Q U I N T E S S E N C E I N T E R N AT I O N A L
PROSTHODONTICS
Sven Rinke
Anterior all-ceramic superstructures: Chance or risk? Sven Rinke, Priv-Doz Dr med dent, MSc, MSc1 The use of zirconia abutments for single-tooth restorations is well documented and supported by clinical studies with observational periods of up to 5 years. However, data for fixed partial dentures (FPDs) on all-ceramic abutments are lacking. Therefore, this indication cannot yet be generally recommended. Based on the available clinical studies, it can be assumed that the treatment results for anterior restorations can be improved by using all-ceramic abutments, especially in situations with a reduced thickness of the peri-implant soft tissues (< 2 mm). Zirconia abutments for single-tooth restorations can be restored with glass-ceramic crowns on a lithium-disilicate
base or crowns with oxide-ceramic structures (alumina or zirconia). If the restorations are cemented adhesively, then all of the cement residues must be carefully removed. Superstructures based on zirconia ceramics can be removed to a certain degree if they are cemented temporarily or screwfixed with directly veneered abutments. However, prior to providing a general recommendation for temporary cementation or screw-fixation of all-ceramic superstructures, additional clinical data are needed. (Originally published in Quintessenz 2014;65(3):289–302; Quintessence Int 2015;46:217–227; doi: 10.3290/j.qi.a33268)
Key words: abutment, clinical performance, esthetics, implant, zirconia
Implant-supported single-tooth restorations are a well-proven therapy, with high survival rates of the implants as well as the superstructures. In a systematic review,1 single-tooth implants showed a 10-year survival rate of 97.2%. Moreover, implant-supported single crowns (1.12%) showed annual failure rates that were similar to conventional fixed partial dentures (FPDs) (1.14%).2 Superstructure losses are mainly due to screw-loosening, loss of retention, or fractures in the veneering ceramics. To date, technical complications predominantly involving titanium abutments are mostly limited to loosening of the abutment screw and demonstrate an anticipated frequency of 5.1% over an observational period of 5 years. However, frac-
1
Private Practice, Hanau, Germany; and Department of Prosthodontics, GeorgAugust-University, Goettingen, Germany.
Correspondence: Dr Sven Rinke, Dr Jablonski & Colleagues, Geleitstr. 68, 63456 Hanau, Germany. Email: [email protected]
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tures of metallic implant abutments are rare. Over an observational period of 5 years, they have an expected incidence of 0.3%.1,3 In an isolated evaluation of studies on anterior implants, 27 studies with more than 900 restorations were included, and no fractures of the metallic abutments could be determined.4 However, the gray color is a potential disadvantage of metallic abutments. A discoloration of the peri-implant mucosa can lead to esthetic limitations. Moreover, in the case of soft tissue recession, exposition of the metallic abutment parts also can lead to esthetic distractions (Fig 1). Therefore, despite their stability, the use of these abutments in esthetically challenging areas, especially in cases with a thin peri-implant mucosa, is not ideal. Thus, the esthetic limitations of visible metallic implant components or discolorations have been documented with a frequency of 6.6% to 7.1% in systematic reviews.1,3 In this context, it is questioned whether
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Fig 1 Esthetic limitation by exposure of the metal abutment after recession of the peri-implant soft tissue.
Fig 2 Restoration of a single-tooth in the canine region with a pre-fabricated all-ceramic abutment.
Fig 3 Clinical situation 5 years after insertion of the veneered crowns made of zirconia ceramic.
these complications can be avoided by using metalfree implant components.5,6 Ceramic abutments, which initially consisted of alumina,3,7 represent an alternative. Clinical studies with observational periods of 1 to 5 years have shown that the presence of peri-implant soft tissue is invariable in the area of the alumina abutment.8-10 Moreover, due to their similarity to the natural tooth color, these abutments provide a more esthetic treatment result compared to titanium abutments. However, these clinical studies documented fractures in the alumina abutments. For single crowns on alumina abutments, Andersson et al9 reported a survival rate of 93% to 100% for the anterior and premolar regions, respectively, over an observational period of 1 to 3 years. Although these abutments provide positive treatment results with regard to biologic and esthetic issues, they clearly bear a risk of fracture during the working process in the dental laboratory and after the final cementation.
Abutments were later constructed of zirconia.11 Laboratory studies revealed a considerably higher static and dynamic stability of zirconia abutments in comparison to alumina abutments.3,7 With the growing establishment of computer-aided design/computer-assisted manufacture (CAD/CAM) systems, individual all-ceramic abutments can be fabricated. Based on the present results derived from animal tests and histologic examinations of human tissues, it can be concluded that with regard to biocompatibility, zirconia is equally as appropriate as titanium for the fabrication of dental implant abutments. However, zirconia displays a reduced tendency towards superficial bacterial plaque accumulation.12,13 Regarding superior treatment results, all-ceramic abutments are used in combination with all-ceramic crowns or FPDs (Figs 2 and 3). As the range of indications for this restoration type is constantly growing, the reliable success of all-ceramic superstructures is a legitimate concern.
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THE CLINICAL SUCCESS OF ALL-CERAMIC ABUTMENTS Two systematic reviews3,7 evaluated the available studies on all-ceramic abutments in comparison to metallic abutments. However, it must be noted that these data were collected from five studies with a total of 166 ceramic abutments and 18 studies with a total of 4,807 metallic abutments. Moreover, the ceramic abutments were re-evaluated over a period of 3.7 years, whereas the observational period for the metallic abutments was 4.8 years. Fractures of the ceramic or metallic abutment occurred rarely and only in cases with alumina abutments. The cumulative incidence after 5 years was 0.3%, and no statistically significant differences between the abutment types could be determined. The survival rate after 5 years was 99.1% for ceramic abutments (alumina and zirconia; 95% confidence interval [CI], 93.8% to 99.9%). It is interesting that the ratio of bone loss > 2 mm was higher for implants on metallic abutments (3.9%; 95% CI, 1.7% to 8.7%) than for implants on ceramic abutments (0%; 95% CI, 1.7% to 8.7%). Regarding metallic abutments, a lack in terms of esthetics has been more frequently reported in the treatment results. Esthetic problems occurred in 0% (95% CI, 0% to 11.3%) of the ceramic abutments but in 6.6% (95% CI, 2% to 22.4%) of the metallic abutments. In another review,4 the clinical performance of metallic and ceramic implant abutments in the anterior region was evaluated, encompassing 951 restorations in 27 studies. While no fractures were documented in the titanium abutments, the fracture rate for the alumina and zirconia abutments was 0.85% and 0.3%, respectively (Fig 4). However, when evaluating these results, it should be noted that most of the available studies on zirconia abutments cover observational periods of less than 5 years for single-tooth restorations only. The suitability of individual zirconia abutments for diameter-reduced implants with a conical connection (Astra MicroThread and Straumann Narrow Neck) is documented in a newer prospective study with an observational period of 5 years and survival rates of 100% for implants and abutments.14
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Fig 4 The fracture of an all-ceramic zirconia abutment rarely occurs in anterior single-tooth restorations.
