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Evaluation of Castable and Premachined Metal Base Abutment/Implant Interfaces Before and After Cyclical Load Renato De Mori, DDS, MSc,* Cyntia Ferreira Ribeiro, DDS, MSc,† Laís Regiane da Silva-Concílio, DDS, MSc, PhD,‡ and Ana Christina Claro Neves, DDS, MSc, PhD‡
mplants are an excellent prosthetic option for the rehabilitation of edentulous areas. The more than 90% success rate after 5 years is directly related to the balance between biological and mechanical factors.1 Notwithstanding the high success rates reported in the literature, the lack of passive fit in implant-retained prostheses and gaps between the implant and the prosthetic abutment can increase bacterial colonization,2,3 cause implant or screw fractures, and distribute occlusal loads inadequately, generating bone loss and possible failure of osseointegration.1,4–7 Abutment/implant fitting is directly related to the precision of manufacturing and to the torque applied on abutment screws. Implant components manufactured under high precision result in lower degrees of abutment rotation and smaller gaps at interfaces, and therefore lower bacterial colonization, tissue alterations, and tension on the retaining screws.6–9 Application of torque causes elongation of the screw steps, generating a preload that leads to surface compression and keeps all the parts together.10–12
I
*Professor, Department of Prosthesis, São Leopoldo Mandic College, Vitória, Brazil. †Graduate Student, Department of Prosthodontics, University of Taubate, São Paulo, Brazil. ‡Associate Professor, Department of Prosthodontics, University of Taubate, São Paulo, Brazil.
Reprint requests and correspondence to: Laís Regiane da Silva-Concílio, DDS, MSc, PhD, Rua Expedicionário Ernesto Pereira, 110. Centro – Taubaté, São Paulo 12020-330, Brazil, Phone: 00-55-12-3625-4149, Fax: 00-55-12-3635-4968, E-mail: [email protected] ISSN 1056-6163/14/02302-212 Implant Dentistry Volume 23 Number 2 Copyright © 2014 by Lippincott Williams & Wilkins DOI: 10.1097/ID.0000000000000058
Purpose: The purpose of this study was to compare the vertical fit of the castable abutment/implant and premachined base metal abutment/ implant interface, before and after cyclic loading. Materials and Methods: Ten UCLA abutments were distributed in 2 groups: castable and premachined with a cobalt-chromium metal base. The abutments were cast in cobalt-chromium alloy and were screwed onto implants (3.75 3 13 mm, external hexagon) with a 32 N$cm torque. The vertical gap (in micrometers) at the interface abutment/implant was evaluated by linear microscope 2 times: after torque and after cyclic load (1 million cycles, 400 N). Results: The mean values of the vertical gap at the castable abut-
ment/implant and premachined abutment/implant interfaces before and after cyclic load were 5.33 to 6.64 mm and 7.36 to 8.16 mm, respectively. The mean values of the vertical gap were statistically analyzed (ANOVA and t test, P # 0.05). Vertical gap values of the castable abutment/implant interface were not significantly different from those of the premachined abutment/ implant interface, before (P ¼ 0.154) and after (P ¼ 0.471) cyclical loading. Conclusion: Castable and premachined base metal abutments showed similar gaps at the implant/ abutment interface, both after applying torque and after cyclic loading. (Implant Dent 2014;23:212–217) Key Words: prosthodontics, implant, torque, gap
When this preload is insufficient, the cyclic load generated by chewing reduces the compression, leading to abutment micromovements and creating gaps at the interface.11,13,14 Cibirka et al15 found that even after 5 million cycles, the implant/abutment interface of the NobelBiocare implant system did not present instability or signs of screw loosening. The UCLA-type abutment, which is characterized by its direct fit on the implant’s prosthetic platform, can be totally plastic before being cast or can
be fitted with a metallic band at its base (premachined). The casting process of UCLA-type abutments involves the lost-wax technique, which requires several laboratory steps. These steps can reduce the fit of casting abutments compared to premachined ones.16 Zervas et al17 studied distortion of frameworks during casting, soldering, and simulated porcelain firing, and a significant difference was detected in the distortion during casting and high-fusing soldering; however, these values are clinically insignificant. Different from Zervas et al,17 this
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study aims to evaluate if the prefabricated pillars (premachined base metal abutments) and the decrease of laboratory steps can positively influence the fit. Considering the large clinical application of UCLA-type abutments and the possibility of larger gaps forming during technical procedures, the objective of this study was to compare the misfit of castable and premachined base metal abutments at the implant/abutment interface before and after cyclic loading. The null hypothesis tested in this study is that there would be no difference in misfit at the implant/abutment castable and premachined base metal interfaces before and after cyclic loading.
MATERIALS
Fig. 1. Castable and premachined base metal abutments (UCLA type).
Fig. 2. Implants were attached to a cast surveyor and then partially inserted into epoxy resinglass fiber composite. Two steps of the implant thread were kept out of the resin, simulating a bone resorption of 2.5 mm.
