JJOD 2266 1–8 journal of dentistry xxx (2014) xxx–xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/jden 1 2 3
In vitro performance of zirconia and titanium implant/abutment systems for anterior application
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Martin Rosentritt, Anna Hagemann, Sebastian Hahnel, Michael Behr, Verena Preis *
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Department of Prosthetic Dentistry, University Medical Center Regensburg, 93042 Regensburg, Germany
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Q1
article info
abstract
Article history:
Objectives: To investigate the type of failure and fracture resistance behaviour of different
Received 26 September 2013
zirconia and titanium implant/abutment systems for anterior application.
Received in revised form
Methods: Eight groups of implant–abutment combinations (n = 8/system) were restored with
5 March 2014
identical full-contour zirconia crowns. The systems represented one-piece and multi-piece
Accepted 23 March 2014
zirconia (Z) or titanium (T) implants/abutments with different types of connection (scre-
Available online xxx
wed = S, bonded = B). The following combinations (implant–abutment-connection) were investigated: ZZS, ZZB, ZZZB (three-piece), ZTS, TTS, TTS reference, and Z (one-piece,
Keywords:
2). To simulate clinical anterior loading situations the specimens were mounted into
Implant
the chewing simulator at an angle of 1358 and subjected to thermal cycling (2 3000 58/
Abutment
55 8C) and mechanical loading (1.2 106 50 N; 1.6 Hz). Fracture resistance and maximum
Zirconia
bending stress were determined for all specimens that survived ageing. Data were statisti-
Titanium
cally analyzed with the Kolmogorov–Smirnov-test and one-way ANOVA (a = 0.05). Survival
Chewing simulation
performance was calculated with the Kaplan–Meier Log-Rank test.
Fracture resistance
Results: Independent of the material combinations screwed systems showed partly failures of the screws during simulation (ZZS: 3, ZTS: 8, TTS: 3). Screw failures were combined with implant/abutment fractures of zirconia systems. Zirconia one-piece implants and the reference system did not show any failures, and only one specimen of the systems with a bonded connection (ZZZB) fractured. Mean (standard deviation) fracture forces and maximum bending stresses differed significantly (p = 0.000) between 187.4 42.0 N/ 250.0 56.0 N/mm2 (ZZZB) and 524.3 43.1 N/753.0 61.0 N/mm2 (Z). Conclusions: Both material (zirconia or titanium) and the type of connection influenced failure resistance during fatigue testing, fracture force, and maximum bending stress. Clinical significance: Different material combinations for implants and abutments as well as different types of connection achieved acceptable or even good failure and fracture resistance that may be satisfactory for anterior clinical application. # 2014 Published by Elsevier Ltd.
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Yttria-stabilized zirconia (Y-TZP) ceramics have proven their suitability for many clinical applications in dentistry. Zirconia
Introduction
has high potential to be successfully used for fixed partial dentures, root posts, and implant abutments.1,2 However, the few scientific data on the performance of zirconia implants available mainly consist of case reports and are restricted to
* Corresponding author. Tel.: +49 941 944 6055; fax: +49 941 944 6171. E-mail address: [email protected] (V. Preis). http://dx.doi.org/10.1016/j.jdent.2014.03.010 0300-5712/# 2014 Published by Elsevier Ltd.
Please cite this article in press as: Rosentritt M, et al. In vitro performance of zirconia and titanium implant/abutment systems for anterior application. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.03.010
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short clinical observation periods.3–8 The clinical success of two-piece titanium implants has been well proven over many years, but zirconia implants still seem to be at the developmental stage and are not yet recommended for routine clinical application.3,9,10 Zirconia is marked by comparable osseointegration as for titanium implants,6,11 good stabilization of the soft tissues, low plaque retention,12–15 as well as tooth-like colour. These characteristics make zirconia an attractive alternative to titanium, especially in the aesthetically demanding anterior tooth region. In case of unfavourable tissue conditions, such as thin peri-implant mucosa or soft tissue recession, the more natural appearance of zirconia implants/abutments in comparison to titanium is of particular interest, as is the possibility of staining zirconia in tooth or gingival colours. A further advantage is the high hardness of the inert zirconia surface, allowing the easy removal of residual cement. In contrast, titanium implant coatings may be damaged by metallic scalers. However, because the mechanical properties of zirconia ceramics differ from metals, common geometries of titanium implants cannot be directly transferred to viable zirconia implant designs. Although providing high strength and structural reliability, zirconia is vulnerable to bending and subcritical crack growth,16,17 thus requiring round design elements without sharp edges or thin peaked areas. Because implant and abutment connections by screws, as common in titanium implants, are difficult to obtain, most zirconia implants available are one-piece or two-piece systems, in which the abutment is bonded to the implant. The clinical indication and user popularity of one-piece implants are limited because of the high demands on perfect anatomical positioning, load-free transgingival healing periods,5 and the necessity of intraoral abutment preparation. Two-piece bonded implants may overcome these problems but do not allow any reversible connection between implant and abutment. Furthermore, the inert character of zirconia18 may limit the long-term stability of such bonding. Because neither of these zirconia implant designs allows replacing the abutment, the entire implant must be removed in the case of catastrophic abutment failure. To overcome these limitations, efforts to develop a reliable screwed implant–abutment connection have resulted in the recent introduction of two-piece zirconia implants with different connecting screws. Ceramic screws are difficult to realize, and insufficient fitting of the implant–abutment combination may lead to failure; therefore, metal screws that may allow ‘‘cold welding’’ could be applied. Prior to the routine clinical application of new implant/ abutment designs and materials, in vitro thermal cycling and mechanical loading are supposed to allow first predictions of their mechanical performance and resistance against hydrolytic effects in a simulated clinical situation. Even in cases without any catastrophic failure during oral application, ageing and deterioration effects might occur, thus reducing strength and fracture resistance. In these cases, a subsequent static fracture test may help locate initiated weak points. The hypothesis of this investigation was that different zirconia and titanium implant/abutment systems show comparable in vitro performance and fracture resistance and may be appropriate for anterior application.
2.
