Retention Forces between Titanium and Zirconia Components of Two-Part Implant Abutments with Different Techniques of Surface Modification Nadine Freifrau von Maltzahn, Dr. med. dent.;* Jan Holstermann, dent.;† Philipp Kohorst, Dr. med. dent.‡
ABSTRACT Background: The adhesive connection between titanium base and zirconia coping of two-part abutments may be responsible for the failure rate. A high mechanical stability between both components is essential for the long-term success. Purpose: The aim of the present in-vitro study was to evaluate the influence of different surface modification techniques and resin-based luting agents on the retention forces between titanium and zirconia components in two-part implant abutments. Materials and Methods: A total of 120 abutments with a titanium base bonded to a zirconia coping were investigated. Two different resin-based luting agents (Panavia F 2.0 and RelyX Unicem) and six different surface modifications were used to fix these components, resulting in 12 test groups (n = 10). The surface of the test specimens was mechanically pretreated with aluminium oxide blasting in combination with application of two surface activating primers (Alloy Primer, Clearfil Ceramic Primer) or a tribological conditioning (Rocatec), respectively. All specimens underwent 10,000 thermal cycles between 5°C and 55°C in a moist environment. A pull-off test was then conducted to determine retention forces between the titanium and zirconia components, and statistical analysis was performed (two-way anova). Finally, fracture surfaces were analyzed by light and scanning electron microscopy. Results: No significant differences were found between Panavia F 2.0 and RelyX Unicem. However, the retention forces were significantly influenced by the surface modification technique used (p < 0.001). For both luting agents, the highest retention forces were found when adhesion surfaces of both the titanium bases and the zirconia copings were pretreated with aluminium oxide blasting, and with the application of Clearfil Ceramic Primer. Conclusion: Surface modification techniques crucially influence the retention forces between titanium and zirconia components in two-part implant abutments. All adhesion surfaces should be pretreated by sandblasting. Moreover, a phosphate-based primer serves to enhance long-term retention of the components. KEY WORDS: abutments, bonding, CAD/CAM technology, ceramics, surface properties, titanium, zirconia
INTRODUCTION
*Research associate, Department of Prosthetic Dentistry and Biomedical Materials Science, Hannover Medical School, Hannover, Germany; †research associate, Department of Prosthetic Dentistry and Biomaterials, Faculty of Medicine, Saarland University, Homburg, Germany; ‡prof., head, professor, Department of Prosthetic Dentistry and Biomaterials, Faculty of Medicine, Saarland University, Homburg, Germany
Zirconia ceramic is often used for the fabrication of dental implant abutments. This ceramic exhibits several characteristics which are crucial for its use as abutment material, for example, its aesthetic tooth color, excellent biocompatibility, and high mechanical stability.1,2 Due to these characteristics, abutments made of zirconia ceramics are especially recommended for the aesthetic zone, but there is first evidence that they may also be applicable for implant-based restorations in the highly loaded posterior region.3–5 Zirconia implant abutments can generally be divided into two different forms: standardized (stock) abutments and customized
Corresponding Author: Prof. Philipp Kohorst, Department of Prosthetic Dentistry and Biomaterials, Faculty of Medicine, Saarland University, Kirrberger Str. 100, Homburg 66421, Germany; e-mail: [email protected] Conflict of Interest: The authors have no conflict of interest related to the content of the submission. © 2015 Wiley Periodicals, Inc. DOI 10.1111/cid.12352
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abutments.6 Standardized abutments are offered in different sizes, shapes, and angulations. These abutments have the advantage that their handling is easy, and the costs are relatively low; however, the possibility of individualization is limited. Furthermore, the load-bearing capacity of standardized abutments is lower than for customized zirconia abutments.7 Customized abutments are produced by CAD/CAM methods, as adapted for the individual soft tissue and biomechanical characteristics of each patient.6,8 Customized zirconia abutments can be fabricated in two different ways.9,10 Either the abutments are made homogeneously from one material in a one-part design or they are made with a two-part design. With the two-part-design, a standardized titanium base is connected to an individually fabricated zirconia superstructure by using a resin-based luting agent.11,12 A disadvantage of one-part zirconia abutments is the higher rate of failures, due to fractures in the implant-abutment connection or in the transmucosal part of the abutment. Foong and colleagues13 found that under laboratory conditions, the load-bearing capacity of one-part zirconia abutments is significantly lower than that of titanium one-part abutments. Stimmelmayr and colleagues analyzed the wear of titanium implant fixtures at the implant-abutment interface under cyclic loading conditions and when they are connected to either titanium or zirconia one-part abutments.14 Implant fixtures showed significantly greater wear when connected to one-part zirconia abutments. The authors concluded that the application of one-part zirconia abutments may be detrimental for the long-term stability of the components, due to this damage to the inner parts of the implant fixtures. Analogously to these results, other studies have analyzed the load capacity of two-part abutments. For example, Kim and colleagues evaluated the maximum load-bearing capacity of different types of zirconia abutments and showed that the two-part abutments had the highest values.15 This was also confirmed by investigations of Butz and colleagues16 and Aramouni and colleagues.17 Therefore, the use of two-part zirconia abutments may be beneficial with regard to the mechanical stability and absence of wear phenomena. However, the weak point of these abutments is the adhesive connection between the titanium base and the zirconia build-up, and this is crucial for long-term clinical success.18 A few investigations have shown that the retention force between the zirconia and titanium components is
influenced not only by the fixture materials, but also by the surface characteristics of the components.12,18–20 Ebert and colleagues18 analyzed two different methods of conditioning the surface of zirconia copings bonded to titanium abutments with composite resin cement. They investigated whether pretreatment by airborne particle abrasion of the zirconia copings can improve retention between the two components of two-part abutments during water storage and thermal cycling. Their results showed that surface treatment methods significantly influence retention forces. Gehrke and colleagues12 investigated different types of composite resin cements used to bond zirconia copings and titanium bases. All surfaces were air-abraded and disinfected with alcohol. After bonding, all test specimens underwent thermal cycling. The authors concluded that surface treatment with air abrasion of the titanium bases and the zirconia copings leads to stable adhesion within twopart abutments. However, the differences between the different bonding materials are not statistically significant. On the other hand, there is hardly any information about how retention forces can be improved by specific surface modification by mechanical/tribological conditioning or by application of surface activating primers to enhance long-term stability under clinical conditions. Therefore, the aim of the present in-vitro study was to investigate the influence of different techniques of surface modification in combination with different adhesive fixation resins on the retention forces between titanium bases and zirconia copings in two-part implant abutments. Before final pull-off testing, all test specimens underwent cyclic thermal loading in moisture to simulate conditions of the oral environment. It was hypothesized that both surface modification and the adhesive resin significantly influence the retention forces between the components of two-part implant abutments. MATERIALS AND METHODS Test Specimens Test specimens were composed of standardized titanium bases (S 1020, Medentika, Hügelsheim, Germany) and zirconia copings, fabricated by CAD/CAM (CADSPEED GmbH, Nienhagen, Germany) from a presintered Y-TZP material (Zirkon Biostar, Siladent, Goslar, Germany) and with a customized design for the subsequent pull-off tensile test. The luting gap in the copings
Retention Forces of Two-Part Implant Abutments
Figure 1 Components of the test specimens: prefabricated titanium base (left) and zirconia coping customized for pull-off testing.