AND
METHODS
Ten implants (3.75 3 13 mm, external hexagon; Revolution Implant; SIN– System Implants, São Paulo, Brazil) and 10 UCLA-type abutments (Revolution Implant; SIN–System Implants) were distributed in 2 groups (n ¼ 5)7,18: plastic abutments (castable) and premachined with a cobalt-chromium base metal (premachined base metal) (Fig. 1). The implants were attached to a cast surveyor (Parallelometer; Bio-Art, São Paulo, Brazil) using an implant screwdriver (Revolution Implant; SIN–System Implants) and then partially inserted into epoxy resin-glass fiber composite (NEMA Grade G-10 rod; Piedmont Plastics, Inc., Charlotte, NC) in a polyvinyl chloride (PVC) tube (½ inch 3 19 mm) (Tigre Ltda., São Paulo, Brazil). This embedment material has an appropriate elastic modulus for a bone analog material (approximately 20 GPa), is easily machined, and is sufficiently tough for cyclic testing.19 Two steps of the implant thread were kept out of the resin, simulating a bone resorption of 2.5 mm1 (Fig. 2). The lateral surfaces of the PVC were planed to ensure that the samples would be at the same position. The abutment wax-up was performed on UCLA-type abutments (plastic cylinder) with a standardized height of 8.0 3 7.5 mm without cusps19 (Fig. 3), with red acrylic resin (Duralay; Reliance Dental Mfg. Co, Worth, IL) and using a silicon mold (Silicon-Fluid; Ultralub Química Ltda., São Paulo, Brazil) for duplicating. Five identical wax crowns were obtained for each group.
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Fig. 3. Abutment wax-up was performed on castable and premachined base metal abutments (UCLA type).
The crowns were included in a phosphate investment mold (Bellavest SH; Bego Dental, Bremen, Germany) and cast in cobalt-chromium alloy (DanCeramalloy; Nihon Shika Kinzoku Co., Ltd., Takaishi, Osaka, Japan) in an induction oven (Neutrodyn Easyti HD; Manfredi Srl., Torino, Italy). All procedures were performed by the same technician, according to the manufacturers’ instructions. Finishing and polishing procedures were performed
using 50-mm aluminum oxide airborne-particle abrasion (Assler, São Paulo, Brazil), aluminum oxide stone (JON, São Paulo, Brazil), and ultrasonic cleaning (Thornton T7; Inpec Eletronics Ltda., São Paulo, Brazil) in distilled water for 10 minutes. The crowns were marked at 4 sites using a carbide bur in a high-speed handpiece to identify the measurement areas. The cast abutments were attached to the implants with a torque of 32 N$cm
(value recommended by the manufacturer of the implant system used in this study) measured with a previously calibrated electronic torque controller device20 (Torque Controller; Nobel Biocare, Gothenberg, Sweden). The measurements were performed with a measure linear microscopedMeasuring Microscope STM (Olympus Co., Tokyo, Japan), with precision of 0.0005 mm and 330 magnification, by a single investigator. Each region previously identified was measured 3 times to reach a mean. Using the eyepiece crosshair reticule as a reference, the vertical misfit was defined as the vertical gap measured from the zero point on a line through the most external point of the implant (not considering the rounding of the outer contour of the implant) and the same area of the abutment (Fig. 4). This procedure was conducted 2 times: after torque and after cyclical load. After the initial measurements, the samples were subjected to 1 million cyclical loads11,14,15,18 using a hydraulic machine Instron 8800 with a load cell of 1 kN capacity (Instron, Grove City, PA).21 The force applied was 400 N22 and a frequency of 15 Hz.17 The number of cycles was recorded by software connected to the universal testing machine. After this, the samples were measured again, following the above-described procedure. The means of the interface gap values of the castable and premachined metal base abutments/implant groups before and after cyclical load were calculated, and the Kolmogorov-Smirnov test was performed to confirm that the means were normally distributed (a ¼ 0.05). For intragroup and intergroup comparisons, the means were statistically analyzed by ANOVA 1-way and Student t test (P , 0.05) using Bioestat software (Bioestat 5.0; Maringá, Paraná, Brazil).
RESULTS
Fig. 4. Gap region was measured in the linear microscope.
The average gap of the samples after torque was 5.33 mm for the castable group and 7.36 mm for the premachined base metal group, whereas the gap after cyclic loading was 6.64 and 8.16 mm for the castable and premachined base metal
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Table 1. Average Gap, SD (in Micrometers), and P-value Between Implant and Abutment After Torque Castable Premachined base metal P
After Cyclic Load
Mean
SD
Mean
SD
P
5.33 Aa 7.36 Aa 0.154
1.60 2.40
6.64 Aa 8.16 Aa 0.471
1.93 4.04
0.231 0.652
Comparison of values before and after cycling is shown statistically using different capital letters in horizontal (t test, P # 0.05), no differences were observed. Comparison of values between the 2 types of abutments used is shown statistically using different lower case letters in vertical (t test, P # 0.05), no differences were observed.
groups, respectively. Table 1 shows descriptive data of the samples. Statistical analysis revealed no statistically significant differences between castable and premachined base metal abutments before and after cyclical loading, in both intragroup and intergroup comparisions.