Apart from the clinical performance, clinical studies also evaluated the potential esthetic advantages of allceramic abutments by spectroscopic processes and visual assessment. Three clinical studies15-17 investigated color changes in peri-implant soft tissue after the insertion of metallic and all-ceramic abutments using colorspectrometric measurements. In comparison with the soft tissue of the adjacent natural teeth, the all-ceramic abutments induced significantly less color change than the titanium abutments. In contrast, another clinical study documented no significant influence of the abutment material on the degree of discoloration.18 In a fifth clinical study that also utilized a spectroscopic measurement, it was stated that color changes due to titanium and zirconia abutments cannot be distinguished by the human eye if the peri-implant tissues exceed a thickness of more than 2 mm.19 During a visual evaluation of the dentists who performed the treatment, the use of all-ceramic abutments led to a lower tendency to experience discoloration in peri-implant soft tissues.
Clinical relevance The use of zirconia abutments in single-tooth restorations has been documented and supported in clinical studies with observational periods of up to 5 years. However, data for FPD restorations on all-ceramic abutments are lacking. Therefore, this indication cannot yet be recommended. Based on previously published clinical studies, it can be assumed that the treatment results can be improved by using all-ceramic abutments in the anterior region, especially in cases with a reduced thickness (< 2 mm) of the peri-implant soft tissue. Moreover, in cases of recession of
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Fig 5 In the case of soft tissue recession, the use of tooth-colored all-ceramic abutments in comparison to titanium abutments only leads to a limited esthetic limitation.
Fig 6 Restoration of two anterior implants with prefabricated all-ceramic abutments. The abutment design is significantly different from the shape of a prepared anterior tooth and offers only limited support for peri-implant soft tissue.
Fig 7 Conventionally cemented zirconia crowns after 4 years in situ.
the peri-implant soft tissue, the esthetic complications will be less compromising if all-ceramic abutments are used instead of titanium abutments (Fig 5).
ments were only available in white (Figs 6 and 7), colored zirconia abutments are currently available. They have a dentin-like color, thus offering ideal conditions for an all-ceramic superstructure. Prefabricated all-ceramic abutments are especially suitable for clinical situations in which the abutment shape already allows a near optimum abutment geometry for subsequent prosthetic restoration and only minimal grinding of the zirconia structure is required. When finishing the structure, the zirconia structure must be ground only with water-cooled diamond instruments to avoid damage to the partially stabilized zirconia, thus avoiding a reduction of the fatigue resistance. In general, post-processing should be reduced to a minimum. Above all, the minimum wall thickness of the zirconia structures must not fall below 0.5 to 0.7 mm. This guideline is especially important for angled one-piece abutments with a lower fracture resistance than straight abutments (Figs 8 to 11).
FABRICATION TECHNIQUES USED FOR ZIRCONIA ABUTMENTS All-ceramic zirconia abutments are available in three different styles: • prefabricated abutments consisting of pure zirconia; the only metal part is a center holding screw • individual CAD/CAM-fabricated zirconia abutments • two-piece zirconia abutments that are connected to the implant with a metallic abutment base and then luted with an individual zirconia structure. Prefabricated zirconia abutments have been clinically applied for several years. While the first zirconia abut-
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a Figs 8a and 8b
b Well-shaped soft tissue with a long-term provisional restoration at 12 weeks after exposition.
Fig 9 Impression-taking with a screw-fixed transfer post and polyvinylsiloxane (Aquasil Ultra XLV, Aquasil Monophase, Dentsply DeTrey) using a simultaneous mixing technique.
a
b
c
Figs 10a to 10c The prefabricated dentin-colored zirconia abutment (Cercon Balance, Dentsply Implants) can be adapted to the gap and soft tissue situation with minimal post-processing.
a Figs 11a and 11b
b Treatment results after insertion of the all-ceramic superstructure.
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a
b Fig 12 Well-shaped peri-implant soft tissue after 12 weeks with provisional restoration, allowing for the final prosthetic restoration with an individual abutment.
c
Figs 13a to 13c The abutment is fabricated with CAD software (Cercon Art 3.3, DeguDent) after production of a working model and scan using a reference structure. The one-piece abutment is fabricated in a central production unit.
Fig 14 Insertion of a one-piece zirconia abutment. A good adaptation to the tooth-shaped soft tissue situation was achieved. The slightly subgingival position of the preparation limit later allows easy removal of excess cement residues.
Fig 15 Because of the individual abutment design, the cross-section of the abutment can emulate the natural tooth shape. The abutment is therefore similar to the shape of a prepared tooth.
The support of individually shaped peri-implant soft tissue is essential for an optimum esthetic result. Prefabricated abutments are limited because the given sizes cannot always be adapted to the individual shape of the soft tissues. Moreover, the position of the preparation limit cannot always be adapted to the marginal gingival rim. In the context of good esthetics with invisible crowns together with easy removal of cement residues, the preparation limit should be approximately 1 mm under the marginal gingiva rim. Individually fabricated abutments have the advantage that they can be adapted to the soft tissue situation with regard to support of the tissue and a slightly subgingival preparation limit.
In cases with a pronounced angulation that requires a massive grinding of the abutment, or if the shape of the prefabricated structure deviates sharply from the natural abutment geometry and the shaped soft tissue profile, individually fabricated all-ceramic structures should be used. Different techniques are available to fabricate individual all-ceramic zirconia structures. Due to their complex contact geometry, one-piece individual zirconia structures can only be fabricated to sufficient precision with CAD/CAM systems that include central production. Today, numerous dental CAD/CAM systems allow the construction of individual ceramic abutments based on digital data (model scan or digital impressiontaking) in the dental laboratory (Figs 12 to 15). The CAD
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Fig 16 The CAD file for production release of a one-piece abutment (Atlantis, Dentsply Implants).
