Effect of Abutment Screw Surface Treatment on Reliability of Implant-Supported Crowns Rodolfo Bruniera Anchieta, DDS, MS, PhD1/Lucas Silveira Machado, DDS, MS, PhD1/ Estevam Augusto Bonfante, DDS, MS, PhD2/Ronaldo Hirata, DDS, MS, PhD3/ Amilcar Chagas Freitas Jr, DDS, MS, PhD4/Paulo G. Coelho, DDs, MS, PhD5 Purpose: To evaluate and compare the reliability of implant-supported single crowns cemented onto abutments retained with coated (C) or noncoated (NC) screws and onto platform-switched abutments with coated screws. Materials and Methods: Fifty-four implants (DT Implant 4-mm Standard Platform, Intra-Lock International) were divided into three groups (n = 18 each) as follows: matching-platform abutments secured with noncoated abutment screws (MNC); matching-platform abutments tightened with coated abutment screws (MC); and switched-platform abutments secured with coated abutment screws (SC). Screws were characterized by scanning electron microscopy and x-ray photoelectron spectroscopy (XPS). The specimens were subjected to step-stress accelerated life testing. Use-level probability Weibull curves and reliability for 100,000 cycles at 200 N and 300 N (90% two-sided confidence intervals) were calculated. Polarized light and scanning electron microscopes were used for fractographic analysis. Results: Scanning electron microscopy revealed differences in surface texture; noncoated screws presented the typical machining grooves texture, whereas coated screws presented a plastically deformed surface layer. XPS revealed the same base components for both screws, with the exception of higher degrees of silicon in the SiO 2 form for the coated samples. For 100,000 cycles at 300 N, reliability values were 0.06 (0.01 to 0.16), 0.25 (0.09 to 0.45), and 0.25 (0.08 to 0.45), for MNC, MC, and SC, respectively. The most common failure mechanism for MNC was fracture of the abutment screw, followed by bending, or its fracture, along with fracture of the abutment or implant. Coated abutment screws most commonly fractured along with the abutment, irrespective of abutment type. Conclusion: Reliability was higher for both groups with the coated screw than with the uncoated screw. Failure modes differed between coated and uncoated groups. Int J Oral Maxillofac Implants 2014;29:585–592. doi: 10.11607/jomi.3387 Key words: abutment screw, biomechanics, dental implants, reliability, Weibull curves 1Researcher,
Department of Dental Materials and Prosthodontics, Universidade Estadual Paulista, Araçatuba Dental School, Araçatuba, SP, Brazil; Department of Biomaterials and Biomimetics, New York University, College of Dentistry, New York, New York. 2 Assistant Professor, Department of Prosthodontics, University of São Paulo, Bauru College of Dentistry, Bauru, SP, Brazil. 3Assistant Professor, Department of Biomaterials and Biomimetics, New York University, College of Dentistry, New York, New York. 4 Assistant Professor, Postgraduate Program in Dentistry, Universidade Potiguar, Department of Health Sciences, aNatal, Rio Grande do Norte, Brazil. 5Associate Professor, Department of Biomaterials and Biomimetics, New York University, College of Dentistry, New York, New York; Director for Research, Department of Periodontology and Implant Dentistry, New York University College of Dentistry, New York, New York. Correspondence to: Paulo G. Coelho, Department of Biomaterials and Biomimetics, New York University, 345 E. 24th Street, 10010, New York, NY. Fax: +212-995-4244. Email: [email protected] ©2014 by Quintessence Publishing Co Inc.
T
o optimize biomechanical stress distribution and consequently decrease the amount of bone and soft tissue loss during the years after implant placement, the platform-switching concept has been used frequently.1–4 The platform-switched configuration appears to contribute to bone preservation by moving the gap between the implant and the abutment away from the marginal bone. Platform switching is characterized by the placement of smaller-diameter prosthetic components on wider-diameter implants.5 Such a configuration seems to preserve the bone height and soft tissue levels as the result of better stress distribution along the length of the implant body.4,6 It has been suggested that the degree of marginal bone resorption appears to be inversely related to the extent of the implant-abutment mismatch.6 However, in spite of bone and soft tissue preservation, implants with platform-switched restorations are more susceptible to higher stress concentrations in the abutment7 and prosthetic screw.7 It has been reported that the failure location is related to the The International Journal of Oral & Maxillofacial Implants 585
© 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
Anchieta et al
design characteristics of the implant-abutment system, which is commonly located in the threaded region or areas that represent a critical point for prosthetic component endurance because of the shift in geometry along the length of the implant-abutment assembly and subtle alterations in cross-sectional area.8 In addition, whereas this setup is beneficial for bone maintenance, the higher the level of mismatch, the more likely the integrity of the prosthetic components, especially the abutment screws of external-connection implants, will be challenged.8 Because oblique loading is resisted mainly by the abutment screw, with a small amount of stress being dissipated by the connection, early screw fracture or loosening may be anticipated. Because the connecting screw has been regarded as the weakest link at the implant-abutment interface joint,9 potential alternatives to improve the biomechanical loading capabilities of the platform-switched unit have been explored. Different component alloys,10 component geometric designs,11 and abutment screw surface treatments11,12 have all been suggested. The rationale for abutment screw surface treatment resides in attempts to improve its fit and mechanical behavior as mechanical movements occur between parts. Such an interaction between two metal surfaces sliding against each other in dry contact generates the galling phenomenon, in which, during loading, the softer material adheres locally to the opposite surface, forming hard “galls” that improve the overall mechanical strength of the system.13 In general, metal coating improves the mechanical behavior of bolted joints through high hardness, strength, and the ability to generate low friction to improving wear resistance and friction level.13 Clinical studies have shown that abutment screw loosening represents the most common technical complication with implant-supported prostheses, averaging 8.8%,14–16 and that abutment or prosthesis screw fracture represents around 4% of complications in a 5-year period,17,18 which may demand extra time for repair and/or maintenance. Because one potential source of screw fracture is undetected screw loosening,19 improvements in the mechanisms of abutment screw retention are likely to result in an overall mechanical advantage for implantsupported prostheses. The aim of the present study was to evaluate the reliability and failure modes of anatomically designed implant-supported single crowns restored with two different types of abutment screws (coated and noncoated) under step-stress accelerated life testing (SSALT). The postulated null hypothesis was that there would be no differences in reliability between matching implant-abutment platforms secured with coated and noncoated screws and a platform-switched abutment secured with a coated screw.
MATERIALS AND METHODS Experimental Design
Two different abutment platforms (regular [V-IAA02] and switched [IAA02], Intra-Lock International) were used to restore single-unit implants with internal-connection geometry and flat-top surface (Blossom DT, 4.0 mm, Intra-Lock) and two different abutment screws (coated [HRS2] and noncoated [HRS], Intra-Lock International) as follows: MNC = matching platform tightened with a noncoated abutment screw (n = 18), MC = matching platform tightened with a coated abutment screw (n = 18), and SC = switched platform tightened with a coated abutment screw (n = 18) (Fig 1). All abutment screws were made of titanium-aluminum-vanadium alloy (Ti-6Al-4V) and are standard commercial screws. The geometry and surface of the coated screws were identical to those of the noncoated screws except for the proprietary surface treatment applied to the former.
Screw Morphology and Chemical Characterization
Scanning electron microscope (SEM) images (S-3500N, Hitachi) of the two types of screws were acquired at ×100 and ×10,000. To evaluate the chemical composition of the two types of screw surfaces, x-ray photoelectron spectroscopy (XPS) (PHI Quantum 2000) was used. XPS data are quantified using relative sensitivity factors and a model that assumes a homogenous layer. The analysis volume is the product of the analysis area (spot size or aperture size) and the depth of information. Photoelectrons are generated within the x-ray penetration depth (typically many microns), but only the photoelectrons within the top three photoelectron escape depths are detected. Escape depths are on the order of 15 to 35 Å, which leads to an analysis depth of about 50 to 100 Å. Typically, 95% of the signal originates from within this depth. The x-ray source utilized was a monochromatic aluminum-potassium alpha 1,486.6 eV at a ± 23-degree acceptance angle and a take-off angle of 45 degrees. An analysis area 50 µm in diameter was utilized, and charge correction for C1s (284 eV) was used.
