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Ceramic Veneers Susana Morimoto, dds, msd, phd Marcelo A. Calamita, dds, msd, phd Christian Coachman, dds, cdt Galip G端rel, dds, msd
Dental ceramics have individual characteristics with regard to structure, properties, manufacturing processes, and, consequently, clinical indications. In view of the increasing number of material options available on the market, the dental practitioner needs to understand some essential concepts and classifications. Extensive information about the composition and structure of porcelain and glass-ceramic materials is presented in other chapters (see, for example, chapters 1, 4, 5, and 9). Therefore, this chapter is focused on the methods of fabrication and clinical aspects of ceramic veneer restorations in order to guide clinicians in their choice of materials, treatment approaches, and clinical procedures.
Materials Selection When the dental treatment plan includes the placement of veneers, the ultimate goal is the fabrication of restorations that provide the most esthetic appearance with minimal reduction of the dental structure. Therefore, this discussion focuses on only two types of ceramic for the fabrication of adhesively bonded veneers: porcelains and glass-ceramics. These ceramics have a balanced vitreous and crystalline structure that results in exceptional esthetics and permits acid etching. This characteristic makes it feasible to effectively bond the veneers to the tooth and to perform more conservative tooth preparations. After bonding, both materials show excellent clinical results.1,2
Methods of fabrication By definition, all ceramics are initially fabricated by means of treatment at extremely high temperatures (casting), when the main chemical reactions occur. This makes the thermal process fundamental in determining the final properties of the ceramic.3–6 The raw components from nature, such as feldspar, kaolin, and quartz, must be subjected to extremely high temperatures, leading to the fusion of these components and thereby creating a new product, labeled ceramic, with a melting point lower than that of the original raw components.5,6 This reduction in melting point facilitates the use of ceramics in dental laboratories by allowing the utilization of simpler techniques.
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Ceramic Veneers Manufacturers make ceramics available to dental laboratories in the form of powder, paste (for retouching color), ingots, and blocks. Therefore, ceramic restorations may be fabricated using the powder/liquid technique, pressed, or machined by the dental laboratory.3,4,7 With powder/liquid ceramics, the vitreous phase undergoes softening (pyroplastic flow), acquiring a more pasty consistency due to the presence of various materials with different melting points. Approximation of the particles (sintering) occurs, but there is no melting or complete union of the components, and there are no important chemical reactions.7,8 The porcelain is modeled during the fabrication of a restoration but undergoes sinterization shrinkage, which is largely related to the loss of water and densification of particles and may generate marginal deficiencies and porosities.7 Nevertheless, technical and technologic improvements will compensate for this misfit, such as using the stratification process, glazing, a vacuum furnace, and the pressable technique. Ceramics fabricated by the powder/liquid technique are porcelains, glass-ceramic (to re-cover a glass-ceramic or metal coping), and some crystalline ceramics. Veneers may be fabricated using the platinum foil technique, built up on a refractory die, or layered on metal or ceramic substructures (copings). With pressable ceramics, melting or fusion—the passage from the solid to liquid state at high temperatures— takes place. The ceramic components undergo a process of complete interaction, frequently forming new chemical elements. With machinable ceramics, porcelain, glass-ceramic, or crystalline ceramic blocks are milled by means of a computerized system (computer-aided design/computer-assisted manufacture [CAD/CAM]) to produce the final veneer restoration. Most CAD/CAM–fabricated veneers are made of glass-ceramic systems, although polycrystalline ceramics have been proposed as a potential material.4–8
Methods to increase ceramic strength The strength of ceramics is a relevant factor for the longevity of veneer restorations, and the resurgence in the use of ceramics in dentistry is largely attributable to improvements in this property. The strength of ceramics is determined by (1) intrinsic factors that are inherent to ceramics and (2) extrinsic factors that are not inherent to the composition of ceramics or the treatments these undergo to increase their own strength. Extrinsic factors are coadjuvant but essential.