Materials and methods
Eight groups of implant–abutment combinations were investigated (n = 8 per group), representing different one-piece and multi-piece (two- or three-piece) zirconia or titanium systems (Table 1). Most of the implants/abutments represented experimental systems. A proven two-piece titanium system (Semados S) served as a reference. Depending on the availability, standard implant diameters between 4.0 and 4.5 mm were chosen. The implants were accurately positioned in polyoxymethlyene (POM) resin, allowing a consistent length (13.0 0.5 mm) between the embedding resin and the incisal edge of the prosthetic crown for the different systems. Thus, a constant lever arm during loading was guaranteed. For two-piece zirconia and titanium implant/abutment systems with a screw-retained connection, the respective prefabricated straight abutments were tightened with a torque gauge using titanium abutment screws according to the manufacturer’s instructions (25–35 N cm). For two-piece or three-piece bonded zirconia systems, the corresponding straight abutments were connected to the implants with a resin-based composite cement (Panavia F 2.0, Kuraray, Okayama, Japan). 64 Full-contour anterior crowns (tooth 11) of nearly identical external shape were manufactured of yttria-stabilized zirconia (Cercon HT, DeguDent, Hanau, Germany) by means of the CAD/CAM (computer aided design/computer aided manufacturing) technique (Cercon eye/art/brain/heat plus, DeguDent). The dimensions of the crowns were designed in such a way that the abutments did not or only minimally require preparation, which might have weakened the material. Therefore, the inner and cervical geometry as well as the thickness of the crowns were modified for the different groups. The zirconia crowns were glazed with the corresponding glazing material (Cercon glaze, DeguDent) and adhesively fixed onto the implant/abutment systems with a resin-based composite cement (Panavia F 2.0). For simulating clinically relevant anterior loading situations, the specimens were mounted into a chewing simulator (EGO, Regensburg, Germany) at an angle of 1358 between the long axis of the implant and the horizontal plane of the sample holder (Fig. 1), representing an average interincisal angle of people with normal occlusion.19,20 Steatite balls (CeramTec, Plochingen, Germany) with a diameter of 12 mm served as antagonists and were placed in maximal contact position on the palatal surface 2 mm from the incisal edge of the anterior crowns. Thermal cycling (TC: 2 3000 5 8C/55 8C; 2 min each cycle) and simultaneous mechanical loading (ML: 1.2 106; 50 N; 1.6 Hz; mouth opening: 2 mm) was performed. These parameters were based on the literature data on zirconia and ceramic restorations, stating that chewing simulations using these parameters might simulate a maximum of five years of oral service.21,22 During the simulation, all implant/abutment systems were optically monitored daily by examiners wearing binocular magnifying (2.8) glasses. Apparent failures were documented, and failed specimens were excluded from the further simulation process. To exclude any defects or imperfections before TCML as well as undetected failures after TCML, all specimens were inspected with a light
Please cite this article in press as: Rosentritt M, et al. In vitro performance of zirconia and titanium implant/abutment systems for anterior application. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.03.010
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Table 1 – Overview of implant/abutment systems. Group
138 139 140 141 142 143 144 145 146 147 148 149 150
Name/Manufacturer
Number of pieces
Implant
Abutment
Connection
Implant diameter/length
ZZS
Experimental system
Two-piece
Zirconia
Zirconia
Screwed
4.1 mm/10 mm
ZZB
Ceramic Implant Prototype Straumann, G
Two-piece
Zirconia
Zirconia
Bonded
4.0 mm/10 mm
ZZZB
Experimental implant
Three-piece
Zirconia
Zirconia (two-piece)
Bonded
4.1 mm/11 mm
ZTS
Experimental implant
Two-piece
Zirconia
Titanium
Screwed
4.1 mm/14 mm
TTS
Experimental system
Two-piece
Titanium
Titanium
Screwed
4.0 mm/10 mm
TTS reference
Semados S Bego, G
Two-piece
Titanium
Titanium
Screwed
4.1 mm/15 mm
Za
OMNIS Creamed, G
One-piece
Zirconia
4.5 mm/12 mm
Zb
White Sky 4.0 Bredent, G
One-piece
Zirconia
4.0 mm/10 mm
microscope (magnification: 20; M420, Wild, Heerbrugg, Switzerland) before and after fatigue testing. Implant/abutment systems that survived TCML were subsequently loaded until failure with a testing machine (Zwick 1446, Ulm, Germany; v = 1 mm/min). In analogy to chewing simulation, load was applied at an angle of 1358 with the loading stamp centrally positioned at the palatal surface 2 mm from the incisal edge. A tin foil (0.25 mm, Dentaurum, Ispringen, Germany) between crown and loading stamp prevented force peaks. Implant/abutment systems were optically examined after fracture testing, and the failure mode was documented.
Fig. 1 – Design of testing apparatus and loading situation.
Scheme
Maximum bending stress sb was calculated using the formula: sb ¼
150 151 152
M ; W
with M being the bending moment: M = F l sin 458, and W being the moment of resistance: W = p/32 D3 (D: diameter of the implant).
153 154 155 156
Calculations and statistical analysis were done with SPSS 19.0 for Windows (SPSS Inc., Chicago, IL, USA). Power calculation (G*Power 3.1.3, Kiel, Germany) provided an estimated power of >90% using eight specimens per group. Data distribution was controlled with the Kolmogorov–Smirnov-test. Means and standard deviations of fracture force and bending stress were calculated and analyzed using one-way analysis of variance (ANOVA) and the Bonferroni-test for posthoc analysis. Survival performance was calculated with the Kaplan–Meier Log-Rank test. The level of significance was set to a = 0.05.
157 158 159 160 161 162 163 164 165 166
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Results
Independent of the material combination, some screws in screw-retained systems came loose or ruptured during the chewing simulation. For the TTS systems, screw rupture was only observed in one TTS group, whereas the reference group
Please cite this article in press as: Rosentritt M, et al. In vitro performance of zirconia and titanium implant/abutment systems for anterior application. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.03.010
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Table 2 – Number of failed implant/abutment systems and description of failures during thermal cycling and mechanical loading (TCML) and subsequent fracture testing. Group
172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189
TCML
Fracture test (exclusion of failed crowns during TCML)
Number of failed implants/ abutments
Number of chewing cycles: type of failure
Fracture force [N] Mean SD
Maximum bending stress [N/mm2] Mean SD
ZZS
3
269.6 52.8
359.6 70.4
3 screw loosening/crack of implant/fracture of abutment 2 screw loosening/fracture of abutment
ZZB
0
5000: screw loosening/fracture of implant 425,194: screw rupture/fracture of implant and abutment 1,200,000: screw rupture/ fracture of implant and abutment –
297.4 32.4
427.1 46.6
ZZZB
1
187.4 42.0
250.0 56.0
ZTS
8
TTS
3
TTS Reference Za Zb
0
890,314: fracture of implant and abutment 249,000: screw rupture/fracture of implant 490,000: screw rupture/fracture of implant 490,000: screw rupture/fracture of implant 735,000: screw rupture/fracture of implant 735,000: screw rupture/fracture of implant 932,000: screw rupture/fracture of implant 735,000: screw rupture/fracture of implant 1,200,000: screw rupture/ fracture of implant 454,562: screw rupture 562,373: screw rupture 850,000: screw rupture –
7 fracture of abutment 1 fracture of implant and abutment 7 fracture of implant and abutment
0 0
– –
TTS survived TCML without any failures. All specimens of the zirconia/titanium system connected by a screw (ZTS) failed because of screw rupture, and, in each case, the buccal implant neck fractured. Some specimens of the screw-retained zirconia/ zirconia system (ZZS) showed screw loosening or rupture and, in one specimen, a fracture of the implant body below the level of the embedding resin. Two other ZZS specimens failed because of screw rupture in combination with a fracture of both the implant and abutment at the implant–abutment interface. For the bonded zirconia systems, only one specimen in the ZZZB group showed a fracture of the implant and abutment at the level of the implant neck, whereas no failures were observed in the ZZB group. Both one-piece zirconia systems did not show any failure at all. Except for slight wear facets, full-contour zirconia crowns remained intact in all groups. Table 2 gives an overview of the failure modes and the number of chewing cycles at failure. The Kaplan–Meier Log-Rank test showed significant (p = 0.000) differences between the eight groups (Fig. 2).
Type of failure
No fracture test
377.6 30.2
542.4 43.4
2 screw rupture 3 screw deformation
394.1 20.2
525.7 27.0
8 screw rupture
372.4 39.7 524.3 43.1
375.6 40.0 753.0 61.9
8 fracture of implant 7 fracture of implant 1 fracture of implant and crown
Mean (SD) fracture forces (Table 2) for implant/abutment systems that survived TCML varied widely between 187.4 42.0 N (ZZZB) and 524.3 43.1 N (Zb), and showed significant (p = 0.000) differences between the various groups (Table 3). The one-piece zirconia system Zb had the significantly highest mean fracture force, followed by the two screwretained titanium systems TTS and the one-piece zirconia implant Za. The mean fracture forces of the multi-piece zirconia implant–abutment combinations ranged in the lower third, showing the lowest value for the three-piece bonded implant/abutment system ZZZB. The two-piece bonded zirconia system ZZB and the screw-retained zirconia system ZZS showed comparable fracture forces that were significantly higher than those of ZZZB. The ZTS group was excluded from the fracture test because all specimens had already failed during TCML. Fracture forces of the different groups showed good correlation with the respective bending stresses for systems
Please cite this article in press as: Rosentritt M, et al. In vitro performance of zirconia and titanium implant/abutment systems for anterior application. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.03.010
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Fig. 2 – Kaplan–Meier survival curve for the different implant/abutment systems.
