An In Vitro Comparison of Fracture Load of Zirconia Custom Abutments with Internal Connection and Different Angulations and Thickness: Part I Abdalah Albosefi, BDT, MSc,1 Matthew Finkelman, PhD,2 & Roya Zandparsa, DDS, MSc, DMD3 1
Former resident, Prosthodontic Division and Advanced Education in Esthetic Dentistry, Department of Prosthodontics and Operative Dentistry, Tufts University School of Dental Medicine, Boston, MA 2 Assistant Professor, Department of Public Health and Community Service, Tufts University School of Dental Medicine, Boston, MA 3 Clinical Professor, Prosthodontic Division and Advanced Education in Esthetic Dentistry, Department of Prosthodontics and Operative Dentistry, Tufts University School of Dental Medicine, Boston, MA
Keywords Ceramic abutment stability; shear stress; implant/abutment connection; ceramics. Correspondence Dr. Roya Zandparsa, Prosthodontic Division and Advanced Education in Esthetic Dentistry, Department of Prosthodontics and Operative Dentistry, Tufts University School of Dental Medicine, 1 Kneeland St., Boston, MA 02111. E-mail: [email protected] Presented at the American College of Prosthodontists local meeting, Boston, MA, April 2012. Research sponsored by Straumann USA LLC, Andover, MA. Accepted August 12, 2013 doi: 10.1111/jopr.12118
Abstract Purpose: The purpose of this in vitro study was to compare the fracture load of one-piece zirconia custom abutments with different thicknesses and angulations. Materials and Methods: Forty zirconia custom abutments were divided into four groups. Group A-1 and group B-1 simulated a clinical situation with an ideal implant position, which allows for the use of straight zirconia custom abutments with two thicknesses (0.7 and 1 mm). Groups A-2 and B-2 simulated a situation with a compromised implant position requiring 15◦ angulated abutments with different thicknesses (0.7 and 1 mm). Implant replicas were mounted in self-cure acrylic jigs to support the abutments in all groups. The zirconia custom abutments were engaged in the implant replicas using a manual torque wrench. Each jig was secured and mounted in a metallic vice 30◦ relative to a mechanical indenter. All groups were subjected to shear stress until failure using a universal testing machine with a 0.5 mm/min crosshead speed with the force transferred to the lingual surface of the zirconia custom abutments 2 mm below the top surface. The universal testing machine was controlled via a computer software system that also completed the stress-strain diagram and recorded the breaking fracture load. The fracture loads were recorded for comparison among the groups and subjected to statistical analysis (two-way ANOVA). Results: The mean fracture load of zirconia custom abutments across the groups (A-1 through B-2) ranged from 160 ± 60 to 230 ± 95 N. The straight zirconia custom abutment exhibited the highest fracture load among the groups (p = 0.009); however, the thickness of the zirconia custom abutment had no influence on the strength of any of the specimens (p = 0.827). Conclusions: There was no statistically significant difference in fracture strength between the 0.7 and 1.0 mm groups; however, angulated zirconia custom abutments had the lowest fracture load. Clinical Implication: The results of this in vitro study will help dental practitioners with their decision-making process in selecting the type of custom abutment to be used clinically.
The influence of connection type on the long-term stability of the abutment/implant complex has been analyzed for metallic abutments in several studies.1,2 In contrast, to date, the stability of one-piece internally connected zirconia abutments has not been specifically investigated. It might be expected that onepiece zirconia abutments exhibit different resistance to loading as a result of different thicknesses and angulations. Following the functional and biological success of implant-supported 296
restorations, esthetic aspects gain primary importance. Standardized titanium abutments exhibit high survival rates because of their excellent physical properties.3 However, their application can impair the esthetic result. In the case of soft tissue recession, exposure of the gray titanium abutment can lead to failure of the reconstruction in highly visible anterior regions.4 Furthermore, when titanium abutments are used in patients with a thin labial mucosa, a grayish discoloration of the mucosa can
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 23 (2014) 296–301
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An In Vitro Comparison of Fracture Load of Zirconia Custom Abutments
Figure 1 Group distribution.
occur, owing to the gray metal color showing through.4,5 The esthetic shortcomings of titanium led to the development of ceramic materials as an alternative for esthetically demanding patients.5 In the search for ceramic abutment material with improved physical properties, yttria-stabilized zirconia was introduced in 1996.6 The bending strength of zirconia is 900 MPa, and its fractural toughness reaches 9 MPa/m2 .7 Zirconia abutments showed resistance to a high load of up to 730 N in one in vitro study.8 In comparison, the naturally occurring mean incisoocclusal loads in anterior regions amount to 110 N for teeth and 370 N for implants.9,10 With these data from in vitro studies, zirconia was expected to reduce the risk of fracture; however, the type and architecture of the implant/abutment connection, thickness, and angulations might have a substantial influence on the stability and fixation of brittle ceramic abutments. Implants are designed with different types of implant/abutment connections. The abutments can either be fixed onto an external connecting part of the implant or internally into the implant.11 The internal connection of zirconia abutments can be accomplished either by the abutment itself (one-piece) or by means of secondary components (two-piece). One-piece abutments are made entirely of ceramic, whereas for two-piece abutments the internal connecting part can be either a secondary titanium abutment or separate metallic insert mounted on the implant together with the abutment and fixed by means of one abutment screw.11 The aim of this in vitro study was to compare the fracture load of one-piece zirconia custom abutments with different thicknesses and angulations.
