Pf11cap05

5

Individual Ceramic Crowns for Teeth Estevam A. Bonfante, dds, msc, phd Stefano Gracis, dmd, msd

The use of ceramics in dentistry has been responsible for great advances in the quality and esthetic appearance of restorations delivered to patients worldwide. However, since their earliest applications, such as the complete dentures of the apothecary Alexis Duchateau made by the Parisian dentist Nicholas Dubois de ChĂŠmant (1774), ceramics have faced many challenges.

Their brittle nature has constituted a huge obstacle to the fabrication of complication-free prostheses. It was only in the early 1960s that a porcelain frit with expansion and contraction coefficients matching those of any dental alloy1 was developed, thus paving the way to the more reliable metalceramic restorations. In the past 50 years, as metal-ceramic restorations became the gold standard,2–5 the quest for more esthetic restorations has increased tremendously, fostering the development of all-ceramic materials that could match or outperform the esthetic and mechanical performance hitherto experienced.

This chapter describes the most commonly used high-strength ceramic materials for tooth-supported crown applications from clinical, laboratory, and performance perspectives. The chapter presents the materials’ properties and discusses how fabrication processes and clinical procedures may affect their clinical performance. Fortunately, current evidence is available to assure clinicians regarding the safe use of all-ceramic restorations as well as recent improvements in restoration longevity. The aim of this chapter is to provide a rationale for selecting materials based on the research, clinical, and laboratory aspects of all-ceramic single crowns.

Evidence-Based Criteria for Material Selection Metal-ceramic restorations, regardless of alloy or porcelain type, are indicated for both anterior and posterior areas. Some all-ceramic materials used for single-unit tooth-supported crowns have shown significantly lower survival rates when used in the posterior region. For example, glass-ceramic and glass-infiltrated crowns have shown a 5-year survival rate of 84.4% and 90.4%, respectively.6 Because this chapter focuses mainly on high-strength ceramics, special attention will be devoted to currently available evidence from laboratory and clinical studies.

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5

Individual Ceramic Crowns for Teeth

Classification Many different classifications of the ceramics used in dentistry have been proposed, among them classification according to their indication or use, composition, processing method, firing temperature, microstructure, translucency, and fracture resistance.7–10 Although each one of these classifications could be useful, they may not be straightforward to the clinician for purposes of communication, indication, and handling. Attempts to classify dental ceramics based on composition, for instance, could be useful for historical purposes but, due to continuous advances, are already too broad and impractical.

An often used classification11 describes dental ceramics according to glass content: (1) predominantly glassy materials; (2) particle-filled glasses; and (3) polycrystalline ceramics. In this particular classification, the clinician may be confused by the subjectivity of the amount of glass phase required for the ceramic to be included either as “predominantly glassy” or “particle filled”. Furthermore, it does not recognize the switch that has occurred in the ceramics industry from natural feldspath to synthetic feldspath.

The latter has allowed different batches of each particular ceramic material to be more standardized and of more uniform quality.

Herein, the authors propose to classify all-ceramic materials in only two families, based on their ability to be etched as well as on their indication as a substructure material: (1) the particle-filled glasses and (2) the polycrystalline ceramics. This categorization can help the clinician and the dental technician understand more readily some of the implications of the selection of a particular material in terms of intrinsic strength and need for adhesive cementation, as will be explained later in the chapter.

Ceramic-polymer compound materials are an emerging class of materials that must be acknowledged in this chapter. They are currently classified on the market as ceramics, although a significant percentage of their composition is made up of glass or oxide ceramic particles embedded in a resin matrix. From a strict material science definition of ceramics (ie, nonmetallic inorganic material), their composition would exclude them from the family of ceramics.

However, aside from the controversy of definition, these materials (eg, Vita Enamic [Vident], Lava ultimate [3M ESPE]) represent a very interesting restorative opportunity that combines tailored mechanical and optical properties with an interface that is user friendly for computer-assisted machining as well as clinical adjustment. Because they are still new in the market and there are no data available on their clinical longevity as single crowns, the authors will not elaborate on them in this chapter.

64

The particle-filled glasses encompass three subgroups that are very different in nature: (1) the feldspathic ceramics, (2) the synthetic ceramics, and (3) the glass-infiltrated ceramics. The polycrystalline ceramics include aluminum oxide ceramics and stabilized zirconia ceramics.

Particle-filled glasses Feldspathic ceramics. Dental porcelains commonly used

as a veneering material are based on a ternary materials system comprising clay/kaolin (hydrated aluminosilicate), quartz (silica), and feldspar (potassium and sodium alumino­ silicates). This group of feldspathic ceramics is composed of naturally occurring materials (feldspar), such as mixtures of sodium feldspar (Na2Al2Si6O16) and potassium feldspar (K2Al2Si6O16). The latter forms leucite crystals (crystalline phase), which depending on the amount not only increase the resistance of the restoration but also make this porcelain suitable for veneering on metal substructures; the co­ efficient of thermal expansion is approximately 10% or less below that of the substructure.7,9,10 Examples of feldspathic ceramics include IPS Empress Esthetic and IPS Classic (Ivoclar Vivadent) and Vita VMK Master (Vident).

Synthetic ceramics. To become less dependent on natu-

ral resources as raw materials, such as feldspar, the ceramic industry has used synthetic-based materials. The compositions vary among manufacturers but commonly include base materials such as silicon dioxide (SiO2), potassium oxide (K2O), sodium oxide (NaO), and aluminum oxide (Al2O3).

Their crystal phases may be enhanced with apatite crystals in addition to leucite for thermal expansion compatibility with metals and for improved strength. When applied to allceramic frameworks, some groups of synthetic porcelains must match the coefficient of thermal expansion of their respective frameworks (eg, Cerabien [Noritake] or Vita VM 7 [Vident] for polycrystalline alumina and glass-infiltrated ceramics and Vita VM 9 [Vident] for polycrystalline zirconia).

For enhanced mechanical properties and use as substruc­ ture material, manufacturers have introduced feldspathic porcelains composed of 63% SiO2, 17% Al2O3, 11.2% K2O, 4.6% Na2O, 1.6% cerium oxide (CeO2), and less than 1% boron trioxide (B2O3), calcium oxide (CaO), barium oxide (BaO), and titanium dioxide (TiO2), such as IPS Empress (Ivoclar Vivadent). Another composition (IPS Empress 2, Ivoclar Viva­ dent) contains approximately 70% lithium disilicate (57% to 80% SiO2; 0% to 5% Al2O3; 0.1 to 6% lanthanum oxide [La2O3]; 0% to 5% magnesium oxide [MgO]; 0% to 8% zinc oxide [ZnO]; 0% to 13% K2O; 11% to 19% lithium oxide [Li2O]; and 0% to 11% phosphorus pentoxide [P2O5]). A further devel-

Evidence-Based Criteria for Material Selection opment within the lithium disilicate system adds zirconium dioxide (ZrO2) to improve mechanical properties; IPS e.max (SiO2-Li2O-K2O-ZnO-P2O5-Al2O3-ZrO2; Ivoclar Vivadent) has been introduced for use as inlays, onlays, crowns, and three-unit fixed dental prostheses in the anterior or pre­ molar region.

Glass-infiltrated ceramics. In-Ceram Alumina (Vident),

the first glass-infiltrated material, introduced in 1989, is fabricated with the slip-casting technique. A slurry of densely packed Al2O3 is sintered to a refractory die; after a porous skeleton of alumina particles is formed, infiltration with lanthanum glass is performed in a second firing to eliminate porosity and increase strength. Three different particle sizes of alumina are observed, including large elongated grains (10.0 to 12.0 µm long and 2.5 to 4.0 µm wide), faceted particles (1 to 4 µm diameter), and spherical grains of less than 1.0 µm in diameter. Because of the opacity of In-Ceram Alumina, subsequent layering with a porcelain veneer is mandatory. The composition, according to the manufacturer, is Al2O3 (82%), La2O3 (12%), SiO2 (4.5%), CaO (0.8%), and other oxides (0.7%).

In-Ceram Spinell (Vident), introduced in 1994, is processed similarly, where the glass is infiltrated in a synthetically produced MgAl2O4 porous core.

