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Journal of Prosthetic Dentistry

Accuracy of digital complete-arch, multi-implant scans made in the edentulous jaw with gingival movement simulation: An in vitro study

Open AccessPublished:February 18, 2021DOI:https://doi.org/10.1016/j.prosdent.2020.12.037

      Abstract

      Statement of problem

      The use of computer-aided design and computer-aided manufacturing (CAD-CAM) technologies is widely established, with single restorations or short fixed partial dentures having similar accuracy when generated from digital scans or conventional impressions. However, research on complete-arch scanning of edentulous jaws is sparse.

      Purpose

      The purpose of this pilot in vitro study was to compare the accuracy of a digital scan with the conventional method in a workflow generating implant-supported complete-arch prostheses and to establish whether interference from flexible soft tissue segments affects accuracy.

      Material and methods

      An edentulous maxillary master cast containing 6 angled implant analogs was used and digitized with mounted scan bodies by using a high-precision laboratory scanner. The master cast was then scanned 10 times with 4 different intraoral scanners: TRIOS 3 with a complete-arch scanning strategy (TRI1) or implant-scanning strategy (TRI2), TRIOS Color (TRC), CEREC Omnicam (CER), and CEREC Primescan (PS). The same procedure was repeated with 4 different levels of free gingiva (G0–G3). Ten conventional impressions were obtained. Differences in implant position and direction were evaluated at the implant shoulder as mean values for trueness and interquartile range (IQR) for precision. Statistical analysis was performed by using the Kruskal–Wallis and post hoc Conover tests (α=.05).

      Results

      At G0, position deviations ranged from 34.8 μm (IQR 23.0 μm) (TRC) to 68.3 μm (12.2 μm) (CER). Direction deviations ranged from 0.34 degrees (IQR 0.18 degrees) (conventional) to 0.57 degrees (IQR 0.37 degrees) (TRI2). For digital systems, the position deviation ranged from 48.4 μm (IQR 5.9 μm) (PS) to 76.6 μm (IQR 8.1 μm) (TRC) at G1, from 36.3 μm (IQR 9.3 μm) (PS) to 79.9 μm (IQR 36.1 μm) (TRI1) at G2, and from 51.8 μm (IQR 14.3 μm) (PS) to 257.5 μm (IQR 106.3 μm) (TRC) at G3. The direction deviation ranged from 0.45 degrees (IQR 0.15 degrees) (CER) to 0.64 degrees (IQR 0.20 degrees) (TRC) at G1, from 0.38 degrees (IQR 0.05 degrees) (PS) to 0.925 degrees (IQR 0.09 degrees) (TRI) at G2, and from 0.44 degrees (IQR 0.07 degrees) (PS) to 1.634 degrees (IQR 1.08 degrees) (TRI) at G3. Statistical analysis revealed significant differences among the test groups for position (G0: P<.001; G1: P<.05; G2: P<.001; G3: P<.001) and direction (G0: P<.005; G1: P<.001; G2: P<.001; G3: P<.001).

      Conclusions

      Without soft tissue interference, the accuracy of certain digital scanning systems was comparable with that of the conventional impression technique. The amount of flexible soft tissue interference affected the accuracy of the digital scans.
      Clinical Implications
      With improvements in both hardware and software, the accuracy of digital scans has improved considerably. Digital scanning appears to have become a valid alternative to conventional impression techniques for specific implant-supported indications.
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      as well as the operator’s protocol.
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      The only in vivo study included in Rutkunas et al’s review
      • Rutkūnas V.
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      • Vaitiekūnas M.
      Accuracy of digital implant impressions with intraoral scanners. A systematic review.
      concluded that the main source of error for intraoral optical scanning was the surrounding movable soft tissue, which led to unreliable height scanning and false image stitching. To the authors’ knowledge, none of the in vitro or in vivo studies investigated the impact of surrounding soft tissue on the accuracy of the scans. The aim of this explorative in vitro study was to examine the accuracy of acquiring multiple implant positions in an edentulous master cast with different configurations of fixed and movable gingiva-like surfaces. The null hypothesis was that the accuracy of digital implant scans with different amounts of movable tissue interference is similar to that of conventional implant impressions in terms of implant position and direction.