A1
A2
B1
B2
Figure 2 Cares regular cross-fit zirconia custom abutments connected to regular platform lab analog 4.1 mm × 12 mm, A1 = straight (0◦ angle) zirconia custom abutment (0.7-mm thick), A2 = angulated (15◦ angle) zirconia custom abutment (0.7-mm thick), B1 = straight (0◦ angle) zirconia custom abutment (1-mm thick), B2 = angulated (15◦ angle) zirconia custom abutment (1-mm thick).
zirconia custom abutments (Cares Bone level regular cross-fit [RC] 4.1 mm × 12 mm; Straumann LLC, Andover, MA) were divided into four groups. Groups A-1 and B-1 simulated a clinical situation with an ideal implant position from a prosthetic point of view, which allows for the use of straight zirconia custom abutments with two thicknesses (0.7 and 1 mm). Groups A-2 and B-2 simulated a situation with a compromised implant position requiring 15◦ angulated abutments with different thicknesses (0.7 and 1 mm; Fig 2). Specimen preparation
Materials and methods Sample size and power calculation
For the sample size calculation, some studies related to this study were reviewed, and the closest study was chosen to measure the strength of the relationship between variables (effect size).1,12 With a sample size of 40 (10 per group) 90% power was achieved to detect a difference among the groups. Statistical software (R 2.11.1) was used to calculate the sample size required to achieve an α = 0.05 and a power of 90%. Experimental design
This in vitro study measured and compared the fracture load of one-piece zirconia custom abutments made in two angulations (0◦ , 15◦ ) and two thicknesses (0.7 and 1 mm; Fig 1). Forty
Forty implant replicas (10 for each subgroup) (Straumann implant replica, 4.1 mm × 12 mm RC octagon restorative platform; Straumann LLC) were placed in cubic self-cure acrylic jigs (Caulk Orthodontic Resin; Dentsply Caulk, York, PA). The acrylic jigs were standardized with 2.5 × 2.5 × 2.5 cm3 dimensions. To make sure that the implant replicas were ad◦ justed to be perpendicular to the jig’s surface (90 ), each replica was attached to a laboratory surveyor (Dentsply Neytech, Yucaipa, CA) by using a guide pin (Impression Post RC4.1mm; Strauman LLC). A water scale was used to adjust the implant replicas with the surveyor’s pen. The implant replicas placed in cubic acrylic jigs were scanned to design custom abutments digitally at the manufacturer’s facility by a single operator using a surface scanner (Straumann Cares Scan CS2; Straumann LLC). This scanner uses a laser R
R
R
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2mm
A
Figure 3 (A and B) Direction of load applied to the specimens using Instron machine. Note that, t = torque vector (implant/abutment connection), r = length of the lever arm vector = 7.1 mm (the distance between the force being applied on the abutment’s surface and the implant abutment connection), f = force vector (the force applied by the Instron machine), θ = angle between the force vector and the lever arm vector.
B
beam to trace a polymer scan-body and render a digitized image of the implant analog; it then designed the custom abutments digitally.13 The finish line was set and adjusted as necessary using 3D imaging design software (Straumann Cares Visual 6.0; Strauman LLC). The scanned information was transferred electronically to the production facility. After zirconia custom abutments were made by the manufacturer, they were engaged to the implant replicas into the cubic acrylic jig, using a manual torque wrench based on manufacturer’s recommendations (35 Ncm).13 The acrylic jigs were mounted and adjusted at 30◦ relative to the mechanical indenter for all groups. The indenter was covered by a resilient material (Durasoft; Scheu Dental GmbH, Iserlohn, Germany). The indenter contacts the entire mesiodestal-occluding surface in a contact width of approximately 2 to 4 mm. The resilient material is a coextrusion compound material consisting of a hard polycarbonate base and soft polyester urethane, which was used to reduce localized contact stress intensities and to distribute stress over the complete testing unit, including screws and abutments. Then, the shear stress was measured by loading the specimens to failure with a 0.5 mm/min crosshead speed with the force transferred to the lingual surface of the zirconia custom abutments, 2 mm below the top surface using a universal testing machine (Model 5566; Instron, Canton, MA; Fig 3). The universal testing machine was controlled via a computer software system (Bluehill 2 Software, Canton, MA), which also completed the stress-strain diagram and recorded the breaking loads. R
R
R
Statistical analysis
Descriptive statistics were reported for each group (means, standard deviations, minimum, and maximum values). A two-way ANOVA was performed to assess the statistical significance of each factor. A Kolmogorov-Smirnov test was also performed to check the normal distribution of residuals across the groups.
