Ceramic Applications to Restore Implants
8
Joerg R. Strub, dmd, dr med dent, dr hc, phd Michael V. Swain, bsc, phd
Fixed implant reconstructions such as single implant-supported crowns and multiple-unit fixed dental prostheses (FDPs) are well documented in the literature and are fully accepted as a treatment option for the replacement of single or multiple missing teeth.1,2 The osseointegration of dental implants has been thoroughly investigated and found to be highly predictable.3,4 Implant-supported reconstructions exhibit excellent clinical survival rates. In a recent systematic review, implant-supported crowns and FDPs also had high survival rates. resembling those of tooth-borne reconstructions, amounting to 95% at 5 years.1 However, the clinical success of implant reconstructions depends not only on the survival rates but also on the extent of biologic and technical complications occurring during clinical function. To improve the clinical success, the materials and techniques for implant-supported reconstructions are constantly analyzed to determine which provide the most predictable outcomes.1,2 In addition, the ideal connection between the implant and the reconstruction is also frequently questioned.5,6 The purpose of this chapter is to address the current level of knowledge regarding available ceramic materials and their properties, clinical outcomes of various ceramic materials, and the ideal connection to restore implants. In addition, alternative materials and fabrication techniques are presented and future perspectives discussed.
Available Ceramic Materials A wide range of ceramic materials have been developed for clinical restorative purposes. These range from nearly 100% glass to dense, fine-grained, sintered ceramics. Traditionally, the former were the basis of the veneer and the latter the supporting coping material. However, the situation today has become more diverse as both high-strength zirconia and glassy materials are being used in situations where direct contact with the opposing dentition occurs. This section briefly outlines the various types of materials available with a specific focus on their microstructure and resultant properties.
149
8
Ceramic Applications to Restore Implants
a
2 µm
b
2 µm
Fig 8-1 (a) Scanning electron microscopic (SEM) image of the fracture surface of a somewhat more opaque porcelain, showing the presence of different crystallographic inclusions. (b) SEM image of the fracture surface of a more translucent glassy porcelain, showing the presence of pores and the underlying structure of zirconia substrate.
Feldspathic porcelains have had a long association with dentistry.7 In fact, they were part of the initial development of crowns by Dubois de Chémant in 1789.8 These materials have traditionally been obtained from naturally occurring orthoclase feldspars and subjected to various melting, fritting, blending, and pigmenting processes. More recently, chemically derived sources of porcelains have become available; they offer a more consistent composition and are generally more translucent. The mechanical, thermal expansion, and optical properties of feldspathic porcelains may be adjusted by utilizing additives and adjusting their composition. All these factors are important because the porcelain must match the thermal expansion of the substrate to which it is bonded, hide the underlying color of the substructure, and match the shade of the adjacent dentition. Matching the color and form of the adjacent teeth has traditionally been achieved through the art and creativity of the dental technician. With the advent of computer-aided design/computer-assisted manufacture (CAD/CAM) milling systems, more complex, multilayer shaded blocks are available to achieve this outcome without the manual build-up process associated with conventional powder-based porcelains. Typical observations of the microstructure of dentin and incisal feldspathic porcelains are shown in Fig 8-1. The porcelain in Fig 8-1a shows the presence of numerous inclusions containing various additives, including tin oxide, titanium oxide, and zirconium oxide. These inclusions have higher refractive indexes than the feldspathic glasses, thereby scattering the light and imparting a degree of opacity to 150
the porcelain to hide the underlying substructure. In the incisal edge porcelain (Fig 8-1b), which has much higher translucency, there are limited inclusions that must closely match the optical properties of the natural teeth and impart a degree of fluorescence similar to that of the natural teeth. Porcelains are now available in a range of forms for prosthetic use, from those that involve traditional powder build-up and sintering to CAD/CAM milling and glazing to pressing. The mechanical properties of porcelains can be altered by the composition, the nature of the inclusions, the extent of air bubble entrapment, and the extent of residual stresses developed during cooling. This issue is taken up in more detail later in the chapter. Leucite (K2O∙Al2O3∙4SiO2) has long been used as an additive to porcelain. Its refractive index is very close to that of feldspathic porcelain, but it has a much higher coefficient of thermal expansion. It has traditionally been added to feldspathic porcelains to adjust their thermal expansion to match that of the underlying substructure. Porcelains used for bonding to noble metals require a relatively high percentage of leucite; however, it has been shown that the size of the leucite particles is critical for both the resultant strength and the opacity of the porcelain.9 Coarse leucite grains, with their much higher coefficient of expansion, undergo twinning and circumferential cracking on cooling. The presence of leucite within feldspathic porcelain was found to improve toughness but reduces strength.9 About 25 years ago, a pressable leucite-containing glass-ceramic with 20% to 25% leucite by volume was developed; the material showed greater strength and fracture toughness than feldspathic porcelains.9
Available Ceramic Materials
a
b
Fig 8-2 (a) SEM image of IPS e.max Press material (Ivoclar Vivadent). (b) SEM image of IPS e.max CAD material prior to ceramming. (c) SEM image of IPS e.max CAD material after ceramming. (Courtesy of Ivoclar Vivadent.)
c
Lithium disilicate–based glass-ceramics have been available for more than 40 years. However, they have been used in dentistry only for the past 20 years. These materials were developed by Hölland et al,10 and the associated processing technology enabled them to be pressed directly into the desired form in a manner similar to the lost wax casting process, resulting in a net shape-forming capability. These materials have very a high percentage (65% to 70%) of needle-shaped lithium disilicate crystals that impart substantial strength and toughness. A specific feature of the pressed version of this material was the alignment of the crystals as they flowed through the sprue and narrow regions for formation of the tooth restoration.9 The high strength and toughness along with associated difficulty in milling lithium disilicate–based materials have more recently led to the release of a precrystallized block that can be readily machined. After milling, the restoration is cerammed so that the lithium disilicate crystals are coarsened, resulting in a two- to threefold increase in strength and toughness.10 Typical examples of the microstructure of these materials are shown in Fig 8-2.
