Assessment of Fit on Ten Screw-Retained FrameworksRealized through Digital Full-Arch Implant Impression

: Background: Discordant opinions have emerged among clinicians and researchers regarding a digital impression for full-arch implant-supported ﬁxed dental prostheses (FDPs). The purpose of this study was to assess the ﬁt of screw-retained milled frameworks on six implants realized from digital impressions through the Shefﬁeld test. Methods: One patient received a maxillary full-arch implant-supported FDP. Six months after the surgical procedure, ten intraoral full-arch digital impressions were performed to mill ten frameworks. To clinically assess the ﬁt, the Shefﬁeld test was applied for all frameworks. The gaps among the frameworks and the implant analogs were measured using a microscope on the master model realized with a traditional impression. The Wilcoxon sum-rank test was used to compare the misﬁt value among the different implant positions. Results: The Shefﬁeld test did not show gaps in the framework–implant interfaces when the screw was completely tightened on the more distal implant for all the milled frameworks. The mean misﬁt value calculated after microscope examination was 38 ± 5 µ m. Differences that were statistically signiﬁcant emerged when the misﬁt values of central positions were compared with other values. Conclusions: The use of full-arch implant digital impressions represents a viable alternative to traditional impressions for the fabrication of implant-supported FDPs.


Introduction
Intraoral scanners (IOS) are becoming a commonly used tool in dental clinical practice [1]. A decrease in operative time and patient discomfort and increased accuracy represent the main advantages that digital technology has introduced into clinical practice [2,3]. Regarding the digital impression for single crowns [4,5] or three-unit fixed dental prostheses (FDPs) [6], several authors have demonstrated the better performance of a digital workflow. However, discordant opinions have emerged among clinicians and researchers regarding a digital impression for full-arch implant-supported FDPs.
In the literature, most studies that have investigated this topic were conducted in vitro and the findings showed that not all IOS were suitable for digital impressions in full-arch implant-supported FDPs [7][8][9][10][11][12][13][14]. The correct execution of an impression in prosthodontics is fundamental to avoid a misfit among the interface of the implant and the prosthesis and consequently an increased risk of mechanical and biological complications [15]. An acceptable misfit value was reported by several authors; however, there are different opinions. Branemark et al. [16] classified a misfit as a value above 10 µm, Jemt [17] declared as acceptable any values less than 150 µm, and, conversely, Di Fiore et al. [7] believed that the misfit value should be below 30-50 µm.

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To the best of the authors' knowledge, there are three articles that have investigated in vivo the accuracy of the digital impression for full-arch implant-supported FDPs [18][19][20]. In two articles, the authors compared the implant-supported FDPs on four implants realized using digital and traditional impressions through panoramic radiographs during the follow-up examinations. No signs of misfit were identified among the framework and implants [18,19]. However, the third clinical trial compared the accuracy of digital versus conventional full-arch implant impressions of edentulous patients [20]. The authors demonstrated the possibility of fabricating a maxillary fixed complete denture using a digital impression. However, the authors did not realize frameworks but calculated the 3D deviations between the Standard Tessellation Language (STL) files from the intraoral digital scan and the STL files from extraoral digitalized final stone casts.
Clinical assessment of the passive fit between framework and implants is difficult [21]. Several methods have been suggested, but they all have their limitations. However, the Sheffield test, radiographs, visual inspection, and tactile sensation are considered the most common clinical evaluation methods [21][22][23] and are sometimes used in laboratory research [24]. The fit of implant-supported FDPs made from the conventional impression is well described in the literature [15,24], while there is a lack of research regarding the clinical evaluation of the fit of full-arch implant-supported FDPs realized through the digital workflow.
Therefore, the aim of this clinical trial was to evaluate the fit of screw-retained milled frameworks on six implants realized from digital impressions through the Sheffield test and radiographs. Moreover, the misfit value was measured using a microscope on a printed master model. The null hypothesis was that all the frameworks would show comparable fit without any difference.

