Vertical Microgap and Passivity of Fit of Three-Unit Implant-Supported Frameworks Fabricated Using Different Techniques Halina Massignan Berejuk, DDS, MSc1/Roberto Hideo Shimizu, DDS, MSc, PhD2/ Ivete Aparecida de Mattias Sartori, DDS, MSc, PhD3/ Laiz Valgas, BEng, MSc, PhD4/Rodrigo Tiossi, DDS, MSc, PhD5 Purpose: This study compared the vertical microgaps of milled zirconia and cobalt-chromium (Co-Cr) implantsupported fixed dental prosthesis frameworks to those seen in one-piece frameworks cast with different techniques. Materials and Methods: Two threaded implants were used to simulate the rehabilitation of a maxillary partially edentulous space from the second premolar to the second molar. Three-unit screwretained prosthetic frameworks were fabricated and divided into the following groups (n = 10 in each group): 1 = cast in Co-Cr using burnout cylinders; 2 = cast in Co-Cr using cast-on Co-Cr cylinders; 3 = onepiece cast in Co-Cr using a passive fitting technique; 4 = milled Co-Cr framework; 5 = milled zirconia framework. The microgap was measured under an optical microscope at ×25. Readings were made with one screw tightened and with both screws tightened. Data were submitted to statistical analysis to enable comparison between groups (α = .05). Results: All frameworks presented microgaps less than 70 µm under all reading conditions, except for group 1 with the readings made on the side opposite the tightened side (124.22 µm). With one screw tightened, the microgap on the tightened side was significantly smaller in groups 2, 3, and 4. On the opposite side, groups 3, 4, and 5 presented significantly smaller microgaps compared to the other groups. When both screws were tightened, the microgap was significantly smaller in groups 2, 3, and 4. Conclusion: One-piece cast frameworks associated with a passive fitting technique and milled Co-Cr frameworks presented smaller microgaps than the other fabrication methods and materials that were tested. Int J Oral Maxillofac Implants 2014;29:1064–1070. doi: 10.11607/jomi.3421 Key words: computer-aided design, computer-assisted manufacture, dental implants, implant-supported dental prosthesis, prosthetic fit, prosthodontics, screw retention
I
mplant-supported oral rehabilitations aim to restore function without compromising adjacent teeth and the supporting tissues. Important factors in the success of implant-supported restorations include the fit between the framework and the implants or prosthetic 1Former
Student, Latin American Institute for Dental Research and Education, Curitiba, PR, Brazil. 2Professor, Latin American Institute for Dental Research and Education, Curitiba, PR, Brazil. 3Director and Professor, Latin American Institute for Dental Research and Education, Curitiba, PR, Brazil. 4Engineer, Neodent, Curitiba, PR, Brazil. 5Assistant Professor, Fluminense Federal University, Niterói, RJ, Brazil. Correspondence to: Rodrigo Tiossi, Fluminense Federal University, School of Dentistry, Department of Prosthodontics (MOT). Rua Mário Santos Braga, 28, Campus do Valonguinho, Centro, Niterói, RJ, Brazil. 24020-140. Fax: +55 21 2629-9901. Email: [email protected] ©2014 by Quintessence Publishing Co Inc.
abutments, a well-adjusted occlusion, and biocompatible materials.1–3 A poorly fitting prosthesis framework may induce tensile, compressive, and bending forces when the prosthesis is connected, possibly leading to mechanical complications such as loosening or fracture of the prosthetic screws.1,4–6 The microgaps between the framework and abutments can also be colonized by bacteria, which may influence the remodeling of the peri-implant crestal bone and the long-term health of the peri-implant tissues.7 Different techniques to fabricate implant-supported restorations have been used successfully for more than 30 years.8,9 Currently, several methods and materials are available to fabricate implant-supported frameworks. It was previously reported10 that a 10-µm misfit with the abutments would be the maximum misfit recommended to allow bone remodeling and maturation in response to the occlusal loads. However, it is difficult to achieve such small microgaps.11,12 Also, other studies showed that a greater misfit would not necessarily lead to biologic complications.13
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Berejuk et al
Fig 1 Waxed three-unit FDP framework.
Fig 2 Silicone impression to standardize the waxing of the specimens.
