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Analysis of the misfit of dental implant-supported prostheses made with three manufacturing processes Marc Fernández, MSc,a Luis Delgado, MSc,a Meritxell Molmeneu, MSc,a David García, CDT,b and Daniel Rodríguez, PhDc Technical University of Catalonia, Barcelona, Spain Statement of problem. The microgap between implant components has been associated with complications such as screw loosening or adverse biologic responses. Purpose. The purpose of this study was to quantify the microroughness of the mating surfaces of implant components manufactured with different processes, to quantify the microgap between implant components, and to determine whether a correlation exists between microroughness and the microgap. Material and methods. Nine dental implants with a standard external connection were paired with 3 milled, 3 cast, and 3 sintered compatible cobalt-chromium alloy abutments. The abutment surface was examined, and the roughness parameter Sz was measured by using a white-light interferometric microscope at 10 to 100 magnification. The abutment surface and the microgap of the implant-abutment connection were observed with scanning electron microscopy, and the microgap width was quantified from micrographs made of each implant-abutment pair. The mean and standard deviation of roughness and microgap were evaluated. A 1-way ANOVA (a¼.05) was used to assess the influence of the manufacturing process on roughness and microgap. The Pearson correlation was used to check dependence between roughness and microgap. Results. The milled abutments possessed a connection geometry with defined edges and a mean roughness of 29 mm, sintered abutments showed a blurred but functional connection with a roughness of 115 mm, and cast abutments showed a connection with a loss of axial symmetry and a roughness of 98 mm. A strong correlation was found between the roughness values on the mating surfaces and the microgap width. Conclusions. The milled components were smoother than the cast or sintered components. A correlation was found between surface roughness and microgap width. (J Prosthet Dent 2014;111:116-123) Clinical Implications The microgap of implant-abutment connections could be reduced with smoother mating surfaces. The long-term success of dental implants has been well established and is extensively documented in relation to biologic factors,1,2 surgical procedures, and restorative principles that influence the effectiveness of oral implants.3,4 In spite of their success, dental implants may present problems associated with the loosening and fracture of the prosthetic screw that clamps the dental prosthesis to the implant. The screw tightens the prosthetic abutment to the dental implant with controlled torque, usually through an antirotation geometry, either protruded (external connection) or sunken (internal connection).5-7 The torque generates a force (preload) in the screw equal in magnitude to the clamping force of the abutment to implant minus friction and local deformation forces on the mating surfaces.8 Loosening or fracture of the screw occurs when the joint that separates the forces that act on the screw joint are greater than the clamping forces that hold the screw joint or greater than the mechanical resistance of the screw.5,9-11 Screw-related failures can be associated with implant-abutment connection misalignments.7,12,13 Different variables a Research Technician, Biomaterials, Biomechanics and Tissue Engineering Group, Department of Materials Science, Technical University of Catalonia. b Private practice, Barcelona, Spain. c Assistant Professor, Biomaterials, Biomechanics and Tissue Engineering group, Department of Materials Science, Technical University of Catalonia. The Journal of Prosthetic Dentistry Fernández et al February 2014 have been studied as primary causes of screw loosening, such as the errors accumulated in the multiple fabrication steps required for an implant-supported fixed prosthesis,14-16 the characteristics of the materials used in the reconstructions,17-19 or the surface irregularities among the mating surfaces of implant, abutment, and screw.12,13,17,20 Surface irregularities are closely related to the presence of a microgap between implant components. The microgap has been linked to periimplant inflammation and bacteria infiltration.4,21,22 The fabrication and surface finishing processes of the connection components are major factors of surface roughness.23 Most implant components are precision milled or