Intrinsic strength The introduction of new, more fracture-resistant ceramics and strengthening techniques has improved the final 102
strength of these materials, culminating in the creation of high-strength ceramics that can be used for veneers.3,9–11 Incorporation of metal oxides. The inclusion of metal oxide in a ceramic enhances its hardness and fracture toughness, so that cracks do not propagate between crystals as they do in glass materials. Ceramic strengthening techniques. Ceramics may be strengthened by means of various processes: • Compressive stress generation by means of exchanging smaller ions (eg, sodium ions) for larger ions (eg, potassium or rubidium ions; potassium is approximately 25% larger than sodium) on the surface of feldspathic porcelains. This process, which is called chemical tempering, increases flexural strength by approximately 43%.12,13 • Sequential polishing, which increases strength by approximately 22%.13 • Different rates of cooling contraction between the glaze and internal layers will generate compressive stress, which can result in appreciable strengthening by inhibiting crack growth from the surface through the body of the porcelain.13 • Crystal growth (ceramification) within ceramified glasses. • Improvements in fabrication techniques, such as vacuum furnaces that result in an increase in the final strength of ceramic parts. Machinable and pressable systems have much higher fracture resistance than powder/liquid systems.10 Impurities and porosities have an influence on the translucence and strength of the restoration. Porosities of 10% by volume can diminish the flexural strength by one-half. Defects such as cracks, fissures, and very deep indentations or preparations that have sharp irregularities may initiate fractures, as they act as points for the concentration of stresses. Abrupt alterations in shape, lack of rounded angles, and inadequate thickness of restorations are also points of stress. The concentration of stress at these points could be reduced by simply rounding off irregularities or filling them with a chemical glaze or resin cement.7,10 However, if ceramics are strengthened only by alterations to the surface, the improvements may be diminished or eliminated by wear.3,7,9,11
Extrinsic strength For many years, when the intrinsic strength of veneer ceramics was limited, attempts were made to develop external means of improving the final strength of the veneer restoration. Even today these adjuncts are of great importance.
Clinical Procedures Metal substructure. For many years, adding a metal substructure was the only predictable method to obtain a strong ceramic restoration. Metal-ceramic restorations are considered to offer high mechanical resistance with satisfactory esthetics.6–8,11,14 Bonding. The chemical-mechanical bond obtained with the use of acids, adhesives, and silane has became so strong and stable that it frequently exceeds the intrinsic (cohesive) strength of the ceramic itself as well as that of the tooth. When attempts are made to separate porcelain from resin, fracture of the resin or porcelain occurs, but the bond interface will frequently be maintained. The use of resin cement with insoluble, adhesive characteristics ensures the retention, marginal sealing, and reinforcement of the tooth and restoration; reduces postoperative sensitivity; and contributes to the final esthetic appearance.6,7,14,15 Ceramic, known to be a material with a high modulus of elasticity (rigidity), becomes less friable after the bonding procedures. Adhesive bonding is well suited for laminates, allowing thin ceramic restorations to become extremely resistant to fracture or dislodgment.
Selection parameters For clinicians who are planning to place veneer restorations, this question is fundamental: Which ceramics perform better, specifically, for veneers, and why? It is our opinion that two parameters, with an equal degree of importance, must be considered when a ceramic material is selected for veneers: (1) esthetics and (2) adhesion. Strength of the material can be ranked third in importance for several reasons: • Generally, teeth where veneers are indicated should have adequate dental structure, particularly enamel, to maintain good fracture strength. If this condition cannot be met, complete-coverage restorations might be the preferred treatment alternative. • The tooth-ceramic interface becomes very strong after adhesive cementation. Bonding reinforces the ceramic and restores the strength of the tooth.16 • Ceramics that allow bonding require less tooth reduction. The larger the quantity of enamel preserved, the less the tooth will flex in function and therefore the lower the failure rate (debonding, chipping, fractures, and leakage) of the ceramic veneers and teeth will be.
Clinical Procedures As Gürel has observed previously, “Esthetic dentistry is a delicate combination of scientific principles and artistic
abilities.”14 An understanding of the appropriate choice of material for ceramic veneers is part of the treatment planning. However, the clinician also must properly design a desirable smile.
Preclinical consultation Various predetermining factors play important roles in the evaluation and decision-making process of treatment planning for each case. Where porcelain laminate veneers (PLVs) are planned, many factors should be thoroughly assessed before the actual treatment begins. These details must be carefully analyzed to minimize difficult situations that may arise during the treatment process and to avoid possible postoperative complications or complaints from patients.17 The dentist’s perception of a desirable smile should be considered along with the patient’s personal needs, wishes, and thoughts on his or her appearance.14 It is paramount that patients be fully informed about the possibilities, limits, and prognosis of their particular case.