Each step needed for the fabrication of implantsupported fixed dental prosthesis (FDP) frameworks introduces a certain degree of inaccuracy.14 One-piece castings have shown poorly fitting frameworks, and sectioning followed by welding is recommended to improve fit.9,15,16 In the attempt to achieve passive fit of one-piece cast frameworks, a passive fitting technique for framework fabrication has been proposed.17–19 In such technique, the framework is waxed over a brass cylinder that is dimensionally larger than the titanium cylinder, over which the cast framework is then luted with resin cement. Recently, a well-developed industrialized engineering approach by means of computer-aided design and computer-assisted manufacture (CAD/CAM) was introduced to minimize distortion and casting inaccuracies.14 CAD/CAM metal-based frameworks have shown better fit compared to cast gold20,21 and cast silver-palladium frameworks.22 The constant demand for highly esthetic restorations has led to the research and development of framework materials with improved optical properties. Since their introduction, zirconia-based restorations have been widely researched and have shown sufficient mechanical strength for use in the stress-bearing posterior regions.23 Zirconia has the advantages of being esthetic, highly biocompatible, and unsupportive of plaque accumulation.14,24 It has a flexural strength of 900 to 1,400 MPa and fracture toughness of up to 10 MPa/m0.5.14,25 However, there are differences in milling metal and zirconia frameworks. Most CAD/CAM systems use “soft” machining to mill an oversized presintered zirconia block to compensate for the 25% shrinkage that occurs after sintering.14,26 The extent of the sintering shrinkage demands that the CAD software reliably calculate the framework size to the required dimensions after the sintering process, which could lead to prosthesis misfit. Metal-based milled frameworks do not go through any additional process after their fabrication, thus minimizing distortion. The “hard” machining of fully sintered zirconia blocks could eliminate the shrinkage distortion of zirconia.
However, it was previously shown to contain a significant amount of monoclinic zirconia, which is usually associated with surface microcracking, higher susceptibility to low temperature degradation, and lower reliability.27,28 This study compared milled zirconia and cobaltchromium (Co-Cr) frameworks to one-piece frameworks cast with different techniques. The one-piece casting techniques compared were: casting plastic burnout cylinders, casting cast-on Co-Cr cylinders, and the aforementioned passive fitting technique. The tested null hypothesis was that no significant differences would be found between the different fabrication procedures for the implant-supported frameworks that were compared.
MATERIALS AND METHODS A metal model of a partially edentulous maxilla was machined to simulate the restoration of an edentulous space from the second premolar to the second molar with a three-unit FDP supported by two implants. Two threaded implants (Titamax Ti, Neodent; 3.75 mm diameter × 13 mm length) were placed parallel to each other inside perforations prepared in the metal model and fixed with cyanoacrylate adhesive (Super Bonder, Loctite Brasil).9 Two prosthetic abutments (4 mm high, SF mini-conical abutment, Neodent) were screwed to the implants with 32 Ncm of torque, and a three-unit FDP framework was waxed (Fig 1). A silicone impression of the waxing was performed to standardize the waxing of the specimens (Fig 2). Ten specimens were fabricated for each different procedure, and five groups were tested in this study: • Group 1 used plastic burnout cylinders screwed to the prosthetic abutments and were cast in one piece in Co-Cr alloy (FitCast Cobalto, Talmax). • Group 2 used cast-on Co-Cr cylinders and were again cast as one piece using a Co-Cr alloy (FitCast Cobalto). The International Journal of Oral & Maxillofacial Implants 1065
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a
b
c
d
Fig 3 Sequence for framework fabrication using the passive fitting technique. (a) Brass and burnout plastic cylinders for casting; (b) titanium cylinders; (c) one-piece cast framework before cementation; (d) framework cementation using resin cement.
(3Shape Dental System, 3Shape A/S) to fabricate the frameworks. The specimens for group 4 were milled in a Co-Cr alloy, and the specimens for group 5 were milled in presintered zirconia (yttria-stabilized tetragonal zirconia polycrystal) blocks that were later submitted to the sintering process.
Fig 4 The angled metal matrix base allowed for microgap measurements in proximal regions.
• The specimens for group 3 were fabricated using a passive fitting technique.17–19 First, the framework was waxed over brass cylinders and cast as one piece in Co-Cr alloy (FitCast Cobalto); then the framework was luted to titanium cylinders with smaller dimensions using resin cement (Panavia F 2.0, Kuraray Noritake Dental) (Figs 3a to 3d). • Groups 4 and 5 were fabricated using a CAD/CAM system (Neoshape, Neodent). The framework waxing was digitized (D-700, 3Shape A/S) and the digital images were transferred to specialized software
The vertical microgaps between the framework and the abutments were evaluated using the one-screw tightening test.15,29 For the first reading, the screw located on the second premolar was tightened to 10 Ncm with a torque controller (Neodent), and the microgap was measured on both tightened and nontightened sides. The screw tightening location was then changed to the other abutment (second molar), and the second reading was made as previously described. Both screws were tightened to 10 Ncm with the same torque controller device for the third reading. The vertical microgaps were measured in an optical comparator microscope (Marcel Aubert SA CH250, Marcel Aubert) at ×25 magnification with a precision of 1 µm. Three readings were made of each microgap on the buccal, lingual, and proximal aspects. A metal matrix was designed to allow the exact placement of the specimens in the same reading location between each measurement. The matrix also provided a 45-degree angled base to allow microgap measurements in the proximal regions (Fig 4).29 The intraobserver reliability was tested in a pilot study. The same investigator measured the same
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Table 1 Microgap Values (Means ± SDs, in µm) and Results of Comparison of Framework Microgaps Under All Reading Conditions Group
Opposite side
Group 1 124.22 ± 37.02A Group 2 64.78 ±
46.69B
Group 3 12.33 ±
16.73C
Group 4
3.53 ± 5.29C
Group 5 37.76 ±
24.73B,C
Tightened side
Both tightened
9.63 ± 10.29A,B
26.03 ± 18.43A
2.37 ±
5.39B
4.86 ± 5.38B,C
2.04 ±
1.57B
3.80 ± 2.43B,C
1.59 ± 1.34B 17.68 ±
10.47A
Microgap (um)
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140 120 100 80 60 40 20 0
Group 1 Group 2 Group 3 Group 4 Group 5
Opposite side
1.85 ± 1.50C 14.87 ± 9.83A,B
Within columns, levels with different uppercase letters were significantly different (P < .05; Tukey test).
