Effects of Implant Diameter and Prosthesis Retention System on the Reliability of Single Crowns Estevam A. Bonfante, DDS, MS, PhD1/Erika O. Almeida, DDS, MS, PhD2/ Fabio C. Lorenzoni, DDS, MS, PhD3/Paulo G. Coelho, DDS, BS, MS, MSMtE, PhD4 Purpose: The probability of survival of implant-supported prostheses may be affected by the interplay between different implant diameters supporting screwed or cemented crowns. The purpose of this study was to investigate the effect of implant diameter and prosthesis retention system on the reliability and failure modes of single crowns. Materials and Methods: Internal-hexagon implants were divided into six groups (n = 21 each) according to implant diameter (3.3, 4.0, or 5.0 mm) and crown retention system (screwed or cemented). Abutments were torqued to the implants, and crowns were then fixed and subjected to step-stress accelerated life testing in water. Use-level probability Weibull curves and reliability for missions of 50,000 cycles at 100, 150, and 200 N were calculated. Failure analysis was performed. Results: Cemented systems presented higher reliability than screwed ones, except between 3.3-mm-diameter cemented and screwed systems at a load of 100 or 150 N. Failure modes were restricted to the abutment screw and varied with implant diameter only in the cement-retained groups. Conclusion: Higher reliability was observed for cementretained crowns and implants of larger diameter compared to screw-retained and smaller diameter. Failure modes differed between groups. Int J Oral Maxillofac Implants 2015;30:95–101. doi: 10.11607/jomi.3545 Key words: dental implant, fatigue, fractography, reliability, step-stress accelerated life testing, Weibull analysis
C
linical studies involving retrospective or prospective follow-up evaluations of dental implant rehabilitations have focused extensively on implant survival, especially on parameters including bone stability and peri-implant soft tissue health.1,2 The performance of different prosthesis systems and especially the mechanical complications related to
1 Assistant
Professor, Department of Prosthodontics, University of São Paulo – Bauru College of Dentistry, Bauru, São Paulo, Brazil. 2 Assistant Professor, Department of Dentistry, Universidade Federal do Rio Grande do Norte, Campus Universitário Lagoa Nova, Natal, Rio Grando do Norte, Brazil. 3Department of Prosthodontics, UNIGRAN University, Dourados, MS, Brazil. 4Department of Biomaterials and Biomimetics, New York University, New York, New York; Director of Research, Department of Periodontology and Implant Dentistry, New York University College of Dentistry, New York, New York, USA; Affiliated Faculty, Division of Engineering, New York University Abu Dhabi, Abu Dhabi, United Arab Emirates. Correspondence to: Dr Estevam A. Bonfante, Department of Prosthodontics, University of São Paulo – Bauru College of Dentistry, Alameda Octávio Pinheiro Brisola 9-75, Bauru, SP, Brasil – 17012-901. Fax: +55-14-3234-2566. Email: [email protected] ©2015 by Quintessence Publishing Co Inc.
the connection components (eg, screw loosening or fracture) and veneering materials have been less explored.3 Recent systematic reviews of full-arch prostheses4 and single crowns5 reported that abutment screw loosening is the most common prosthodontic complication. The calculated cumulative complication rate was 20.8% for full-arch prostheses,4 and the cumulative incidence of screw or abutment loosening was 12.7% after 5 years for single crowns.5 Along with the plethora of implant-abutment connection designs available in the market, such as external hexagon, octagonal, internal cone, and Morse taper, the fixation modes of the implant-supported restoration can be either screwed, cemented, or a combination of both.6 Although these crown retention systems have been widely employed, there has been controversy regarding which design may provide not only a particular set of clinical advantages, but more important, improved mechanical and biologic performance. The rationale for screw retention of restorations relies mainly on retrievability, which prevents the need to undertake complex procedures during prosthesis removal should maintenance, hygiene, repairs, or abutment screw tightening be required.7 On the other hand, cement-retained prostheses have been claimed to overcome the esthetic shortcomings of visible occlusal screw access holes and to improve The International Journal of Oral & Maxillofacial Implants 95
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occlusal contacts.6 In some situations, such as poorly aligned implants or limited occlusal space, cementation may be the only treatment option. Also claimed as an advantage is the more conventional manufacturing process that does not demand an opening on the porcelain and the substructure to enable abutment screw access.6 Disadvantages are that excess cement could lead to peri-implant soft tissue inflammation if not properly removed8 and that retrieval of cementretained restorations may necessitate destruction of a restoration.9,10 Previous research has shown that fatigue reliability is significantly improved by crown cementation,11 even with external-hexagon implant-abutment connections, where most of the stresses are borne by the abutment screw.12 Because an internal-hexagon connection is expected to result in improved reliability given that loading stresses are also dissipated within the implant walls, the additional variable of implant diameter—and consequently implant wall thickness— may also play a role in the reliability and failure mode of implant-supported crowns. As the implant diameter decreases, the integrity of the implant’s internal walls may be challenged by fatigue, potentially resulting in fractures at the cervical area as the chief failure mode, especially when crowns are cemented.11 In addition to the failure modes of implants with different diameters when subjected to step-stress accelerated life testing (SSALT), the role of crown fixation mode (ie, screw or cement retained) remains unclear when such interplay is considered. Therefore, this study was conducted to assess the reliability and failure modes of screw- and cement-retained implant-supported single crowns connected to implants with three different diameters when subjected to SSALT. The research hypotheses tested were: (1) crown fixation mode (screw or cement retained) will not result in different reliability within the same implant diameter; and (2) failure mode will not differ between screw- and cement-retained crowns, regardless of implant diameter.