      Material and Methods

      An edentulous maxillary stone cast containing 6 implant analogs (Biomet 3i Certain 3.4; Zimmer Biomet) in the regions of the first molars, first premolars, and lateral incisors was used as a reference model (Fig. 1A). The cast was covered with a silicone gingival mask (Gingifast Elastic; Zhermack GmbH) (Fig. 1B). For all digital scanning procedures, single-piece scan bodies (Elos Accurate IO Scan Body; Elos Medtech) were used (Fig. 1C). For conventional impression making, pick-up copings (Biomet 3i Pick-up; Zimmer Biomet) were used. Each test group consisted of 10 repeated scans or impressions (n=10).
      Figure thumbnail gr1
      Figure 1A, Master model. B, With gingival mask. C, With mounted scan bodies. D, Image made with high-precision laboratory scanner.
      For the reference scan evaluation, the master model was scanned 10 times from different directions with a high-precision laboratory scanner (EOLS) (inEos X5; Dentsply Sirona) to evaluate the EOLS accuracy (Fig. 1D).
      • Ender A.
      • Mehl A.
      Accuracy of complete-arch dental impressions: a new method of measuring trueness and precision.
      The data sets were superimposed and evaluated as described subsequently. As EOLS showed highly accurate scanning results, one scan was selected to be used as a reference for later comparisons with all test groups.
      Digital scans were made with 4 different intraoral scanners (Fig. 2A): TRIOS 3 (TRI1 – complete-arch scanning strategy–and TRI2 – implant-scanning strategy; 3Shape A/S; software version 1.5.1.3), TRIOS Color (TRC; 3Shape A/S; software version 1.4.7.2), CEREC Omnicam (CER; Dentsply Sirona; software version 4.6), and CEREC Primescan (PS; Dentsply Sirona; software version 5.0.0). All scans were obtained by using the scanning methods suggested by the manufacturers (Table 1).
      Figure thumbnail gr2
      Figure 2Intraoral scans from group TRI1. A, Gingival level G0. B, Gingival level G1. C, Gingival level G2. D, Gingival level G3.
      Table 1Impression/scanning procedures used for test groups to generate standard tessellation language files
      Test GroupSystem/SoftwareImpression Making/Scan StrategyPostprocessing
      RefscanSirona inEos X5Scan bodies: Elos Accurate IO Scan Body (Elos Medtech)

      Digitization
      Direct export to STL
      COIdentium Light + Identium Medium, open-tray splinted conventional impression making
      • custom-made open tray (Profibase; VOCO GmbH)
      • copings (Biomet 3i Pick-up; Zimmer Biomet)
      • splinted using a pattern resin (DuraLay; CG Lab Technologies Inc)
      • impressions made using the sandwich technique (Identium Light and Medium; Kettenbach GmbH & Co KG)
      • 10 min setting time
      • gingival mask (rema Sil; DENTAURUM GmbH & Co KG)
      • 8 h storage time before pouring
      • pouring with Type IV gypsum (Fujirock EP; CG Lab Technologies Inc)
      • 48 h storage time before digitizing
      Digitized with inEos X5 (inLAB 16), direct export to STL
      CERCEREC Omnicam, CEREC SW 4.6Scan bodies: Elos Accurate IO Scan Body (Elos Medtech)

      Scan strategy: as suggested by manufacturer for complete-arch scanning (Fig. 2B) (https://manuals.sirona.com/home.HomeDmsDocument.download.html?id=21927)
      Direct export to STL
      TRI1TRIOS 3, TRIOS software 1.5.1.3Scan bodies: Elos Accurate IO Scan Body (Elos Medtech)

