- ration should be pretreated.
2.2.1 Pretreatment of Tooth Structure
Resin cements mainly adhere to the tooth structure through micromechanical reten- tion. To achieve this micromechanical retention, the usual adhesive steps of etching, priming, and bonding should be performed on the enamel and dentin to form a sta- ble hybrid layer. Most resin cement systems come with their proprietary adhesives to avoid incompatibilities between adhesives and cements. Some cements use etch- and-rinse adhesive systems (etch-and-rinse or total etch resin cements), while other cements use adhesives containing self-etch primers (self-etch resin cements). Newer resin cements, the so-called self-adhesive resin cements, have their monomers and adhesives incorporated in the cement itself eliminating the need for pretreatment procedures. As cements adhere to tooth structure through resin bonding, care should be taken that the bonding substrates are clean and free from fluid contamination.
2.2.2 Pretreatment of the Internal Surface of the Restoration
The cement serves as a bridge between the tooth and the restoration. While tooth bonding procedures ensure that the cement adheres well to the tooth, pretreatment of the internal surface of the restoration ensures that the cement will adhere to the res- toration as well. A good adhesion to the internal surface of the restoration requires
(1) roughening of the internal surface of the restoration to increase the surface area for bonding and (2) increasing the wettability of the cement to the restoration and forming chemical bonds between the ceramic, the fillers, and the cement. Depending on the restoration material, the first procedure is done through air abrasion, sand- blasting, or etching with a hydrofluoric acid (for ceramic and composite restorations) or application of an alloy primer (for restorations with a metal subsurface).
The second procedure is achieved by applying a silanating agent on the etched porcelain or composite. The silane makes the ceramic chemically adhere to the resin cement through covalent and hydrogen bonds (Horn 1983). Silanating the internal surface of indirect composite restorations ensures that the fillers of the composite react and adhere with the resin cement (Calamia and Simonsen 1985).
As restoration pretreatments differ from material to material, knowledge of the different types of tooth-colored materials (composites and ceramics) used in den- tistry can simplify pretreatment procedures for tooth-colored indirect restorations.
2.3 Classification of Dental Ceramics
There are different ways of classifying ceramics or different terms for different types of ceramics. To simplify it, dental ceramics can be classified into two broad groups based on their composition: the silica-based ceramics and the non-silica-based ceramics (Blatz and Kern 2003). Since the physical and mechanical properties of ceramics depend mainly on their composition, silica-based ceramics are also referred to as low- to-moderate-strength ceramics, and non-silica-based ceramics are the high-strength ceramics. Based on their structural component and phases, silica-based ceramics are also called glass-ceramic systems, and non-silica-based ceramics are called polycrys- talline ceramics. The silica-based ceramics are further classified into feldspathic porce- lains, leucite-reinforced ceramics, and lithium disilicate ceramics (Table 2.1).
2.3.1 Pretreatment for Ceramics Based on Their Classification
Dental ceramics, because of their differences in composition and phases, therefore require different pretreatment procedures. Silica-based ceramics will require either etching with hydrofluoric acid or sandblasting and subsequent silanization to improve adhesion to the resin cement.
Hydrofluoric acids (HF) roughen the internal surface of the restoration. They are available in varying concentrations from 2.5 to 10 %, and etching time is usu- ally 2–3 min (Chen et al. 1998). Etching ceramic with hydrofluoric acid renders the surface microscopically porous, increases the surface energy resulting in a microretentive surface (Hussain et al. 1979; Suliman et al. 1993). Care should be taken not to over-etch the porcelain with hydrofluoric acid as it can weaken the bond between the ceramic and resin cement. After HF etching, a white residue sometimes forms on the surface of the porcelain. This white residue is a potential contami- nant and should be removed prior to silane application. Recommended methods of removing this residue include immersing in an ultrasonic cleaner for 5 min, steam cleaning, or using an alcohol solution (Alex 2008).
Silane-coupling agents, or simply silane, ensure a good bond between the hydroxyl groups of the ceramic and the organic portion of the resin cement. They are available in two forms (Manso et al. 2011): (1) pre-hydrolyzed single-bottle solutions or (2) two-bottle solutions. Silanes have a rather short shelf life and, once exceeded, are virtually ineffective and unusable. Clinically, a milky-colored solution indicates that the silane is well past its expiration date and should be dis- carded (Blatz and Kern 2003). This is especially true for the two-bottle systems. Unfortunately, since one-bottle silanes are alcoholic, they stay transparent which makes it difficult to gauge whether they are still usable. Clinicians should strictly respect expiration date and follow manufacturer’s instructions when using silanes.