Sample Preparation for Mechanical Testing
Implants were connected to their respective abutments and then vertically embedded in polymethyl methacrylate resin (Orthodontic Resin, Dentsply Caulk), with 1 mm below the implant-abutment finishing line left exposed (Fig 1). Groups were restored with standardized central incisor metallic crowns (cobaltchromium alloy, Wirobond 280, BEGO; n = 63), which were cemented (Rely X Unicem, 3M ESPE) to the abutments. The crowns were identical and milled from the
586 Volume 29, Number 3, 2014 © 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
Anchieta et al
Fig 1a The implants and abutments were tightened. The pieces were mounted in a polyvinylchloride tube and polymethyl methacrylate resin was added, with the resin level left 1 mm below the implant-abutment junction.
Fig 1b The samples were mounted in the SSALT apparatus at a 30-degree axial inclination.
same stereolithographic file onto a five-axis mill. The machined commercial abutments, made with Ti-6Al4V (Internal Connection System, Intra-Lock International), were tightened with a torque gauge according to the manufacturer’s instructions at 35 Ncm.
Mechanical Testing and Reliability Analysis
To test the implant-abutment connection in a challenging scenario, mechanical testing was undertaken with all specimens placed at a 30-degree axial inclination, per International Organization for Standardization guideline 14801:2007 (Fig 1). The indenter remained in contact with the crown surface during SSALT in an attempt to provide a bending component during loading. SSALT profiles were based on the mean load to failure of previous studies9,20–23 (Fig 1). The profiles were designated as mild, moderate, and aggressive, with the number of specimens assigned to each group in the ratio 3:2:1, respectively. Therefore, of the 18 samples in each group, nine were allocated for mild, six for moderate, and three for the aggressive profile. Mild, moderate, or aggressive profiles refer to the increasingly stepwise rapidity in which a specimen is fatigued to reach a certain level of load, meaning that specimens assigned to a mild profile will be cycled longer to reach the same load of a specimen assigned to a moderate or aggressive profile.23 The rationale for using at least three profiles for this type of testing was based on the need to distribute failure across different step loads and allow better prediction statistics, narrowing confidence bounds. The prescribed fatigue method was SSALT under water at 9 Hz with a servo–all-electric system (Test Resources 800L). The specimens were evaluated under light microscopy at the completion of each block of fatigue cycling. Criteria used for failure were bending or fracture
Fig 1c Approximated view of the indenter contacting the crown during SSALT.
of the abutment screw, partial or complete fracture of the abutment, and fracture of the implant. Based upon the step-stress distribution of the failures (number of performed cycles and load in Newtons), use-level probability Weibull curves (probability of failure vs cycles) with use stress of 300 N at 90% twosided confidence intervals were calculated and plotted (Alta Pro 7, ReliaSoft) using a power law relationship for damage accumulation. For the mission reliability (100,000 cycles at 200 N and 300 N) and β parameters calculated in the present study, the 90% confidence interval ranges were calculated as follows: IC = E(G) ± Zαsqrt(Var(G)), where IC is the confidence bound, E(G) is the mean estimated reliability for the mission calculated from Weibull statistics, Zα is the Z value concerning the given IC level of significance, and Var(G) is the value calculated by the Fisher Information matrix.24,25
Failure Analysis
The failed samples were inspected under a polarized light microscope (MZ-APO stereomicroscope, Carl Zeiss MicroImaging) and classified according to the proposed failure criteria for comparisons between groups. To identify fractographic markings and to characterize the origin of failure and direction of propagation, the most representative failed samples of each group were inspected under SEM (S-3500N, Hitachi)20,25 (Fig 1).
RESULTS Figure 2 shows the morphology of the noncoated and coated abutment screws before testing. Typical machining marks can be observed in the noncoated
The International Journal of Oral & Maxillofacial Implants 587 © 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
Anchieta et al
a
b
d
c
Fig 2 (a and c) SEM images of noncoated screw (×100 and ×10,000, respectively). (b and d) Images of coated screw (×100 and ×10,000, respectively).
Table 1 Atomic Concentrations (in %) Screw type
C
O
Na
Al
Si
P
Noncoated
9.3
67.7
–
1.9
0.4
1.0
Coated
5.9
68.6
0.6
1.4
11.5
1.1
K – < 0.1
Ca
Ti
V
?
19.1
0.7
0.2
10.7
–
Numbers are normalized to 100% of the elements detected. XPS does not detect H or He. – = element not detected; < = accurate quantification cannot be made due to weak signal intensity; ? = species may be present at or near the detection limit of the technique.
Reliability (%)
100 90 80 70 60 50 40 30 20 10 0
MNC
MC
Reliability 200 N
SC Reliability 300 N
Fig 3 Calculated reliability and 90% confidence bounds for 200 N and 300 N at 100,000 cycles.
screw, whereas the coated screw presents a plastically deformed textured surface. The XPS analysis showed that both samples were composed primarily of titanium (Ti), oxygen (O), and carbon (C). The coated screw surface also contained high levels of silicon (Si); in contrast, only trace levels of Si were detected on the noncoated sample. The two samples also differed in that the Ti and C levels were notably lower on the coated samples. Several additional elements were detected in low to trace levels on one or both of the samples; these included sodium, aluminum, phosphorus, potassium, calcium, and vanadium (Table 1). The high levels of Si were observed on the coated sample primarily as SiO2. Ti was found as TiO2 on both samples. The noncoated samples also contained trace levels of metallic Ti (TiO). The fact that metallic Ti is observed indicates that the TiO2 film thickness is thinner than the XPS information depth (~60 to
80 Å) on this sample. Trace levels of V were only detected on the noncoated sample (as VOx). Phosphorus was found as phosphate and aluminum was found as Al2O3 on both samples. Trace levels of calcium, sodium, and potassium were detected on the coated samples. These elements do not exhibit significant shifts in binding energy with bonding state. For both screw types, C was observed primarily as hydrocarbon (C-C, C-H) with lower levels of oxidized carbon forms. At the low level, the observed C is likely nearly entirely attributable to adventitious C (ie, C adsorbed from atmospheric exposure, packaging, and handling). O was observed over a range of binding energies, attributable to metal oxides and organic species on the “machined only” sample. On the coated samples, a large fraction of O was also observed as SiO2 (Table 1). All specimens failed during SSALT, and collected data included the number of cycles, load, and profile of the specimen. The SSALT reliability calculation depicted mean β values (confidence interval range) and associated upper and lower bounds derived from use-level probability Weibull calculations: 2.05 (1.26 to 3.32), 1.54 (0.68 to 3.49), and 1.41 (0.52 to 3.85) for groups MNC, MC, and SC, respectively, indicating that fatigue accelerated the failure of all groups. The calculated reliability (probability of the implantsupported incisor operating for a given amount of time without failure) with 90% confidence intervals for a mission of 100,000 cycles at 200 N showed that the cumulative damage from loads reaching 200 N would lead to 86% survival of MNC, 93% of MC, and 92% of SC. Simulations at 300 N estimated a 6% survival rate for MNC and approximately 25% for MC and SC (Fig 3). A use-level probability Weibull plot for the probability of failure as a function of number of cycles (time) given a mission of 100,000 cycles at 300 N is presented in Fig 4. The β values suggest that overall failures were influenced by fatigue.