specimen from group 5 three times. This was repeated for two other specimens from the same group. A Pearson correlation test was performed for the three measurements done of each specimen, and the R values were as follows (specimens 1 to 3, respectively): 0.9162, 0.8638, and 0.9503. The rationale for the statistical analysis was also determined by the pilot study using the specimens from group 5. Before all the measurements were averaged, each measurement from each side of the specimens was statistically compared. The measured sides presented no significant differences between each other (P = .6654). Since the microgaps on the different sides of the same framework were similar, the measurements were then averaged together for the statistical analysis. The results found for the groups after each reading condition were submitted to the Kolmogorov-Smirnov test to verify whether the data reflected Gaussian distributions and the results passed the normality test. Analysis of variance was used to indicate whether statistically significant differences (α = .05) were found, and the Tukey test was used for multiple comparisons. Data were processed using statistical software (JMP 6.0, SAS Institute).
RESULTS The mean values, standard deviations (SDs), and Tukey test results for all microgaps are presented in Table 1 and Fig 5. A post hoc power analysis computed the achieved power for the results found in this study. The achieved power for the number of specimens, considering the SD that was found within each group, was above 99% (α = .05; effect size f = 0.6256432). The results found with one of the screws tightened and with the microgap read on the tightened side found that cast plastic burnout cylinders (group 1) and milled zirconia (group 5) presented the largest microgaps with the abutments (P < .05; Table 1). When the
Tightened side
Both screws tightened
Reading conditions Fig 5 Mean values and SDs found for the groups tested in the study under all reading conditions.
reading was made on the side opposite to the tightened side, one-piece cast frameworks (groups 1 and 2) presented the largest microgaps with the abutments (P < .05; Table 1). When both screws were tightened, groups 1 and 5 presented the largest microgaps (P < .05; Table 1).
DISCUSSION This study compared milled zirconia and Co-Cr frameworks to one-piece cast frameworks fabricated with different techniques. The results support rejection of the null hypothesis, since significant differences were found between the groups tested in the study. Cast frameworks presented with larger microgaps compared to milled Co-Cr and casting associated with a passive fitting technique. Milled zirconia presented microgaps of intermediate size among the tested fabrication methods. These differences can be attributed to the technical difficulties inherent to cast frameworks and to sintering shrinkage of the zirconia frameworks. Framework distortion could be caused by the number, position, and configuration of the implants and prosthesis span and design.30 The fit of a framework is also determined by the impression technique, impression material, and dimensional stability of the master cast.1 Previous studies found that the pickup impression technique was more precise than the repositioning technique with a closed tray.31–33 When snap-on plastic impression cylinders were used in a closedtray impression, a larger interabutment distance was present, irrespective of the impression material.32 In addition, studies have shown that the impression material can add significant distortion to the framework.33 Some studies found that silicone was more precise than polyether when a deep subgingivally positioned implant was molded,34 and more accurate casts were obtained with polyvinylsiloxane with the The International Journal of Oral & Maxillofacial Implants 1067
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indirect technique than with polyether.35 However, other studies found no significant differences between these two materials.36–39 The precision of CAD/ CAM restorations is limited by the accuracy of the master cast, especially when metal-based frameworks are milled. Thus, newer technologies, such as intraoral scanning, seem to be a promising approach for future studies.40 Because the main focus of this study was to compare the different framework fabrication techniques, the implants were placed parallel to each other, thus minimizing variation in the experimental design and eliminating possible inaccuracies caused by impression distortion. The absence of the periodontal ligament in implant-supported restorations limits micromovement of the implants.41 An excessive vertical microgap in a screw-retained implant-supported restoration could therefore lead to the transmission of stresses to the supporting implants and bone structures after the prosthetic screws are tightened.42,43 A previous study found that increased levels of misfit resulted in high levels of stress in the peri-implant area.40 The higher preload induced by a reduced passive fit of the screwretained framework may explain the greater frequency of abutment screw loosening.44,45 A correlation between excessive vertical microgaps and screw loosening or fracture was previously reported.19 Prosthetic fit is also considered, among other factors, as essential to the long-term success of implant-supported restorations.40,46 This study used the one-screw tightening test to evaluate the passivity of fit of the FDPs; in this method, the microgap is measured with one screw tightened, and readings are made on the tightened side and on the side opposite the tightened side.15 The results of the current study support the use of this method to evaluate passive fit of the FDPs, since the vertical microgap was smaller when both screws were tightened compared to the results on the nontightened side when only one screw was tightened (Table 1). Comparison of the groups only when both screws are tightened would require the measurement of strain.29,40 All frameworks that were tested in this study presented misfit levels with the abutments below 70 µm in all reading conditions, except for group 1 with the readings on the side opposite the tightened side (124.22 µm). This is in agreement with studies that found similar vertical microgaps for one-piece castings.1,15,30 Sectioning of one-piece cast frameworks for laser welding also contributes to smaller microgaps.15,16 When cast-on Co-Cr cylinders were used (group 2), a smaller microgap was found when both screws were tightened and on the tightened side with only one screw tightened. This is in agreement with previous studies that used cast-on cylinders.30,47 This