Fig 17
Fig 18 Construction of a two-piece zirconia abutment (DentalDesigner 2013, 3Shape).
Fig 19 Titanium base and laboratory-fabricated zirconia structure (Cercon base, DeguDent) with components that are luted with composite cement.
data are then dispatched to a central fabrication unit. In addition, it is possible to forward the working model for the fabrication of an abutment (eg, Atlantis, Dentsply Implants). After receiving the master model, the suggested design is delivered to the practice by email. The dentist can check the structure of the design with a special program and, if necessary, order the necessary modifications. After approval of the design, the individual structure is fabricated and returned together with the working model (Figs 16 and 17). Despite the proven good long-term stability of onepiece abutments, up-to-date in vitro studies document potential limitations. One-piece zirconia abutments lead to an increase in fretting wear at the implant-
abutment joint.20,21 Furthermore, one-piece zirconia abutments have a reduced fitting quality in comparison with titanium abutments.22 To date, the clinical consequences of these observations have not yet been clarified, and therefore the indication for one-piece zirconia abutments should only be applied under strict adherence to clinically verified application areas. Direct fabrication of two-piece zirconia abutments in the dental laboratory is possible with most common dental CAD/CAM systems. After scanning the master model, a titanium base fit for the applied implant system is selected. The individual zirconia structure is then milled from a pre-sintered zirconia blank using common dental milling units (Figs 18 to 20). The densely
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Insertion of a one-piece zirconia abutment.
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Figs 20a and 20b Clinical situation after insertion of a two-piece abutment (a) and the all-ceramic crown (b).
sintered abutment is glued to the titanium base and then finished and polished. To glue the titanium base and zirconia structure, the respective surfaces should be roughened by air abrasion (50 μm alumina at a maximum pressure of 2 bars). It is recommended that the metal base be conditioned with a primer (eg, Alloy Primer, Kuraray Europe). The gluing surfaces of the zirconia structure should also be pre-treated with a zirconia primer (Clearfil Ceramic Primer, Kuraray Europe). If glues containing phosphate monomers are used, then additional primers are not necessary.23 In in vitro studies using two-piece abutments,24 standard dual-hardening composite cements (Panavia F2.0, Kuraray Europe; and Multilink Implant, Ivoclar Vivadent) showed good adhesive results after water storage and thermocycling. Therefore, they can be recommended for clinical applications. Although the long-term stability of the gluing joint under clinical conditions is discussed as a potential weak point of two-piece abutments, to date no failure of the gluing is documented in clinical studies. In a study investigating the clinical performance of twopiece zirconia abutments on a titanium base fixed with a composite cement, Canullo25 examined 25 patients who were restored with a total of 30 implants and implant-supported all-ceramic single crowns. The mean observational period was 40 months. During the obser-
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vational period of clinical function, neither abutment fractures nor screw loosening were observed, leading to a cumulative survival rate of 100%. Moreover, in vitro studies showed that a significantly higher fracture strength was obtained for diameter-reduced implants with two-piece abutments compared with one-piece abutments.26 An influence of the implant-abutment connection on the fracture strength also can be documented; for conical internal joints in particular, twopiece abutments showed a higher fracture strength than one-piece zirconia abutments.27 These results should be considered during the process of treatment planning, especially if an increased biomechanical stress is anticipated, for example due to a long clinical crown.
Clinical relevance Clinical studies are available for prefabricated28,29 as well as individual14,30 one- and two-piece zirconia abutments25 in the anterior region with a maximum observational period of up to 5 years. Similar survival rates for anterior single-tooth restorations are documented for all three types, and their clinical reliability can therefore be considered equivalent. The type of material should be selected according to its clinical features. In addition to the best possible support of the peri-implant soft tissue, optimal prepar-
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ation limit, and tooth-shaped abutment design, the expected load situation and implant design should be considered. For a good treatment result, the periimplant soft tissues should be shaped at an early stage, allowing an emergence profile similar to the natural tooth. In the anterior region, it is most effective to apply a laboratory-fabricated provisional restoration, always considering that it is worn 10 to 12 weeks prior to being replaced with the final prosthetic restoration.
THE PERFORMANCE OF ALLCERAMIC SINGLE CROWNS ON IMPLANTS To achieve optimum treatment results, all-ceramic crowns are appropriate for the prosthetic restoration of all-ceramic abutments. However, this leads to the question of whether dental ceramic materials are best suited for this indication. Due to the small number of available studies on all-ceramic superstructures, evidence-based data are limited. In the available studies, leucite-reinforced glassceramics (Empress 1, Ivoclar Vivadent), lithium-disilicate ceramics (e.max, Ivoclar Vivadent), and all-ceramic crowns with alumina or zirconia structures were applied. Glauser et al28 evaluated the combination of 36 zirconia superstructures and crowns made of a leucitereinforced glass ceramic (Empress 1) in the anterior and premolar regions. After an observational period of 4 years, they documented a survival rate of 100% for the implant superstructures; three crowns displayed minor fractures of the veneering ceramic that could be polished. Vanlioglu et al14 investigated the clinical performance of lithium-disilicate anterior crowns that were adhesively luted on individual zirconia abutments. After an observational period of 5 years, the 11 reevaluated restorations displayed neither ceramic fractures nor a loss of retention. The clinical performance of zirconia crowns on ceramic abutments in the anterior region was evaluated in a prospective study.29 After an observational period of 3 years, 64 anterior crowns showed two ceramic fractures (3.1%). One crown had