588 Volume 29, Number 3, 2014 © 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
Anchieta et al
99.000
Use level MC
Probability of failure
90.000
Cumulative damage Weibull 150 F = 16 | S = 0 Data points Use level line
50.000
Cumulative damage Weibull 150 F = 16 | S = 0 Data points Use level line
MNC
10.000
Cumulative damage Weibull 150 F = 18 | S = 0 Data points Use level line
5.000
1.000 10,000.00
SC
100,000.00
1,000,000.00 1.000E + 7 1.000E + 8
Time MC: Beta = 1.5462; Alpha(0) = 38.5493; Alpha(1) = 4.7752. MNC: Beta = 2.0527; Alpha(0) = 30.8911; Alpha(1) = 3.4822. SC: Beta = 1.4169; Alpha(0) = 39.8513; Alpha(1) = 5.0093.
Fig 4 Use-level probability Weibull for tested groups showing the probability of failure as a function of number of cycles (time) given a mission of 100,000 cycles at 300 N.
Failure Modes All specimens failed after SSALT. When component failures were evaluated together, failures comprised different combinations of abutment screw bending or fracture, abutment fracture, and implant fracture. Observed failure modes are described in Table 2. For all groups, failure predominantly involved abutment and abutment screw fracture. Observation of the polarized light and SEM micrographs of the fractured surface of the abutment screws allowed the consistent identification of fractographic marks, such as compression curl, fatigue striations, fracture origin, and the direction of crack propagation (Figs 5 to 8).
DISCUSSION The abutment screw is a key element that keeps the implant-abutment-prosthesis system assembled and bears occlusal loads; it therefore plays a significant role in the long-term stability of a joint.26 To evaluate the effect of a novel abutment screw surface treatment, implant-supported crowns with or without platformswitched prosthetic components were subjected to accelerated life testing in a challenging scenario. Previous investigations have shown that lower reliability of an implant system may be expected for platformswitched prostheses compared to regular platforms in external-connection systems.8 The SSALT8,9,22,23,26–39 testing results showed that accumulated fatigue damage accelerated the failures for all groups in the present study, as evidenced by the resulting β > 1 (also called the Weibull shape factor). The β value describes failure rate changes over time (β
Table 2 Failure Modes After Mechanical Testing (SSALT) Failure mode Implant fracture Abutment fracture
MNC (%) MC (%) SC (%) 0
0
0
0
0
0
Screw fracture
50
0
0
Screw bending
12.5
0
0
Abutment bending
0
Abutment + screw fractures
18.75
Implant + screw fractures
12.5
Implant + abutment + screw fractures
6.25
15 85
100
0
0
0
0
< 1 indicates that the failure rate is decreasing over time, commonly associated with “early failures” or failures that occur as the result of egregious flaws; β ~ 1 is a failure rate that does not vary over time, associated with failures of a random nature; β > 1 is a failure rate that is increasing over time, associated with failures related to accumulation of damage). It has been suggested that the abutment screw acts as a safety mechanism in protecting the implant and the surrounding structure from bending overload,26 especially in implants with internal connections. In this situation, implants will present a minimum wall thickness at the connecting part, which may be regarded as a weak mechanical link as a result of the high stress distribution and long-term fatigue loading at the implant crest.40 In an attempt to improve the prosthetic system’s overall mechanical performance, a proprietary surface The International Journal of Oral & Maxillofacial Implants 589
© 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
Anchieta et al
Figs 5 and 6 Representative fractured screw after SSALT for a group without coating (MNC). Fig 5a Fractured abutment.
Fig 5b Screw fracture in the threaded region (×60). The white dotted outline shows a compression curl, which is evidence of the fracture origin at the opposing tensile side (white box) indicating the direction of crack propagation (dcp) (white arrow). dcp
a
b
b
a Fig 6a Higher magnification (×400) of the white boxed area shown in Fig 5b, showing the fracture origin.
d
c
Figs 6b and 6c Higher magnifications (×100 and ×2,500, respectively) of the fractured surface showing typical fractographic features of metallic materials as fatigue striations and dimpled surface appearance.
Fig 6d Higher magnification of compression curls area (×1,500).
Figs 7 and 8 Representative fractured screw after SSALT, depicting a group with coating (MC). Fig 7a Fractured abutment (×35).
Fig 7b Screw fracture in the threaded region (×60). The white dotted outline shows a compression curl, evidence of the fracture origin at the opposing tensile side (white box), indicating the direction of crack propagation (dcp) (white arrow).
dcp
b
a
a Fig 8a Higher magnification (×600) of the white boxed area in Fig 7b showing the fracture origin.
b
c
Figs 8b and 8c Higher magnifications (×600 and ×3,000, respectively) of the fractured surface showing typical fractographic features of metallic materials as fatigue striations and a dimpled appearance.
d Fig 8d Higher magnification of compression curls area (×200).
590 Volume 29, Number 3, 2014 © 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
Anchieta et al
treatment was delivered to the Ti-6Al-4V abutment screws of platform-matched and platform-switched groups. The rationale for such treatment is to improve the surface interaction between the screw and abutment and to reduce the galling phenomenon.41 Our results show that the surface treatment resulted in increased reliability when coated screws were used to connect both matching and the more challenging platform-switched abutments versus implants in a platform-matched configuration using a noncoated screw. Reliability was higher for the coated-screw groups than for the noncoated abutment screws, irrespective of platform configuration, for missions at 200 and 300 N. A potential reason for this is that the coating surface treatment improved the screw contact interaction with its matching surface, which therefore resisted unclamping forces during mechanical testing. However, when this observation is considered along with the failure modes, it is interesting to note that, whereas 50% of the noncoated screws fractured during fatigue testing, none of the coated screws tested presented this type of failure in either regular or platform-switched groups. Instead, a combination of abutment and screw fractures (85%) or abutment bending (15%) was observed for the regular platform with the coated abutment screw. On the other hand, the reduction in the abutment-engaging area in the platformswitched group with the coating treatment led to the confinement of all failures to both abutment and screw. Implant fractures were observed only for the regular platform with a noncoated abutment screw. Although the abutment screw of switched platforms has been associated with higher stress levels than abutment screws with regular platforms in implants with internal connections,8 the current results showed that reliability of switched platforms with coated screws (SC) was higher than that of regular platforms with noncoated screws (MNC). Clinically, the association of a switched platform and coated screws seems to confer the biologic benefit of the switched platform and the increased mechanical strength of the coated screw. Because the present study was designed to evaluate the effect of abutment screw surface treatment and different platforms, only one type of implant-abutment connection was used to limit the variables. The same holds true for the prosthesis material, which was metallic. Future studies incorporating other implant-abutment connections are warranted.
CONCLUSION The postulated null hypothesis, that the two types of abutment screws in regular-platform implants and the coated screw in platform-switched implants would
provide the same reliability in anterior single crowns subjected to step-stress-accelerated life testing, was rejected. Coated screws rendered higher reliability levels at 100,000 cycles and at both 200 N and 300 N. Failure modes were also different between groups.
ACKNOWLEDGMENTs The authors reported no conflicts of interest related to this study.