Results The mean fracture load of one-piece zirconia custom abutments across the groups (A-1 through B-2) ranged from 160 ± 60 to 230 ± 95 N where the numbers were rounded to the nearest 10. The p-value of the Kolmogorov-Smirnov test was p = 0.301, 298
Table 1 Fracture load means, SDs, minimum, and maximum values (n = 10/group) Groups Thickness Angulations Mean SD Min Max
A1
A2
B1
B2
0.7 mm 0◦ 227 N 70 141 N 360 N
0.7 mm 15◦ 160 N 60 104 N 314 N
1 mm 0◦ 230 N 95 111 N 406 N
1 mm 15◦ 168 N 59 109 N 308 N
p = 0.827 between A1 and A2, B1 and B2. p = 0.009 between A1 and B1, A2 and B2.
meaning there was no evidence that the assumption of normal distribution of the residuals is violated. On the basis of this, a two-way ANOVA was performed for this group. Straight abutments showed a higher fracture load than angulated abutments. The mean fracture load from a highest to lowest was 230 ± 95 (B-1), 227 ± 70 (A-1), 168 ± 59 (B-2), 160 ± 60 (A-2). When the test was conducted for the first time, the interaction was not significant between the variables (p = 0.981), so the test was performed for a second time without the interaction, and it was found that the straight zirconia custom abutments exhibited a significantly higher fracture load than angulated zirconia custom abutments (p = 0.009; Table 1). No statistically significant difference was found between groups with different thicknesses (group A-1/group A-2; group B-1/group B-2; p = 0.827).
Discussion Researchers highlighted the importance of whether the evaluated specimens are zirconia dioxide abutments with a metallic insertion or all-ceramic components.14-17 The majority of published investigations on all-ceramic implant abutments made from zirconia dioxide examined simulated single incisor replacements.8,18-20 These papers reported fracture load between 429 and 793 N under load angles that range from 30◦ to 60◦ . It seems that there is a strong correlation between measured fracture load and the type of implant/abutment connection.11 In this study, the mean fracture load for 0◦ and 15◦ in one-piece
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An In Vitro Comparison of Fracture Load of Zirconia Custom Abutments
Figure 4 The relation between the lever arm vector and the fracture behavior of zirconia custom abutments. (A) The length of the lever arm vector = 12.6 mm. (B) The length of the implant/abutment connection = 5.5 mm. (C) The thickness of implant abutment connection (C1 = 0.6 mm and C2 = 0.4 mm).
zirconia custom abutments ranged from 160 ± 60 to 230 ± 95 N. It is difficult to compare our fracture load values with results from other studies, due to the various ranges of angles, difference in force application, and different study designs. The moment of force (torque moment) played an important role in this study and had an important effect on the fracture load of the zirconia custom abutments. The strength of the specimens might be affected by three quantities: the force applied to the specimens, the length of the lever arm connecting the axis to the point of force application, and the angle between the force vector and the lever arm21 (Fig 3). For one-piece zirconia custom abutments, the length of the lever arm vector was 12.6 mm, with 0.4 and 0.6 mm thicknesses in the area of implant/abutment connection (Fig 4). This thickness is designed by the manufacturer and is standard and unchangeable. The thinnest part in one-piece zirconia custom abutments is in the implant/abutment connection area (0.4 and 0.6 mm). In addition, this area is the most critical area in the abutments because of the force concentration that might be one
of the reasons for failures in this area. This finding might mean that the thickness of zirconia custom abutments in this design had no influence on the fracture load, since the thickness of implant/abutment connection is standard; however, the angulation had a negative influence on the fracture load, because when the force was applied on angulated abutments, the angle (θ ) between the force vector (f) and the lever arm (r) increased, decreasing the force required to break the specimens. This could have been one of the reasons for failure in this design. To estimate the failure risk associated with implant-supported restorative concepts consistent with their fracture loads as determined in an in vitro setting, it is important to separately consider the forces expected in actual clinical situations. Ferrario et al measured single-tooth occlusion forces in healthy young adults and reported results of 150 and 140 N for men in the central and lateral incisors, respectively.22 Higher occlusion forces are to be expected in individuals with functional disorders such as bruxism.23 In this study, static loading was applied, and the force was applied slowly with a 0.5 mm/min
Figure 5 Views of the fracture pattern of the zirconia custom abutments, appearing as an oblique line below the abutment’s shoulder.
