In vivo trueness of full-arch implant-supported CAD/CAM restorations and models based on conventional impressions

Objectives: To evaluate a method for in situ reference acquisition of implant positions in complete edentulous maxillae using an industrial scanner. To assess in vivo trueness of full-arch implant-supported fixed dentures (IFD) and dental models based on conventional impressions. Methods: In five subjects, scan-bodies were mounted to six maxillary implants and scanned three times using an industrial scanner (REF). Original impression-based models used to manufacture existing IFDs, (MOD1), and models fabricated from new polyether impressions, (MOD2), were scanned three times with a laboratory scanner. Scan-bodies were aligned and exported with analogue positions corresponding to implant positions. Implant analogues were mounted onto existing IFDs and scanned three times (BRIDGE). CAD files of scan-bodies with inter-aligned CAD-analogues were geometry-aligned to REF. CAD-analogues were aligned to exported files of MOD1 and MOD2, and to BRIDGE. Resulting six CAD-analogues were Globally Aligned using a consistent geometry-based alignment. Deviations were computed after a Reference Point System Alignment at the implant/ prosthetic platform for Cartesian axes and a linear Resultant. Results: REF precision was 9.3 ± 1 µ m. In vivo trueness for Resultant was MOD1: 36 ± 16 µ m, MOD2: 28 ± 7 µ m and BRIDGE: 70 ± 23 µ m, where MOD1 and MOD2 were statistically significantly different from BRIDGE. In vitro manufacturing trueness of Resultant when MOD1 acted reference for BRIDGE was: 69 ± 22. Conclusions: This method can be applied for assessing in vivo trueness. CAD/CAM processed IFD showed deviations twice that of impression-based models, however, errors from impressions and subsequent model scans were not additive to the entire workflow.