cast, but new manufacturing techniques, for example, laser sintering, are becoming available. The control of roughness on the mating surfaces of implant components before their use could reduce screw loosening as well as the microgap between implant components.24 Previous studies have analyzed the microgap of the implant-abutment connection by using an optical microscope and by scanning electron microscopy (SEM).25-27 However, these methodologies are not well suited for a fast and accurate quantification of the misalignment between components or the microroughness on mating surfaces with different fabrication processes. One of the techniques used in quality-control processes that measures surface roughness with adequate accuracy and speed is coherence scanning interferometry (also known as white-light interferometry). This technique can produce high-quality 3-dimensional surface maps of a macroscopic surface with vertical resolutions up to 0.1 nm.28 The purpose of this study was to quantify the microroughness of the mating surfaces of implant components manufactured with 3 different processes (milled, laser sintered, and cast) with coherence scanning interferometry, to quantify the microgap between implant components with SEM, and to determine whether a correlation exists between microroughness and microgap. The null hypotheses were that no difference would be found in the Fernández et al 117 surface roughness or microgap among surfaces manufactured with different processes and no correlation would be found between the surface roughness and microgap of implant components. MATERIAL AND METHODS Nine dental implants with a standard (4.1-mm diameter) external hexagonal connection (Avinent Implant System; Avinent) were paired with 9 compatible cobalt-chromium alloy (CoCr) abutments. Three abutments were chosen within the same production batch from 3 different manufacturing processes. Abutment groups were named according to the abutment manufacturing process. Milled abutments were manufactured with a high-speed milling process (Core3dcentres; Core3d Protech SL) with cutting tools that gradually shaped the component by removing excess material. Sintered abutments were processed from raw material in powder form deposited in the working tray (EOSINT; EOS GmbH Electro Optical System). A temperature increase of the powder in the working tray Table I. Quantitative abutment surface analysis The abutment surface was examined by using a white-light microscope equipped with Michelson/Mirau interferometric objectives (Wyko NT9300 Optical Profilometer; Veeco Instruments) in vertical scanning interferometry mode at 10 to 100 magnification. Three measurements were made for each abutment specimen of each type of manufacturing process. A matrix of 811 stitched images Characteristics of specimens Material Manufacturing Method Applied Torque, Ncm Milled CoCr High-speed milling 35 Sintered CoCr Laser sintering 35 CoCr Casting 35 Process Description MIL SIN CAS Cast Group in a given spot with a laser beam caused the particles of the material to bind locally, and, through repetition of the process, the desired structure was created layer by layer. Cast abutments were fabricated with the lost wax casting technique (Laboratorio de Prótesis Dental Garbident) by preparing the structure with different entry channels and adding reservoirs to improve solidification of the material. The 9 abutments were each fixed to an implant with a new screw and tightened with a torque wrench at the prescribed load (Table I). MIL, milled abutment; CoCr, cobalt-chromium alloy; SIN, sintered abutment; CAS, cast abutment. 1 Scanning electron microscopy image of abutment mating surface with area of analysis (10). 118 Volume 111 Issue 2 of 635479 mm each, with 20% of overlapping, was performed, which defined an analysis area of 4.54.5 mm. The surface analyzed is shown as the area between the dashed circles (Fig. 1). Data filtering and analysis were performed with specific image analysis software (Wyko Vision 32; Veeco Instruments). A Gaussian filter was used to eliminate tilt from every surface analysis. The Sz roughness parameter (average difference between the 5 highest peaks and 5 lowest valley s) was evaluated for all specimens28: P5 Sz ¼ 1 absðPeak HeightsÞ þ 5 P5 1 absðValley DepthsÞ Qualitative abutment surface evaluation The abutment surface and the microgap of the implant-abutment connection for the 3 groups were observed with SEM (6400 Scanning Microscope; Jeol Ltd). The microgap was observed at different magnifications, depending on the type of abutment (Fig. 2). Microgap measurements The microgap was measured by using 5 SEM images made from each implant-abutment pair. A given implant-abutment pair was introduced in the SEM with a position predefined by the SEM holder. Once the microgap region was brought into the field of view, a picture was made, and a predefined axial rotation was applied to the unit. The process was repeated until 5 regions of interest were documented. Seventytwo measurements per image of the microgap were evaluated with the aid of image analysis software (OmniMet; Buehler). 