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crosshead speed. This corresponds to the load in a parafunctional situation rather than during chewing. In this study, the mean fracture load for all of the zirconia custom abutments exceeded the above-mentioned occlusion forces; however, we note continued uncertainty in predicting the performance of one-piece zirconia custom abutments in individuals with functional disorders. Special attention might need to be paid to the occlusal relationship of the lower and upper jaw whenever possible. It was recommended that keeping such abutments free from dynamic occlusion might extend the lifespan of such abutments.24-26 The straight zirconia custom abutments exhibited statistically significantly higher mean fracture loads than angulated abutments did. These findings lead us to reject our original null hypothesis. It is possible that in one-piece zirconia custom abutments, loading forces are higher in the area of the apical hexagon, colocalized with the thinnest portion of the abutment.24-27 In this study, artificial dynamic thermal aging was not applied to the specimens due to the failure to exert a statistically significant influence on the fracture load of either straight or angulated abutments in previous studies.28,29 Static loading was performed at an angle of 30◦ to the long axis of the abutments to simulate a parafunctional situation. In this study, the most typical fracture pattern (95% of the specimens among the groups) was an oblique fracture line below the implant shoulder (Fig 5). This finding confirmed Adatia et al’s observations, which indicate that certain grinding procedures above the level of the implant shoulder for the purpose of abutment individualization have no impact on fracture load.20 It is difficult to compare data between studies on the fractural stability of zirconia custom abutments because of different study designs. In this study, the implants and abutments were embedded in orthodontic acrylic resin jigs, simulating horizontal bone loss, whereas in the other investigations the implant-supported restorations were embedded up to the implant shoulder.8 As a result, the loads were applied with different lever arms. Furthermore, variations in the angle of the applied load, static or dynamic testing methods, and the size and shape of the abutments and restoration can have an important influence on the results. A variation in the fracture pattern was also observed in other studies reporting on alumina, zirconia, and titanium abutments with internal connections.12,16 In only one of those studies, however, an implant neck distortion was found in a specimen bearing a titanium abutment.16 In another investigation testing alumina and zirconia abutments, fracture of the abutment, and/or fracture of both the abutment and crown were the main reasons for failure.12 In contrast to these results, zirconia custom abutment components failed by fracture in both groups with different thicknesses and angulations, but the fracture was significantly higher in the groups with 15◦ angulations.
Conclusions Within the limitations of this in vitro study, the following conclusions can be drawn: 1. Angulated zirconia custom abutments had the lowest fracture load of the present investigations; however, there 300
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was no statistically significant difference in the thicknesses between the 0.7 and 1.0 mm group. 2. The thickness of one-piece zirconia custom abutments had no influence on the fracture load.
Acknowledgments The authors extend sincere thanks to the Straumann USA, LLC for their generous materials support and Ms. Jennifer Jackson and Ms. Kelly Jork for all their help and support.
References 1. Sailer I, Sailer T, Stawarczyk B: In vitro study of the influence of the type of connection on the fracture load of zirconia abutments with internal and external implant abutment connection. Int J Oral Maxillofac Implants 2009;24:850-858 2. Buser D, Mericske R, Bernard JP: Long term evaluation of non submerged ITI implants. Clin Oral Implants Res 1997;8:161-172 3. Anderson B, Odman P, Lindvall AM, et al: Cemented single crowns on osseointegrated implants after 5 years. Int J Prosthodont 1998;11:212-218 4. Prestipino V, Ingber A: All ceramic implant abutments: esthetic indecations. J Esthet Dent 1996;8:255-262 5. Jung RE, Sailer I, Hammerle CHF: In vitro color changes of soft tissue caused by restorative materials. Int J Periodontics Restorative Dent 2007;27:251-257 6. Wohlwend A, Studer S, Schaere P: Zirconia oxide abutments. Quintessence Dent Tech 1996;22:364-381 7. Riger W: Medical Applications of Ceramics. London, Academic Press, 1989 8. Yildirim M, Fischer H, Marx R, et al: In vivo fracture resistance of implant-supported all-ceramic restorations. J Prosthet Dent 2003;90:325-331 9. Haraldson T, Carlsson GE, Ingervall B: Functional state bite force and postural muscle activity in patients with osseointegrated oral implant bridges. Acta Odontol Scand 1979;37:195-206 10. Paphangkorakit J, Osborn JW: The effect of pressure on maximum incisal bite force in men. Archs Oral Biol 1997;42:11-17 11. Sailer I, Philipp A, Zembic A, et al: A systematic review of the performance of ceramic and metal implant abutments supporting fixed implant reconstructions. Clin Oral Implants Res 2009;20:4-31 12. Att W, Kurun S, Gerds T: Fracture resistance of single tooth implant supported all ceramic restorations. J Prosthet Dent 2006;95:111-116 13. Straumann Product Catalog, 2013:80, www.straumann.us 14. Butz F, Heydecke G, Okutan M, et al: Survival rate, fracture strength and failure mode of ceramic implant abutments after chewing simulation. J Oral Rehabil 2005;32:838-843 15. Kolbeck C, Behr M, Rosentritt M, et al: Fracture force of tooth–tooth- and implant–tooth-supported all-ceramic fixed partial dentures using titanium vs. customised zirconia implant abutments. Clin Oral Implants Res 2008;19: 1049-1053 16. Canullo L, Morgia P, Marinotti F: Preliminary laboratory evaluation of bicomponent customized zirconia abutments. Int J Prosthodont 2007;20:486-488