The silane is applied on the internal ceramic surface and then air-dried. There is no consensus on the duration of silane application as it may range from 5 min to 2 h. The usual application time is between 60 and 90 s (Anagnostopoulos et al. 1993; Martinlinna et al. 2004; Alex 2008). This application forms the so-called interphase layer, which is actually three layers. The outermost layer and the middle layer are hydrolyzable and can adversely affect adhesion of the ceramic to the resin cement. These two layers should be removed. The innermost layer, closest to the internal surface of the restoration, is a monolayer, which is chemically bonded to the silica phase of the ceramic and is actually responsible for adhesion to the resin cement.
The silanated ceramic should appear dull and not shiny. A shiny surface is indic- ative of excessive silane and can affect the bond of the ceramic to the resin cement. The silanated surface is then air-dried preferably with warm air. This method of drying, together with the contaminants during the try-in procedure, usually removes the hydrolyzable outermost and middle layers (Ishida 1985).
One important thing to remember is that a hydrofluoric acid-etched ceramic is very prone to contamination with oral fluids. The laboratory usually does the hydrofluoric acid etching. During try-in, the hydrofluoric acid-etched ceramic restoration can be contaminated with saliva. One suggestion to prevent contamination is to apply the silanating agent immediately after hydrofluoric acid etching and prior to try-in as the silane renders the etched ceramic hydrophobic and thus more resistant to fluid con- tamination. Another advantage of silanating prior to try-in is that the try-in procedure removes the hydrolyzable outermost and middle layers of the silane, rendering the internal surface more conducive to bonding with the resin cement (Manso et al. 2011). Non-silica-based ceramics such as alumina and zirconia have polycrystalline phase and should not be etched as they are highly resistant to chemical attack from HF (Sorensen et al. 1991; Valandro et al. 2005; Ozcan and Vallitu 2003) or silanated as it might destroy the crystalline structure and weaken the material. Other studies find no improvement in adhesion when alumina and zirconia are etched and silanated prior to cementation. This explains why achieving high and durable bond strengths to alumina and zirconia ceramics is difficult.
The preferred pretreatments for alumina or aluminum oxide ceramics include (1) airborne abrasion with 50–110 μm aluminum oxide particles at 2.5 bars, (2) use of an MDP-containing resin cement (Panavia 21, Kuraray, Japan; Single Bond Universal (3 M Espe, Germany)), or (3) silicoating through tribochemical surface treatment (Rocatec, 3 M Espe, Germany) followed by application of a conventional bis-GMA resin cement (Blatz and Kern 2003; Kern et al. 2009; Kitayama et al. 2010; Yun et al. 2010).
Several surface treatments have been studied to improve bonding with zirco- nia ceramics. These include APA (airborne particle abrasion) or wet hand grind- ing and tribochemical silicoating. APA or wet grinding roughens the surface of the zirconia which was thought to improve bonding. However, some studies have shown that grinding or sandblasting may create surface defects and sharp cracks that render the zirconia prone to cracking or fracture during function (Zhang et al. 2004). Tribochemical silicoating was introduced in an attempt to improve bond without compromising the physical and mechanical properties of zirconia (Kern and Thompson 1994; 1995). In tribochemical silicoating, the internal surface of the zirconia is air abraded with aluminum trioxide particles with silica to coat the zirco- nia with silica aluminum. This renders the internal surface of the restoration chemi- cally adhere to the resin cement. Studies done on tribochemical silicoating however showed decreased bond strengths with resin cements during aging and thermocy- cling (Kern and Wegner 1998; Wegner and Kern 2000; Ozcan and Vallitu 2003).
Resin cements and primers containing the acidic monomer 10-MDP are the recommended cements for zirconia ceramics as MDP can chemically bond with zirconia (Tanaka et al. 2008; Oyague et al. 2009). Examples of such cements and primers are Panavia F 2.0, SE Bond, SA Luting Cement (Kuraray, Osaka, Japan) and the newer Scotchbond Universal adhesive (3 M Espe, Germany). Aside from these 10 MDP-containing primers, primers such as Metal/Zirconia Primer (Ivoclar), Z-Primer (Bisco), and AZ Primer (Shofu) which contain phosphoric acid monomers can also be used to promote the adhesion of alumina and zirconia due to chemical bond formation (Kern et al. 2009; Kitayama et al. 2010).