REFERENCES 1. Canullo L, Fedele GR, Iannello G, Jepsen S. Platform switching and marginal bone-level alterations: The results of a randomized-controlled trial. Clin Oral Implants Res 2010;21:115–121. 2. Canullo L, Iannello G, Gotz W. The influence of individual bone patterns on peri-implant bone loss: Preliminary report from a 3-year randomized clinical and histologic trial in patients treated with implants restored with matching-diameter abutments or the platform-switching concept. Int J Oral Maxillofac Implants 2011;26:618–630. 3. Tabata LF, Assunção WG, Barão VAR, de Sousa EA, Gomes EA, Delben JA. Implant platform switching: Biomechanical approach using two-dimensional finite element analysis. J Craniofac Surg 2010;21:182–187. 4. Gurgel-Juarez NC, de Almeida EO, Rocha EP, et al. Regular and platform switching: Bone stress analysis varying implant type. J Prosthodont 2012;21:160–166. 5. Lazzara RJ, Porter SS. Platform switching: A new concept in implant dentistry for controlling postrestorative crestal bone levels. Int J Periodontics Restorative Dent 2006;26:9–17. 6. Atieh MA, Ibrahim HM, Atieh AH. Platform switching for marginal bone preservation around dental implants: A systematic review and meta-analysis. J Periodontol 2010;81:1350–1366. 7. Maeda Y, Miura J, Taki I, Sogo M. Biomechanical analysis on platform switching: Is there any biomechanical rationale? Clin Oral Implants Res 2007;18:581–584. 8. Freitas AC Jr, Rocha EP, Bonfante EA, et al. Biomechanical evaluation of internal and external hexagon platform switched implant-abutment connections: An in vitro laboratory and three-dimensional finite element analysis. Dent Mater 2012;28:e218–228. 9. Freitas AC Jr, Bonfante EA, Rocha EP, Silva NR, Marotta L, Coelho PG. Effect of implant connection and restoration design (screwed vs. cemented) in reliability and failure modes of anterior crowns. Eur J Oral Sci 2011;119:323–330. 10. Stuker RA, Teixeira ER, Beck JC, da Costa NP. Preload and torque removal evaluation of three different abutment screws for single standing implant restorations. J Appl Oral Sci 2008;16:55–58. 11. Prado CJ, Neves FD, Soares CJ, Dantas KA, Dantas TS, Naves LZ. Influence of abutment screw design and surface coating on bending flexural strength of implant set. J Oral Implantol 2012 Jan 17. doi: 10.1563/AAID-JOI-D-11-00116 [epub ahead of print]. 12. Park JK, Choi JU, Jeon YC, Choi KS, Jeong CM. Effects of abutment screw coating on implant preload. J Prosthodont 2010;19:458–464. 13. Vitos L, Larsson K, Johansson B, Hanson A, Hogmark S. An atomistic approach to the initiation mechanism of galling. Compl Mater Sci 2006;37:193–197. 14. Berglundh T, Persson L, Klinge B. A systematic review of the incidence of biological and technical complications in implant dentistry reported in prospective longitudinal studies of at least 5 years. J Clin Periodontol 2002;29(suppl 3):197–212. 15. Sailer I, Philipp A, Zembic A, Pjetursson BE, Hammerle CH, Zwahlen M. A systematic review of the performance of ceramic and metal implant abutments supporting fixed implant reconstructions. Clin Oral Implants Res 2009;20(suppl 4):4–31.
The International Journal of Oral & Maxillofacial Implants 591 © 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
Anchieta et al
16. Jung RE, Zembic A, Pjetursson BE, Zwahlen M, Thoma DS. Systematic review of the survival rate and the incidence of biological, technical, and aesthetic complications of single crowns on implants reported in longitudinal studies with a mean follow-up of 5 years. Clin Oral Implants Res 2012;23(suppl 6):2–21. 17. Kreissl ME, Gerds T, Muche R, Heydecke G, Strub JR. Technical complications of implant-supported fixed partial dentures in partially edentulous cases after an average observation period of 5 years. Clin Oral Implants Res 2007;18:720–726. 18. Goodacre CJ, Bernal G, Rungcharassaeng K, Kan JY. Clinical complications with implants and implant prostheses. J Prosthet Dent 2003;90:121–132. 19. Walia MS, Arora S, Luthra R, Walia PK. Removal of fractured dental implant screw using a new technique: A case report. J Oral Implantol 2012;38:747–750. 20. Freitas AC Jr, Almeida EO, Bonfante EA, Silva NR, Coelho PG. Reliability and failure modes of internal conical dental implant connections. Clin Oral Implants Res 2013;24:197–202. 21. Coelho PG, Silva NR, Bonfante EA, Guess PC, Rekow ED, Thompson VP. Fatigue testing of two porcelain-zirconia all-ceramic crown systems. Dent Mater 2009;25:1122–1127. 22. Martins LM, Bonfante EA, Zavanelli RA, et al. Fatigue reliability of 3 single-unit implant-abutment designs. Implant Dent 2012;21:67–71. 23. Bonfante EA, Coelho PG, Navarro JM Jr, et al. Reliability and failure modes of implant-supported Y-TZP and MCR three-unit bridges. Clin Implant Dent Relat Res 2010;12:235–243. 24. Abernethy RB. The New Weibull Handbook. Reliability & Statistical Analysis for Predicting Life, Safety, Risk, Support Costs, Failures, and Forecasting Warranty Claims, Substantiation and Accelerated Testing, Using Weibull, Log Normal, Crow-AMSAA , Probit, and KaplanMeier Models. North Palm Beach, FL: Dr Robert B. Abernethy, 2006. 25. Manda MG, Psyllaki PP, Tsipas DN, Koidis PT. Observations on an invivo failure of a titanium dental implant/abutment screw system: A case report. J Biomed Mater Res B Appl Biomater 2009;89:264–273. 26. Cehreli MC, Akca K, Iplikcioglu H, Sahin S. Dynamic fatigue resistance of implant-abutment junction in an internally notched Morse-taper oral implant: Influence of abutment design. Clin Implant Dent Relat Res 2004;15:459–465. 27. Piermatti J, Yousef H, Luke A, Mahevich R, Weiner S. An in vitro analysis of implant screw torque loss with external hex and internal connection implant systems. Implant Dent 2006;15:427–435. 28. Bonfante EA, Coelho PG, Guess PC, Thompson VP, Silva NR. Fatigue and damage accumulation of veneer porcelain pressed on Y-TZP. J Dent 2010;38:318–324.
29. Bonfante EA, Rafferty B, Zavanelli RA, et al. Thermal/mechanical simulation and laboratory fatigue testing of an alternative yttria tetragonal zirconia polycrystal core-veneer all-ceramic layered crown design. Eur J Oral Sci 2010;118:202–209. 30. Bonfante EA, Sailer I, Silva NR, Thompson VP, Dianne Rekow E, Coelho PG. Failure modes of Y-TZP crowns at different cusp inclines. J Dent 2010;38:707–712. 31. Bonfante EA, Suzuki M, Lubelski W, et al. Abutment design for implant-supported indirect composite molar crowns: Reliability and fractography. J Prosthodont 2012;21:596–603. 32. Freitas AC Jr, Bonfante EA, Rocha EP, Silva NR, Marotta L, Coelho PG. Effect of implant connection and restoration design (screwed vs. cemented) in reliability and failure modes of anterior crowns. Eur J Oral Sci 2011;119:323–330. 33. Freitas AC Jr, Bonfante EA, Martins LM, Silva NR, Marotta L, Coelho PG. Effect of implant diameter on reliability and failure modes of molar crowns. Int J Prosthodont 2011;24:557–561. 34. Guess PC, Bonfante EA, Silva NR, Coelho PG, Thompson VP. Effect of core design and veneering technique on damage and reliability of Y-TZP-supported crowns. Dent Mater 2013;29:307–316. 35. Guess PC, Zavanelli RA, Silva NR, Bonfante EA, Coelho PG, Thompson VP. Monolithic CAD/CAM lithium disilicate versus veneered Y-TZP crowns: Comparison of failure modes and reliability after fatigue. Int J Prosthodont 2010;23:434–442. 36. Silva NR, Bonfante EA, Zavanelli RA, Thompson VP, Ferencz JL, Coelho PG. Reliability of metalloceramic and zirconia-based ceramic crowns. J Dent Res 2010;89:1051–1056. 37. Silva NR, Coelho PG, Fernandes CA, Navarro JM, Dias RA, Thompson VP. Reliability of one-piece ceramic implant. J Biomed Mater Res B Appl Biomater 2009;88:419–426. 38. Silva NR, de Souza GM, Coelho PG, et al. Effect of water storage time and composite cement thickness on fatigue of a glassceramic trilayer system. J Biomed Mater Res B Appl Biomater 2008;84:117–123. 39. Almeida EO, Freitas AC Jr, Bonfante EA, Rocha EP, Silva NR, Coelho PG. Effect of microthread presence and restoration design (screw versus cemented) in dental implant reliability and failure modes. Clin Oral Implants Res 2013;24:191–196. 40. Maeda Y, Satoh T, Sogo M. In vitro differences of stress concentrations for internal and external hex implant-abutment connections: A short communication. J Oral Rehabil 2006;33:75–78. 41. Karlsson P, Eriksson J, Gåård A, Krakhmalev P, Olsson M, Bergström J. Galling resistance evaluation of tool steels by two different laboratory test methods for sheet metal forming. Lubric Sci 24: 263–72.