In-Ceram Zirconia (Vident) is a modification of In-Ceram Alumina in which partially stabilized zirconium oxide is added to the slip composition of the Al2O3 to strengthen the ceramic. According to the manufacturer, its composition is Al2O3 (62%), ZnO (20%), La2O3 (12%), SiO2 (4.5%), CaO (0.8%), and other oxides (0.7%) Applications. Given the presence of glass in the family

of ceramics described in this section, all of these materials may be conventionally etched with hydrofluoric acid and silanized, irrespective of their use as a veneer, substructure, or monolithic restoration. Regarding their use as a substructure material, a direct relationship can be observed between improved mechanical properties and an increased amount of fillers (and consequently a reduced glass phase).

Clinical evidence of their performance and indications for single crowns in the anterior and posterior regions have been discussed previously.6

Polycrystalline ceramics Aluminum oxide ceramics. These ceramics are com-

posed of high-purity aluminum oxide (99.5% Al2O3). Examples of aluminum oxide ceramics include Procera AllCeram (Nobel Biocare) and In-Ceram AL (Vident).

Stabilized zirconia ceramics. Pristine zirconia is found

in three allotropic forms: (1) a monoclinic form, which is stable up to 1,170°C, where it transforms to (2) a tetragonal form, and (3) a cubic form, when temperatures are between 2,370°C and 2,680°C.12,13 The monoclinic to tetragonal transformation is accompanied by shear strain and an increase in volume. To improve toughness, pure zirconia must be alloyed with oxides such as yttrium, magnesium, calcium, and cerium that will fully or partially stabilize either the tetragonal or cubic phase at room temperature.14 A classification of zirconia ceramics has been proposed according to their microstructure: fully stabilized zirconia (FSZ), partially stabilized zirconia (PSZ), and tetragonal zirconia polycrystals (TZP).15 In FSZ, zirconia is in its cubic form and more than 8 mol% yttrium oxide. PSZ is formed by nanosized tetragonal or monoclinic particles in a cubic matrix, and TZPs are monoliths mainly of tetragonal phase, stabilized most commonly with yttria or ceria.15 Examples of stabilized zirconia-based ceramics include NobelProcera Zirconia (Nobel Biocare), Lava (3M ESPE), In-Ceram YZ (Vident), DC-Zirkon (DCS Dental), Cercon (Dentsply), Prettau Zirconia (Zirkonzahn), IPS e.max ZirCAD (Ivoclar Vivadent).

Translucency Generally speaking, in progressing from the feldspathic ceramics to glass-infiltrated ceramics to polycrystalline ceramics, translucency decreases and intrinsic strength increases; however, there are exceptions among the individual products, and novel developments such as transparent nanocrystalline zirconia challenge this rather simplistic concept.16 In an in vitro study in which the translucency of six all-ceramic system core materials of a clinically appropriate thickness (0.5 and 0.8 mm) was compared,17 it was found that Procera Alumina (Nobel Biocare) is as translucent as IPS Empress and IPS Empress 2, a leucite-based and a disilicate-based particle-filled glass, respectively, and more translucent than In-Ceram Alumina and In-Ceram Zirconia.

In-Ceram Zirconia was found to be as opaque as the metal control. The analysis was then repeated after the specimens were veneered with a layer of porcelain,18 and it was found that their opacity had increased. It is important, however, to point out that the differences noted under transmitted light might not be as significant under reflected light, which is the condition in which these materials would be observed clinically.

Whereas the feldspathic materials are most commonly used as a veneer for ceramic frameworks or as stand-alone esthetic veneers, the other glass-ceramics (mainly lithium disilicate) and the polycrystalline ceramics can be used as

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Individual Ceramic Crowns for Teeth TABLE 5-1 Mean maximum voluntary biting forces in the molar area in the permanent dentition of adults Unilateral force (N/cm) Study

Male or all

Female

Male or all

Female

No. of Subjects (M/F or all)

Ahlberg et al

878

690

—

—

196/188

24

Bakke et al

480

—

694

—

19

Bakke et al25

522

441

—

—

59/63

—

—

814

615

86/56

Ferrario et al

306

234

—

—

36/16

Gibbs et al

—

—

725

—

20

Hagberg29

—

395

—

—

9

Haraldson et al30

383

—

—

—

10

Helkimo et al

444

357

—

—

28/16

Ikebe et al

—

—

512

442

444/376

Miyaura et al33

—

—

491

—

590

Shinogaya et al34

553

—

1,110

—

17

Thompson et al

520

—

—

—

13

Tortopidis et al

429

—

579

—

8

van der Bilt et al37

490

418

652

553

13/68

Waltimo and Könönen38

847

597

—

—

22/24

23

Braun et al26 27

28

31

32

35

36

substructures or as full-contour crowns and therefore will be considered high-strength ceramics.

Occlusal forces Because material properties are commonly presented as predictors of clinical performance, it seems crucial to describe the range of loads to which ceramic restorations are subjected in the oral cavity. During jaw closure, two phases can be observed in the early chewing stages prior to swallowing: (1) fast closure occurs before the teeth come in contact with the food bolus, and (2) slow closure takes place when the teeth meet resistance by the food, which will increase masticatory muscle activity depending on the consistency and texture of the bolus.19,20 Although maximum voluntary bite force varies depending on sex,

66

Bilateral force (N/cm)

age, location in the mouth (anterior versus posterior), type of prosthesis, and opposing dentition,21 its value becomes a relevant parameter to predict worst-case loading scenarios to which all-ceramic restorations can be subjected.

Given that fracture rates of prosthetic reconstructions increase from the anterior to the posterior region (3% for anterior teeth, 7% for premolars, and 21% for molars),22 Table 5-1 lists maximum voluntary bite forces at the molar region, as reported in the literature. In the following sections, the mechanical properties of different high-strength all-ceramic materials are based on their performance in the molar region. Several studies are cited because of the large variability between values resulting from different bite force–measuring tools.25 Also, unilateral and bilateral forces are presented because unilateral measurements tend to be 30% to 40% lower than bilateral.21,39

Evidence-Based Criteria for Material Selection

a

b

Fig 5-1  (a) These anterior glass-infiltrated alumina and magnesium ceramic crowns had been cemented in this patient’s oral cavity about 6 years prior to the time in which this catastrophic failure occurred. (b) Besides the fracture and detachment of the palatal portion of the crown on the maxillary right canine, the other crowns show crack lines and minor chipping of the ceramic.

Material properties The role of materials science in characterizing all-ceramic materials is complex because prediction of their clinical performance involves variables that are not easily controlled and reproduced in a laboratory setting. In addition, one single material property cannot reliably be used to predict long-term clinical performance. Instead, it has been reported that a combination of properties, in tandem with initial conditions, fabrication operations, and environmental conditions, can influence clinical longevity.40 Among other relevant properties, the combined roles of fracture toughness, hardness, modulus of elasticity, and strength are initial parameters to predict the potential failure modes of crowns.41

Fracture toughness Fracture toughness is the ability of a material to resist crack propagation from an initiating flaw until final failure.

A brief discussion of this particular property improves the understanding of the difference between failure modes observed for polycrystalline zirconia ceramics and particlefilled glasses used for all-ceramic restorations. Whereas cracks for the latter class of ceramics, once initiated, propagate unimpeded across the restoration until final failure, in polycrystalline zirconia ceramics, stress-induced cracks lead

to phase transformation within the polycrystalline zirconia grains, resulting in an incremental increase in toughness (known as R-curve behavior) until a plateau is reached.42 This property is sufficiently high for polycrystalline zirconia to minimize or almost extinguish cementation surface failures that would lead to bulk fractures of crowns, as is commonly the case for ceramic systems with lower fracture toughness.

Instead, in clinical studies, failure has been shown to occur from cohesive fractures of porcelain veneer.8,43–46 Glass-infiltrated zirconia as well as alumina crowns have also failed clinically from occlusal wear resulting in porcelain veneer fractures (Fig 5-1), as evidenced by fractography.47 However, core fractures are more likely to occur with particle-filled materials used as framework for all-ceramic crowns than with polycrystalline zirconia because of the lower fracture toughness of the former. These fractures are of major clinical significance because they demand restoration replacement. Fractographic analysis of a clinically failed alumina crown, for instance, has shown a fracture originating from the crown margin likely generated from hoop stresses.48 Fractographic analysis of another alumina framework from a clinically failed molar crown revealed that the origin of the fracture was the cementation surface.47 Thus far, the clinical studies of zirconia crowns are shortterm, but they have shown that failures coming from the cementation surface or any other origin leading to framework fracture (in which the restoration fractures into two 67

5

Individual Ceramic Crowns for Teeth Fig 5-2 Complete fracture of a zirconia-supported crown is an extremely rare event. This crown fractured as soon as the patient occluded on it during the try-in.

a

b

Fig 5-3 (a) This zirconia-supported crown cemented with glass-ionomer cement fractured after 2 years in service. (b) The broken fragment of the crown shows a clear fracture line.

pieces) are an extremely rare event49 and do not currently represent the chief failure mode (Figs 5-2 and 5-3). Cohesive failures in porcelain-fused-to-zirconia restorations (Figs 5-4 to 5-6) have commonly originated from rough occlusal surface areas, either as a result of occlusal adjustments followed by poor final polishing or occlusal wear resulting from parafunctional activity (bruxism).50,51

Hardness Hardness measures the material’s ability to locally resist plastic deformation. It controls quasiplasticity, which is a yield process that determines the intensity of the shear stress responsible for damage initiation. Considering the crown as a complete contour or layered structure, a high hardness value may hinder crack initiation, but other properties and conditions must be considered.