Indirect composite or laboratory composites were developed in an attempt to improve on the physical and mechanical properties of direct composites as well as facilitate carving of adequate proximal contours and contacts and occlusal anat- omy. Indirect composites have microhybrid fillers and are highly filled with less of the organic matrix to minimize polymerization shrinkage (Nandini 2010). This class of composites undergoes secondary curing either by heat polymerization or high-intensity light polymerization. Secondary curing has been found to decrease bonding of the restoration to the resin cement as secondary curing leaves no avail- able monomer for subsequent bonding to resin cements (Kildal and Ruyter 1994). Suggested pretreatments for indirect composites include sandblasting followed by phosphoric acid etching the internal surface of the restoration. The sandblast- ing roughens the internal surface, while phosphoric acid etching cleans the sand- blasted surface of debris (Jivraj et al. 2006). Other authors recommend sandblasting followed by application of a silane (Soares et al. 2005) (Table 2.2).
2.4 Physical and Mechanical Properties of the Resin Cement
The following physical and mechanical properties directly affect the clinical perfor- mance of resin cements (McCabe and Walls 2008):
1. Compressive strength
2. Flexural strength
3. Film thickness
4. Solubility and water sorption
2.4.1 Compressive Strength
Luting cements should have good compressive strength to be able to withstand masticatory forces in the mouth. As resin cements are bonded to both the tooth struc- ture and the restoration, a high compressive strength of the cement also increases the fracture resistance of the restoration, particularly brittle materials such as ceramics. This is particularly true for the first-generation silica-based feldspathic ceramics, which have very low flexural strength (65–120 MPa) (Powers et al. 2013).
2.4.2 Flexural Strength
Flexural strength is that property of a material to withstand bending forces without breaking. In a tooth-cement-restoration assembly, the cement should have adequate flexural strength to be able to transmit the stresses between the tooth and restoration without breaking. This will protect the brittle restorative material. Moreover, the closer the elastic modulus of the cement to that of the dentin, the less will be the stress concentrations at the cement tooth interface and will result to a more durable bond. Resin cements are approximately 20× stronger and 130× tougher in flexure than conventional cements, which make them the material of choice in the cementa- tion of all-ceramic restorations (Chun and White 1999).
2.4.3 Film Thickness and Viscosity
Considerable differences in film thickness occur between resin cements (Varjao et al. 2002). As a rule, luting cements should exhibit low film thickness. A low cement film thickness improves seating of the restoration and decreases marginal discrepancies which in turn will help reduce plaque accumulation, periodontal disease, cement dis- solution, and eventual secondary caries formation. Resin cements have been shown to exhibit a somewhat higher film thickness than conventional cements (Yu et al. 1995). Although resin cements are less soluble in oral fluids, which will compensate for this higher film thickness, a high film thickness can prevent proper seating of the restora- tion. Studies have also shown that increased film thickness can decrease the tensile strength of cast restorations (Scherrer et al. 1994). An increased film thickness of greater than 300 μm has also been shown to cause gradual decrease in fracture strength resulting to cracks (Levine 1989) and lower bond strengths in all-ceramic restorations (Cekic-Nagas et al. 2010). Evidence shows that a lower cement film thickness (less than 50 μm) is more advantageous for all-ceramic restorations (Levine 1989).
According to the American Dental Association (ADA) Specification for luting agents, a film thickness of 25 μm is required for Type I cements and 40 μm for Type II cements. Type I cements because of their low film thickness are recommended for precision restorations such as inlays, while Type II cements are commonly used for fixed partial prostheses. When using cements that fall within the Type II category, a thicker die relief is recommended to compensate for the higher film thickness of the cement (Fusayama et al. 1964).
The film thickness of resin cements can usually be found in the products literature that come with the cement.
2.4.4 Solubility and Water Sorption
Although resin cements are insoluble to oral fluids, being resins, they absorb water. When resin cements absorb water, their flexural strength is decreased (Oysaed and Ruyter 1986). The thicker the cement, the greater will be the decrease in flexural strength (plasticizing effect) which makes the cement unable to dissipate stresses from masticatory function between tooth and restoration. This may result to even- tual fracture of the ceramic. It is thus important that resin cement layers be kept to a thin layer to minimize the plasticizing phenomenon or resin cements (Ferracane et al. 1998).
2.5 Bond Strength
Since the function of the resin cement is to retain the restoration through adhesion, adequate bond strengths to the underlying tooth structure are very important. Resin cements are also classified according to mechanism of adhesion. Different types of resin cements will exhibit different bond strengths to enamel and to dentin. The choice of resin cement greatly depends on the degree of retention needed. The more retention is needed (such as short crowns, preparations with too much taper, etc.); cements with higher bond strengths are better.
2.6 Depth of Cure and Degree of Polymerization
Resin cements can be classified according to mode of polymerization.