DISCUSSION The average gap of the samples after torque was 5.33 mm for the castable group and 7.36 mm for the premachined base metal group; whereas the gap after cyclic loading was 6.64 and 8.16 mm for the castable and premachined base metal groups, respectively. Table 1 shows descriptive data of the samples. Statistical analysis revealed no statistically significant differences between castable and premachined base metal abutments before and after cyclical loading, in both intragroup and inter group comparisions. Jemt and Pettersson5 believe that the high screw loosening rate of abutment screws is attributable to their material or to their structural properties. Modifying these properties should allow for application of increased loads during screw fixation and, thus, greater stability of the implant/prosthetic abutment interface. It has been suggested that torque is less in premachined metal abutment than in cast plastic abutment. Casting procedures decrease the percentage of applied torque, which may influence final screw joint stability.23 The force recommended by manufacturers varies according to the type of screw, its design, and diameter. However, to avoid deforming plastic, the maximum stress values allowed during screwing procedures must be lower than the alloy’s strength. The ideal force for every kind of fixation screw is 75% of its alloy resistance.11,12 In this study,
we used an electronic torque controller implementing ideal torque of 32 N$cm as recommended by the manufacturer and according to the materials used for the fabrication of 2 abutments used in this study (castable and premachined), thus developing the ideal tension on the screw and reducing the likelihood of micromovements of the abutments and screw loosening during the cycling load.20 The use of premade components has been highly recommended to improve or even assure a better fit. The high cost of this type of framework has led to the development of plastic components, allowing the use of alternative alloys like base metal alloys. The material used to cast the UCLA abutments and base of premachined abutments in this study was the Co-Cr alloy, and 2 abutments showed similar results (Table 1). This material is widely used in every day practice and offers many advantages. They are relatively inexpensive when compared to gold and offer a favorable combination of biocompatibility, resistance to corrosion, castability, weight, and stiffness.24 This can be used in the abutment of implant-supported prosthodontics, and there are already companies that manufacture premachined abutment base metal with this alloy.23,25–27 Resistance to corrosion results from the adding of chromium, which further improves the resistance to corrosion. Cobalt-chromium alloys have a high modulus of elasticity (approximately 228 MN/m2) and high yield strength (500–700 MN/m2), which makes the frameworks stiff.24 Byrne et al16 studied marginal fitting between implants and prosthetic premachined and castable abutments, before and after casting and ceramic application. Premachined abutments showed better results, whereas castable
abutments were more susceptible to instability and presented a higher tendency to unscrew. The experimental groups subjected to ceramic application after casting showed no significant difference in marginal fitting compared to the other groups. According to the results of these authors, castable abutment fixation screws were subjected to a higher torque than premachined abutment fixation screws. It was hypothesized that the difference in tension generated by this added load should contribute to worsening the fit of castable abutments. The results of this study showed no significant difference between castable and premachined base metal abutments. This result differed from those reported by Byrne et al,16 whose study showed castable abutments presenting worse results than premachined ones. This difference may be due to torque used by the authors in castable abutments (20 N$cm) was lower than that of other groups and this study (32 N$cm). However, our results agree with those found by Hecker et al28 who concluded that differences in fit between the implantsupported prosthesis and the machined abutments were not significantly different when abutments worn through loading were replaced with new as-manufactured abutments. Vigolo et al29 evaluated the changes in the interface implant/prosthetic abutment prefabricated before and after the overcasting procedure of the noble alloy and porcelain application in prefabricated UCLA pillars and found that the original adaptation was not significantly changed during the laboratory processes of casting and porcelain application. These results corroborate the study, which did not observe difference after casting misfit even when they used a non-noble alloy of chromium-cobalt.
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After cyclic loading, the gaps between abutment and implant increased in both groups, although the increases were not statistically significant. This result corroborates earlier findings that cyclic loading by masticatory forces reduces the compression between components. This reduction leads to micromovements of abutments, resulting in inadequate abutment-implant fit.11,13,14 No standard tests for cyclic loading, cyclic frequency, and number of cycles were found in the literature reviewed for this study.11,14,15,18 Masticatory force varies in the oral areas where it was recorded, showing an intensity of 880 N in the molar, 453 N in the premolar, and 222 N in the incisor areas. The maximum bite load limit could be as high as 2440 N.11,30 The works reviewed for this study reported cyclic loads ranging from 20 to 200 N, emphasizing the absence of a standard for this item.7,11,14,15,19,31,32 With regard to implant fatigue resistance, some studies showed that implants with a 4-mm-diameter and a 0.7- and 1-mm external hexagon withstand an average load of 242 and 350 N, respectively. For inside antirotational retentions, the load is even higher, ranging from 367 to 400 N.31 In this study, implants with a 0.7-mm external hexagon withstood a cyclic load of 400 N without showing signs of fracture, demonstrating acceptable fatigue resistance. Lee et al14 and Binon32 reported that cycle frequency varies from 1 to 19 Hz. The number of cycles varies from 50,000 to 5 million cycles.15 Three daily meals, each lasting 15 minutes at 60 cycles per minute (1 Hz), add up to 2700 masticatory cycles per day, or a million cycles per year.11 In this work, applied cyclic loads ranging from 400 N, similar to those the premolar area, is normally used.11,30 This study also applied a million cycles to simulate a year of masticatory function. Because little attention has so far focused on the effect of cyclic frequency on fatigue, this study used 15 Hz, as did Boggan et al.30 Although this study applied higher cyclic loads and a larger number of cycles than those reported in earlier studies,7,13 we found no screw loosening or fractures in implants or other components.