was adjusted to 30 μm. Titanium bases were 7.8 mm in height, with an upper aperture of 3.4 mm (Figure 1). The zirconia copings were 11 mm in height, with a bore diameter of 3.5 mm and a major diameter of 5 mm (Figure 1). To produce the test specimens, laboratory implants (S 52, Medentika) were embedded into a socket of polyurethane (AlphaDie MF, Schütz Dental Group, Rosbach, Germany). Then, all titanium bases were screwed into the laboratory implants with a torque of 25 Ncm. Surfaces of the different components were then modified, and components were connected by using two different resin-based luting agents as described in the following.
Figure 2 Light microscopic view of a titanium base blasted with aluminium oxide.
Test Procedure Surface Treatments. The surface treatments of all test specimens are shown in Table 1. The bonding surfaces of 120 titanium bases (group A–J) and of 80 zirconia copings (group C–J) were blasted with aluminium oxide powder (Shera, Lemförde, Germany) (Figure 2). This procedure was conducted with an average particle size of 110 μm and a pressure of 2 bar. The titanium bases and zirconia copings of all groups were then cleaned in a bath of acetone and blown dry. Then, the surfaces were treated with different bonding systems. For the titanium bases of groups E, F, G and H and for the zirconia
TABLE 1 Surface Modifications and Fixture Resins Used for Different Test Groups
Group
A (n = 10) B (n = 10) C (n = 10) D (n = 10) E (n = 10) F (n = 10) G (n = 10) H (n = 10) I (n = 10) J (n = 10) K (n = 10) L (n = 10)
Titanium Bases
Zirconia Copings
Surface Modification
Surface Modification
Mechanical
110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3
Chemical
Alloy Primer Alloy Primer Alloy Primer Alloy Primer Clearfil Clearfil Rocatec Rocatec
3
Mechanical
110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3
Chemical
Luting Agent
Clearfil Clearfil Alloy Primer Alloy Primer Clearfil Clearfil Rocatec Rocatec
Panavia F 2.0 RelyX Unicem Panavia F 2.0 RelyX Unicem Panavia F 2.0 RelyX Unicem Panavia F 2.0 RelyX Unicem Panavia F 2.0 RelyX Unicem Panavia F 2.0 RelyX Unicem
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copings of groups G and H, the adhesion promoter Alloy Primer (Kuraray Europe, Frankfurt/Main, Germany) was used. According to the manufacturer’s instructions, Alloy Primer is a metal conditioning agent indicated for bonding between dental metals and resin-based materials. Alloy Primer consists of acetone, 10-methacryloyloxydecyl dihydrogen phosphate and 6-(4-vinylbenzyl-n-propyl) amino-1,3,5-triazine-2,4dithione. The titanium bases of groups I and J and the zirconia copings of groups E, F, I, and J were coated with the silane-coupling agent Clearfil Ceramic Primer (Kuraray Europe). This adhesion promoter was composed of 3-methacryloxypropyl trimethoxy silane, 10-methacryloyloxydecyl dihydrogen phosphate, and ethanol. Application of Clearfil Ceramic Primer is indicated for the surface treatments of ceramics, hybrid ceramics, or composite resins. Both components of the abutments of group K and L were pretreated with the Rocatec System (3M Espe, Seefeld, Germany). The Rocatec System is indicated for the coating of metal, ceramic, and composite restorations before adhesive cementing. Application of a silicate layer and a silane leads to the development of a chemical bond to the resin material that then requires no mechanical retention. The Rocatec System is based on three different components: Rocatec Pre (ultrapure aluminium oxide 110 μm), Rocatec Plus (ultrapure aluminium oxide 110 μm modified with silicic acid), and ESPE Sil (silane in ethanol). Fixation of the Components. The resin-based luting agents Panavia F 2.0 (Panavia F2.0, Kuraray Europe GmbH, Frankfurt am Main, Germany) or RelyX Unicem (RelyX Unicem, 3M ESPE) was applied according to the manufacturers’ information, and a constant film was applied to the outer surfaces of the titanium bases and to the inner surfaces of the zirconia copings (Table 1). The titanium bases and zirconia copings of the groups A, C, E, G, I, and K were fixed with Panavia F 2.0. Panavia F 2.0 is a dual-hardening fixation resin which is based on two components. After mixing these components, a constant layer was applied to the outer surfaces of the titanium bases and the inner surfaces of the zirconia copings. The build-ups and the bases were pressed together by hand, using a constant pressure. The test specimens of groups B, D, F, H, J, and L were fixed with RelyX Unicem. This resin is a self-adhesive, dual-hardening material composed of two components delivered in a special dis-
Figure 3 Final installation of test specimens. Laboratory implant fixed in a base of polyurethane and surrounded by an aluminium cartridge. Titanium base bolted into the laboratory implant by a holding screw. Zirconia coping connected to the titanium base by use of a resin-based luting agent.
penser. With this dispenser, the exact amount of resin was applied to the outer surfaces of the titanium bases and to the inner surfaces of the zirconia copings. After this procedure, both components of the abutment were again pressed together manually (Figure 3). In all groups, the surpluses were removed. All specimens were light-cured for 90 seconds to initiate self-polymerization (Uni XS, Heraeus Kulzer, Hanau, Germany). The final hardness was achieved by temporary storage in a heating cabinet of 23°C for 24 hours. Simulated Aging. To simulate the humid atmosphere and variations in temperature in the oral cavity, all specimens were exposed to alternating thermal loads in a moist atmosphere. A total of 10,000 thermal cycles were conducted with temperatures of 5°C and 55°C. Within each cycle, the test specimens were submerged for 30 seconds in a temperature bath at each temperature and were then exposed to room air for 10 seconds during transportation to the other bath. All cycles were performed automatically. After the 10,000 cycles, the specimens were returned to the heating cabinet of 23°C for 24 hours. Pull-Off Test. After simulated aging, all specimens were subjected to a pull-off test to separate zirconia copings from the titanium bases. For this reason, the specimens
Retention Forces of Two-Part Implant Abutments
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Statistical Analysis Statistical analysis was performed using SPSS for Windows, version 19.0 (IBM, Ehringen, Germany). The normal distribution of data and homogeneity of variance were checked using the Kolmogorov–Smirnov and Levene tests, respectively. From the measured values of all groups, the arithmetric mean, the standard deviation, and the minimum and the maximum were calculated. The influence of the luting resins and surface modifications on the bond strength was analyzed by two-way analysis of variance (anova). For direct comparison of the different groups, the post-hoc Scheffé test was performed. The level of significance was set at 0.05 for all analyses. RESULTS Figure 4 Test specimen installed into a universal test machine by using a special jig for pull-off tensile testing.