2.6.1 Self-Curing Resin Cements
These cements set through chemical reaction and are especially useful in areas that are difficult to reach with light. Examples are metal restorations, porcelain fused to metals, and thick ceramic restorations (Simon and Darnell 2012). These cements contain the tertiary amine benzoyl peroxide that initiates polymerization. The per- oxide molecules are the ones responsible for color shift during aging.
2.6.2 Dual-Cured Resin Cements
These cements cure by both light curing and chemical curing, hence the name “dual.” These types of cements contain both a self-cured initiator (benzoyl perox- ide) and a light-cured initiator (camphoroquinone). The initial set is usually achieved with light curing to quickly seal the gingival margins (Vohra et al. 2013). The self- curing component ensures that the cement will cure underneath restorations that are too thick or too opaque to allow transmission of light through it (Pegoraro et al.2007). Dual-cured resin cements although they can set through chemical reaction alone still require light curing to achieve a high degree of polymerization.
2.6.3 Light-Cured Resin Cements
These cements set exclusively through light polymerization. The most common photoinitiator is camphoroquinone although some cements may contain a different photoinitiator. Because of this, the clinician should be aware of the type of photoini- tiator present in the resin cement as some curing lights may not match the spectrum of absorption of the photoinitiator.
As most, if not all, resin cements have a light curing component, the depth of cure and degree of polymerization is a very important factor to consider. Insufficient polymerization of the resin cement can lead to increased solubility especially at the margins leading to marginal gaps and secondary caries, marginal discoloration, pulpal reactions, and increased fluid absorption which can lead to hygroscopic expansion and changes in color (color shift) due to the unreacted camphoroquinone photoinitiators. Insufficiently polymerized resin cement has decreased hardness, fracture toughness, and wear resistance and can also lead to lower bond strengths (Vohra et al. 2013). Reducing the exposure time for dual-cured and light-cured resin cements to 75 % of that recommended by the manufacturer will likewise increase water sorption (Pearson and Longman 1989).
Several factors affect the depth of cure and degree of polymerization of resin cements (Table 2.3). Factors related to the restoration include restoration thickness, opacity, and shade. Factors related to the resin cement include mode of polymerization (light cured, dual cured), opacity of the cement, film thickness, filler particle size, and filler loading. Factors related to the light source include distance, duration of exposure, light intensity, and wavelength that matches the spectrum of absorption of the cement’s photoinitiator.
2.7 Color Shift
Resins that contain the tertiary amine benzoyl peroxide in self-cured and dual-cured resins tend to darken with time. The photoinitiator camphoroquinone in light-cured cements is more color stable. However, the cement should be sufficiently polymer- ized as unreacted camphoroquinone turns yellow with age. Some resin cements use a different kind of photoinitiator other than the tertiary amines to prevent any form of color shift.
2.8 Chemical Interactions Between the Adhesive Resin and the Cement
Incompatibilities between simplified adhesives and dual-cured resin cements exist, especially for etch-and-rinse single-bottle adhesives and the seventh-generation all- in-one self-etch adhesives. These adhesives are inherently acidic and hydrophilic.
Incompatibilities occur because the oxygen-inhibited layer of the acidic simplified adhesives reacts with the tertiary amine of the dual-cured resin cement creating a so- called acid dissolution zone that does not completely set and eventually result to poor bonding (Sanares et al. 2001) (Fig. 2.2). Also, as these adhesives are hydrophilic, they are still somewhat permeable even after polymerization, which further compro- mises the bond (Tay et al. 2002). This incompatibility becomes more significant when the cement takes harder to cure by light such as in situations where distance from the light is greater, when cementing thick restorations and in the cementation of posts, as it takes more time for the cement to attain complete polymerization allow- ing for more formation of the acid-inhibited zone (Manso et al. 2011).
To avoid adverse interaction between the adhesive system and the dual-cured cement, three-step etch-prime-bond systems or two-step self-etch systems are rec- ommended (Tay et al. 2003; Carvalho et al. 2005). The application of a separate layer of pure hydrophobic bonding resin forms a barrier between the adhesive’s oxygen- inhibited layer and the amine of the resin cement (King et al. 2005). Some resin cements come with a dual-cured activator, which acts as a barrier between the acidic monomer and tertiary amine to prevent incompatibilities. Incompatibilities can also be avoided by using the self-adhesive resin cements instead as they do not require that the tooth be treated first with adhesives prior to cementation (Manso et al. 2011).
Understanding how all these factors are interrelated will minimize errors and enhance the longevity of bonded indirect restorations. This is intelligent cementation.