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Modulus of elasticity Modulus of elasticity (E), a material’s tendency to deform elastically when a force is applied to it, is an important property but secondary when high-strength ceramics are analyzed. The higher modulus of alumina (E = 300 GPa) relative to that of zirconia (E = 205 GPa) should not be taken alone as an advantage, considering the stress-induced transformation toughening mechanism of zirconia described earlier. Actually, higher maximum principal stresses have been observed in alumina crown frameworks than in zirconia frameworks,52 leading to alumina framework fracture in laboratory fatigue testing as well as in some clinically failed alumina crowns.47,48 Also of interest is the fact that, for the first glass-ceramic available for dental use (Dicor, Dentsply), the mismatch between its higher modulus of elasticity (E = 75 GPa) and that

Evidence-Based Criteria for Material Selection Fig 5-4 The zirconia-supported crown of this patient with Class II deep bite occlusion chipped 1 day after definitive cementation with a selfadhesive composite resin cement, despite confirmation that the occlusal contact was accurate.

Fig 5-5 The adhesive failure of the veneering ceramic of this zirconiasupported crown occurred in the same patient shown in Fig 5-3, 1 year later. This crown had to be replaced as well.

Fig 5-6 This chipping of the veneering ceramic is once again of adhesive nature and occurred after 4 years of use in a patient with parafunctional activity. Note the surface roughness at the level of the buccal cusps, which may have contributed to the failure.

of the dentin core (E = 16 GPa) became a failure predictor after clinical studies had detected a superior survival rate of the same crown system when it was supported by a metal core (cast post).53 Because the high modulus of elasticity of the zirconia framework may contribute to a decrease in tensile stresses at the cementation surface from occlusal loading, the modulus of elasticity of the substrate seems to be less important.

with caution because they seem to reflect a conditional rather than an inherent material property.54

Strength Strength, evaluated in single-load-to-failure testing, traditionally has been used as a predictor of a material’s clinical performance. Mean values obtained under static loading are commonly much higher than those obtained from materials failing as a result of repeated loading, as in fatigue testing. Therefore, values for strength should be examined

Implications An understanding of fracture toughness, hardness, modulus of elasticity, and strength helps to explain the direct relationship between the enhancement of ceramic properties by the ceramic manufacturers and the tremendous increase in the use of computer-assisted design/computer assisted manufacture (CAD/CAM). More than 20 zirconia and a few aluminum oxide CAD/CAM systems55 are available in the market. Lithium disilicate, for example, is available both for pressing and CAD/CAM. The properties of various ceramic materials are presented in Table 5-2. These data are presented in an attempt to elicit two major reflections. First, the properties of all-ceramic

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Individual Ceramic Crowns for Teeth TABLE 5-2 Properties of all-ceramic materials and tooth structures Modulus of elasticity (GPa)

Hardness (GPa)

Fracture toughness (MPa × m1/2)

Strength (MPa)

Coefficient of thermal expansion (X × 10–6 °C)

Vitablocks (feldspathic)

45

NA

NA

154

9.4

Lava Ceram

78

5.3

1.1

100

10.5

60–70

5.4

NA

90

9.5

65

5.4

NA

110

10.5–11

IPS e.max Press (lithium disilicate)

95

5.8

2.75

400

10.2–10.5

IPS e.max CAD (lithium disilicate)

95

5.8

2.25

360

10.2–10.5

4

In-Ceram Alumina

280

20

3.5

500

7.2

30

Procera Alumina

340

17

3.2

695

7.0

30

Cercon

210

12

9

1,300

10.5

2

IPS e.max ZirCAD

210

13

5.5

900

10.8

2

Lava

210

14

5.9

1,048

10.5

2

DC-Zirkon

210

12

7

1,200

10.4

2

In-Ceram YZ

210

12

5.9

> 900

10.5

2

Procera Zirconia

210

14

6

1,200

10.4

2

Prettau Zirconia

210

12.5

NA

1,000

10

2

Dentin

16

0.6

3.1

—

11–14

—

Enamel

94

3.2

0.3

—

2–8

—

Material

Thermal conduction (Wm–1 K–1)

Veneering ceramic

IPS e.max Ceram IPS e.max ZirPress (fluorapatite) Glass-ceramic

Alumina

Zirconia

Tooth

NA, not applicable.

Manufacturers: Vitablocks, Vident; Lava materials, 3M ESPE; IPS e.max materials, Ivoclar Vivadent; In-Ceram materials, Vident; Procera, Nobel Biocare; Cercon, Dentsply; DC-Zirkon, DSC Dental; Prettau Zirconia, Zirkonzahn.

materials have improved over time, but the failure rates did not decrease proportionally until a better understanding of the limitations of each material was acquired. This became possible thanks to recently published information from industrial, academic, and materials science sources, including specialized technical and clinical handling instructions.

Second, the overall mechanical properties reported for high-strength ceramic materials are substantially higher

70

than those found for natural teeth and, although intact teeth may also crack,56 enamel has equivocally been considered a brittle structure. Comprehensive work has shown that enamel has a stress-strain response comparable to that of predominantly base metal alloys57 and exhibits viscoplastic and viscoelastic behavior closely matching those of bone, which is relevant for stress redistribution during loading.58 Also, a remarkable R-curve behavior (ie, an increase in crack growth resistance, as mentioned previously

Improving the Long-Term Performance

Box 5-1

Possible causes of chipping of the veneering ceramic over zirconia frameworks related to fabrication method and materials employed

• Insufficient support of the veneering material by the framework design60–62 • Mismatch of coefficient of thermal expansions between veneering and framework ceramics63 • Rapid cooling of the veneering porcelain64,65 • Unfavorable surface and heat treatment of the zirconia framework and associated phase transformation (high-pressure air abrasion with aluminum oxide)66 • Strength degradation of ceramics67

for zirconia) has been described for crack propagation from the outer to the inner enamel.59 Such characteristics may explain the strength and fatigue-resistant nature of human teeth.

Improving the Long-Term Performance Chipping, porcelain veneer cohesive fracture, and other nonstandardized terminology have been used to describe failures in single-unit zirconia crowns and also in conventional fixed dental prostheses, which have been far more investigated clinically than any other type of prostheses. Because a tremendous amount of recently acquired information has increased the profession’s understanding of the multifactorial nature of zirconia porcelain veneer failure (Box 5-1), it is hoped that present and future laboratory and clinical studies will lead to reduced fracture rates. Current manufacturer guidelines for laboratory handling of porcelain-fused-tozirconia ceramics have changed since the first version because of the unexpected early failures, almost exclusively limited to the porcelain veneer. Although numerous factors have been considered responsible for veneer failure, an important aspect that has been reviewed is the cooling rate.64 In the last few years, most companies have been suggesting a slow cooling regimen as part of the porcelain veneer firing protocol.