Introduction
The central part of manufacturing implant-supported fixed dentures (IFD) begins with the acquisition of the inter-implant relationship in the three-dimensional (3D) space and the soft-tissues in the immediate proximity.Models derived from impressions have been used either to manufacture cast frameworks or indirectly digitised with dental laboratory scanners for Computer-Aided Design / Computer-Aided Manufacturing (CAD/CAM) [1,2].Each step in the restorative process will undeniably introduce a certain error, however not every step may contribute equally or additively to the final misfit.
Tolerances in implants and prosthetic components are necessary from a production and handling point, and ought to have only a small impact on the total misfit.A study has shown that different implant connections may lead to varying misfit between impression copings and implant analogues [3].The mean misfit was 2.8 µm for an internal flat to flat connection, 4.3 µm for an external flat-to-flat connection, and 21 µm for an internal conical connection.Internal conical connections can further lead to an increased rotational displacement because of the friction-fit interface [4].
Most studies investigating misfit of impressions include the production of models.Therefore, the results will include all errors introduced by clinicians, dental technicians, and the associated materials and methods [5].Reviews investigating misfit in full-arch impressions have been inconclusive regarding the use of different trays, acquisition method, impression materials, use of splints, and impressions at implant or abutment-level.A factor that has been found to increase the degree of in vitro misfit is the degree of implant angulations [6,7].
There are a multitude of manufacturing processes available to fabricate a full-arch framework [1].Several studies have shown that frameworks based on CAD/CAM processes have a lower misfit than cast or laser-welded frameworks [8][9][10].
Although it is not realistic to expect a perfect or passive fit when restoring multiple implants with IFD, there is no absolute limit for a clinically acceptable misfit at the implant/prosthetic interface [1,2,11,12].
An axial misfit of 150 µm has been discussed in previous publications and commonly referred to [13].The rationale being that the distance of 150 µm equals half a turn of a prosthetic gold-screw at abutment level which can be clinically related to when performing a fit check.However, the gold-screw has been deprecated for nearly two decades and modern prosthetic screws on either abutment or implant level varies greatly between manufacturers in design and tensile properties.
The biological response to misfit varies between tooth and implants as implants lack an adaptable periodontal ligament.Implants respond with a limited 3-5 µm axial mobility with fulcrum stress concentrated at the crestal bone [16].This increased stress may create micro-fractures in the adjacent bone with potential biological complications such as bone-loss [2,12].Although the biological response and adaptation to stress is not fully understood, the consensus to minimise misfit stands [12,17].
In vitro framework misfit has been evaluated in several studies [2,11].Human and animal studies are few and use different techniques, from relatively crude visual inspection, to microscopy, torque control, strain gauging, stereo photogrammetry, and 3D Compare Analysis [12,18].
A common method for analysing the 3D fit in industrial manufacturing and quality control is through best-fit alignment comparing the measured data to a reference measurement or to the actual CAD drawing.The process is commonly followed by a 3D Compare Analysis with deviations displayed in a colour histogram [19][20][21].This method has been used extensively in dentistry when analysing free-form shapes of anatomical structures [22][23][24][25][26].
Scan-bodies used in implant dentistry carry a combination of specific geometries such as planes, cylinders, and hemispheres [27].
A method using such geometries, Datums, allows for the alignment of CAD-files of scan-bodies and analogues to their counterpart in the scanned files through Datum Alignment.The subsequent pair-wise fitanalysis can be investigated at the actual implant/prosthetic interface through a Reference Point System (RPS) Alignment using identifiable geometries.
This method is frequently found in 3D inspection and quality control of production in several industries and does not carry the same limitations seen in 3D Compare Analysis of free-form shapes [19,20].The described method has a further advantage over best-fit alignments of free-form shapes as specific areas of scan-bodies, such as sharp edges, are prone to aliasing artefacts and phantom points which may affect the 3D Analysis [28].
This study uses the ISO-5725 terminology to describe trueness and precision [29], where trueness is defined as the closeness of agreement between the arithmetic mean of a large number of test results and the true or accepted reference value, and precision is defined as the closeness of agreement between test results.
The aim of this pilot study was to investigate the possibility to acquire an in vivo reference measurement of implants in full-arch implant treatments and to report the trueness of impression-based models taken at different occasions and previously manufactured IFD.Furthermore, the aim was to evaluate in vitro trueness of the manufactured IFD using the original working model as a reference.
The primary null hypothesis was that there were no differences between in vivo trueness of two impression-based models and the manufactured IFD.
Second, there were no differences between the model used to manufacture the framework and the actual IFD.

Ethical approval
The study was conducted in accordance with ethics approval (Dnr 2016/020; Regional Ethical Review Board, Uppsala) and conforming to the standard of the Declaration of Helsinki.

Inclusion and exclusion criteria
To avoid the increased misfit seen in components with conical connections, the inclusion criteria were six maxillary implants with buttjoint external hexagonal regular platform (RP) of either Brånemark System (Nobel Biocare AB, Gothenburg, Sweden) or Biohelix (Brånemark Integration, Gothenburg, Sweden) [3,4].
The requirements of the existing IFD were an abutment-free CAD/ CAM manufactured titanium framework based on milling or lasersintering with straight or angulated screw channels.
Subjects who fulfilled the inclusion criteria and had received fullarch IFD in the edentulous maxillae at a private specialist centre of dental implantology between the years 2012 and 2017 were identified in the patient register.
The full workflow is depicted in Fig. 1.Abbreviations are listed in Table 1.Specific software commands and protocols are presented in the Appendix.
All scans using an industrial-grade scanner were performed by a specialist in oral prosthetics (first author) with training and several years' experience of handling the system.Scans were conducted without operator light and with dimmed indirect ambient lighting.The same clinician conducted the impression in direct connection to the referencescans.Handling of impressions and scans of models were conducted by experienced certified dental technicians.
All instruments were calibrated according to manufacturer's recommendations.