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with implant diameters of 4.0 and 4.1 mm (Table 2). Here, oneway ANOVA and post hoc Bonferroni test showed corresponding significance levels for comparisons between the groups (Table 3). Only Za with an implant diameter of 4.5 mm showed comparably low bending stress despite high fracture force that was reflected in different p-values for comparisons of fracture force and bending stress (Table 3). In accordance to failure modes observed during TCML, systems connected by screws mainly failed because of rupture, deformation, or loosening of the screw, irrespective of the material combination. Zirconia one-piece systems failed because of fracture of the implant and bonded zirconia systems because of fracture of the abutment or implant (Table 2). In contrast to titanium systems, in which failure patterns were restricted to the connecting screw (deformation or rupture), screw loosening in the ZZS specimens was always combined with damage to the abutment or implant at the level of the implant neck. Failures of bonded zirconia systems were either sole fractures of the abutment in proximity to the crown
margin (ZZB) or combined fractures of the abutment and implant at the level of the implant neck (ZZB, ZZZB). Fullcontour zirconia crowns were not damaged during fracture testing in any group except for one specimen of the series Zb, in which the crown fractured together with the implant. Apart from this particular specimen, the fracture patterns of onepiece zirconia implants were uniform with the fracture starting at the palatal implant head close to the crown margin and proceeding diagonally to the compression zone at the buccal side near the level of the embedding resin.
227 228 229 230 231 232 233 234 235 236
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Discussion
The hypothesis of this investigation that different zirconia and titanium implant/abutment systems show comparable in vitro performance and fracture resistance cannot be confirmed. The results indicated that the types of connection as well as the materials of the implant–abutment combination affect a system’s resistance to failure. Most of the implant/abutment systems in the present study are experimental systems, whose mechanical properties have not yet been investigated under clinical conditions. Therefore, chewing simulation with thermal cycling and mechanical loading may allow the preclinical evaluation of their suitability for clinical application. Once osseointegration has been achieved and preserved, resistance against fatigue and the stability of the implant– abutment connection become crucial factors for clinical success. An average loading force of 50 N was chosen in this study, because clinical loading situations physiologically vary between 12 and 70 N.23,24 Although such low physiological forces may not necessarily surpass the fracture threshold of the assembly, they might, if repeatedly applied, lead to fatigue of the implant or abutment materials and their connecting mechanism and finally to failure. According to the common clinical position of anterior teeth, the specimens were mounted at an angle of 1358, resulting in a 458-off-axisloading, as described by Kohal.25,26 In this respect, the present loading scenario differs from other studies referring to occlusal load application at an angle of 308 according to ISO 14801.27–29The abutment titanium screw is commonly the weakest link in this system, and the region around the screw
Table 3 – p-Values (one-way ANOVA, post hoc Bonferroni test) for comparisons between fracture force (first line) or bending stress (second line) of different groups.
ZZB ZZZB TTS TTS reference Za Zb
ZZS
ZZB
ZZZB
TTS
1.000 0.466 0.012* 0.011* 0.001* 0.000* 0.000* 0.000* 0.000* 1.000 0.000* 0.000*
0.000* 0.000* 0.012* 0.005* 0.000* 0.006* 0.006* 0.950 0.000* 0.000*
0.000* 0.000* 0.000* 0.000* 0.000* 0.000* 0.000* 0.000*
1.000 1.000 1.000 0.000* 0.000* 0.000*
TTS Reference
1.000 0.000* 0.000* 0.000*
Za
0.000* 0.000*
* Indicates significant differences ( p 0.05).
Please cite this article in press as: Rosentritt M, et al. In vitro performance of zirconia and titanium implant/abutment systems for anterior application. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.03.010
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head is the area of the highest torque and stress concentration.30 Therefore, failures of such two-piece implant/abutment systems always involve the connecting screw. For the titanium system TTS, these forces led to rupture of the screws without affecting the titanium implant or abutment. In contrast, the same forces caused fractures of the zirconia implant or abutment besides screw failure (loosening, rupture) in systems with a zirconia component. This result may be explained by the low tolerance of zirconia against tensile forces occurring in the area around the screw that has been proven to be critical for the stability of ceramic abutments in laboratory studies.31–33 Cyclic loading is assumed to result in loosening, the plastic deformation of the screw thread and in the subsequent deflection of the entire construction. As a consequence, fatigue-assisted crack initiation and growth lead to fractures within the thin parts of the ceramic construction.34 Because the abutments were bent over the zirconia implants after the loss of the connection by screw loosening or rupture, the specimens fractured at the implant–abutment interface or the implant neck. In contrast to zirconia abutments in ZZS, no additional fractures of the titanium abutments were found in ZTS. Nevertheless, the breakdown of all ZTS specimens during TCML critically questions the suitability of a system that connects titanium abutments to zirconia implants. In addition to fatigue by mechanical loading, yttria-stabilized zirconia ceramics are sensitive towards thermal ageing in the presence of water or the moisture in the oral environment and may become critically brittle.17 Zirconia is thus more susceptible towards cracking than titanium. Because of the brittleness of ceramic materials and the difficulties associated with connecting zirconia abutments and implants, the first zirconia implants consisted of one-piece systems. Accordingly, both one-piece zirconia implants investigated were more reliable against fatigue than the two-piece system ZZS and did not fail during TCML. High resistance of the one-piece zirconia implant design againstageing has also been reported in previous in vitro studies25,26,35 and may be attested by first clinical observations.5,36 However, it should be kept in mind that abutment preparation – that is usually unavoidable in clinical situations but was not or only minimally done in the present study – might increase the risk of failure and decrease fracture strength.10,26 Connecting zirconia abutments and implants by bonding was shown to have almost comparable high resistance towards fatigue as one-piece zirconia implants. However, the inert character of zirconia18 may limit the stability of the bonding over longer periods of time. In accordance to the failure modes observed in the present study, technical complications with the abutment screws (loosening, rupture) have been the most often reported problems for two-piece systems in other laboratory or clinical studies.30 So far, no comprehensive studies are available on zirconia implants and abutments connected by screws. However, studies on two-piece titanium implants with either metal or ceramic abutments showed that abutment screws fracture regularly in case of metal abutments.30,34 Ceramic abutments showed similar rates of loosening or deformation of the connecting screws as metal abutments. However, fracture of the abutments was primarily described instead of screw rupture. Therefore, the fracture of the zirconia abutment is assumed to occur before the fracture of the abutment screw.30
Failure patterns for two-piece zirconia systems, as investigated in the present study, seem to be even more complex. Considering failures of ZZS after fatigue and fracture testing, screw loosening or rupture were always combined with either failure of the abutment or the implant, or both. Furthermore, the screw preload is supposed to be an important factor for avoiding screw loosening or subsequent screw rupture.37,38 The high rates of screw failures during TCML in the present study may have been the result of loss of the preload because the screws were not retightened after some minutes as recommended in daily clinical routine.39 The number of mechanical cycles at failure may give important clues about the underlying failure reason. Failures at the beginning of the endurance test might indicate the existence of material or processing defects, as may be assumed for the early implant fracture of the ZZS group (5000 mechanical cycles). The presence of a pre-existing defect of this specimen is further confirmed by the untypical fracture of the implant body, whereas other specimens typically failed at the implant neck. In return, failures at higher mechanical cycles may be more typical for fatigue mechanisms of the connecting screws and materials. Because screw failures occurred at mechanical cycles higher than 249,000 in both titanium and zirconia systems, annual dental visits to control and – if indicated – retighten the connecting screw may contribute to avoiding or reducing the risk of further complications (rupture, fracture) in clinical use. However, ageing and deterioration often occur without any evidence of failure. In these cases, a subsequent static fracture test may help locate weak points and permit further differentiation of implant and abutment materials as well as design variations. Undoubtedly, fracture data cannot be directly related to clinical survival. However, in relation to clinically proven systems – as in the reference group TTS – such data may provide helpful information on the principal suitability of new systems. Maximum chewing forces in anterior dentition have been reported to range between about 150 N and 300 N.40,41 Therefore, not all groups investigated might withstand force peaks considered to be within the physiological range. In comparison to the two-piece titanium systems, the fracture resistance and maximum bending stress of the two-piece zirconia systems with either a screwed or a bonded connection were lower but seem to be acceptable (ZZS, ZZB) or marginally acceptable in the case of ZZZB. Despite similar fracture forces for bonded and screw-retained zirconia systems, the higher failure frequency of ZZS during TCML than of ZZB/ZZZB needs to be taken into consideration. In contrast, one-piece zirconia implants showed high reliability during TCML and fracture forces comparable or even higher than those of the reference group. Despite a smaller diameter, Zb showed significantly higher force values than Za, and even twice the maximum bending stress of Za. As Za was the implant with the largest diameter in this study, calculation of the maximum bending stress allowed a fair comparison of the groups, irrespective of the individual implant diameters. Considering both fracture force and maximum bending stress, the results showed that even zirconia implants with small diameters might be successfully applied in anterior tooth regions.