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to be renewed due to the fracture size. In their study, the crowns were adhesively luted to the prefabricated zirconia abutments. During radiographic re-evaluation, two crowns showed cement residues. This finding confirms the clinical relevance of residue removal in adhesive cementation. In a retrospective study,30 crowns with alumina ceramic structures and directly veneered zirconia abutments were re-evaluated over an observational period of up to 3 years. The fracture rate was found to be 1% and no fractures of the alumina structures could be documented, while 2% of the restorations showed a fracture of the veneering ceramic within the first year. With regard to clinical data for glass ceramics, materials on a lithium-disilicate base should be preferred because they are significantly stronger than leucite-reinforced ceramics. However, this material has limitations because, to date, only data for adhesive cementation are available. The adhesive cementation of all-ceramic restorations on implants bears the risk of peri-implantitis caused by the remaining cement residues.29 Therefore, adhesive cementation can only be recommended if good accessibility of the preparation limit guarantees the safe and complete removal of excess cement residues. This precondition is fulfilled best in situations with individually fabricated abutments with a slightly subgingival preparation limit. With regard to oxide ceramics, alumina as well as zirconia seem to be appropriate for use in implantsupported anterior single crowns. Both materials show a high reliability of the structures, and technical complications are expected as fractures of the veneering ceramics. To avoid this phenomenon, findings obtained for tooth-supported oxide-ceramic restorations should be transferred to implant-supported restorations, ie a consistent layer thickness of the veneering ceramics must be guaranteed by the tooth-analog abutment and structure design. During the veneering process, internal strain should be reduced, for example by longterm cooling.31 Because screw-loosening is a common technical complication in implant-supported restorations,1,2 it must be considered that re-fixation of the abutment screw in adhesively cemented restorations is only pos-
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sible after removal or trepanation of the crown. Here, there are two possible solutions: a temporary cementation of the crown, or a centrally screw-fixed crown with a directly veneered zirconia abutment. If a sufficient height of the individually fabricated abutment or the integration of a retention-improving design (grooves) guarantees sufficient retention of the all-ceramic restoration, temporary cementation (at least for zirconia-based restorations) can be considered. Temporary cementation permits destruction-free removal of the restoration in the case of screw-loosening or fracture of the veneering ceramics. Because the cumulative incidence of these two events is approximately 7.5% within the first 5 years, the ability to revise a superstructure is an important aspect in the choice of materials and methods. However, it must be considered that until now, clinical studies have not provided any results for temporarily cemented all-ceramic superstructures. Data for centrally screw-fixed direct abutments are available for zirconia abutments with a maximum observational period of 3 years. The advantages of screw fixation are the same as those for temporary cementation: screw fixation allows a safe removal in the case of technical complications. Moreover, the induction of iatrogenic peri-implantitis in response to excess cement residues can be safely avoided. However, this type of restoration is limited because it can only be applied in the anterior maxilla if a palatal shifted implant position allows the positioning of the screw canal on the palatal surface. A screw-fixed superstructure must be planned at an early stage, ie prior to insertion of the implant.
Clinical relevance Zirconia abutments for single-tooth restorations can be restored either with glass-ceramic crowns on a lithiumdisilicate base or with crowns on oxide-ceramic structures (alumina or zirconia). If they are cemented adhesively, safe removal of the excess cement residues is essential. For superstructures based on zirconia ceramics, limited removability can be achieved by temporary
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cementation or screw-fixation of the directly veneered abutments. The first clinical results reported for screwfixed single-crown restorations showed no tendency towards an increased technical complication rate. However, prior to providing a general recommendation for temporary cementation or screw fixation of all-ceramic superstructures, additional clinical data are needed.
REFERENCES 1. Jung RE, Zembic A, Pjetursson BE, Zwahlen M, Thoma DS. Systematic review of the survival rate and the incidence of biological, technical, and aesthetic complications of single crowns on implants reported in longitudinal studies with a mean follow-up of 5 years. Clin Oral Implants Res 2012;23:2–21. 2. Albrektsson T, Donos N. Working Group 1. Implant survival and complications. The Third EAO consensus conference 2012. Clin Oral Implants Res 2012;23: 63–65. 3. Sailer I, Philipp A, Zembic A, Pjetursson BE, Hämmerle CH, Zwahlen M. A systematic review of the performance of ceramic and metal implant abutments supporting fixed implant reconstructions. Clin Oral Implants Res 2009;20:4–31. 4. Bidra AS, Rungruanganunt P. Clinical outcomes of implant abutments in the anterior region: a systematic review. J Esthet Restor Dent 2013;25:159–176. 5. Guess PC, Att W, Strub JR. Zirconia in fixed implant prosthodontics. Clin Implant Dent Relat Res 2010;14:633–645. 6. Sailer I, Zembic A, Jung RE, Hämmerle CH, Mattiola A. Single-tooth implant reconstructions: esthetic factors influencing the decision between titanium and zirconia abutments in anterior regions. Eur J Esthet Dent 2007;2:296–310. 7. Nakamura K, Kanno T, Milleding P, Ortengren U. Zirconia as a dental implant abutment material: a systematic review. Int J Prosthodont 2010;23:299–309. 8. Andersson B, Odman P, Lindvall AM, Brånemark PI. Cemented single crowns on osseointegrated implants after 5 years: results from a prospective study on CeraOne. Int J Prosthodont 1998;11:212–218. 9. Andersson B, Taylor A, Lang BR, et al. Alumina ceramic implant abutments used for single-tooth replacement: a prospective 1- to 3-year multicenter study. Int J Prosthodont 2001;14:432–438. 10. Scheller H, Urgell JP, Kultje C, et al. A 5-year multicenter study on implantsupported single crown restorations. Int J Oral Maxillofac Implants 1998;13:212–218. 11. Wohlwend A, Studer S, Schärer P. Das Zirkonoxidabutment – ein neues vollkeramisches Konzept zur ästhetischen Verbesserung der Suprastruktur in der Implantologie. Quintessenz Zahntech 1996;22:364–381. 12. De Medeiros RA, Vechiato-Filho AJ, Pellizzer EP, Mazaro JV, dos Santos DM, Goiato MC. Analysis of the peri-implant soft tissues in contact with zirconia abutments: an evidence-based literature review. J Contemp Dent Pract 2013;14:567–572. 13. Van Brakel R, Meijer GJ, Verhoeven JW, Jansen J, de Putter C, Cune MS. Soft tissue response to zirconia and titanium implant abutments: an in vivo withinsubject comparison. J Clin Periodontol 2012;39:995–1001. 14. Vanlioglu BA, Özkan Y, Evren B, Özkan YK. Experimental custom-made zirconia abutments for narrow implants in esthetically demanding regions: a 5-year follow-up. Int J Oral Maxillofac Implants 2012;27:1239–1242. 15. Bressan E, Paniz G, Lops D, Corazza B, Romeo E, Favero G. Influence of abutment material on the gingival color of implant supported all-ceramic restorations: a prospective multicenter study. Clin Oral Implants Res 2011;22:631–637. 16. Happe A, Schulte-Mattler V, Strassert C, et al. In vitro color changes of soft tissues caused by dyed fluorescent zirconia and nondyed, nonfluorescent zirconia in thin mucosa. Int J Periodontics Restorative Dent 2013;33:e1–e8.