Acquisition and virtual models 2.3.1. Reference scan
The only officially distributed scan-body by the implant manufacturer (Elos Accurate IO 6A-B; Elos Medtech) were connected and handtightened onto the implants in the oral cavity with its 40-degree top angled plane oriented facially (Fig. 2A).
To limit movement during scanning with the reference-scanner, the head and neck of the subject was fixated with an orthopaedic vacuum pillow (223940000; Camp Scandinavia AB, Helsingborg, Sweden) with the chair raised at a 30-degree angle.Clear self-retractors were used to aid the visualisation of all scan-bodies (Adult Self Retracting; Photomed, USA).
To evaluate the precision of the scanner, three complete scans were conducted for each subject with each scan comprising 5-7 sequences depicting sufficient data for further scan-body alignment.The first sequence was initiated centrally with the scanner subsequently moved to an eccentric position at an angle to capture the scan-bodies' cylindrical part, faceted part, and the top partial circular surface (Fig. 2) in the oral cavity, (Fig. 1).
Whenever the software would warn of excessive micro-movement,

Analogue impressions
After removing scan-bodies from implants, original open tray impression copings (Brånemark System; Nobel Biocare AB) were hand tightened to the implants in each subject.A non-splinted impression was taken in an open tray (Position Tray, 3M, St Paul, USA) using polyether material (Impregum Penta; 3M).After disinfection (MD 520; Dürr Dental AG, Bietigheim-Bissingen, Germany) and according to the manufacturer's recommendation, the impression was gently air-dried.Original analogues were attached, and impressions were poured within 24-48 hours using type IV dental stone (Fujirock EP; GC Europe, Leuven, Belgium).All handling and storage of impressions and models took place at room temperature (+18 to +22 • C), (Fig. 1).
Original impression-based models taken by three different specialists in oral prosthetics used in the manufacturing of the IFD 33 to 73 months prior were acquired from room-tempered storage (+18 to +22 • C).The acquisition and pouring of models had been conducted using similar procedure.

Bridge scan
To overcome that scanners cannot successfully reproduce prosthetic connections of IFD frameworks, the only officially distributed 3D print implant analogues by the implant manufacturer (Elos Medtech Accurate Model Analogue MA-BRA41-1; Elos Medtech) were hand-tightened onto the IFD using existing bridge-screws.Because of the reflective surface of the analogues and titanium framework, the scanning procedure required an ultra-thin-coating and the use of an industrial grade scanner capable of such acquisition.A mixture of titanium dioxide and methylated spirit according to the ATOS manufacturer's recommendation was applied using an airbrush (Iwata HP-TR1; Iwata Medea, Inc, Portland, USA) at 1.0 ± 0.2 Bar onto the analogues and exposed framework (Appendix 2.1).Multiple proprietary sticker reference points (ATOS Core 80 reference points; GOM) were attached to the IFD to allow for automatic reference point identification and assisted transformation.
To evaluate the precision of the system, the attached analogues and bridge framework were scanned three times with ATOS, each comprising of 8-10 sequences.After post-processing and reference-point assisted transformation, the three scans were polygonised in the software with details set to the highest level and exported as STL files, (BRIDGE-S1, BRIDGE-S2, and BRIDGE-S3), (Fig. 1).

Model scanning and virtual 3D print models
Identical scan-bodies as used in the reference-scan were handtightened to model analogues.To digitise the models and allow for precision analysis, each model was scanned three times using a dental laboratory scanner (D1000; 3Shape, Copenhagen, Denmark) with proprietary software (3Shape Dental System Premium, version 18.2 and 3Shape ScanIT Dental 2017, version 1.17.5.1), (Fig. 1).
Open library scan-bodies (Elos Accurate -Single Abutment_7.0.0.dme, Elos Medtech) referred to as Scan-Flags in the software, were aligned to the six scan-bodies in each scan's mesh using a semiautomatic three-point alignment.
To extract the analogue positions of the aligned Scan-Flags, a virtual 3D print model was created (3Shape Dental Designer, version 18.2 and 3Shape Model Builder, version 18.2) through exact transposition of Scan-Flags to 3D print analogue fittings of primitive mesh type, (Appendix 3.1).
To simplify the alignment procedure and handling, CAD files were defeatured, removing the complex internal threading of CAD-AN and the hexagonal connection not in use for multiple-unit restorations for CAD-SB and CAD-AN (Geomagic Design X, version 2019.0.0 64-bit; 3D Systems), (Fig. 2B-C), (Appendix 4.2).
All conducted modifications and defeaturing were based on exact geometries in the CAD files and did not affect the inter-alignment of the two objects in any of the subsequent alignments.
The resulting files were exported separately in CAD STP file format as Virtual Scan-Body (VSB) and Virtual Analogue (VAN) whilst maintaining their global inter-alignment.