were fixed into a special jig (Figure 4) which was installed in a universal test machine (Type 20K, UTS Testsysteme, Ulm-Einsingen, Germany). The design of the extractor device ensured that the specimens were stably and reproducibly positioned. The universal test machine moved vertically at a crosshead speed of 1 mm/ min until the zirconia copings were completely separated from the titanium bases. During loading, data for crosshead displacement and load were collected (Programm Phoenix, UTS Testsysteme), and the recorded maximum force was defined as the retention force between the components. Fractographical Analysis. After the pull-off tests, the fracture surfaces of all components were analyzed by reflected light microscopy (M3Z, Wild, Heerbrugg, Switzerland). Characteristics of the fracture surfaces were photographically recorded with a connected digital camera (ProgRes C12 plus, Jenoptik, Jena, Germany). The residues of the resin-based luting agents on the adhesion surfaces of the components were evaluated and classified into three different failure modes: residues only on the titanium base, residues only on the zirconia copings, and residues on both components. Additionally, representative specimens of each group were investigated in a scanning electron microscope (Philips SEM 505, Philips, Eindhoven, the Netherlands).
Pull-Off Test There were no differences between the two fixation resins and the different surface treatments in the characteristics of the pull-off tests (in particular, the load displacement curves). The single pull-off forces are shown in Table 2. Statistical analysis of retention force data revealed no significant differences between the two fixation resins (p = 0.913). However, different surface treatments had a statistically significant influence on the pull-off forces (p < 0.001) (Table 2). Test specimens only
TABLE 2 Pull-Off Forces for Different Test Groups Pull-Off Forces (N)
Group
A B C D E F G H I J K L
Mean
Standard Deviation
Minimum
Maximum
222.3a 258.7a,b 319.3a,b,c 364.9a,b,c 499.0a,b,c 360.3a,b,c 532.4a,b,c 543.7a,b,c 598.6c 555.8b,c 431.3a,b,c 538.9a,b,c
67.6 106.1 95.6 109.9 129.5 124.0 209.3 208.4 173.7 144.4 244.3 208.4
138.5 153.0 217.9 219.0 340.1 156.1 267.6 289.6 370.4 329.4 239.1 290.7
357.0 503.1 482.3 627.1 756.6 638.0 927.7 849.1 859.4 762.3 929.2 900.6
Mean, standard deviation, minimum, and maximum are given. Values with the same superscript do not differ with statistical significance (posthoc Scheffé, p < .05).
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blasted with aluminium oxide powder at the titanium components (groups A and B) showed mean pull-off forces of 222.3 N and 258.7 N, respectively. Due to additional blasting of the zirconia coping and chemical conditioning by application of Clearfil Ceramic Primer to the connecting surfaces, retention forces increased statistically significantly to 598.6 N (group I) and 555.8 N (group J), respectively. By tribochemical preconditioning (Rocatec, groups K and L), the pull-off forces were slightly lower than the maximum values in this investigation (Table 2). Analysis of Fracture Patterns The analysis of fracture patterns showed only adhesive fractures between the components and the fixation resin, but there was no cohesive fracture which ran within the resin layer (Table 3). Adhesive fractures between the zirconia copings and the resin were detected in each group. In groups A, B, C, and L, only adhesive fractures between the zirconia surfaces and the resin occurred (Figures 5 and 6). In the other groups, fracture patterns were found with resin residues on both the zirconia and titanium surfaces (mixed fracture pattern) (Figure 7). No specimen was found to contain adhesive fractures exclusively between the titanium surface and the resin. This fracture distribution indicates that the bonding between the zirconia surface and the resin is the weakest point of the construction.
TABLE 3 Distribution of Fracture Patterns
Figure 5 Light microscopic view onto the adhesion surface of a zirconia specimen (test group C) after pull-off testing. No residues of the fixation material are visible on the surface.
DISCUSSION Zirconia abutments are commonly used in implant dentistry, as they possess both aesthetic and biological advantages. Several recent studies have described problems with one-part abutments, including abutment fracture under functional loading, wear of the implantabutment interface, and marginal misfit to the implant.13,14,21 There may therefore be advantages in using two-part implant abutments with a zirconia coping bonded to a titanium base.9,22 However, even if the adhesive connection between the zirconia and titanium components may be a weak point in these abutments, only a few studies have investigated the strength of this connection and the factors influencing it.12,18
Number of Different Fracture Patterns in Each Group
Group
A B C D E F G H I J K L
Adhesive Fracture Only between Zirconia Surface and Resin
Adhesive Fracture between Zirconia Surface and Resin as well as between Titanium Surface and Resin (Mixed)
10 10 10 4 5 7 5 9 5 8 7 10
— — — 6 5 3 5 1 5 2 3 —
Figure 6 Scanning electron microscopic view of the titanium base of the specimen shown in Figure 5. The adhesion surface of the base is covered by a homogeneous layer of the fixation resin.
Retention Forces of Two-Part Implant Abutments
Figure 7 Representative example of a mixed fracture pattern. The titanium surface is only partially covered with residues of the fixation resin.
