Cyclic loading of implant-supported prostheses: Changes in component fit over time Donna M. Hecker, DDS, MS,a and Steven E. Eckert, DDS, MSb School of Dentistry, University of Minnesota, Minneapolis, Minn.; Mayo Clinic, Mayo Graduate School of Medicine, Rochester, Minn. Statement of problem. Dental literature suggests that an implant-supported prosthesis must exhibit a passive fit to prevent implant fracture, component breakage, and screw loosening. From a practical standpoint, passive fit is impossible to achieve; instead, minimal misfit may be the clinical goal. To date no specific range of misfit (below which problems are minimal and above which catastrophic failure occurs) has been established. Purpose. The purpose of this study was to determine whether the fit of an implant-supported prosthesis changes through cyclic loading and to quantify the amount of change between the gold cylinder and implant abutment over time. Materials and methods. Fifteen implant-supported frameworks were fabricated with conventional casting techniques and were cyclically loaded under 3 different loading conditions. Five frameworks were loaded on the anterior portion of the framework, 5 were loaded on the left unilateral posterior cantilever, and 5 were loaded bilaterally on the posterior cantilevers with a servohydraulic testing machine. A cyclical load of 200 N was applied to each framework for up to 200,000 cycles. Linear measurements were made in micrometers of the gap between the prosthetic cylinder and the implant-supported abutment at 4 predetermined reference points. These measurements were recorded before the application of the cyclical load, after 50,000 cycles, and after 200,000 cycles. A repeated measures of variance model was fit separately to the data for each load location (P⬍.05). Results. There was a significant (P⫽.024) decrease in gap dimensions at individual reference points and a significant (P⫽.031) decrease in the average gap when the load was applied to the anterior portion of the framework. When the load was applied unilaterally or bilaterally on the posterior cantilever, significant gap closure was not observed (P⫽.33 and P⫽.35, respectively). Conclusion. Within the limitations of this study, the fit between the prosthetic superstructure and the implantsupported abutment changed when simulated functional loading of the anterior portion of the prosthesis was performed. Simulated functional loading applied unilaterally or bilaterally to the posterior cantilever portion of the prosthesis did not result in changes in the measured gap sizes. (J Prosthet Dent 2003;89:346-51.)
CLINICAL IMPLICATIONS This in vitro study suggests that force application to an implant-supported prosthesis may alter the fit of the prosthesis to its respective supporting components. These changes may be cumulative over time, having clinical implications toward maintenance of screw joint integrity.
T
he recognition of a biocompatible union between bone and alloplastic implant material creates a number of new applications to the field of dentistry. Osseointegration offers new treatment options for the edentulous and partially edentulous patient. The use of dental implants to support and retain dental prostheses has been demonstrated to be clinically efficacious.1-4 Although the bone-to-implant interface may be reliable,5-7 clinical complications can and do occur at the prosthetic level.
Presented at the Academy of Prosthodontics Meeting, Santa Fe, NM, May 2001. a Director of Maxillofacial Prosthetic Services and Assistant Clinical Dental Specialist, Department of Restorative Sciences, University of Minnesota. b Associate Professor, Mayo Graduate School of Medicine. 346 THE JOURNAL OF PROSTHETIC DENTISTRY
Technical problems continue to frustrate clinicians. These include inability to achieve intimate fit on fabrication of the prosthetic framework, inability to correct the misfits, and difficulty in developing an occlusal scheme that will not overload the assembly.8 The objective of achieving passive fit of the prosthesis (gold cylinder) to the abutment is addressed by several authors.9-18 Branemark13 suggested that components have no more than a 10-m misfit, but modern dental technology is unlikely to consistently achieve this level of accuracy. Many manufactured components do not demonstrate this level of accuracy of fit.19,20 Consequently, misfit of prostheses is a clinical reality, but the amount of misfit that can be tolerated without adverse mechanical21-30 or biologic9,31-34 complications has yet to be determined. VOLUME 89 NUMBER 4
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Fig. 1. Model testing system included 5 implants arranged in arc, with uniform (10 mm) inter-implant spacing and fulcrum line-anterior abutment distance of 9 mm.
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Fig. 2. Cylindrical sleeve custom-machined to ensure uniform identification of reference points scribed on abutments at every 60 degrees.
This study evaluated the changes that take place in the gap between the implant and the prosthetic components after cyclic loading of prostheses. The goal was to evaluate changes qualitatively and quantitatively.
