I
t has been reported that an implant-supported pros-thesis is the first choice of treatment for patients who have lost their teeth because of its good esthetics and high functional success rate.1–4 However, a lack ofpas-sive fit, mechanical failures of prosthetic components and implants, increased marginal openings, and abut-ment screw loosening are potential complications of this modality of therapy.5,6
Torque plays an important role in maintaining the integrity of the implant-abutment interface and in reducing the tendency for screw loosening and in-creased marginal openings when it is correctly
ap-plied with accurate technique. In a recently published systematic review of implant-related complications, Jung et al calculated the cumulative incidence of connection-related complications (screw loosening, 12.7%; screw fracture, 0.35%) during 5 years of clini-cal service.7 The crucial cause of the loosening of the
implant-abutment connection is the loss of preload at the abutment screw and the resulting loosening or fa-tigue failure of the screw material. It has been reported that mechanical complications occur more frequently in single-tooth restorations and in the posterior region than in the anterior region.8,9
Abutment screw loosening, an inconvenient situ-ation for both patients and clinicians,6–10 leads to
implant-abutment joint opening and soft tissue and mechanical complications.11–13 To minimize these
problems, some procedures, such as the addition of a diamondlike carbon (DLC) coating to abutment screws, have been used to reduce the friction at the interface and to improve component sliding, enhancing pre-load and hence reducing the incidence of screw loos-ening.14–16 A DLC film has properties similar to those
of real diamonds, including hardness, wear resistance, and chemical stability. Low friction resistance and ex-1 Graduate Student, Department of Prosthodontics, University
of Taubate, Taubate, São Paulo, Brazil.
2 Associate Professor, Dental Clinic Department, University
Federal of Bahia, Salvador, BA, Brazil.
3 Associate Professor, Department of Prosthodontics, University
of Taubate, Taubate, SP, Brazil.
Correspondence to: Laís Regiane da Silva-Concílio, Rua Expedicionário Ernesto Pereira, 110 - Centro – Taubaté, São Paulo, BRAZIL, 12020-330. Fax: +55-12-3635-4968. Email: regiane1@yahoo.com
Implant-Abutment Interface
Juliana Socas Vanoni Diez, MSc1/Vinicius Carvalho Brigagão, MSc1/
Leonardo Gonçalves Cunha, PhD2/Ana Christina Claro Neves, PhD3/Laís Regiane da Silva-Concílio, PhD3
Purpose: To evaluate the implant-abutment interface area and the abutment screw loosening value when diamondlike carbon (DLC)–coated or titanium screws were used before and after cyclic loading. Materials and Methods: Thirty-six implants were divided into four groups according to the type of connection (external hexagon [EH] or internal hexagon [IH]) and the type of abutment screw (with [EHD/IHD] or without [EHT/IHT] DLC coating). The implants were placed in epoxy resin–glass fiber composite, and crowns cast in a metal alloy were screwed to the implants. The implant-abutment interface was measured before (VG1) and after (VG2) cyclic loading. The removal torque values were recorded. Results: In groups with titanium screws, there was an increase in the implant-abutment interface area from VG1 to VG2, whereas in groups with DLC-coated screws, the interface area was reduced (EHT = 4.49%, IHT = 24.32%, EHD = –1.05%, IHD = –9.95%). In the IHT group only, the implant-abutment interface area showed a statistically significant difference between VG1 and VG2. The Pearson correlation indicated no significant differences among the studied factors, where R = –0.11 for EHT, 0.14 for EHD, 0.07 for IHT, and 0.43 for IHD. Conclusions: The implant-abutment interface areas in groups with an EH connection were larger than those in groups with an IH connection, regardless of the type of screws used. The screw loosening values decreased in all groups after cyclic loading. No correlation between the implant-abutment interface area and the screw loosening value was seen. Int J Oral MaxIllOfac IMplants 2012;27:xxx–xxx.