Indications The following situations are indications for placement of PLVs: • Changes in shape, contour, or position of teeth, such as correction of conical teeth, rotated teeth, and poor positioning as well as closure of diastemas • Esthetic correction of congenital or acquired structural defects, such as extensive enamel caries, multiple restorations, fractures, size discrepancies, imperfect amelogenesis, erosion, abrasion, and abfraction • Color changes for pigmented teeth that do not respond satisfactorily to dental bleaching, such as teeth affected by fluorosis, pigmentation by tetracycline, enamel dysplasia, and iatrogenic discoloration from endodontic treatment • Functional restoration, such reestablishment of contacts on occlusal surfaces and incisal guidance
Limitations or contraindications The following situations are not ideal for placement of PLVs: • Absence of enamel and structural integrity. The presence of tooth enamel assures more effective bonding procedures and provides a rigid substrate ideal for veneers.1,2,12,18–25 • Poor tooth positioning for misalignment. Tooth preparation is often necessary to provide an adequate thickness of ceramic veneers.20,26 Thus, generally speaking, darker, 103
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Ceramic Veneers labially positioned teeth, eroded teeth, extruded teeth, and teeth needing alterations in contour will require less conservative preparations that extend into dentin. This type of preparation compromises the tooth structure and bonding procedures in the long term. • Occlusal concerns. Patients with parafunctional habits and inadequate maxillomandibular relationships are poor candidates for PLVs.
such as the interpapilary, interalar, and intercomissural lines.29,35 • Maxillary gingival plane. The height of gingival contours must be symmetric, continuous in shape, and parallel to the outline of the upper lip, and the interdental papillae must fill the interdental spaces. Gingival surgery, alone or in association with orthodontic traction, may be indicated for the correction of some irregularities.36
Esthetic evaluation
The same sequence must be used for evaluation of the mandibular occlusal plane when this arch requires treatment. After initial evaluation, the photographic record is made, and pretreatment impressions are taken. The diagnostic casts are mounted on an articulator, and the planned alterations are made on the casts by means of diagnostic waxing (Fig 6-1f ). The wax-up is transferred to the mouth using a silicone index, which is esthetically and functionally tested in the mouth. Once approved by the restorative team and the patient, these mock-ups (aesthetic pre-evaluative temporaries [APTs])14 are used as a precise guideline to prepare the tooth structure, based on the intended final tooth contour (Fig 6-1g).
The authors recommend that the esthetic evaluation be performed “from the whole to the parts” or “from the outside in.” The dentist should start the examination by evaluating the patient’s face, observing the balance between the different thirds of the face and noting if there are relevant asymmetries (Figs 6-1a to 6-1c). Furthermore, the clinician should analyze how the horizontal reference lines, such as the interpupillary (Fig 6-1d), interalar, and intercomissural lines, are related to the horizontal plane. This assessment is critical to evaluating the incisal plane and the plane of occlusion. Next, the dentist should locate the midline, since it can interfere with the design of the new restorations.27–29 These analyses are more accurate when performed with the aid of standardized pictures and movies. The use of the Digital Smile Design30,31 facilitates the evaluation process and provides predictability and consistency to the treatment planning process (Fig 6-1e). Analysis must focus on: • The three-dimensional position of the incisal edges of maxillary central incisors in relation to the face and lips. This is the focal point that guides the entire planning process. The position of the incisal edges of the maxillary central incisors must be observed with the patient’s lips at rest, in a posed smile, and in a spontaneous smile. These incisal edges must be positioned in accordance with established esthetic parameters.32,33 However, these parameters are individualized for each patient according to his or her characteristics, desires, and expectations. When reestablishment of the incisal edge of maxillary teeth is indicated, this can be previously tested by means of an immediate mock-up,34 performed directly in the mouth using composite resin, to guide a more precise waxing. • Position of the incisal edges of the maxillary lateral incisors, canines, premolars, and molars (maxillary occlusal plane). The incisal edges of central and lateral incisors and canines and the buccal cusps of premolars and molars should be parallel to the lower lip when this is symmetric. In cases of patients with an asymmetric facial pattern or irregular occlusal plane, the image should be digitally aligned according to horizontal references 104
Tooth preparation Basically, there are two different approaches to tooth preparation for veneers: The traditional approach, which is based on following the contour of the existent tooth structure,20 and APT, which is guided by the planned final contour of the restoration, with the use of indexes and mock-ups.14,37 Meticulous tooth preparation is required to maximize esthetics, improve fracture resistance, optimize laboratory procedures, and maintain soft tissue health for PLVs.38 The use of the APT technique may guide diagnosis, communication with the patient and dental technician, and the tooth preparation to facilitate a predictable final result. Use of controlled depth-cutting burs through the APTs ensures that the tooth structure will undergo only the minimal preparation necessary, perhaps even needing no preparation in certain areas, according to the preestablished goals. Although PLVs can be applied with very little tooth preparation, this type of restoration should not be misconstrued as a simple procedure; in fact, it actually requires great skill. It is beyond the scope of this chapter to provide all the steps associated with correct tooth preparation for veneers, but it is important to highlight some parameters (Fig 6-2): • Each tooth should be properly reduced according to its structural integrity, position in the arch, desired final contour, color, gingival level, and occlusion.