Given that very few clinical studies of single crowns are available,45,49 the effect of changes in protocol and design were first evaluated in the laboratory setting. A significant contribution from multiple research groups has resulted in a number of publications in the field. To address the topic from a sequentially logical and informed platform, most of the studies described in this section will be those performed at the Department of Biomaterials and Biomimetics

at New York University College of Dentistry in collaboration with Dr Van P. Thompson, Dr E. Dianne Rekow, Dr Paulo G. Coelho, Dr Nelson Silva, Dr Petra Guess, Dr Yu Zhang, and several visiting scholars and clinical or research collaborators. A method capable of simulating clinically observed failures in the laboratory was developed and applied to several anatomically relevant prosthetic materials, especially in the molar region.68,69 One surprising and remarkable finding of a systematic review was that metal-ceramic crowns, in spite of their more than 50 years of use, have been sparsely investigated with controlled, prospective studies, unlike all-ceramic materials. Because metal-ceramic restorations are con­ sidered the gold standard, high-strength ceramics have been compared to them. In essence, zirconia molar crowns fabricated with early porcelain veneer firing protocols (and evaluated in 2008) presented significantly lower reliability (probability of survival when subjected to fatigue testing) than did metal-ceramic crowns.70 Therefore, reliability and characteristic strength targets were set based on the values observed for metal-ceramics and desired for high-strength ceramics.

Some of the fatigue testing was mainly performed on the buccal cusp of the mandibular molar crown, where the occlusal contacts are normally expected to occur. However, observations of clinically failed zirconia prostheses showed that porcelain cohesive fractures also occurred in the lingual cusp of mandibular posterior teeth.50,71–73 For this reason, the probabilities of failure of the lingual and buccal cusps were compared; although no difference in reliability was observed between cusps, the finding that porcelain cohesive failures were much more extensive (virtually unrepairable) on the lingual than on the buccal cusp was of clinical significance.60 It became clear that frameworks for prostheses fabricated by CAD/CAM, when designed as default with even thicknesses, led to discrepancies in porcelain veneer thicknesses after the final anatomy was

71

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Individual Ceramic Crowns for Teeth Fig 5-7 To reduce or eliminate the chance of porcelain chipping, zirconia frameworks should be shaped to support an even and reduced thickness of veneering ceramic, similar to metal-ceramic frameworks. In this single crown, note the lingual collar and the proximal ledge.

completed for function and esthetics. Therefore, framework design modifications that incorporate changes to minimize such effects have been suggested.60 The rationale for a specific framework design for an allceramic crown has been, in general, empirically suggested or simplistically derived from metal-ceramic frameworks when issues observed with the porcelain veneer compromised the survival of these restorations and a similar learning curve was required. Although several framework design modifications have been proposed since 1962 for metalceramic restorations,74 they mainly comprise a lingual collar of varied heights that extends proximally75–78 or a supportive framework for the final crown anatomy, known as anatomical design.61,79,80 Fatigue testing of porcelain-fused-to-zirconia crowns with uniform framework thickness compared to frame­ works where a lingual collar extending proximally was used for porcelain support showed that, in the latter, not only was reliability significantly improved, but also an important trend toward reduced porcelain veneer fracture sizes was consistently observed.62,81 Therefore, the benefits of improved characteristic strength and of reduced fracture sizes seem to be of clinical significance when framework design modification, including the lingual collar extending proximally, is considered (Fig 5-7). The main drawback is that such design modification provides changes only at the lingual aspect of the crown. Also, because these porcelains were fired according to protocols recommended in 2008, it is not surprising that, despite the greater reliability of the modified framework compared to the conventionally designed even-thickness framework, it was still significantly lower than that of the metal-ceramic crowns.70,82 Perhaps the most clinically relevant framework design modification for high-strength ceramics is anatomical design, involving use of a coping that is similar to the final crown anatomy, assuring an even and reduced thickness 72

of the veneering material compared to a coping of even thickness. For zirconia fixed dental prostheses, at a time where failure rates were commonly high, two clinical studies using this design demonstrated reduced compli­cations of porcelain veneer fracture.79,80 The use of an anatomically designed framework for zirconia crowns was investigated under fatigue testing and compared to a conventional framework design with uniform thickness, with either hand-layered or pressed veneering porcelain. The lowest reliability and strength were observed for the pressed veneer porcelain zirconia crown over the con­ventionally designed framework; a significant improvement was found if the same porcelain was pressed on the anatomically designed framework. Reliability was not significantly improved when a hand-layered veneering porcelain was fired onto the anatomically designed framework, because probability of survival was already at its highest levels, even in the conventionally designed coping. How­ever, irrespective of porcelain veneering method (pressed or hand-layered), the anatomical frame­work, besides improving the characteristic strength and reliability, always resulted in reduced porcelain veneer fracture size; repair or, sometimes, repolishing would return them to function. Such benefits were extended to all cusps.61 A proposal for a further evolution of the anatomical coping design has been advanced recently by a dentist–dental technician team who have been using zirconia as a prosthetic material for more than a decade.83 The modifications have been introduced for both posterior and anterior prostheses to improve their reliability while maintaining the esthetic advantages of a veneered restoration over a monolithic zirconia prosthesis. For posterior units, the zirconia coping is enriched with ridges and fins that support and reinforce the veneering ceramic at the cusp tips and circumferentially around the axial surfaces. The location and height of these ridges and fins are determined with the aid

Improving the Long-Term Performance Fig 5-8  Aesthetic functional area protection (AFAP) concept for the prevention of ceramic chipping, as applied to posterior zirconia frameworks. (arrows) Fins that extend inside the cusps almost to the surface provide a wall-contained area where the veneering ceramic is always placed under compression, even if the opposing teeth were to guide on the cusps’ inner inclines. The ridges on the axial surfaces of the coping support the veneering ceramic, which is no longer loaded with shearing forces but is placed in compression instead. This is important especially proximally, where the ridges are prone to fracture, even when the thickness of the veneering ceramic is not excessive, because of the absence of a supporting ledge.

(Courtesy of Dr Mauro Broseghini and Cristiano Broseghini, CDT, Trento, Italy.)

a

b

c

d

e

f

g Fig 5-9  (a to c) The zirconia framework for a molar crown is shown after sintering. (d to f ) Different firings of the veneering ceramic on the corrugated coping. (g) Completed molar crown. (Courtesy of Dr Mauro Broseghini and Cristiano Broseghini, CDT, Trento, Italy.)

of a complete-contour wax-up. These additions to the basic coping design are shaped in such a way that they are just short of the surface of the completed restoration, so that they are entirely covered by the veneering material (Figs

5-8 and 5-9). Because zirconia is an esthetic material due to its color and relative translucency, in the critical areas, it can be brought almost to the surface without compromising the esthetic outcome of the restoration.

73

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Individual Ceramic Crowns for Teeth

a

b

c

d

Fig 5-10 (a to d) The ceramic at the incisal margin of several anterior crowns of this metal-ceramic rehabilitation has chipped after 2 years in function.

In anterior restorations, the most vulnerable area, that is the one with a higher incidence of chipping, is the incisal edge (Fig 5-10). For this reason, this functional area of the crown and, possibly, the entire palatal surface of maxillary crowns ideally should be made of the stronger substructure material with little or no veneering ceramic layered on it. Once again, zirconia allows the technician to satisfy this requirement without negatively affecting the esthetic appearance of the completed crown. A very precise complete-contour wax-up of the crown is made. When the wax-up is cut back to create space for the veering ceramic, the incisal margin is kept intact, but a window is created below it (Fig 5-11). In this way, light is allowed to pass through, mediated by the more translucent veneering ceramic instead of being reflected by the framework material, which is much more opaque.

Both posterior and anterior framework concepts have been named the aesthetic functional area protection (AFAP) frame­work designs.83 For both configurations, properly designed in vitro studies are needed to compare the fracture and chipping resistance of the veneering ceramic on this 74

framework design with that on other commonly used configurations.

Comprehensive studies aimed at describing the reasons for cohesive porcelain fractures of layered- or pressedzirconia restorations have provided an enormous contri­ bution to the understanding of the role of porcelain thickness, coefficient of thermal expansion mismatches, and cooling rates on the resulting internal residual stresses within porcelain. The poor thermal conductivity of zirconia (2 Wm–1 K–1; see Table 5-2) compared to metal (200 Wm–1 K–1 for gold alloy) along with thick layers of porcelain veneer and rapid cooling of a layered zirconia restoration present a favorable scenario to generate internal residual stresses in the porcelain that can be exposed during occlusal adjustments or contacts, eventually resulting in the porcelain fractures observed clinically.64 Several studies incorporating a slow cooling protocol have shown that the reliability of zirconia crowns can be significantly improved, and specific firing protocols including this modification have been adopted by manufacturers.65,84–86 Within this context, combining framework design modification (such as AFAP, or an

Improving the Long-Term Performance

a

b

e

g

c

d

f

h

Fig 5-11  (a and b) Full wax-up of an anterior crown. Once the position, height, and volume of the restoration have been defined, a selective cutback can be carried out, either manually or digitally after the wax-up has been scanned. (c and d) The framework of the crown has been made out of a presintered zirconia blank using a pantographic milling system. The intact incisal margin is connected to the crown coping, and there is a wide open window between the two. (e and f ) The same framework is shown after the sintering process. (g) The veneering ceramic is applied. (h) The finished crown is placed. The incisal margin is entirely in zirconia without any overlying ceramic. (Courtesy of Dr Mauro Broseghini and Cristiano Broseghini, CDT, Trento, Italy.)