2 Scanning electron microscopy image of implant-abutment gap. A, General view (15). B, Detailed view (500). check the dependence between the Sz parameter and the microgap. The P value was calculated (a¼.05) for the null hypotheses that no correlation would be found between the surface roughness and microgap of implant components. All statistical analyses were done with a statistical software package (Minitab; Minitab Inc). Statistical analysis RESULTS Means and standard deviations of the Sz parameter and the microgap of the 3 groups were determined. A 1-way ANOVA (a¼.05) was used to assess the influence of the manufacturing process on the Sz parameter and the microgap. The Pearson correlation was used to of the components produced by laser sintering and casting, with a mean Sz of 29 mm (Fig. 3). The sintered components had the highest Sz (115 mm), and the cast components had an Sz of 98 mm. Statistically significant differences were found among the 3 groups (milled, laser sintered, and cast) (P¼.002). However, no statistically significant differences were found between the sintered and cast components (P¼.269) (Fig. 4). Quantitative surface analysis Qualitative surface evaluation The measurements made with the interferometric equipment indicated that the mating surface of the milled components was smoother than that The Journal of Prosthetic Dentistry The milled abutment surface showed accurate connection geometry with defined edges (Fig. 5A). The sintered Fernández et al February 2014 119 3 Reconstructed surfaces with interferometric microscopy of abutment mating surfaces. A, Milled. B, Sintered. C, Cast. abutment showed a blurred but functional connection (Fig. 5B). The cast abutment, however, showed a connection that lacked axial symmetry (Figs. 3C, 5C). 160 140 Sz (µm) 120 Microgap measurements 100 80 60 40 20 0 MIL SIN CAS 4 Roughness parameter Sz values of abutment mating surfaces. Fernández et al The mean value of the microgap of the implant-milled abutment system measured 0.73 mm, which was smaller than the gap of sintered (11.30 mm) and cast (9.09 mm) abutments (Fig. 6). SEM micrographs agreed with the measured values (Fig. 7). A statistically significant difference of the factor “manufacturing technique” was 120 Volume 111 Issue 2 5 Scanning electron microscopy images of abutment mating surfaces (10). A, Milled. B, Sintered. C, Cast. 25 was 0.96, which implied a strong correlation between microroughness on the mating surfaces and the width of the microgap. The P value for the null hypothesis was P<.001. Thus, the null hypothesis was rejected, and a positive significant relationship was identified between microroughness and microgap width. Gap (µm) 20 15 10 5 0 DISCUSSION MIL SIN CAS 6 Implant-abutment microgap values for each study group. found by 1-way ANOVA (P¼.01). However, a post hoc test revealed no statistically significant difference between the sintered and cast abutments (P¼.26). Correlation between roughness and microgap The Pearson correlation between the Sz parameter and the microgap The Journal of Prosthetic Dentistry The null hypotheses were rejected based on the results, which implied that coherence scanning interferometry can be used to measure surface irregularities on the mating surfaces of implant components. Surface irregularities and connection misalignments between implant components have Fernández et al February 2014 121 7 SEM images of implant (left)-abutment (right) microgap. A, Milled (2250). B, Sintered (750). C, Cast (750). been considered as a possible cause of mechanical complications such as screw loosening and/or fractures.5,8 Achieving a passive fit (no shear stresses at the connection level) is important for implant success and is generally considered acceptable for a microgap of less than 10 mm.12,18,19,27 Microroughness on the mating surfaces of implant components plays an important role on insertion forces and the geometry of the connection because the energy required