Please cite this article in press as: Rosentritt M, et al. In vitro performance of zirconia and titanium implant/abutment systems for anterior application. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.03.010
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Failure modes of groups already failing during TCML were similar after fracture testing: two-piece titanium systems failed because of screw rupture or deformation, whereas ZZS showed additional damage to the abutment or the implant. These results indicate that even fracture testing may be used to replicate clinically relevant failure modes. Although bonded zirconia implants/abutments survived TCML without any failure (ZZB) and only one failure (ZZZB) respectively, comparably low fracture forces during static loading led to catastrophic failures of the implants and abutments. Therefore, the weakest point of these groups does not seem to be the bonding itself, but primarily the design of the connecting parts. Thin ceramics parts that may be especially used in the three-piece implant–abutment combination are supposed to be prone to cracking and fracture. Use of a higher number of connected pieces obviously increases the importance of efficient and strong bonding to achieve monoblocks. However, the inertness of zirconia may not allow any long-term stability of this bonding. In contrast, one-piece zirconia implants also failed in a catastrophic manner but at fracture forces that are supposed to exceed chewing forces of anterior teeth. The crack pathway from the lingual to the buccal face is typical for bending fractures and has already been described in previous in vitro studies.26,42 Monolithic zirconia restorations would usually not be applied in the anterior tooth region because of their inferior aesthetics compared to feldspathic porcelains or glass ceramics. In this study, however, they were chosen because of their high fracture resistance.43,44 In veneered restorations, chipping or fracture of the prosthetic crowns might have occurred before failure of the implants or abutments that would have complicated the evaluation and differentiation of the implant/abutment systems. In a study by Yildirim,27 the glass-ceramic crowns partly failed before the destruction of the abutments. According to a study by Martı´nez-Rus,29 both the abutment material and the ceramic crown system affected the results. Nevertheless, Sailer28 showed that restoring different abutments with leucite-reinforced glass-ceramic crowns did not influence the bending moments. Different degrees of severity of failure patterns have to be differentiated. In clinical application, implant systems may be preferable that have a predetermined breaking point and allow the uncomplicated replacement of an abutment. This concept has been shown to be successful in two-piece titanium systems: for both TTS groups, the connecting screw failed (rupture or deformation) before any damage to the abutment or implant could occur. Therefore, two-piece screwed zirconia implant–abutment combinations may have great future potential if further enhancements in the design of the connection may protect the zirconia implant and abutment in the case of any overloading or unexpected failure. Different implant diameters, loading scenarios, material compositions, and the embedding materials may influence the outcome of in vitro studies. Therefore, the present results cannot be transferred to the clinical situation without limitations. For example, embedding resins with a modulus of elasticity similar to human bone (10–18 GPa) might be preferable. In addition, the manifold possibilities of varying the design of implants, abutments, and their connections require further research into this topic.
7
This in vitro study was a first approach to investigate the principal suitability of different zirconia and titanium implant/abutment systems for anterior application. Two-piece titanium implants may still be recommended as standard implants for most indications. Nevertheless, different zirconia implant/abutment systems might be appropriate for anterior application with limitations. However, further research to improve the connecting design before routine clinical application is suggested because of the partly high failure rates of the zirconia implants and abutments during TCML and the comparably low fracture resistance of some connections.
447 448 449 450 451 452 453 454 455 456 457
5.
458
Conclusions
Both the material combination (zirconia and titanium) and the type of connection influenced the failure resistance during chewing simulation, the fracture force, as well as the maximum bending stress. Irrespective of the implant or abutment material, failures of screw-retained systems always involved the connecting titanium screw (loosening, rupture). Screwed connections with a zirconia component additionally showed fractures of the implant or the abutment, or both. One-piece and bonded multi-piece zirconia implants/abutments as well as the proven two-piece titanium system showed good in vitro performance during TCML. Bonded or screwed systems with zirconia implants/abutments showed lower fracture resistance than one-piece zirconia implants and two-piece titanium implants.
459 460 461 462 463 464 465 466 467 468 469 470 471 472 473
Acknowledgement
474
We would like to thank the manufacturers for providing the materials.