Aluminum oxide (Al2O3), or alumina, is a high-strength technical ceramic that has a wide range of applications, including hip prostheses. This material is typically a finegrained powder that is pressed and then sintered at 1,300°C to 1,500°C to achieve high density. It has high elastic modulus and hardness values and exhibits reasonably high strength and fracture toughness values. In addition, alumina has a slightly higher refractive index than porcelain. The strength of an alumina ceramic is dependent on the grain size and the density achieved after sintering. In the case of orthopedic hip prostheses, strengths of 500 to 600 MPa are typical with grain sizes between 2 and 4 μm. It is possible to achieve much finer-grained sintered alumina materials, and these have been developed for ceramic orthodontic brackets. These materials have a submicron grain size, increased translucency, and strengths up to 1 GPa. Despite the use of alumina for orthopedic prostheses, the adoption of alumina for dental restorative purposes has been hampered by the significant shrinkage of alumina
151
8
Ceramic Applications to Restore Implants
a
b Fig 8-3 (a) SEM image of the typical coarse microstructure of MgO– partially stabilized zirconia (Mg-PSZ), showing minor presence of a monoclinic phase at the grain boundaries. (inset) There is a high density of ellipsoidal fine tetragonal precipitates of the highest strength or peaked age (PA) material within the coarse cubic matrix that are metastable and undergo tetragonal-monoclinic transformation on grinding or at a crack tip. A crack passing through this microstructure shows multiple branching as well as phase transformation and follows a highly tortuous path because of the fine tetragonal precipitates. (b) SEM image of Y-TZP. (c) SEM image of In-Ceram Alumina-Zirconia. (Courtesy of Dr Richard Hannink, Melbourne, Australia.)
c
during sintering and the difficulty of machining it. During the past decade, with the advent of scanning systems and electronic transfer of files, as well as the development of CAD/CAM, this limitation has been overcome, and blocks of partially sintered material can be machined oversized and then sintered to full density with resultant shrinkage in size. Prior to the availability of dense alumina structures and abutments for implants, porous alumina (as well as spinel [MgO∙Al2O3] and alumina-zirconia) materials were available; these were produced by slip casting or copy milling from presintered porous blocks that were subsequently infiltrated with a lanthanum-based glass. These innovative materials, developed by Sadoun,11 have been used to fabricate very successful all-ceramic restorations with clinical follow-up of almost 20 years.12 Zirconium oxide (ZrO2), or zirconia, is a material that has attracted considerable attention during the past 30 years. For the past decade or so, it has experienced increased utilization and there has been considerable debate over its role
152
in dentistry. In its pure state, this material is unsuitable for structural applications because it undergoes phase changes on heating and cooling. There are substantial changes in volume (3% to 4%) as the monoclinic phase undergoes a transformation to the tetragonal phase at 1,100°C. However, the addition of divalent and trivalent metallic ions such as magnesium (Mg), calcium (Ca), and yttrium (Y) stabilizes the high-temperature tetragonal or cubic phases. The most critical discovery leading to greater acceptance of this material was that, when the tetragonal phase is stabilized, it is possible to achieve a stress-induced transformation around the crack tip. This results in a highly significant increase in the fracture toughness and strength. This finding was reported by Garvie et al,13 and since then there has been considerable development of these phase-transforming materials.14 These materials are generally grouped into three principal classes: partially stabilized zirconia (PSZ), stabilized tetragonal zirconia polycrystals (TZP), and composite mate-
Available Ceramic Materials
a
b
c
d
e
f
g
h
i
j
k
l
Fig 8-4  (a to c) Pretreatment condition. The patient is missing several permanent teeth and thus retains some primary teeth. The canine, first and second premolars, and third molars are missing bilaterally in the maxilla. In the mandible, the first and second premolars and both third molars are missing bilaterally. (d) NobelReplace implants placed in the areas of the maxillary canine and first and second premolars bilaterally. (e) NobelProcera Zirconia modified abutments. (f ) NobelProcera Zirconia cemented crowns. (g) NobelReplace implants, placed in the areas of the first and second premolars bilaterally with NobelProcera Zirconia regular abutments. (h) Panoramic radiograph. (i) NobelProcera Zirconia cemented implant-supported crowns. (j to l) Restorations after 4 years.
rials such as alumina-zirconia. The PSZ materials generally have a relatively coarse cubic grain structure containing Mg or Ca ions with tetragonal precipitates (Fig 8-3a), while the TZP materials have a fine-grained, primarily tetragonalgrained structure containing yttrium ions (Fig 8-3b), and
the alumina-zirconia contains ceria-zirconia (white grains) and alumina porous network structure infiltrated with glass (Fig 8-3c). A clinical example of the use of zirconia-based cement-retained crowns to replace missing teeth is shown in Fig 8-4. 153
8
Ceramic Applications to Restore Implants
Biologic and Mechanical Properties of Zirconia Ceramics Recent studies and critical reviews of plaque accumulation on zirconia ceramics have been presented by a number of authors. Hisbergues et al15 explored the biocompatibility of zirconia ceramics and stated that zirconia has the ability to reduce plaque accumulation. Degidi et al16 investigated soft tissue attachment to zirconia and titanium healing caps and observed that zirconia resulted in a reduced inflammatory response and less blood microvessel development than titanium; there were also implications that the amount of bacterial colonization was lower around zirconia than around titanium. In another study, Bianchi et al17 investigated a transmucosal zirconia collar and found it provided better tissue stabilization than titanium. Rimondini et al18 and Scarano et al19 investigated the in vivo responses of zirconia and titanium disks of the same surface roughness attached to the molar area of patients for 24 hours. They found that zirconia had fewer bacteria attached than titanium. More recently, Scotti et al20 evaluated the effect of glazing and polishing of zirconia on early dental plaque formation and found no differences in bacterial presence at polished and glazed surfaces. The flexural strength of ceramics and brittle materials is a property that is frequently discussed, but often the consequences of various typical clinical procedures on such values are not addressed. Most brittle materials, including glass, can be made to have extremely attractive properties provided that the presence of defects is minimized. However, the processes of fabrication, such as shaping, grinding, or grit blasting followed by acid etching, may result in strengths far different from those obtained with the pristine polished surfaces that are often the basis for the ranking of materials. A recent study by Scherrer et al21 investigated the strengths of a variety of zirconia (yttrium-stabilized tetragonal zirconia polycrystal [Y-TZP]) ceramics after sintering and found that both the strength and variation in strength, as addressed using the Weibull analysis, varied significantly. Weibull analysis has a very important function, because in principle it enables the effect of volume under test to be appreciated and explains why three-point bending results in higher strengths than four-point bending.22 In the case of Y-TZP, the presence of phase transformation toughening generated by grit blasting can result in a strength increase, provided that the compressive stresses generated by grit blasting are not released by heating the ceramic above 600°C. Such heating transforms the monoclinic phase back to the tetragonal variant. The latter transformation process generally results in reduced strength, because the defects 154