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17. Jung RE, Holderegger C, Sailer I, Khraisat A, Suter A, Hämmerle CH. The effect of all-ceramic and porcelain-fused-to-metal restorations on marginal periimplant soft tissue color: a randomized controlled clinical trial. Int J Periodontics Restorative Dent 2008;28:357–365. 18. Zembic A, Bösch A, Jung RE, Hämmerle CH, Sailer I. Five-year results of a randomized controlled clinical trial comparing zirconia and titanium abutments supporting single-implant crowns in canine and posterior regions. Clin Oral Implants Res 2013;24:384–390. 19. Van Brakel R, Noordmans HJ, Frenken J, de Roode R, de Wit GC, Cune MS. The effect of zirconia and titanium implant abutments on light reflection of the supporting soft tissues. Clin Oral Implants Res 2011;22:1172–1178. 20. Klotz MW, Taylor TD, Goldberg AJ. Wear at the titanium-zirconia implant-abutment interface: a pilot study. Int J Oral Maxillofac Implants 2011;26:970-975. 21. Stimmelmayr M, Edelhoff D, Güth JF, Erdelt K, Happe A, Beuer F. Wear at the titanium-titanium and the titanium-zirconia implant-abutment interface: a comparative in vitro study. Dent Mater 2012;28:1215–1220. 22. Baldassarri M, Hjerppe J, Romeo D, Fickl S, Thompson VP, Stappert CF. Marginal accuracy of three implant-ceramic abutment configurations. Int J Oral Maxillofac Implants 2012;27:537–543. 23. Ebert A, Hedderich J, Kern M. Retention of zirconia ceramic copings bonded to titanium abutments. Int J Oral Maxillofac Implants 2007;22:921–927. 24. Gehrke P, Alius J, Fischer C, Erdelt KJ, Beuer F. Retentive strength of two-piece CAD/CAM zirconia implant abutments [Epub ahead of print 25 Mar 2013]. Clin Implant Dent Relat Res.
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25. Canullo L. Clinical outcome study of customized zirconia abutments for single-implant restorations. Int J Prosthodont 2007;20:489–493. 26. Stimmelmayr M, Sagerer S, Erdelt K, Beuer F. In vitro fatigue and fracture strength testing of one-piece zirconia implant abutments and zirconia implant abutments connected to titanium cores. Int J Oral Maxillofac Implants 2013;28:488–493. 27. Sailer I, Sailer T, Stawarczyk B, Jung RE, Hämmerle CH. In vitro study of the influence of the type of connection on the fracture load of zirconia abutments with internal and external implant-abutment connections. Int J Oral Maxillofac Implants 2009;24:850–858. 28. Glauser R, Sailer I, Wohlwend A, Studer S, Schibli M, Schärer P. Experimental zirconia abutments for implant-supported single-tooth restorations in esthetically demanding regions: 4-year results of a prospective clinical study. Int J Prosthodont 2004;17:285–290. 29. Hosseini M, Worsaae N, Schiødt M, Gotfredsen K. A 3-year prospective study of implant-supported, single-tooth restorations of all-ceramic and metalceramic materials in patients with tooth agenesis. Clin Oral Implants Res 2013;24:1078–1087. 30. Ekfeldt A, Fürst B, Carlsson GE. Zirconia abutments for single-tooth implant restorations: a retrospective and clinical follow-up study. Clin Oral Implants Res 2011;22:1308–1314. 31. Rinke S, Schäfer S, Lange K, Gersdorff N, Roediger M. Practice-based clinical evaluation of metal-ceramic and zirconia molar crowns: 3-year results. J Oral Rehabil 2013;40:228–237.
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Sven Rinke
Anterior all-ceramic superstructures: Chance or risk? Sven Rinke, Priv-Doz Dr med dent, MSc, MSc1 The use of zirconia abutments for single-tooth restorations is well documented and supported by clinical studies with observational periods of up to 5 years. However, data for fixed partial dentures (FPDs) on all-ceramic abutments are lacking. Therefore, this indication cannot yet be generally recommended. Based on the available clinical studies, it can be assumed that the treatment results for anterior restorations can be improved by using all-ceramic abutments, especially in situations with a reduced thickness of the peri-implant soft tissues (< 2 mm). Zirconia abutments for single-tooth restorations can be restored with glass-ceramic crowns on a lithium-disilicate
base or crowns with oxide-ceramic structures (alumina or zirconia). If the restorations are cemented adhesively, then all of the cement residues must be carefully removed. Superstructures based on zirconia ceramics can be removed to a certain degree if they are cemented temporarily or screwfixed with directly veneered abutments. However, prior to providing a general recommendation for temporary cementation or screw-fixation of all-ceramic superstructures, additional clinical data are needed. (Originally published in Quintessenz 2014;65(3):289–302; Quintessence Int 2015;46:217–227; doi: 10.3290/j.qi.a33268)
Key words: abutment, clinical performance, esthetics, implant, zirconia
Implant-supported single-tooth restorations are a well-proven therapy, with high survival rates of the implants as well as the superstructures. In a systematic review,1 single-tooth implants showed a 10-year survival rate of 97.2%. Moreover, implant-supported single crowns (1.12%) showed annual failure rates that were similar to conventional fixed partial dentures (FPDs) (1.14%).2 Superstructure losses are mainly due to screw-loosening, loss of retention, or fractures in the veneering ceramics. To date, technical complications predominantly involving titanium abutments are mostly limited to loosening of the abutment screw and demonstrate an anticipated frequency of 5.1% over an observational period of 5 years. However, frac-
1
Private Practice, Hanau, Germany; and Department of Prosthodontics, GeorgAugust-University, Goettingen, Germany.
Correspondence: Dr Sven Rinke, Dr Jablonski & Colleagues, Geleitstr. 68, 63456 Hanau, Germany. Email: [email protected]
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tures of metallic implant abutments are rare. Over an observational period of 5 years, they have an expected incidence of 0.3%.1,3 In an isolated evaluation of studies on anterior implants, 27 studies with more than 900 restorations were included, and no fractures of the metallic abutments could be determined.4 However, the gray color is a potential disadvantage of metallic abutments. A discoloration of the peri-implant mucosa can lead to esthetic limitations. Moreover, in the case of soft tissue recession, exposition of the metallic abutment parts also can lead to esthetic distractions (Fig 1). Therefore, despite their stability, the use of these abutments in esthetically challenging areas, especially in cases with a thin peri-implant mucosa, is not ideal. Thus, the esthetic limitations of visible metallic implant components or discolorations have been documented with a frequency of 6.6% to 7.1% in systematic reviews.1,3 In this context, it is questioned whether
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Fig 1 Esthetic limitation by exposure of the metal abutment after recession of the peri-implant soft tissue.