Analogously to our results, Gehrke and colleagues found that the kind of luting resin used has no significant influence on the pull-off forces.12 Even though the force values in the present study are lower than those found by Gehrke and colleagues, differences between the various luting resins within the two studies show the same tendency. Variations between the two studies may most likely be explained by differences in the implant and abutment components used or by differences in the thermal cycling procedure and the crosshead speed during pull-off testing. However, based on the results, it can be concluded that both resin-based luting agents investigated (Panavia F2.0 and RelyX Unicem) are generally suitable for connecting zirconia and titanium components in two-part abutments, even if the chemical composition of the resin materials differs significantly.23–26 In another survey, Ebert and colleagues analyzed the effect of the luting gap size on the retention between zirconia and titanium components. They found that zirconia copings bonded with a small luting gap of about 30 μm achieved significantly higher retention than those bonded with a greater luting gap.18 Therefore, in the present study, the design of the copings was adjusted to a luting gap size of 30 μm to ensure the best possible conditions. As in the present study, Ebert and colleagues reported that pretreating the zirconia surface with airborne particle abrasion increases retention between the components.18 It can be concluded that blasting with aluminium oxide is one of the most effective surface preparation methods to achieve efficient adhesion
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between component surfaces and luting resins as also shown in several other studies.27,28 Various published studies used aluminium oxide with an average particle size of 110 μm and a pressure of 2.5 bar.29–31 In the present investigation, a pressure of only 2 bar was applied in order to simulate a procedure which, under real clinical conditions, would be more effective in preventing detrimental microcracks in the zirconia surface.32,33 Nevertheless, even under application of lower pressure, sandblasting has been discussed to be detrimental related to phase structure and subsequent mechanical properties of zirconia. Recent investigations revealed that sandblasting indeed results in a significant transformation from the tetragonal to the monoclinic phase on the zirconia surface accompanied by increasing compressive residual stresses.34,35 However, these phenomena seem to have a beneficial effect on both the flexural strength36 and the aging resistance of Y-TZP ceramic under simulated clinical conditions in a humid atmosphere.37 Aside from the effect of mechanical pretreatment, we also investigated the influence of additional chemical or tribochemical conditioning of the bonding surfaces. It was then found that the different surface conditioning techniques had significant effects (Table 2). The highest retention forces were found after application of Clearfil Ceramic Primer on both the titanium bases and on the zirconia copings. Analogous to our results, in several studies, highest bond strength was found with the combination of blasting with aluminium oxide and Clearfil Ceramic Primer.38,39 Some other test specimens in the present analysis were preconditioned with aluminium oxide blasting and application of Alloy Primer which is typically used for enhancing the bond strength between metal alloys and resin-based materials but is also applied for the preconditioning of zirconia surfaces. Komine and colleagues40 investigated the influence of different primers, including Alloy Primer, on the bond strength of resin materials to zirconia. They showed highest bond adhesion with Alloy Primer; however, they utilized 50-μm particles for aluminium oxide blasting and test specimens just passed through 5,000 cycles of thermo-cycling. Kern and colleagues compared Clearfil Ceramic Primer and Alloy Primer with respect to adhesion to zirconia.41 Their findings confirmed our results that Clearfil Ceramic Primer achieves the best adhesion to zirconia. Ozcan and Valandro evaluated the effect of different primers on the
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adhesion of resins to titanium surfaces. They showed that blasting with aluminium oxide (50 μm) and coating with Alloy Primer lead to the highest adhesion forces.42 In this context, Kim and colleagues reported that the use of surface coupling primers is generally recommended to improve bond adhesion to titanium.43 The Rocatec system was also been evaluated in the present study. Published data on this system are inconsistent. Some surveys reported significant improvements in adhesion with this tribochemical method,31,44–47 whereas other studies favored other preconditioning methods. The analysis of fracture patterns showed that the bonding between the zirconia surface and the resin seemed to be the weakest point of construction. In particular, without the use of a primer, the connection between both components is challenging. A reason for this could be that the surface hardness of zirconia is higher, and consequently, a blasting with aluminium oxide is not as effective as on a titanium surface. A current meta-analysis of Inokoshi and colleagues48 confirms our estimations. The authors stated that in particular, the combination of mechanical and chemical preconditioning contributes to the durability of the bond of resin cements to zirconia ceramics. The results of the present study showed that retention forces between the titanium and zirconia components of two-part abutments can be significantly improved by application of selected preconditioning techniques which are not currently recommended by the manufacturers of the components. However, significant differences between different combinations of luting agent, surface treatment, and adhesion promoter indicate that more investigations are necessary to identify the best possible retention between the components of two-part abutments. CONCLUSIONS Within the limitations of this study, it could be concluded that 1. The procedure of surface preconditioning has a significant influence on the retention forces between the components of two-part implant abutments. 2. The highest retention forces in titanium/zirconia two-part abutments are achieved by applying a single phosphate-based ceramic primer on all connecting surfaces.
3. Preconditioning of surfaces should be included in the processing guidelines for two-part abutments. REFERENCES 1. Hisbergues M, Vendeville S, Vendeville P. Zirconia: established facts and perspectives for a biomaterial in dental implantology. J Biomed Mater Res B Appl Biomater 2009; 88:519–529. 2. Manicone PF, Rossi Iommetti P, Raffaelli L. An overview of zirconia ceramics: basic properties and clinical applications. J Dent 2007; 35:819–826. 3. Nothdurft F, Pospiech P. Prefabricated zirconium dioxide implant abutments for single-tooth replacement in the posterior region: evaluation of peri-implant tissues and superstructures after 12 months of function. Clin Oral Implants Res 2010; 21:857–865. 4. Nothdurft FP, Pospiech PR. Zirconium dioxide implant abutments for posterior single-tooth replacement: first results. J Periodontol 2009; 80:2065–2072. 5. Nothdurft FP, Nonhoff J, Pospiech PR. Pre-fabricated zirconium dioxide implant abutments for single-tooth replacement in the posterior region: success and failure after 3 years of function. Acta Odontol Scand 2014; 72:392–400. 6. Hamilton A, Judge RB, Palamara JE, Evans C. Evaluation of the fit of CAD/CAM abutments. Int J Prosthodont 2013; 26:370–380. 7. Park JI, Lee Y, Lee JH, Kim YL, Bae JM, Cho HW. Comparison of fracture resistance and fit accuracy of customized zirconia abutments with prefabricated zirconia abutments in internal hexagonal implants. Clin Implant Dent Relat Res 2013; 15:769–778. 8. Fuster-Torres MA, Albalat-Estela S, Alcaniz-Raya M, Penarrocha-Diago M. CAD/CAM dental systems in implant dentistry: update. Med Oral Patol Oral Cir Bucal 2009; 14:E141–E145. 9. Sailer I, Sailer T, Stawarczyk B, Jung RE, Hammerle 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. 10. Pozzi A, Sannino G, Barlattani A. Minimally invasive treatment of the atrophic posterior maxilla: a proof-of-concept prospective study with a follow-up of between 36 and 54 months. J Prosthet Dent 2012; 108:286–297. 11. Kerstein RB, Radke J. A comparison of fabrication precision and mechanical reliability of 2 zirconia implant abutments. Int J Oral Maxillofac Implants 2008; 23:1029–1036. 12. Gehrke P, Alius J, Fischer C, Erdelt KJ, Beuer F. Retentive strength of two-piece CAD/CAM Zirconia implant abutments. Clin Implant Dent Relat Res 2014; 16:920–925. 13. Foong JK, Judge RB, Palamara JE, Swain MV. Fracture resistance of titanium and zirconia abutments: an in vitro study. J Prosthet Dent 2013; 109:304–312.