      Scan strategy: as suggested by manufacturer for complete-arch scanning (Fig. 2A) (https://www.youtube.com/watch?v=M_KbWcCianY) (user manual)
      Direct export to STL
      TRI2TRIOS 3, TRIOS software 1.5.1.3Scan bodies: Elos Accurate IO Scan Body (Elos Medtech)

      Scan strategy: as suggested by manufacturer for implant scanning (Fig. 2A) (https://www.youtube.com/watch?v=AwemiJ654Ko) (user manual)
      Direct export to STL
      TRCTRIOS Color, TRIOS software 1.4.7.2Scan bodies: Elos Accurate IO Scan Body (Elos Medtech)

      Scan strategy: as suggested by manufacturer for implant scanning (Fig. 2A) (https://www.youtube.com/watch?v=AwemiJ654Ko) (user manual)
      Direct export to STL
      PSCEREC Primescan, CEREC SW 5.0.0Scan bodies: Elos Accurate IO Scan Body (Elos Medtech)

      Scan strategy: as suggested by manufacturer for complete-arch scanning (Fig. 2B) (https://www.youtube.com/watch?v=TChcmvQiGJk)
      Direct export to STL
      At gingival level G0, the master model was scanned without changes (Fig. 2A). For the digital scans at gingival level G1-G3, a dental dam was fixed to the silicone gingival mask with a cyanoacrylate adhesive (Renfert) to simulate different levels of free gingiva (Fig. 2B–D). The free tissue movements were simulated by using a hanger assembly and by manually tilting and rotating the model during digital scanning. The scanning procedure was repeated with each scanner at 3 levels of simulated free gingiva, reducing the amount of fixed soft tissue at every step. At G1, the total surface area of fixed gingiva averaged 1764.7 mm2, decreasing to 1008.9 mm2 at G2 and to 423.6 mm2 at G3 (Fig. 2B–D).
      Conventional impressions were made at gingival level G0 as described in Table 1 and displayed in Figure 3 and served as the control group. The schematic workflow for all impression procedures and reference scan evaluations is displayed in Figure 4.
      Figure thumbnail gr3
      Figure 3A, Splinting of copings with pattern resin. B, Custom tray for conventional impression making. C, Intaglio view of impression with copings visible. D, Poured cast with implant analogs. E, Cast with mounted scan bodies on implant analogs. F, Reference model digitized with high-precision laboratory scanner.
      Two parameters were evaluated: the position and direction of the scanned implants. Position was defined as the center point of a modeled plane at the implant shoulder. Direction was defined as the surface normal vector (perpendicular to the surface) of this modeled plane, which equaled the implant axis.
      Each impression or scan resulted in a standard tessellation language (STL) file (Fig. 5A). This file was imported into a software program (Dental Designer, v16.2.0; 3Shape A/S). Through digital 3-point surface scan-post matching, the exact implant positions were calculated (Fig. 5B). A joint bar superstructure was created (Fig. 5C), and the output data of this joint bar included the position (mm) and direction (degrees) of each implant in relation to the 3 coordinate system axes (x, y, z) at the abutment shoulder level and the center point of the implant (Fig. 5D).