Fig 2 Restoration of a single-tooth in the canine region with a pre-fabricated all-ceramic abutment.
Fig 3 Clinical situation 5 years after insertion of the veneered crowns made of zirconia ceramic.
these complications can be avoided by using metalfree implant components.5,6 Ceramic abutments, which initially consisted of alumina,3,7 represent an alternative. Clinical studies with observational periods of 1 to 5 years have shown that the presence of peri-implant soft tissue is invariable in the area of the alumina abutment.8-10 Moreover, due to their similarity to the natural tooth color, these abutments provide a more esthetic treatment result compared to titanium abutments. However, these clinical studies documented fractures in the alumina abutments. For single crowns on alumina abutments, Andersson et al9 reported a survival rate of 93% to 100% for the anterior and premolar regions, respectively, over an observational period of 1 to 3 years. Although these abutments provide positive treatment results with regard to biologic and esthetic issues, they clearly bear a risk of fracture during the working process in the dental laboratory and after the final cementation.
Abutments were later constructed of zirconia.11 Laboratory studies revealed a considerably higher static and dynamic stability of zirconia abutments in comparison to alumina abutments.3,7 With the growing establishment of computer-aided design/computer-assisted manufacture (CAD/CAM) systems, individual all-ceramic abutments can be fabricated. Based on the present results derived from animal tests and histologic examinations of human tissues, it can be concluded that with regard to biocompatibility, zirconia is equally as appropriate as titanium for the fabrication of dental implant abutments. However, zirconia displays a reduced tendency towards superficial bacterial plaque accumulation.12,13 Regarding superior treatment results, all-ceramic abutments are used in combination with all-ceramic crowns or FPDs (Figs 2 and 3). As the range of indications for this restoration type is constantly growing, the reliable success of all-ceramic superstructures is a legitimate concern.
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THE CLINICAL SUCCESS OF ALL-CERAMIC ABUTMENTS Two systematic reviews3,7 evaluated the available studies on all-ceramic abutments in comparison to metallic abutments. However, it must be noted that these data were collected from five studies with a total of 166 ceramic abutments and 18 studies with a total of 4,807 metallic abutments. Moreover, the ceramic abutments were re-evaluated over a period of 3.7 years, whereas the observational period for the metallic abutments was 4.8 years. Fractures of the ceramic or metallic abutment occurred rarely and only in cases with alumina abutments. The cumulative incidence after 5 years was 0.3%, and no statistically significant differences between the abutment types could be determined. The survival rate after 5 years was 99.1% for ceramic abutments (alumina and zirconia; 95% confidence interval [CI], 93.8% to 99.9%). It is interesting that the ratio of bone loss > 2 mm was higher for implants on metallic abutments (3.9%; 95% CI, 1.7% to 8.7%) than for implants on ceramic abutments (0%; 95% CI, 1.7% to 8.7%). Regarding metallic abutments, a lack in terms of esthetics has been more frequently reported in the treatment results. Esthetic problems occurred in 0% (95% CI, 0% to 11.3%) of the ceramic abutments but in 6.6% (95% CI, 2% to 22.4%) of the metallic abutments. In another review,4 the clinical performance of metallic and ceramic implant abutments in the anterior region was evaluated, encompassing 951 restorations in 27 studies. While no fractures were documented in the titanium abutments, the fracture rate for the alumina and zirconia abutments was 0.85% and 0.3%, respectively (Fig 4). However, when evaluating these results, it should be noted that most of the available studies on zirconia abutments cover observational periods of less than 5 years for single-tooth restorations only. The suitability of individual zirconia abutments for diameter-reduced implants with a conical connection (Astra MicroThread and Straumann Narrow Neck) is documented in a newer prospective study with an observational period of 5 years and survival rates of 100% for implants and abutments.14
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Fig 4 The fracture of an all-ceramic zirconia abutment rarely occurs in anterior single-tooth restorations.
Apart from the clinical performance, clinical studies also evaluated the potential esthetic advantages of allceramic abutments by spectroscopic processes and visual assessment. Three clinical studies15-17 investigated color changes in peri-implant soft tissue after the insertion of metallic and all-ceramic abutments using colorspectrometric measurements. In comparison with the soft tissue of the adjacent natural teeth, the all-ceramic abutments induced significantly less color change than the titanium abutments. In contrast, another clinical study documented no significant influence of the abutment material on the degree of discoloration.18 In a fifth clinical study that also utilized a spectroscopic measurement, it was stated that color changes due to titanium and zirconia abutments cannot be distinguished by the human eye if the peri-implant tissues exceed a thickness of more than 2 mm.19 During a visual evaluation of the dentists who performed the treatment, the use of all-ceramic abutments led to a lower tendency to experience discoloration in peri-implant soft tissues.
Clinical relevance The use of zirconia abutments in single-tooth restorations has been documented and supported in clinical studies with observational periods of up to 5 years. However, data for FPD restorations on all-ceramic abutments are lacking. Therefore, this indication cannot yet be recommended. Based on previously published clinical studies, it can be assumed that the treatment results can be improved by using all-ceramic abutments in the anterior region, especially in cases with a reduced thickness (< 2 mm) of the peri-implant soft tissue. Moreover, in cases of recession of
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Fig 5 In the case of soft tissue recession, the use of tooth-colored all-ceramic abutments in comparison to titanium abutments only leads to a limited esthetic limitation.
Fig 6 Restoration of two anterior implants with prefabricated all-ceramic abutments. The abutment design is significantly different from the shape of a prepared anterior tooth and offers only limited support for peri-implant soft tissue.