592 Volume 29, Number 3, 2014 © 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
Department of Dental Materials and Prosthodontics, Universidade Estadual Paulista, Araçatuba Dental School, Araçatuba, SP, Brazil; Department of Biomaterials and Biomimetics, New York University, College of Dentistry, New York, New York. 2 Assistant Professor, Department of Prosthodontics, University of São Paulo, Bauru College of Dentistry, Bauru, SP, Brazil. 3Assistant Professor, Department of Biomaterials and Biomimetics, New York University, College of Dentistry, New York, New York. 4 Assistant Professor, Postgraduate Program in Dentistry, Universidade Potiguar, Department of Health Sciences, aNatal, Rio Grande do Norte, Brazil. 5Associate Professor, Department of Biomaterials and Biomimetics, New York University, College of Dentistry, New York, New York; Director for Research, Department of Periodontology and Implant Dentistry, New York University College of Dentistry, New York, New York. Correspondence to: Paulo G. Coelho, Department of Biomaterials and Biomimetics, New York University, 345 E. 24th Street, 10010, New York, NY. Fax: +212-995-4244. Email: [email protected] ©2014 by Quintessence Publishing Co Inc.
T
o optimize biomechanical stress distribution and consequently decrease the amount of bone and soft tissue loss during the years after implant placement, the platform-switching concept has been used frequently.1–4 The platform-switched configuration appears to contribute to bone preservation by moving the gap between the implant and the abutment away from the marginal bone. Platform switching is characterized by the placement of smaller-diameter prosthetic components on wider-diameter implants.5 Such a configuration seems to preserve the bone height and soft tissue levels as the result of better stress distribution along the length of the implant body.4,6 It has been suggested that the degree of marginal bone resorption appears to be inversely related to the extent of the implant-abutment mismatch.6 However, in spite of bone and soft tissue preservation, implants with platform-switched restorations are more susceptible to higher stress concentrations in the abutment7 and prosthetic screw.7 It has been reported that the failure location is related to the The International Journal of Oral & Maxillofacial Implants 585
© 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
Anchieta et al
design characteristics of the implant-abutment system, which is commonly located in the threaded region or areas that represent a critical point for prosthetic component endurance because of the shift in geometry along the length of the implant-abutment assembly and subtle alterations in cross-sectional area.8 In addition, whereas this setup is beneficial for bone maintenance, the higher the level of mismatch, the more likely the integrity of the prosthetic components, especially the abutment screws of external-connection implants, will be challenged.8 Because oblique loading is resisted mainly by the abutment screw, with a small amount of stress being dissipated by the connection, early screw fracture or loosening may be anticipated. Because the connecting screw has been regarded as the weakest link at the implant-abutment interface joint,9 potential alternatives to improve the biomechanical loading capabilities of the platform-switched unit have been explored. Different component alloys,10 component geometric designs,11 and abutment screw surface treatments11,12 have all been suggested. The rationale for abutment screw surface treatment resides in attempts to improve its fit and mechanical behavior as mechanical movements occur between parts. Such an interaction between two metal surfaces sliding against each other in dry contact generates the galling phenomenon, in which, during loading, the softer material adheres locally to the opposite surface, forming hard “galls” that improve the overall mechanical strength of the system.13 In general, metal coating improves the mechanical behavior of bolted joints through high hardness, strength, and the ability to generate low friction to improving wear resistance and friction level.13 Clinical studies have shown that abutment screw loosening represents the most common technical complication with implant-supported prostheses, averaging 8.8%,14–16 and that abutment or prosthesis screw fracture represents around 4% of complications in a 5-year period,17,18 which may demand extra time for repair and/or maintenance. Because one potential source of screw fracture is undetected screw loosening,19 improvements in the mechanisms of abutment screw retention are likely to result in an overall mechanical advantage for implantsupported prostheses. The aim of the present study was to evaluate the reliability and failure modes of anatomically designed implant-supported single crowns restored with two different types of abutment screws (coated and noncoated) under step-stress accelerated life testing (SSALT). The postulated null hypothesis was that there would be no differences in reliability between matching implant-abutment platforms secured with coated and noncoated screws and a platform-switched abutment secured with a coated screw.
MATERIALS AND METHODS Experimental Design
Two different abutment platforms (regular [V-IAA02] and switched [IAA02], Intra-Lock International) were used to restore single-unit implants with internal-connection geometry and flat-top surface (Blossom DT, 4.0 mm, Intra-Lock) and two different abutment screws (coated [HRS2] and noncoated [HRS], Intra-Lock International) as follows: MNC = matching platform tightened with a noncoated abutment screw (n = 18), MC = matching platform tightened with a coated abutment screw (n = 18), and SC = switched platform tightened with a coated abutment screw (n = 18) (Fig 1). All abutment screws were made of titanium-aluminum-vanadium alloy (Ti-6Al-4V) and are standard commercial screws. The geometry and surface of the coated screws were identical to those of the noncoated screws except for the proprietary surface treatment applied to the former.
Screw Morphology and Chemical Characterization
Scanning electron microscope (SEM) images (S-3500N, Hitachi) of the two types of screws were acquired at ×100 and ×10,000. To evaluate the chemical composition of the two types of screw surfaces, x-ray photoelectron spectroscopy (XPS) (PHI Quantum 2000) was used. XPS data are quantified using relative sensitivity factors and a model that assumes a homogenous layer. The analysis volume is the product of the analysis area (spot size or aperture size) and the depth of information. Photoelectrons are generated within the x-ray penetration depth (typically many microns), but only the photoelectrons within the top three photoelectron escape depths are detected. Escape depths are on the order of 15 to 35 Å, which leads to an analysis depth of about 50 to 100 Å. Typically, 95% of the signal originates from within this depth. The x-ray source utilized was a monochromatic aluminum-potassium alpha 1,486.6 eV at a ± 23-degree acceptance angle and a take-off angle of 45 degrees. An analysis area 50 µm in diameter was utilized, and charge correction for C1s (284 eV) was used.
Sample Preparation for Mechanical Testing
Implants were connected to their respective abutments and then vertically embedded in polymethyl methacrylate resin (Orthodontic Resin, Dentsply Caulk), with 1 mm below the implant-abutment finishing line left exposed (Fig 1). Groups were restored with standardized central incisor metallic crowns (cobaltchromium alloy, Wirobond 280, BEGO; n = 63), which were cemented (Rely X Unicem, 3M ESPE) to the abutments. The crowns were identical and milled from the
586 Volume 29, Number 3, 2014 © 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
Anchieta et al
Fig 1a The implants and abutments were tightened. The pieces were mounted in a polyvinylchloride tube and polymethyl methacrylate resin was added, with the resin level left 1 mm below the implant-abutment junction.