can be attributed, in such cases, to the premachined interface of the framework with the abutments, in comparison to a cast interface when burnout plastic cylinders are used. However, a larger microgap was found on the side opposite the tightened side, indicating a less passive framework. This can be expected when a one-piece cast fabrication method is used, as the result of investing and casting distortions. The association of one-piece castings with a passive fitting technique (group 3) resulted in the smallest microgaps among the cast frameworks fabricated for this study. This is in agreement with earlier studies that introduced the technique.19,48 The passive fitting technique is capable of compensating for the distortions inherent to one-piece castings by luting the framework to a titanium cylinder with smaller dimensions, compared to the brass cylinder that is used when the framework is waxed. The results found for group 3 on the nontightened side with only one screw tightened are in agreement with a previous study that used a similar passive fitting technique and found less strain development, indicating a more passively fitting framework.1 It should be stated that the titanium cylinder will not be covered by porcelain, and the 1-mm height of the cylinder will be exposed, thus limiting the use of this technique to posterior regions. It is recommended that frameworks be luted to the titanium cylinders in a moisture-controlled environment, ie, a working cast. This technique is also limited by the quality and amount of resin cement that is used and by the cementation procedures used. The CAD/CAM technique eliminates some of the steps needed for conventional framework fabrication, such as waxing, investing, and casting.40 The milled Co-Cr frameworks (group 4) tested in this study presented the smallest microgaps compared to the other groups in the study, and the results were comparable only to the passive fitting–technique group (group 3). This is in agreement with previous reports.14,40,43 Milled zirconia showed larger microgaps with the abutments. The CAD/CAM system used for this study mills an enlarged presintered zirconia block, and the final sintering process of zirconia is probably the cause of such distortions. This step may limit fabrication accuracy.43,49 Despite this, the microgaps seen with the zirconia frameworks were comparable to those reported in recent studies.14,40,43,50 To reduce the microgap between zirconia frameworks and their abutments, the association of milled zirconia with the passive fitting technique could be recommended, as previously reported.40 However, to ensure and improve bonding between zirconia and resin cement, adequate surface treatment is recommended.51 Again, the height of the collar of the titanium cylinder may limit the use of this technique to posterior regions.
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CONCLUSIONS Some of the fabrication techniques currently available can result in smaller microgaps and more passively fitting implant-supported frameworks, as was the case with milled or cast cobalt-chromium associated with a passive fitting technique in this study. Complete passivity, however, cannot be attained; when only one screw was tightened, microgaps were found for all frameworks on the nontightened side. Further clinical follow-up studies are recommended to assess the longterm survival of fixed partial denture frameworks fabricated using the techniques tested in this study.
ACKNOWLEDGMENTS The authors wish to thank Neodent for providing the implants, implant components, and CAD/CAM frameworks used for this study. The authors reported no conflicts of interest related to this study.
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36. Lorenzoni M, Pertl C, Penkner K, Polansky R, Sedaj B, Wegscheider WA. Comparison of the transfer precision of three different impression materials in combination with transfer caps for the Frialit-2 system. J Oral Rehabil 2000;27:629–638. 37. Liou AD, Nicholls JI, Yuodelis RA, Brudvik JS. Accuracy of replacing three tapered transfer impression copings in two elastomeric impression materials. Int J Prosthodont 1993;6:377–383. 38. Aguilar ML, Elias A, Vizcarrondo CE, Psoter WJ. Analysis of threedimensional distortion of two impression materials in the transfer of dental implants. J Prosthet Dent 2010;103:202–209. 39. Wenz HJ, Hertrampf K. Accuracy of impressions and casts using different implant impression techniques in a multi-implant system with an internal hex connection. Int J Oral Maxillofac Implants 2008;23:39–47. 40. Karl M, Taylor TD. Effect of material selection on the passivity of fit of implant-supported restorations created with computer-aided design/computer-assisted manufacture. Int J Oral Maxillofac Implants 2011;26:739–745. 41. Weinberg LA. The biomechanics of force distribution in implantsupported prostheses. Int J Oral Maxillofac Implants 1993;8:19–31. 42. de Torres EM, Rodrigues RC, de Mattos Mda G, Ribeiro RF. The effect of commercially pure titanium and alternative dental alloys on the marginal fit of one-piece cast implant frameworks. J Dent 2007;35:800–805. 43. Katsoulis J, Mericske-Stern R, Rotkina L, Zbaren C, Enkling N, Blatz MB. Precision of fit of implant-supported screw-retained 10-unit computer-aided-designed and computer-aided-manufactured frameworks made from zirconium dioxide and titanium: An in vitro study. Clin Oral Implants Res 2014;25:165–174.