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Fig 6-1  (a to c) Preoperative clinical situation. The patient is not satisfied with her profile, tilted plane of occlusion, tooth alignment, or overall esthetic appearance. (d and e) After orthognathic surgery, the new incisal plane and esthetic design of the restorations are planned with the aid of the Digital Smile Design. (f ) After the two-dimensional information gathered from the Digital Smile Design is transferred to the cast, a three-dimensional representation of the planning is fabricated in wax. (g) The new smile design is tested in vivo. This mock-up will serve as a “test drive� for esthetic and functional parameters, such as the position of the incisal edge of the maxillary central incisor, the plane of occlusion, lip support, and phonetics, as well as a precise guide for tooth preparations. (FIgs 6-1d to 6-1f reprinted from Coachman and Calamita30 with permission.)
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Fig 6-2 (a and b) Teeth are reprepared according to the APT technique. The controlled depth-cutting burs are applied through the mock-up, providing minimally invasive preparations according to preoperative goals. The final color and tooth contours determine the exact amount of tooth reduction to reach the esthetic and functional results desired. Removal of the APTs reveals the minimal amount of tooth structure that has been removed. (Fig 6-2a reprinted from Coachman and Calamita30 with permission.)
• Preparations should be of uniform thickness with rounded angles. • Well-defined, cervical chamfered finish lines should be created to facilitate the technical procedures. • Placement of margins in areas of stress concentration should be avoided. There is no consensus in the literature about whether or not the incisal edge should be included in the preparation. Laboratory studies have evaluated incisal coverage with regard to longevity and failure of veneers. In one study, tooth preparation without incisal overlap (window preparation) showed better results than the preparation with the incisal edge overlapped.39 If incisal coverage is indicated for occlusal or esthetic reasons, in vitro studies have shown that a shoulder with a palatal chamfer margin design increased the fatigue failure cycle count.40 On the other hand, a clinical study demonstrated that the overlapped incisal edge had a significantly positive effect on the survival rate.20 Another study reported that no significant differences could be observed between the outcomes of veneers with incisal porcelain coverage and those designed with an uncovered incisal edge.25
Impression After tooth preparation has been completed, completearch impressions should be taken with polyvinyl siloxane or polyether material or taken digitally. Tissue management is key, and it is recommended that small-diameter retraction cords be used to gently expose the margins. Alginate impressions can be made for the provisional restorations and opposing arch. 106
Bite registration If the patient does not present any sign or symptom related to occlusal or temporomandibular joint disorders, the casts can be mounted in centric occlusion or maximum intercuspation position. In most cases where PLVs are placed to address esthetic concerns, only the maxillary anterior teeth are treated with PLVs; because the tooth reductions are limited to the facial surfaces, almost all the teeth will be in contact when the two stone casts are brought into contact. The casts are then mounted on the articulator with the use of a facebow aligned with a reliable horizontal reference line on the face. In cases of digital impressions, the arch relationship can also be assessed digitally.
Laboratory communication Communication with the dental laboratory professionals is discussed in the section on laboratory procedures (Fig 6-3).
Provisional restoration Depending on the extent of tooth preparation, it may not be necessary to make provisional restorations. Whenever necessary, they can be made directly in the mouth, with the use of the silicone index derived from the diagnostic wax-up and bisacrylic resins, or indirectly on the stone casts in the laboratory, using acrylic or bisacrylic resins. It is important that the provisional restorations have appropriate contours, so that the patient can experience the appearance of the “new smile” and to allow adequate hygiene in the area (see Fig 6-1g).
Clinical Procedures
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Fig 6-3  (a and b) Substrate color and definitive restoration color are required. It is crucial to adequately communicate color to the technician. Good-quality photographs with different shade tabs can improve the quality of communication. Use of different photography angles can also communicate the correct surface texture to the technician. (c to e) Wax-up for the veneers. This wax-up is reduced to provide space for layered veneering ceramics. This kind of bilayered restoration provides superior esthetics, excellent clinical fit, and good strength. (f ) Wax-ups are sprued, invested, and burnt out prior to the ceramic pressing. (g) Ceramic veneers after pressing.
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Clinical Procedures
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Fig 6-3  (cont) (h to p) Sequence of ceramic buildup over the lithium disilicate structure. (q and r) The veneers are 0.2 to 0.3 mm thick. The strength and esthetic properties of the material allow the fabrication of conservative “contact lenses� (IPS e.max, Ivoclar Vivadent). (s to u) Veneers on the cast. Before the restorations are tried in, the clinician should evaluate them to confirm that all the esthetic and functional parameters are in accord with the treatment plan.