MATERIALS AND METHODS Sample Preparation
One hundred twenty-six titanium-aluminum-vanadium (Ti-6Al-4V) dental implants with an internalhexagon configuration (Colosso ECI Evolution, Emfils Sistema de Implante Itú, SP, Brazil) were divided into three groups (n = 42 each) according to implant diameter (3.3, 4.0, and 5.0 mm). Each group was then subdivided into another two groups (n = 21 each) according to crown fixation system as follows: group S3, 3.3 mm implant diameter (ECIE 3310) and screwed crown; group C3, 3.3 mm implant diameter (ECIE 3310)
with cemented crown; group S4, 4.0 mm implant diameter (ECIE 4010) with screwed crown; group C4, 4.0 mm implant diameter (ECIE 4010) with cemented crown; group S5, 5.0 mm implant diameter (ECIE 5010) with screwed crown; and group C5, 5.0 mm implant diameter (ECIE 5010) with cemented crown. All implants were embedded vertically in acrylic resin (Orthoresin, Degudent), which was poured into a 25-mm-diameter plastic tube, with the implant platform left at the level of the resin surface. Maxillary central incisor crowns were waxed up and cast in a cobalt-chrome metal alloy (Wirobond 280, BEGO). Impressions were taken of the waxed crowns to standardize the anatomy of both screwed and cemented crowns (n = 21 for each group) throughout the study. The internal surfaces of the crowns were designed to fit the screwed or cemented abutments. The proprietary prefabricated abutments were torqued to the respective implants with a torque gauge according to the manufacturer’s instructions (30 N.cm). Subsequently, the internal surface of each cement-retained crown was sandblasted with 50-µm aluminum oxide (Al2O3) particles, cleaned with 100% ethanol, and dried with oil-free steam. They were then cemented with a self-adhesive dual-curing resin cement (Rely X Unicem, 3M, ESPE) to their respective prefabricated abutments (CPPL 3314, 4014, 5014 Adjustable Standard Abutments, Emfils Sistema de Implante). For the screw-retained crown groups (S3, S4, and S5), the abutments (CPTC 3304, 4004, 5004 Titanium Transmucosal Abutments, Emfils Sistema de Implante) were tightened to their respective implants with a torque gauge (30 N.cm). Then a 10-N.cm torque was applied to tighten the crowns to their respective abutments.
Mechanical Testing
Three specimens of each group underwent single load to fracture (SLF) testing in a 30-degree off-axis loading orientation in a universal testing machine (Model 5666, Instron) equipped with a 10-kN load cell at a crosshead speed of 1 mm/min. The load was applied to the specimens at their incisal edge through a flat tungsten carbide indenter.13–16 The SLF test is conducted so that step-stress fatigue loading profiles can be designed. Based on the mean load to failure from the SLF results, three profiles for SSALT were established for the remaining crowns (n = 18 each group), which were randomly distributed as follows: mild (n = 9), moderate (n = 6), and aggressive (n = 3).13–15,17,18 The SSALT was carried out on a servo–all-electric system (TestResources 800L) underwater at 9 Hz.13 All samples were subjected to fatigue until failure (bending or fracture of the fixation screw and/or of the abutment or implant) or survival (no failure at the end of step-stress profiles, where maximum loads were 500
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N).18–20 The load was applied at the incisal edge of the crowns with a flat tungsten carbide indenter. Use-level probability Weibull curves (probability of failure versus number of cycles) using a cumulative damage and power law relationship were calculated (Alta Pro 7, Reliasoft). The reliability (the probability of an item functioning for a given amount of time without failure) for completion of a mission of 50,000 cycles (using 90% two-sided confidence intervals) was determined for group comparisons. The reliability for the C3 and S3 groups was determined based on loads of 100 N and 150 N. For the remaining groups, loads of 100 N and 200 N were used to estimate the reliability.21 This difference in reliability calculations for only the greater loads (150 N and 200 N) was made on the basis of indications for small-diameter implants, ie, for areas where forces are of lower magnitude. If the use-level probability Weibull calculated beta (β) values were lower than 1 for any group, then a probability Weibull contour plot (Weibull modulus [m] vs characteristic strength [η]) was calculated using final load to failure or survival of groups (95% confidence intervals). Alternatively, Kruskal-Wallis along with Duncan’s multiple-comparison post hoc test at a 95% level of significance was carried out based on the load to failure of all samples tested, also under SSALT.13,15
Failure Analyses
All specimens were first inspected under a polarized light microscope (MZ-APO Stereomicroscope, Carl Zeiss MicroImaging) to analyze and classify the modes of failure. Most representative failed samples were also imaged under a scanning electron microscope (SEM) (Model S-3500N, Hitachi) for qualitative fractography.
RESULTS SLF and Reliability
The SLF mean values (± standard deviations) were 318.99 ± 139.67 N for C3, 489.41 ± 37 N for S3, 642.93 ± 188.88 N for C4, 569.47 ± 26.25 N for S4, 975.13 ± 190.77 N for C5, and 873.97 ± 159.26 N for S5. Abutment screw fracture was the only observed failure mode for all groups after SLF. The plot and summary statistics for step-stress–derived use-level probability Weibull with use stress of 150 N are presented in Fig 1a and Table 1, respectively. The β value means (confidence interval bounds) derived from use-level probability Weibull calculations (probability of failure versus number of cycles) were 0.60 for C3, 3.29 for S3, 1.64 for C4, 0.59 for S4, 2.74 for C5, and 1.59 for S5. The resulting β values indicated that the fatigue was a strong accelerating factor for all groups, except for C3 and S4. The β value describes
failure rate changes over time (β < 1: failure rate is decreasing over time, commonly associated with “early failures” or failures that occur because of egregious flaws; β > 1: failure rate is increasing over time, associated with failures related to damage accumulation; β ∙ 1: failure rate does not vary over time, associated with failures of a random nature). Thus, the resulting β values for C3 and S4 indicated that, regardless of the step-stress level at which samples were fatigued, failures were associated with load (stress level) and that fatigue damage did not appear to accumulate. As two groups showed β values lower than 1, the probability Weibull distribution was determined using fatigue load to failure data of the groups. The probability Weibull distribution presented values of 9.67 for C3, 10.24 for S3, 10.65 for C4, 5.5 for S4, 8.21 for C5, and 9.26 for S5. The highest η was 369.96 N, recorded for group C5. The Weibull parameter contour plot (Fig 1b) presents this information graphically and detects whether these data sets are from different populations (based upon nonoverlap of confidence bounds). The Kruskal-Wallis test showed that there were statistically significant differences between the evaluated groups (P < .0001). Overall, multiple comparisons between groups showed a statistically higher fatigue load to failure for the C5 group than for the other groups, and groups S3, C3, and C4 displayed statistically similar behavior (Fig 2). The step-stress accelerated fatigue technique permits estimates of reliability at a given load level. The calculated reliability (Table 1) with 90% confidence intervals for missions of 50,000 cycles at 100-, 150-, and 200-N loads indicated that the cumulative damage for higher loads and smaller-diameter implant-abutment connections would lead to a lower survival, as well as for larger-diameter connections when screwed crowns were used. When pairwise comparisons between cemented and screwed crowns presenting the same diameter were performed, the cemented systems always presented higher reliability than screwed ones, except for the comparisons between C3 and S3 at a 100- or 150-N load (not statistically different). The calculated reliability was significantly higher for C4 than for S4 at the 100- and 200-N load levels and was also significantly higher for C5 than for S5 at 200 N (not different at 100 N). Conversely, C5 presented very close calculated reliability for load levels of 100 and 200 N.