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tetragonal zirconia polycrystal ceramic and a self-adhesive resin cement. Oper Dent 2015; 40:63–71. Shin YJ, Shin Y, Yi YA, et al. Evaluation of the shear bond strength of resin cement to Y-TZP ceramic after different surface treatments. Scanning 2014; 36:479–486. Kern M, Wegner SM. Bonding to zirconia ceramic: adhesion methods and their durability. Dent Mater 1998; 14:64–71. Blatz MB, Sadan A, Martin J, Lang B. In vitro evaluation of shear bond strengths of resin to densely-sintered high-purity zirconium-oxide ceramic after long-term storage and thermal cycling. J Prosthet Dent 2004; 91:356–362. Blatz MB, Chiche G, Holst S, Sadan A. Influence of surface treatment and simulated aging on bond strengths of luting agents to zirconia. Quintessence Int 2007; 38:745–753. Zhang Y, Lawn BR, Rekow ED, Thompson VP. Effect of sandblasting on the long-term performance of dental ceramics. J Biomed Mater Res B Appl Biomater 2004; 71:381–386. Guazzato M, Albakry M, Quach L, Swain MV. Influence of grinding, sandblasting, polishing and heat treatment on the flexural strength of a glass-infiltrated alumina-reinforced dental ceramic. Biomaterials 2004; 25:2153–2160. Chintapalli RK, Marro FG, Jimenez-Pique E, Anglada M. Phase transformation and subsurface damage in 3Y-TZP after sandblasting. Dent Mater 2013; 29:566–572. Chintapalli RK, Mestra Rodriguez A, Garcia Marro F, Anglada M. Effect of sandblasting and residual stress on strength of zirconia for restorative dentistry applications. J Mech Behav Biomed Mater 2014; 29:126–137. Souza RO, Valandro LF, Melo RM, Machado JP, Bottino MA, Ozcan M. Air-particle abrasion on zirconia ceramic using different protocols: effects on biaxial flexural strength after cyclic loading, phase transformation and surface topography. J Mech Behav Biomed Mater 2013; 26:155–163. Inokoshi M, Vanmeensel K, Zhang F, et al. Aging resistance of surface-treated dental zirconia. Dent Mater 2015; 31:182– 194. Inokoshi M, Kameyama A, De Munck J, Minakuchi S, Van Meerbeek B. Durable bonding to mechanically and/or chemically pre-treated dental zirconia. J Dent 2012; 41:170– 179. Yang B, Barloi A, Kern M. Influence of air-abrasion on zirconia ceramic bonding using an adhesive composite resin. Dent Mater 2010; 26:44–50. Komine F, Kobayashi K, Saito A, Fushiki R, Koizumi H, Matsumura H. Shear bond strength between an indirect composite veneering material and zirconia ceramics after thermocycling. J Oral Sci 2009; 51:629–634. Kern M, Barloi A, Yang B. Surface conditioning influences zirconia ceramic bonding. J Dent Res 2009; 88:817– 822. Ozcan M, Valandro L. Effect of silane coupling agents and alloy primers on adhesion to titanium. Minerva Stomatol 2011; 60:427–434.
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43. Kim SS, Vang MS, Yang HS, Park SW, Lim HP. Effect of adhesive primers on bonding strength of heat cure denture base resin to cast titanium and cobalt-chromium alloy. J Adv Prosthodont 2009; 1:41–46. 44. Guggenberger R. [Rocatec system – adhesion by tribochemical coating]. Dtsch Zahnarztl Z 1989; 44:874– 876. 45. Atsu SS, Kilicarslan MA, Kucukesmen HC, Aka PS. Effect of zirconium-oxide ceramic surface treatments on the bond strength to adhesive resin. J Prosthet Dent 2006; 95:430–436.
46. Lohbauer U, Zipperle M, Rischka K, Petschelt A, Muller FA. Hydroxylation of dental zirconia surfaces: characterization and bonding potential. J Biomed Mater Res B Appl Biomater 2008; 87:461–467. 47. Nothdurft FP, Motter PJ, Pospiech PR. Effect of surface treatment on the initial bond strength of different luting cements to zirconium oxide ceramic. Clin Oral Investig 2009; 13:229–235. 48. Inokoshi M, De Munck J, Minakuchi S, Van Meerbeek B. Meta-analysis of bonding effectiveness to zirconia ceramics. J Dent Res 2014; 93:329–334.
ABSTRACT Background: The adhesive connection between titanium base and zirconia coping of two-part abutments may be responsible for the failure rate. A high mechanical stability between both components is essential for the long-term success. Purpose: The aim of the present in-vitro study was to evaluate the influence of different surface modification techniques and resin-based luting agents on the retention forces between titanium and zirconia components in two-part implant abutments. Materials and Methods: A total of 120 abutments with a titanium base bonded to a zirconia coping were investigated. Two different resin-based luting agents (Panavia F 2.0 and RelyX Unicem) and six different surface modifications were used to fix these components, resulting in 12 test groups (n = 10). The surface of the test specimens was mechanically pretreated with aluminium oxide blasting in combination with application of two surface activating primers (Alloy Primer, Clearfil Ceramic Primer) or a tribological conditioning (Rocatec), respectively. All specimens underwent 10,000 thermal cycles between 5°C and 55°C in a moist environment. A pull-off test was then conducted to determine retention forces between the titanium and zirconia components, and statistical analysis was performed (two-way anova). Finally, fracture surfaces were analyzed by light and scanning electron microscopy. Results: No significant differences were found between Panavia F 2.0 and RelyX Unicem. However, the retention forces were significantly influenced by the surface modification technique used (p < 0.001). For both luting agents, the highest retention forces were found when adhesion surfaces of both the titanium bases and the zirconia copings were pretreated with aluminium oxide blasting, and with the application of Clearfil Ceramic Primer. Conclusion: Surface modification techniques crucially influence the retention forces between titanium and zirconia components in two-part implant abutments. All adhesion surfaces should be pretreated by sandblasting. Moreover, a phosphate-based primer serves to enhance long-term retention of the components. KEY WORDS: abutments, bonding, CAD/CAM technology, ceramics, surface properties, titanium, zirconia
INTRODUCTION
*Research associate, Department of Prosthetic Dentistry and Biomedical Materials Science, Hannover Medical School, Hannover, Germany; †research associate, Department of Prosthetic Dentistry and Biomaterials, Faculty of Medicine, Saarland University, Homburg, Germany; ‡prof., head, professor, Department of Prosthetic Dentistry and Biomaterials, Faculty of Medicine, Saarland University, Homburg, Germany
Zirconia ceramic is often used for the fabrication of dental implant abutments. This ceramic exhibits several characteristics which are crucial for its use as abutment material, for example, its aesthetic tooth color, excellent biocompatibility, and high mechanical stability.1,2 Due to these characteristics, abutments made of zirconia ceramics are especially recommended for the aesthetic zone, but there is first evidence that they may also be applicable for implant-based restorations in the highly loaded posterior region.3–5 Zirconia implant abutments can generally be divided into two different forms: standardized (stock) abutments and customized
Corresponding Author: Prof. Philipp Kohorst, Department of Prosthetic Dentistry and Biomaterials, Faculty of Medicine, Saarland University, Kirrberger Str. 100, Homburg 66421, Germany; e-mail: [email protected] Conflict of Interest: The authors have no conflict of interest related to the content of the submission. © 2015 Wiley Periodicals, Inc. DOI 10.1111/cid.12352