Fig 7 Conventionally cemented zirconia crowns after 4 years in situ.
the peri-implant soft tissue, the esthetic complications will be less compromising if all-ceramic abutments are used instead of titanium abutments (Fig 5).
ments were only available in white (Figs 6 and 7), colored zirconia abutments are currently available. They have a dentin-like color, thus offering ideal conditions for an all-ceramic superstructure. Prefabricated all-ceramic abutments are especially suitable for clinical situations in which the abutment shape already allows a near optimum abutment geometry for subsequent prosthetic restoration and only minimal grinding of the zirconia structure is required. When finishing the structure, the zirconia structure must be ground only with water-cooled diamond instruments to avoid damage to the partially stabilized zirconia, thus avoiding a reduction of the fatigue resistance. In general, post-processing should be reduced to a minimum. Above all, the minimum wall thickness of the zirconia structures must not fall below 0.5 to 0.7 mm. This guideline is especially important for angled one-piece abutments with a lower fracture resistance than straight abutments (Figs 8 to 11).
FABRICATION TECHNIQUES USED FOR ZIRCONIA ABUTMENTS All-ceramic zirconia abutments are available in three different styles: • prefabricated abutments consisting of pure zirconia; the only metal part is a center holding screw • individual CAD/CAM-fabricated zirconia abutments • two-piece zirconia abutments that are connected to the implant with a metallic abutment base and then luted with an individual zirconia structure. Prefabricated zirconia abutments have been clinically applied for several years. While the first zirconia abut-
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a Figs 8a and 8b
b Well-shaped soft tissue with a long-term provisional restoration at 12 weeks after exposition.
Fig 9 Impression-taking with a screw-fixed transfer post and polyvinylsiloxane (Aquasil Ultra XLV, Aquasil Monophase, Dentsply DeTrey) using a simultaneous mixing technique.
a
b
c
Figs 10a to 10c The prefabricated dentin-colored zirconia abutment (Cercon Balance, Dentsply Implants) can be adapted to the gap and soft tissue situation with minimal post-processing.
a Figs 11a and 11b
b Treatment results after insertion of the all-ceramic superstructure.
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a
b Fig 12 Well-shaped peri-implant soft tissue after 12 weeks with provisional restoration, allowing for the final prosthetic restoration with an individual abutment.
c
Figs 13a to 13c The abutment is fabricated with CAD software (Cercon Art 3.3, DeguDent) after production of a working model and scan using a reference structure. The one-piece abutment is fabricated in a central production unit.
Fig 14 Insertion of a one-piece zirconia abutment. A good adaptation to the tooth-shaped soft tissue situation was achieved. The slightly subgingival position of the preparation limit later allows easy removal of excess cement residues.
Fig 15 Because of the individual abutment design, the cross-section of the abutment can emulate the natural tooth shape. The abutment is therefore similar to the shape of a prepared tooth.
The support of individually shaped peri-implant soft tissue is essential for an optimum esthetic result. Prefabricated abutments are limited because the given sizes cannot always be adapted to the individual shape of the soft tissues. Moreover, the position of the preparation limit cannot always be adapted to the marginal gingival rim. In the context of good esthetics with invisible crowns together with easy removal of cement residues, the preparation limit should be approximately 1 mm under the marginal gingiva rim. Individually fabricated abutments have the advantage that they can be adapted to the soft tissue situation with regard to support of the tissue and a slightly subgingival preparation limit.
In cases with a pronounced angulation that requires a massive grinding of the abutment, or if the shape of the prefabricated structure deviates sharply from the natural abutment geometry and the shaped soft tissue profile, individually fabricated all-ceramic structures should be used. Different techniques are available to fabricate individual all-ceramic zirconia structures. Due to their complex contact geometry, one-piece individual zirconia structures can only be fabricated to sufficient precision with CAD/CAM systems that include central production. Today, numerous dental CAD/CAM systems allow the construction of individual ceramic abutments based on digital data (model scan or digital impressiontaking) in the dental laboratory (Figs 12 to 15). The CAD
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Fig 16 The CAD file for production release of a one-piece abutment (Atlantis, Dentsply Implants).
Fig 17
Fig 18 Construction of a two-piece zirconia abutment (DentalDesigner 2013, 3Shape).
Fig 19 Titanium base and laboratory-fabricated zirconia structure (Cercon base, DeguDent) with components that are luted with composite cement.
data are then dispatched to a central fabrication unit. In addition, it is possible to forward the working model for the fabrication of an abutment (eg, Atlantis, Dentsply Implants). After receiving the master model, the suggested design is delivered to the practice by email. The dentist can check the structure of the design with a special program and, if necessary, order the necessary modifications. After approval of the design, the individual structure is fabricated and returned together with the working model (Figs 16 and 17). Despite the proven good long-term stability of onepiece abutments, up-to-date in vitro studies document potential limitations. One-piece zirconia abutments lead to an increase in fretting wear at the implant-
abutment joint.20,21 Furthermore, one-piece zirconia abutments have a reduced fitting quality in comparison with titanium abutments.22 To date, the clinical consequences of these observations have not yet been clarified, and therefore the indication for one-piece zirconia abutments should only be applied under strict adherence to clinically verified application areas. Direct fabrication of two-piece zirconia abutments in the dental laboratory is possible with most common dental CAD/CAM systems. After scanning the master model, a titanium base fit for the applied implant system is selected. The individual zirconia structure is then milled from a pre-sintered zirconia blank using common dental milling units (Figs 18 to 20). The densely
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Insertion of a one-piece zirconia abutment.
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Figs 20a and 20b Clinical situation after insertion of a two-piece abutment (a) and the all-ceramic crown (b).
sintered abutment is glued to the titanium base and then finished and polished. To glue the titanium base and zirconia structure, the respective surfaces should be roughened by air abrasion (50 μm alumina at a maximum pressure of 2 bars). It is recommended that the metal base be conditioned with a primer (eg, Alloy Primer, Kuraray Europe). The gluing surfaces of the zirconia structure should also be pre-treated with a zirconia primer (Clearfil Ceramic Primer, Kuraray Europe). If glues containing phosphate monomers are used, then additional primers are not necessary.23 In in vitro studies using two-piece abutments,24 standard dual-hardening composite cements (Panavia F2.0, Kuraray Europe; and Multilink Implant, Ivoclar Vivadent) showed good adhesive results after water storage and thermocycling. Therefore, they can be recommended for clinical applications. Although the long-term stability of the gluing joint under clinical conditions is discussed as a potential weak point of two-piece abutments, to date no failure of the gluing is documented in clinical studies. In a study investigating the clinical performance of twopiece zirconia abutments on a titanium base fixed with a composite cement, Canullo25 examined 25 patients who were restored with a total of 30 implants and implant-supported all-ceramic single crowns. The mean observational period was 40 months. During the obser-
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vational period of clinical function, neither abutment fractures nor screw loosening were observed, leading to a cumulative survival rate of 100%. Moreover, in vitro studies showed that a significantly higher fracture strength was obtained for diameter-reduced implants with two-piece abutments compared with one-piece abutments.26 An influence of the implant-abutment connection on the fracture strength also can be documented; for conical internal joints in particular, twopiece abutments showed a higher fracture strength than one-piece zirconia abutments.27 These results should be considered during the process of treatment planning, especially if an increased biomechanical stress is anticipated, for example due to a long clinical crown.