Materials and Methods
A 58-year-old woman with no problematic medical history (ASA 1) presented to the dental office with the chief complaint of dental pain, tooth mobility, and unsatisfactory esthetics ( Figure 1).
in vivo the accuracy of the digital impression for full-arch implant-suppo 20]. In two articles, the authors compared the implant-supported FDPs on realized using digital and traditional impressions through panoramic radio the follow-up examinations. No signs of misfit were identified among the f implants [18,19]. However, the third clinical trial compared the accuracy o conventional full-arch implant impressions of edentulous patients [20] demonstrated the possibility of fabricating a maxillary fixed complete d digital impression. However, the authors did not realize frameworks but 3D deviations between the Standard Tessellation Language (STL) files from digital scan and the STL files from extraoral digitalized final stone casts.
Clinical assessment of the passive fit between framework and impla [21]. Several methods have been suggested, but they all have their limitati the Sheffield test, radiographs, visual inspection, and tactile sensation are most common clinical evaluation methods [21][22][23] and are sometimes used research [24]. The fit of implant-supported FDPs made from the conventio is well described in the literature [15,24], while there is a lack of research clinical evaluation of the fit of full-arch implant-supported FDPs realized th ital workflow.
Therefore, the aim of this clinical trial was to evaluate the fit of screw-r frameworks on six implants realized from digital impressions through th and radiographs. Moreover, the misfit value was measured using a mi printed master model. The null hypothesis was that all the frameworks wo parable fit without any difference.