      Figure thumbnail gr5
      Figure 5A, Data file imported into 3Shape Dental Designer software program. B, Digital scan-post matching. C, Digitally generated superstructure. D, Digitally calculated implant positions with 3D orientation of implants.
      The position and direction data of each implant were imported into a computer-aided design (CAD) software program (Geomagic Qualify software 12; 3D Systems Inc). A standardized 2×2-mm plane was created as a Feature Object for every implant shoulder level by using these data for plane orientation. All scans were processed in this way to generate STL files for deviation comparisons with 6 separate planes at the respective implant (Fig. 6A).
      Figure thumbnail gr6
      Figure 6A, Modeled planes at implant shoulder. B, Best-fit match of modeled planes between test group and reference model file. C, Calculated differences of matched planes for each implant at implant shoulder level and central axis with differences in position and direction in x-y-z coordinate system.
      For trueness evaluation, each processed STL file was superimposed with the reference scan using the Geomagic best-fit algorithm applied to the Feature Objects (Fig. 6B). The output data of the best-fit match display the position deviation of each implant shoulder in all 3 dimensions—x, y, and z—separately and the normal deviation (Fig. 6C). To obtain a single position value per implant that includes the accumulated deviation in all 3 dimensions, the total position deviation was calculated. The direction deviation value per implant was calculated as the angle between the plane normals.
      As every implant was measured separately, a total of 6 position and normal deviations per impression or scan were obtained. With 10 repeated impressions or scans, a total of 60 difference values for position and direction were obtained for each test group and gingival level.
      All statistical analyses and plots were performed with a statistical software program (R version 3.5.1; The R Foundation),
      R Core Team
      R: A language and environment for statistical computing.
      including the packages ggplot2
      • Wickham H.
      ggplot2: Elegant graphics for data analysis.
      and PMCMRplus.
      • Pohlert T.
      PMCMRplus: calculate pairwise multiple comparisons of mean rank sums extended. R package version 1.4.0.
      The accuracy of the different impression techniques was assessed by trueness measured by the median deviation from the reference scan and by precision measured by the IQR of the repeated measurements (n=60 per impression technique and gingival level). At each gingival level, Kruskal–Wallis tests were applied to investigate whether the impression techniques performed the same with respect to trueness of position and direction. In cases of significant differences, pairwise post hoc Conover tests were used to check which pairs of techniques showed significantly different performance. P values were adjusted for multiple testing with the Holm procedure (α=.05 for all tests).