MATERIAL AND METHODS A model system was designed to allow placement of five 3.75- ⍝ 10-mm implants (Branemark System; Nobel Biocare, Yorba Linda, Calif.) into a rigid base. The system used 5 implants arranged in an arc, with uniform (10 mm) interimplant spacing, a fulcrum line-anterior abutment distance of 9 mm, and a distal extension cantilever of 18 mm (Fig. 1). The model system included load cells (Model MB-250; Interface, Scottsdale, Ariz.) attached to each implant for the purpose of measuring applied loads and detecting any changes in load caused by change in fit of the prosthesis. Polysiloxane (Reprosil; Caulk/Dentsply, York, Pa.) impressions of the implants were made by use of an open custom tray to engage direct implant transfer abutments (Nobel Biocare) that had been rigidly connected with autopolymerizing resin (GC pattern resin; GC America, Chicago, Ill.). After the fabrication of master casts, new standard 4-mm abutments were placed on each of the implant analogues. The abutments were labeled and marked to ensure the exact position on each hex could be duplicated on the model testing system. Fifteen frameworks were waxed, cast, finished, and polished with conventional lost-wax laboratory techniques. Frameworks incorporated standard 4-mm gold cylinders (Nobel Biocare). Framework design used an L-shaped configuration in cross-section, with a lingual vertical rise of 6 mm and horizontal dimension of 8 mm.35-37 No attempts were made to improve the fit of the frameworks by sectioning and soldering. A manual engraving pen was used to mark lines on the abutment at APRIL 2003
Fig. 3. Graphic representation of reference points labeled a, b, c, and e. Reference points d and f eliminated because of lack of access with microscope.
4 different positions, indicating preselected reference points. A cylindrical sleeve with a slot every 60 degrees was custom-machined to ensure uniformity between placement of the engraved lines on the abutments (Fig. 2). Three engraved lines were made on the facial aspect of the abutment and 1 on the lingual aspect. These positions were consistent on all abutments and were labeled a, b, c, and e, respectively (Fig. 3). The abutments were attached to the implants in the predetermined position and tightened to a torque of 20 Ncm with a torque driver (Machine, standard 21 mm, Nobel Biocare). The frameworks were attached to the abutments with the gold retaining screws supplied by the manufacturer (Nobel Biocare) and were tightened manually with the slotted driver (Slot, 22/37; Nobel Biocare). An electronic torque wrench (Torque measurement device, custom; NK Biotechnical Corp, Minneapolis, Minn.) with digital read-out capability was used to tighten the gold screws to the recommended preload force of 10 Ncm. These values were recorded before load application. 347
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Fig. 5. View through microscope demonstrates 10-m vertical discrepancy (gap).
Fig. 4. Frameworks were evaluated for vertical discrepancies between components with micron microscope.
Fig. 6. View through microscope demonstrates 147-m vertical discrepancy (gap).
A micron microscope (Model # FS-60; Mitutoyo Corp, Kawasaki, Japan) with a traveling stage and digital measurement read-out system (VRZ-720-B X-Y; Heidenhain, Schaumberg, Ill.) was used to visually aid in quantifying the vertical space between the abutment and gold cylinder at the mating surface (Fig. 4). The vertical discrepancies were measured and recorded before cycling, after 50,000 cycles, and after 200,000 cycles. Measurement values were recorded in micrometers to a resolution of 0.0005 in/0.01 mm (Figs. 5 and 6). The measurements were made at 4 different locations around the abutment, to quantify the change in misfit at specific locations. These individual points could also be averaged to quantify change in a planar dimension. The values were recorded in a laboratory notebook and then entered into a spreadsheet for subsequent analysis. The summary of the distributions of the gap values (averaged over reference points, implant positions, and frames by load location and cycling interval) is shown in Table I. The summary statistics are based on 25 values (5
implants within each of the 5 frameworks). A repeated measures analysis of variance model was fit separately to the data for each of the 3 load locations. In this analysis, the gap changes that occur with cycling were different between the 4 reference points (P⬍.05).