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Individual Ceramic Crowns for Teeth

a

c

f

b

d

e

g Fig 5-12  (a) Complete wax-up of a single mandibular molar. A special wax has been used to capture the margin of the preparation. (b) Occlusal view of the wax-up. A handle has been added. (c) The wax-up is sprued and connected to the sprue former. (d) After the investment has hardened in the cylinder, the wax is burned in the heating oven, and the cylinder is then placed in the special oven, where the chosen ingot of lithium disilicate is heated and pressed. (e) The casting is shown after removal of the investment material with glass beads under air pressure. (f ) The crown is positioned and adapted on the master cast. (g) Occlusal view of the crown.

The handle has been reproduced by the glass-infiltrated ceramic. (h) Stains to be used for coloring the surface of the restoration.

h

76

Improving the Long-Term Performance

i

j

k

l

Fig 5-12  (cont) (i) The finished crown is shown after the crystallization phase. (j) Occlusal view of the finished crown. (k) The crown has been cemented adhesively with a dual-cured composite resin. Use of rubber dam was made possible by the supragingival margins. (l) Occlusal view of the finished crown in situ. (m) Radiograph of the cemented crown. (Figures 5-12a to 5-12j courtesy of Luca Vailati, CDT, Tronzano Vercellese, Italy.)

m

anatomically designed framework) with slow cooling will result in improved support and a more even porcelain veneer–core thickness ratio; the consequences will be minimal residual stress and likely reduced fracture sizes, should they be caused by fatigue or parafunction.

To simplify the fabrication of high-strength ceramic posterior restorations, the exclusion of the porcelain veneer layer has been attempted in lithium disilicate crowns (Fig 5-12) as well as in zirconia crowns (Fig 5-13). Monolithic (complete-contour) high-strength all-ceramic crowns eliminate the weaker porcelain, resulting in improved 77

5

78

Individual Ceramic Crowns for Teeth

Fig 5-13 Monolithic zirconia crown. The surface has been stained and glazed in the oven.

Fig 5-14 Scalloped preparations present a challenge for the dental technician when fabricating metal-ceramic crowns. The larger the difference between the buccal and the proximal levels of the preparation, the more difficult it is to manufacture a metal-ceramic crown that maintains circumferentially acceptable marginal adaptation throughout the fabrication phases.

strength.87,88 However, the final thickness of the restoration plays an important role in crown reliability. Completecontour lithium disilicate molar crowns of 2.0-mm thickness at the occlusal surface and 0.5-mm thickness at the buccal surface have shown characteristic strength levels even higher than those of metal-ceramic restorations when subjected to fatigue. When the lithium disilicate crown was reduced to a 1.0-mm thickness to simulate limited occlusal clearance, the characteristic strength was not significantly different from that of the 2.0-mm crown but sufficiently lower to become similar to that of a metal-ceramic crown.89 Thus far, although clinical results are short-term, promising survival rates have been observed with monolithic lithium disilicate crowns.90,91 Similarly favorable results have been observed in vitro for complete-contour zirconia crowns, especially if glazed; similar translucency, contact wear of the opposing tooth, and an additional significant gain in strength have been observed compared to the properties of a porcelainlayered zirconia crown92 (see Fig 5-13). Although phase transformation (tetragonal-monoclinic) has been detected in a layer only 6 µm below the zirconia surface, mechanical properties have shown to be compromised by hydrothermal degradation, resulting in a 30% reduction of Young modulus of elasticity and hardness.67 Therefore, future clinical studies are warranted to ascertain if low-temperature degradation and aging of zirconia affect its long-term performance.

Clinical Criteria for Material Selection for Single Crowns To make a rational material selection between metal-­ ceramic crowns (with a metal margin or with a porcelain butt margin) and the different all-ceramic systems, the clinician and the dental technician can take into consideration a number of criteria. Material properties, as explained earlier, are the basis for understanding whether a new material has the potential to withstand the mechanical and thermal stresses of the oral environment. However, other clinical criteria can also influence the selection: • Circumferential position of the preparation margin: Does the preparation margin follow the gingival margin, and thus is it scalloped, or does it follow the cementoenamel junction, and is it at a relatively horizontal level circumferentially?

• Appearance of the abutment: Is the color of the abutment within a normal range, or is it dark, or does it have a metal post?

• Position in the arch: Is the tooth to be restored an incisor, canine, or premolar, or is it a molar?

Clinical Criteria for Material Selection for Single Crowns

Porcelain

Porcelain

Porcelain

Metal Metal

Metal

Unstable

Stable a

b

Shrinkage during casting, high friction, and adaptation

Unstable Distortion during porcelain baking, marginal opening

c

Opening at the withdrawal of the wax pattern, distortion during baking

Fig 5-15 (a) In the fabrication of a metal-ceramic crown, having the margins approximately at the same level circumferentially helps the casting to remain stable during the firing of the veneering porcelain. Therefore, it is easier to produce a well-fitting crown. (b and c) With scalloped preparations, the crown is subjected to distortions during both casting and porcelain baking, and thus it is much more difficult to obtain a crown with well-adapted margins circumferentially. (Reprinted from Yamamoto96 with permission.)

Fig 5-16 Microscopic observation reveals the lack of marginal adaptation of a completed metal-ceramic crown with a disappearing metal margin.

Circumferential position of the preparation margin Marginal adaptation The work of a restorative dentist often consists in replacing previously made restorations. In those instances, the old preparation design and position can limit the freedom of choice as far as the selection of the material for the new crown is concerned. This is true especially when the margin is purposely “hidden” in the sulcus. An intrasulcular position of the crown margin often affects the restoration’s marginal precision and integrity in several ways: (1) it complicates the capture of an accurate impression of the finish line, therefore increasing session chair time; (2) it may hinder the operator in obtaining a satisfactory marginal seal during the cementation procedure; and (3) if the margin has followed the scallop of the gingiva, the preparation is scalloped as

well; this can make it difficult to fabricate metal-ceramic crowns with well-adapted margins around the circumference of the abutment.

Several authors,93–96 recognized masters in the fabrica­ tion of metal-ceramic restorations, have pointed out the technical difficulties associated with obtaining and main­ taining optimal marginal adaptation throughout the manu­facturing process of the restoration. This is especially true when the finish line of a preparation has a pronounced scallop, that is, a marked difference between the buccal and proximal levels of the preparation (Fig 5-14). This scalloped preparation produces a crown with unsupported margins that are prone to distortion during the different firings in the porcelain oven (Figs 5-15 and 5-16). The request to fabricate a metal-ceramic crown with a porcelain butt margin increases further the level of difficulty for the technician, thus making circumferential marginal adapta­tion an almost impossible-to-reach goal or at least one that requires a high

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Individual Ceramic Crowns for Teeth

a

b

c

d

Fig 5-17 (a) The fit of an alumina crown is observed on the stone die of a scalloped preparation of a mandibular premolar. (b) The fit of the crowns is demonstrated by a thin, uniform film of silicone disclosing paste. (c) The finished alumina crowns have been cemented with a glass-ionomer cement. (d) The same alumina crowns are shown after 10 years in function. There is slight discoloration at the margins.

level of competency and skill. When a relatively horizontal course is developed for the preparation margin, in contrast, metal-ceramic crowns demonstrate a more consistently satisfactory fit.

Scalloped preparations for single crowns, therefore, may suggest the use of all-ceramic materials instead of metalceramic systems, certainly those fabricated through a heatpressed technique (leucite and lithium disilicate ceramics) but also those milled through CAD/CAM tech­nology (lithium disilicate ceramics, polycrystalline alumina, and zirconia). A potential advantage of polycrystalline ceramics is that they demonstrate good dimensional stability during all firing cycles of the veneering ceramic. Several studies97–101 have examined CAD/CAM crowns and have pointed out the factors that, beside preparation design and cementation procedure, may influence marginal adaptation: • Software and hardware limitations • Scanner type • Machining technology 80

• Single crown versus multiple-unit fixed partial denture • Sintered versus nonsintered material The evidence published so far for many CAD/CAM systems on the market demonstrates a level of marginal precision that is clinically acceptable.102–104 Therefore, if the CAD/ CAM system utilized by the dental technician is able to produce a coping with clinically acceptable marginal adaptation, then the application of the veneering ceramic should not alter that relationship (Fig 5-17). This can represent a savings in time, which is an advantage for the dental technician, dentist, and patient.