and flaws generated by the grit blasting are now bigger and more prevalent than for the as-sintered or polished surface. Also, the firing to porcelain-sintering temperatures does not heal cracks, although the presence of porcelain may result in glass diffusing into the flaws generated. An area of particular interest at present is the consequence of grinding and adjusting of the surface of Y-TZP on the resultant strength of the material. Recent studies by a number of authors have suggested that the extent of the tetragonal-monoclinic phase transformation is relatively limited. Instead, a significant rhombohedral phase forms, which has a much reduced volumetric dilation and is a phase that has not been observed in high-resolution transmission electron microscopy of Y-TZP ceramics.23 Fracture toughness (K1c) is another area about which there has been much discussion in the literature. It is possible to generate significantly different K1c values, depending on the technique used to measure it. For instance, the values generated by indentation tests are notoriously optimistic, especially at lower loads, because of the compressive transformation stresses generated in zirconia-containing ceramics by indentation. Other tests that necessitate notching of a specimen may also generate phase transformations around the notch tip that develop localized compressive stresses, resulting in very optimistic values for toughness being reported. The most realistic values of toughness are those generated using far more expensive stable crack extension sample geometries, such as the double cantilever beam and chevron notch tests. Typically, values obtained for the toughness of zirconia when these approaches are used are approximately 5 MPa · m1/2, but slow crack growth in moist conditions occurs at values well below this.24 The transformation toughening mechanism involves tensile stress–induced transformation around the tip of a loaded crack. In the case of zirconia, the volume dilation associated with the metastable tetragonal-monoclinic phase change results in a volume dilation of 3% to 4%. The extent of the toughness increase associated with this mechanism is dependent on both the volume fraction of the tetragonal phase that transforms as well as the size of the transformed zone. The size of the transformed zone is dependent on the critical stress to trigger the tetragonal-monoclinic transformation. This outcome results in a compromise between the strength and toughness of transformation-toughened ceramics. This is a very tough transformation-toughened ceramics “yield,” whereas high-strength ceramics are flaw-size dependent and have a relatively lower fracture toughness.25 Another feature of transformation-toughened ceramics that is often not appreciated is that their strength and toughness decline with increasing temperature because of the stability of the tetragonal phase at elevated temperatures.26 Low-temperature degradation (LTD) is a unique
Current Issues phenomenon that involves the slow transformation of the metastable tetragonal phase to monoclinic phase in the presence of moisture. This was appreciated more than 30 years ago.27 It is far more prevalent in yttria (Y-TZP) ceramics and follows a classic time-temperature-transformation response in that the process has an activation energy and the maximum rate takes place at about 200째C but still occurs at body temperatures. The process is also highly dependent on the microstructure and composition of the Y-TZP; lower yttria content, porosity, and larger grain size contribute to enhanced metastability. There is limited evidence currently available to suggest that the phenomenon of LTD has been instrumental in the failure of dental prosthodontic systems; however, a decade ago it caused a major catastrophe in the orthopedic community, when more than 500 hip prostheses failed in vivo from inattention to the microstructure and porosity that resulted from sintering and hot isostatic pressing.28 To limit the extent of LTD, manufacturers have used various additives such as alumina and other stabilizers such as ceria (CeTZP) and alloys.29
Current Issues Despite the fact that zirconia ceramics, and in particular Y-TZP compositions, have been available for more than 30 years, there are some perennial issues that are still important and other specific issues that have arisen with dental usage of these materials.
Aging of zirconia Ceramic materials, because of their highly inert nature, are generally considered very stable in moist environments. Some glasses, typically more alkaline porcelains, may exhibit minor solubility issues and release various ions. However, the metastability required so that transformation toughening can occur in zirconia ceramics results in aging or the LTD process described earlier. This feature was also belatedly appreciated by the orthopedic community with the result that polymeric acetabular cups had higher wear rates than alumina ceramics because of the enhanced roughness of Y-TZP that resulted from grain uplift and surface microcracking. The influence of the LTD process results in the presence of monoclinic nucleation at the surface, which gradually extends deeper into the Y-TZP ceramic. Initially there is an increase of strength of the ceramic as the volume dilation associated with this transformation induces compressive stresses at the surface. However, as the depth of the transformation increases, microcracking becomes more dominant and there is a decrease in strength
as well as a decrease in hardness and surface elastic modulus. These detrimental effects lead to enhanced wear of not only the ceramic but also the materials with which it is in contact. In the case of Y-TZP materials in the dental environment, a number of clinical and laboratory procedures may contribute to aging and initiation of the LTD process. For instance, it is common for materials to be steam cleaned prior to use, which would assist in the more rapid LTD nucleation of the transformation. Also, with the baking of porcelain on various frameworks, it is typical to grit blast, steam clean, and then place a wet porcelain layer on the surface; this layer is dried in an accelerated manner sitting beneath the open furnace prior to sintering. All of these processes will contribute to the presence of monoclinic zirconia on the surface of Y-TZP ceramics.30 Another factor that is anticipated to result in enhanced aging is the high-temperature sintering of Y-TZP materials that do not contain alumina. This procedure has become popular to enhance the translucency of Y-TZP ceramics and thereby to attain more esthetic outcomes.