44. Karl M, Taylor TD, Wichmann MG, Heckmann SM. In vivo stress behavior in cemented and screw-retained five-unit implant FPDs. J Prosthodont 2006;15:20–24. 45. 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. 46. Eliasson A, Arnelund CF, Johansson A. A clinical evaluation of cobalt-chromium metal-ceramic fixed partial dentures and crowns: A three- to seven-year retrospective study. J Prosthet Dent 2007;98:6–16. 47. do Nascimento C, Miani PK, Watanabe E, Pedrazzi V, de Albuqerque RF Jr. In vitro evaluation of bacterial leakage along the implantabutment interface of an external-hex implant after saliva incubation. Int J Oral Maxillofac Implants 2011;26:782–787. 48. Sellers GC. Direct assembly framework for osseointegrated implant prosthesis. J Prosthet Dent 1989;62:662–668. 49. Kunii J, Hotta Y, Tamaki Y, et al. Effect of sintering on the marginal and internal fit of CAD/CAM-fabricated zirconia frameworks. Dent Mater J 2007;26:820–826. 50. Kohorst P, Brinkmann H, Li J, Borchers L, Stiesch M. Marginal accuracy of four-unit zirconia fixed dental prostheses fabricated using different computer-aided design/computer-aided manufacturing systems. Eur J Oral Sci 2009;117:319–325. 51. Dias de Souza GM, Thompson VP, Braga RR. Effect of metal primers on microtensile bond strength between zirconia and resin cements. J Prosthet Dent 2011;105:296–303.
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I
mplant-supported oral rehabilitations aim to restore function without compromising adjacent teeth and the supporting tissues. Important factors in the success of implant-supported restorations include the fit between the framework and the implants or prosthetic 1Former
Student, Latin American Institute for Dental Research and Education, Curitiba, PR, Brazil. 2Professor, Latin American Institute for Dental Research and Education, Curitiba, PR, Brazil. 3Director and Professor, Latin American Institute for Dental Research and Education, Curitiba, PR, Brazil. 4Engineer, Neodent, Curitiba, PR, Brazil. 5Assistant Professor, Fluminense Federal University, Niterói, RJ, Brazil. Correspondence to: Rodrigo Tiossi, Fluminense Federal University, School of Dentistry, Department of Prosthodontics (MOT). Rua Mário Santos Braga, 28, Campus do Valonguinho, Centro, Niterói, RJ, Brazil. 24020-140. Fax: +55 21 2629-9901. Email: [email protected] ©2014 by Quintessence Publishing Co Inc.
abutments, a well-adjusted occlusion, and biocompatible materials.1–3 A poorly fitting prosthesis framework may induce tensile, compressive, and bending forces when the prosthesis is connected, possibly leading to mechanical complications such as loosening or fracture of the prosthetic screws.1,4–6 The microgaps between the framework and abutments can also be colonized by bacteria, which may influence the remodeling of the peri-implant crestal bone and the long-term health of the peri-implant tissues.7 Different techniques to fabricate implant-supported restorations have been used successfully for more than 30 years.8,9 Currently, several methods and materials are available to fabricate implant-supported frameworks. It was previously reported10 that a 10-µm misfit with the abutments would be the maximum misfit recommended to allow bone remodeling and maturation in response to the occlusal loads. However, it is difficult to achieve such small microgaps.11,12 Also, other studies showed that a greater misfit would not necessarily lead to biologic complications.13
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Berejuk et al
Fig 1 Waxed three-unit FDP framework.
Fig 2 Silicone impression to standardize the waxing of the specimens.
Each step needed for the fabrication of implantsupported fixed dental prosthesis (FDP) frameworks introduces a certain degree of inaccuracy.14 One-piece castings have shown poorly fitting frameworks, and sectioning followed by welding is recommended to improve fit.9,15,16 In the attempt to achieve passive fit of one-piece cast frameworks, a passive fitting technique for framework fabrication has been proposed.17–19 In such technique, the framework is waxed over a brass cylinder that is dimensionally larger than the titanium cylinder, over which the cast framework is then luted with resin cement. Recently, a well-developed industrialized engineering approach by means of computer-aided design and computer-assisted manufacture (CAD/CAM) was introduced to minimize distortion and casting inaccuracies.14 CAD/CAM metal-based frameworks have shown better fit compared to cast gold20,21 and cast silver-palladium frameworks.22 The constant demand for highly esthetic restorations has led to the research and development of framework materials with improved optical properties. Since their introduction, zirconia-based restorations have been widely researched and have shown sufficient mechanical strength for use in the stress-bearing posterior regions.23 Zirconia has the advantages of being esthetic, highly biocompatible, and unsupportive of plaque accumulation.14,24 It has a flexural strength of 900 to 1,400 MPa and fracture toughness of up to 10 MPa/m0.5.14,25 However, there are differences in milling metal and zirconia frameworks. Most CAD/CAM systems use “soft” machining to mill an oversized presintered zirconia block to compensate for the 25% shrinkage that occurs after sintering.14,26 The extent of the sintering shrinkage demands that the CAD software reliably calculate the framework size to the required dimensions after the sintering process, which could lead to prosthesis misfit. Metal-based milled frameworks do not go through any additional process after their fabrication, thus minimizing distortion. The “hard” machining of fully sintered zirconia blocks could eliminate the shrinkage distortion of zirconia.