cellent wear resistance make DLC a superior material for use as a protectant and lubricant.14 DLC films with
various atomic bond structures and compositions are being used in orthopedic, cardiovascular, and dental applications.17
Additionally, with implants expected to serve for decades, fatigue failure may continue to be an issue.18
Over 10 years, the reported success rate for implant-supported single crowns is 89.4%, and the most com-mon failures are fracture of the veneering material and loosening of the abutment screw.19 Moreover,
dy-namic loading forces during physiologic activities that do not exceed the maximum resistance of an implant/ abutment connection, or even those that are far below this threshold, might gradually loosen the implant-abutment connection or cause a sudden failure of this connection as a result of fatigue.18
The purpose of this study was to evaluate the im-plant-abutment interface area and screw loosening values when DLC-coated screws or conventional tita-nium screws were used before and after cyclic loading. The hypothesis was that DLC coating could decrease abutment screw loosening and, consequently, lead to a smaller interface and smaller differences between initial and final torque values.
MATERIALS AND METHODS
Manufacturing of Samples
In all, 36 implants, 36 UCLA-type abutments, 18 con-ventional titanium screws, and 18 DLC-coated screws (Neodent) were used in this study. The samples were divided into four groups of nine specimens each ac-cording to the type of implant connection and screw used: EHT = external hexagon and conventional tita-nium screw, EHD = external hexagon and DLC-coated screw, IHT = internal hexagon and conventional titani-um screw, and IHD = internal hexagon and DLC-coated screw. Table 1 provides additional information about the materials used in this study.
The dental implants were fixed with the vertical rod of a cast surveyor (Bioart) in an epoxy resin–glass fiber com-posite (NEMA grade G-10 rod, Piedmont Plastics). This embedment material has an appropriate elastic modulus for a bone analog material (about 20 GPa) that is easily machined and sufficiently tough for cyclic testing.20
The abutment wax-up was performed on the abut-ments with a standardized height of 8.0 mm without cusps21 (Fig 1) and was then duplicated in a polyvinyl
siloxane matrix. A nickel-chromium-titanium alloy (Tal-ladium) was used to fabricate the crowns. Before the torque procedure, the crowns were marked at four sites using a carbide bur in a high-speed handpiece to iden-tify the areas that would be used for measurement.
Testing and Imaging Protocols
The screws were tightened initially by hand and then with an analog torque meter (Tohnichi BTG60CN), ac-cording to the manufacturer’s specifications: 32 Ncm was used for the EH groups and 20 Ncm was used for the IH groups. In vitro studies have demonstrated that inter-nal connections are more stable mechanically than ex-ternal flat connections22,23; this explains the differences
in recommended tightening torques recommended by the manufacturer. After 10 minutes, the screws were re-tightened to compensate for the initial loss of preload.24
The interfaces were analyzed using a stereoscope (Aus Jena, Carl Zeiss) with 100× magnification and were captured by a digital camera (Nikon D70) at the previously defined four sites. The vertical gaps (VGs) were measured three times (Image J Software, Nation-al Institutes of HeNation-alth), and the means were cNation-alculated (VG1) (Fig 2).
Next, the samples were mounted in a universal test-ing machine (Instron 8800) and subjected to a load of 400 N at a frequency of 8 Hz for 1 million cycles. This was done to simulate 1 year of function.20,25,26
After mechanical cycling, the abutment-implant VGs were measured again (VG2) and means were cal-culated. The crowns were unscrewed, and the removal torque values were measured (RT).
Table 1 Overview of Investigated Materials
Group Connection Screw type Torque Abutment screw diameter Diameter of screw hole
EHT External hex Titanium 32 Ncm 1.75 mm 1.8 mm
EHD External hex DLC-coated 32 Ncm 1.75 mm 1.8 mm
IHT Internal hex Titanium 20 Ncm 1.65 mm 1.6 mm
Statistical Analysis
The values of VG1, VG2, and RT were statistically ana-lyzed by two-way analysis of variance and t test with P ≤ .05 as the level of significance. A possible correlation was determined with r ≤ 0.8 for the Pearson correlation coefficient and P ≤ .05.