Materials and Methods
A 58-year-old woman with no problematic medical history (ASA 1) p dental office with the chief complaint of dental pain, tooth mobility, and esthetics ( Figure 1). The clinical and radiographic examination revealed several missing t plaque, and periodontal disease (Stage IV, Grade C). The patient did not re ular or muscular symptoms. The preliminary treatment plan focused on t chart and non-surgical periodontal treatment in both arches. After re-eva diagnostic wax-up, different possible definitive treatments were discusse tient, including risks, benefits, and costs. The patient wished to rehabilitate arch through a full-arch implant-supported FDP. Cone beam computed tom prescribed to plan the implant placements. The presented study was perfor The clinical and radiographic examination revealed several missing teeth, bleeding, plaque, and periodontal disease (Stage IV, Grade C). The patient did not report any articular or muscular symptoms. The preliminary treatment plan focused on the periodontal chart and non-surgical periodontal treatment in both arches. After re-evaluations and a diagnostic wax-up, different possible definitive treatments were discussed with the patient, including risks, benefits, and costs. The patient wished to rehabilitate only the upper arch through a full-arch implant-supported FDP. Cone beam computed tomography was prescribed to plan the implant placements. The presented study was performed in private practice, in accordance with the Declaration of Helsinki of 1964, as revised in 2013, as Good Clinical Research Practice requires. The operator asked the patient to give full and informed consent before inclusion within the protocol.
In the first surgery step, all teeth except elements 13 and 23 were extracted and dental implants (Tapered Screw-Vent Implant, Zimmer Biomet, Conegliano, Italy) in locations 16,14,12,22,24, and 16 were placed. Teeth 13 and 23 were prepared with an abutment tooth to support a temporary prosthesis realized by using the diagnostic wax-up. After 4 months, temporary abutments were screwed into the implants, the temporary prosthesis was relined using the abutment teeth as support, and elements 13 and 23 were extracted. After a 1-month period to allow for the healing of the soft tissue, the digital impressions were performed using the IOS (iTero Align, Align Technology, Tempe, AZ, USA). Six scan-bodies (Gentek, Zimmer Biomet, Conegliano, Italy) were tightened with a special screwdriver (Zimmer Biomet, Conegliano, Italy) and a manual torque-controlled ratchet (15 Ncm), and 10 full-arch digital implant impressions were acquired according to the manufacturer's instructions ( Figure 2). After scanning the antagonists, a buccal scan with teeth in maximum intercuspation position as a bite registration was performed. During scanning, a dry field was maintained using a dental aspirator. The implant-supported temporary prosthesis was acquired to transfer the vertical dimension, shape, and occlusal morphology to the definitive prosthesis [5]. The 10 STL files were uploaded to CAD software (Exocad, Exocad DentaCad, Darmstadt, Germany) to design the frameworks. Successively, 10 titanium frameworks were realized with a 5-axis milling machine (Zfx in-house 5X, Zimmer Biomet, Conegliano, Italy) ( Figure 3). In the first surgery step, all teeth except elements 13 and 23 were extracted and dental implants (Tapered Screw-Vent Implant, Zimmer Biomet, Conegliano, Italy) in locations 16,14,12,22,24, and 16 were placed. Teeth 13 and 23 were prepared with an abutment tooth to support a temporary prosthesis realized by using the diagnostic wax-up. After 4 months, temporary abutments were screwed into the implants, the temporary prosthesis was relined using the abutment teeth as support, and elements 13 and 23 were extracted. After a 1-month period to allow for the healing of the soft tissue, the digital impressions were performed using the IOS (iTero Align, Align Technology, Tempe, AZ, USA). Six scan-bodies (Gentek, Zimmer Biomet, Conegliano, Italy) were tightened with a special screwdriver (Zimmer Biomet, Conegliano, Italy) and a manual torque-controlled ratchet (15 Ncm), and 10 full-arch digital implant impressions were acquired according to the manufacturer's instructions ( Figure 2). After scanning the antagonists, a buccal scan with teeth in maximum intercuspation position as a bite registration was performed. During scanning, a dry field was maintained using a dental aspirator. The implant-supported temporary prosthesis was acquired to transfer the vertical dimension, shape, and occlusal morphology to the definitive prosthesis [5]. The 10 STL files were uploaded to CAD software (Exocad, Exocad DentaCad, Darmstadt, Germany) to design the frameworks. Successively, 10 titanium frameworks were realized with a 5-axis milling machine (Zfx in-house 5X, Zimmer Biomet, Conegliano, Italy) ( Figure 3).  Moreover, a traditional impression by using coping pick-up and the open tray technique was used to realize a master model [15].
The passive fit was assessed by the Sheffield test, screwing each framework onto the mouth of the patient. According to this test, the framework presented a passive fit when one screw on the distal abutment was completely tightened without creating a gap among the other framework-implant interfaces. If the fit was not sufficient, the superstructure was lifted when a screw was tightened, creating a gap at the level of one or more abutment analogs [21,22] (Figure 4). Moreover, a traditional impression by using coping pick-up and the open tray technique was used to realize a master model [15].
The passive fit was assessed by the Sheffield test, screwing each framework onto the mouth of the patient. According to this test, the framework presented a passive fit when one screw on the distal abutment was completely tightened without creating a gap among the other framework-implant interfaces. If the fit was not sufficient, the superstructure was lifted when a screw was tightened, creating a gap at the level of one or more abutment analogs [21,22] (Figure 4). In addition, the frameworks were screwed onto the master model to measure the interface among frameworks and implants by using a digital microscope (AM7915MZL, Dino-Lite Microscope, Almere, The Netherlands) at 150× magnification ( Figure 5). For each framework, we obtained 6 measurements. The average of the 6 measurements was considered as the overall misfit value for the framework. Descriptive statistical analysis was performed. Average and standard deviations were calculated. Comparative statistical analysis was performed to compare the misfit value among the different implant positions. The non-parametric Wilcoxon sum-rank test was used with α = 0.05 and statistical power of 80%. Regarding the radiograph examination, a random selection of only one framework was chosen. A customized X-ray holder was realized and the film was placed as perpendicular as possible to the long axis of the implant-framework interface to improve the accuracy of the radiographs. After the fit evaluation of the frameworks, one of these was used to realize the definitive implant-supported FDP. In addition, the frameworks were screwed onto the master model to measure the interface among frameworks and implants by using a digital microscope (AM7915MZL, Dino-Lite Microscope, Almere, The Netherlands) at 150× magnification ( Figure 5). Moreover, a traditional impression by using coping pick-up and the open tray tech nique was used to realize a master model [15].
The passive fit was assessed by the Sheffield test, screwing each framework onto th mouth of the patient. According to this test, the framework presented a passive fit when one screw on the distal abutment was completely tightened without creating a gap amon the other framework-implant interfaces. If the fit was not sufficient, the superstructur was lifted when a screw was tightened, creating a gap at the level of one or more abutmen analogs [21,22] (Figure 4). In addition, the frameworks were screwed onto the master model to measure th interface among frameworks and implants by using a digital microscope (AM7915MZL Dino-Lite Microscope, Almere, The Netherlands) at 150× magnification ( Figure 5). For each framework, we obtained 6 measurements. The average of the 6 measure ments was considered as the overall misfit value for the framework. Descriptive statistica analysis was performed. Average and standard deviations were calculated. Comparativ statistical analysis was performed to compare the misfit value among the different im plant positions. The non-parametric Wilcoxon sum-rank test was used with α = 0.05 and statistical power of 80%. Regarding the radiograph examination, a random selection o only one framework was chosen. A customized X-ray holder was realized and the film was placed as perpendicular as possible to the long axis of the implant-framework inter face to improve the accuracy of the radiographs. After the fit evaluation of the frame works, one of these was used to realize the definitive implant-supported FDP. For each framework, we obtained 6 measurements. The average of the 6 measurements was considered as the overall misfit value for the framework. Descriptive statistical analysis was performed. Average and standard deviations were calculated. Comparative statistical analysis was performed to compare the misfit value among the different implant positions. The non-parametric Wilcoxon sum-rank test was used with α = 0.05 and statistical power of 80%. Regarding the radiograph examination, a random selection of only one framework was chosen. A customized X-ray holder was realized and the film was placed as perpendicular as possible to the long axis of the implant-framework interface to improve the accuracy of the radiographs. After the fit evaluation of the frameworks, one of these was used to realize the definitive implant-supported FDP.