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Ceramic Veneers Fig 6-4  During the try-in, the need for some characterization is evaluated. This material permits extrinsic and intrinsic characterization in a very straightforward way.
Try-in After the provisional restorations, cement, and adhesive remnants are removed, the prepared teeth are first cleaned with fine pumice and water. The PLVs must be checked, individually and collectively, to evaluate their fit, contours, interproximal contact points, and color (Fig 6-4). The use of try-in pastes helps to keep the veneers in place and enables the clinician to evaluate the color match. If possible, this tryin is best performed without anesthesia so that the patient retains sensation to feel the relationship of the new veneers to the lips and face.
Cementation After the PLVs are approved, small-diameter retraction cords must be inserted in the gingival sulcus to withdraw the gingival margins, preventing the flow of crevicular fluid in the direction of the margins and preventing the flow of cement to intrasulcular areas.
Preparation of the veneers The intaglio surface of the veneers must be etched in accordance with manufacturer specifications. Stangel et al41 suggested an optimum etching time of 2.5 minutes, using 20% hydrofluoric acid (HF); however, porcelains may be etched with 9% to 10% HF for 60 to 120 seconds, depending on the concentration of the etching liquid and fabrication of the porcelain restoration.14,42 The speed of the HF reaction on the ceramics also depends on the situation of the surface to be etched; glazed surfaces or those fabricated on platinum foil are very smooth (requiring etching for approximately 4 to 5 minutes), whereas surfaces that have been fabricated on a lining and subsequently abraded with airborne particles 110
provide a broader contact area for the action of the acids.5 Leucite- and lithium disilicate–reinforced ceramics should be etched with 10% HF for 60 and 20 seconds, respectively. Porcelain and glass-ceramic have highly retentive surfaces after being etched with HF, favoring resin cementation. After acid etching, the surface is rinsed and dried; however, scanning electron microscopic studies have shown that even after the etched surface is rinsed with copious amounts of water, a large number of acid crystals remain deposited on the etched surface, which may affect the bond strength.5 To eliminate these crystals, the veneers should be put in the ultrasonic cleaner. After ultrasonic cleaning the porosities are exposed, and penetration of adhesive in these porosities facilitates bonding. The sheer bond strength that has been improved from the mean 600 to 3,000 MPa by acid etching can be further increased with the application of a silane coupling agent. The silane coupling agent is the second component of the classic conditioning methods for ceramic restorations and must be applied for approximately 60 seconds.14,43 This coupling agent makes the retention of the bonded ceramictooth interface possible because of its high wettability and chemical contribution to bonding. Once a dry surface is obtained after silanization, the adhesive of choice is applied inside the veneer with the help of a brush.
Preparation of the tooth surface Luting procedures require meticulous attention to every detail. The tooth has to be thoroughly cleansed after the try-in stage, using pumice stone, water, and a Robinson brush kept at low speed. After a transparent strip is placed to protect the adjacent teeth, 37% phosphoric acid is applied to the prepared area for 15 to 20 seconds on dentin and for 20 to 60 seconds on enamel. The tooth is then washed thoroughly and dried
Clinical Procedures
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Fig 6-5  (a to c) Final characterization and surface texture of the high-strength PLVs. Note the positive biologic response of the gingival tissue to the fit of the restorations.
without dehydrating the dentin. If a three-step system has been selected, the primer is applied on the exposed dentin area, left in place for 30 seconds, and gently dried. The bonding agent can be applied to both dentin and enamel, dried, and light polymerized according to manufacturer instructions. The adhesive cement of choice is applied on the internal surface of the previously prepared veneer, the veneer is seated on the tooth, and the restoration is light polymerized for 3 seconds. Gross excess cement is removed, and all the veneers are light polymerized one tooth at a time, on each surface, for 20 seconds. The cementation procedures are addressed in more detail in chapter 9.
Occlusal adjustment, refinishing, and polishing Occlusal adjustment must be carefully performed, because excursive protrusive movements are critical for veneers, particularly when there has been an increase in the incisal edge length. Surface irregularities make the restoration prone to fractures; in addition, roughness accelerates wear of the opposing tooth.5 Veneers designed and fabricated with meticulous attention to detail achieve excellent biologic and esthetic integration in terms of form, texture, and color (Fig 6-5).
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f Fig 6-5 (cont) (d to f ) Final view of the restorations in place. The patient is very satisfied with the results. Incisal edge position, plane of occlusion, tooth morphology, and color were all correctly addressed in a systematic way, surpassing the patient’s expectations.