Veneer failure During the past 5 years or more, researchers have reported clear evidence that veneer fracture occurs at a higher rate on zirconia cores than on most other materials.31,32 The extent of this failure has in some instances been as high as 50%. As a consequence, there has been intense research and speculation regarding the possible causes of this behavior. Some of the mechanisms suggested for such failure include core design, the presence of residual stresses associated with thermal expansion mismatch between the veneering material and the zirconia core, and cooling procedures that induce so-called tempering stresses following the sintering or final glaze firing cycle. Fischer et al33 investigated a range of porcelains used for veneering to zirconia and showed that there was a weak correlation between the shear bond strength and thermal expansion mismatch. Swain,34 on the other hand, suggested that the major contributor was the presence of tempering stresses associated with final cooling; he contended that the very low thermal conductivity of zirconia resulted in large temperature gradients between the surface and interior parts of the crown. This was particularly the case with thick sections of porcelain, such as at the cuspal regions of crowns, where chipping was most prevalent. Other researchers have argued that the cause of the problem is the limited support offered by the traditionally shaped core, which is typically 0.5 mm thick. They recommended use of an anatomical design framework with a much thinner layer of veneering porcelain.35,36 This ap155
8
Ceramic Applications to Restore Implants
a
b
2 mm
Fig 8-5 SEM images of veneer failure after occlusal adjustment. (a) Occlusal view of veneer failure (circle). (b) Detail of failure (arrows). (Courtesy of Dr Irene Sailer, Geneva, Switzerland.)
proach certainly results in less extensive chipping, although the outcome of such crowns may be less esthetic. These issues and the advances possible with CAD/CAM systems have led to the development of newer systems in which the porcelain or glass-ceramic veneer is separately milled and bonded with either a low temperature–fusing glass or resin to the milled substrate. This approach allows lower sintering temperatures or even elimination of a final firing to generate the crown. Choi et al37 showed that substantial residual stresses were developed as a consequence of faster cooling, and these could be directly associated with the temperature difference between the surface and core of the porcelain during cooling through the glass transition temperature.
zirconia ceramic, a monoclinic phase will be observed on the treated surface. This monoclinic transformation will increase the strength of the restoration.40,41 However, if a crack initiates in that area, there will be no transformation toughening mechanism available anymore to oppose crack propagation, because the tetragonal phase has already transformed.42 Therefore, special attention has to be paid to the static and dynamic occlusion of zirconia-based, implant-supported restorations.43 Occlusal adjustment should be performed with fine-grain diamonds followed by a polishing sequence (Fig 8-5).
Handling
The majority of implant manufacturers offer zirconia abutments for esthetic implant-supported restorations because it was reported that metallic restorations can cause dis coloration of the mucosa.44 Several authors have shown that all-ceramic restorations on ceramic abutments provide a better color match to unrestored adjacent teeth than porcelain-fused-to-metal restorations.45–47 Zirconia abut ments are available in prefabricated or customizable forms
Clinicians and laboratory technicians should follow precise treatment steps when fabricating zirconia-based restorations.38 Zirconia as a framework material is highly susceptible to surface modifications resulting from improper laboratory and clinical handling.39 If any subtractive procedure (sandblasting or grinding) is performed after sintering of 156
Zirconia Abutments
Zirconia Abutments TABLE 8-1 In vitro fracture strengths of zirconia abutments Study
Implant system
Abutment
Restoration/tooth
Restorative material
Yildirim et al52
Brånemark standard
Wohlwend ZrO2
Zirconia
SC/maxillary incisor
Glass-ceramic
No
738
Butz et al53
Osseotite
ZiReal
Zirconia with titanium insert
SC/maxillary incisor
Nonprecious metal
TCML
NR
Att et al54
NobelReplace
Aesthetic Zirconia
Zirconia
SC/maxillary incisor
Procera Alumina
TCML
470
NobelReplace
Aesthetic Zirconia
Zirconia*
SC/maxillary incisor
Procera Zirconia
TCML
593
Gehrke et al
XiVE XiVE
Cercon Cercon
Zirconia Zirconia
Spherical caps/ maxillary incisor
TCML No
269 672
Canullo et al57
ProUnic
Custom-made zirconia
Zirconia
NR
NR
No
436
Att et al
55
56
Wiskott et al58
Fatigue loading
Mean fracture load (N)
Abutment material
Replace Select
Aesthetic Zirconia
Zirconia
NR
NR
TCML
59
Kerstein and Radke
Brånemark standard
Atlantis
Zirconia
NR
NR
740
830
60
Sundh and Sjögren
Straumann Straumann
Denzir M Denzir
Magnesia-zirconia Zirconia
Ceramic copy
Ceramic
No
430 470
Sailer et al61
Straumann standard Brånemark standard NobelReplace Straumann standard
CARES Procera Procera Zirabut
Zirconia Zirconia Zirconia Zirconia
SC/maxillary incisor
All-ceramic
No
378 416 490 246
Adatia et al62
Astra Tech
Y-TZP (reduction)
Zirconia
NR
NR
NR
429–547
Albrecht et al63
Straumann
Prototype
Zirconia
NR
NR
NR
705
Att et al64
NobelReplace
Aesthetic Zirconia (reduction)
Zirconia
SC/maxillary incisor
Nonprecious metal
TCML
451–491
Nothdurft et al65
OsseoSpeed
ZirDesign (reduction)
Zirconia
SC/maxillary incisor
Nonprecious metal
TCML
269–355
Koutayas et al
OsseoSpeed
ZirDesign (reduction)
Zirconia
SC/maxillary incisor
Glass-ceramic
No
294–384
66
56
SC, single crowns; TCML, thermocycling and mechanical loading; NR, not reported. Manufacturers: Brånemark standard, Nobel Biocare; ZrO2, Wohlwend Innovative; Osseotite, Biomet 3i; ZiReal, Biomet 3i; NobelReplace, Nobel Biocare; Aesthetic Zirconia abutments, Nobel Biocare; Procera, Nobel Biocare; XiVE, Dentsply; Cercon, Dentsply; ProUnic, Impladent; Custom-made zirconia abutments, Zirkonzahn; Replace Select, Nobel Biocare; Atlantis, Dentsply; Straumann implants, Straumann; Denzir, Decim; CARES, Straumann; Zirabut, Wohlwend Innovative; Astra Tech, Dentsply; OsseoSpeed, Dentsply; ZirDesign, Dentsply.