475 476
references
477
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26. Kohal RJ, Wolkewitz M, Tsakona A. The effects of cyclic loading and preparation on the fracture strength of zirconium-dioxide implants: an in vitro investigation. Clinical Oral Implants Research 2011;22:808–14. 27. Yildirim M, Fischer H, Marx R, Edelhoff D. In vivo fracture resistance of implant-supported all-ceramic restorations. Journal of Prosthetic Dentistry 2003;90:325–31. 28. Sailer I, Sailer T, Stawarczyk B, Jung RE, Ha¨mmerle CHF. 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. The International Journal of Oral and Maxillofacial Implants 2009;24:850–8. ¨ zcan M, Bartolome´ JF, Pradı´es 29. Martı´nez-Rus F, Ferreiroa A, O G. Fracture resistance of crowns cemented on titanium and zirconia implant abutments: a comparison of monolithic versus manually veneered all-ceramic systems. The International Journal of Oral and Maxillofacial Implants 2012;27:1448–55. 30. Sailer I, Philipp A, Zembic A, Pjetursson BE, Ha¨mmerle CHF, Zwahlen M. A systematic review of the performance of ceramic and metal implant abutments supporting fixed implant reconstructions. Clinical Oral Implants Research 2009;20:4–31. 31. Tripodakis AP, Strub JR, Kappert HF, Witkowski S. Strength and mode of failure of single implant all-ceramic abutment restorations under static load. International Journal of Prosthodontics 1995;8:265–72. 32. Att W, Kurun S, Gerds T, Strub JR. Fracture resistance of single-tooth implant-supported all-ceramic restorations after exposure to the artificial mouth. Journal of Oral Rehabilitation 2006;33:380–6. 33. Att W, Kurun S, Gerds T, Strub JR. Fracture resistance of single-tooth implant-supported all-ceramic restorations: an in vitro study. Journal of Prosthetic Dentistry 2006;95:111–6. 34. Foong JKW, Judge RB, Palamara JE, Swain MV. Fracture resistance of titanium and zirconia abutments: an in vitro study. Journal of Prosthetic Dentistry 2013;109:304–12. 35. Kohal R, Klaus G, Strub JR. Zirconia-implant-supported allceramic crowns withstand long-term load: a pilot investigation. Clinical Oral Implants Research 2006;17:565–71. 36. Lambrich M, Iglhaut G. Comparison of the survival rates for zirconia and titanium implants. Zeitschrift fu¨r Zahna¨rztliche Implantologie 2008;24:182–91. 37. Siamos G, Winkler S, Boberick KG. Relationship between implant preload and screw loosening on implant-supported prostheses. Journal of Oral Implantology 2002;28:67–73. 38. Gracis S, Michalakis K, Vigolo P, Vult von Steyern P, Zwahlen M, Sailer I. Internal vs. external connections for abutments/ reconstructions: a systematic review. Clinical Oral Implants Research 2012;23:202–16. 39. Winkler S, Ring K, Ring JD, Boberick KG. Implant screw mechanics and the settling effect: overview. Journal of Oral Implantology 2003;29:242–5. 40. Serra CM, Manns AE. Bite force measurements with hard and soft bite surfaces. Journal of Oral Rehabilitation 2013;40:563–8. 41. Fontijn-Tekamp FA, Slagter AP, Van Der Bilt A, van ‘THof MA, Witter DJ, Kalk W, et al. Biting and chewing in overdentures, full dentures, and natural dentitions. Journal of Dental Research 2000;79:1519–24. 42. Silva NRFA, Coelho PG, Fernandes CAO, Navarro JM, Dias RA, Van Thompson P. Reliability of one-piece ceramic implant. Journal of Biomedical Materials Research Part B Applied Biomaterials 2009;88:419–26. 43. Beuer F, Stimmelmayr M, Gueth J, Edelhoff D, Naumann M. In vitro performance of full-contour zirconia single crowns. Dental Materials 2011;28:449–56. 44. Preis V, Behr M, Hahnel S, Handel G, Rosentritt M. In vitro failure and fracture resistance of veneered and full-contour zirconia restorations. Journal of Dentistry 2012;40:921–8.
Please cite this article in press as: Rosentritt M, et al. In vitro performance of zirconia and titanium implant/abutment systems for anterior application. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.03.010
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/jden 1 2 3
In vitro performance of zirconia and titanium implant/abutment systems for anterior application
4
6
Martin Rosentritt, Anna Hagemann, Sebastian Hahnel, Michael Behr, Verena Preis *
7
Department of Prosthetic Dentistry, University Medical Center Regensburg, 93042 Regensburg, Germany
5
Q1
article info
abstract
Article history:
Objectives: To investigate the type of failure and fracture resistance behaviour of different
Received 26 September 2013
zirconia and titanium implant/abutment systems for anterior application.
Received in revised form
Methods: Eight groups of implant–abutment combinations (n = 8/system) were restored with
5 March 2014
identical full-contour zirconia crowns. The systems represented one-piece and multi-piece
Accepted 23 March 2014
zirconia (Z) or titanium (T) implants/abutments with different types of connection (scre-
Available online xxx
wed = S, bonded = B). The following combinations (implant–abutment-connection) were investigated: ZZS, ZZB, ZZZB (three-piece), ZTS, TTS, TTS reference, and Z (one-piece,
Keywords:
2). To simulate clinical anterior loading situations the specimens were mounted into
Implant
the chewing simulator at an angle of 1358 and subjected to thermal cycling (2 3000 58/
Abutment
55 8C) and mechanical loading (1.2 106 50 N; 1.6 Hz). Fracture resistance and maximum
Zirconia
bending stress were determined for all specimens that survived ageing. Data were statisti-
Titanium
cally analyzed with the Kolmogorov–Smirnov-test and one-way ANOVA (a = 0.05). Survival
Chewing simulation
performance was calculated with the Kaplan–Meier Log-Rank test.
Fracture resistance
Results: Independent of the material combinations screwed systems showed partly failures of the screws during simulation (ZZS: 3, ZTS: 8, TTS: 3). Screw failures were combined with implant/abutment fractures of zirconia systems. Zirconia one-piece implants and the reference system did not show any failures, and only one specimen of the systems with a bonded connection (ZZZB) fractured. Mean (standard deviation) fracture forces and maximum bending stresses differed significantly (p = 0.000) between 187.4 42.0 N/ 250.0 56.0 N/mm2 (ZZZB) and 524.3 43.1 N/753.0 61.0 N/mm2 (Z). Conclusions: Both material (zirconia or titanium) and the type of connection influenced failure resistance during fatigue testing, fracture force, and maximum bending stress. Clinical significance: Different material combinations for implants and abutments as well as different types of connection achieved acceptable or even good failure and fracture resistance that may be satisfactory for anterior clinical application. # 2014 Published by Elsevier Ltd.
10 89 11 12 13
1.
14 15
Yttria-stabilized zirconia (Y-TZP) ceramics have proven their suitability for many clinical applications in dentistry. Zirconia
Introduction
has high potential to be successfully used for fixed partial dentures, root posts, and implant abutments.1,2 However, the few scientific data on the performance of zirconia implants available mainly consist of case reports and are restricted to
* Corresponding author. Tel.: +49 941 944 6055; fax: +49 941 944 6171. E-mail address: [email protected] (V. Preis). http://dx.doi.org/10.1016/j.jdent.2014.03.010 0300-5712/# 2014 Published by Elsevier Ltd.
Please cite this article in press as: Rosentritt M, et al. In vitro performance of zirconia and titanium implant/abutment systems for anterior application. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.03.010
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short clinical observation periods.3–8 The clinical success of two-piece titanium implants has been well proven over many years, but zirconia implants still seem to be at the developmental stage and are not yet recommended for routine clinical application.3,9,10 Zirconia is marked by comparable osseointegration as for titanium implants,6,11 good stabilization of the soft tissues, low plaque retention,12–15 as well as tooth-like colour. These characteristics make zirconia an attractive alternative to titanium, especially in the aesthetically demanding anterior tooth region. In case of unfavourable tissue conditions, such as thin peri-implant mucosa or soft tissue recession, the more natural appearance of zirconia implants/abutments in comparison to titanium is of particular interest, as is the possibility of staining zirconia in tooth or gingival colours. A further advantage is the high hardness of the inert zirconia surface, allowing the easy removal of residual cement. In contrast, titanium implant coatings may be damaged by metallic scalers. However, because the mechanical properties of zirconia ceramics differ from metals, common geometries of titanium implants cannot be directly transferred to viable zirconia implant designs. Although providing high strength and structural reliability, zirconia is vulnerable to bending and subcritical crack growth,16,17 thus requiring round design elements without sharp edges or thin peaked areas. Because implant and abutment connections by screws, as common in titanium implants, are difficult to obtain, most zirconia implants available are one-piece or two-piece systems, in which the abutment is bonded to the implant. The clinical indication and user popularity of one-piece implants are limited because of the high demands on perfect anatomical positioning, load-free transgingival healing periods,5 and the necessity of intraoral abutment preparation. Two-piece bonded implants may overcome these problems but do not allow any reversible connection between implant and abutment. Furthermore, the inert character of zirconia18 may limit the long-term stability of such bonding. Because neither of these zirconia implant designs allows replacing the abutment, the entire implant must be removed in the case of catastrophic abutment failure. To overcome these limitations, efforts to develop a reliable screwed implant–abutment connection have resulted in the recent introduction of two-piece zirconia implants with different connecting screws. Ceramic screws are difficult to realize, and insufficient fitting of the implant–abutment combination may lead to failure; therefore, metal screws that may allow ‘‘cold welding’’ could be applied. Prior to the routine clinical application of new implant/ abutment designs and materials, in vitro thermal cycling and mechanical loading are supposed to allow first predictions of their mechanical performance and resistance against hydrolytic effects in a simulated clinical situation. Even in cases without any catastrophic failure during oral application, ageing and deterioration effects might occur, thus reducing strength and fracture resistance. In these cases, a subsequent static fracture test may help locate initiated weak points. The hypothesis of this investigation was that different zirconia and titanium implant/abutment systems show comparable in vitro performance and fracture resistance and may be appropriate for anterior application.