Failure Modes
All specimens failed after SLF and SSALT tests. Failures comprised abutment screw fracture, regardless of crown fixation system. Failures of screw-retained crowns were mainly observed at the first or second thread, irrespective of implant diameter. The cemented crowns of groups C3 and C4 presented with abutment screw fractures located before the abutment screw The International Journal of Oral & Maxillofacial Implants 97
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CB 90% - two-sided cumulative damage Weibull C3 F = 17 | S = 0 Data points Use level line S3 F = 18 | S = 0 Data points Use level line C4 F = 18 | S = 0 Data points Use level line S4 F = 18 | S = 0 Data points Use level line C5 F = 18 | S = 0 Data points Use level line S5 F = 18 | S = 0 Data points Use level line
Probability of failure
90 50
10 5
1 100
1,000
10,000
100,000
20 Weibull modulus
99
C3 m = 9.67, η = 173.48 P = .94 95% S3 m = 10.24, η = 162.62 P = .95 95% C4 m = 10.65, η = 256.95 P = .92 95% S4 m = 5.50, η = 202.66 P = .98 95% C5 m = 8.21, η = 360.96 P = .93 95% S5 m = 9.26, η = 259.54 P = .88 95%
16 12 8 4 1
1,000,000 1.000E+7
0
100
200
(No. of cycles) Fig 1a Use-level probability Weibull showing probability of failure as a function of time.
Table 1
300
400
500
Strength (η) Fig 1b Contour plot (Weibull modulus vs characteristic strength [η, in N]) for group comparisons. Note that there is no statistical difference where contour plots overlap.
Calculated Reliability for a Given Mission of 50,000 Cycles According to Load C3
Result
100 N
S3 150 N
100 N
C4 150 N
100 N
S4 200 N
100 N
C5 200 N
100 N
S5 200 N
100 N
200 N
Upper bound
0.98
0.57
0.97
0.91
1
0.8
0.97
0.23
1
0.99
0.99
0.8
Reliability
0.95
0.35
0.90
0.72
0.99
0.67
0.92
0.11
1
0.99
0.99
0.67
Lower bound
0.79
0.13
0.65
0.29
0.99
0.49
0.79
0.03
0.99
0.97
0.99
0.50
β
0.60
3.29
1.64
0.59
2.74
1.59
400 341.66***
350 Load (N)
300 245.83** 247.22**
250
C3
C4
C5
S3
S4
S5
200 164.72* 165.29* 187.50* 150 100 50 0
S3
C3
S4
Group
C4
S5
C5
Fig 2 Mean load to failure values of each group. Different numbers of asterisks represent statistically significant differences in between-group comparisons.
came into contact with the internal threads of the implant. Conversely, crowns in the C5 group depicted failures occurring at the head of the abutment screw (fracture), along with bending of both the screw and the antirotational mechanism of the abutment in some samples (Fig 3). No implants fractured during either SLF or SSALT.
Fig 3 Polarized light micrographs showing the main failure modes for each group. Whereas screw-retained crowns presented similar fracture patterns, cemented crowns displayed failures at different loci of the abutment screw, especially in C5, where it chiefly occurred at the head of the abutment screw.
SEM observation of the fractured abutment screws allowed for consistent identification of fractographic marks, including compression curl, beach and ratchet marks, which indicated the origins and nature (fatigue) of the fractures, as well as the direction of crack propagation (Figs 4 to 6).
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Bonfante et al
Compression curl dcp
dcp
dcp dcp
*
**
* Fig 4 Representative SEMs of failed abutment screws of (left column) C3 and (right column) S3 after SSALT. (Top row) Overviews of C3 and S3, showing a ductile fracture with an evident compression curl (black arrows), which suggests a bending component, with its opposite tensile side indicating the fracture origin (from lingual to buccal). The yellow arrows in both images show semielliptic fatigue marks, known as beach marks, delimiting a smoother fracture zone closer to the origin and a coarser zone closer to the compression curl. The white arrows point in the direction of crack propagation (dcp). (Second row) Fatigue striations are shown, confirming the fatigue fracture type. Asterisks represent the slow- and fast-speed failure zones (smoother and coarser areas, respectively). (Third row) Ratchet marks (arrows) indicate the fracture origin. (Bottom row) The dimpled surface appearance (asterisk) caused by the coalescence of microvoids (pointer) during fatigue. Arrows = dcp.
Fig 5 SEMs showing representative fractured abutment screws from (left column) C4 and (right column) S4 after SSALT. (Top row) Overviews of C4 and S4 showing compression curls (black arrows) and dcp (white dotted arrow) from lingual to buccal. (Second row) Fatigue striations (white arrows). (Third row) Ratchet marks (arrows) indicate the fracture origin. They also suggest that multiple failure origins are present. (Bottom row) Micrographs depict the dimpled surface appearance of the fractured surface. Fatigue striations (white arrows) are associated with fatigue crack growth.