and can be prepared in the dental laboratory by hand or by CAD/CAM techniques. Zirconia abutments are successors to the densely sintered high-purity alumina abutments. Compared with the latter, zirconia abutments are more radiopaque and demonstrated significantly higher fracture resistance.48 It is well known that ceramics, including zirconia, are highly biocompatible and are less prone to plaque accumulation than metal substrates.16,18,19 It is commonly agreed that ceramic abutments should show proper resistance to masticatory forces generated during chewing or swallowing.49 Researchers have reported a mean loading force of 206 N and maximum biting forces of up to 290 N in the esthetic zone.50,51 Several laboratory studies have evaluated the fracture strength values of different zirconia abutments (Table 8-1). No implantsupported FDPs with zirconia abutments were tested; all
studies identified used implant-supported single crowns. The resistance-to-fracture values ranged from 246 to 737 N for specimens not subjected to fatigue loading and from 56 to 593 N for specimens subjected to fatigue loading.52–66 In four studies, the wall thicknesses of zirconia abutments were reduced by grinding and compared with unmodified abutments.62,64–66 In three of the four studies, there were no statistically significant differences in the fracture strength values between the test and control groups. Nevertheless, there is a need to explore the effect of grinding procedures on the resistance of zirconia abutments as well as to identify the minimal wall thickness that guarantees long-term stability. The lack of knowledge about the outcome of zirconia abutments in restorative systems other than single crowns, as well as the effect of the abutment’s design on its resistance, highlights the necessity for further evaluation of 157
8
Ceramic Applications to Restore Implants TABLE 8-2 Clinical outcome of zirconia abutments Survival rate Observa- AbutRestoration period ments tions
Study design/ restoration/ material
No. of restorations
Resin cement
Prospective/ SC/press ceramic
54 abutments/ NR
4y
100%
NR
NR
Zirconia abutments with titanium connection
Resin cement
Prospective/ SC/zirconia
30
3.3 y
100%
100%
NR
XiVE
Zirconia (Cercon)
Resin modified glass-ionomer cement
Prospective/ SC/ posterior Y-TZP
40
6 mo
100%
NR
Zembic et al69
Brånemark
Zirconia (Procera)
Resin cement
RCT/SC/ glass-ceramic, alumina, or zirconia
18
3y
100%
100%
NR
Ekfeldt et al70
Replace Select
Zirconia (Procera)
Different cements
Retrospective/SC/ alumina or zirconia
185
3–5 y
99%
100%
NR
Implant system
Abutment
Cement
Glauser et al67
Brånemark
Zirconia
Canullo68
TSA
Nothdurft and Pospiech43
Study
Complications
Chipping of veneering ceramic: 7.5%
SC, single crowns; NR, not reported; RCT, randomized controlled trial. Manufacturers: Brånemark, Nobel Biocare; Zirconia abutment, Nobel Biocare; TSA, Impladent; XiVE, Dentsply; Cercon, Dentsply; Procera, Nobel Biocare; Replace Select, Nobel Biocare.
TABLE 8-3 In vitro fracture strengths of implant-supported zirconia-based crowns and FDPs Restoration/ framework material
Veneering material
Fatigue loading
Mean fracture load (N)
VM9
TCML
593
Posterior 3-unit FDPs/ Creation Procera Zirconia Three different framework/bar designs
No fatigue
Initial crack/ final fracture load Straight bar: 644/1,292 Occlusal curve: 476/1,398 Gingival curve: 722/1,040
Posterior 3-unit FDPs/Cercon Base Three different framework/bar designs
Cercon Ceram S
No fatigue
Initial crack/ final fracture load Regular pontic: 682/916 Occlusal curve: 439/1,691 Gingival curve: 945/1,516
Resin cement
Posterior 3-unit FDPs/Lava
Lava Ceram
Accelerated step-stress fatigue
Prior to fatigue: 693 Load at which 63.2% would fail: 497
Glass-ionomer cement
Posterior 3-unit FDPs/Cercon Base
No veneer
No fatigue TCML
Standard abutment/ Individual abutment No fatigue: 473/424 TCML: 647/556
Study
Implant system Abutment
Cementation
Att et al55
Nobel Replace or Nobel Select
Y-TZP Abutments (Aesthetic Zirconia)
Resin cement
Anterior SC/Procera Zirconia
Kokubo et al71
NR, Straumann
NR
Resin cement
Tsumita et al72
Brånemark Mk III
Titanium (Procera)
NR
Bonfante et al73
NR, Nobel Biocare
Abutment Replicas (Replica Snappy Abutment)
Nothdurft et al74
XiVE S
Y-TZP (Cercon) Standard or individualized
SC, single crowns; NR, not reported; TCML, thermocycling and mechanical loading. Manufacturers: NobelReplace, Nobel Biocare; Nobel Select, Nobel Biocare; Aesthetic Zirconia Abutments, Nobel Biocare; Procera Zirconia, Nobel Biocare; VM9, Vident; Straumann implants, Straumann; Creation, Geller; Brånemark Mk III, Nobel Biocare; Cercon, Dentsply; Replica Snappy Abutment, Nobel Biocare; Lava, 3M ESPE; XiVE S, Dentsply.