RESULTS
Table 2 shows the VG values in each group before and after mechanical cycling (VG1 and VG2), as well as
com-parisons within and between groups. The groups with DLC-coated screws (EHT and IHT) showed increased VG2 values, whereas the groups of titanium screws (EHD and IHD) showed decreased VG2 values.
Table 3 shows values for torque, RT, and the differ-ences between them. The initial torque decreased in all groups versus the baseline values (Table 3), indicating that, initially, the preload was equally compensated among all groups (torque values). With respect to RT values, statistically significant differences were ob-served between all groups except for the IHT group. Fig 1 (left) The implant is fixed and the
abutment is positioned.
Fig 2 (right) Analysis of gap region (red arrow).
Table 2 Values for VG1 and VG2 (in Microns) and Within-Group and Between-Group Differences Group VG1 VG2 VG2 – VG1 % difference EHT 59.81 (10.81)Aa 62.50 (9.95)Aa 2.70 4.49 EHD 73.73 (15.85)Ab 72.95 (13.85)Ab –0.77 –1.05 IHT 18.13 (2.97)Ac 22.54 (7.36)Bc 4.20 24.32 IHD 16.07 (4.97)Ad 14.47 (3.99)Ad –1.60 –9.95
* Means and standard deviations shown. Different uppercase letters indicate within-group differences; lowercase letters indicate between-group differences (t test, P ≤ .05).
Table 3 Values for T and RT (in Ncm) and Within-Group and Between-Group Differences Group T RT* RT – T % difference EHT 32Aa 21.25 (3.28)Ba –10.75 –33.59 EHD 32Aa 18.25 (2.96)Bb –13.75 –42.96 IHT 20Aa 11.25 (4.71)Bcd –8.75 –43.75 IHD 20Aa 13.62 (1.68)Bd –6.37 –31.90
Different uppercase letters indicate within-group differences; lowercase letters indicate between-group differences (t test, P ≤ .05).
*Mean (SD) shown.
With respect to the differences in the mean values of the implant-abutment interface area (VG1 and VG2) and the differences between torque and RT, no tions were seen for any comparison (Pearson correla-tion coefficient, r ≥ 0.8; P ≤ .05) (Table 4).
DISCUSSION
The implant-abutment interface has been reported as a primary factor in stress distribution and in adverse biologic responses and other prosthetic complica-tions.6,7,11,13 A correct fit between an implant and its
abutment is directly related to the precision of manu-facturing procedures and the torque applied to the abutment screws.10 It can be evaluated by the gap
depth, vertical and horizontal misfit, and rotational free-dom.27–30 This study used the vertical gap because it is
the ideal parameter for evaluating cyclic loading, where-as evaluation of the horizontal gap is normally used to determine the precision of casting procedures.27
Several studies have reported different values for the implant-abutment interface area. It is agreed, how-ever, that cast abutments generate the largest gaps, followed by cast-on abutments and machined abut-ments.11,13 There is no perfect fit between implant and
abutment6,31; however, the smaller gap is, the better
the fit is. Marginal misfit between 30 and 200 µm is con-sidered clinically acceptable.32 Machinable abutments
generate gaps around 10 µm33; gaps of 36 to 86 µm
are typical for cast-on abutments and 118.8 µm for cast abutments.27 It is known that gaps can cause problems
because they allow bacterial growth and, consequent-ly, peri-implant soft tissue damage.11–13 In this study,
vertical misfit values were between 59.81 and 73.73 µm for EH assemblies and between 14.47 and 22.54 µm for IH assemblies. The EH groups showed higher values before and after cyclic loading, in agreement with pre-vious studies that evaluated cast-on abutments.27,29,34
Another important factor affecting the passivity of the system is the screw itself. When the screw is first tightened, the contact area between the screw and the implant thread generates a compression force—the preload—which is responsible for keeping the implant and the abutment united.5,9,10,25,35
To minimize screw loosening, previous studies have proposed new techniques and products.9,14,16,21,36,37
The DLC-coated screw film possesses properties similar to those of real diamonds, including hardness, wear re-sistance, and chemical stability. Moreover, low friction resistance and excellent wear resistance make DLC one of the best materials for a protectant and lubricant.14,16
Some studies have shown that the stability of the connection and RT may be influenced by the ini-tial torque,5,35,38 the screw material,12,37 masticatory
strength,25,26,39 and relaxation of the fit. When initial
torque is applied, leveling and accommodation be-tween the screw thread and the implant occur, result-ing in a reduction of preload.5 However, according to
Dixon et al,12 a reduction in the RT value, in relation to
the initial torque applied, is not necessarily harmful if it is not progressive and if the remaining torque is suf-ficient to prevent failure of the connection.