2.2 Pretreatments Prior to the Cementation Procedure
The resin cement bonds the underlying tooth structure to the internal surface of the restoration. Regardless of the type of resin cement, a bond should exist between the dentin and the cement (tooth-cement interface) and between the cement and the internal surface of the restoration (cement-restoration interface) (Fig. 2.1). For these bonds to form, the tooth and the internal surface of the resto- ration should be pretreated.
2.2.1 Pretreatment of Tooth Structure
Resin cements mainly adhere to the tooth structure through micromechanical reten- tion. To achieve this micromechanical retention, the usual adhesive steps of etching, priming, and bonding should be performed on the enamel and dentin to form a sta- ble hybrid layer. Most resin cement systems come with their proprietary adhesives to avoid incompatibilities between adhesives and cements. Some cements use etch- and-rinse adhesive systems (etch-and-rinse or total etch resin cements), while other cements use adhesives containing self-etch primers (self-etch resin cements). Newer resin cements, the so-called self-adhesive resin cements, have their monomers and adhesives incorporated in the cement itself eliminating the need for pretreatment procedures. As cements adhere to tooth structure through resin bonding, care should be taken that the bonding substrates are clean and free from fluid contamination.
2.2.2 Pretreatment of the Internal Surface of the Restoration
The cement serves as a bridge between the tooth and the restoration. While tooth bonding procedures ensure that the cement adheres well to the tooth, pretreatment of the internal surface of the restoration ensures that the cement will adhere to the res- toration as well. A good adhesion to the internal surface of the restoration requires
(1) roughening of the internal surface of the restoration to increase the surface area for bonding and (2) increasing the wettability of the cement to the restoration and forming chemical bonds between the ceramic, the fillers, and the cement. Depending on the restoration material, the first procedure is done through air abrasion, sand- blasting, or etching with a hydrofluoric acid (for ceramic and composite restorations) or application of an alloy primer (for restorations with a metal subsurface).
The second procedure is achieved by applying a silanating agent on the etched porcelain or composite. The silane makes the ceramic chemically adhere to the resin cement through covalent and hydrogen bonds (Horn 1983). Silanating the internal surface of indirect composite restorations ensures that the fillers of the composite react and adhere with the resin cement (Calamia and Simonsen 1985).
As restoration pretreatments differ from material to material, knowledge of the different types of tooth-colored materials (composites and ceramics) used in den- tistry can simplify pretreatment procedures for tooth-colored indirect restorations.
2.3 Classification of Dental Ceramics
There are different ways of classifying ceramics or different terms for different types of ceramics. To simplify it, dental ceramics can be classified into two broad groups based on their composition: the silica-based ceramics and the non-silica-based ceramics (Blatz and Kern 2003). Since the physical and mechanical properties of ceramics depend mainly on their composition, silica-based ceramics are also referred to as low- to-moderate-strength ceramics, and non-silica-based ceramics are the high-strength ceramics. Based on their structural component and phases, silica-based ceramics are also called glass-ceramic systems, and non-silica-based ceramics are called polycrys- talline ceramics. The silica-based ceramics are further classified into feldspathic porce- lains, leucite-reinforced ceramics, and lithium disilicate ceramics (Table 2.1).
2.3.1 Pretreatment for Ceramics Based on Their Classification
Dental ceramics, because of their differences in composition and phases, therefore require different pretreatment procedures. Silica-based ceramics will require either etching with hydrofluoric acid or sandblasting and subsequent silanization to improve adhesion to the resin cement.
Hydrofluoric acids (HF) roughen the internal surface of the restoration. They are available in varying concentrations from 2.5 to 10 %, and etching time is usu- ally 2–3 min (Chen et al. 1998). Etching ceramic with hydrofluoric acid renders the surface microscopically porous, increases the surface energy resulting in a microretentive surface (Hussain et al. 1979; Suliman et al. 1993). Care should be taken not to over-etch the porcelain with hydrofluoric acid as it can weaken the bond between the ceramic and resin cement. After HF etching, a white residue sometimes forms on the surface of the porcelain. This white residue is a potential contami- nant and should be removed prior to silane application. Recommended methods of removing this residue include immersing in an ultrasonic cleaner for 5 min, steam cleaning, or using an alcohol solution (Alex 2008).
Silane-coupling agents, or simply silane, ensure a good bond between the hydroxyl groups of the ceramic and the organic portion of the resin cement. They are available in two forms (Manso et al. 2011): (1) pre-hydrolyzed single-bottle solutions or (2) two-bottle solutions. Silanes have a rather short shelf life and, once exceeded, are virtually ineffective and unusable. Clinically, a milky-colored solution indicates that the silane is well past its expiration date and should be dis- carded (Blatz and Kern 2003). This is especially true for the two-bottle systems. Unfortunately, since one-bottle silanes are alcoholic, they stay transparent which makes it difficult to gauge whether they are still usable. Clinicians should strictly respect expiration date and follow manufacturer’s instructions when using silanes.