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17. Att W, Kurun S, Gerds T, et al: Fracture resistance of single-tooth implant-supported all-ceramic restorations after exposure to the artificial mouth. J Oral Rehabil 2006;33: 380-386 18. Gehrke P, Dhom G, Brunner J, et al: Zirconium implant abutments: fracture strength and influence of cyclic loading on retaining-screw loosening. Quintessence Int 2006;37:19-26 19. Aramouni P, Zebouni E, Tashkandi E, et al: Fracture resistance and failure location of zirconium and metallic implant abutments. J Contemp Dent Pract 2008;9:41-48 20. Adatia ND, Bayne SC, Cooper LF, et al: Fracture resistance of yttria-stabilized zirconia dental implant abutments. J Prosthodont 2009;18:17-22 21. Hall AS, Archer FE, Gilbert RI: Engineering Statics (ed 2). Sydney, Australia, UNSW Press, 2005, pp. 39-227 22. Ferrario VF, Sforza C, Serrao G, et al: Single tooth bite forces in healthy young adults. J Oral Rehabil 2004;31:18-22 23. Nishigawa K, Bando E, Nakano M: Quantitative study of bite force during sleep associated bruxism. J Oral Rehabil 2001;28:485-491
An In Vitro Comparison of Fracture Load of Zirconia Custom Abutments
24. Balshi TJ, Ekfeldt A, Stenberg T, et al: Three-year evaluation of Branemark implants connected to angulated abutments. Int J Oral Maxillofac Implants 1997;12:52-58 25. Sethi A, Kaus T, Sochor P: The use of angulated abutments in implant dentistry: five-year clinical results of an ongoing prospective study. Int J Oral Maxillofac Implants 2000;15:801-810 26. Sethi A, Kaus T, Sochor P, et al: Evolution of the concept of angulated abutments in implant dentistry: 14-year clinical data. Implant Dent 2002;11:41-51 27. Torbjorner A, Fransson B: A literature review on the prosthetic treatment of structurally compromised teeth. Int J Prosthodont 2004;17:369-376 28. Heydecke G, Butz F, Hussein A, et al: Fracture strength after dynamic loading of endodontically treated teeth restored with different post-and-core systems. J Prosthet Dent 2002; 87:438-445 29. Nothdurft FP, Doppler KE, Erdelt KJ, et al: Fracture behavior of straight or angulated zirconia implant abutments supporting anterior single crowns. Clin Oral Investig 2011;15:157-163
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Former resident, Prosthodontic Division and Advanced Education in Esthetic Dentistry, Department of Prosthodontics and Operative Dentistry, Tufts University School of Dental Medicine, Boston, MA 2 Assistant Professor, Department of Public Health and Community Service, Tufts University School of Dental Medicine, Boston, MA 3 Clinical Professor, Prosthodontic Division and Advanced Education in Esthetic Dentistry, Department of Prosthodontics and Operative Dentistry, Tufts University School of Dental Medicine, Boston, MA
Keywords Ceramic abutment stability; shear stress; implant/abutment connection; ceramics. Correspondence Dr. Roya Zandparsa, Prosthodontic Division and Advanced Education in Esthetic Dentistry, Department of Prosthodontics and Operative Dentistry, Tufts University School of Dental Medicine, 1 Kneeland St., Boston, MA 02111. E-mail: [email protected] Presented at the American College of Prosthodontists local meeting, Boston, MA, April 2012. Research sponsored by Straumann USA LLC, Andover, MA. Accepted August 12, 2013 doi: 10.1111/jopr.12118
Abstract Purpose: The purpose of this in vitro study was to compare the fracture load of one-piece zirconia custom abutments with different thicknesses and angulations. Materials and Methods: Forty zirconia custom abutments were divided into four groups. Group A-1 and group B-1 simulated a clinical situation with an ideal implant position, which allows for the use of straight zirconia custom abutments with two thicknesses (0.7 and 1 mm). Groups A-2 and B-2 simulated a situation with a compromised implant position requiring 15◦ angulated abutments with different thicknesses (0.7 and 1 mm). Implant replicas were mounted in self-cure acrylic jigs to support the abutments in all groups. The zirconia custom abutments were engaged in the implant replicas using a manual torque wrench. Each jig was secured and mounted in a metallic vice 30◦ relative to a mechanical indenter. All groups were subjected to shear stress until failure using a universal testing machine with a 0.5 mm/min crosshead speed with the force transferred to the lingual surface of the zirconia custom abutments 2 mm below the top surface. The universal testing machine was controlled via a computer software system that also completed the stress-strain diagram and recorded the breaking fracture load. The fracture loads were recorded for comparison among the groups and subjected to statistical analysis (two-way ANOVA). Results: The mean fracture load of zirconia custom abutments across the groups (A-1 through B-2) ranged from 160 ± 60 to 230 ± 95 N. The straight zirconia custom abutment exhibited the highest fracture load among the groups (p = 0.009); however, the thickness of the zirconia custom abutment had no influence on the strength of any of the specimens (p = 0.827). Conclusions: There was no statistically significant difference in fracture strength between the 0.7 and 1.0 mm groups; however, angulated zirconia custom abutments had the lowest fracture load. Clinical Implication: The results of this in vitro study will help dental practitioners with their decision-making process in selecting the type of custom abutment to be used clinically.