158
Implant-Supported Zirconia-Based Fixed Restorations TABLE 8-4 Clinical performance of implant-supported zirconia-based crowns and FDPs Study
Implant system
Abutment
Cementation
Study design/ restoration/ material
No. of restorations
Observation period
Incidence of fracture (%) Abutment Framework Veneer fracture fracture fracture
Larsson et al75
Titanium (Astra Tech Standard)
Zinc phosphate Titanium cement Abutments (Astra Tech ST)
Prospective/posterior 2- to 5-unit FDPs/ Denzir with Esprident Triceram
13 FDPs
12 mo
0
0
53
Kohal et al76
Y-TZP one-piece
NA
Glass-ionomer cement
Prospective/SC and 3-unit FDPs/ Procera Zirconia
65 SC 27 FDPs
15.3 mo 13 mo
NA
0
18.5 41
Nothdurft and Pospiech43
Titanium (XiVE S)
Y-TZP Abutments (Cercon)
Resin-modified glass-ionomer cement
Prospective/posterior SC/Cercon
40 SC
6 mo
0
0
7.5
Larsson et al77
Titanium (Astra Tech)
Titanium Abutments (Astra Tech)
Zinc phosphate cement
Prospective/ 2- to 5-unit FDPs/ Denzir with Esprident Triceram or In-Ceram with Vitadur Alpha
25 FDPs
60 mo
0 0
0 0
69 17
Larsson et al78
Titanium (Astra Tech)
Titanium Abutments (Astra Tech)
Resin cement
Prospective/ 9- to 10-unit FDPs/ Cercon
10 FDPs
36 mo
0
0
34
Hosseini et al79
Titanium (Astra Tech Standard)
ZirDesign (Astra Tech)
Zinc phosphate cement or resin cement
RCT/posterior SC/ KaVo Zirconia or Procera Zirconia
38 SC
12 mo
0
0
0
Schwarz et al80
Titanium (Tissue Level, Bone Level, or NobelReplace)
NR
Different cements Retrospective/ SC/Cercon
53 SC
24 mo
NR
0
24.5
SC, single crowns; RCT, randomized controlled trial; NA, not applicable; NR, not reported. Manufacturers: Astra Tech, Dentsply; Denzir, Decim; Esprident Triceram, Dentaurum; Procera Zirconia, Nobel Biocare; XiVE S, Dentsply; Cercon, Dentsply; ZirDent, Zirkonzahn; In-Ceram, Vident; Vitadur Alpha, Vident; KaVo Zirconia, KaVo Dental; Tissue Level implants, Straumann; Bone Level implants, Straumann.
these parameters under laboratory conditions before these devices receive widespread clinical application. Less information is available on the clinical outcome of zirconia abutments (Table 8-2). Over observation periods between 6 months and 4 years, the survival rates of zirconia abutments were 100%.43,67–70 A systematic review estimated a 5-year survival rate of 99.1% for zirconia abutments, which was similar to that estimated for metal abutments (97.4%).49 Despite encouraging short-term data, there is a need for long-term data concerning the treatment outcomes of zirconia abutments.
Implant-Supported Zirconia-Based Fixed Restorations The fracture strength of implant-supported zirconia-based restorations has been evaluated in a small number of laboratory studies (Table 8-3). Only five investigations with vari-
ous testing protocols and study designs could be identified.55,71–73 For zirconia-based implant-supported single crowns, the resistance-to-fracture values amounted to 593 N. For zirconia-based FDPs, the resistance-to-fracture values ranged between 424 N and 1,691 N. Initial restoration failure was caused by failure of the veneering ceramic. Clinical data concerning treatment outcomes of zirconiabased implant-supported restorations are still scarce. Apart from case reports, only seven short-term clinical studies on zirconia-based implant-supported crowns and FDPs could be identified43,75–80 (Table 8-4). Fracture rates within the veneering ceramic of implant-supported zirconia-based single crowns ranged from 0% to 18.5% after 6 and 24 months. Even higher failure rates (17% to 69% after 12 to 60 months) were reported with implant-supported zirconia-based FDPs. None of the studies revealed fractures of zirconia frameworks or implant-supported single crowns or FDPs. In summary, implant-supported zirconia-based crowns and FDPs exhibited an unacceptable number of veneer chipping failures (Fig 8-6; see Table 8-4). Impaired proprio159
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Ceramic Applications to Restore Implants
a
b
Fig 8-6 Chipping of veneering materials on zirconia frameworks. (a) NobelRondo crown. (b) Three-unit Procera Zirconia FDP.
ception and rigidity of osseointegrated implants, resulting in higher functional impact forces, might further exacerbate porcelain fractures.
Connection Between Implant and Reconstruction An implant-supported reconstruction can be screw retained on the implant or abutment or cemented on abutments. Initially, screw retention was used for multiple-unit completearch FDPs in edentulous patients.81 Single crowns were generally cemented on prefabricated abutments.82 Both types of reconstruction exhibited satisfactory clinical longterm outcomes.81,83 Both cementation and screw retention have their benefits and shortcomings in clinical application.5,6 Cemented implant reconstructions are easier to fabricate and manipulate in the patient’s mouth. One shortcoming of cementretained crowns and FDPs is the difficulty with the removal of the excess cement. In vitro investigations have shown that excess cement always remains at the tested specimens, irrespective of the submucosal position of the crown margin.84,85 Wilson86 showed that excess cement causes peri-implantitis. Another shortcoming of cement-retained reconstructions is that, if complications arise, they are very difficult or virtually impossible to remove without damage or destruction of the reconstruction. The major benefit of screw-retained reconstructions is their retrievability.87,88 Technical complications such as loosening of retaining screws or fracture of the veneering ceramic have been reported.89 To date, very little scientific information is available com paring the advantages and disadvantages of cement re160
tention and screw retention.5,90 Sailer et al91 showed in a systematic review that neither of the fixation methods is clearly advantageous relative to the other. Cement-retained reconstructions exhibited more biologic complications (implant loss), while screw-retained reconstructions exhibited more technical problems. Because screw-retained reconstructions are retrievable, however, technical problems can be solved. These reconstructions therefore seem to be better from the biologic perspective. High costs of technical maintenance have not been taken into account, however.
Alternative Materials Aluminum ceramics The clinical data for implant-supported aluminum oxide ceramic restorations are very limited. Current evidence consists of reported 100% framework survival and 2% veneer fractures of Procera Alumina crowns after 3 years70 and 100% framework survival and 17% veneer fractures of InCeram Zirconia FDPs after 5 years.77
Monolithic CAD/CAM lithium disilicate (fatigue behavior) Monolithic CAD/CAM–fabricated, full anatomical lithium disilicate glass-ceramic crown restorations have recently been explored with promising results.92 The in vitro strength values of CAD/CAM lithium disilicate crowns on zirconia abutments were published by Albrecht et al.63 Short-term clinical experiences are promising (Figs 8-7 and 8-8).
Alternative Materials
a
b
d
c
e
Fig 8-7  (a to e) Straumann implant- and tooth-supported cemented CAD/CAM lithium disilicate crowns made using an intraoral digital scanner. They replace the mandibular left second premolar and first molar and restore the second molar.
161
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Ceramic Applications to Restore Implants
a
b
c
d
e
f
Fig 8-8  (a to c) Extraction of the hopeless maxillary left central incisor. (d to f ) NobelActive implant-supported all-ceramic restoration.