The silane is applied on the internal ceramic surface and then air-dried. There is no consensus on the duration of silane application as it may range from 5 min to 2 h. The usual application time is between 60 and 90 s (Anagnostopoulos et al. 1993; Martinlinna et al. 2004; Alex 2008). This application forms the so-called interphase layer, which is actually three layers. The outermost layer and the middle layer are hydrolyzable and can adversely affect adhesion of the ceramic to the resin cement. These two layers should be removed. The innermost layer, closest to the internal surface of the restoration, is a monolayer, which is chemically bonded to the silica phase of the ceramic and is actually responsible for adhesion to the resin cement.
The silanated ceramic should appear dull and not shiny. A shiny surface is indic- ative of excessive silane and can affect the bond of the ceramic to the resin cement. The silanated surface is then air-dried preferably with warm air. This method of drying, together with the contaminants during the try-in procedure, usually removes the hydrolyzable outermost and middle layers (Ishida 1985).
One important thing to remember is that a hydrofluoric acid-etched ceramic is very prone to contamination with oral fluids. The laboratory usually does the hydrofluoric acid etching. During try-in, the hydrofluoric acid-etched ceramic restoration can be contaminated with saliva. One suggestion to prevent contamination is to apply the silanating agent immediately after hydrofluoric acid etching and prior to try-in as the silane renders the etched ceramic hydrophobic and thus more resistant to fluid con- tamination. Another advantage of silanating prior to try-in is that the try-in procedure removes the hydrolyzable outermost and middle layers of the silane, rendering the internal surface more conducive to bonding with the resin cement (Manso et al. 2011). Non-silica-based ceramics such as alumina and zirconia have polycrystalline phase and should not be etched as they are highly resistant to chemical attack from HF (Sorensen et al. 1991; Valandro et al. 2005; Ozcan and Vallitu 2003) or silanated as it might destroy the crystalline structure and weaken the material. Other studies find no improvement in adhesion when alumina and zirconia are etched and silanated prior to cementation. This explains why achieving high and durable bond strengths to alumina and zirconia ceramics is difficult.
The preferred pretreatments for alumina or aluminum oxide ceramics include (1) airborne abrasion with 50–110 μm aluminum oxide particles at 2.5 bars, (2) use of an MDP-containing resin cement (Panavia 21, Kuraray, Japan; Single Bond Universal (3 M Espe, Germany)), or (3) silicoating through tribochemical surface treatment (Rocatec, 3 M Espe, Germany) followed by application of a conventional bis-GMA resin cement (Blatz and Kern 2003; Kern et al. 2009; Kitayama et al. 2010; Yun et al. 2010).
Several surface treatments have been studied to improve bonding with zirco- nia ceramics. These include APA (airborne particle abrasion) or wet hand grind- ing and tribochemical silicoating. APA or wet grinding roughens the surface of the zirconia which was thought to improve bonding. However, some studies have shown that grinding or sandblasting may create surface defects and sharp cracks that render the zirconia prone to cracking or fracture during function (Zhang et al. 2004). Tribochemical silicoating was introduced in an attempt to improve bond without compromising the physical and mechanical properties of zirconia (Kern and Thompson 1994; 1995). In tribochemical silicoating, the internal surface of the zirconia is air abraded with aluminum trioxide particles with silica to coat the zirco- nia with silica aluminum. This renders the internal surface of the restoration chemi- cally adhere to the resin cement. Studies done on tribochemical silicoating however showed decreased bond strengths with resin cements during aging and thermocy- cling (Kern and Wegner 1998; Wegner and Kern 2000; Ozcan and Vallitu 2003).
Resin cements and primers containing the acidic monomer 10-MDP are the recommended cements for zirconia ceramics as MDP can chemically bond with zirconia (Tanaka et al. 2008; Oyague et al. 2009). Examples of such cements and primers are Panavia F 2.0, SE Bond, SA Luting Cement (Kuraray, Osaka, Japan) and the newer Scotchbond Universal adhesive (3 M Espe, Germany). Aside from these 10 MDP-containing primers, primers such as Metal/Zirconia Primer (Ivoclar), Z-Primer (Bisco), and AZ Primer (Shofu) which contain phosphoric acid monomers can also be used to promote the adhesion of alumina and zirconia due to chemical bond formation (Kern et al. 2009; Kitayama et al. 2010).
