Clinical methods for evaluating implant framework fit

doi:10.1016/S0022-3913(99)70229-5

The Journal of Prosthetic Dentistry

Volume 81, Issue 1, January 1999, Pages 7-13

Presented at the Pacific Coast Society of Prosthodontists Annual Meeting, Sacramento, Calif., June 1998.

https://doi.org/10.1016/S0022-3913(99)70229-5 Get rights and content

Abstract

Statement of problem. Passive fit of implant-supported–prosthesis frameworks has been suggested as a prerequisite for successful long-term osseointegration. However, there are no scientific guidelines as to what is passive fit and how to achieve and measure it. Purpose. The purpose of this article is to discuss passive fit and to review the various clinical methods that have been suggested for evaluating implant framework fit. Methods. The dental literature was reviewed to identify the clinical methods that have been used to evaluate implant framework fit. Conclusions. The suggested levels of passive fit are empirical. Numerous techniques have been advocated to evaluate the prosthesis-implant interface, but none individually provides objective results. It is suggested that clinicians use a combination of the available methods to minimize misfits. (J Prosthet Dent 1999;81:7-13.)

Section snippets

ACCEPTABLE LEVELS OF FIT

Many authors have attempted to define an acceptable level of implant prosthesis fit.1, 34, 35 In 1983, Brånemark was the first to define passive fit and he proposed that it should exist at the 10 μm level to enable bone maturation and remodeling in response to occlusal loads.¹ In 1985, Klineberg and Murray³⁴ suggested that castings with discrepancies greater than 30 μm over more than 10% of the circumference of the abutment interface were unacceptable. In 1991, Jemt³⁵ defined passive fit as a

FACTORS AFFECTING FRAMEWORK FIT EVALUATION

The accuracy and validity of clinically evaluating framework fit can be affected by factors such as implant number and distribution, framework rigidity, ability of the screw to close the gap, and/or margin location. Clelland et al.³⁶ demonstrated that marginal gaps up to 500 μm for 2-implant frameworks were not detectable with an explorer when the framework screws were tightened to 10 Ncm, which suggests that passive fit may appear to be present because screw tightening has closed a gap.

METHODS FOR EVALUATING FRAMEWORK FIT

Methods for evaluating implant framework fit can be categorized according to the assessment method.

INSTRUMENTS

Jemt et al.⁵² described 4 systems that quantify framework misfit 3-dimensionally: the Mylab, University of Washington, 3-D photogrammetric, and University of Michigan systems. Discrepancies can be accurately measured to the nearest 10 μm. However, these systems are technique sensitive, expensive, and require special equipment. Furthermore, except for 3-D photogrammetric, these systems can only be used extra-orally and therefore limit their clinical applications.

An in vitro study by May et al.⁵³

BIOLOGIC TOLERANCE

In 1994, Kallus and Bessing¹⁰ retrospectively evaluated 236 patients who were wearing implant-supported prostheses for at least 5 years. Although there appeared to be a clinically significant correlation between prostheses discrepancies and loose gold screws, neither clinical nor radiographic findings indicated that these misfits affected the long-term osseointegration or maintenance of the bone level.

Recent studies were designed to correlate degrees of framework misfit and bone response.

CONCLUSIONS

On the basis of what is known, the relative misfit with the available fit evaluation methods cannot be accurately assessed and determined. In the absence of such quantitative fit guidelines, achieving passive fit may be of emotional reasons rather than of evidence-based science. However, implant components and bone appear to tolerate a degree of misfit without adverse biomechanical problems. The level of this misfit has yet to be determined. Therefore improving clinical techniques such as the

Acknowledgements

We would like to acknowledge Dr. John B. Holmes for his assistance in editing the manuscript.