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The findings obtained in this in vitro study support the null hypothesis because the results revealed no statistically significant differences in gap (abutment/implant) between castable and premachined base metal groups before and after cyclical loading. A satisfactory marginal adaptation between implant and abutment depends on several factors, and it is directly related to the success of biological and biomechanical properties of dental implants.22,24,33 The results of this study show that castable and premachined metal base abutment of cobalt-chromium are similar in adaptation even after the mechanical cycling load. Further experiments are required to better evaluate the stability of the implant/abutment interface with a larger number of cycles and with different manufacturers.
CONCLUSIONS Considering the methodology applied and based on results of this study, we concluded that castable and premachined base metal abutments showed similar gaps at the implant/ abutment interface, both after applying torque and after cyclic loading. The differences between the 2 types were not statistically significant.
DISCLOSURE The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
REFERENCES 1. Adell R, Lekholm U, Branemark PI. A 15 year study of osseointegrated implants in the treatment of edentulous jaw. Int J Oral Surg. 1981;10:387–416. 2. Dias EC, Bisognin ED, Harari ND, et al. Evaluation of implant-abutment microgap and bacterial leakage in five externalhex implant systems: An in vitro study. Int J Oral Maxillofac Implants. 2012;27: 346–351. 3. Baldassarri M, Hjerppe J, Romeo D, et al. Marginal accuracy of three implantceramic abutment configurations. Int J Oral Maxillofac Implants. 2012;27:537–543. 4. Jemt T, Laney WR, Harris D, et al. Osseointegrated implants for single tooth replacement: A one year report from
a multicenter prospective study. Int J Oral Maxillofac Implants. 1991;6:29–36. 5. Jemt T, Pettersson PA. A 3-year follow-up study on single implant treatment. J Dent. 1993;21:203–208. 6. Binon PP, Sutter F. The role of screw in implant system. Int J Oral Maxillofac Implants. 1994;9:48–63. 7. Binon PP, Mchugh MJ. The effect of eliminating implant/abutment rotational misfit on screw joint stability. Int J Prosthodont. 1996;9:511–519. 8. Binon PP. Evaluation of machining accuracy and consistency of selected implants, standard abutments, and laboratory analogs. Int J Prosthodont. 1995;8: 162–178. 9. Gross M, Kozak D, Laufer BZ, et al. Manual closing torque in five implant abutment systems: An in vitro comparative study. J Prosthet Dent. 1999;81:574–578. 10. 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. 11. Gratton DG, Aquilino SA, Stanford CM. Micromotion and dynamic fatigue properties of the dental implant-abutment interface. J Prosthet Dent. 2001;85:47–52. 12. Lang LA, Rang B, Wang RF, et al. Finite element analysis to determine implant preload. J Prosthet Dent. 2003; 90:539–546. 13. Hoyer SA, Stanford CM, Buranadham S, et al. Dynamic fatigue properties of the dental implant-abutment interface: Joint opening in wide-diameter versus standard-diameter hex-type implants. J Prosthet Dent. 2001;85:599–607. 14. Lee J, Kim YS, Kim CW, et al. Wave analysis of implant screw loosening using an air cylindrical cyclic loading device. J Prosthet Dent. 2002;88:402–408. 15. Cibirka RM, Nelson SK, Lang BR, et al. Examination of implant abutment interface after fatigue testing. J Prosthet Dent. 2001;85:268–275. 16. Byrne D, Houston F, Cleary R, et al. The fit of cast and premachined implant abutments. J Prosthet Dent 1998;80:184–192. 17. Zervas PJ, Papazoglou E, Beck FM, et al. Distortion of three-unit implant frameworks during casting, soldering, and simulated porcelain firing. J Prosthodont. 1999;8:171–179. 18. Park J-K, Choi JU, Jeon YC, et al. Effects of abutment screw coating on implant preload. J Prosthodont. 2010;19: 458–464. 19. Lee CK, Karl M, Kelly JR. Evaluation of test protocol variable for dental implant fatigue research. Dent Mater. 2009;25:1419–1425. 20. Alikhasi M, Monzavi A, Bassir SH, et al. A comparison of precision of fit,