The influence of connection type on the long-term stability of the abutment/implant complex has been analyzed for metallic abutments in several studies.1,2 In contrast, to date, the stability of one-piece internally connected zirconia abutments has not been specifically investigated. It might be expected that onepiece zirconia abutments exhibit different resistance to loading as a result of different thicknesses and angulations. Following the functional and biological success of implant-supported 296
restorations, esthetic aspects gain primary importance. Standardized titanium abutments exhibit high survival rates because of their excellent physical properties.3 However, their application can impair the esthetic result. In the case of soft tissue recession, exposure of the gray titanium abutment can lead to failure of the reconstruction in highly visible anterior regions.4 Furthermore, when titanium abutments are used in patients with a thin labial mucosa, a grayish discoloration of the mucosa can
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 23 (2014) 296–301
Albosefi et al
An In Vitro Comparison of Fracture Load of Zirconia Custom Abutments
Figure 1 Group distribution.
occur, owing to the gray metal color showing through.4,5 The esthetic shortcomings of titanium led to the development of ceramic materials as an alternative for esthetically demanding patients.5 In the search for ceramic abutment material with improved physical properties, yttria-stabilized zirconia was introduced in 1996.6 The bending strength of zirconia is 900 MPa, and its fractural toughness reaches 9 MPa/m2 .7 Zirconia abutments showed resistance to a high load of up to 730 N in one in vitro study.8 In comparison, the naturally occurring mean incisoocclusal loads in anterior regions amount to 110 N for teeth and 370 N for implants.9,10 With these data from in vitro studies, zirconia was expected to reduce the risk of fracture; however, the type and architecture of the implant/abutment connection, thickness, and angulations might have a substantial influence on the stability and fixation of brittle ceramic abutments. Implants are designed with different types of implant/abutment connections. The abutments can either be fixed onto an external connecting part of the implant or internally into the implant.11 The internal connection of zirconia abutments can be accomplished either by the abutment itself (one-piece) or by means of secondary components (two-piece). One-piece abutments are made entirely of ceramic, whereas for two-piece abutments the internal connecting part can be either a secondary titanium abutment or separate metallic insert mounted on the implant together with the abutment and fixed by means of one abutment screw.11 The aim of this in vitro study was to compare the fracture load of one-piece zirconia custom abutments with different thicknesses and angulations.
A1
A2
B1
B2
Figure 2 Cares regular cross-fit zirconia custom abutments connected to regular platform lab analog 4.1 mm × 12 mm, A1 = straight (0◦ angle) zirconia custom abutment (0.7-mm thick), A2 = angulated (15◦ angle) zirconia custom abutment (0.7-mm thick), B1 = straight (0◦ angle) zirconia custom abutment (1-mm thick), B2 = angulated (15◦ angle) zirconia custom abutment (1-mm thick).
zirconia custom abutments (Cares Bone level regular cross-fit [RC] 4.1 mm × 12 mm; Straumann LLC, Andover, MA) were divided into four groups. Groups A-1 and B-1 simulated a clinical situation with an ideal implant position from a prosthetic point of view, which allows for the use of straight zirconia custom abutments with two thicknesses (0.7 and 1 mm). Groups A-2 and B-2 simulated a situation with a compromised implant position requiring 15◦ angulated abutments with different thicknesses (0.7 and 1 mm; Fig 2). Specimen preparation
Materials and methods Sample size and power calculation
For the sample size calculation, some studies related to this study were reviewed, and the closest study was chosen to measure the strength of the relationship between variables (effect size).1,12 With a sample size of 40 (10 per group) 90% power was achieved to detect a difference among the groups. Statistical software (R 2.11.1) was used to calculate the sample size required to achieve an α = 0.05 and a power of 90%. Experimental design
This in vitro study measured and compared the fracture load of one-piece zirconia custom abutments made in two angulations (0◦ , 15◦ ) and two thicknesses (0.7 and 1 mm; Fig 1). Forty
Forty implant replicas (10 for each subgroup) (Straumann implant replica, 4.1 mm × 12 mm RC octagon restorative platform; Straumann LLC) were placed in cubic self-cure acrylic jigs (Caulk Orthodontic Resin; Dentsply Caulk, York, PA). The acrylic jigs were standardized with 2.5 × 2.5 × 2.5 cm3 dimensions. To make sure that the implant replicas were ad◦ justed to be perpendicular to the jig’s surface (90 ), each replica was attached to a laboratory surveyor (Dentsply Neytech, Yucaipa, CA) by using a guide pin (Impression Post RC4.1mm; Strauman LLC). A water scale was used to adjust the implant replicas with the surveyor’s pen. The implant replicas placed in cubic acrylic jigs were scanned to design custom abutments digitally at the manufacturer’s facility by a single operator using a surface scanner (Straumann Cares Scan CS2; Straumann LLC). This scanner uses a laser R
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2mm
A
Figure 3 (A and B) Direction of load applied to the specimens using Instron machine. Note that, t = torque vector (implant/abutment connection), r = length of the lever arm vector = 7.1 mm (the distance between the force being applied on the abutment’s surface and the implant abutment connection), f = force vector (the force applied by the Instron machine), θ = angle between the force vector and the lever arm vector.