Fig 1b The samples were mounted in the SSALT apparatus at a 30-degree axial inclination.
same stereolithographic file onto a five-axis mill. The machined commercial abutments, made with Ti-6Al4V (Internal Connection System, Intra-Lock International), were tightened with a torque gauge according to the manufacturer’s instructions at 35 Ncm.
Mechanical Testing and Reliability Analysis
To test the implant-abutment connection in a challenging scenario, mechanical testing was undertaken with all specimens placed at a 30-degree axial inclination, per International Organization for Standardization guideline 14801:2007 (Fig 1). The indenter remained in contact with the crown surface during SSALT in an attempt to provide a bending component during loading. SSALT profiles were based on the mean load to failure of previous studies9,20–23 (Fig 1). The profiles were designated as mild, moderate, and aggressive, with the number of specimens assigned to each group in the ratio 3:2:1, respectively. Therefore, of the 18 samples in each group, nine were allocated for mild, six for moderate, and three for the aggressive profile. Mild, moderate, or aggressive profiles refer to the increasingly stepwise rapidity in which a specimen is fatigued to reach a certain level of load, meaning that specimens assigned to a mild profile will be cycled longer to reach the same load of a specimen assigned to a moderate or aggressive profile.23 The rationale for using at least three profiles for this type of testing was based on the need to distribute failure across different step loads and allow better prediction statistics, narrowing confidence bounds. The prescribed fatigue method was SSALT under water at 9 Hz with a servo–all-electric system (Test Resources 800L). The specimens were evaluated under light microscopy at the completion of each block of fatigue cycling. Criteria used for failure were bending or fracture
Fig 1c Approximated view of the indenter contacting the crown during SSALT.
of the abutment screw, partial or complete fracture of the abutment, and fracture of the implant. Based upon the step-stress distribution of the failures (number of performed cycles and load in Newtons), use-level probability Weibull curves (probability of failure vs cycles) with use stress of 300 N at 90% twosided confidence intervals were calculated and plotted (Alta Pro 7, ReliaSoft) using a power law relationship for damage accumulation. For the mission reliability (100,000 cycles at 200 N and 300 N) and β parameters calculated in the present study, the 90% confidence interval ranges were calculated as follows: IC = E(G) ± Zαsqrt(Var(G)), where IC is the confidence bound, E(G) is the mean estimated reliability for the mission calculated from Weibull statistics, Zα is the Z value concerning the given IC level of significance, and Var(G) is the value calculated by the Fisher Information matrix.24,25
Failure Analysis
The failed samples were inspected under a polarized light microscope (MZ-APO stereomicroscope, Carl Zeiss MicroImaging) and classified according to the proposed failure criteria for comparisons between groups. To identify fractographic markings and to characterize the origin of failure and direction of propagation, the most representative failed samples of each group were inspected under SEM (S-3500N, Hitachi)20,25 (Fig 1).
RESULTS Figure 2 shows the morphology of the noncoated and coated abutment screws before testing. Typical machining marks can be observed in the noncoated
The International Journal of Oral & Maxillofacial Implants 587 © 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
Anchieta et al
a
b
d
c
Fig 2 (a and c) SEM images of noncoated screw (×100 and ×10,000, respectively). (b and d) Images of coated screw (×100 and ×10,000, respectively).
Table 1 Atomic Concentrations (in %) Screw type
C
O
Na
Al
Si
P
Noncoated
9.3
67.7
–
1.9
0.4
1.0
Coated
5.9
68.6
0.6
1.4
11.5
1.1
K – < 0.1
Ca
Ti
V
?
19.1
0.7
0.2
10.7
–
Numbers are normalized to 100% of the elements detected. XPS does not detect H or He. – = element not detected; < = accurate quantification cannot be made due to weak signal intensity; ? = species may be present at or near the detection limit of the technique.
Reliability (%)
100 90 80 70 60 50 40 30 20 10 0
MNC
MC
Reliability 200 N
SC Reliability 300 N
Fig 3 Calculated reliability and 90% confidence bounds for 200 N and 300 N at 100,000 cycles.
screw, whereas the coated screw presents a plastically deformed textured surface. The XPS analysis showed that both samples were composed primarily of titanium (Ti), oxygen (O), and carbon (C). The coated screw surface also contained high levels of silicon (Si); in contrast, only trace levels of Si were detected on the noncoated sample. The two samples also differed in that the Ti and C levels were notably lower on the coated samples. Several additional elements were detected in low to trace levels on one or both of the samples; these included sodium, aluminum, phosphorus, potassium, calcium, and vanadium (Table 1). The high levels of Si were observed on the coated sample primarily as SiO2. Ti was found as TiO2 on both samples. The noncoated samples also contained trace levels of metallic Ti (TiO). The fact that metallic Ti is observed indicates that the TiO2 film thickness is thinner than the XPS information depth (~60 to
80 Å) on this sample. Trace levels of V were only detected on the noncoated sample (as VOx). Phosphorus was found as phosphate and aluminum was found as Al2O3 on both samples. Trace levels of calcium, sodium, and potassium were detected on the coated samples. These elements do not exhibit significant shifts in binding energy with bonding state. For both screw types, C was observed primarily as hydrocarbon (C-C, C-H) with lower levels of oxidized carbon forms. At the low level, the observed C is likely nearly entirely attributable to adventitious C (ie, C adsorbed from atmospheric exposure, packaging, and handling). O was observed over a range of binding energies, attributable to metal oxides and organic species on the “machined only” sample. On the coated samples, a large fraction of O was also observed as SiO2 (Table 1). All specimens failed during SSALT, and collected data included the number of cycles, load, and profile of the specimen. The SSALT reliability calculation depicted mean β values (confidence interval range) and associated upper and lower bounds derived from use-level probability Weibull calculations: 2.05 (1.26 to 3.32), 1.54 (0.68 to 3.49), and 1.41 (0.52 to 3.85) for groups MNC, MC, and SC, respectively, indicating that fatigue accelerated the failure of all groups. The calculated reliability (probability of the implantsupported incisor operating for a given amount of time without failure) with 90% confidence intervals for a mission of 100,000 cycles at 200 N showed that the cumulative damage from loads reaching 200 N would lead to 86% survival of MNC, 93% of MC, and 92% of SC. Simulations at 300 N estimated a 6% survival rate for MNC and approximately 25% for MC and SC (Fig 3). A use-level probability Weibull plot for the probability of failure as a function of number of cycles (time) given a mission of 100,000 cycles at 300 N is presented in Fig 4. The β values suggest that overall failures were influenced by fatigue.
588 Volume 29, Number 3, 2014 © 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
Anchieta et al
99.000
Use level MC
Probability of failure
90.000
Cumulative damage Weibull 150 F = 16 | S = 0 Data points Use level line
50.000
Cumulative damage Weibull 150 F = 16 | S = 0 Data points Use level line
MNC
10.000
Cumulative damage Weibull 150 F = 18 | S = 0 Data points Use level line
5.000
1.000 10,000.00
SC
100,000.00
1,000,000.00 1.000E + 7 1.000E + 8
Time MC: Beta = 1.5462; Alpha(0) = 38.5493; Alpha(1) = 4.7752. MNC: Beta = 2.0527; Alpha(0) = 30.8911; Alpha(1) = 3.4822. SC: Beta = 1.4169; Alpha(0) = 39.8513; Alpha(1) = 5.0093.
Fig 4 Use-level probability Weibull for tested groups showing the probability of failure as a function of number of cycles (time) given a mission of 100,000 cycles at 300 N.