to flatten these irregularities affects the clamping forces generated by the retaining screw and the irregularities interfere with a perfect contact between mating surfaces.10,18,27 Fernández et al White-light interferometry is a fast and accurate technique for measuring the surface microroughness of implant components and is suitable for use in the quality control of implant components. The analysis of the surface roughness of abutments made with 3 manufacturing techniques (milled, laser sintered, and cast) with this method allowed the differentiation of the surfaces based on the measured roughness and their correlation with the microgap implantabutment measured by SEM.25 Some studies have reported different variables that affect the implant-abutment interface. Prosthetic frameworks, for example, can present distortions due to inaccuracies compounded by the multiple fabrication steps that compromise the implant-abutment interface fitting.14-16 The results of the present study indicate that the manufacturing technique is also a variable that influences the presence of a microgap, probably because of the different surface roughness produced by each manufacturing method. A rough mating surface inevitably produces a microgap between implant and abutment and hinders the achievement of a passive fit.12,13,17,20 Milled surfaces have a better fit and a larger number of contacts with the implant mating surface than cast and sintered surfaces, which allows a better closure of the microgap between implant components.10-12 122 Volume 111 Issue 2 The presence of a microgap seems to allow bacterial infiltration, which leads to clinical alterations in the periimplant tissues.21 Although the mean values measured in this study are less than12 mm, reported values in other studies of microgap sizes range from 0 to 135 mm, with bacterial leakage reduced for the minor microgap sizes.22 Therefore, a minor effect of bacteria on surrounding tissues is more likely for milled components than for cast or sintered ones. Statistically significant results were obtained from this study. A possible limitation of the results, however, is related to the number of specimens included. No power analysis was performed to determine adequate sample size because the inclusion of more specimens would have posed significant practical difficulties. The sample size was similar to those of other studies of roughness and microgap,23,24 and the compelling correlation between surface roughness and microgap is an applicable result of this study. Another possible limitation of this study is that the results were established with external connection implants, in which the contact geometry of the microgap is evident. The results, however, may be extended to dental implants with internal connection, because the closure of the microgap is closely related to the flattening of the surface irregularities of the mating surfaces of implant, screw, and abutment.10 The results of the present study were measured on cobalt-chromium specimens but are presumably related to the manufacturing process, not only to the characteristics of the material used in the prostheses.16,17 The effect of the processing techniques on other materials could also affect the surface roughness and connection misfit, thereby affecting the microgap.18,19 Further investigations could look for a possible correlation among the materials, roughness of mating surfaces, microgap presence, and torque forces applied to implant components. Because existing studies are not conclusive, future studies could also focus on the possible clinical effects of abutments fabricated with the 3 methods to see if the differences found in the present study have clinical relevance.21,22 CONCLUSIONS Within the limitations of this study, the following conclusions were drawn: 1. Coherence scanning interferometry can measure the surface roughness of components manufactured with different processes with sufficient accuracy (P¼.002) to differentiate them. 2. Milled implant components have smoother surfaces than cast or sintered components. 3. Surface roughness correlates (P<.001) with the microgap width between external connection implants and abutments. REFERENCES 1. Taylor TD, Agar JR. Twenty years of progress in implant prosthodontics. J Prosthet Dent 2002;88:89-95. 2. Zarb GA, Schmitt A. The edentulous predicament. I: a prospective study of the effectiveness of implant-supported fixed prostheses. J Am Dent Assoc 1996;127:59-65. 