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abutments.6 Standardized abutments are offered in different sizes, shapes, and angulations. These abutments have the advantage that their handling is easy, and the costs are relatively low; however, the possibility of individualization is limited. Furthermore, the load-bearing capacity of standardized abutments is lower than for customized zirconia abutments.7 Customized abutments are produced by CAD/CAM methods, as adapted for the individual soft tissue and biomechanical characteristics of each patient.6,8 Customized zirconia abutments can be fabricated in two different ways.9,10 Either the abutments are made homogeneously from one material in a one-part design or they are made with a two-part design. With the two-part-design, a standardized titanium base is connected to an individually fabricated zirconia superstructure by using a resin-based luting agent.11,12 A disadvantage of one-part zirconia abutments is the higher rate of failures, due to fractures in the implant-abutment connection or in the transmucosal part of the abutment. Foong and colleagues13 found that under laboratory conditions, the load-bearing capacity of one-part zirconia abutments is significantly lower than that of titanium one-part abutments. Stimmelmayr and colleagues analyzed the wear of titanium implant fixtures at the implant-abutment interface under cyclic loading conditions and when they are connected to either titanium or zirconia one-part abutments.14 Implant fixtures showed significantly greater wear when connected to one-part zirconia abutments. The authors concluded that the application of one-part zirconia abutments may be detrimental for the long-term stability of the components, due to this damage to the inner parts of the implant fixtures. Analogously to these results, other studies have analyzed the load capacity of two-part abutments. For example, Kim and colleagues evaluated the maximum load-bearing capacity of different types of zirconia abutments and showed that the two-part abutments had the highest values.15 This was also confirmed by investigations of Butz and colleagues16 and Aramouni and colleagues.17 Therefore, the use of two-part zirconia abutments may be beneficial with regard to the mechanical stability and absence of wear phenomena. However, the weak point of these abutments is the adhesive connection between the titanium base and the zirconia build-up, and this is crucial for long-term clinical success.18 A few investigations have shown that the retention force between the zirconia and titanium components is
influenced not only by the fixture materials, but also by the surface characteristics of the components.12,18–20 Ebert and colleagues18 analyzed two different methods of conditioning the surface of zirconia copings bonded to titanium abutments with composite resin cement. They investigated whether pretreatment by airborne particle abrasion of the zirconia copings can improve retention between the two components of two-part abutments during water storage and thermal cycling. Their results showed that surface treatment methods significantly influence retention forces. Gehrke and colleagues12 investigated different types of composite resin cements used to bond zirconia copings and titanium bases. All surfaces were air-abraded and disinfected with alcohol. After bonding, all test specimens underwent thermal cycling. The authors concluded that surface treatment with air abrasion of the titanium bases and the zirconia copings leads to stable adhesion within twopart abutments. However, the differences between the different bonding materials are not statistically significant. On the other hand, there is hardly any information about how retention forces can be improved by specific surface modification by mechanical/tribological conditioning or by application of surface activating primers to enhance long-term stability under clinical conditions. Therefore, the aim of the present in-vitro study was to investigate the influence of different techniques of surface modification in combination with different adhesive fixation resins on the retention forces between titanium bases and zirconia copings in two-part implant abutments. Before final pull-off testing, all test specimens underwent cyclic thermal loading in moisture to simulate conditions of the oral environment. It was hypothesized that both surface modification and the adhesive resin significantly influence the retention forces between the components of two-part implant abutments. MATERIALS AND METHODS Test Specimens Test specimens were composed of standardized titanium bases (S 1020, Medentika, Hügelsheim, Germany) and zirconia copings, fabricated by CAD/CAM (CADSPEED GmbH, Nienhagen, Germany) from a presintered Y-TZP material (Zirkon Biostar, Siladent, Goslar, Germany) and with a customized design for the subsequent pull-off tensile test. The luting gap in the copings
Retention Forces of Two-Part Implant Abutments
Figure 1 Components of the test specimens: prefabricated titanium base (left) and zirconia coping customized for pull-off testing.
was adjusted to 30 μm. Titanium bases were 7.8 mm in height, with an upper aperture of 3.4 mm (Figure 1). The zirconia copings were 11 mm in height, with a bore diameter of 3.5 mm and a major diameter of 5 mm (Figure 1). To produce the test specimens, laboratory implants (S 52, Medentika) were embedded into a socket of polyurethane (AlphaDie MF, Schütz Dental Group, Rosbach, Germany). Then, all titanium bases were screwed into the laboratory implants with a torque of 25 Ncm. Surfaces of the different components were then modified, and components were connected by using two different resin-based luting agents as described in the following.
Figure 2 Light microscopic view of a titanium base blasted with aluminium oxide.
Test Procedure Surface Treatments. The surface treatments of all test specimens are shown in Table 1. The bonding surfaces of 120 titanium bases (group A–J) and of 80 zirconia copings (group C–J) were blasted with aluminium oxide powder (Shera, Lemförde, Germany) (Figure 2). This procedure was conducted with an average particle size of 110 μm and a pressure of 2 bar. The titanium bases and zirconia copings of all groups were then cleaned in a bath of acetone and blown dry. Then, the surfaces were treated with different bonding systems. For the titanium bases of groups E, F, G and H and for the zirconia
TABLE 1 Surface Modifications and Fixture Resins Used for Different Test Groups
Group
A (n = 10) B (n = 10) C (n = 10) D (n = 10) E (n = 10) F (n = 10) G (n = 10) H (n = 10) I (n = 10) J (n = 10) K (n = 10) L (n = 10)
Titanium Bases
Zirconia Copings
Surface Modification
Surface Modification
Mechanical
110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3
Chemical
Alloy Primer Alloy Primer Alloy Primer Alloy Primer Clearfil Clearfil Rocatec Rocatec
3
Mechanical
110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3 110 μm Al2O3
Chemical
Luting Agent
Clearfil Clearfil Alloy Primer Alloy Primer Clearfil Clearfil Rocatec Rocatec
Panavia F 2.0 RelyX Unicem Panavia F 2.0 RelyX Unicem Panavia F 2.0 RelyX Unicem Panavia F 2.0 RelyX Unicem Panavia F 2.0 RelyX Unicem Panavia F 2.0 RelyX Unicem
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copings of groups G and H, the adhesion promoter Alloy Primer (Kuraray Europe, Frankfurt/Main, Germany) was used. According to the manufacturer’s instructions, Alloy Primer is a metal conditioning agent indicated for bonding between dental metals and resin-based materials. Alloy Primer consists of acetone, 10-methacryloyloxydecyl dihydrogen phosphate and 6-(4-vinylbenzyl-n-propyl) amino-1,3,5-triazine-2,4dithione. The titanium bases of groups I and J and the zirconia copings of groups E, F, I, and J were coated with the silane-coupling agent Clearfil Ceramic Primer (Kuraray Europe). This adhesion promoter was composed of 3-methacryloxypropyl trimethoxy silane, 10-methacryloyloxydecyl dihydrogen phosphate, and ethanol. Application of Clearfil Ceramic Primer is indicated for the surface treatments of ceramics, hybrid ceramics, or composite resins. Both components of the abutments of group K and L were pretreated with the Rocatec System (3M Espe, Seefeld, Germany). The Rocatec System is indicated for the coating of metal, ceramic, and composite restorations before adhesive cementing. Application of a silicate layer and a silane leads to the development of a chemical bond to the resin material that then requires no mechanical retention. The Rocatec System is based on three different components: Rocatec Pre (ultrapure aluminium oxide 110 μm), Rocatec Plus (ultrapure aluminium oxide 110 μm modified with silicic acid), and ESPE Sil (silane in ethanol). Fixation of the Components. The resin-based luting agents Panavia F 2.0 (Panavia F2.0, Kuraray Europe GmbH, Frankfurt am Main, Germany) or RelyX Unicem (RelyX Unicem, 3M ESPE) was applied according to the manufacturers’ information, and a constant film was applied to the outer surfaces of the titanium bases and to the inner surfaces of the zirconia copings (Table 1). The titanium bases and zirconia copings of the groups A, C, E, G, I, and K were fixed with Panavia F 2.0. Panavia F 2.0 is a dual-hardening fixation resin which is based on two components. After mixing these components, a constant layer was applied to the outer surfaces of the titanium bases and the inner surfaces of the zirconia copings. The build-ups and the bases were pressed together by hand, using a constant pressure. The test specimens of groups B, D, F, H, J, and L were fixed with RelyX Unicem. This resin is a self-adhesive, dual-hardening material composed of two components delivered in a special dis-
Figure 3 Final installation of test specimens. Laboratory implant fixed in a base of polyurethane and surrounded by an aluminium cartridge. Titanium base bolted into the laboratory implant by a holding screw. Zirconia coping connected to the titanium base by use of a resin-based luting agent.