DISCUSSION The present study evaluated the reliability and failure modes of screwed and cemented implant-supported incisor crowns under SSALT of implants with different diameters. A general trend for better reliability of cement-retained prostheses on implants with larger diameter was observed. These findings reflected in a difference in failure modes; screw-retained prostheses, failing at lower loads during fatigue testing, presented with abutment screw fractures at the first or second thread,
in contrast to cement-retained crowns, in which higher fatigue loads eventually led to failures at different loci, such as the head of the abutment screw. From a clinical standpoint, failure modes of either group could be considered repairable, since they did not affect the implant’s integrity per se, as observed in previous fatigue11 and clinical studies.22–25 As a consequence, expected long-term maintenance of the investigated system would likely be limited to the prosthetic components. The International Journal of Oral & Maxillofacial Implants 99
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dcp
a
b
c
d
e
f
Traditionally, the clinical decision to choose a screwor cement-retained prostheses has been made on the basis of personal preference7 and clinical restraints, with negligible concern for the longevity of the restoration or its components. Typical advantages of screwretained prostheses are retrievability and marginal integrity, with the disadvantages including the need for ideal implant placement and open screw-access holes that may interfere with occlusion, esthetics, and integrity of the porcelain veneer. Although some of these disadvantages may be overcome with cemented prostheses, cemented restorations also present some shortcomings that should not be overlooked, such as difficult retrievability and the need for extremely careful removal of excess cement.3,26,27 From a mechanical standpoint, the finding concerning higher reliability for cement-retained implant-supported prostheses compared to screw-retained prostheses has been previously reported in fatigue studies of the authors’ group11,13 and was recently confirmed by a clinical study.28 In a split-mouth study, implant-supported screw-retained prostheses placed in the posterior arches and followed for up to 15 years showed remarkably higher complication rates than cemented prostheses in terms of fractures of the porcelain veneering material (38% vs 4%, respectively), abutment screw loosening (32% vs 9%, respectively), mean gingival index (0.48 vs 0.09, respectively), and mean marginal bone loss (1.4 mm vs 0.69 mm, respectively).28 Therefore, cement-retained prostheses are expected to present fewer complications,
Fig 6 Representative failed abutment screws from (left column) C5 and (right column) S5 groups after SSALT. (a) The fracture commonly occurred at the head of the abutment screw; the white dotted arrows show the dcp. (b) The black arrows point to the compression curl, located on the side opposite the fracture origin. (c and d) Higher magnifications of Figs 6a and 6b, respectively, depicting ratchet marks (arrows), which indicate the fracture origin. (e) Before catastrophic fracture, the crack propagation produced a secondary origin (white arrows, ratchet marks), which is located on the opposite side of a compression curl (here, its counterpart is displayed; yellow arrows). (f) The dimpled surface appearance of a ductile fracture.
not only from a mechanical standpoint, but relevant biologic parameters also seem to be less strongly affected over the long term. In an extensive review, the survival of cement-retained single crowns was 93%, compared to 83% for implant-supported screwed crowns for follow-up periods commonly longer than 6 years.29 In addition to fixation mode, different implant diameters influenced both the reliability and characteristic strength; this is shown clearly in the contour plot, in which the smallest implant (3.3-mm-diameter) was located in a significantly different position (nonoverlap between the contours) compared to the intermediate (4.0-mm-diameter) implants, regardless of retention system. Only the 5.0-mm-diameter screwed group overlapped both 4.0-mm-diameter groups as well as the 5.0-mm cemented group, meaning that survival is likely similar within the same populations, with no significant differences between them. On the other hand, unaltered (and the highest) reliability was observed for the 5.0-mm cemented group for simulations when loads were increased from 100 to 200 N, whereas a significant decrease in reliability was observed in the 5.0-mm screw-retained group for the same simulation. Such an observation may be a result of the increased internal wall thickness provided by the 5.0-mm-diameter implants, along with the reduced motion between the components, likely caused by the filling of the spaces between the crown and abutment in the cemented group.
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Although it was not the aim of this study to perform quantitative fractography, a visually larger fast fracture zone was observed in the fractured abutment screws of the S3 group compared to the C3 group, according to the imaging results. Since the reliability and characteristic strength levels were not different between these groups, it is suggested that different fracture kinetics likely occurred for these systems. Another relevant fractographic mark consistently observed in the samples was the beach marks on the cyclically grown portion of the fracture; these are typical of failed metal surfaces subjected to varied load ranges, as occurring in chewing and simulated in SSALT fatigue blocks.30 Since the maximal bite force in the incisor area may vary from 108 to 190 N,31–33 the loads used for reliability calculation fell within the physiologic range, as did the resulting characteristic strength values for all groups.
CONCLUSION The postulated hypothesis that crown fixation modes (screw or cement retention) would not result in different reliability within the same implant diameter was partially accepted. The second hypothesis was rejected, since failure modes differed between screw- and cement-retained crowns, irrespective of implant diameter.
ACKNOWLEDGMENTS The authors reported no conflicts of interest related to this study.
REFERENCES 1. Sherif S, Susarla SM, Hwang JW, Weber HP, Wright RF. Clinician- and patient-reported long-term evaluation of screw- and cementretained implant restorations: A 5-year prospective study. Clin Oral Invest 2011;15:993–999. 2. Bartlett D. Implants for life? A critical review of implant-supported restorations. J Dent 2007;35:768–772. 3. Chaar MS, Att W, Strub JR. Prosthetic outcome of cement-retained implant-supported fixed dental restorations: A systematic review. J Oral Rehabil 2011;38:697–711. 4. Papaspyridakos P, Chen CJ, Chuang SK, Weber HP, Gallucci GO. A systematic review of biologic and technical complications with fixed implant rehabilitations for edentulous patients. Int J Oral Maxillofac Implants 2012;27:102–110. 5. Jung RE, Pjetursson BE, Glauser R, Zembic A, Zwahlen M, Lang NP. A systematic review of the 5-year survival and complication rates of implant-supported single crowns. Clin Oral Implants Res 2008;19:119–130. 6. Michalakis KX, Hirayama H, Garefis PD. Cement-retained versus screw-retained implant restorations: A critical review. Int J Oral Maxillofac Implants 2003;18:719–728. 7. Chee W, Jivraj S. Screw versus cemented implant supported restorations. Br Dent J 2006;201:501–507. 8. Pauletto N, Lahiffe BJ, Walton JN. Complications associated with excess cement around crowns on osseointegrated implants: A clinical report. Int J Oral Maxillofac Implants 1999;14:865–868.