However, it was previously shown to contain a significant amount of monoclinic zirconia, which is usually associated with surface microcracking, higher susceptibility to low temperature degradation, and lower reliability.27,28 This study compared milled zirconia and cobaltchromium (Co-Cr) frameworks to one-piece frameworks cast with different techniques. The one-piece casting techniques compared were: casting plastic burnout cylinders, casting cast-on Co-Cr cylinders, and the aforementioned passive fitting technique. The tested null hypothesis was that no significant differences would be found between the different fabrication procedures for the implant-supported frameworks that were compared.
MATERIALS AND METHODS A metal model of a partially edentulous maxilla was machined to simulate the restoration of an edentulous space from the second premolar to the second molar with a three-unit FDP supported by two implants. Two threaded implants (Titamax Ti, Neodent; 3.75 mm diameter × 13 mm length) were placed parallel to each other inside perforations prepared in the metal model and fixed with cyanoacrylate adhesive (Super Bonder, Loctite Brasil).9 Two prosthetic abutments (4 mm high, SF mini-conical abutment, Neodent) were screwed to the implants with 32 Ncm of torque, and a three-unit FDP framework was waxed (Fig 1). A silicone impression of the waxing was performed to standardize the waxing of the specimens (Fig 2). Ten specimens were fabricated for each different procedure, and five groups were tested in this study: • Group 1 used plastic burnout cylinders screwed to the prosthetic abutments and were cast in one piece in Co-Cr alloy (FitCast Cobalto, Talmax). • Group 2 used cast-on Co-Cr cylinders and were again cast as one piece using a Co-Cr alloy (FitCast Cobalto). The International Journal of Oral & Maxillofacial Implants 1065
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a
b
c
d
Fig 3 Sequence for framework fabrication using the passive fitting technique. (a) Brass and burnout plastic cylinders for casting; (b) titanium cylinders; (c) one-piece cast framework before cementation; (d) framework cementation using resin cement.
(3Shape Dental System, 3Shape A/S) to fabricate the frameworks. The specimens for group 4 were milled in a Co-Cr alloy, and the specimens for group 5 were milled in presintered zirconia (yttria-stabilized tetragonal zirconia polycrystal) blocks that were later submitted to the sintering process.
Fig 4 The angled metal matrix base allowed for microgap measurements in proximal regions.
• The specimens for group 3 were fabricated using a passive fitting technique.17–19 First, the framework was waxed over brass cylinders and cast as one piece in Co-Cr alloy (FitCast Cobalto); then the framework was luted to titanium cylinders with smaller dimensions using resin cement (Panavia F 2.0, Kuraray Noritake Dental) (Figs 3a to 3d). • Groups 4 and 5 were fabricated using a CAD/CAM system (Neoshape, Neodent). The framework waxing was digitized (D-700, 3Shape A/S) and the digital images were transferred to specialized software
The vertical microgaps between the framework and the abutments were evaluated using the one-screw tightening test.15,29 For the first reading, the screw located on the second premolar was tightened to 10 Ncm with a torque controller (Neodent), and the microgap was measured on both tightened and nontightened sides. The screw tightening location was then changed to the other abutment (second molar), and the second reading was made as previously described. Both screws were tightened to 10 Ncm with the same torque controller device for the third reading. The vertical microgaps were measured in an optical comparator microscope (Marcel Aubert SA CH250, Marcel Aubert) at ×25 magnification with a precision of 1 µm. Three readings were made of each microgap on the buccal, lingual, and proximal aspects. A metal matrix was designed to allow the exact placement of the specimens in the same reading location between each measurement. The matrix also provided a 45-degree angled base to allow microgap measurements in the proximal regions (Fig 4).29 The intraobserver reliability was tested in a pilot study. The same investigator measured the same
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Table 1 Microgap Values (Means ± SDs, in µm) and Results of Comparison of Framework Microgaps Under All Reading Conditions Group
Opposite side
Group 1 124.22 ± 37.02A Group 2 64.78 ±
46.69B
Group 3 12.33 ±
16.73C
Group 4
3.53 ± 5.29C
Group 5 37.76 ±
24.73B,C
Tightened side
Both tightened
9.63 ± 10.29A,B
26.03 ± 18.43A
2.37 ±
5.39B
4.86 ± 5.38B,C
2.04 ±
1.57B
3.80 ± 2.43B,C
1.59 ± 1.34B 17.68 ±
10.47A
Microgap (um)
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140 120 100 80 60 40 20 0
Group 1 Group 2 Group 3 Group 4 Group 5
Opposite side
1.85 ± 1.50C 14.87 ± 9.83A,B
Within columns, levels with different uppercase letters were significantly different (P < .05; Tukey test).