      Results

      The accuracy of the digital scans was assessed in 2 steps, first at G0 without free gingiva and then with interference from different amounts of free gingiva (G1–G3). All results and statistically significant differences are displayed in Table 2. Deviations are expressed as median value (IQR).
      Table 2Deviation of implant position from reference model indicated as median and interquartile range (IQR) of position (μm) and direction (degrees)
      Test GroupGingival LevelPosition (μm) Median [IQR]Direction (degrees) Median [IQR]
      CO048.3 [25.9]A,B,C,D0.34 [0.18]A,B,C,D
      CER068.3 [12.2]E,F0.45 [0.04]A,E,F,G,H
      PS054.8 [6.7]A,E,G,H0.40 [0.04]B,E,I,K,L
      TRI1063.7 [24.8]B,F,G,I0.48 [0.04]F,I,M
      TRI2044.4 [19.3]C,H,I,K0.57 [0.37]C,G,K,M
      TRC034.8 [23.0]D,K0.36 [0.08]D,H,L
      CER171.3 [8.7]A,B,C,D0.45 [0.15]A,B
      PS148.4 [5.9]A,E,F0.45 [0.02]A,C
      TRI1166.3 [36.4]B,E,G,H0.51 [0.11]B,C,D,E
      TRI2153.0 [21.8]C,F,G,I0.61 [0.08]D,F
      TRC176.6 [8.1]D,H,I0.64 [0.20]E,F
      CER278.0 [27.2]A,B,C0.48 [0.15]
      PS236.3 [9.3]0.38 [0.05]
      TRI1279.9 [36.1]A,D,E0.64 [0.07]A
      TRI2270.9 [24.6]B,D,F0.93 [0.09]
      TRC270.7 [11.3]C,E,F0.58 [0.41]A
      CER364.8 [15.8]0.46 [0.07]A
      PS351.8 [14.3]0.44 [0.07]A
      TRI1379.5 [15.3]0.60 [0.19]
      TRI23257.5 [106.3]1.63 [1.08]
      TRC3106.7 [39.7]0.72 [0.12]
      CER, CEREC Omnicam; CO, conventional impression; IQR, interquartile range; PS, CEREC Primescan; TRC, TRIOS Color; TRI1, TRIOS 3 with complete-arch scanning strategy; TRI2, TRIOS 3 with implant-scanning strategy.
      Values with same uppercase letter within same column and gingival level indicate nonstatistically significant differences (pairwise comparisons using Conover test for multiple comparisons of independent samples, P>.05).
      The reference scan evaluation resulted in a median (IQR) position deviation of 6 μm (10.3 μm). The median (IQR) deviation of the direction was 0.09 degrees (0.07 degrees).
      At gingival level G0, the position median (IQR) deviations ranged from 34.8 μm (23.0 μm) (TRC) to 68.3 μm (12.2 μm) (CER). The direction median (IQR) deviations ranged from 0.34 degrees (0.18 degrees) (conventional) to 0.57 degrees (0.37 degrees) (TRI2) (Fig. 7). Statistical analysis revealed significant differences between the test groups for position (P<.001, Kruskal-Wallis chi-squared=24.2, degrees of freedom [df] =5) and direction (P<.005, Kruskal-Wallis chi-squared=19.0, df=5) (Table 2).
      Figure thumbnail gr7
      Figure 7Boxplots of median direction and position deviations at gingival level G0. Whiskers represent ×1.5 interquartile range (IQR) or minima and maxima if below 1.5×IQR. Dots represent individual measurements. IQR, interquartile range.
      At gingival level G0, the position median (IQR) deviations ranged from 48.4 μm (5.9 μm) (PS) to 76.6 μm (8.1 μm) (TRC). The direction median (IQR) deviations ranged from 0.45 degrees (0.15 degrees) (CER) to 0.64 degrees (0.20 degrees) (TRC) (Fig. 8). Statistical analysis revealed significant differences between the test groups for position (P<.05, Kruskal-Wallis chi-squared=13.8, df=4) and direction (P<.001, Kruskal-Wallis chi-squared=23.6, df=4) (Table 2).
      Figure thumbnail gr8
      Figure 8Boxplots of median direction and position deviations at all gingival levels. Whiskers represent ×1.5 interquartile range (IQR) or minima and maxima if below 1.5×IQR. Dots represent individual measurements. IQR, interquartile range.
      At gingival level G0, the position median (IQR) deviations ranged from 36.3 μm (9.3 μm) (PS) to 79.9 μm (36.1 μm) (TRI1). The direction median (IQR) deviations ranged from 0.378 degrees (0.05 degrees) (PS) to 0.925 degrees (0.09 degrees) (TRI2) (Fig. 8). Statistical analysis revealed significant differences among the test groups for position (P<.001, Kruskal-Wallis chi-squared=22.1, df=4) and direction (P<.001, Kruskal-Wallis chi-squared=33.5, df=4) (Table 2).
      At gingival level G0, the position median (IQR) deviations ranged from 51.8 μm (14.3 μm) (PS) to 257.5 μm (106.3 μm) (TRC). The direction median (IQR) deviations ranged from 0.441 degrees (0.07 degrees) (PS) to 1.634 degrees (1.08 degrees) (TRI2) (Fig. 8). Statistical analysis revealed significant differences among the test groups for position (P<.001, Kruskal-Wallis chi-squared=40.6, df=4) and direction (P<.001, Kruskal-Wallis chi-squared=40.0, df=4) (Table 2).