Failure Modes All specimens failed after SSALT. When component failures were evaluated together, failures comprised different combinations of abutment screw bending or fracture, abutment fracture, and implant fracture. Observed failure modes are described in Table 2. For all groups, failure predominantly involved abutment and abutment screw fracture. Observation of the polarized light and SEM micrographs of the fractured surface of the abutment screws allowed the consistent identification of fractographic marks, such as compression curl, fatigue striations, fracture origin, and the direction of crack propagation (Figs 5 to 8).
DISCUSSION The abutment screw is a key element that keeps the implant-abutment-prosthesis system assembled and bears occlusal loads; it therefore plays a significant role in the long-term stability of a joint.26 To evaluate the effect of a novel abutment screw surface treatment, implant-supported crowns with or without platformswitched prosthetic components were subjected to accelerated life testing in a challenging scenario. Previous investigations have shown that lower reliability of an implant system may be expected for platformswitched prostheses compared to regular platforms in external-connection systems.8 The SSALT8,9,22,23,26–39 testing results showed that accumulated fatigue damage accelerated the failures for all groups in the present study, as evidenced by the resulting β > 1 (also called the Weibull shape factor). The β value describes failure rate changes over time (β
Table 2 Failure Modes After Mechanical Testing (SSALT) Failure mode Implant fracture Abutment fracture
MNC (%) MC (%) SC (%) 0
0
0
0
0
0
Screw fracture
50
0
0
Screw bending
12.5
0
0
Abutment bending
0
Abutment + screw fractures
18.75
Implant + screw fractures
12.5
Implant + abutment + screw fractures
6.25
15 85
100
0
0
0
0
< 1 indicates that the failure rate is decreasing over time, commonly associated with “early failures” or failures that occur as the result of egregious flaws; β ~ 1 is a failure rate that does not vary over time, associated with failures of a random nature; β > 1 is a failure rate that is increasing over time, associated with failures related to accumulation of damage). It has been suggested that the abutment screw acts as a safety mechanism in protecting the implant and the surrounding structure from bending overload,26 especially in implants with internal connections. In this situation, implants will present a minimum wall thickness at the connecting part, which may be regarded as a weak mechanical link as a result of the high stress distribution and long-term fatigue loading at the implant crest.40 In an attempt to improve the prosthetic system’s overall mechanical performance, a proprietary surface The International Journal of Oral & Maxillofacial Implants 589
© 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
Anchieta et al
Figs 5 and 6 Representative fractured screw after SSALT for a group without coating (MNC). Fig 5a Fractured abutment.
Fig 5b Screw fracture in the threaded region (×60). The white dotted outline shows a compression curl, which is evidence of the fracture origin at the opposing tensile side (white box) indicating the direction of crack propagation (dcp) (white arrow). dcp
a
b
b
a Fig 6a Higher magnification (×400) of the white boxed area shown in Fig 5b, showing the fracture origin.
d
c
Figs 6b and 6c Higher magnifications (×100 and ×2,500, respectively) of the fractured surface showing typical fractographic features of metallic materials as fatigue striations and dimpled surface appearance.
Fig 6d Higher magnification of compression curls area (×1,500).
Figs 7 and 8 Representative fractured screw after SSALT, depicting a group with coating (MC). Fig 7a Fractured abutment (×35).
Fig 7b Screw fracture in the threaded region (×60). The white dotted outline shows a compression curl, evidence of the fracture origin at the opposing tensile side (white box), indicating the direction of crack propagation (dcp) (white arrow).
dcp
b
a
a Fig 8a Higher magnification (×600) of the white boxed area in Fig 7b showing the fracture origin.
b
c
Figs 8b and 8c Higher magnifications (×600 and ×3,000, respectively) of the fractured surface showing typical fractographic features of metallic materials as fatigue striations and a dimpled appearance.
d Fig 8d Higher magnification of compression curls area (×200).
590 Volume 29, Number 3, 2014 © 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
Anchieta et al
treatment was delivered to the Ti-6Al-4V abutment screws of platform-matched and platform-switched groups. The rationale for such treatment is to improve the surface interaction between the screw and abutment and to reduce the galling phenomenon.41 Our results show that the surface treatment resulted in increased reliability when coated screws were used to connect both matching and the more challenging platform-switched abutments versus implants in a platform-matched configuration using a noncoated screw. Reliability was higher for the coated-screw groups than for the noncoated abutment screws, irrespective of platform configuration, for missions at 200 and 300 N. A potential reason for this is that the coating surface treatment improved the screw contact interaction with its matching surface, which therefore resisted unclamping forces during mechanical testing. However, when this observation is considered along with the failure modes, it is interesting to note that, whereas 50% of the noncoated screws fractured during fatigue testing, none of the coated screws tested presented this type of failure in either regular or platform-switched groups. Instead, a combination of abutment and screw fractures (85%) or abutment bending (15%) was observed for the regular platform with the coated abutment screw. On the other hand, the reduction in the abutment-engaging area in the platformswitched group with the coating treatment led to the confinement of all failures to both abutment and screw. Implant fractures were observed only for the regular platform with a noncoated abutment screw. Although the abutment screw of switched platforms has been associated with higher stress levels than abutment screws with regular platforms in implants with internal connections,8 the current results showed that reliability of switched platforms with coated screws (SC) was higher than that of regular platforms with noncoated screws (MNC). Clinically, the association of a switched platform and coated screws seems to confer the biologic benefit of the switched platform and the increased mechanical strength of the coated screw. Because the present study was designed to evaluate the effect of abutment screw surface treatment and different platforms, only one type of implant-abutment connection was used to limit the variables. The same holds true for the prosthesis material, which was metallic. Future studies incorporating other implant-abutment connections are warranted.
CONCLUSION The postulated null hypothesis, that the two types of abutment screws in regular-platform implants and the coated screw in platform-switched implants would
provide the same reliability in anterior single crowns subjected to step-stress-accelerated life testing, was rejected. Coated screws rendered higher reliability levels at 100,000 cycles and at both 200 N and 300 N. Failure modes were also different between groups.
ACKNOWLEDGMENTs The authors reported no conflicts of interest related to this study.
REFERENCES 1. Canullo L, Fedele GR, Iannello G, Jepsen S. Platform switching and marginal bone-level alterations: The results of a randomized-controlled trial. Clin Oral Implants Res 2010;21:115–121. 2. Canullo L, Iannello G, Gotz W. The influence of individual bone patterns on peri-implant bone loss: Preliminary report from a 3-year randomized clinical and histologic trial in patients treated with implants restored with matching-diameter abutments or the platform-switching concept. Int J Oral Maxillofac Implants 2011;26:618–630. 3. Tabata LF, Assunção WG, Barão VAR, de Sousa EA, Gomes EA, Delben JA. Implant platform switching: Biomechanical approach using two-dimensional finite element analysis. J Craniofac Surg 2010;21:182–187. 4. Gurgel-Juarez NC, de Almeida EO, Rocha EP, et al. Regular and platform switching: Bone stress analysis varying implant type. J Prosthodont 2012;21:160–166. 5. Lazzara RJ, Porter SS. Platform switching: A new concept in implant dentistry for controlling postrestorative crestal bone levels. Int J Periodontics Restorative Dent 2006;26:9–17. 6. Atieh MA, Ibrahim HM, Atieh AH. Platform switching for marginal bone preservation around dental implants: A systematic review and meta-analysis. J Periodontol 2010;81:1350–1366. 7. Maeda Y, Miura J, Taki I, Sogo M. Biomechanical analysis on platform switching: Is there any biomechanical rationale? Clin Oral Implants Res 2007;18:581–584. 8. Freitas AC Jr, Rocha EP, Bonfante EA, et al. Biomechanical evaluation of internal and external hexagon platform switched implant-abutment connections: An in vitro laboratory and three-dimensional finite element analysis. Dent Mater 2012;28:e218–228. 9. Freitas AC Jr, Bonfante EA, Rocha EP, Silva NR, Marotta L, Coelho PG. Effect of implant connection and restoration design (screwed vs. cemented) in reliability and failure modes of anterior crowns. Eur J Oral Sci 2011;119:323–330. 10. Stuker RA, Teixeira ER, Beck JC, da Costa NP. Preload and torque removal evaluation of three different abutment screws for single standing implant restorations. J Appl Oral Sci 2008;16:55–58. 11. Prado CJ, Neves FD, Soares CJ, Dantas KA, Dantas TS, Naves LZ. Influence of abutment screw design and surface coating on bending flexural strength of implant set. J Oral Implantol 2012 Jan 17. doi: 10.1563/AAID-JOI-D-11-00116 [epub ahead of print]. 12. Park JK, Choi JU, Jeon YC, Choi KS, Jeong CM. Effects of abutment screw coating on implant preload. J Prosthodont 2010;19:458–464. 13. Vitos L, Larsson K, Johansson B, Hanson A, Hogmark S. An atomistic approach to the initiation mechanism of galling. Compl Mater Sci 2006;37:193–197. 14. Berglundh T, Persson L, Klinge B. A systematic review of the incidence of biological and technical complications in implant dentistry reported in prospective longitudinal studies of at least 5 years. J Clin Periodontol 2002;29(suppl 3):197–212. 15. Sailer I, Philipp A, Zembic A, Pjetursson BE, Hammerle CH, Zwahlen M. A systematic review of the performance of ceramic and metal implant abutments supporting fixed implant reconstructions. Clin Oral Implants Res 2009;20(suppl 4):4–31.