9. Kent DK, Koka S, Froeschle ML. Retention of cemented implantsupported restorations. J Prosthodont 1997;6:193–196. 10. Hebel KS, Gajjar RC. Cement-retained versus screw-retained implant restorations: Achieving optimal occlusion and esthetics in implant dentistry. J Prosthet Dent 1997;77:28–35. 11. 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. 12. Pessoa RS, Muraru L, Junior EM, et al. Influence of implant connection type on the biomechanical environment of immediately placed implants - CT-based nonlinear, three-dimensional finite element analysis. Clin Implant Dent Relat Res 2010;12:219–234. 13. 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 Implant Res 2013;24:191–196. 14. Freitas AC Jr, Bonfante EA, Silva NR, Marotta L, Coelho PG. Effect of implant-abutment connection design on reliability of crowns: Regular vs. horizontal mismatched platform. Clin Oral Implants Res 2012;23:1123–1126. 15. Freitas AC Jr, Almeida EO, Bonfante EA, Silva NR, Coelho PG. Reliability and failure modes of internal conical dental implant connections. Clin Oral Implant Res 2013;24:197–202. 16. 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. 17. Freitas AC Jr, Bonfante EA, Martins LM, Silva NR, Marotta L, Coelho PG. Reliability and failure modes of anterior single-unit implantsupported restorations. Clin Oral Implants Res 2012;23:1005–1011. 18. Nelson W. Accelerated Testing: Statistical Models, Test Plans and Data Analysis. New York: John Wiley & Sons, 2004. 19. Coelho PG, Bonfante EA, Silva NR, Rekow ED, Thompson VP. Laboratory simulation of Y-TZP all-ceramic crown clinical failures. J Dent Res 2009;88:382–386. 20. 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. 21. Abernethy R. The New Weibull Handbook, ed 5. North Palm Beach, FL: Dr Robert B. Abernethy, 2006. 22. Piattelli A, Piattelli M, Scarano A, Montesani L. Light and scanning electron microscopic report of four fractured implants. Int J Oral Maxillofac Implants 1998;13:561–564. 23. Brånemark PI, Hansson BO, Adell R, et al. Osseointegrated implants in the treatment of the edentulous jaw. Experience from a 10-year period. Scand J Plast Reconstr Surg Suppl 1977;16:1–132. 24. Adell R, Lekholm U, Rockler B, Brånemark PI. A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Surg 1981;10:387–416. 25. Velasquez-Plata D, Lutonsky J, Oshida Y, Jones R. A close-up look at an implant fracture: A case report. Int J Periodontics Restorative Dent 2002;22:483–491. 26. Chen CJ, Papaspyridakos P, Guze K, Singh M, Weber HP, Gallucci GO. Effect of misfit of cement-retained implant single crowns on crestal bone changes. Int J Prosthodont 2013;26:135–137. 27. Wilson TG, Jr. The positive relationship between excess cement and peri-implant disease: A prospective clinical endoscopic study. J Periodontol 2009;80:1388–1392. 28. Nissan J, Narobai D, Gross O, Ghelfan O, Chaushu G. Long-term outcome of cemented versus screw-retained implant-supported partial restorations. Int J Oral Maxillofac Implants 2011;26:1102–1107. 29. Weber HP, Sukotjo C. Does the type of implant prosthesis affect outcomes in the partially edentulous patient? Int J Oral Maxillofac Implants 2007;22(suppl):140–172. 30. ASM-Handbook Committee. Fractography ASM Handbook. Materials Park, OH: ASM International, 1987. 31. Helkimo E, Carlsson GE, Helkimo M. Bite force and state of dentition. Acta Odontol Scandinavica 1977;35:297–303. 32. Helkimo E, Ingervall B. Bite force and functional state of the masticatory system in young men. Swed Dent J 1978;2:167–175. 33. Hagberg C. Assessment of bite force: A review. J Craniomandib Disord Facial Oral Pain 1987;1:162–169.
The International Journal of Oral & Maxillofacial Implants 101 © 2015 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.
C
linical studies involving retrospective or prospective follow-up evaluations of dental implant rehabilitations have focused extensively on implant survival, especially on parameters including bone stability and peri-implant soft tissue health.1,2 The performance of different prosthesis systems and especially the mechanical complications related to
1 Assistant
Professor, Department of Prosthodontics, University of São Paulo – Bauru College of Dentistry, Bauru, São Paulo, Brazil. 2 Assistant Professor, Department of Dentistry, Universidade Federal do Rio Grande do Norte, Campus Universitário Lagoa Nova, Natal, Rio Grando do Norte, Brazil. 3Department of Prosthodontics, UNIGRAN University, Dourados, MS, Brazil. 4Department of Biomaterials and Biomimetics, New York University, New York, New York; Director of Research, Department of Periodontology and Implant Dentistry, New York University College of Dentistry, New York, New York, USA; Affiliated Faculty, Division of Engineering, New York University Abu Dhabi, Abu Dhabi, United Arab Emirates. Correspondence to: Dr Estevam A. Bonfante, Department of Prosthodontics, University of São Paulo – Bauru College of Dentistry, Alameda Octávio Pinheiro Brisola 9-75, Bauru, SP, Brasil – 17012-901. Fax: +55-14-3234-2566. Email: [email protected] ©2015 by Quintessence Publishing Co Inc.
the connection components (eg, screw loosening or fracture) and veneering materials have been less explored.3 Recent systematic reviews of full-arch prostheses4 and single crowns5 reported that abutment screw loosening is the most common prosthodontic complication. The calculated cumulative complication rate was 20.8% for full-arch prostheses,4 and the cumulative incidence of screw or abutment loosening was 12.7% after 5 years for single crowns.5 Along with the plethora of implant-abutment connection designs available in the market, such as external hexagon, octagonal, internal cone, and Morse taper, the fixation modes of the implant-supported restoration can be either screwed, cemented, or a combination of both.6 Although these crown retention systems have been widely employed, there has been controversy regarding which design may provide not only a particular set of clinical advantages, but more important, improved mechanical and biologic performance. The rationale for screw retention of restorations relies mainly on retrievability, which prevents the need to undertake complex procedures during prosthesis removal should maintenance, hygiene, repairs, or abutment screw tightening be required.7 On the other hand, cement-retained prostheses have been claimed to overcome the esthetic shortcomings of visible occlusal screw access holes and to improve The International Journal of Oral & Maxillofacial Implants 95
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occlusal contacts.6 In some situations, such as poorly aligned implants or limited occlusal space, cementation may be the only treatment option. Also claimed as an advantage is the more conventional manufacturing process that does not demand an opening on the porcelain and the substructure to enable abutment screw access.6 Disadvantages are that excess cement could lead to peri-implant soft tissue inflammation if not properly removed8 and that retrieval of cementretained restorations may necessitate destruction of a restoration.9,10 Previous research has shown that fatigue reliability is significantly improved by crown cementation,11 even with external-hexagon implant-abutment connections, where most of the stresses are borne by the abutment screw.12 Because an internal-hexagon connection is expected to result in improved reliability given that loading stresses are also dissipated within the implant walls, the additional variable of implant diameter—and consequently implant wall thickness— may also play a role in the reliability and failure mode of implant-supported crowns. As the implant diameter decreases, the integrity of the implant’s internal walls may be challenged by fatigue, potentially resulting in fractures at the cervical area as the chief failure mode, especially when crowns are cemented.11 In addition to the failure modes of implants with different diameters when subjected to step-stress accelerated life testing (SSALT), the role of crown fixation mode (ie, screw or cement retained) remains unclear when such interplay is considered. Therefore, this study was conducted to assess the reliability and failure modes of screw- and cement-retained implant-supported single crowns connected to implants with three different diameters when subjected to SSALT. The research hypotheses tested were: (1) crown fixation mode (screw or cement retained) will not result in different reliability within the same implant diameter; and (2) failure mode will not differ between screw- and cement-retained crowns, regardless of implant diameter.