      Discussion

      The present study was designed to evaluate the accuracy of digital complete-arch edentulous implant scanning in comparison with conventional impression making. Additionally, the influence of the different extents of surrounding movable soft tissue on the accuracy of digital scans was evaluated. The results of this study showed that in specific situations, and for specific scanners, the difference in accuracy of implant position and direction was not significantly different from that of conventional impression methods. In contrast, the distribution of discrepancies varied widely among different systems and scanning strategies in the digital test groups (Table 2). Therefore, the null hypothesis was rejected. A trend could be observed where, with increased free gingival height, accuracy declined for certain digital scan groups. This might be caused by interference from movable tissue and more difficulties in the process of overlapping images because of missing reference structures.
      The comparison of digital scanning and conventional impressions included the digitization of the conventional impressions with an extraoral scanning device. The systematic error occurring from the digitization of the gypsum cast must be considered. Therefore, the EOLS (reference scanner) used to digitize the gypsum cast was evaluated. It showed high trueness with a median position deviation of 6 μm, high precision with an IQR of 10.3 μm, and a median direction deviation of 0.09 degrees with an IQR of 0.07 degrees. By comparing these results with the deviations measured for the conventional impression technique of 48.3 μm (IQR 25.9 μm) for position and 0.34 degrees (IQR 0.18 degrees) for direction, it can be concluded that the systematic error of the EOLS did not contribute substantially to the results of the study.
      A discrepancy threshold for the passive fit of implant-supported fixed partial dentures has not been consistent. Recent studies have suggested a discrepancy threshold of 59 to 72 μm for creating clinically acceptable complete-arch restorations,
      • Papaspyridakos P.
      • Benic G.I.
      • Hogsett V.L.
      • White G.S.
      • Lal K.
      • Gallucci G.O.
      Accuracy of implant casts generated with splinted and non-splinted impression techniques for edentulous patients: an optical scanning study.
      whereas an earlier study stated that 150 μm at the implant shoulder was an acceptable discrepancy.
      • Jemt T.
      • Lie A.
      Accuracy of implant-supported prostheses in the edentulous jaw: analysis of precision of fit between cast gold-alloy frameworks and master casts by means of a three-dimensional photogrammetric technique.
      The results of the present study showed that, in specific situations, these values can be reached with both digital scanning systems and the conventional impression method. While research has supported the use of CAD-CAM workflows for single-tooth restorations, increasing the scanned arch and the number of scanned implants increases distortions to critical values.
      • Ender A.
      • Mehl A.
      In-vitro evaluation of the accuracy of conventional and digital methods of obtaining full-arch dental impressions.
      ,
      • Ender A.
      • Attin T.
      • Mehl A.
      In vivo precision of conventional and digital methods of obtaining complete-arch dental impressions.
      ,
      • Chew A.A.
      • Esguerra R.J.
      • Teoh K.H.
      • Wong K.M.
      • Ng S.D.
      • Tan K.B.
      Three-dimensional accuracy of digital implant impressions: effects of different scanners and implant level.
      High-quality digital scans require correct scanning strategies which are unique to each scanning system.
      • Ender A.
      • Mehl A.
      Influence of scanning strategies on the accuracy of digital intraoral scanning systems.
      In the present study, the scanning strategies suggested by the manufacturers were used. For TRIOS 3, the complete arch (TRI1) and implant (TRI2) scanning strategies were applied. Comparing the results for these groups, the TRI2 showed significantly less accurate outcomes at G3 (Fig. 8). Test group TRC showed a similar behavior, with lower accuracy at the G3 level, but with a less pronounced decrease. TRC implements the same scanning strategy as TRI2 but uses different hardware components. TRC also has a larger scan window. The decreased accuracy of TRI2 and TRC at the G3 level may have been caused by the specific scan strategy. In the first scan step, it may have arisen from problematic image stitching algorithms when fixed and movable areas were scanned at the same time. This effect may have been enhanced in the second step by stitching inaccuracies of the scan body site to the surrounding soft tissue because of the lack of geometric structures. The adapted scanning strategy TRI1, scanning all the structures in one turn, did not substantially decrease the accuracy at the G3 level. Iturrate et al
      • Iturrate M.
      • Eguiraun H.
      • Etxaniz O.
      • Solaberrieta E.
      Accuracy analysis of complete-arch digital scans in edentulous arches when using an auxiliary geometric device.