The International Journal of Oral & Maxillofacial Implants 591 © 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
Anchieta et al
16. Jung RE, Zembic A, Pjetursson BE, Zwahlen M, Thoma DS. Systematic review of the survival rate and the incidence of biological, technical, and aesthetic complications of single crowns on implants reported in longitudinal studies with a mean follow-up of 5 years. Clin Oral Implants Res 2012;23(suppl 6):2–21. 17. Kreissl ME, Gerds T, Muche R, Heydecke G, Strub JR. Technical complications of implant-supported fixed partial dentures in partially edentulous cases after an average observation period of 5 years. Clin Oral Implants Res 2007;18:720–726. 18. Goodacre CJ, Bernal G, Rungcharassaeng K, Kan JY. Clinical complications with implants and implant prostheses. J Prosthet Dent 2003;90:121–132. 19. Walia MS, Arora S, Luthra R, Walia PK. Removal of fractured dental implant screw using a new technique: A case report. J Oral Implantol 2012;38:747–750. 20. Freitas AC Jr, Almeida EO, Bonfante EA, Silva NR, Coelho PG. Reliability and failure modes of internal conical dental implant connections. Clin Oral Implants Res 2013;24:197–202. 21. Coelho PG, Silva NR, Bonfante EA, Guess PC, Rekow ED, Thompson VP. Fatigue testing of two porcelain-zirconia all-ceramic crown systems. Dent Mater 2009;25:1122–1127. 22. Martins LM, Bonfante EA, Zavanelli RA, et al. Fatigue reliability of 3 single-unit implant-abutment designs. Implant Dent 2012;21:67–71. 23. Bonfante EA, Coelho PG, Navarro JM Jr, et al. Reliability and failure modes of implant-supported Y-TZP and MCR three-unit bridges. Clin Implant Dent Relat Res 2010;12:235–243. 24. Abernethy RB. The New Weibull Handbook. Reliability & Statistical Analysis for Predicting Life, Safety, Risk, Support Costs, Failures, and Forecasting Warranty Claims, Substantiation and Accelerated Testing, Using Weibull, Log Normal, Crow-AMSAA , Probit, and KaplanMeier Models. North Palm Beach, FL: Dr Robert B. Abernethy, 2006. 25. Manda MG, Psyllaki PP, Tsipas DN, Koidis PT. Observations on an invivo failure of a titanium dental implant/abutment screw system: A case report. J Biomed Mater Res B Appl Biomater 2009;89:264–273. 26. Cehreli MC, Akca K, Iplikcioglu H, Sahin S. Dynamic fatigue resistance of implant-abutment junction in an internally notched Morse-taper oral implant: Influence of abutment design. Clin Implant Dent Relat Res 2004;15:459–465. 27. Piermatti J, Yousef H, Luke A, Mahevich R, Weiner S. An in vitro analysis of implant screw torque loss with external hex and internal connection implant systems. Implant Dent 2006;15:427–435. 28. Bonfante EA, Coelho PG, Guess PC, Thompson VP, Silva NR. Fatigue and damage accumulation of veneer porcelain pressed on Y-TZP. J Dent 2010;38:318–324.
29. Bonfante EA, Rafferty B, Zavanelli RA, et al. Thermal/mechanical simulation and laboratory fatigue testing of an alternative yttria tetragonal zirconia polycrystal core-veneer all-ceramic layered crown design. Eur J Oral Sci 2010;118:202–209. 30. Bonfante EA, Sailer I, Silva NR, Thompson VP, Dianne Rekow E, Coelho PG. Failure modes of Y-TZP crowns at different cusp inclines. J Dent 2010;38:707–712. 31. Bonfante EA, Suzuki M, Lubelski W, et al. Abutment design for implant-supported indirect composite molar crowns: Reliability and fractography. J Prosthodont 2012;21:596–603. 32. Freitas AC Jr, Bonfante EA, Rocha EP, Silva NR, Marotta L, Coelho PG. Effect of implant connection and restoration design (screwed vs. cemented) in reliability and failure modes of anterior crowns. Eur J Oral Sci 2011;119:323–330. 33. Freitas AC Jr, Bonfante EA, Martins LM, Silva NR, Marotta L, Coelho PG. Effect of implant diameter on reliability and failure modes of molar crowns. Int J Prosthodont 2011;24:557–561. 34. Guess PC, Bonfante EA, Silva NR, Coelho PG, Thompson VP. Effect of core design and veneering technique on damage and reliability of Y-TZP-supported crowns. Dent Mater 2013;29:307–316. 35. Guess PC, Zavanelli RA, Silva NR, Bonfante EA, Coelho PG, Thompson VP. Monolithic CAD/CAM lithium disilicate versus veneered Y-TZP crowns: Comparison of failure modes and reliability after fatigue. Int J Prosthodont 2010;23:434–442. 36. Silva NR, Bonfante EA, Zavanelli RA, Thompson VP, Ferencz JL, Coelho PG. Reliability of metalloceramic and zirconia-based ceramic crowns. J Dent Res 2010;89:1051–1056. 37. Silva NR, Coelho PG, Fernandes CA, Navarro JM, Dias RA, Thompson VP. Reliability of one-piece ceramic implant. J Biomed Mater Res B Appl Biomater 2009;88:419–426. 38. Silva NR, de Souza GM, Coelho PG, et al. Effect of water storage time and composite cement thickness on fatigue of a glassceramic trilayer system. J Biomed Mater Res B Appl Biomater 2008;84:117–123. 39. Almeida EO, Freitas AC Jr, Bonfante EA, Rocha EP, Silva NR, Coelho PG. Effect of microthread presence and restoration design (screw versus cemented) in dental implant reliability and failure modes. Clin Oral Implants Res 2013;24:191–196. 40. Maeda Y, Satoh T, Sogo M. In vitro differences of stress concentrations for internal and external hex implant-abutment connections: A short communication. J Oral Rehabil 2006;33:75–78. 41. Karlsson P, Eriksson J, Gåård A, Krakhmalev P, Olsson M, Bergström J. Galling resistance evaluation of tool steels by two different laboratory test methods for sheet metal forming. Lubric Sci 24: 263–72.
592 Volume 29, Number 3, 2014 © 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