MATERIALS AND METHODS Sample Preparation
One hundred twenty-six titanium-aluminum-vanadium (Ti-6Al-4V) dental implants with an internalhexagon configuration (Colosso ECI Evolution, Emfils Sistema de Implante Itú, SP, Brazil) were divided into three groups (n = 42 each) according to implant diameter (3.3, 4.0, and 5.0 mm). Each group was then subdivided into another two groups (n = 21 each) according to crown fixation system as follows: group S3, 3.3 mm implant diameter (ECIE 3310) and screwed crown; group C3, 3.3 mm implant diameter (ECIE 3310)
with cemented crown; group S4, 4.0 mm implant diameter (ECIE 4010) with screwed crown; group C4, 4.0 mm implant diameter (ECIE 4010) with cemented crown; group S5, 5.0 mm implant diameter (ECIE 5010) with screwed crown; and group C5, 5.0 mm implant diameter (ECIE 5010) with cemented crown. All implants were embedded vertically in acrylic resin (Orthoresin, Degudent), which was poured into a 25-mm-diameter plastic tube, with the implant platform left at the level of the resin surface. Maxillary central incisor crowns were waxed up and cast in a cobalt-chrome metal alloy (Wirobond 280, BEGO). Impressions were taken of the waxed crowns to standardize the anatomy of both screwed and cemented crowns (n = 21 for each group) throughout the study. The internal surfaces of the crowns were designed to fit the screwed or cemented abutments. The proprietary prefabricated abutments were torqued to the respective implants with a torque gauge according to the manufacturer’s instructions (30 N.cm). Subsequently, the internal surface of each cement-retained crown was sandblasted with 50-µm aluminum oxide (Al2O3) particles, cleaned with 100% ethanol, and dried with oil-free steam. They were then cemented with a self-adhesive dual-curing resin cement (Rely X Unicem, 3M, ESPE) to their respective prefabricated abutments (CPPL 3314, 4014, 5014 Adjustable Standard Abutments, Emfils Sistema de Implante). For the screw-retained crown groups (S3, S4, and S5), the abutments (CPTC 3304, 4004, 5004 Titanium Transmucosal Abutments, Emfils Sistema de Implante) were tightened to their respective implants with a torque gauge (30 N.cm). Then a 10-N.cm torque was applied to tighten the crowns to their respective abutments.
Mechanical Testing
Three specimens of each group underwent single load to fracture (SLF) testing in a 30-degree off-axis loading orientation in a universal testing machine (Model 5666, Instron) equipped with a 10-kN load cell at a crosshead speed of 1 mm/min. The load was applied to the specimens at their incisal edge through a flat tungsten carbide indenter.13–16 The SLF test is conducted so that step-stress fatigue loading profiles can be designed. Based on the mean load to failure from the SLF results, three profiles for SSALT were established for the remaining crowns (n = 18 each group), which were randomly distributed as follows: mild (n = 9), moderate (n = 6), and aggressive (n = 3).13–15,17,18 The SSALT was carried out on a servo–all-electric system (TestResources 800L) underwater at 9 Hz.13 All samples were subjected to fatigue until failure (bending or fracture of the fixation screw and/or of the abutment or implant) or survival (no failure at the end of step-stress profiles, where maximum loads were 500
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N).18–20 The load was applied at the incisal edge of the crowns with a flat tungsten carbide indenter. Use-level probability Weibull curves (probability of failure versus number of cycles) using a cumulative damage and power law relationship were calculated (Alta Pro 7, Reliasoft). The reliability (the probability of an item functioning for a given amount of time without failure) for completion of a mission of 50,000 cycles (using 90% two-sided confidence intervals) was determined for group comparisons. The reliability for the C3 and S3 groups was determined based on loads of 100 N and 150 N. For the remaining groups, loads of 100 N and 200 N were used to estimate the reliability.21 This difference in reliability calculations for only the greater loads (150 N and 200 N) was made on the basis of indications for small-diameter implants, ie, for areas where forces are of lower magnitude. If the use-level probability Weibull calculated beta (β) values were lower than 1 for any group, then a probability Weibull contour plot (Weibull modulus [m] vs characteristic strength [η]) was calculated using final load to failure or survival of groups (95% confidence intervals). Alternatively, Kruskal-Wallis along with Duncan’s multiple-comparison post hoc test at a 95% level of significance was carried out based on the load to failure of all samples tested, also under SSALT.13,15
Failure Analyses
All specimens were first inspected under a polarized light microscope (MZ-APO Stereomicroscope, Carl Zeiss MicroImaging) to analyze and classify the modes of failure. Most representative failed samples were also imaged under a scanning electron microscope (SEM) (Model S-3500N, Hitachi) for qualitative fractography.