      reported that accuracy was improved by using an auxiliary geometry device. However, they did not apply the 2-step scanning procedure with the 3Shape TRIOS scanner. The effect of using this kind of additional device for complete-arch implant scanning could be investigated by including the variables used in the current test setup.
      With digital scanning systems, greater interimplant distance can lead to imprecision because of missing intraoral landmarks and difficulty in distinguishing between identical scan posts.
      • Andriessen F.S.
      • Rijkens D.R.
      • van der Meer W.J.
      • Wismeijer D.W.
      Applicability and accuracy of an intraoral scanner for scanning multiple implants in edentulous mandibles: a pilot study.
      The geometry of the scan bodies may also play a role in image stitching during the scanning process. In the present study, the intraoral scanners used showed no difficulties in distinguishing among the different implants and their scan bodies.
      In conventional impression making, the clinical routine is a crucial factor for impression quality. The learning curve is steep, and the required impression quality is harder to reach than in digital scanning.
      • Joda T.
      • Lenherr P.
      • Dedem P.
      • Kovaltschuk I.
      • Bragger U.
      • Zitzmann N.U.
      Time efficiency, difficulty, and operator's preference comparing digital and conventional implant impressions: a randomized controlled trial.
      In conventional impression making, implant angulation can cause inaccuracies, probably because of the greater forces required to remove the impression when the implants are not placed parallel to each other.
      • Sorrentino R.
      • Gherlone E.F.
      • Calesini G.
      • Zarone F.
      Effect of implant angulation, connection length, and impression material on the dimensional accuracy of implant impressions: an in vitro comparative study.
      ,
      • Shim J.S.
      • Ryu J.J.
      • Shin S.W.
      • Lee J.Y.
      Effects of implant angulation and impression coping type on the dimensional accuracy of impressions.
      In the digital workflow, this factor is negligible. In the reference model used in this study, all implants were placed at different angulations and heights. By applying the standard impression making method, the conventional impression group in this study used individual impression trays, splinted copings, and the open-tray technique, which resulted in implant position deviations below 50 μm.
      In contrast with most recent studies, accuracy measurements in the present study were not made by scan surface superposition based on best-fit algorithms.
      • Papaspyridakos P.
      • Gallucci G.O.
      • Chen C.J.
      • Hanssen S.
      • Naert I.
      • Vandenberghe B.
      Digital versus conventional implant impressions for edentulous patients: accuracy outcomes.
      ,
      • Vandeweghe S.
      • Vervack V.
      • Dierens M.
      • De Bruyn H.
      Accuracy of digital impressions of multiple dental implants: an in vitro study.
      ,
      • Stimmelmayr M.
      • Erdelt K.
      • Güth J.F.
      • Happe A.
      • Beuer F.
      Evaluation of impression accuracy for a four-implant mandibular model--a digital approach.
      The 3D implant position and direction were instead calculated with CAD software by using the center point of the implant shoulder for distance measurements since this is a crucial area for creating a passively fitting superstructure. Additionally, this method of comparison is not restricted to a specific implant scan-post geometry within 1 test arrangement since the scan-post geometry is not used for the difference measurements.
      Intraoral conditions have a high impact on the quality of the digital workflow, while conventional impressions show similar precision whether they are applied in vitro or in vivo.
      • Ender A.
      • Attin T.
      • Mehl A.
      In vivo precision of conventional and digital methods of obtaining complete-arch dental impressions.
      Little clinical research has been done with edentulous patients. One study reported high deviations when using the iTero scanner with software version 3.5.0 (Cadent Inc).
      • Andriessen F.S.
      • Rijkens D.R.
      • van der Meer W.J.
      • Wismeijer D.W.
      Applicability and accuracy of an intraoral scanner for scanning multiple implants in edentulous mandibles: a pilot study.
      Digital scans in the current study showed accuracy similar to that of conventional impressions at the G0 gingival level. At higher gingival levels, greater deformations and inaccuracies in digital scans occurred. Further in vivo studies are needed to verify the present findings, while further improvement of digital intraoral scanner systems is needed to reach consistently highly accurate scanning results for multiple implant scans.

      Conclusions

      Based on the findings of this in vitro study, the following conclusions were drawn:
      • 1.
        Without soft tissue interference, the accuracy of certain digital scanning systems is comparable with that of the conventional impression technique.
      • 2.
        Results indicate a trend of decreasing accuracy of digital scanning systems with increasing soft tissue interference.
      • 3.
        There are significant differences in the accuracy of different intraoral scanners.

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