When a tooth is prepared for a crown for the first time, the approach of the clinician may be different. If the color of the abutment matches the adjacent and remaining natural teeth or prosthesis and no proximal spaces have to be closed, the dentist has the choice to keep the preparation margin supragingival and to follow the cementoenamel junction as the guide to the preparation instead of the gingival margin. The use of all-ceramic materials potentially al-

Clinical Criteria for Material Selection for Single Crowns

a

b

d

c

e

Fig 5-18  (a) This vital maxillary second premolar is prepared for a complete crown because of the fracture of the palatal cusp and the presence of a large mesio-occlusodistal composite resin restoration. (b and c) Because the cervical area on the buccal side is not involved, the preparation margin is kept at the level of the cementoenamel junction circumferentially. (d and e) The stone cast poured from the final impression of the tooth reveals the supragingival margins. (f ) Finished monolithic lithium disilicate crown.

f

lows the clinician to avoid intrasulcular placement of the preparation margins to hide the prosthetic margins, as is traditionally done with metal-ceramic crowns because of the opacity of the cervical area. These decisions not only can generate significant time savings during relining of a provisional restoration, impression taking, try-in, and cementation of the definitive crown, but also contribute to the preservation of tooth structure, especially the enamel around the margin, greatly benefiting the quality of the

marginal seal obtained with the cementation procedure and avoiding traumatic damage of the gingiva during preparation, impression, and cementation. Furthermore, if synthetic glass ceramics or the particle-filled glass ceramics are used, the prosthetic margins can be almost undetectable to the eye, enhancing the esthetic outcome. Therefore, whenever possible, preparations should follow the cementoenamel junction and avoid intrasulcular margin placement (Fig 5-18).

81

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82

Individual Ceramic Crowns for Teeth

g

h

i

j

k

l

m

n

Clinical Criteria for Material Selection for Single Crowns

o

p

q

r Fig 5-18  (cont) (g and h) The crown is positioned on the stone cast. (i) The tooth is easily isolated with rubber dam. A waxed floss helps to keep the dam tucked cervically. (j) Because the tooth is isolated and the mouth protected, it is possible to clean the tooth with a light, short spray of aluminum oxide powder. (k) Cleaned tooth. (l and m) Although a self-adhesive cement is going to be used, the enamel margin is acid etched with 37% orthophosphoric acid for 30 seconds. (n) The abutment has been rinsed with water and dried. (o) The dual-curing resin cement has been light polymerized for a few seconds on both the buccal and the palatal sides. (p) The excess cement has been removed, and final light polymerization has been completed. (q) Buccal view of the finished crown. (r) After 4 years in situ, slight discoloration is visible at the margin.

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Individual Ceramic Crowns for Teeth TABLE 5-3 Possible choices of cement types for complete crowns made of different substrates Prosthesis substrate

Cement RMGIC DC-CR

ZnP

GIC

✓ ✓

✓ ✓

✓ ✓

✓ ✓

✓ ✓

Densely sintered alumina

✗ ✓

✗ ✓

Densely sintered zirconia

✓

✓

High-noble alloys Noble alloys Predominantly base alloys Titanium and titanium alloys Glass-ceramics

LC-CR

CC-CR

✗ ✗

✗ ✗

✓ ✓

✓ ✓

✗ ✗

✗ ✗

✓ ✓

?

✓

?*

✓

?

✗

✓ ✓

✓

?

✗

✓

ZnP, zinc phosphate; GIC, glass ionomer; RMGIC, resin-modified glass ionomer; DC, dual-cured; CR, composite resin; LC, light-cured; CC, chemically cured. ✓, indicated; ✗, contraindicated; ?, possible, but not ideal. *If the thickness of the restoration is not more than 1 to 1.5 mm.

Cementation of single crowns Although cementation procedures and materials for highstrength ceramics are discussed in chapter 9, it is important to emphasize key aspects of the cementation procedures for single-unit, high-strength ceramic restorations. Cement selection and the ability to isolate the abutment to perform the cementation procedure properly are related factors that strongly influence the selection of crown material105,106 (Table 5-3). When a prosthesis has to be permanently cemented to a natural abutment, it is important to provide the proper conditions for the luting agent to set optimally.

For example, a clinician interested in taking advantage of the superior translucency of glass-ceramics, aware that proper adhesive cementation is necessary to enhance their strength so that they can perform well clinically,107 may be in doubt whether to select such a material if the preparation margin is located apical to the gingival margin and the gingiva is not optimally healthy. At a time when composite resin cementation has become so popular and prescribed for any prosthetic substrate because of its superior retention, convenient packaging, and reduced setting time compared with zinc phosphate and glass-ionomer cements, the issue of selecting the best luting agent for intrasulcular preparations still is a dilemma for many clinicians.

In principle, a luting agent should fulfill, among others, some basic requirements: provide the prosthesis with the necessary retention; be resistant to dissolution; present high strength in tension, shear, and compression; be user

84

friendly (adequate working and setting times); be biologically acceptable; and be adequate to seal the interface between prosthesis and tooth to avoid bacterial infiltration.7,9,108 For the last requirement to be fulfilled, the cement should not come in contact with saliva or blood during setting because part of the cement may be contaminated before it has had the chance to set, and the seal at the margin may be compromised. If the preparation margin is supragingival or at the gingival margin, keeping it isolated is fairly simple (see Fig 5-18i). If the margin is intrasulcular, rubber dam isolation may be a very time-consuming procedure and one that still may not guarantee total fluid control (Fig 5-19).

Traditionally, metal-ceramic crowns have been cemented with zinc phosphate or glass-ionomer cements. These same agents have been recommended for the nonetchable ceramics, alumina and zirconia. The main drawbacks of these cements are their relatively high solubility in the early stages of setting and their prolonged setting times. Thus, in the presence of fluids, it is difficult to protect their integrity for the time needed to reach a sufficiently insoluble state.

The advantage of the resin cements is that they set faster; in case of the dual-cured resin cements, their light-curing portion is within the control of the operator. Some clinicians feel that resin cements, therefore, can provide a more reliable marginal seal even in less than ideal conditions.

Although there is no scientific evidence that an optimal seal is necessary to increase the longevity of a restoration, the lack of such a seal may increase the susceptibility of the

Clinical Criteria for Material Selection for Single Crowns

a

b

c

d

e

f

g

h

Fig 5-19  (a and b) Despite the healthy gingival tissues, the intrasulcular placement of the preparation margins makes it difficult for ideal isolation of the field to allow the cement to set in optimal conditions. In these instances, the use of a composite resin cement that has a light-curing component may be advantageous because the cement can be stabilized as soon as the placement of the restoration is deemed satisfactory. (c to e) Zirconia crowns have been cemented with a self-adhesive, dual-curing cement after a retraction cord was placed in the sulcus of each abutment. (Prosthetic work by Luca Vailati, CDT, Tronzano Vercellese, Italy.) (f to h) Periapical radiographs of the teeth show the definitive restorations in situ.

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Individual Ceramic Crowns for Teeth TABLE 5-4 Surface treatment of different prosthetic substrates to be luted with a composite resin cement* Substrate High-noble alloy Noble alloy Predominantly base alloy Titanium and titanium alloy

Surface treatment Air-particle abrasion

With what 50-μm alumina particles

Pressure

Activator

Sample product name

~3 bar

Specific for alloys or universal

Alloy Primer Signum Metal Bond or Scotchbond Universal† Monobond Plus†

30-μm silica-coated alumina particles Rocatec Soft or CoJet

~3 bar

Silane coating

RelyX Ceramic Primer Clearfil Ceramic Primer

or

Lithium disilicate glass-ceramic Leucite glass-ceramic Feldspathic ceramic

Acid etching

5% hydrofluoric acid: 20 s for disilicate, 60 s for leucite, and 120 s for feldspathic

NA

Silane coating

RelyX Ceramic Primer Clearfil Ceramic Primer

Densely sintered alumina

Air-particle abrasion

50-μm alumina particles

~3 bar

Silane coating

RelyX Ceramic Primer Clearfil Ceramic Primer

Densely sintered zirconia

Air-particle abrasion

50-μm alumina particles

~1.5 bar

Specific for ceramics or universal

RelyX Ceramic Primer Clearfil Ceramic Primer or Scotchbond Universal† Monobond Plus†

~1.5 bar

Silane coating

RelyX Ceramic Primer Clearfil Ceramic Primer

or 30-μm silica-coated alumina particles Rocatec Soft or CoJet

NA, not applicable. *Do not use orthophosphoric acid for the surface cleaning and decontamination of alloys, etched ceramics, and polycrystalline ceramics tried in the oral cavity just before final cementation with a composite resin cement, because orthophosphoric acid deactivates the surfaces of the crown substrates for adhesive resin cementation. † Contains methacryloyloxydecyl dihydrogen phosphate (MDP).