Indirect composite or laboratory composites were developed in an attempt to improve on the physical and mechanical properties of direct composites as well as facilitate carving of adequate proximal contours and contacts and occlusal anat- omy. Indirect composites have microhybrid fillers and are highly filled with less of the organic matrix to minimize polymerization shrinkage (Nandini 2010). This class of composites undergoes secondary curing either by heat polymerization or high-intensity light polymerization. Secondary curing has been found to decrease bonding of the restoration to the resin cement as secondary curing leaves no avail- able monomer for subsequent bonding to resin cements (Kildal and Ruyter 1994). Suggested pretreatments for indirect composites include sandblasting followed by phosphoric acid etching the internal surface of the restoration. The sandblast- ing roughens the internal surface, while phosphoric acid etching cleans the sand- blasted surface of debris (Jivraj et al. 2006). Other authors recommend sandblasting followed by application of a silane (Soares et al. 2005) (Table 2.2).
2.4 Physical and Mechanical Properties of the Resin Cement
The following physical and mechanical properties directly affect the clinical perfor- mance of resin cements (McCabe and Walls 2008):
1. Compressive strength
2. Flexural strength
3. Film thickness
4. Solubility and water sorption
2.4.1 Compressive Strength
Luting cements should have good compressive strength to be able to withstand masticatory forces in the mouth. As resin cements are bonded to both the tooth struc- ture and the restoration, a high compressive strength of the cement also increases the fracture resistance of the restoration, particularly brittle materials such as ceramics. This is particularly true for the first-generation silica-based feldspathic ceramics, which have very low flexural strength (65–120 MPa) (Powers et al. 2013).
2.4.2 Flexural Strength
Flexural strength is that property of a material to withstand bending forces without breaking. In a tooth-cement-restoration assembly, the cement should have adequate flexural strength to be able to transmit the stresses between the tooth and restoration without breaking. This will protect the brittle restorative material. Moreover, the closer the elastic modulus of the cement to that of the dentin, the less will be the stress concentrations at the cement tooth interface and will result to a more durable bond. Resin cements are approximately 20× stronger and 130× tougher in flexure than conventional cements, which make them the material of choice in the cementa- tion of all-ceramic restorations (Chun and White 1999).
2.4.3 Film Thickness and Viscosity
Considerable differences in film thickness occur between resin cements (Varjao et al. 2002). As a rule, luting cements should exhibit low film thickness. A low cement film thickness improves seating of the restoration and decreases marginal discrepancies which in turn will help reduce plaque accumulation, periodontal disease, cement dis- solution, and eventual secondary caries formation. Resin cements have been shown to exhibit a somewhat higher film thickness than conventional cements (Yu et al. 1995). Although resin cements are less soluble in oral fluids, which will compensate for this higher film thickness, a high film thickness can prevent proper seating of the restora- tion. Studies have also shown that increased film thickness can decrease the tensile strength of cast restorations (Scherrer et al. 1994). An increased film thickness of greater than 300 μm has also been shown to cause gradual decrease in fracture strength resulting to cracks (Levine 1989) and lower bond strengths in all-ceramic restorations (Cekic-Nagas et al. 2010). Evidence shows that a lower cement film thickness (less than 50 μm) is more advantageous for all-ceramic restorations (Levine 1989).
According to the American Dental Association (ADA) Specification for luting agents, a film thickness of 25 μm is required for Type I cements and 40 μm for Type II cements. Type I cements because of their low film thickness are recommended for precision restorations such as inlays, while Type II cements are commonly used for fixed partial prostheses. When using cements that fall within the Type II category, a thicker die relief is recommended to compensate for the higher film thickness of the cement (Fusayama et al. 1964).
The film thickness of resin cements can usually be found in the products literature that come with the cement.
2.4.4 Solubility and Water Sorption
Although resin cements are insoluble to oral fluids, being resins, they absorb water. When resin cements absorb water, their flexural strength is decreased (Oysaed and Ruyter 1986). The thicker the cement, the greater will be the decrease in flexural strength (plasticizing effect) which makes the cement unable to dissipate stresses from masticatory function between tooth and restoration. This may result to even- tual fracture of the ceramic. It is thus important that resin cement layers be kept to a thin layer to minimize the plasticizing phenomenon or resin cements (Ferracane et al. 1998).
2.5 Bond Strength
Since the function of the resin cement is to retain the restoration through adhesion, adequate bond strengths to the underlying tooth structure are very important. Resin cements are also classified according to mechanism of adhesion. Different types of resin cements will exhibit different bond strengths to enamel and to dentin. The choice of resin cement greatly depends on the degree of retention needed. The more retention is needed (such as short crowns, preparations with too much taper, etc.); cements with higher bond strengths are better.