Datum alignment with modified CAD files
The purpose of the Datum Alignment was to transfer all scan-bodies or analogues in the scans to corresponding VAN using specific geometries or Datums.The Datums were created in 3D inspection software Geomagic Control X (Software version 2018.1.1 64-bit; 3D Systems), (Fig. 3 and Fig. 4), (Appendix 5.1-5.3).
In the following Datum Alignment protocol, Datums from Reference Data in VSB or VAN, was paired to Datums for each scan-body or analogue in Measured Data, (Fig. 1).
For REF, VSB with an inter-aligned VAN, was Datum Aligned to each scan-body as one entity, resulting in an indirectly aligned VAN.For BRIDGE, the VAN was directly Datum Aligned to the scanned analogues.Similarly, for virtual 3D print models MOD1 and MOD2, VAN

Global alignment
Because of inherently different 3D orientation of models between scans and scanners, a Global Alignment was performed using a consistent protocol based on geometries in AVAN files (Figs. 1, 5A, B). (Appendix 6.1).

RPS alignment
AVAN files assigned as Reference Data for cross-comparison analysis of trueness and precision at the implant/prosthetic interface were prepared by selecting automatically identifiable Datums based on circular geometry.
The order of the created geometry was from upper right posterior VAN (Position 1) to the upper left posterior VAN (Position 6), (Fig. 5).

Precision
For each subject, the Globally Aligned S1 assigned as Reference Data: REF-AVAN, MOD1-AVAN, MOD2-AVAN and BRIDGE-AVAN, was RPS Aligned to its respective S2 and S3 file imported as tessellated CAD in Measured Data.Similarly, each Globally Aligned S2, set as Reference Data, was RPS Aligned to its respective S3, set as Measured Data.

Trueness to REF
To establish in vivo trueness, the Globally Aligned REF-AVAN-S1, was assigned Reference Data, and RPS-aligned to file S1 through S3 of MOD1-AVAN, MOD2-AVAN and BRIDGE-AVAN imported as tessellated CAD in Measured Data.The comparison process was repeated for Globally Aligned REF-AVAN S2 and S3 set as Reference Data.

Manufacturing trueness of bridge vs MOD1
To establish the in vitro trueness, the Globally Aligned MOD1-AVAN-S1 was assigned as Reference Data and RPS Aligned to files S1 through S3 of BRIDGE-AVAN imported as tessellated CAD in Measured Data.The process was repeated for Globally Aligned MOD1-AVAN-S2 and S3 set as Reference Data and cross compared to BRIDGE-AVAN S1 through S3.

Statistical methods
Trueness and precision were evaluated using a mixed linear model with patient ID as a random factor, using the R package nlme [30].Residuals were assessed graphically, and the model fit was deemed to be adequate.Confidence intervals for the effect estimates were computed using t-statistics.To test whether accuracies were greater than 10 micrometres, a t-statistic was used.A Wald test was used to test for differences in trueness.Level of significance was set at p < 0.05.
Because there is no prior in vivo precision reported for the industrial scanner acting as a reference or in vivo trueness of models based on fullarch implant impressions and the fabricated restoration, a priori power analysis could not be performed as it would have to be based on assumptions of deviations in three dimensions.A post hoc power analysis using a Monte Carlo simulation was conducted.