2.
Materials and methods
Eight groups of implant–abutment combinations were investigated (n = 8 per group), representing different one-piece and multi-piece (two- or three-piece) zirconia or titanium systems (Table 1). Most of the implants/abutments represented experimental systems. A proven two-piece titanium system (Semados S) served as a reference. Depending on the availability, standard implant diameters between 4.0 and 4.5 mm were chosen. The implants were accurately positioned in polyoxymethlyene (POM) resin, allowing a consistent length (13.0 0.5 mm) between the embedding resin and the incisal edge of the prosthetic crown for the different systems. Thus, a constant lever arm during loading was guaranteed. For two-piece zirconia and titanium implant/abutment systems with a screw-retained connection, the respective prefabricated straight abutments were tightened with a torque gauge using titanium abutment screws according to the manufacturer’s instructions (25–35 N cm). For two-piece or three-piece bonded zirconia systems, the corresponding straight abutments were connected to the implants with a resin-based composite cement (Panavia F 2.0, Kuraray, Okayama, Japan). 64 Full-contour anterior crowns (tooth 11) of nearly identical external shape were manufactured of yttria-stabilized zirconia (Cercon HT, DeguDent, Hanau, Germany) by means of the CAD/CAM (computer aided design/computer aided manufacturing) technique (Cercon eye/art/brain/heat plus, DeguDent). The dimensions of the crowns were designed in such a way that the abutments did not or only minimally require preparation, which might have weakened the material. Therefore, the inner and cervical geometry as well as the thickness of the crowns were modified for the different groups. The zirconia crowns were glazed with the corresponding glazing material (Cercon glaze, DeguDent) and adhesively fixed onto the implant/abutment systems with a resin-based composite cement (Panavia F 2.0). For simulating clinically relevant anterior loading situations, the specimens were mounted into a chewing simulator (EGO, Regensburg, Germany) at an angle of 1358 between the long axis of the implant and the horizontal plane of the sample holder (Fig. 1), representing an average interincisal angle of people with normal occlusion.19,20 Steatite balls (CeramTec, Plochingen, Germany) with a diameter of 12 mm served as antagonists and were placed in maximal contact position on the palatal surface 2 mm from the incisal edge of the anterior crowns. Thermal cycling (TC: 2 3000 5 8C/55 8C; 2 min each cycle) and simultaneous mechanical loading (ML: 1.2 106; 50 N; 1.6 Hz; mouth opening: 2 mm) was performed. These parameters were based on the literature data on zirconia and ceramic restorations, stating that chewing simulations using these parameters might simulate a maximum of five years of oral service.21,22 During the simulation, all implant/abutment systems were optically monitored daily by examiners wearing binocular magnifying (2.8) glasses. Apparent failures were documented, and failed specimens were excluded from the further simulation process. To exclude any defects or imperfections before TCML as well as undetected failures after TCML, all specimens were inspected with a light
Please cite this article in press as: Rosentritt M, et al. In vitro performance of zirconia and titanium implant/abutment systems for anterior application. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.03.010
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Table 1 – Overview of implant/abutment systems. Group
138 139 140 141 142 143 144 145 146 147 148 149 150
Name/Manufacturer
Number of pieces
Implant
Abutment
Connection
Implant diameter/length
ZZS
Experimental system
Two-piece
Zirconia
Zirconia
Screwed
4.1 mm/10 mm
ZZB
Ceramic Implant Prototype Straumann, G
Two-piece
Zirconia
Zirconia
Bonded
4.0 mm/10 mm
ZZZB
Experimental implant
Three-piece
Zirconia
Zirconia (two-piece)
Bonded
4.1 mm/11 mm
ZTS
Experimental implant
Two-piece
Zirconia
Titanium
Screwed
4.1 mm/14 mm
TTS
Experimental system
Two-piece
Titanium
Titanium
Screwed
4.0 mm/10 mm
TTS reference
Semados S Bego, G
Two-piece
Titanium
Titanium
Screwed
4.1 mm/15 mm
Za
OMNIS Creamed, G
One-piece
Zirconia
4.5 mm/12 mm
Zb
White Sky 4.0 Bredent, G
One-piece
Zirconia
4.0 mm/10 mm
microscope (magnification: 20; M420, Wild, Heerbrugg, Switzerland) before and after fatigue testing. Implant/abutment systems that survived TCML were subsequently loaded until failure with a testing machine (Zwick 1446, Ulm, Germany; v = 1 mm/min). In analogy to chewing simulation, load was applied at an angle of 1358 with the loading stamp centrally positioned at the palatal surface 2 mm from the incisal edge. A tin foil (0.25 mm, Dentaurum, Ispringen, Germany) between crown and loading stamp prevented force peaks. Implant/abutment systems were optically examined after fracture testing, and the failure mode was documented.
Fig. 1 – Design of testing apparatus and loading situation.
Scheme
Maximum bending stress sb was calculated using the formula: sb ¼
150 151 152
M ; W
with M being the bending moment: M = F l sin 458, and W being the moment of resistance: W = p/32 D3 (D: diameter of the implant).
153 154 155 156
Calculations and statistical analysis were done with SPSS 19.0 for Windows (SPSS Inc., Chicago, IL, USA). Power calculation (G*Power 3.1.3, Kiel, Germany) provided an estimated power of >90% using eight specimens per group. Data distribution was controlled with the Kolmogorov–Smirnov-test. Means and standard deviations of fracture force and bending stress were calculated and analyzed using one-way analysis of variance (ANOVA) and the Bonferroni-test for posthoc analysis. Survival performance was calculated with the Kaplan–Meier Log-Rank test. The level of significance was set to a = 0.05.
157 158 159 160 161 162 163 164 165 166
3.
167
Results
Independent of the material combination, some screws in screw-retained systems came loose or ruptured during the chewing simulation. For the TTS systems, screw rupture was only observed in one TTS group, whereas the reference group
Please cite this article in press as: Rosentritt M, et al. In vitro performance of zirconia and titanium implant/abutment systems for anterior application. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.03.010
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Table 2 – Number of failed implant/abutment systems and description of failures during thermal cycling and mechanical loading (TCML) and subsequent fracture testing. Group
172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189
TCML
Fracture test (exclusion of failed crowns during TCML)
Number of failed implants/ abutments
Number of chewing cycles: type of failure
Fracture force [N] Mean SD
Maximum bending stress [N/mm2] Mean SD
ZZS
3
269.6 52.8
359.6 70.4
3 screw loosening/crack of implant/fracture of abutment 2 screw loosening/fracture of abutment
ZZB
0
5000: screw loosening/fracture of implant 425,194: screw rupture/fracture of implant and abutment 1,200,000: screw rupture/ fracture of implant and abutment –
297.4 32.4
427.1 46.6
ZZZB
1
187.4 42.0
250.0 56.0
ZTS
8
TTS
3
TTS Reference Za Zb
0
890,314: fracture of implant and abutment 249,000: screw rupture/fracture of implant 490,000: screw rupture/fracture of implant 490,000: screw rupture/fracture of implant 735,000: screw rupture/fracture of implant 735,000: screw rupture/fracture of implant 932,000: screw rupture/fracture of implant 735,000: screw rupture/fracture of implant 1,200,000: screw rupture/ fracture of implant 454,562: screw rupture 562,373: screw rupture 850,000: screw rupture –
7 fracture of abutment 1 fracture of implant and abutment 7 fracture of implant and abutment
0 0
– –
TTS survived TCML without any failures. All specimens of the zirconia/titanium system connected by a screw (ZTS) failed because of screw rupture, and, in each case, the buccal implant neck fractured. Some specimens of the screw-retained zirconia/ zirconia system (ZZS) showed screw loosening or rupture and, in one specimen, a fracture of the implant body below the level of the embedding resin. Two other ZZS specimens failed because of screw rupture in combination with a fracture of both the implant and abutment at the implant–abutment interface. For the bonded zirconia systems, only one specimen in the ZZZB group showed a fracture of the implant and abutment at the level of the implant neck, whereas no failures were observed in the ZZB group. Both one-piece zirconia systems did not show any failure at all. Except for slight wear facets, full-contour zirconia crowns remained intact in all groups. Table 2 gives an overview of the failure modes and the number of chewing cycles at failure. The Kaplan–Meier Log-Rank test showed significant (p = 0.000) differences between the eight groups (Fig. 2).