B
beam to trace a polymer scan-body and render a digitized image of the implant analog; it then designed the custom abutments digitally.13 The finish line was set and adjusted as necessary using 3D imaging design software (Straumann Cares Visual 6.0; Strauman LLC). The scanned information was transferred electronically to the production facility. After zirconia custom abutments were made by the manufacturer, they were engaged to the implant replicas into the cubic acrylic jig, using a manual torque wrench based on manufacturer’s recommendations (35 Ncm).13 The acrylic jigs were mounted and adjusted at 30◦ relative to the mechanical indenter for all groups. The indenter was covered by a resilient material (Durasoft; Scheu Dental GmbH, Iserlohn, Germany). The indenter contacts the entire mesiodestal-occluding surface in a contact width of approximately 2 to 4 mm. The resilient material is a coextrusion compound material consisting of a hard polycarbonate base and soft polyester urethane, which was used to reduce localized contact stress intensities and to distribute stress over the complete testing unit, including screws and abutments. Then, the shear stress was measured by loading the specimens to failure with a 0.5 mm/min crosshead speed with the force transferred to the lingual surface of the zirconia custom abutments, 2 mm below the top surface using a universal testing machine (Model 5566; Instron, Canton, MA; Fig 3). The universal testing machine was controlled via a computer software system (Bluehill 2 Software, Canton, MA), which also completed the stress-strain diagram and recorded the breaking loads. R
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Statistical analysis
Descriptive statistics were reported for each group (means, standard deviations, minimum, and maximum values). A two-way ANOVA was performed to assess the statistical significance of each factor. A Kolmogorov-Smirnov test was also performed to check the normal distribution of residuals across the groups.
Results The mean fracture load of one-piece zirconia custom abutments across the groups (A-1 through B-2) ranged from 160 ± 60 to 230 ± 95 N where the numbers were rounded to the nearest 10. The p-value of the Kolmogorov-Smirnov test was p = 0.301, 298
Table 1 Fracture load means, SDs, minimum, and maximum values (n = 10/group) Groups Thickness Angulations Mean SD Min Max
A1
A2
B1
B2
0.7 mm 0◦ 227 N 70 141 N 360 N
0.7 mm 15◦ 160 N 60 104 N 314 N
1 mm 0◦ 230 N 95 111 N 406 N
1 mm 15◦ 168 N 59 109 N 308 N
p = 0.827 between A1 and A2, B1 and B2. p = 0.009 between A1 and B1, A2 and B2.
meaning there was no evidence that the assumption of normal distribution of the residuals is violated. On the basis of this, a two-way ANOVA was performed for this group. Straight abutments showed a higher fracture load than angulated abutments. The mean fracture load from a highest to lowest was 230 ± 95 (B-1), 227 ± 70 (A-1), 168 ± 59 (B-2), 160 ± 60 (A-2). When the test was conducted for the first time, the interaction was not significant between the variables (p = 0.981), so the test was performed for a second time without the interaction, and it was found that the straight zirconia custom abutments exhibited a significantly higher fracture load than angulated zirconia custom abutments (p = 0.009; Table 1). No statistically significant difference was found between groups with different thicknesses (group A-1/group A-2; group B-1/group B-2; p = 0.827).