IMPLANT DENTISTRY / VOLUME 23, NUMBER 2 2014 rotational freedom, and torque loss with copy-milled zirconia and prefabricated titanium abutments. Int J Oral Maxillofac Implants. 2013;28:996–1002. 21. Koren Z, Botstein O, Ustilovsky S. Static and Cycling Loading Tests of Dental Implants. Haifa, IL: Israel Institute of Metal, The Technion Research & Development Foundation, Laboratory Metallurgical; 2006; No.27353 (cited August 28, 2010). Available at: http://www.ardsimplants. com/ardsimplants/files/Type_S_Fatigue_ report.pdf. 22. Jung RE, Pjetursson BE, Glauser R, et al. A systematic review of the 5-year survival and complication rates of implantsupported single crowns. Clin Oral Implants Res. 2008;19:119–130. 23. Kano SC, Binon P, Bonfante G, et al. Effect of casting procedures on screw loosening in UCLA-type abutment. J Prosthodont. 2006;15:77–81. 24. Hulterström M, Nilsson U. Cobaltchromium as a framework material in implant-supported fixed prostheses: A
preliminary report. Int J Oral Maxillofac Implants. 1991;6:475–480. 25. do Nascimento C, Barbosa RES, Issa JPM, et al. Bacterial leakage along the implant–abutment interface of premachined or cast components. Int J Oral Maxillofac Surg. 2008;37:177–180. 26. Kano SC, Bonfante G, Hussne R, et al. Use of base metal casting alloys for implant framework: Marginal accuracy analysis. J Appl Oral Sci. 2004;12:337–343. 27. Moraes LMC, Rossetti PHO, Rossetti LMN, et al. Marginal fit at cylinderabutment interface before and after overcasting procedure. J Appl Oral Sci. 2005; 13:366–371. 28. Hecker DM, Eckert SE, Choi YG. Cyclic loading of implant-supported prostheses: Comparison of gaps at the prostheticabutment interface when cycled abutments are replaced with as-manufactured abutments. J Prosthet Dent. 2006; 95:26–32. 29. Vigolo P, Majzoub Z, Cordioli G. Measurement of the dimensions and
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abutment rotational freedom of goldmachined 3i UCLA-type abutments in the as-received condition, after casting with a noble metal alloy and porcelain firing. J Prosthet Dent. 2000;84:548–553. 30. Boggan RS, Strong JT, Misch GE, et al. Influence of hex geometry and prosthetic table width on static and fatigue strength of dental implants. J Prosthet Dent. 1999;82:436–440. 31. Binon PP. Evaluation of the effectiveness of a technique to prevent screw loosening. J Prosthet Dent. 1998;79: 430–432. 32. Binon PP. The effect of implant/ abutment hexagonal misfit on screw joint stability. Int J Prosthodont. 1996;9: 149–160. 33. Chowdhary R, Halldin A, Jimbo R, et al. Evaluation of stress pattern generated through various thread designs of dental implants loaded in a condition of immediately after placement and on osseointegrationdAn FEA study. Implant Dent. 2013;22:91–96.
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Evaluation of Castable and Premachined Metal Base Abutment/Implant Interfaces Before and After Cyclical Load Renato De Mori, DDS, MSc,* Cyntia Ferreira Ribeiro, DDS, MSc,† Laís Regiane da Silva-Concílio, DDS, MSc, PhD,‡ and Ana Christina Claro Neves, DDS, MSc, PhD‡
mplants are an excellent prosthetic option for the rehabilitation of edentulous areas. The more than 90% success rate after 5 years is directly related to the balance between biological and mechanical factors.1 Notwithstanding the high success rates reported in the literature, the lack of passive fit in implant-retained prostheses and gaps between the implant and the prosthetic abutment can increase bacterial colonization,2,3 cause implant or screw fractures, and distribute occlusal loads inadequately, generating bone loss and possible failure of osseointegration.1,4–7 Abutment/implant fitting is directly related to the precision of manufacturing and to the torque applied on abutment screws. Implant components manufactured under high precision result in lower degrees of abutment rotation and smaller gaps at interfaces, and therefore lower bacterial colonization, tissue alterations, and tension on the retaining screws.6–9 Application of torque causes elongation of the screw steps, generating a preload that leads to surface compression and keeps all the parts together.10–12
I
*Professor, Department of Prosthesis, São Leopoldo Mandic College, Vitória, Brazil. †Graduate Student, Department of Prosthodontics, University of Taubate, São Paulo, Brazil. ‡Associate Professor, Department of Prosthodontics, University of Taubate, São Paulo, Brazil.