Manufacturers: Rocatec Soft or CoJet, 3M ESPE; Alloy Primer, Kuraray; Signum Metal Bond, Heraeus Kulzer; Scotchbond Universal, 3M ESPE; Monobond Plus, Ivoclar Vivadent; RelyX Ceramic Primer, 3M ESPE; Clearfil Ceramic Primer, Kuraray.

patient to a failure because of the recurrence of secondary caries or loss of retention. An optimal marginal seal is enhanced whenever the crown’s substrate has been treated appropriately109 (Table 5-4), a proper field isolation has been provided, and the correct procedure has been carried out for the luting agent selected (Table 5-5). Ideally, rubber dam should be applied, but, if the preparation margin is intrasulcular and the gingiva is healthy, the placement of a retraction cord may be clinically acceptable as well.

86

In addition, appropriate selection of the adhesive system is of utmost importance, irrespective of manufacturer recommendation. Incompatibility and permeability issues between simplified adhesive systems (one-step self-etch and two-step etch and rinse systems) and self- and dualcure resin cements have been extensively reported in the literature.110–113 Instead, conventional total-etch three-step or two-step self-etch adhesive systems are indicated for cementation with dual- or self-cure resin cements.108,114

Clinical Criteria for Material Selection for Single Crowns TABLE 5-5 Examples of resin cements, their categorization, and products for the pretreatment of the dental surfaces*

Cement with separate adhesive Total or selective etching (etch and rinse)

Self-etching (etch and dry)

✓

✓

Scotchbond Universal*

Clearfil Esthetic Cement

✓

ED Primer†

Panavia F2.0*

✓

ED Primer†

Product

Self-adhesive cement

Name of adhesive

Dual-cured RelyX Unicem2

✓

Clearfil SA Cement

✓

SpeedCem

✓

RelyX Ultimate

Variolink II

✓

Excite F DSC or Syntac Classic

Enacem

✓

Enabond

Light-cured Variolink Veneer

✓

RelyX Veneer

✓

Excite F DSC or Syntac Classic ✓

Scotchbond Universal*

Multilink‡

✓

Multilink Primer A/B

Panavia 21†

✓

ED Primer†

Self-cured

*Self-adhesive and self-cured cements are indicated for luting metal or metal-ceramic, alumina, and zirconia crowns. † Contains MDP. ‡ Can also be light-cured but, according to the manufacturer, only for an easier removal of excess cement.

Manufacturers: RelyX materials, 3M ESPE; Clearfil materials, Kuraray; SpeedCem, Ivoclar Vivadent; Panavia materials, Kuraray; Variolink materials, Ivoclar Vivadent; Enacem, Micerium; Multilink, Ivoclar Vivadent.

Appearance of the abutment Because of the differing relative translucencies of different all-ceramic materials (Fig 5-20), the color of the underlying structure may be of relevance in the selection of the proper material. If the abutments are of normal color, the best material to use may be the more translucent ones, that is, the feldspathic and synthetic glass ceramics, especially if residual enamel is preserved, at least around the preparation margin. With these materials, it is possible to obtain an invisible margin between restoration and tooth substrate (Figs 5-21 and 5-22).

Discolored teeth, or teeth with metal posts or dark preprosthetic restorations (Fig 5-23a), on the other hand, may need to be masked by an opaque core. Studies on core translucency16,17 have pointed out the inadequacy of the glass-ceramics to mask underlying dark-colored substrates.

However, the outcome depends also on the amount of space that the technician has available on the buccal side for the crown, especially in the marginal area. If the space exceeds 1.3 mm, there is a better chance to mask dark backgrounds with the all-ceramic materials. Moreover, in recent years, a number of opaque cores have been introduced in the disilicate family; these can reach the same degree of

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Individual Ceramic Crowns for Teeth

Fig 5-20  All-ceramic materials may exhibit different translucencies at the same thickness. Dental technicians must know the masking abilities of different materials when there is a request to fabricate an all-ceramic restoration on a discolored or metal substrate. This image highlights the different translucencies of four lithium disilicate samples of the same thickness (0.8 mm) but of increasing opacities. (Courtesy of Prof Daniel Edelhoff, Munich, Germany.)

a

b

c Fig 5-21  (a) A 19-year-old patient requested the replacement of composite resin restorations applied about 10 years earlier after a traumatic fracture of the two central incisors as well as closure of the diastema. (b) The composite resin has been removed, and the teeth have been prepared in enamel for porcelain veneers. The preparation margin is supragingival in both teeth. (c) After adhesive cementation of the two feldspathic ceramic veneers, the interface between the supragingival restorations and the remaining tooth structure is hardly visible. (Prosthetic work by Marco Cossu, CDT, Lugano, Switzerland.)

88

Clinical Criteria for Material Selection for Single Crowns

a

b

c

d

Fig 5-22  (a) A 30-year-old woman had a lemon-sucking habit for several years. When eating and drinking started to cause severe sensitivity of her teeth, despite some conservative attempts to cover the eroded tooth structure, the patient asked for a more comprehensive approach. (b) To keep the mandibular incisors vital, after the removal of the existing composite resin restorations, a very conservative circumferential preparation of the incisors was carried out. The preparations end about 1 mm coronal to the cementoenamel junction. (c) The leucite-based ceramic crowns are shown shortly after cementation with a light-cured composite resin cement. (Prosthetic work by Fabrizio Tordi, CDT, and Roberto Nobili, CDT, Milan, Italy.) (d) The same restorations remain indistinguishable from remaining tooth structure after 14 years in service.

masking as the polycrystalline ceramics of similar thickness17 (Figs 5-23b and 5-23c).

It is extremely helpful for the technician to see the color of the prepared teeth. The technician can be provided with photographic images of the prepared abutments to allow him or her to evaluate their color to aid in the selection of the material for the restoration. In esthetically demanding areas, it is strongly suggested that the copings be tried in prior to the application of the veneering ceramic (see Fig 5-23b).

When the abutment is very dark and the discoloration extends down the root, often the esthetic result is inadequate even when a totally opaque material is used, because the gingiva, especially if it is of a thin biotype, is unable to conceal the grayish appearance115 (Fig 5-24). At times, when opaque cores are used to raise the value of the restorations on endodontically treated teeth, even if the color of the residual tooth structure seems to be in a normal range, a shadow is generated in the cervical area (Fig 5-25). A possible solution may be to cut back the core in the area of the buccal margin to allow more passage of light.

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a

c

a

b

Fig 5-23  (a) The right central incisor presents a cast gold post and core and moderately discolored cervical dentin, while the other three incisors are vital and have a normal-­colored substrate. This abutment has lost more tooth structure on the buccal side than has the contralateral tooth; therefore, more space is available for this crown. A glass-ceramic material will be used for all four crowns. (b) The try-in of the all-ceramic copings reveals the difference in color of the coping for the right central incisor compared to those for the other incisors. A more opaque and higher value blank has been used for the right central incisor. (c) The finished lithium disilicate crowns have been cemented with a self-adhesive composite resin cement. The color match at the crown is quite satisfactory, but at the gingival level, a grayish look remains. (Prosthetic work by Luca Vailati, CDT, Tronzano Vercellese, Italy.)

b

Fig 5-24  (a) The left central incisor of this young woman not only has a cast gold post and core but also presents an extremely dark dentin both at the supragingival and subgingival levels. (b) Two alumina crowns have been manufactured for these abutments. Despite the fact that the left crown is thicker on the buccal side and has been cemented with an opaque luting agent, the gingival area, as expected, displays a grayish shadow.