Clinical relevance Clinical studies are available for prefabricated28,29 as well as individual14,30 one- and two-piece zirconia abutments25 in the anterior region with a maximum observational period of up to 5 years. Similar survival rates for anterior single-tooth restorations are documented for all three types, and their clinical reliability can therefore be considered equivalent. The type of material should be selected according to its clinical features. In addition to the best possible support of the peri-implant soft tissue, optimal prepar-
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ation limit, and tooth-shaped abutment design, the expected load situation and implant design should be considered. For a good treatment result, the periimplant soft tissues should be shaped at an early stage, allowing an emergence profile similar to the natural tooth. In the anterior region, it is most effective to apply a laboratory-fabricated provisional restoration, always considering that it is worn 10 to 12 weeks prior to being replaced with the final prosthetic restoration.
THE PERFORMANCE OF ALLCERAMIC SINGLE CROWNS ON IMPLANTS To achieve optimum treatment results, all-ceramic crowns are appropriate for the prosthetic restoration of all-ceramic abutments. However, this leads to the question of whether dental ceramic materials are best suited for this indication. Due to the small number of available studies on all-ceramic superstructures, evidence-based data are limited. In the available studies, leucite-reinforced glassceramics (Empress 1, Ivoclar Vivadent), lithium-disilicate ceramics (e.max, Ivoclar Vivadent), and all-ceramic crowns with alumina or zirconia structures were applied. Glauser et al28 evaluated the combination of 36 zirconia superstructures and crowns made of a leucitereinforced glass ceramic (Empress 1) in the anterior and premolar regions. After an observational period of 4 years, they documented a survival rate of 100% for the implant superstructures; three crowns displayed minor fractures of the veneering ceramic that could be polished. Vanlioglu et al14 investigated the clinical performance of lithium-disilicate anterior crowns that were adhesively luted on individual zirconia abutments. After an observational period of 5 years, the 11 reevaluated restorations displayed neither ceramic fractures nor a loss of retention. The clinical performance of zirconia crowns on ceramic abutments in the anterior region was evaluated in a prospective study.29 After an observational period of 3 years, 64 anterior crowns showed two ceramic fractures (3.1%). One crown had
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to be renewed due to the fracture size. In their study, the crowns were adhesively luted to the prefabricated zirconia abutments. During radiographic re-evaluation, two crowns showed cement residues. This finding confirms the clinical relevance of residue removal in adhesive cementation. In a retrospective study,30 crowns with alumina ceramic structures and directly veneered zirconia abutments were re-evaluated over an observational period of up to 3 years. The fracture rate was found to be 1% and no fractures of the alumina structures could be documented, while 2% of the restorations showed a fracture of the veneering ceramic within the first year. With regard to clinical data for glass ceramics, materials on a lithium-disilicate base should be preferred because they are significantly stronger than leucite-reinforced ceramics. However, this material has limitations because, to date, only data for adhesive cementation are available. The adhesive cementation of all-ceramic restorations on implants bears the risk of peri-implantitis caused by the remaining cement residues.29 Therefore, adhesive cementation can only be recommended if good accessibility of the preparation limit guarantees the safe and complete removal of excess cement residues. This precondition is fulfilled best in situations with individually fabricated abutments with a slightly subgingival preparation limit. With regard to oxide ceramics, alumina as well as zirconia seem to be appropriate for use in implantsupported anterior single crowns. Both materials show a high reliability of the structures, and technical complications are expected as fractures of the veneering ceramics. To avoid this phenomenon, findings obtained for tooth-supported oxide-ceramic restorations should be transferred to implant-supported restorations, ie a consistent layer thickness of the veneering ceramics must be guaranteed by the tooth-analog abutment and structure design. During the veneering process, internal strain should be reduced, for example by longterm cooling.31 Because screw-loosening is a common technical complication in implant-supported restorations,1,2 it must be considered that re-fixation of the abutment screw in adhesively cemented restorations is only pos-
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sible after removal or trepanation of the crown. Here, there are two possible solutions: a temporary cementation of the crown, or a centrally screw-fixed crown with a directly veneered zirconia abutment. If a sufficient height of the individually fabricated abutment or the integration of a retention-improving design (grooves) guarantees sufficient retention of the all-ceramic restoration, temporary cementation (at least for zirconia-based restorations) can be considered. Temporary cementation permits destruction-free removal of the restoration in the case of screw-loosening or fracture of the veneering ceramics. Because the cumulative incidence of these two events is approximately 7.5% within the first 5 years, the ability to revise a superstructure is an important aspect in the choice of materials and methods. However, it must be considered that until now, clinical studies have not provided any results for temporarily cemented all-ceramic superstructures. Data for centrally screw-fixed direct abutments are available for zirconia abutments with a maximum observational period of 3 years. The advantages of screw fixation are the same as those for temporary cementation: screw fixation allows a safe removal in the case of technical complications. Moreover, the induction of iatrogenic peri-implantitis in response to excess cement residues can be safely avoided. However, this type of restoration is limited because it can only be applied in the anterior maxilla if a palatal shifted implant position allows the positioning of the screw canal on the palatal surface. A screw-fixed superstructure must be planned at an early stage, ie prior to insertion of the implant.
Clinical relevance Zirconia abutments for single-tooth restorations can be restored either with glass-ceramic crowns on a lithiumdisilicate base or with crowns on oxide-ceramic structures (alumina or zirconia). If they are cemented adhesively, safe removal of the excess cement residues is essential. For superstructures based on zirconia ceramics, limited removability can be achieved by temporary
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cementation or screw-fixation of the directly veneered abutments. The first clinical results reported for screwfixed single-crown restorations showed no tendency towards an increased technical complication rate. However, prior to providing a general recommendation for temporary cementation or screw fixation of all-ceramic superstructures, additional clinical data are needed.
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