Reprint requests and correspondence to: Laís Regiane da Silva-Concílio, DDS, MSc, PhD, Rua Expedicionário Ernesto Pereira, 110. Centro – Taubaté, São Paulo 12020-330, Brazil, Phone: 00-55-12-3625-4149, Fax: 00-55-12-3635-4968, E-mail: [email protected] ISSN 1056-6163/14/02302-212 Implant Dentistry Volume 23 Number 2 Copyright © 2014 by Lippincott Williams & Wilkins DOI: 10.1097/ID.0000000000000058
Purpose: The purpose of this study was to compare the vertical fit of the castable abutment/implant and premachined base metal abutment/ implant interface, before and after cyclic loading. Materials and Methods: Ten UCLA abutments were distributed in 2 groups: castable and premachined with a cobalt-chromium metal base. The abutments were cast in cobalt-chromium alloy and were screwed onto implants (3.75 3 13 mm, external hexagon) with a 32 N$cm torque. The vertical gap (in micrometers) at the interface abutment/implant was evaluated by linear microscope 2 times: after torque and after cyclic load (1 million cycles, 400 N). Results: The mean values of the vertical gap at the castable abut-
ment/implant and premachined abutment/implant interfaces before and after cyclic load were 5.33 to 6.64 mm and 7.36 to 8.16 mm, respectively. The mean values of the vertical gap were statistically analyzed (ANOVA and t test, P # 0.05). Vertical gap values of the castable abutment/implant interface were not significantly different from those of the premachined abutment/ implant interface, before (P ¼ 0.154) and after (P ¼ 0.471) cyclical loading. Conclusion: Castable and premachined base metal abutments showed similar gaps at the implant/ abutment interface, both after applying torque and after cyclic loading. (Implant Dent 2014;23:212–217) Key Words: prosthodontics, implant, torque, gap
When this preload is insufficient, the cyclic load generated by chewing reduces the compression, leading to abutment micromovements and creating gaps at the interface.11,13,14 Cibirka et al15 found that even after 5 million cycles, the implant/abutment interface of the NobelBiocare implant system did not present instability or signs of screw loosening. The UCLA-type abutment, which is characterized by its direct fit on the implant’s prosthetic platform, can be totally plastic before being cast or can
be fitted with a metallic band at its base (premachined). The casting process of UCLA-type abutments involves the lost-wax technique, which requires several laboratory steps. These steps can reduce the fit of casting abutments compared to premachined ones.16 Zervas et al17 studied distortion of frameworks during casting, soldering, and simulated porcelain firing, and a significant difference was detected in the distortion during casting and high-fusing soldering; however, these values are clinically insignificant. Different from Zervas et al,17 this
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study aims to evaluate if the prefabricated pillars (premachined base metal abutments) and the decrease of laboratory steps can positively influence the fit. Considering the large clinical application of UCLA-type abutments and the possibility of larger gaps forming during technical procedures, the objective of this study was to compare the misfit of castable and premachined base metal abutments at the implant/abutment interface before and after cyclic loading. The null hypothesis tested in this study is that there would be no difference in misfit at the implant/abutment castable and premachined base metal interfaces before and after cyclic loading.
MATERIALS
Fig. 1. Castable and premachined base metal abutments (UCLA type).
Fig. 2. Implants were attached to a cast surveyor and then partially inserted into epoxy resinglass fiber composite. Two steps of the implant thread were kept out of the resin, simulating a bone resorption of 2.5 mm.
AND
METHODS
Ten implants (3.75 3 13 mm, external hexagon; Revolution Implant; SIN– System Implants, São Paulo, Brazil) and 10 UCLA-type abutments (Revolution Implant; SIN–System Implants) were distributed in 2 groups (n ¼ 5)7,18: plastic abutments (castable) and premachined with a cobalt-chromium base metal (premachined base metal) (Fig. 1). The implants were attached to a cast surveyor (Parallelometer; Bio-Art, São Paulo, Brazil) using an implant screwdriver (Revolution Implant; SIN–System Implants) and then partially inserted into epoxy resin-glass fiber composite (NEMA Grade G-10 rod; Piedmont Plastics, Inc., Charlotte, NC) in a polyvinyl chloride (PVC) tube (½ inch 3 19 mm) (Tigre Ltda., São Paulo, Brazil). This embedment material has an appropriate elastic modulus for a bone analog material (approximately 20 GPa), is easily machined, and is sufficiently tough for cyclic testing.19 Two steps of the implant thread were kept out of the resin, simulating a bone resorption of 2.5 mm1 (Fig. 2). The lateral surfaces of the PVC were planed to ensure that the samples would be at the same position. The abutment wax-up was performed on UCLA-type abutments (plastic cylinder) with a standardized height of 8.0 3 7.5 mm without cusps19 (Fig. 3), with red acrylic resin (Duralay; Reliance Dental Mfg. Co, Worth, IL) and using a silicon mold (Silicon-Fluid; Ultralub Química Ltda., São Paulo, Brazil) for duplicating. Five identical wax crowns were obtained for each group.
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Fig. 3. Abutment wax-up was performed on castable and premachined base metal abutments (UCLA type).