162
Alternative Materials
CAD/CAM materials sintered to zirconia To overcome chipping fractures of veneered zirconia restorations, Beuer et al93 have suggested a novel approach. They asserted that sintering a CAD/CAM lithium disilicate veneer cap on the zirconia coping can significantly increase the mechanical strength of crowns and FDPs and represents a cost-effective way of fabricating all-ceramic restorations. To date, no published clinical studies have looked at this method, and further in vitro studies are needed before this type of restoration can be utilized clinically.
Zirconia cores and CAD/CAM veneers Studies have shown that CAD/CAM–guided application of lithium disilicate ceramic veneer on zirconia cores resulted in fatigue-resistant crowns (2,699 N), whereas pressed (1,507 N) and hand-layered veneer crowns (1,195 N) were prone to veneer chipping after exposure to the artificial environment.93,94 Single load to failure of pressed and layered veneer crowns resulted in veneer chipping, whereas CAD/CAM veneer crowns showed bulk fractures exposing the resin preparation.94 No clinical data have been published on the outcome of CAD/CAM application of lithium disilicate ceramic veneer on zirconia cores.
Composite resin abutments and bonded CAD/CAM veneers There are some concerns about the use of zirconia abutments, such as difficulty bonding to this substrate, the risk of propagating fractures while trimming prefabricated abutments, and the absence of shock absorption during occlusal loading. The aim of a study published by Magne et al95 was to assess in vitro the fatigue resistance and failure mode of type III veneers (porcelain versus composite resin) bonded to CAD/CAM composite resin implant abutments. The researchers were able to show that porcelain veneers bonded to custom composite resin implant abutments presented a higher survival rate than composite resin veneers. The survival probability of composite resin abutments did not differ from that of zirconia ones.
Monolithic zirconia restorations Complete anatomical zirconia restorations with subsequent surface characterization and glazing have been developed (Zirkonzahn).96 Monolithic zirconia was used to decrease the risk of chipping or fracture. Monolithic zirconia restorations are claimed to have the following advantages:
• There is no issue with chipping. • There is no need for veneer application. • Thermal mismatch between core and veneer is avoided. • Only color glazing is needed. • There is no wear on zirconia. Monolithic zirconia also has several disadvantages: • Esthetic characterization options are limited. • Aging of zirconia is a possibility. • In vivo data on outcomes are limited. Rosentritt et al97 compared the wear of dental porcelain and substructure oxide ceramics after exposure to enamel in a chewing simulator. They did not find any wear of the enamel with glass-infiltrated ceramics and zirconia in the as-polished condition. From the point of wear testing, zirconia may be used for the fabrication of FDPs without veneering; however, the influence of surface adjustment with burs, intraoral polishing, and food abrasion is unknown. Similarly, little experience has been reported for application of monolithic zircona in removable dentures. Bühler et al98 presented examples of the clinical and technical fabrication of zirconia bars on implants and of the corresponding zirconia complete denture. To date, no clinical data regarding the performance of monolithic zirconia restorations on implants are available. Monolithic zirconia restorations have several potential issues. They have esthetic limitations, because the very high refractive index and relatively high opacity of zirconia may cause issues when the clinician is attempting to match the new restoration to existing restorations or natural teeth. Moreover, aging of zirconia or LTD in the oral cavity, as mentioned earlier, may cause surface roughness, especially in some of the current monolithic materials, because they are being sintered at higher temperatures and without alumina additives. These two factors result in larger tetragonal grain sizes, which are more metastable and more likely to experience LTD at a faster rate. The result is a rougher occlusal surface that will cause greater wear of the opposing surface. Currently, data are limited because these materials are relatively new to the marketplace.
Polymethyl methacrylate–based CAD/CAM materials Table 8-5 gives an overview of the polymethyl methacrylate (PMMA) products that are available on the market. To date clinical information regarding the longevity and survival rates of these products is lacking. However, their reported 163
8
Ceramic Applications to Restore Implants TABLE 8-5 PMMA-based CAD/CAM materials PMMA material
CAD/CAM system
artBloc TEMP (Merz Dental)
Cerec (Sirona)
Biotec CP (Teamziereis)
Datron D5 (Datron) TZ450i (imes-icore) MDX40a (Roland)
CAD-TEMP (Vident)
Cerec (Sirona) Everest (KaVo)
CARA-PMMA Prov (Heraeus Kulzer)
CARA System (Heraeus Kulzer)
Ceramill TEMP units (Amann Girrbach)
Ceramill motion (Amann Girrbach)
Cercon Base PMMA (Dentsply)
Cercon brain expert (Dentsply)
BeCe Temp (Bego)
Bego CAD/CAM (Bego)
New Outline CAT (Anaxdent)
Organical (R+K) Zenotec (Wieland)
Polycon AE (Straumann)
CAD/CAM (Straumann)
Quattro Disc Eco PMMA (Goldquadrat)
Quattro Mill Comfort/Maxi5X/Easy (Goldquadrat)
SHERA eco-Disc PM (Shera Werkstoff Technologie)
SHERA-eco Mill 40/50/80 (Shera)
Telio CAD (Ivoclar Vivadent)
Procera (Nobel Biocare) Cerec (Sirona) Planmeca PlanScan (E4D Technologies) PlanMill (E4D Technologies)
TEMP Basic (Zirkonzahn)
5-Tec (Zirkonzahn)
Zenotec PMMA (Wieland)
All Zenotec (Wieland)
Tizian PMMA Blanks (Schütz)
Tizian CAD/CAM (Schütz)
flexural strength ranges from 50 to 130 MPa, and their modulus of elasticity ranges from 2,000 to 3,200 MPa.99,100 The advantages of PMMA materials are their high fracture resistance and low susceptibility to aging. They are available in six shades. These materials are indicated for use as provisional restorations for up to 6 months (Fig 8-9).