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Radiographic examination revealed no gaps at implant-prosthetic connection of the definitive IFCDPs. The 3D-printed casts showed an overall average distance deviation within the clinically acceptable range of errors of 150 µm. Quantitative occlusal relationship analysis with IOS showed well-distributed contacts.
Within the limitations of this study, the following conclusions can be drawn:
(1) A 3-unit toolset with ISBs, LPAs and OPAs allows to register the implant position and maxillomandibular relationship in one single visit;
(2) the 2-stage clinical workflow with the CAPS system facilitates the IOS data acquisition for fabrication of an interim IFCDP at the same day;
(3) a passive fit was demonstrated for the interim and the definitive IFCDPs.
The CAPS system can help clinicians to register the implant position and the maxillomandibular relationship in one single visit for the fabrication of an IFCDP.
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The accuracy of intraoral scanners (IOSs) in recording edentulous jaws has improved recently. However, improvement in accuracy does not necessarily imply the clinical validity of the scans, and limited information is available regarding the manufacture of passively fitting prostheses.
The purpose of this in vitro study was to analyze the passivity of complete arch screw-retained frameworks fabricated using different acquisition techniques.
A 3-dimensional maxillary edentulous model to receive all-on-4 screw-retained frameworks was prototyped. Eighteen polymethylmethacrylate (PMMA) frameworks were fabricated with a 5-axis milling machine and divided into 3 groups according to the acquisition technique (n=6): scanned by using an IOS (CEREC Primescan; Dentsply Sirona), scanned with the aid of an auxiliary device by using the same IOS, and by using a conventional impression and then scanning the stone cast with an extraoral scanner (EOS). The passivity of fit of the frameworks was tested with the 1-screw test, the terminal screw of the framework assembly was tightened on the multiunit abutment (MUA), and the vertical marginal gap (µm) was measured at the other 3 framework-to-abutment interfaces by using a digital microscope at ×40 magnification. A modification to the 1-screw test was analyzed by tightening all screws and then unscrewing all except 1 of the anterior abutments. Data were explored for normality by using the theoretical quantile-quantile (Q-Q) plots and the Shapiro-Wilk test of normality. The Friedman test compared data between the different acquisition techniques; the tightening methods and locations (buccal and palatal) were used as the block variable. The post hoc Dunn test was used when the Friedman test was significant. The Kruksal-Wallis test compared the data from the 2 groups of the tightening methods and the 2 location groups. The aligned rank transformation (ART) ANOVA test was used for the interaction effects among the 3 variables. A multiway ANOVA was applied to the ranked data. (α=.05 for all tests).
Significant differences were found among all groups (P<.001). Regarding the passivity of fit, the mean vertical marginal gap was 50 µm for frameworks fabricated from an intraoral scan with the aid of an auxiliary device, 62 µm for frameworks fabricated by using an IOS, and 140 µm for frameworks fabricated by using an EOS. No significant difference was found among all groups regarding the tightening method (P=.355) or location measured (P=.175).
Digital scanning with the aid of an auxiliary device resulted in the best fit; however, digital approaches with or without the auxiliary device resulted in a more accurate fit with a smaller marginal gap than with the conventional impression.
Misfit simulation on implant prostheses with different combinations of engaging and nonengaging titanium bases. Part 2: Screw resistance test
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Prosthesis fit is 1 of the main factors influencing the success and survival of an implant-supported screw-retained restoration. However, scientific validation of the performance of engaging and nonengaging components in a fixed partial denture (FPD) and the effect of their combinations on the fit of FPDs is lacking. The screw resistance test has been used for the fit assessment of screw-retained FPDs. However, objective assessments by using analog and digital devices are now available.
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Thirty 2-implant-supported bar-shaped zirconia frameworks cemented on two 2-mm titanium bases were fabricated and divided into 3 groups (n=10) according to different abutment combinations: both engaging, engaging and nonengaging, both nonengaging. The fit of each framework was tested on the control cast and on 6 definitive casts simulating 50-, 100-, and 150-μm vertical and 35-, 70-, and 100-μm horizontal misfit levels. The abutment screws were tightened on each implant, and the screw rotation angle was measured both digitally, with a custom-made digital torque wrench and a computer software program, and conventionally, with an analog torque wrench and protractor. Clearly ill-fitting specimens were excluded. The data were statistically analyzed by 1-way analysis of variance (ANOVA) and the Tukey post hoc test (α=.05).
Both engaging specimens on the 100-μm horizontal misfit group and on all vertical misfit groups were clearly ill-fitting and excluded. Statistically significant differences among groups with different combinations of abutments were found (P<.05). The engaging abutments had a higher angle of rotation than the nonengaging abutments on all casts. In the horizontal misfit group, both engaging specimens had the highest angle of rotation, followed by engaging and nonengaging and both engaging specimens. In the vertical misfit group, the engaging and nonengaging specimens had the highest angle of rotation on the side of the engaging abutment. The angle of rotation increased with the increasing level of misfit.
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A predictable protocol for accurately scanning implants in a complete edentulous arch has not been established.
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This single center, nonrandomized, clinical trial included a total of 19 arches. Definitive casts with scan bodies were fabricated and scanned with a laboratory scanner as the reference (control) scan. Each participant received 2 intraoral scans, the first with unsplinted scan bodies and the second with resin-splinted scan bodies. The scan time was also recorded for each scan. To compare the accuracy of the scans, the standard tessellation language (STL) files of the 2 scans were superimposed on the control scan, and positional and angular deviations were analyzed by using a 3-dimensional (3D) metrology software program. The Mann-Whitney U test was used to compare the distance and angular deviations between the splinted group and the unsplinted group with the control. The ANOVA test was conducted to examine the effect of the scan technique on trueness (distance deviation and angular deviation) and scan time (α=.05 for all tests).
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Clinical methods for evaluating implant framework fit☆,☆☆,★

Abstract

Section snippets

ACCEPTABLE LEVELS OF FIT

FACTORS AFFECTING FRAMEWORK FIT EVALUATION

METHODS FOR EVALUATING FRAMEWORK FIT

INSTRUMENTS

BIOLOGIC TOLERANCE

CONCLUSIONS

Acknowledgements

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