In this study, higher values for torque loss were ob-served in the EHD (42.96%) and IHT (43.75%) groups. The least amount of torque loss was observed in the EHT group (33.59%) and the IHD group (31.9%). Most in vitro studies have demonstrated that internal con-nections are more stable mechanically than external connections.22,23 The general focus is clearly on deep
internal connections, in which the screw takes little or no load and provides intimate contact with the implant walls to resist micromovement.22,23 Thus, we
can assume that the higher RT values seen in the EHD group are a result of the fact that a longer screw was required than for the IHD group. [AU: Have I edited the previous sentence correctly? The original word-ing was unclear to me.]
Table 4 Comparisons of Mean Differences Between VG1 and VG2 and Torque (T) and RT Values
Groups VG2 – VG1 RT – T Pearson r P value
EHT 2.70 –10.75 –0.11 .76
EHD –0.77 –13.75 0.14 .70
IHT 4.20 –8.75 0.07 .85
IHD –1.60 –6.37 0.43 .24
In this study, all groups showed a loss of torque af-ter mechanical cycling, and screw loosening occurred regardless of the type of connection and screw used. With respect to the RT values of the EHT and EHD groups, both of which featured an EH connection, the EHD group, in which DLC-coated screws were used, lost more torque after mechanical testing. Because DLC coating, an engineering process, reduces the coefficient of friction, therefore making the surface smoother,37,40 this effect may account for the observed
significant decrease in RT in the EHD group. In con-trast, with respect to the RT seen in the IH groups, the greater loss of torque was seen in the IHT group, but the difference versus the IHD group was not statistical-ly significant. According to similar studies in the litera-ture, the number of samples is satisfactory; thus, the absence of statistical difference may only be consid-ered a finding. [AU: Please clarify previous sentence. “The absence of statistical difference may only be considered a ________ finding”—what kind of find-ing? Is it a valid or meaningful findfind-ing? Seems like a word is missing here, and the “only” would seem to point to an invalid or incidental finding.]
When comparing the percentages of torque loss ac-cording to the type of connection, the results showed that the EH groups lost about 38.27% of the initial torque and the EH group lost about 37.85%. In a recent study of titanium screws, Barbosa et al41 observed that,
after the initial torque loss of about 50% and succes-sive retightening, the amount of torque lost decreased and tended to stabilize around 25% after the second tightening. The present study did not assess successive tightenings after cyclic loading, but based on the re-sults, the possible stabilization of these values can be validated, contributing to preload maintenance.
The present results indicated no positive correlation between the RT and the mean implant-abutment in-terface area (Pearson coefficient, r ≥ 0.8). This lack of a positive correlation between the interface area and the screw loosening value suggests that the higher RT values observed may be merely a coincidence.
This investigation did not address whether bacteria would penetrate the implant-abutment interface of the systems used in this study and lead to coloniza-tion. However, the tightening of abutment screws at the torque recommended by the manufacturers could help to minimize microgaps and the potentially ad-verse effects of microleakage.
In this study, DLC coating did not reduce the RT value and, consequently, did not reduce the implant-abutment interface area and RT. Therefore, other variables should be explored to improve the under-standing and potential role of DLC-coated screws in the field of implant dentistry.
CONCLUSION
Given the conditions and limitations of this study, the following conclusions can be drawn:
1. The implant-abutment interface areas in assem-blies with external-hexagon implants were larger than those of internal-hexagon assemblies, both before and after cyclic loading.