Composite resin–based CAD/CAM materials Composite resin dental restorations represent a unique class of biomaterials with severe limitations related to biocompatibility, curing behavior, esthetics, and ultimate mechanical properties. The use of these materials is limited by shrinkage and polymerization-induced shrinkage stress, limited toughness, the presence of unreacted monomer that remains following polymerization, and several other factors. In recent years, the performance of these restorations has been improved by changes in the initiation system, monomers, fillers, coupling agents, and polymerization strategies.101 Prefabricated blocks can be used, in combination with the CAD/CAM systems, for the fabrication of dental restorations. 164
Andriani et al102 tested the strength to failure and fracture mode of three indirect composite resin materials (Tescera, Bisco; Ceramage, Shofu; and Diamond crown, DRM) applied directly to titanium implant abutments and compared the data with those of cemented porcelain-fused-to-metal crowns. All crowns were loaded to failure by an indenter placed at one of the cusp tips. The single loads to failure recorded were between 1,155 and 1,081 N. The three indirect composite and porcelain-fused-to-metal systems fractured at higher loads than those typically associated with normal function. No significant differences in single-loadto-fracture resistance were found among composite resin systems and porcelain-fused-to-metal crowns. In an in vitro study, Santing et al103 analyzed the fracture strength and failure mode of maxillary implant-supported screw-retained composite resin single crowns on polyetheretherketone (PEEK) and titanium abutments. The researchers concluded that provisional crowns on PEEK abutments showed fracture strengths similar to those observed with titanium abutments, except for central incisors. Maxillary central incisor composite resin crowns on PEEK abutments fractured below the mean masticatory loading force.
Alternative Materials
a
b
Fig 8-9  (a to c) CAD/CAM implant-supported, threeunit PMMA provisional restoration replacing the mandibular left first, second, premolars, and the first molar.
c
Suzuki et al104 used two different composite resin materials (Ceramage and Diamond Crown) to restore molars on titanium abutments. The crowns were then loaded until failure occurred. The fracture strength values were high (1,099 and 1,155 N, respectively), but no significant differences could be observed between the two materials. Failure modes comprised composite veneer chipping.
Novel ceramic–composite resin materials Innovative ceramic–composite resin compound materials may combine the esthetic features of ceramic with the favorable load-bearing mechanical properties of the composite resin component, according to He and Swain.105 They investigated the properties of an interpenetrating network material in which a porous ceramic structure is infiltrated with composite resin rather than glass, as occurs in the InCeram system (Vident).
There have been other developments in this area, with the release of a higher-percentage ceramic-loaded composite resin: Paradigm MZ 100 Block (3M ESPE) and Enamic (Vita) for Cerec (Sirona). This is available as a CAD/CAM composite for inlay, onlay, veneer, and crown applications. Early clinical results are promising (Fig 8-10). Advantages associated with these materials over allceramic systems include more toothlike properties: The compound materials are resilient and shock absorbent, not brittle, and have a dentinlike elastic modulus of 12 to 20 GPa. They offer good esthetics with 12 available shades and a toothlike fluorescence. Studies of mechanical properties105,106 have shown that the properties of interpenetrating network composite materials are superior to those of composite resin materials; the former have higher strength and toughness values than porcelain materials, an elastic modulus that is midway between that of enamel and dentin (25 to 30 GPa), and hardness that is less than that of enamel. Because of their lower
165
8
Ceramic Applications to Restore Implants
a
b
c Fig 8-10  (a to e) Permanent hybrid ceramic implant-supported Enamic crowns replacing the mandibular left first and second premolars.
d
e
hardness and higher toughness values, these materials can be more rapidly machined with sharper margins. As yet, no reports of clinical trials have been presented. An in vitro study by Guess et al94 showed that although the strength of the interpenetrating structure was comparable to that of existing glass-ceramic materials, the interpenetrating structure exhibited no cracking during chewing simulator
166
studies, whereas the glass-ceramics exhibited more than 40% incidence of cracking. Preliminary interpretation of these results suggests that the lower stiffness contributed to the better outcome of the interpenetrating ceramic– composite resin material. Figure 8-11 summarizes the clinical data that are available for different materials used to restore dental implants.
Conclusion
Ceramic implants and abutments
Zirconia bilayer
Zirconia core and CAD/CAM veneer
Monolithic lithium disilicate and zirconia
Permanent: resin nanoceramic/hybrid
✓ Reliable and esthetic Mid-term data
✓ Reliable core High veneer failure
✓ CAD/CAM veneer No clinical data
✓ Reliable construction No clinical data
✓ Favorable properties Short-term data
Fig 8-11 Summary of the current status of ceramic applications for dental implants.
Future Perspectives
Conclusion
Inkjet printing of zirconia prostheses
Implant-supported crowns and multiple-unit FDPs have proven to be a reliable treatment option for the replacement of missing teeth. Over the years, a variety of ceramic materials have been used with great success to fabricate esthetic and functional implant-supported restorations. Nevertheless, the profession is always striving to improve outcomes through the development of new materials and techniques. In recent years, the potential of zirconia ceramics for use as implants, abutments, and restorative materials has been investigated. Fracture of veneering ceramics and the susceptibility of zirconia to aging are major concerns for the clinical long-term success of zirconia in fixed implant prosthodontics. Presently, there are very limited clinical data evaluating the performance of zirconia abutments and implant-supported fixed restorations. Therefore, like other novel materials and devices, zirconia implants can only be recommended for use in daily private practice with caution.
CAD/CAM milling systems provide a rapid and individual method for the manufacture of zirconia dental restorations. However, the disadvantages of these systems include limited accuracy, possible introduction of microscopic cracks, and a waste of material because of the subtractive process. Ebert et al107 and van Noort108 showed that direct inkjet printing of zirconia prostheses has the potential to produce cost-effective, all-ceramic dental restorations with high accuracy, good physical properties, and a minimum of wasted material during manufacturing. These zirconia-based materials have the following characteristics: novel generative manufacturing procedures, the ability to tailor properties and shades, a characteristic strength of 763 MPa, and a fracture toughness of 6.7 MPa ∙ m1/2.
Solid freeform fabrication of threedimensional structures In the future, robocasting technology that generates threedimensional, custom-made layered structures may be a promising fabrication method for zirconia in dental applications. A variety of structures with changing or graded configurations can be produced by using colloidal pastes, slurries, or inks with different compositions of alumina and zirconia to control the specific mechanical and esthetic characteristics of the final product.109
Acknowledgments The authors acknowledge the support of Dr Petra Guess, Mrs Ulrike Soldat, and Mr Cumhur Yörük, Freiburg, Germany.
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Ceramic Applications to Restore Implants
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