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RESULTS There was a significant decrease in gap measurements considering each reference point separately when the load was applied to the anterior portion of the framework (P⫽.024), but not when the load was applied unilaterally on the posterior cantilever (P⫽.33) or when the load was applied bilaterally on the posterior cantilever (P⫽.35). When the load was applied to the anterior portion of the framework, there was a significant change with cycling in the gap measurements at reference point e, which corresponds to the lingual aspect of the abutment (P⫽.023). The gap measurements did not change significantly with cycling at any of the other 3 reference points. VOLUME 89 NUMBER 4
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Table I. Summary of the distributions of gap measurements (averaged over reference points, implant positions and frames) by load location and cycling interval Load location
Unilateral Posterior Unilateral Posterior Unilateral Posterior Anterior Anterior Anterior Bilateral Posterior Bilateral Posterior Bilateral Posterior
Cycle
Geometric mean
Median
Mean (std)
Minimum
Maximum
25th percentile
75th percentile
Pre-load 50,000 200,000 Pre-load 50,000 200,000 Pre-load 50,000 200,000
0.067 0.062 0.066 0.054 0.052 0.049 0.047 0.042 0.041
0.076 0.068 0.069 0.047 0.049 0.048 0.052 0.050 0.055
0.108 (0.092) 0.105 (0.095) 0.104 (0.091) 0.082 (0.077) 0.082 (0.082) 0.075 (0.070) 0.078 (0.060) 0.075 (0.059) 0.071 (0.058)
0.009 0.007 0.010 0.014 0.013 0.014 0.005 0.007 0.006
0.334 0.334 0.325 0.346 0.378 0.299 0.199 0.212 0.209
0.038 0.033 0.038 0.039 0.035 0.025 0.034 0.034 0.027
0.142 0.150 0.131 0.115 0.110 0.123 0.124 0.122 0.116
When the load was applied bilaterally to the posterior cantilevers, there was a significant change with cycling in the gap measurements at reference point e (P⫽.003). The gap measurements did not change significantly with cycling at any of the other 3 reference points. In an attempt to summarize the gaps across the 4 reference points on the abutment, the average of these gap measurements per abutment was used. Averaging the 4 individual reference points allows inferences to be made about changes occurring in the planar dimension. A repeated measures analysis of variance model was fit separately to the data for each of the 3 load locations. There was evidence of a significant decrease in the average gaps due to cycling when the load was applied to the anterior portion of the framework (P⫽.031); the average gaps were significantly less after 200,000 cycles than the average gaps before testing.
DISCUSSION The purpose of this study was to investigate changes that occur in fit at the gold cylinder to implant abutment interface over time. The investigation was initiated in response to the recognition that there is little likelihood of achieving a perfect fit between these components with current dental technology. Given the inevitable nature of imperfection in fit, it seems prudent to understand the effect of cyclic loading on the fitting surfaces of the implant-supported prosthesis. In evaluating the determinants of a passively fitting prosthesis, several factors should be considered. These include the definition of passive fit and the accuracy of the procedures used to achieve this degree of passivity. Other factors include the method by which the fit is evaluated and the role of the premachined components. Numerous methods exist for evaluating framework fit. Different methods apply to the clinical and laboratory settings. Both settings involve use of visual or tactile inspection. Visual inspection may include the 1-screw test,14 use of disclosing medium,15 fabrication of a cast verification jig,20 or use of radiographic films.22 All of APRIL 2003
these methods require visualization of the space between 2 components. Tactile evaluation of fit is performed by placing the prosthesis in contact with its respective abutments and feeling for even seating of the prosthesis. Once the initial seating is confirmed, retaining screws are alternately tightened and loosened to determine whether movement of the prosthesis occurs while the individual screws are secured. The visual and tactile methods of evaluation attempt to bring objectivity to the evaluation process, but both retain a level of subjectivity. Conversely, laboratory methods for measuring fit 3-dimensionally at the implant-prosthodontic interface may eliminate this level of subjectivity. There are essentially 3 methods for laboratory assessment: stylus contact techniques,16 laser videography,17 and photogrammetry.18 Each method provides precise measurements to the level of approximately 3.5 m. Of course, any discrepancy between the oral cavity and the master cast will continue to create clinical misfit of the prosthetic frame. Although there are many ways to assess 3-dimensional fit of prostheses to abutments,16-18 the sophisticated testing equipment required for such testing is not readily available. The use of more common measuring techniques still provides information on the relative fit or misfit of a prosthesis while recognizing the fact that this is not as precise as the other methods. Another factor that must be considered in the fabrication of prostheses is the reproducible fit of the premachined components. These components are machined by the manufacturer to ensure a specific tolerance. Machining tolerances define the limit of acceptability when assessing the measurement of a component feature in relation to the specified design value target. There can be no consistently greater accuracy of the final prosthesis fit to its abutments than the level of machining tolerance. Ma et al21 demonstrated a range of tolerance for the implant components (abutment, gold cylinder, impression coping, and brass abutment analog) in the horizontal plane from 22 to 100 m. This study also dem349
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onstrated a discrepancy between the machining tolerances of the prosthetic components and the laboratory analogs. The machining tolerance of brass analog and gold cylinder was significantly larger compared with that of the abutment and gold cylinder. This level of discrepancy may result in a prosthesis that appears to fit in the laboratory, whereas the same prosthesis may have a larger misfit in the clinical setting. Rubenstein and Ma19 found similar results when comparing components for titanium laser-welded and standard cast implant components. A range of 23.1 to 51.7 â?Žm for machining tolerances was reported. This study demonstrated that under specific loading conditions, the gap between the implant abutment and the prosthesis can undergo significant change. In this study, the change resulted in smaller gaps after cyclic loading. This change is probably due to wear at the component interface, because some point of contact must prevent the components from achieving the manufactured fit. Once this wear occurs and prosthetic retaining screws are retightened to ideal levels, it is possible that the net effect may be favorable toward future component loosening or breakage. However, if components do become dislodged, the orientation of the worn surfaces could be altered. The consequences of such a change are currently being investigated in a subsequent study.