2. The implant-abutment interface in the internal-hexagon group with titanium screws was enlarged after cyclic loading.
3. There was a loss of torque after cyclic loading in all groups.
4. No correlations were observed between the im-plant-abutment interface area and the removal torque or screw loosening value.
5. Application of a diamondlike carbon coating to abutment screws did not result in a statistically significant decrease in the implant-abutment in-terface area.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the donation of material by Neodent, which allowed this research.
REFERENCES
1. Adell R. Clinical results of osseointegrated implants supporting fixed prostheses in edentulous jaws. J Prosthet Dent 1983;50:251–254. 2. Albrektsson T, Zarb GA, Worthington P, Eriksson A. The long-term
efficacy of currently used implants: A review and proposed criteria of success. Int J Oral Maxillofac Implants 1986;1:11–25.
3. Brånemark P-I, Svensson B, van Steenberghe D. Ten-year survival rates of fixed prostheses on four or six implants ad modum Bråne-mark in full edentulism. Clin Oral Implants Res 1995;6:227–231. 4. Khraisat A, Jebreen SE, Baqain ZH, Smadi L, Bakaeen L,
Abu-Hammad O. Multicenter retrospective study of cement-retained implant-supported anterior partial prostheses: Success and restora-tion evaluarestora-tion. Int J Oral Maxillofac Implants 2008;23:705–708. 5. Breeding LC, Dixon DL, Nelson EW, Tietge JD. Torque required to
loosen single-tooth implant abutment screws before and after simulated function. Int J Prosthodont 1993;6:435–439.
6. Taylor TD. Prosthodontic problems and limitations associated with osseointegration. J Prosthet Dent 1998;79:74–78.
7. 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.
8. Theoharidou A, Petridis HP, Tzannas K, Garefis P. Abutment screw loosening in single implant restorations: A systematic review. Int J Oral Maxillofac Implants 2008;23:681–690.
9. Stüker RA, Teixeira ER, Beck JCP, 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. 10. Binon PP. The role of screws in implant systems. Int J Oral Maxillofac
Implants 1994;9:48–63.
11. Byrne D, Houston F, Cleary R, Claffey N. The fit of cast and prema-chined implant abutments. J Prosthet Dent 1998;80:184–192.
12. Dixon DL, Breeding LC, Sadler JP, McKay ML. Comparison of screw loosening, rotation and deflection among three implant designs. J Prosthet Dent 1995;74:270–278.
13. Jansen VK, Conrads G, Richter EJ. Microbial leakage and marginal fit of the implant-abutment interface. Int J Oral Maxillofac Implants 1997;12:527–540.
14. Kim SK, Lee JB, Koak JY, et al. An abutment screw loosening study of a diamond like carbon-coated CP titanium implant. J Oral Rehabil 2005;32:346–350.
15. Lang LA, Kang B, Wang RF, Lang BR. Finite element analysis to determine implant preload. J Prosthet Dent 2003;90:539–546. 16. Park CI, Choe HC, Chung CH. Effects of surface coating on the screw loosening of dental abutment screws.Metals Mater Int 2004;10:549–553.
17. Roy RK, Lee KR. Biomedical applications of diamond-like carbon coatings: A review. J Biomed Mater Res B Appl Biomater 2007 Oct;83(1):72–84.
18. Steinebruner L, Wolfart S, Ludwig K, Kern M. Implant-abutment interface design affects fatigue and fracture strength of implants. Clin Oral Implants Res 2008;19:1276–1284.
19. Pjetursson BE, Brägger U, Lang NP, Zwahlen M. Comparison of survival and complication rates of tooth-supported fixed dental prostheses (FDPs) and implant-supported FDPs and single crowns (SCs). Clin Oral Implants Res 2007;18:97–113.
20. Lee Cornell K, Karl M, Kelly R. Evaluation of test protocol variables for dental implant fatigue research. Dent Mater 2009;25:1419–1425. 21. Binon PP. Evaluation of the effectiveness of a technique to prevent
screw loosening. J Prosthet Dent 1998;79:430–432.