Type of failure
No fracture test
377.6 30.2
542.4 43.4
2 screw rupture 3 screw deformation
394.1 20.2
525.7 27.0
8 screw rupture
372.4 39.7 524.3 43.1
375.6 40.0 753.0 61.9
8 fracture of implant 7 fracture of implant 1 fracture of implant and crown
Mean (SD) fracture forces (Table 2) for implant/abutment systems that survived TCML varied widely between 187.4 42.0 N (ZZZB) and 524.3 43.1 N (Zb), and showed significant (p = 0.000) differences between the various groups (Table 3). The one-piece zirconia system Zb had the significantly highest mean fracture force, followed by the two screwretained titanium systems TTS and the one-piece zirconia implant Za. The mean fracture forces of the multi-piece zirconia implant–abutment combinations ranged in the lower third, showing the lowest value for the three-piece bonded implant/abutment system ZZZB. The two-piece bonded zirconia system ZZB and the screw-retained zirconia system ZZS showed comparable fracture forces that were significantly higher than those of ZZZB. The ZTS group was excluded from the fracture test because all specimens had already failed during TCML. Fracture forces of the different groups showed good correlation with the respective bending stresses for systems
Please cite this article in press as: Rosentritt M, et al. In vitro performance of zirconia and titanium implant/abutment systems for anterior application. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.03.010
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Fig. 2 – Kaplan–Meier survival curve for the different implant/abutment systems.
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with implant diameters of 4.0 and 4.1 mm (Table 2). Here, oneway ANOVA and post hoc Bonferroni test showed corresponding significance levels for comparisons between the groups (Table 3). Only Za with an implant diameter of 4.5 mm showed comparably low bending stress despite high fracture force that was reflected in different p-values for comparisons of fracture force and bending stress (Table 3). In accordance to failure modes observed during TCML, systems connected by screws mainly failed because of rupture, deformation, or loosening of the screw, irrespective of the material combination. Zirconia one-piece systems failed because of fracture of the implant and bonded zirconia systems because of fracture of the abutment or implant (Table 2). In contrast to titanium systems, in which failure patterns were restricted to the connecting screw (deformation or rupture), screw loosening in the ZZS specimens was always combined with damage to the abutment or implant at the level of the implant neck. Failures of bonded zirconia systems were either sole fractures of the abutment in proximity to the crown
margin (ZZB) or combined fractures of the abutment and implant at the level of the implant neck (ZZB, ZZZB). Fullcontour zirconia crowns were not damaged during fracture testing in any group except for one specimen of the series Zb, in which the crown fractured together with the implant. Apart from this particular specimen, the fracture patterns of onepiece zirconia implants were uniform with the fracture starting at the palatal implant head close to the crown margin and proceeding diagonally to the compression zone at the buccal side near the level of the embedding resin.
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Discussion
The hypothesis of this investigation that different zirconia and titanium implant/abutment systems show comparable in vitro performance and fracture resistance cannot be confirmed. The results indicated that the types of connection as well as the materials of the implant–abutment combination affect a system’s resistance to failure. Most of the implant/abutment systems in the present study are experimental systems, whose mechanical properties have not yet been investigated under clinical conditions. Therefore, chewing simulation with thermal cycling and mechanical loading may allow the preclinical evaluation of their suitability for clinical application. Once osseointegration has been achieved and preserved, resistance against fatigue and the stability of the implant– abutment connection become crucial factors for clinical success. An average loading force of 50 N was chosen in this study, because clinical loading situations physiologically vary between 12 and 70 N.23,24 Although such low physiological forces may not necessarily surpass the fracture threshold of the assembly, they might, if repeatedly applied, lead to fatigue of the implant or abutment materials and their connecting mechanism and finally to failure. According to the common clinical position of anterior teeth, the specimens were mounted at an angle of 1358, resulting in a 458-off-axisloading, as described by Kohal.25,26 In this respect, the present loading scenario differs from other studies referring to occlusal load application at an angle of 308 according to ISO 14801.27–29The abutment titanium screw is commonly the weakest link in this system, and the region around the screw
Table 3 – p-Values (one-way ANOVA, post hoc Bonferroni test) for comparisons between fracture force (first line) or bending stress (second line) of different groups.
ZZB ZZZB TTS TTS reference Za Zb
ZZS
ZZB
ZZZB
TTS
1.000 0.466 0.012* 0.011* 0.001* 0.000* 0.000* 0.000* 0.000* 1.000 0.000* 0.000*
0.000* 0.000* 0.012* 0.005* 0.000* 0.006* 0.006* 0.950 0.000* 0.000*
0.000* 0.000* 0.000* 0.000* 0.000* 0.000* 0.000* 0.000*
1.000 1.000 1.000 0.000* 0.000* 0.000*
TTS Reference
1.000 0.000* 0.000* 0.000*
Za
0.000* 0.000*
* Indicates significant differences ( p 0.05).