Discussion Researchers highlighted the importance of whether the evaluated specimens are zirconia dioxide abutments with a metallic insertion or all-ceramic components.14-17 The majority of published investigations on all-ceramic implant abutments made from zirconia dioxide examined simulated single incisor replacements.8,18-20 These papers reported fracture load between 429 and 793 N under load angles that range from 30◦ to 60◦ . It seems that there is a strong correlation between measured fracture load and the type of implant/abutment connection.11 In this study, the mean fracture load for 0◦ and 15◦ in one-piece
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Figure 4 The relation between the lever arm vector and the fracture behavior of zirconia custom abutments. (A) The length of the lever arm vector = 12.6 mm. (B) The length of the implant/abutment connection = 5.5 mm. (C) The thickness of implant abutment connection (C1 = 0.6 mm and C2 = 0.4 mm).
zirconia custom abutments ranged from 160 ± 60 to 230 ± 95 N. It is difficult to compare our fracture load values with results from other studies, due to the various ranges of angles, difference in force application, and different study designs. The moment of force (torque moment) played an important role in this study and had an important effect on the fracture load of the zirconia custom abutments. The strength of the specimens might be affected by three quantities: the force applied to the specimens, the length of the lever arm connecting the axis to the point of force application, and the angle between the force vector and the lever arm21 (Fig 3). For one-piece zirconia custom abutments, the length of the lever arm vector was 12.6 mm, with 0.4 and 0.6 mm thicknesses in the area of implant/abutment connection (Fig 4). This thickness is designed by the manufacturer and is standard and unchangeable. The thinnest part in one-piece zirconia custom abutments is in the implant/abutment connection area (0.4 and 0.6 mm). In addition, this area is the most critical area in the abutments because of the force concentration that might be one
of the reasons for failures in this area. This finding might mean that the thickness of zirconia custom abutments in this design had no influence on the fracture load, since the thickness of implant/abutment connection is standard; however, the angulation had a negative influence on the fracture load, because when the force was applied on angulated abutments, the angle (θ ) between the force vector (f) and the lever arm (r) increased, decreasing the force required to break the specimens. This could have been one of the reasons for failure in this design. To estimate the failure risk associated with implant-supported restorative concepts consistent with their fracture loads as determined in an in vitro setting, it is important to separately consider the forces expected in actual clinical situations. Ferrario et al measured single-tooth occlusion forces in healthy young adults and reported results of 150 and 140 N for men in the central and lateral incisors, respectively.22 Higher occlusion forces are to be expected in individuals with functional disorders such as bruxism.23 In this study, static loading was applied, and the force was applied slowly with a 0.5 mm/min
Figure 5 Views of the fracture pattern of the zirconia custom abutments, appearing as an oblique line below the abutment’s shoulder.
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crosshead speed. This corresponds to the load in a parafunctional situation rather than during chewing. In this study, the mean fracture load for all of the zirconia custom abutments exceeded the above-mentioned occlusion forces; however, we note continued uncertainty in predicting the performance of one-piece zirconia custom abutments in individuals with functional disorders. Special attention might need to be paid to the occlusal relationship of the lower and upper jaw whenever possible. It was recommended that keeping such abutments free from dynamic occlusion might extend the lifespan of such abutments.24-26 The straight zirconia custom abutments exhibited statistically significantly higher mean fracture loads than angulated abutments did. These findings lead us to reject our original null hypothesis. It is possible that in one-piece zirconia custom abutments, loading forces are higher in the area of the apical hexagon, colocalized with the thinnest portion of the abutment.24-27 In this study, artificial dynamic thermal aging was not applied to the specimens due to the failure to exert a statistically significant influence on the fracture load of either straight or angulated abutments in previous studies.28,29 Static loading was performed at an angle of 30◦ to the long axis of the abutments to simulate a parafunctional situation. In this study, the most typical fracture pattern (95% of the specimens among the groups) was an oblique fracture line below the implant shoulder (Fig 5). This finding confirmed Adatia et al’s observations, which indicate that certain grinding procedures above the level of the implant shoulder for the purpose of abutment individualization have no impact on fracture load.20 It is difficult to compare data between studies on the fractural stability of zirconia custom abutments because of different study designs. In this study, the implants and abutments were embedded in orthodontic acrylic resin jigs, simulating horizontal bone loss, whereas in the other investigations the implant-supported restorations were embedded up to the implant shoulder.8 As a result, the loads were applied with different lever arms. Furthermore, variations in the angle of the applied load, static or dynamic testing methods, and the size and shape of the abutments and restoration can have an important influence on the results. A variation in the fracture pattern was also observed in other studies reporting on alumina, zirconia, and titanium abutments with internal connections.12,16 In only one of those studies, however, an implant neck distortion was found in a specimen bearing a titanium abutment.16 In another investigation testing alumina and zirconia abutments, fracture of the abutment, and/or fracture of both the abutment and crown were the main reasons for failure.12 In contrast to these results, zirconia custom abutment components failed by fracture in both groups with different thicknesses and angulations, but the fracture was significantly higher in the groups with 15◦ angulations.
Conclusions Within the limitations of this in vitro study, the following conclusions can be drawn: 1. Angulated zirconia custom abutments had the lowest fracture load of the present investigations; however, there 300
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was no statistically significant difference in the thicknesses between the 0.7 and 1.0 mm group. 2. The thickness of one-piece zirconia custom abutments had no influence on the fracture load.
Acknowledgments The authors extend sincere thanks to the Straumann USA, LLC for their generous materials support and Ms. Jennifer Jackson and Ms. Kelly Jork for all their help and support.
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