Results
For the clinical examinations, the Sheffield test did not show gaps among frameworkimplant interfaces when the screw was completely tightened on the more distal implant for all the milled frameworks. The same result emerged also after the examination of the master model. After microscope analysis, the mean misfit value was 38 ± 5 µm (min = 34 Max = 44) for all the frameworks. The results are reported in Table 1. Differences that were statistically significant emerged among the misfit values of the implant in positions: 1 versus 3 (p = 0.004), 1 versus 4 (p < 0.001), 2 versus 3 (p < 0.001), 2 versus 4 (p < 0.001), 3 versus 5 (p = 0.006), 3 versus 6 (p = 0.002), 4 versus 5 (p = 0.002), and 4 versus 6 (p < 0.001). No statistical differences were found with the remaining combination. Within the limitations of the radiograph examination, the RX did not show any clinical misfit between the framework chosen and the implants ( Figure 6).

Results
For the clinical examinations, the Sheffield test did not show gaps amon work-implant interfaces when the screw was completely tightened on the more d plant for all the milled frameworks. The same result emerged also after the exam of the master model. After microscope analysis, the mean misfit value was 38 ± 5 = 34 Max = 44) for all the frameworks. The results are reported in Table 1. Differences that were statistically significant emerged among the misfit valu implant in positions: 1 versus 3 (p = 0.004), 1 versus 4 (p < 0.001), 2 versus 3 (p < versus 4 (p < 0.001), 3 versus 5 (p = 0.006), 3 versus 6 (p = 0.002), 4 versus 5 (p = 0.0 4 versus 6 (p < 0.001). No statistical differences were found with the remaining c tion. Within the limitations of the radiograph examination, the RX did not show ical misfit between the framework chosen and the implants ( Figure 6).

Discussion
None of the frameworks realized by full-arch digital impressions showed m ther clinically onto the patient or onto the master model. Moreover, the radiograp inations performed on the randomized framework did not reveal any misfit eithe The passive fit can be assessed using two methods: in vivo and in vitro. C various techniques have been introduced; however, none has gained full accepta thorough test [25]. The Sheffield test, finger pressure, visual inspection, tactile sen radiographs, and screw-resistant test are the main in vivo methods described in ature to assess the implant-framework gap, but every procedure has merits an tions [21,22,25].
The major studies published in the literature assessed the marginal fit of crown on an abutment tooth. Di Fiore et al. [26] investigated the range of value marginal and internal fit of a single crown on an abutment tooth. The findings hig

Discussion
None of the frameworks realized by full-arch digital impressions showed misfit, either clinically onto the patient or onto the master model. Moreover, the radiograph examinations performed on the randomized framework did not reveal any misfit either.
The passive fit can be assessed using two methods: in vivo and in vitro. Clinically, various techniques have been introduced; however, none has gained full acceptance as a thorough test [25]. The Sheffield test, finger pressure, visual inspection, tactile sensations, radiographs, and screw-resistant test are the main in vivo methods described in the literature to assess the implant-framework gap, but every procedure has merits and limitations [21,22,25].
The major studies published in the literature assessed the marginal fit of a single crown on an abutment tooth. Di Fiore et al. [26] investigated the range of values for the marginal and internal fit of a single crown on an abutment tooth. The findings highlighted