Laboratory Procedures Communication Close cooperation between the dentist and laboratory is essential to achieve the desired esthetic and functional results. To enable the highest level of communication, both the dentist and the technician should be knowledgeable with regard to current trends and keep the line of communication between them open at all times. In addition, the restorative team of dentist and technician must work together to develop common goals, values, abilities, and
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desires; to this end, the dentist should work to achieve an improved understanding of the indications, limitations, and stages of the laboratory phase, and the technician should have some knowledge of the clinical aspect of dentistry.44 It is paramount to communicate the patient’s preferences or any critical data to the technician in an adequate manner. The use of photographs substantially improves the quality and precision of communication. In this way, the technician can observe “in vivo” all the information needed. At present, with the use of innovative technologies, such as digital images, films, and Digital Smile Design, information can be shared at a distance without significant loss of quality.30,31 All this information may be available digitally to
Evidence-Based Literature Review enable access at any time. Meetings may be scheduled at a distance as well, with the help of technologies such as Skype (Microsoft), Windows Live Messenger (Microsoft), or similar software programs. Esthetic judgment is not an entirely objective process; the dentist must also consider the subjective concerns and the lifestyle of the individual patient when designing a natural smile. The dentist and the dental team must then integrate these criteria, the clinical needs and conditions, and their own personal artistic abilities and subjective feelings to create a smile for the patient. The creativity of the procedure makes each case unique and the dentist’s job pleasingly varied and rewarding.14,45 Thus, to fabricate highly personalized veneers, the technician must have images of the patient’s face, know about the patient’s desires and expectations, and have a visual record of the shade of the tooth preparations as well as the color and texture of the final veneers and the mock-up or provisional restorations approved in the mouth (see Figs 6-3a and 6-3b). There are three laboratory techniques that are most often used for the fabrication of veneers: (1) stratification, (2) pressing and stratification, and (3) machining and stratification.
Monochromatic glass-ceramic ingots are heated to allow the material to flow under pressure into a mold formed by using a conventional lost-wax technique4 (see Figs 6-3c to 6-3f ). This symbiosis between pressed and stratified glass can produce esthetics (see Figs 6-3g to 6-3p) equal to that achieved with the previously described stratification process and has the advantages of better adaptation and strength.46,47 These properties might provide the dentist greater facility in manipulation by the laboratory technician during try-in. These restorations also permit a minimum thickness of 0.2 mm (see Figs 6-3q to 6-3u).
Machining and stratification Machined veneers may be fabricated from various blocks of glass-ceramics (IPS e.max CAD, Ivoclar Vivadent; Authen tic, Ceramay) or porcelain (Vitablocks Mark II, Vident). The blocks are milled to build a substructure that is stratified with compatible porcelains or glass-ceramics. Moreover, the restoration may be characterized by external stains.4 However, the esthetics of monolithic restorations do not equal those achieved by stratification, and the external staining might be removed by wear or toothbrushing over time.
Stratification In the stratification process, powder/liquid porcelain and glass-ceramics are used in the application of sequential layers of ceramics that are taken to the furnace for sinterization, application of a new layer, and finalization with glaze. Ceramic layers can be applied with different degrees of translucence, opacity, and effects (opaque, fluorescent, opalescent, translucent, and high- or low-fusing ceramics, among other characteristics). Stratification makes it possible to fabricate esthetic ceramic restorations that display excellent naturalness. However, this is a technique-sensitive procedure because these restorations are very fragile before cementation. The laboratory technician may work with feldspathic porcelains or ceramified glasses, by means of a paintbrush or powder-liquid technique, on refractory dies or on platinum foil.4
Pressing and stratification In this method of fabricating glass-ceramic restorations, a substructure is constructed by means of the pressable system (ingots); glass-ceramic (powder/liquid) is then layered on this substructure. The restoration may be fabricated with injected ceramic only; however, this type of restoration would have to be superficially stained, and this might not allow as high a degree of esthetics as that achieved with stratification and might be removed by wear and brushing.
Evidence-Based Literature Review Porcelain laminate veneers provide minimally invasive14,24,34,48 esthetic restoration with a high rate of longterm success.18–25 To achieve the best results with these restorations, however, the clinician must understand some of the essential issues involved in their fabrication and apply a clinical protocol that ensures reliable esthetics and longevity. A critical issue for the long-term success of these restorations is the adhesive cementation.49 A stable and longlasting bond does not depend exclusively on the resin cement but rather on an understanding of the complex relationships among three factors at the bond interface. The first factor is the dental substrate. Currently, contemporary adhesive systems have allowed many concepts to be changed, particularly by enabling minimal tooth preparation.15,50,51 The second factor is the ceramic selected. Ceramics that contain a high percentage of vitreous phases, which can be acid etched and silanized, have the best bonding behavior.46,47,52 They also allow more conservative preparations and superior esthetics because of their translucency and color reflectance. Crystalline ceramics do not have a vitreous phase and, therefore, cannot be etched. They are indicated when significant tooth structure is missing, there 113
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Ceramic Veneers is an unfavorable risk of flexure and stress distribution, and it is impossible to obtain and maintain the bond and seal.4 The third factor is the resin cement that is interposed between the ceramic and tooth structure; this factor is interdependent on the other two factors.