22. Maeda Y, Satoh T, Sogo M. In vitrodifferences of stress concentra-tions for internal and external hex implant-abutment connecconcentra-tions: A short communication. J Oral Rehabil 2006;33:75–7 8
23. Norton MR. An in vitroevaluation of the strength of an internal conical interface compared to a butt joint interface in implant design. Clin Oral Implants Res. 1997;8:290-8.
24. Byrne D, Jacobs S, O’Connell B, Houston F, Claffey N. Preloads generated with repeated tightening in three types of screws used in dental implant assemblies. J Prosthodont 2006;15:164–171. 25. Cibirka RM, Nelson SK, Lang RB, Rueggeberg FA. Examination of the
implant-abutment interface after fatigue testing. J Prosthet Dent 2001;85:268–275.
26. Karl M, Kelly RJ. Influence of loading frequency on implant failure under cyclic fatigue conditions Dent Mater 2009;25:1426–1432. 27. Kano SC, Binon PP, Curtis DA. A classification system to measure
the implant-abutment microgap. Int J Oral Maxillofac Implants 2007;22:879–885.
28. Kano SC, Binon P, Bonfante G, Curtis DA. Effect of casting proce-dures on screw loosening in UCLA-type abutments. J Prosthodont 2006;15:77–81.
29. Kano SC, Binon PP, Bonfante G, Curtis DA. The effect of casting procedures on rotational misfit in castable abutments. Int J Oral Maxillofac Implants 2007;22:575–579.
30. Vigolo P, Fonzi F, Majzoub Z, Cordioli G. Evaluation of gold-ma-chined UCLA-type abutments and CAD/CAM titanium abutments with hexagonal external connection and with internal connection. Int J Oral Maxillofac Implants 2008;23:247–252.
31. Kan JYK, Rungcharassaeng K, Bohsali K, Goodacre CJ, Lang BR. Clini-cal methods for Evaluating implant framework fit. J Prosthet Dent 1999;8:7–12.
32. Boeckler AF, Stadler A, Setz JM. The significance of marginal gap and overextension measurement in the evaluation of the fit of complete crowns. J Contemp Dent Pract 2005;6:26–37. 33. Coelho AL, Suzuki M, Dibart S, Da Silva N, Coelho PG.
Cross-sec-tional analysis of the implant-abutment interface. J Oral Rehabil 2007;34:508-516.
34. Kano SC, Bonfante G, Hussne R, Siqueira AF. Use of base metal cast-ing alloys for implant framework: Marginal accuracy analysis. J Appl Oral Sci 2004;12:337–343.
35. Weiss EI, Kozak D, Gross MD. Effect of repeated closure on opening torque value in seven abutment-implant systems. J Prosthet Dent 2000;84:194–199.
36. Korioth TWP, Cardoso AC, Versluis A. Effect of washers on reverse torque displacement of dental implant gold retaining screws. J Prosthet Dent 1999;82:312–361.
37. Martin WC, Woody RD, Miller BH, Miller AW. Implant abutment screw rotations and preloads for four different screw materials and surfaces. J Prosthet Dent 2001;86:24–32.
38. Gratton DG, Aquilino AS, Stanford CM. Micromotion and dynamic fatigue properties of the dental implant-abutment interface. J Prosthet Dent 2001;85:47–52.
39. Cantwell A, Hobkirk JA. Preload loss in gold prosthesis-retaining screws as a function of time. Int J Oral Maxillofac Implants 2004;19:124–132.
40. Haack JE, Sakaguchi RL, Sun T, Coffey JP. Determination of preload stress in dental implant screws [abstract 808]. J Dent Res 1994;73:202.
41. Barbosa GS, Silva-Neto JP, Simamoto PC Jr, Neves FD, Mattos Mda G, Ribeiro RF. Evaluation of screw loosening on new abutment screws and after successive tightening. Braz Dent J 2011;22:51–55.