CONCLUSIONS Within the limitations of this study, cyclic loading of implant-supported frames caused changes in the fit of the frame to the supporting implant abutments. The fit of the superstructure showed significant gap reductions when the prosthesis was loaded on the anterior segment. Fit did not change significantly when the frameworks were loaded on the unilateral or bilateral posterior cantilever portions of the prostheses. REFERENCES 1. Adell R, Lekholm U, Rockler B, Branemark PI. A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Surg 1981;10:387-416. 2. Adell R, Eriksson B, Lekholm U, Branemark PI, Jemt T. Long-term follow-up study of osseointegrated implants in the treatment of totally edentulous jaws. Int J Oral Maxillofac Implants 1990;5:347-59. 3. Cox JF, Zarb GA. The longitudinal clinical efficacy of osseointegrated dental implants: a 3-year report. Int J Oral Maxillofac Implants 1987;2: 91-100. 4. Laney WR, Tolman DE, Keller EE, Desjardins RP, Van Roekel NB, Branemark PI. Dental implants: tissue-integrated prosthesis utilizing the osseointegration concept. Mayo Clin Proc 1986;61:91-7. 5. Branemark PI, Adell R, Breine U, Hansson BO, Lindstrom J, Ohlsson A. Intra-osseous anchorage of dental prostheses. I. Experimental studies. Scand J Plast Reconstr Surg 1969;3:81-100. 6. Carr AB, Gerard DA, Larsen PE. The response of bone in primates around unloaded dental implants supporting prostheses with different levels of fit. J Prosthet Dent 1996;76:500-9. 7. Clelland NL, Carr AB, Gilat A. Comparison of strains transferred to a bone simulant between as-cast and postsoldered implant frameworks for a five-implant-supported fixed prosthesis. J Prosthodont 1996;5:193-200.
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8. 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. 9. Rangert B, Jemt T, Jorneus L. Forces and moments on Branemark implants. Int J Oral Maxillofac Implants 1989;4:241-7. 10. Rangert B, Gunne J, Sullivan DY. Mechanical aspects of a Branemark implant connected to a natural tooth: an in vitro study. Int J Oral Maxillofac Implants 1991;6:177-86. 11. Weinberg LA. The biomechanics of force distribution in implant-supported prostheses. Int J Oral Maxillofac Implants 1993;8:19-31. 12. Zarb GA, Schmitt A. The longitudinal clinical effectiveness of osseointegrated implants: the Toronto study. Part III: Problems and complications encountered. J Prosthet Dent 1990;64:185-94. 13. Branemark PI, Zarb G, Albrektsson T. Tissue-integrated prostheses: osseointegration in clinical dentistry. Special edition for Nobelpharma. Chicago: Quintessence; 1987. p. 268-71. 14. Jemt T. Failures and complications in 391 consecutively inserted fixed prostheses supported by Branemark implants in edentulous jaws: a study of treatment from the time of prosthesis placement to the first annual checkup. Int J Oral Maxillofac Implants 1991;6:270-6. 15. Yanase RT, Binon PP, Jemt T, Gulbransen HJ, Parel SP. Current issues forum: How do you test a cast framework for for a full-arch fixed implantsupported prosthesis? Int J Oral Maxillofac Implants 1994;9:469-74 16. Tan KB, Rubenstein JE, Nicholls JI, Yuodelis RA. Three-dimensional analysis of the casting accuracy of one-piece, osseointegrated implant-retained prostheses. Int J Prosthodont 1993;6:346-63. 17. Riedy SJ, Lang BR, Lang BE. Fit of implant frameworks fabricated by different techniques. J Prosthet Dent 1997;78:596-604. 18. Lie A, Jemt T. Photogrammetric measurements of implant positions. Description of a technique to determine the fit between implants and superstructures. Clin Oral Implants Res 1994;5:30-6. 19. Rubenstein JE, Ma T. Comparison of interface relationships between implant components for laser-welded titanium frameworks and standard cast frameworks. Int J Oral Maxillofac Implants 1999;14:491-5. 