RESULTS SLF and Reliability
The SLF mean values (± standard deviations) were 318.99 ± 139.67 N for C3, 489.41 ± 37 N for S3, 642.93 ± 188.88 N for C4, 569.47 ± 26.25 N for S4, 975.13 ± 190.77 N for C5, and 873.97 ± 159.26 N for S5. Abutment screw fracture was the only observed failure mode for all groups after SLF. The plot and summary statistics for step-stress–derived use-level probability Weibull with use stress of 150 N are presented in Fig 1a and Table 1, respectively. The β value means (confidence interval bounds) derived from use-level probability Weibull calculations (probability of failure versus number of cycles) were 0.60 for C3, 3.29 for S3, 1.64 for C4, 0.59 for S4, 2.74 for C5, and 1.59 for S5. The resulting β values indicated that the fatigue was a strong accelerating factor for all groups, except for C3 and S4. The β value describes
failure rate changes over time (β < 1: failure rate is decreasing over time, commonly associated with “early failures” or failures that occur because of egregious flaws; β > 1: failure rate is increasing over time, associated with failures related to damage accumulation; β ∙ 1: failure rate does not vary over time, associated with failures of a random nature). Thus, the resulting β values for C3 and S4 indicated that, regardless of the step-stress level at which samples were fatigued, failures were associated with load (stress level) and that fatigue damage did not appear to accumulate. As two groups showed β values lower than 1, the probability Weibull distribution was determined using fatigue load to failure data of the groups. The probability Weibull distribution presented values of 9.67 for C3, 10.24 for S3, 10.65 for C4, 5.5 for S4, 8.21 for C5, and 9.26 for S5. The highest η was 369.96 N, recorded for group C5. The Weibull parameter contour plot (Fig 1b) presents this information graphically and detects whether these data sets are from different populations (based upon nonoverlap of confidence bounds). The Kruskal-Wallis test showed that there were statistically significant differences between the evaluated groups (P < .0001). Overall, multiple comparisons between groups showed a statistically higher fatigue load to failure for the C5 group than for the other groups, and groups S3, C3, and C4 displayed statistically similar behavior (Fig 2). The step-stress accelerated fatigue technique permits estimates of reliability at a given load level. The calculated reliability (Table 1) with 90% confidence intervals for missions of 50,000 cycles at 100-, 150-, and 200-N loads indicated that the cumulative damage for higher loads and smaller-diameter implant-abutment connections would lead to a lower survival, as well as for larger-diameter connections when screwed crowns were used. When pairwise comparisons between cemented and screwed crowns presenting the same diameter were performed, the cemented systems always presented higher reliability than screwed ones, except for the comparisons between C3 and S3 at a 100- or 150-N load (not statistically different). The calculated reliability was significantly higher for C4 than for S4 at the 100- and 200-N load levels and was also significantly higher for C5 than for S5 at 200 N (not different at 100 N). Conversely, C5 presented very close calculated reliability for load levels of 100 and 200 N.
Failure Modes
All specimens failed after SLF and SSALT tests. Failures comprised abutment screw fracture, regardless of crown fixation system. Failures of screw-retained crowns were mainly observed at the first or second thread, irrespective of implant diameter. The cemented crowns of groups C3 and C4 presented with abutment screw fractures located before the abutment screw The International Journal of Oral & Maxillofacial Implants 97
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Bonfante et al
CB 90% - two-sided cumulative damage Weibull C3 F = 17 | S = 0 Data points Use level line S3 F = 18 | S = 0 Data points Use level line C4 F = 18 | S = 0 Data points Use level line S4 F = 18 | S = 0 Data points Use level line C5 F = 18 | S = 0 Data points Use level line S5 F = 18 | S = 0 Data points Use level line
Probability of failure
90 50
10 5
1 100
1,000
10,000
100,000
20 Weibull modulus
99
C3 m = 9.67, η = 173.48 P = .94 95% S3 m = 10.24, η = 162.62 P = .95 95% C4 m = 10.65, η = 256.95 P = .92 95% S4 m = 5.50, η = 202.66 P = .98 95% C5 m = 8.21, η = 360.96 P = .93 95% S5 m = 9.26, η = 259.54 P = .88 95%
16 12 8 4 1
1,000,000 1.000E+7
0
100
200
(No. of cycles) Fig 1a Use-level probability Weibull showing probability of failure as a function of time.
Table 1
300
400
500
Strength (η) Fig 1b Contour plot (Weibull modulus vs characteristic strength [η, in N]) for group comparisons. Note that there is no statistical difference where contour plots overlap.
Calculated Reliability for a Given Mission of 50,000 Cycles According to Load C3
Result
100 N
S3 150 N
100 N
C4 150 N
100 N
S4 200 N
100 N
C5 200 N
100 N
S5 200 N
100 N
200 N
Upper bound
0.98
0.57
0.97
0.91
1
0.8
0.97
0.23
1
0.99
0.99
0.8
Reliability
0.95
0.35
0.90
0.72
0.99
0.67
0.92
0.11
1
0.99
0.99
0.67
Lower bound
0.79
0.13
0.65
0.29
0.99
0.49
0.79
0.03
0.99
0.97
0.99
0.50
β
0.60
3.29
1.64
0.59
2.74
1.59
400 341.66***
350 Load (N)
300 245.83** 247.22**
250
C3
C4
C5
S3
S4
S5
200 164.72* 165.29* 187.50* 150 100 50 0
S3
C3
S4
Group
C4
S5
C5
Fig 2 Mean load to failure values of each group. Different numbers of asterisks represent statistically significant differences in between-group comparisons.
came into contact with the internal threads of the implant. Conversely, crowns in the C5 group depicted failures occurring at the head of the abutment screw (fracture), along with bending of both the screw and the antirotational mechanism of the abutment in some samples (Fig 3). No implants fractured during either SLF or SSALT.
Fig 3 Polarized light micrographs showing the main failure modes for each group. Whereas screw-retained crowns presented similar fracture patterns, cemented crowns displayed failures at different loci of the abutment screw, especially in C5, where it chiefly occurred at the head of the abutment screw.
SEM observation of the fractured abutment screws allowed for consistent identification of fractographic marks, including compression curl, beach and ratchet marks, which indicated the origins and nature (fatigue) of the fractures, as well as the direction of crack propagation (Figs 4 to 6).