The crowns were included in a phosphate investment mold (Bellavest SH; Bego Dental, Bremen, Germany) and cast in cobalt-chromium alloy (DanCeramalloy; Nihon Shika Kinzoku Co., Ltd., Takaishi, Osaka, Japan) in an induction oven (Neutrodyn Easyti HD; Manfredi Srl., Torino, Italy). All procedures were performed by the same technician, according to the manufacturers’ instructions. Finishing and polishing procedures were performed
using 50-mm aluminum oxide airborne-particle abrasion (Assler, São Paulo, Brazil), aluminum oxide stone (JON, São Paulo, Brazil), and ultrasonic cleaning (Thornton T7; Inpec Eletronics Ltda., São Paulo, Brazil) in distilled water for 10 minutes. The crowns were marked at 4 sites using a carbide bur in a high-speed handpiece to identify the measurement areas. The cast abutments were attached to the implants with a torque of 32 N$cm
(value recommended by the manufacturer of the implant system used in this study) measured with a previously calibrated electronic torque controller device20 (Torque Controller; Nobel Biocare, Gothenberg, Sweden). The measurements were performed with a measure linear microscopedMeasuring Microscope STM (Olympus Co., Tokyo, Japan), with precision of 0.0005 mm and 330 magnification, by a single investigator. Each region previously identified was measured 3 times to reach a mean. Using the eyepiece crosshair reticule as a reference, the vertical misfit was defined as the vertical gap measured from the zero point on a line through the most external point of the implant (not considering the rounding of the outer contour of the implant) and the same area of the abutment (Fig. 4). This procedure was conducted 2 times: after torque and after cyclical load. After the initial measurements, the samples were subjected to 1 million cyclical loads11,14,15,18 using a hydraulic machine Instron 8800 with a load cell of 1 kN capacity (Instron, Grove City, PA).21 The force applied was 400 N22 and a frequency of 15 Hz.17 The number of cycles was recorded by software connected to the universal testing machine. After this, the samples were measured again, following the above-described procedure. The means of the interface gap values of the castable and premachined metal base abutments/implant groups before and after cyclical load were calculated, and the Kolmogorov-Smirnov test was performed to confirm that the means were normally distributed (a ¼ 0.05). For intragroup and intergroup comparisons, the means were statistically analyzed by ANOVA 1-way and Student t test (P , 0.05) using Bioestat software (Bioestat 5.0; Maringá, Paraná, Brazil).
RESULTS
Fig. 4. Gap region was measured in the linear microscope.
The average gap of the samples after torque was 5.33 mm for the castable group and 7.36 mm for the premachined base metal group, whereas the gap after cyclic loading was 6.64 and 8.16 mm for the castable and premachined base metal
IMPLANT DENTISTRY / VOLUME 23, NUMBER 2 2014
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Table 1. Average Gap, SD (in Micrometers), and P-value Between Implant and Abutment After Torque Castable Premachined base metal P
After Cyclic Load
Mean
SD
Mean
SD
P
5.33 Aa 7.36 Aa 0.154
1.60 2.40
6.64 Aa 8.16 Aa 0.471
1.93 4.04
0.231 0.652
Comparison of values before and after cycling is shown statistically using different capital letters in horizontal (t test, P # 0.05), no differences were observed. Comparison of values between the 2 types of abutments used is shown statistically using different lower case letters in vertical (t test, P # 0.05), no differences were observed.
groups, respectively. Table 1 shows descriptive data of the samples. Statistical analysis revealed no statistically significant differences between castable and premachined base metal abutments before and after cyclical loading, in both intragroup and intergroup comparisions.
DISCUSSION The average gap of the samples after torque was 5.33 mm for the castable group and 7.36 mm for the premachined base metal group; whereas the gap after cyclic loading was 6.64 and 8.16 mm for the castable and premachined base metal groups, respectively. Table 1 shows descriptive data of the samples. Statistical analysis revealed no statistically significant differences between castable and premachined base metal abutments before and after cyclical loading, in both intragroup and inter group comparisions. Jemt and Pettersson5 believe that the high screw loosening rate of abutment screws is attributable to their material or to their structural properties. Modifying these properties should allow for application of increased loads during screw fixation and, thus, greater stability of the implant/prosthetic abutment interface. It has been suggested that torque is less in premachined metal abutment than in cast plastic abutment. Casting procedures decrease the percentage of applied torque, which may influence final screw joint stability.23 The force recommended by manufacturers varies according to the type of screw, its design, and diameter. However, to avoid deforming plastic, the maximum stress values allowed during screwing procedures must be lower than the alloy’s strength. The ideal force for every kind of fixation screw is 75% of its alloy resistance.11,12 In this study,