2.6 Depth of Cure and Degree of Polymerization
Resin cements can be classified according to mode of polymerization.
2.6.1 Self-Curing Resin Cements
These cements set through chemical reaction and are especially useful in areas that are difficult to reach with light. Examples are metal restorations, porcelain fused to metals, and thick ceramic restorations (Simon and Darnell 2012). These cements contain the tertiary amine benzoyl peroxide that initiates polymerization. The per- oxide molecules are the ones responsible for color shift during aging.
2.6.2 Dual-Cured Resin Cements
These cements cure by both light curing and chemical curing, hence the name “dual.” These types of cements contain both a self-cured initiator (benzoyl perox- ide) and a light-cured initiator (camphoroquinone). The initial set is usually achieved with light curing to quickly seal the gingival margins (Vohra et al. 2013). The self- curing component ensures that the cement will cure underneath restorations that are too thick or too opaque to allow transmission of light through it (Pegoraro et al.2007). Dual-cured resin cements although they can set through chemical reaction alone still require light curing to achieve a high degree of polymerization.
2.6.3 Light-Cured Resin Cements
These cements set exclusively through light polymerization. The most common photoinitiator is camphoroquinone although some cements may contain a different photoinitiator. Because of this, the clinician should be aware of the type of photoini- tiator present in the resin cement as some curing lights may not match the spectrum of absorption of the photoinitiator.
As most, if not all, resin cements have a light curing component, the depth of cure and degree of polymerization is a very important factor to consider. Insufficient polymerization of the resin cement can lead to increased solubility especially at the margins leading to marginal gaps and secondary caries, marginal discoloration, pulpal reactions, and increased fluid absorption which can lead to hygroscopic expansion and changes in color (color shift) due to the unreacted camphoroquinone photoinitiators. Insufficiently polymerized resin cement has decreased hardness, fracture toughness, and wear resistance and can also lead to lower bond strengths (Vohra et al. 2013). Reducing the exposure time for dual-cured and light-cured resin cements to 75 % of that recommended by the manufacturer will likewise increase water sorption (Pearson and Longman 1989).
Several factors affect the depth of cure and degree of polymerization of resin cements (Table 2.3). Factors related to the restoration include restoration thickness, opacity, and shade. Factors related to the resin cement include mode of polymerization (light cured, dual cured), opacity of the cement, film thickness, filler particle size, and filler loading. Factors related to the light source include distance, duration of exposure, light intensity, and wavelength that matches the spectrum of absorption of the cement’s photoinitiator.
2.7 Color Shift
Resins that contain the tertiary amine benzoyl peroxide in self-cured and dual-cured resins tend to darken with time. The photoinitiator camphoroquinone in light-cured cements is more color stable. However, the cement should be sufficiently polymer- ized as unreacted camphoroquinone turns yellow with age. Some resin cements use a different kind of photoinitiator other than the tertiary amines to prevent any form of color shift.
2.8 Chemical Interactions Between the Adhesive Resin and the Cement
Incompatibilities between simplified adhesives and dual-cured resin cements exist, especially for etch-and-rinse single-bottle adhesives and the seventh-generation all- in-one self-etch adhesives. These adhesives are inherently acidic and hydrophilic.
Incompatibilities occur because the oxygen-inhibited layer of the acidic simplified adhesives reacts with the tertiary amine of the dual-cured resin cement creating a so- called acid dissolution zone that does not completely set and eventually result to poor bonding (Sanares et al. 2001) (Fig. 2.2). Also, as these adhesives are hydrophilic, they are still somewhat permeable even after polymerization, which further compro- mises the bond (Tay et al. 2002). This incompatibility becomes more significant when the cement takes harder to cure by light such as in situations where distance from the light is greater, when cementing thick restorations and in the cementation of posts, as it takes more time for the cement to attain complete polymerization allow- ing for more formation of the acid-inhibited zone (Manso et al. 2011).
To avoid adverse interaction between the adhesive system and the dual-cured cement, three-step etch-prime-bond systems or two-step self-etch systems are rec- ommended (Tay et al. 2003; Carvalho et al. 2005). The application of a separate layer of pure hydrophobic bonding resin forms a barrier between the adhesive’s oxygen- inhibited layer and the amine of the resin cement (King et al. 2005). Some resin cements come with a dual-cured activator, which acts as a barrier between the acidic monomer and tertiary amine to prevent incompatibilities. Incompatibilities can also be avoided by using the self-adhesive resin cements instead as they do not require that the tooth be treated first with adhesives prior to cementation (Manso et al. 2011).
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