3. Aparicio C. A new method to routinely achieve passive fit of ceramometal prostheses over Brånemark osseointegrated implants: a two-year report. Int J Periodontics Restorative Dent 1994;14:404-19. 4. Esposito M, Hirsch J-M, Lekholm U, Thomsen P. Biological factors contributing to failures of osseointegrated oral implants (I). Success criteria and epidemiology. Eur J Oral Sci 1998;106:527-51. 5. Winkler S, Ring K, Ring JD, Boberick KG. Implant screw mechanics and the settling effect: an overview. J Oral Implant 2003;29:242-5. 6. Coelho AL, Suzuki M, Dibart S, Da Silva N, Coelho PG. Cross sectional analysis of the implant-abutment interface. J Oral Rehabil 2007;34:508-16. 7. Binon PP. Evaluation of the effectiveness of a technique to prevent screw loosening. J Prosthet Dent 1998;79:430-2. 8. Asvanund P, Morgano SM. Photoelastic stress analysis of external versus internal implant-abutment connections. J Prosthet Dent 2011;106:266-71. 9. Alkan I, Sertgoz A, Ekici B. Influence of occlusal forces on stress distribution in preloaded dental implant screws. J Prosthet Dent 2004;91:319-25. 10. Guzaitis KL, Knoernschild KL, Viana MA. Effect of repeated screw joint closing and opening cycles on implant prosthetic screw reverse torque and implant and screw thread morphology. J Prosthet Dent 2011;106: 159-69. The Journal of Prosthetic Dentistry 11. Byrne D, Jacobs S, O’Connell B, Houston F, Claffey N. Preloads generated with repeated tightening in 3 types of screws used in dental implant assemblies. J Prosthodont 2006;15: 164-71. 12. Binon PP. The effect of implant/abutment hexagonal misfit on screw joint stability. Int J Prosthodont 1996;9:149-60. 13. Hecker DM, Eckert SE. Cyclic loading of implant-supported prostheses: changes in component fit over time. J Prosthet Dent 2003;89:346-51. 14. Zervas PJ, Papazoglou E, Beck FM, Carr AB. Distortion of three-unit implant frameworks during casting, soldering, and simulated porcelain firings. J Prosthodont 1999;8:171-9. 15. Nicoll RJ, Sun A, Haney S, Turkyilmaz I. Precision of fit between implant impression coping and implant replica pairs for three implant systems. J Prosthet Dent 2013;109: 37-43. 16. Barbi FC, Camarini ET, Silva RS, Endo EH, Pereira JR. Comparative analysis of different joining techniques to improve the passive fit of cobalt-chromium superstructures. J Prosthet Dent 2012;108: 377-85. 17. Cibirka RM, Nelson SK, Lang BR, Rueggeberg FA. Examination of the implantabutment interface after fatigue testing. J Prosthet Dent 2001;85:268-75. 18. 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;35:800-5. 19. Hulterstrom M, Nilsson U. Cobaltchromium as a framework material in implant-supported fixed prostheses: a preliminary report. Int J Oral Maxillofac Implants 1991;6:475-80. 20. Khraisat A, Stegaroiu R, Nomura S, Miyakawa O. Fatigue resistance of two implant/abutment joint designs. J Prosthet Dent 2002;88:604-10. 21. King GN, Hermann JS, Schoolfield JD, Buser D, Cochran DL. Influence of the size of the microgap on crestal bone levels in nonsubmerged dental implants: a radiographic study in the canine mandible. J Periodontol 2002;73:1111-7. 22. Passos SP, Gressler May L, Faria R, Ozcan M, Bottino MA. Implanteabutment gap versus microbial colonization: clinical significance based on a literature review. J Biomed Mater Res Part B Appl Biomater 2013;101:1381-8. 23. Byrne D, Houston F, Cleary R, Claffey N. The fit of cast and premachined implant abutments. J Prosthet Dent 1998;80: 184-92. 24. Weiss EI, Kozak D, Gross MD. Effect of repeated closures on opening torque values in seven abutmentimplant systems. J Prosthet Dent 2000;84:194-9. 25. 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;22:575-9. Fernández et al February 2014 26. Riedy SJ, Lang BR, Lang BE. Fit of implant frameworks fabricated by different techniques. J Prosthet Dent 1997;78:596-604. 27. Koke U, Wolf A, Lenz P, Gilde H. In vitro investigation of marginal accuracy of implant-supported screw-retained partial dentures. J Oral Rehabil 2004;31:477-82. 28. Veeco Instruments. WYKO NT9800/9300. Profiler setup and operation guide. Tucson: Veeco Instruments; 2007. 123 Corresponding author: Dr Daniel Rodríguez Department of Materials Science e Pavelló E ETSEIB e UPC Av. Diagonal 647 08028 Barcelona SPAIN Acknowledgments The authors thank Avinent Implant System S.L. and Core3d Centres for providing the implants and abutments used for this study and Laboratorio de Prótesis Dental Garbident for providing the prosthetic definitive restorations. Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry. 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