Success and failure rates Variations among materials, dentists, technicians, and patients contribute to clinical failures. Thus, clinical studies are important to evaluate the performance of restorative materials and to determine what issues are strongly related to failures, even when certain intraoral conditions cannot be reproduced in the laboratory.22 The problems that occur during the first year after placement of PLVs are generally related to failure of adhesive cementation; these issues appear to occur most frequently in the first 6 months and afterward decline or stabilize to low rates.53 These bond failures may have an influence on marginal staining, leakage, and ceramic fractures, because incomplete impregnation or polymerization of the adhesive/cement may accelerate the process of hydrolysis in the short term.49 Over time, failures may be more related to fatigue at the bond interface or crack propagation within the ceramics, resulting from masticatory forces, dissolution of the resin matrix in the oral medium, or development of gaps due to hydrolysis of the bonds between the components of the ceramics.18,49,54 Longitudinal evaluations of porcelain veneers have shown excellent results over a period of 5 to 12 years, showing success rates ranging between 85% and 98%.18–22,24,25 In the longest follow-up, which evaluated 3,500 porcelain veneers for 15 years, Fradeani et al22 found a failure rate of only 7%; two-thirds of these were fractures (22%) or leakage and debonding (11%). In a different study,1 580 porcelain laminate veneers were cemented in 66 patients and followed up for a period of 12 years. Data revealed that 42 laminate veneers (7.2%) failed in 23 patients. Of these failed veneers, 20 (48%) fractured, 12 (29%) debonded, 7 (17%) showed microleakage, and the other 3 (7%) presented secondary caries, sensitivity, or indications for root canal treatment. Clinical follow-up of ceramic veneers for long periods has been carried out to obtain more reliable data on the longevity of the ceramic-silane–resin cement–adhesive-tooth interface, and the results have been considered encouraging.48 Etched porcelain veneers offer a predictable and safe treatment modality that preserves a maximum amount of sound tooth structure,21 and they have proved to be one of the most successful treatment modalities that modern dentistry has to offer.55
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Factors affecting the bond interface Dental substrate Several clinical factors may interfere with the success of restorations. However, variations in tooth preparation may explain many of these differences.20 Traditional approaches to veneer preparation can lead to major dentin exposure since the recommended preparation thickness values are frequently close to the average measurements of enamel thickness.37 Tooth preparation should always be as minimal as possible because this will preserve a larger amount of remaining enamel,1,2 providing greater strength, as flexion of the tooth may be related to fractures and debonding.33 Nevertheless, some cases make procedures less straightforward depending on the color of the substrate, type of substrate, position of the tooth, and contour required. For example, teeth with color alterations may demand more tooth reduction, often in dentin, to mask their color. Teeth affected by attrition, erosion, and abrasion may have only a thin layer of enamel, as they have lost some of their original volume and may not permit the entire preparation to be made in enamel. Enamel preservation can still be achieved with bonded porcelain veneer restorations.23,25,34,37,49 Although some studies20 have found no differences in the success rates of veneers placed on teeth with dentin exposure and those with preparations completely confined to enamel, others21,23,49 have emphasized that there is an increased risk of failure when veneers are bonded to large amounts of exposed dentin or on an existing restoration. Nevertheless, more conservative preparations undoubtedly help to preserve tooth vitality and reduce postoperative sensitivity.26 Previous research56 has reported fractures of veneers bonded on teeth with large composite resin restorations. The presence of composite resin restorations had a negative influence on the overall clinical performance but did not increase the loss of veneers in that study. This finding is in agreement with that of Gürel et al,2 who observed failure rates of 10.6% for veneers in teeth with restorations compared with failure rates of 6.6% in teeth without composite resin. Deeper preparation into dentin and a substrate with a lower modulus of elasticity than porcelain result in a less rigid base for restoration placement than enamel. This approach has resulted in higher fracture rates than are found for other previous enamel-supported restorations.1,2,21,57 The residual dentin thickness after preparation may, therefore, influence the life expectancy of the restoration.26,32,55