specimen from group 5 three times. This was repeated for two other specimens from the same group. A Pearson correlation test was performed for the three measurements done of each specimen, and the R values were as follows (specimens 1 to 3, respectively): 0.9162, 0.8638, and 0.9503. The rationale for the statistical analysis was also determined by the pilot study using the specimens from group 5. Before all the measurements were averaged, each measurement from each side of the specimens was statistically compared. The measured sides presented no significant differences between each other (P = .6654). Since the microgaps on the different sides of the same framework were similar, the measurements were then averaged together for the statistical analysis. The results found for the groups after each reading condition were submitted to the Kolmogorov-Smirnov test to verify whether the data reflected Gaussian distributions and the results passed the normality test. Analysis of variance was used to indicate whether statistically significant differences (α = .05) were found, and the Tukey test was used for multiple comparisons. Data were processed using statistical software (JMP 6.0, SAS Institute).
RESULTS The mean values, standard deviations (SDs), and Tukey test results for all microgaps are presented in Table 1 and Fig 5. A post hoc power analysis computed the achieved power for the results found in this study. The achieved power for the number of specimens, considering the SD that was found within each group, was above 99% (α = .05; effect size f = 0.6256432). The results found with one of the screws tightened and with the microgap read on the tightened side found that cast plastic burnout cylinders (group 1) and milled zirconia (group 5) presented the largest microgaps with the abutments (P < .05; Table 1). When the
Tightened side
Both screws tightened
Reading conditions Fig 5 Mean values and SDs found for the groups tested in the study under all reading conditions.
reading was made on the side opposite to the tightened side, one-piece cast frameworks (groups 1 and 2) presented the largest microgaps with the abutments (P < .05; Table 1). When both screws were tightened, groups 1 and 5 presented the largest microgaps (P < .05; Table 1).
DISCUSSION This study compared milled zirconia and Co-Cr frameworks to one-piece cast frameworks fabricated with different techniques. The results support rejection of the null hypothesis, since significant differences were found between the groups tested in the study. Cast frameworks presented with larger microgaps compared to milled Co-Cr and casting associated with a passive fitting technique. Milled zirconia presented microgaps of intermediate size among the tested fabrication methods. These differences can be attributed to the technical difficulties inherent to cast frameworks and to sintering shrinkage of the zirconia frameworks. Framework distortion could be caused by the number, position, and configuration of the implants and prosthesis span and design.30 The fit of a framework is also determined by the impression technique, impression material, and dimensional stability of the master cast.1 Previous studies found that the pickup impression technique was more precise than the repositioning technique with a closed tray.31–33 When snap-on plastic impression cylinders were used in a closedtray impression, a larger interabutment distance was present, irrespective of the impression material.32 In addition, studies have shown that the impression material can add significant distortion to the framework.33 Some studies found that silicone was more precise than polyether when a deep subgingivally positioned implant was molded,34 and more accurate casts were obtained with polyvinylsiloxane with the The International Journal of Oral & Maxillofacial Implants 1067
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indirect technique than with polyether.35 However, other studies found no significant differences between these two materials.36–39 The precision of CAD/ CAM restorations is limited by the accuracy of the master cast, especially when metal-based frameworks are milled. Thus, newer technologies, such as intraoral scanning, seem to be a promising approach for future studies.40 Because the main focus of this study was to compare the different framework fabrication techniques, the implants were placed parallel to each other, thus minimizing variation in the experimental design and eliminating possible inaccuracies caused by impression distortion. The absence of the periodontal ligament in implant-supported restorations limits micromovement of the implants.41 An excessive vertical microgap in a screw-retained implant-supported restoration could therefore lead to the transmission of stresses to the supporting implants and bone structures after the prosthetic screws are tightened.42,43 A previous study found that increased levels of misfit resulted in high levels of stress in the peri-implant area.40 The higher preload induced by a reduced passive fit of the screwretained framework may explain the greater frequency of abutment screw loosening.44,45 A correlation between excessive vertical microgaps and screw loosening or fracture was previously reported.19 Prosthetic fit is also considered, among other factors, as essential to the long-term success of implant-supported restorations.40,46 This study used the one-screw tightening test to evaluate the passivity of fit of the FDPs; in this method, the microgap is measured with one screw tightened, and readings are made on the tightened side and on the side opposite the tightened side.15 The results of the current study support the use of this method to evaluate passive fit of the FDPs, since the vertical microgap was smaller when both screws were tightened compared to the results on the nontightened side when only one screw was tightened (Table 1). Comparison of the groups only when both screws are tightened would require the measurement of strain.29,40 All frameworks that were tested in this study presented misfit levels with the abutments below 70 µm in all reading conditions, except for group 1 with the readings on the side opposite the tightened side (124.22 µm). This is in agreement with studies that found similar vertical microgaps for one-piece castings.1,15,30 Sectioning of one-piece cast frameworks for laser welding also contributes to smaller microgaps.15,16 When cast-on Co-Cr cylinders were used (group 2), a smaller microgap was found when both screws were tightened and on the tightened side with only one screw tightened. This is in agreement with previous studies that used cast-on cylinders.30,47 This