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Bonfante et al
Compression curl dcp
dcp
dcp dcp
*
**
* Fig 4 Representative SEMs of failed abutment screws of (left column) C3 and (right column) S3 after SSALT. (Top row) Overviews of C3 and S3, showing a ductile fracture with an evident compression curl (black arrows), which suggests a bending component, with its opposite tensile side indicating the fracture origin (from lingual to buccal). The yellow arrows in both images show semielliptic fatigue marks, known as beach marks, delimiting a smoother fracture zone closer to the origin and a coarser zone closer to the compression curl. The white arrows point in the direction of crack propagation (dcp). (Second row) Fatigue striations are shown, confirming the fatigue fracture type. Asterisks represent the slow- and fast-speed failure zones (smoother and coarser areas, respectively). (Third row) Ratchet marks (arrows) indicate the fracture origin. (Bottom row) The dimpled surface appearance (asterisk) caused by the coalescence of microvoids (pointer) during fatigue. Arrows = dcp.
Fig 5 SEMs showing representative fractured abutment screws from (left column) C4 and (right column) S4 after SSALT. (Top row) Overviews of C4 and S4 showing compression curls (black arrows) and dcp (white dotted arrow) from lingual to buccal. (Second row) Fatigue striations (white arrows). (Third row) Ratchet marks (arrows) indicate the fracture origin. They also suggest that multiple failure origins are present. (Bottom row) Micrographs depict the dimpled surface appearance of the fractured surface. Fatigue striations (white arrows) are associated with fatigue crack growth.
DISCUSSION The present study evaluated the reliability and failure modes of screwed and cemented implant-supported incisor crowns under SSALT of implants with different diameters. A general trend for better reliability of cement-retained prostheses on implants with larger diameter was observed. These findings reflected in a difference in failure modes; screw-retained prostheses, failing at lower loads during fatigue testing, presented with abutment screw fractures at the first or second thread,
in contrast to cement-retained crowns, in which higher fatigue loads eventually led to failures at different loci, such as the head of the abutment screw. From a clinical standpoint, failure modes of either group could be considered repairable, since they did not affect the implant’s integrity per se, as observed in previous fatigue11 and clinical studies.22–25 As a consequence, expected long-term maintenance of the investigated system would likely be limited to the prosthetic components. The International Journal of Oral & Maxillofacial Implants 99
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dcp
a
b
c
d
e
f
Traditionally, the clinical decision to choose a screwor cement-retained prostheses has been made on the basis of personal preference7 and clinical restraints, with negligible concern for the longevity of the restoration or its components. Typical advantages of screwretained prostheses are retrievability and marginal integrity, with the disadvantages including the need for ideal implant placement and open screw-access holes that may interfere with occlusion, esthetics, and integrity of the porcelain veneer. Although some of these disadvantages may be overcome with cemented prostheses, cemented restorations also present some shortcomings that should not be overlooked, such as difficult retrievability and the need for extremely careful removal of excess cement.3,26,27 From a mechanical standpoint, the finding concerning higher reliability for cement-retained implant-supported prostheses compared to screw-retained prostheses has been previously reported in fatigue studies of the authors’ group11,13 and was recently confirmed by a clinical study.28 In a split-mouth study, implant-supported screw-retained prostheses placed in the posterior arches and followed for up to 15 years showed remarkably higher complication rates than cemented prostheses in terms of fractures of the porcelain veneering material (38% vs 4%, respectively), abutment screw loosening (32% vs 9%, respectively), mean gingival index (0.48 vs 0.09, respectively), and mean marginal bone loss (1.4 mm vs 0.69 mm, respectively).28 Therefore, cement-retained prostheses are expected to present fewer complications,
Fig 6 Representative failed abutment screws from (left column) C5 and (right column) S5 groups after SSALT. (a) The fracture commonly occurred at the head of the abutment screw; the white dotted arrows show the dcp. (b) The black arrows point to the compression curl, located on the side opposite the fracture origin. (c and d) Higher magnifications of Figs 6a and 6b, respectively, depicting ratchet marks (arrows), which indicate the fracture origin. (e) Before catastrophic fracture, the crack propagation produced a secondary origin (white arrows, ratchet marks), which is located on the opposite side of a compression curl (here, its counterpart is displayed; yellow arrows). (f) The dimpled surface appearance of a ductile fracture.
not only from a mechanical standpoint, but relevant biologic parameters also seem to be less strongly affected over the long term. In an extensive review, the survival of cement-retained single crowns was 93%, compared to 83% for implant-supported screwed crowns for follow-up periods commonly longer than 6 years.29 In addition to fixation mode, different implant diameters influenced both the reliability and characteristic strength; this is shown clearly in the contour plot, in which the smallest implant (3.3-mm-diameter) was located in a significantly different position (nonoverlap between the contours) compared to the intermediate (4.0-mm-diameter) implants, regardless of retention system. Only the 5.0-mm-diameter screwed group overlapped both 4.0-mm-diameter groups as well as the 5.0-mm cemented group, meaning that survival is likely similar within the same populations, with no significant differences between them. On the other hand, unaltered (and the highest) reliability was observed for the 5.0-mm cemented group for simulations when loads were increased from 100 to 200 N, whereas a significant decrease in reliability was observed in the 5.0-mm screw-retained group for the same simulation. Such an observation may be a result of the increased internal wall thickness provided by the 5.0-mm-diameter implants, along with the reduced motion between the components, likely caused by the filling of the spaces between the crown and abutment in the cemented group.
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Although it was not the aim of this study to perform quantitative fractography, a visually larger fast fracture zone was observed in the fractured abutment screws of the S3 group compared to the C3 group, according to the imaging results. Since the reliability and characteristic strength levels were not different between these groups, it is suggested that different fracture kinetics likely occurred for these systems. Another relevant fractographic mark consistently observed in the samples was the beach marks on the cyclically grown portion of the fracture; these are typical of failed metal surfaces subjected to varied load ranges, as occurring in chewing and simulated in SSALT fatigue blocks.30 Since the maximal bite force in the incisor area may vary from 108 to 190 N,31–33 the loads used for reliability calculation fell within the physiologic range, as did the resulting characteristic strength values for all groups.
CONCLUSION The postulated hypothesis that crown fixation modes (screw or cement retention) would not result in different reliability within the same implant diameter was partially accepted. The second hypothesis was rejected, since failure modes differed between screw- and cement-retained crowns, irrespective of implant diameter.
ACKNOWLEDGMENTS The authors reported no conflicts of interest related to this study.
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