Evidence-Based Literature Review Several reports have demonstrated that fractures may occur if the surface of the tooth has not been prepared sufficiently to create space for the PLV buildup.1,2,55 On the other hand, deep preparations that expose dentin will increase the risk of microleakage and adhesive fractures.1,2,20,21 By taking into consideration the final volume of the restoration after approval of an additive mock-up, the APT technique14 optimizes the tooth preparation, allowing a higher percentage of dental preparations to be completely confined to enamel (80.5%), thus maintaining the rigidity of the structure, reducing postcementation sensitivity, improving support of the ceramic restoration, and avoiding endodontic intervention.24,40 Without this guide, the dentist resorts to freehand preparation, often exposing dentin20,24 and having difficulties in keeping a uniform thickness. When PLVs are executed according to the previously mentioned optimum guidelines, the least common problems associated with PLVs are marginal discoloration and loss of color stability because all margins are in areas in which hygiene is easy to maintain, the porcelain is often easily finished and polished, and its glazed surface is mostly impervious to extrinsic staining.55 Supragingival preparations also had a positive effect on the survival rate of porcelain veneers.20 In this study, marginal adaptation was considered good or very good (100%), and there was minimal microleakage (1.2%), probably because the preparations were situated at the gingival level, which facilitated impressions and cleaning of the margins. These factors may also have added to the low rate of gingival recession. No gingival recession was observed in 85.7% of PLVs.1
Ceramic material High-strength ceramics of medium and high resistance, such as lithium disilicate glass-ceramics, also provide bonding and esthetics and are an excellent choice at present. However, because the crystalline ceramics do not allow bonding to tooth structures, they may require more aggressive preparation approaches; this does not seem, to the authors, to be in keeping with the main philosophy regarding indications for a veneer. Recently, preparations for veneers have taken increasingly conservative lines, with the fabrication of “contact lenses,” partial veneers, or fragments. The authors believe this to be the better path: stronger ceramics that allow bonding and provide an extremely high degree of esthetics.
Resin cement Clinical follow-up comparing self-etch and total-etch adhesive systems in PLVs showed a similar behavior over the period of 5 years, but a phosphoric acid agent was applied to enamel in all the samples.19 Bonding at the level of dentin is based on a mechanical and chemical bond and has reached bond strength values of approximately 14 to 27 MPa with the use of fourthgeneration adhesives. The bond to enamel fundamentally is a more stable mechanical bond, and its value reportedly ranges from 18 to 31 MPa.58 Examination of contemporary adhesives revealed that the three-step etch-and-rinse adhesives remain the gold standard in terms of durability.59 Because the authors approach envisages interlocking adhesive with enamel, the use of total-etch adhesives is highly recommended. The polymerization and the hardness values of cements are the result of the nature of the cement (dual-, chemical-, or light-polymerizable types), the thickness and opacity of the porcelain, light intensity, exposure time, and distance between the source and restoration.60 A light-polymerized luting composite resin is preferred for cementation of ceramic veneers, because it provides longer working time and greater color stability than dual-cured or chemically polymerized systems; nevertheless, any eventual staining appears to be related to incorrect cement polymerization, when free camphorquinone remains, rather than to the nature of the cement (dual, chemical, or light polymerizable type).37,61,62 Although the usual ceramic thickness for the PLV of 0.5 to 1.0 mm has no significant influence on the hardness of a light-polymerized composite, the use of a dual-cure composite might be preferable in certain cases, especially in ceramics with the use of more opaque substructures.14 The use of a resin cement with adhesive features and insolubility not only ensures retention of the restoration but also contributes to marginal sealing, strengthening of the tooth and restoration, a reduction in postoperative sensitivity, and final esthetics.
Complications The occurrence of complications, such as secondary caries and periodontal disease, has not been reported in many studies,19,57 but it could become a significant factor depending on the patient’s hygiene.24 Despite major advances in materials and techniques, some other clinical factors may be responsible for failures. Occlusal factors and concerns related to the tooth-cementceramic interface have been the most frequently mentioned in the literature.1,2,18–20,22,24 115
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Ceramic Veneers
Conclusion Properly diagnosed, planned, and executed PLV applications have always been satisfactory restorations. When the esthetic perception of the dentist is enhanced by up-todate knowledge and then combined with the artistic skill and devotion of the dental technician, creation of a naturallooking smile is routinely achieved. When details such as anterior-posterior color gradation, translucent areas of the incisors, higher chroma of the gingival third, surface texture, and luster are meticulously applied, the PLVs will be undetectable from the other natural teeth.
Acknowledgments The authors would like to thank Marcos Pitta, DDS, MSC, for the excellent results obtained with the orthognathic surgery and Adriano Shayder, CDT, for the high-quality laboratory work.
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