20. Binon PP. Evaluation of machining accuracy and consistency of selected implants, standard abutments, and laboratory analogs. Int J Prosthodont 1995;8:162-78. 21. Ma T, Nicholls JI, Rubenstein JE. Tolerance measurements of various implant components. Int J Oral Maxillofac Implants 1997;12:371-5. 22. Cox JF, Pharoah M. An alternative holder for radiographic evaluation of tissue-integrated prostheses. J Prosthet Dent 1986;56:338-41. 23. Jemt T, Lekholm U. Measurements of bone and frame-work deformations induced by misfit of implant superstructures: a pilot study in rabbits. Clin Oral Implants Res 1998;9:272-80. 24. Piattelli A, Piattelli M, Scarano A, Montesani L. Light and scanning electron microscopic report of four fractured implants. Int J Oral Maxillofac Implants 1998;13:561-4. 25. Eckert SE, Wollan PC. Retrospective review of 1170 endosseous implants placed in partially edentulous jaws. J Prosthet Dent 1998;79:415-21. 26. Taylor TD. Prosthodontic problems and limitations associated with osseointegration. J Prosthet Dent 1998;79:74-8. 27. Binon PP, McHugh MJ. The effect of eliminating implant/abutment rotational misfit on screw joint stability. Int J Prosthod 1996;9:511-9. 28. Niznick GA. Bending overload and implant fracture: a retrospective clinical analysis. Int J Oral Maxillofac Implants 1996;11:431-2. 29. Rangert B, Krogh PH, Langer B, Van Roekel N. Bending overload and implant fracture: a retrospective clinical analysis. Int J Oral Maxillofac Implants 1995;10:326-34. 30. Kallus T, Bessing C. Loose gold screws frequently occur in full-arch fixed prostheses supported by osseointegrated implants after 5 years. Int J Oral Maxillofac Implants 1994;9:169-78. 31. Goodacre CJ, Kan JY, Rungcharassaeng K. Clinical complications of osseointegrated implants. J Prosthet Dent 1999;81:537-52. 32. Eckert SE, Meraw SJ, Cal E, Ow RK. Analysis of incidence and associated factors with fractured implants: a retrospective study. Int J Oral Maxillofac Implants 2000;15:662-7. 33. Carr AB, Gerard DA, Larsen PE. The response of bone in primates around unloaded dental implants supporting prostheses with different levels of fit. J Prosthet Dent 1996;76:500-9. 34. Jemt T, Lekholm U, Johansson CB. Bone response to implant-supported frameworks with differing degrees of misfit preload: in vivo study in rabbits. Clin Implant Dent Relat Res 2000;2:129-37.
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35. Clelland NL, Papazoglou E, Carr AB, Gilat A. Comparison of strains transferred to a bone simulant among implant overdenture bars with various levels of misfit. J Prosthodont 1995;4:243-50. 36. Michaels GC, Carr AB, Larsen PE. Effect of prosthetic superstructure accuracy on the osteointegrated implant bone interface. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1997;83:198-205. 37. Stewart RB, Desjardins RP, Laney WR, Chao EY. Fatigue strength of cantilevered metal frameworks for tissue-integrated prostheses. J Prosthet Dent 1992;68:83-92. 38. Stewart RB, Staab GH. Cross-sectional design and fatigue durability of cantilevered sections of fixed implant-supported prostheses. J Prosthodont 1995;4:188-94. 39. Staab GH, Stewart RB. Theoretical assessment of cross sections for cantilevered implant-supported prostheses. J Prosthodont 1994;3:23-30.
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Reprint requests to: DR DONNA M HECKER UNIVERSITY OF MINNESOTA ROOM 6-284 MOOS TOWER 515 DELAWARE ST SE MINNEAPOLIS, MN 55455 FAX: 612-624-2660 E-MAIL: hecke003@tc.umn.edu Copyright © 2003 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/2003/$30.00 ⫹ 0 doi:10.1067/mpr.2003.71
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