can be attributed, in such cases, to the premachined interface of the framework with the abutments, in comparison to a cast interface when burnout plastic cylinders are used. However, a larger microgap was found on the side opposite the tightened side, indicating a less passive framework. This can be expected when a one-piece cast fabrication method is used, as the result of investing and casting distortions. The association of one-piece castings with a passive fitting technique (group 3) resulted in the smallest microgaps among the cast frameworks fabricated for this study. This is in agreement with earlier studies that introduced the technique.19,48 The passive fitting technique is capable of compensating for the distortions inherent to one-piece castings by luting the framework to a titanium cylinder with smaller dimensions, compared to the brass cylinder that is used when the framework is waxed. The results found for group 3 on the nontightened side with only one screw tightened are in agreement with a previous study that used a similar passive fitting technique and found less strain development, indicating a more passively fitting framework.1 It should be stated that the titanium cylinder will not be covered by porcelain, and the 1-mm height of the cylinder will be exposed, thus limiting the use of this technique to posterior regions. It is recommended that frameworks be luted to the titanium cylinders in a moisture-controlled environment, ie, a working cast. This technique is also limited by the quality and amount of resin cement that is used and by the cementation procedures used. The CAD/CAM technique eliminates some of the steps needed for conventional framework fabrication, such as waxing, investing, and casting.40 The milled Co-Cr frameworks (group 4) tested in this study presented the smallest microgaps compared to the other groups in the study, and the results were comparable only to the passive fitting–technique group (group 3). This is in agreement with previous reports.14,40,43 Milled zirconia showed larger microgaps with the abutments. The CAD/CAM system used for this study mills an enlarged presintered zirconia block, and the final sintering process of zirconia is probably the cause of such distortions. This step may limit fabrication accuracy.43,49 Despite this, the microgaps seen with the zirconia frameworks were comparable to those reported in recent studies.14,40,43,50 To reduce the microgap between zirconia frameworks and their abutments, the association of milled zirconia with the passive fitting technique could be recommended, as previously reported.40 However, to ensure and improve bonding between zirconia and resin cement, adequate surface treatment is recommended.51 Again, the height of the collar of the titanium cylinder may limit the use of this technique to posterior regions.
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CONCLUSIONS Some of the fabrication techniques currently available can result in smaller microgaps and more passively fitting implant-supported frameworks, as was the case with milled or cast cobalt-chromium associated with a passive fitting technique in this study. Complete passivity, however, cannot be attained; when only one screw was tightened, microgaps were found for all frameworks on the nontightened side. Further clinical follow-up studies are recommended to assess the longterm survival of fixed partial denture frameworks fabricated using the techniques tested in this study.
ACKNOWLEDGMENTS The authors wish to thank Neodent for providing the implants, implant components, and CAD/CAM frameworks used for this study. The authors reported no conflicts of interest related to this study.
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44. Karl M, Taylor TD, Wichmann MG, Heckmann SM. In vivo stress behavior in cemented and screw-retained five-unit implant FPDs. J Prosthodont 2006;15:20–24. 45. 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. 46. Eliasson A, Arnelund CF, Johansson A. A clinical evaluation of cobalt-chromium metal-ceramic fixed partial dentures and crowns: A three- to seven-year retrospective study. J Prosthet Dent 2007;98:6–16. 47. do Nascimento C, Miani PK, Watanabe E, Pedrazzi V, de Albuqerque RF Jr. In vitro evaluation of bacterial leakage along the implantabutment interface of an external-hex implant after saliva incubation. Int J Oral Maxillofac Implants 2011;26:782–787. 48. Sellers GC. Direct assembly framework for osseointegrated implant prosthesis. J Prosthet Dent 1989;62:662–668. 49. Kunii J, Hotta Y, Tamaki Y, et al. Effect of sintering on the marginal and internal fit of CAD/CAM-fabricated zirconia frameworks. Dent Mater J 2007;26:820–826. 50. Kohorst P, Brinkmann H, Li J, Borchers L, Stiesch M. Marginal accuracy of four-unit zirconia fixed dental prostheses fabricated using different computer-aided design/computer-aided manufacturing systems. Eur J Oral Sci 2009;117:319–325. 51. Dias de Souza GM, Thompson VP, Braga RR. Effect of metal primers on microtensile bond strength between zirconia and resin cements. J Prosthet Dent 2011;105:296–303.
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