penser. With this dispenser, the exact amount of resin was applied to the outer surfaces of the titanium bases and to the inner surfaces of the zirconia copings. After this procedure, both components of the abutment were again pressed together manually (Figure 3). In all groups, the surpluses were removed. All specimens were light-cured for 90 seconds to initiate self-polymerization (Uni XS, Heraeus Kulzer, Hanau, Germany). The final hardness was achieved by temporary storage in a heating cabinet of 23°C for 24 hours. Simulated Aging. To simulate the humid atmosphere and variations in temperature in the oral cavity, all specimens were exposed to alternating thermal loads in a moist atmosphere. A total of 10,000 thermal cycles were conducted with temperatures of 5°C and 55°C. Within each cycle, the test specimens were submerged for 30 seconds in a temperature bath at each temperature and were then exposed to room air for 10 seconds during transportation to the other bath. All cycles were performed automatically. After the 10,000 cycles, the specimens were returned to the heating cabinet of 23°C for 24 hours. Pull-Off Test. After simulated aging, all specimens were subjected to a pull-off test to separate zirconia copings from the titanium bases. For this reason, the specimens
Retention Forces of Two-Part Implant Abutments
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Statistical Analysis Statistical analysis was performed using SPSS for Windows, version 19.0 (IBM, Ehringen, Germany). The normal distribution of data and homogeneity of variance were checked using the Kolmogorov–Smirnov and Levene tests, respectively. From the measured values of all groups, the arithmetric mean, the standard deviation, and the minimum and the maximum were calculated. The influence of the luting resins and surface modifications on the bond strength was analyzed by two-way analysis of variance (anova). For direct comparison of the different groups, the post-hoc Scheffé test was performed. The level of significance was set at 0.05 for all analyses. RESULTS Figure 4 Test specimen installed into a universal test machine by using a special jig for pull-off tensile testing.
were fixed into a special jig (Figure 4) which was installed in a universal test machine (Type 20K, UTS Testsysteme, Ulm-Einsingen, Germany). The design of the extractor device ensured that the specimens were stably and reproducibly positioned. The universal test machine moved vertically at a crosshead speed of 1 mm/ min until the zirconia copings were completely separated from the titanium bases. During loading, data for crosshead displacement and load were collected (Programm Phoenix, UTS Testsysteme), and the recorded maximum force was defined as the retention force between the components. Fractographical Analysis. After the pull-off tests, the fracture surfaces of all components were analyzed by reflected light microscopy (M3Z, Wild, Heerbrugg, Switzerland). Characteristics of the fracture surfaces were photographically recorded with a connected digital camera (ProgRes C12 plus, Jenoptik, Jena, Germany). The residues of the resin-based luting agents on the adhesion surfaces of the components were evaluated and classified into three different failure modes: residues only on the titanium base, residues only on the zirconia copings, and residues on both components. Additionally, representative specimens of each group were investigated in a scanning electron microscope (Philips SEM 505, Philips, Eindhoven, the Netherlands).
Pull-Off Test There were no differences between the two fixation resins and the different surface treatments in the characteristics of the pull-off tests (in particular, the load displacement curves). The single pull-off forces are shown in Table 2. Statistical analysis of retention force data revealed no significant differences between the two fixation resins (p = 0.913). However, different surface treatments had a statistically significant influence on the pull-off forces (p < 0.001) (Table 2). Test specimens only
TABLE 2 Pull-Off Forces for Different Test Groups Pull-Off Forces (N)
Group
A B C D E F G H I J K L
Mean
Standard Deviation
Minimum
Maximum
222.3a 258.7a,b 319.3a,b,c 364.9a,b,c 499.0a,b,c 360.3a,b,c 532.4a,b,c 543.7a,b,c 598.6c 555.8b,c 431.3a,b,c 538.9a,b,c
67.6 106.1 95.6 109.9 129.5 124.0 209.3 208.4 173.7 144.4 244.3 208.4
138.5 153.0 217.9 219.0 340.1 156.1 267.6 289.6 370.4 329.4 239.1 290.7
357.0 503.1 482.3 627.1 756.6 638.0 927.7 849.1 859.4 762.3 929.2 900.6
Mean, standard deviation, minimum, and maximum are given. Values with the same superscript do not differ with statistical significance (posthoc Scheffé, p < .05).
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blasted with aluminium oxide powder at the titanium components (groups A and B) showed mean pull-off forces of 222.3 N and 258.7 N, respectively. Due to additional blasting of the zirconia coping and chemical conditioning by application of Clearfil Ceramic Primer to the connecting surfaces, retention forces increased statistically significantly to 598.6 N (group I) and 555.8 N (group J), respectively. By tribochemical preconditioning (Rocatec, groups K and L), the pull-off forces were slightly lower than the maximum values in this investigation (Table 2). Analysis of Fracture Patterns The analysis of fracture patterns showed only adhesive fractures between the components and the fixation resin, but there was no cohesive fracture which ran within the resin layer (Table 3). Adhesive fractures between the zirconia copings and the resin were detected in each group. In groups A, B, C, and L, only adhesive fractures between the zirconia surfaces and the resin occurred (Figures 5 and 6). In the other groups, fracture patterns were found with resin residues on both the zirconia and titanium surfaces (mixed fracture pattern) (Figure 7). No specimen was found to contain adhesive fractures exclusively between the titanium surface and the resin. This fracture distribution indicates that the bonding between the zirconia surface and the resin is the weakest point of the construction.