Please cite this article in press as: Rosentritt M, et al. In vitro performance of zirconia and titanium implant/abutment systems for anterior application. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.03.010
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head is the area of the highest torque and stress concentration.30 Therefore, failures of such two-piece implant/abutment systems always involve the connecting screw. For the titanium system TTS, these forces led to rupture of the screws without affecting the titanium implant or abutment. In contrast, the same forces caused fractures of the zirconia implant or abutment besides screw failure (loosening, rupture) in systems with a zirconia component. This result may be explained by the low tolerance of zirconia against tensile forces occurring in the area around the screw that has been proven to be critical for the stability of ceramic abutments in laboratory studies.31–33 Cyclic loading is assumed to result in loosening, the plastic deformation of the screw thread and in the subsequent deflection of the entire construction. As a consequence, fatigue-assisted crack initiation and growth lead to fractures within the thin parts of the ceramic construction.34 Because the abutments were bent over the zirconia implants after the loss of the connection by screw loosening or rupture, the specimens fractured at the implant–abutment interface or the implant neck. In contrast to zirconia abutments in ZZS, no additional fractures of the titanium abutments were found in ZTS. Nevertheless, the breakdown of all ZTS specimens during TCML critically questions the suitability of a system that connects titanium abutments to zirconia implants. In addition to fatigue by mechanical loading, yttria-stabilized zirconia ceramics are sensitive towards thermal ageing in the presence of water or the moisture in the oral environment and may become critically brittle.17 Zirconia is thus more susceptible towards cracking than titanium. Because of the brittleness of ceramic materials and the difficulties associated with connecting zirconia abutments and implants, the first zirconia implants consisted of one-piece systems. Accordingly, both one-piece zirconia implants investigated were more reliable against fatigue than the two-piece system ZZS and did not fail during TCML. High resistance of the one-piece zirconia implant design againstageing has also been reported in previous in vitro studies25,26,35 and may be attested by first clinical observations.5,36 However, it should be kept in mind that abutment preparation – that is usually unavoidable in clinical situations but was not or only minimally done in the present study – might increase the risk of failure and decrease fracture strength.10,26 Connecting zirconia abutments and implants by bonding was shown to have almost comparable high resistance towards fatigue as one-piece zirconia implants. However, the inert character of zirconia18 may limit the stability of the bonding over longer periods of time. In accordance to the failure modes observed in the present study, technical complications with the abutment screws (loosening, rupture) have been the most often reported problems for two-piece systems in other laboratory or clinical studies.30 So far, no comprehensive studies are available on zirconia implants and abutments connected by screws. However, studies on two-piece titanium implants with either metal or ceramic abutments showed that abutment screws fracture regularly in case of metal abutments.30,34 Ceramic abutments showed similar rates of loosening or deformation of the connecting screws as metal abutments. However, fracture of the abutments was primarily described instead of screw rupture. Therefore, the fracture of the zirconia abutment is assumed to occur before the fracture of the abutment screw.30
Failure patterns for two-piece zirconia systems, as investigated in the present study, seem to be even more complex. Considering failures of ZZS after fatigue and fracture testing, screw loosening or rupture were always combined with either failure of the abutment or the implant, or both. Furthermore, the screw preload is supposed to be an important factor for avoiding screw loosening or subsequent screw rupture.37,38 The high rates of screw failures during TCML in the present study may have been the result of loss of the preload because the screws were not retightened after some minutes as recommended in daily clinical routine.39 The number of mechanical cycles at failure may give important clues about the underlying failure reason. Failures at the beginning of the endurance test might indicate the existence of material or processing defects, as may be assumed for the early implant fracture of the ZZS group (5000 mechanical cycles). The presence of a pre-existing defect of this specimen is further confirmed by the untypical fracture of the implant body, whereas other specimens typically failed at the implant neck. In return, failures at higher mechanical cycles may be more typical for fatigue mechanisms of the connecting screws and materials. Because screw failures occurred at mechanical cycles higher than 249,000 in both titanium and zirconia systems, annual dental visits to control and – if indicated – retighten the connecting screw may contribute to avoiding or reducing the risk of further complications (rupture, fracture) in clinical use. However, ageing and deterioration often occur without any evidence of failure. In these cases, a subsequent static fracture test may help locate weak points and permit further differentiation of implant and abutment materials as well as design variations. Undoubtedly, fracture data cannot be directly related to clinical survival. However, in relation to clinically proven systems – as in the reference group TTS – such data may provide helpful information on the principal suitability of new systems. Maximum chewing forces in anterior dentition have been reported to range between about 150 N and 300 N.40,41 Therefore, not all groups investigated might withstand force peaks considered to be within the physiological range. In comparison to the two-piece titanium systems, the fracture resistance and maximum bending stress of the two-piece zirconia systems with either a screwed or a bonded connection were lower but seem to be acceptable (ZZS, ZZB) or marginally acceptable in the case of ZZZB. Despite similar fracture forces for bonded and screw-retained zirconia systems, the higher failure frequency of ZZS during TCML than of ZZB/ZZZB needs to be taken into consideration. In contrast, one-piece zirconia implants showed high reliability during TCML and fracture forces comparable or even higher than those of the reference group. Despite a smaller diameter, Zb showed significantly higher force values than Za, and even twice the maximum bending stress of Za. As Za was the implant with the largest diameter in this study, calculation of the maximum bending stress allowed a fair comparison of the groups, irrespective of the individual implant diameters. Considering both fracture force and maximum bending stress, the results showed that even zirconia implants with small diameters might be successfully applied in anterior tooth regions.
Please cite this article in press as: Rosentritt M, et al. In vitro performance of zirconia and titanium implant/abutment systems for anterior application. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.03.010
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Failure modes of groups already failing during TCML were similar after fracture testing: two-piece titanium systems failed because of screw rupture or deformation, whereas ZZS showed additional damage to the abutment or the implant. These results indicate that even fracture testing may be used to replicate clinically relevant failure modes. Although bonded zirconia implants/abutments survived TCML without any failure (ZZB) and only one failure (ZZZB) respectively, comparably low fracture forces during static loading led to catastrophic failures of the implants and abutments. Therefore, the weakest point of these groups does not seem to be the bonding itself, but primarily the design of the connecting parts. Thin ceramics parts that may be especially used in the three-piece implant–abutment combination are supposed to be prone to cracking and fracture. Use of a higher number of connected pieces obviously increases the importance of efficient and strong bonding to achieve monoblocks. However, the inertness of zirconia may not allow any long-term stability of this bonding. In contrast, one-piece zirconia implants also failed in a catastrophic manner but at fracture forces that are supposed to exceed chewing forces of anterior teeth. The crack pathway from the lingual to the buccal face is typical for bending fractures and has already been described in previous in vitro studies.26,42 Monolithic zirconia restorations would usually not be applied in the anterior tooth region because of their inferior aesthetics compared to feldspathic porcelains or glass ceramics. In this study, however, they were chosen because of their high fracture resistance.43,44 In veneered restorations, chipping or fracture of the prosthetic crowns might have occurred before failure of the implants or abutments that would have complicated the evaluation and differentiation of the implant/abutment systems. In a study by Yildirim,27 the glass-ceramic crowns partly failed before the destruction of the abutments. According to a study by Martı´nez-Rus,29 both the abutment material and the ceramic crown system affected the results. Nevertheless, Sailer28 showed that restoring different abutments with leucite-reinforced glass-ceramic crowns did not influence the bending moments. Different degrees of severity of failure patterns have to be differentiated. In clinical application, implant systems may be preferable that have a predetermined breaking point and allow the uncomplicated replacement of an abutment. This concept has been shown to be successful in two-piece titanium systems: for both TTS groups, the connecting screw failed (rupture or deformation) before any damage to the abutment or implant could occur. Therefore, two-piece screwed zirconia implant–abutment combinations may have great future potential if further enhancements in the design of the connection may protect the zirconia implant and abutment in the case of any overloading or unexpected failure. Different implant diameters, loading scenarios, material compositions, and the embedding materials may influence the outcome of in vitro studies. Therefore, the present results cannot be transferred to the clinical situation without limitations. For example, embedding resins with a modulus of elasticity similar to human bone (10–18 GPa) might be preferable. In addition, the manifold possibilities of varying the design of implants, abutments, and their connections require further research into this topic.
7
This in vitro study was a first approach to investigate the principal suitability of different zirconia and titanium implant/abutment systems for anterior application. Two-piece titanium implants may still be recommended as standard implants for most indications. Nevertheless, different zirconia implant/abutment systems might be appropriate for anterior application with limitations. However, further research to improve the connecting design before routine clinical application is suggested because of the partly high failure rates of the zirconia implants and abutments during TCML and the comparably low fracture resistance of some connections.
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Conclusions
Both the material combination (zirconia and titanium) and the type of connection influenced the failure resistance during chewing simulation, the fracture force, as well as the maximum bending stress. Irrespective of the implant or abutment material, failures of screw-retained systems always involved the connecting titanium screw (loosening, rupture). Screwed connections with a zirconia component additionally showed fractures of the implant or the abutment, or both. One-piece and bonded multi-piece zirconia implants/abutments as well as the proven two-piece titanium system showed good in vitro performance during TCML. Bonded or screwed systems with zirconia implants/abutments showed lower fracture resistance than one-piece zirconia implants and two-piece titanium implants.
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Acknowledgement
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We would like to thank the manufacturers for providing the materials.
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references
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