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Clinical Criteria for Material Selection for Single Crowns

a

b

c

d

e

f

g Fig 5-25  (a) A 37-year-old woman requested the replacement of her anterior crowns. (b) The four incisor metal-ceramic crowns, inserted about 10 years earlier, are opaque looking and have a visible dark margin. (c) The palatal view of the metal-ceramic crowns reveals areas of exposed opaquer and the high metal collars. (d) All the incisors had already been treated endodontically. The treatment seemed incomplete, but no periapical lesion is visible. (e) A slit has been created in the middle of each crown to allow them to be pried open with an instrument. (f ) After removal of the crowns. (g) The lateral incisor crowns display signs of marginal infiltration, but no caries is detected on the teeth.

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h

i

j

k Fig 5-25  (cont) (h) The preparations have been finalized. (i) All the teeth have been endodontically retreated. New direct preprosthetic reconstructions have been made with composite resin and the placement of glass-fiber posts. (j) Lithium disilicate crowns. (k) On the palatal aspect, the lithium disilicate coping has been left exposed because of the lack of space between the preparation and the opposing dentition.

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Clinical Criteria for Material Selection for Single Crowns

l

m

n Fig 5-25  (cont) (l) The definitive crowns have been cemented with a self-adhesive, self-curing composite resin cement. (m) Despite the use of a metal-free material, a slight gray shadow is visible in the marginal area extending apically. (Prosthetic work by Luca Vailati, CDT, Tronzano, Vercellese, Italy.) (n) Periapical radiographs of the teeth with the definitive restorations in situ.

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Position in the arch When metal-ceramic systems were the main material available for crown fabrication, the location in the arch of the tooth to be restored, whether an anterior tooth, a premolar, or a molar, was a variable that sometimes influenced the extension of the visible metal collars. On the other hand, with all-ceramic materials, this information may influence the clinician’s choice of the specific product.6 As has already been mentioned, the forces that can be generated in the mouth, especially in the posterior area, can be high; therefore, it is important to choose the most reliable materials to withstand long-term use. Different studies and reviews have pointed out that, for anterior teeth, any all-ceramic material can achieve a 5-year survival rate comparable to that of metal-ceramic restorations.6 Even at the premolar level, the performance seems to be just as satisfactory. However, some researchers have stated that, for glass-ceramics to be this successful, adhesive bonding is a prerequisite.60,103 The controversy today relates to the choice of material for the restoration of molars. On one hand, some clinicians recommend the polycrystalline ceramics, and zirconia in particular, as the best solution to reliably withstand loads. If a veneered zirconia crown is to be manufactured, however, then the preparation has to have depths similar to those for a metal-ceramic crown. Monolithic zirconia crowns may be an alternative that not only prevents chipping but also allows for a less invasive preparation.116 There are, however, questions about the wear induced by this material on the opposing dentition and on the procedures to properly adjust and polish the zirconia occlusal surface in the oral cavity,92 with information currently restricted to in vitro studies.108 The use of lithium disilicate in the monolithic configuration is acquiring more acceptance even in the molar region, because a properly performed adhesive cementation procedure should increase the already high reliability of the material,8,89 especially if some enamel is still present. Even in cases with abutment height of more than 4 mm and an angle of convergence less than 10 degrees, and when isolation or humidity control was not feasible, use of conventional glass-ionomer cement did not influence the failure or complication rate of lithium disilicate crowns in an up to 9-year prospective clinical study when compared to adhesive bonding.117 This may become a strong driving force in allowing and promoting less invasive preparation techniques. Furthermore, this material is undoubtedly easier to adjust and polish.

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High-Strength Ceramic Posts and Cores When endodontically treated teeth with significant loss of coronal dentin have to be restored with crowns, the placement of posts and cores to improve retention of the preprosthetic reconstructions and the definitive restorations has been recommended. Traditionally, cast posts have been used. However, prefabricated metallic (eg, stainless steel, titanium) and tooth-colored posts (eg, glass- or polyethylene fiber–reinforced composite resin or zirconia) associated with a direct restoration have become the preferred choice because their use eliminates one clinical appointment.118 A variety of prefabricated post materials and designs are available, but this discussion focuses on tooth-colored posts, specifically on zirconia (Fig 5-26). When zirconia posts are used, the restorative possibilities for core purposes are composite resin core, direct ceramic core heat pressing, and indirect ceramic core processing.

Beside being tooth colored, an ideal post material should ideally present some of the following characteristics already shown to be important when used in combination with allceramic crowns119: good bond strength to resin cements; resistance to flexural forces; modulus of elasticity similar to that of the root dentin (16 GPa); user friendliness during insertion and removal, should endodontic retreatment be necessary; easy adaptation to the root canal without requiring excessive tooth structure removal; and biocompatibility. Although zirconia posts do not fulfill all of these characteristics, some of them can be improved, such as their bond strength to resin cements by silica deposition techniques; others simply cannot, such as ease of removal and modulus of elasticity. Zirconia posts are virtually impossible to grind, and their removal along with root dentin preservation becomes a challenge, if required.116 Only two clinical studies are available concerning the survival rates of zirconia posts, both with follow-up periods of less than 5 years. In one study, 30 zirconia posts (only 5 were in the posterior region) that had heat-pressed ceramic cores and were bonded with glass-ionomer cement were followed for 29 months. No complications such as fracture or loss of retention were reported.120 In another clinical study, the authors evaluated the survival rates of 34 zirconia posts with pressed glass-ceramic cores and 79 zirconia posts with direct composite resin cores after a follow-up period of up to 5 years. No failures were observed for the

High-Strength Ceramic Posts and Cores

a

b

c

d

Fig 5-26  (a) Two vital maxillary lateral incisors exhibit a considerable loss of coronal tooth structure in an elderly patient with severely calcified canals. (b) After tooth reduction and successful endodontic treatment, a post space has been prepared, prefabricated zirconia posts have been fitted in the canals, and a core has been molded with self-polymerizing resin applied with the salt and pepper technique. (c) The cores have been fabricated in a pressable ceramic on the zirconia post and adhesively luted in the canal with a composite resin cement. (d) The definitive pressable leucite-based glass-ceramic crowns have been cemented with a composite resin cement. (Courtesy of Dr Jonathan Ferencz, New York, New York.)

zirconia with composite resin core group, but three failures due to loss of retention were reported in the pressed glass-ceramic group.121 Although results seem promising, the overall evidence from these studies suggesting that zirconia posts are a scientifically validated and safe treatment method should be interpreted with caution. Longer followup periods in well-designed clinical studies are necessary.

An interesting theoretical point concerns the relevance of having posts with similar modulus of elasticity to those of the root dentin. Remarkably, the available clinical studies fail to show any root fractures and, although such a finding may have commonly been found in in vitro studies (mainly observed in single-load-to-failure tests or “crunch

the sample tests� sometimes associated with previous cyclic fatigue),116,122,123 the authors believe that such results should also be interpreted with caution. If root fractures have not been described in clinical studies, but are commonly observed in laboratory studies, it may be appropriate to critically evaluate the information of such studies and reconsider the relevance of the methodologies being used.

Because the majority of studies on zirconia posts are in vitro, limited recommendations can be provided in light of the existing evidence-based information.116 Therefore, future long-term randomized clinical trials comparing the survival rates and complications of zirconia posts with fiberreinforced composite resin and cast posts are warranted.

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Conclusion High-strength all-ceramic materials used for tooth-­ supported single crowns are a reliable treatment option for restoring anterior and posterior teeth. However, for a successful outcome, a multitude of factors must be carefully addressed, including patient selection, clinical procedures, and laboratory techniques. Future incorporation of new ceramic materials and processing technologies into esthetic dentistry must be carefully weighed and should definitely be preceded by relevant laboratory testing prior to clinical use. Science, industry, clinical, and laboratory expertise on high-strength ceramics have evolved to an extent that current knowledge and evidence are abundantly available to promote their successful use. This collaborative effort remains absolutely critical to the future development and adoption of new materials.

Acknowledgments Dr Gracis would like to thank Luca Vailati, CDT, and Marco Cossu, CDT, for their invaluable collaboration and expertise in the delivery of highquality, natural-looking restorations. Dr Bonfante would like to acknowledge grants from Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) (No. 4695/06-2) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (No. 2010/06152-9). The authors would like to specially acknowledge Dr Van P. Thompson and Dr Elizabeth Dianne Rekow for their invaluable leadership and mentorship on ceramics and research.

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