TABLE 3 Distribution of Fracture Patterns
Figure 5 Light microscopic view onto the adhesion surface of a zirconia specimen (test group C) after pull-off testing. No residues of the fixation material are visible on the surface.
DISCUSSION Zirconia abutments are commonly used in implant dentistry, as they possess both aesthetic and biological advantages. Several recent studies have described problems with one-part abutments, including abutment fracture under functional loading, wear of the implantabutment interface, and marginal misfit to the implant.13,14,21 There may therefore be advantages in using two-part implant abutments with a zirconia coping bonded to a titanium base.9,22 However, even if the adhesive connection between the zirconia and titanium components may be a weak point in these abutments, only a few studies have investigated the strength of this connection and the factors influencing it.12,18
Number of Different Fracture Patterns in Each Group
Group
A B C D E F G H I J K L
Adhesive Fracture Only between Zirconia Surface and Resin
Adhesive Fracture between Zirconia Surface and Resin as well as between Titanium Surface and Resin (Mixed)
10 10 10 4 5 7 5 9 5 8 7 10
— — — 6 5 3 5 1 5 2 3 —
Figure 6 Scanning electron microscopic view of the titanium base of the specimen shown in Figure 5. The adhesion surface of the base is covered by a homogeneous layer of the fixation resin.
Retention Forces of Two-Part Implant Abutments
Figure 7 Representative example of a mixed fracture pattern. The titanium surface is only partially covered with residues of the fixation resin.
Analogously to our results, Gehrke and colleagues found that the kind of luting resin used has no significant influence on the pull-off forces.12 Even though the force values in the present study are lower than those found by Gehrke and colleagues, differences between the various luting resins within the two studies show the same tendency. Variations between the two studies may most likely be explained by differences in the implant and abutment components used or by differences in the thermal cycling procedure and the crosshead speed during pull-off testing. However, based on the results, it can be concluded that both resin-based luting agents investigated (Panavia F2.0 and RelyX Unicem) are generally suitable for connecting zirconia and titanium components in two-part abutments, even if the chemical composition of the resin materials differs significantly.23–26 In another survey, Ebert and colleagues analyzed the effect of the luting gap size on the retention between zirconia and titanium components. They found that zirconia copings bonded with a small luting gap of about 30 μm achieved significantly higher retention than those bonded with a greater luting gap.18 Therefore, in the present study, the design of the copings was adjusted to a luting gap size of 30 μm to ensure the best possible conditions. As in the present study, Ebert and colleagues reported that pretreating the zirconia surface with airborne particle abrasion increases retention between the components.18 It can be concluded that blasting with aluminium oxide is one of the most effective surface preparation methods to achieve efficient adhesion
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between component surfaces and luting resins as also shown in several other studies.27,28 Various published studies used aluminium oxide with an average particle size of 110 μm and a pressure of 2.5 bar.29–31 In the present investigation, a pressure of only 2 bar was applied in order to simulate a procedure which, under real clinical conditions, would be more effective in preventing detrimental microcracks in the zirconia surface.32,33 Nevertheless, even under application of lower pressure, sandblasting has been discussed to be detrimental related to phase structure and subsequent mechanical properties of zirconia. Recent investigations revealed that sandblasting indeed results in a significant transformation from the tetragonal to the monoclinic phase on the zirconia surface accompanied by increasing compressive residual stresses.34,35 However, these phenomena seem to have a beneficial effect on both the flexural strength36 and the aging resistance of Y-TZP ceramic under simulated clinical conditions in a humid atmosphere.37 Aside from the effect of mechanical pretreatment, we also investigated the influence of additional chemical or tribochemical conditioning of the bonding surfaces. It was then found that the different surface conditioning techniques had significant effects (Table 2). The highest retention forces were found after application of Clearfil Ceramic Primer on both the titanium bases and on the zirconia copings. Analogous to our results, in several studies, highest bond strength was found with the combination of blasting with aluminium oxide and Clearfil Ceramic Primer.38,39 Some other test specimens in the present analysis were preconditioned with aluminium oxide blasting and application of Alloy Primer which is typically used for enhancing the bond strength between metal alloys and resin-based materials but is also applied for the preconditioning of zirconia surfaces. Komine and colleagues40 investigated the influence of different primers, including Alloy Primer, on the bond strength of resin materials to zirconia. They showed highest bond adhesion with Alloy Primer; however, they utilized 50-μm particles for aluminium oxide blasting and test specimens just passed through 5,000 cycles of thermo-cycling. Kern and colleagues compared Clearfil Ceramic Primer and Alloy Primer with respect to adhesion to zirconia.41 Their findings confirmed our results that Clearfil Ceramic Primer achieves the best adhesion to zirconia. Ozcan and Valandro evaluated the effect of different primers on the
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adhesion of resins to titanium surfaces. They showed that blasting with aluminium oxide (50 μm) and coating with Alloy Primer lead to the highest adhesion forces.42 In this context, Kim and colleagues reported that the use of surface coupling primers is generally recommended to improve bond adhesion to titanium.43 The Rocatec system was also been evaluated in the present study. Published data on this system are inconsistent. Some surveys reported significant improvements in adhesion with this tribochemical method,31,44–47 whereas other studies favored other preconditioning methods. The analysis of fracture patterns showed that the bonding between the zirconia surface and the resin seemed to be the weakest point of construction. In particular, without the use of a primer, the connection between both components is challenging. A reason for this could be that the surface hardness of zirconia is higher, and consequently, a blasting with aluminium oxide is not as effective as on a titanium surface. A current meta-analysis of Inokoshi and colleagues48 confirms our estimations. The authors stated that in particular, the combination of mechanical and chemical preconditioning contributes to the durability of the bond of resin cements to zirconia ceramics. The results of the present study showed that retention forces between the titanium and zirconia components of two-part abutments can be significantly improved by application of selected preconditioning techniques which are not currently recommended by the manufacturers of the components. However, significant differences between different combinations of luting agent, surface treatment, and adhesion promoter indicate that more investigations are necessary to identify the best possible retention between the components of two-part abutments. CONCLUSIONS Within the limitations of this study, it could be concluded that 1. The procedure of surface preconditioning has a significant influence on the retention forces between the components of two-part implant abutments. 2. The highest retention forces in titanium/zirconia two-part abutments are achieved by applying a single phosphate-based ceramic primer on all connecting surfaces.
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