we used an electronic torque controller implementing ideal torque of 32 N$cm as recommended by the manufacturer and according to the materials used for the fabrication of 2 abutments used in this study (castable and premachined), thus developing the ideal tension on the screw and reducing the likelihood of micromovements of the abutments and screw loosening during the cycling load.20 The use of premade components has been highly recommended to improve or even assure a better fit. The high cost of this type of framework has led to the development of plastic components, allowing the use of alternative alloys like base metal alloys. The material used to cast the UCLA abutments and base of premachined abutments in this study was the Co-Cr alloy, and 2 abutments showed similar results (Table 1). This material is widely used in every day practice and offers many advantages. They are relatively inexpensive when compared to gold and offer a favorable combination of biocompatibility, resistance to corrosion, castability, weight, and stiffness.24 This can be used in the abutment of implant-supported prosthodontics, and there are already companies that manufacture premachined abutment base metal with this alloy.23,25–27 Resistance to corrosion results from the adding of chromium, which further improves the resistance to corrosion. Cobalt-chromium alloys have a high modulus of elasticity (approximately 228 MN/m2) and high yield strength (500–700 MN/m2), which makes the frameworks stiff.24 Byrne et al16 studied marginal fitting between implants and prosthetic premachined and castable abutments, before and after casting and ceramic application. Premachined abutments showed better results, whereas castable
abutments were more susceptible to instability and presented a higher tendency to unscrew. The experimental groups subjected to ceramic application after casting showed no significant difference in marginal fitting compared to the other groups. According to the results of these authors, castable abutment fixation screws were subjected to a higher torque than premachined abutment fixation screws. It was hypothesized that the difference in tension generated by this added load should contribute to worsening the fit of castable abutments. The results of this study showed no significant difference between castable and premachined base metal abutments. This result differed from those reported by Byrne et al,16 whose study showed castable abutments presenting worse results than premachined ones. This difference may be due to torque used by the authors in castable abutments (20 N$cm) was lower than that of other groups and this study (32 N$cm). However, our results agree with those found by Hecker et al28 who concluded that differences in fit between the implantsupported prosthesis and the machined abutments were not significantly different when abutments worn through loading were replaced with new as-manufactured abutments. Vigolo et al29 evaluated the changes in the interface implant/prosthetic abutment prefabricated before and after the overcasting procedure of the noble alloy and porcelain application in prefabricated UCLA pillars and found that the original adaptation was not significantly changed during the laboratory processes of casting and porcelain application. These results corroborate the study, which did not observe difference after casting misfit even when they used a non-noble alloy of chromium-cobalt.
216
EVALUATION
OF
ABUTMENT/IMPLANT INTERFACES
After cyclic loading, the gaps between abutment and implant increased in both groups, although the increases were not statistically significant. This result corroborates earlier findings that cyclic loading by masticatory forces reduces the compression between components. This reduction leads to micromovements of abutments, resulting in inadequate abutment-implant fit.11,13,14 No standard tests for cyclic loading, cyclic frequency, and number of cycles were found in the literature reviewed for this study.11,14,15,18 Masticatory force varies in the oral areas where it was recorded, showing an intensity of 880 N in the molar, 453 N in the premolar, and 222 N in the incisor areas. The maximum bite load limit could be as high as 2440 N.11,30 The works reviewed for this study reported cyclic loads ranging from 20 to 200 N, emphasizing the absence of a standard for this item.7,11,14,15,19,31,32 With regard to implant fatigue resistance, some studies showed that implants with a 4-mm-diameter and a 0.7- and 1-mm external hexagon withstand an average load of 242 and 350 N, respectively. For inside antirotational retentions, the load is even higher, ranging from 367 to 400 N.31 In this study, implants with a 0.7-mm external hexagon withstood a cyclic load of 400 N without showing signs of fracture, demonstrating acceptable fatigue resistance. Lee et al14 and Binon32 reported that cycle frequency varies from 1 to 19 Hz. The number of cycles varies from 50,000 to 5 million cycles.15 Three daily meals, each lasting 15 minutes at 60 cycles per minute (1 Hz), add up to 2700 masticatory cycles per day, or a million cycles per year.11 In this work, applied cyclic loads ranging from 400 N, similar to those the premolar area, is normally used.11,30 This study also applied a million cycles to simulate a year of masticatory function. Because little attention has so far focused on the effect of cyclic frequency on fatigue, this study used 15 Hz, as did Boggan et al.30 Although this study applied higher cyclic loads and a larger number of cycles than those reported in earlier studies,7,13 we found no screw loosening or fractures in implants or other components.
DE MORI
ET AL
The findings obtained in this in vitro study support the null hypothesis because the results revealed no statistically significant differences in gap (abutment/implant) between castable and premachined base metal groups before and after cyclical loading. A satisfactory marginal adaptation between implant and abutment depends on several factors, and it is directly related to the success of biological and biomechanical properties of dental implants.22,24,33 The results of this study show that castable and premachined metal base abutment of cobalt-chromium are similar in adaptation even after the mechanical cycling load. Further experiments are required to better evaluate the stability of the implant/abutment interface with a larger number of cycles and with different manufacturers.
CONCLUSIONS Considering the methodology applied and based on results of this study, we concluded that castable and premachined base metal abutments showed similar gaps at the implant/ abutment interface, both after applying torque and after cyclic loading. The differences between the 2 types were not statistically significant.
DISCLOSURE The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
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