Subjects
Of nineteen potential subjects, nine declined participation, one was deceased, one was excluded because of Parkinson's disease, and one was excluded for pronounced implant angulation in the molar region and thus deemed outside the limit of the reference system as established in previous studies [31,32].Seven subjects participated after prior informed consent.Six of the participants had been treated with Brånemark Implants and one subject had received Biohelix implants.
After unmounting existing IFD, two subjects were further excluded.In one subject, scan-bodies could not be attached because of two converging implants.In the subject with Biohelix implants, two implants lacked osseointegration.
The prior prosthetic treatments of the remaining five subjects with Nobel Biocare implants had been provided by three different specialists in prosthodontics and finalised by certified dental technicians at one dental laboratory.Frameworks were laser-sintered (Dentware Scandinavia AB, Kristianstad, Sweden) and designed for angulated screws (Dynamic Abutment Solutions, Lleida, Spain).The restorations had been in function between 33 and 73 months, with a mean of 56 months.

Precision
Precision for in vivo ATOS scans of REF was 9.3 ± 1 micrometres for Resultant.The equivalent for in vitro ATOS scans of BRIDGE was 1.6 ± 0.2 micrometres, and D1000 scans of MOD1 and MOD2 was 4.3 ± 0.5 micrometres, (Table 2).
ATOS scanner used for in vivo REF was statistically significantly different from ATOS scans of BRIDGE and D1000 scans of MOD1 and MOD2, (Table 3).4).
There were no statistically significant differences between MOD1 and MOD2.However, there were differences between MOD1 and MOD2 versus BRIDGE for Resultant and in all DeltaY deviations, (Table 5), (Fig. 7).

Discussion
The results in this pilot study reject both null hypotheses.First, there were in vivo differences in trueness between MOD1 and MOD2 versus BRIDGE for multiple deviations.Second, there were in vitro differences between MOD1 and BRIDGE when MOD1 served as reference.
The present pilot study uses an industrial-grade scanner to acquire an in vivo reference to which the manufacturing workflow in full-arch implant-supported treatments can be evaluated.The existing IFDs show a twofold increase in deviation over models based on polyether impressions when compared to the in vivo reference.Similarly, the deviation is twofold when comparing the IFDs to the original model used to manufacture the IFD.
There are several limitations in this pilot.First, the study uses an industrial grade scanner as a reference scanner.Second, there is a potential for bias when using multiple scanners.Last, the strict inclusion criteria and the willingness to participate in a four-hour long investigation limited the number of available subjects.
To evaluate trueness, a reference measurement will act as ground truth and requires an instrument with considerably higher trueness than the instrument which is to be investigated.The ATOS scanner is a hightrueness, high-precision industrial-grade optical scanner.Although not intended for clinical use, ATOS and similar industrial scanners have been used in vivo to acquire reference measurements [31,32].
The higher precision seen for ATOS BRIDGE versus ATOS REF (Table 2) is most likely the result of an acquisition under optimal conditions in vitro versus conditions in vivo.ATOS scans of BRIDGE allowed for the use of reference points stickers with assisted transformation, optimal angles, and scan-sequences from all necessary directions.ATOS REF acquisition was conducted on a subject where micro-movement was present, with limited accessibility, relying on best-fit alignment without assisted transformation and only capturing part of the scan-body.As a result, it was presumed that the trueness of the ATOS scanner may change accordingly, which was considered in the applied statistical method.
To acquire precision measurements below 10 µm in vivo is not only challenging due to anatomical and physiological reasons.To the authors' knowledge the measurements are within a range of what is possible to acquire with existing technology and methods.
The use of different scanners and scanning techniques may introduce operator and system bias.ATOS was used for scanning the highly reflective analogues and frameworks based on the capability of acquiring scans with ultra-thin airbrush-coating.Since the D1000    4.

Table 6
Manufacturing Trueness BRIDGE vs MOD1 as reference.scanner is not suited to capture highly reflective materials, it will require a denser and thicker coating delivered by aerosol sprays.This can lead to potentially uneven build-up.The D1000 was used to scan MOD1 and MOD2 as part of the digital workflow and to align Scan-Flags to the scan-bodies using dedicated dental laboratory software and real-world practice.Indifferent of which scanner was used in vitro, it should be noted that both ATOS and D1000 operated with precision in the range of CMM.
Limiting the inclusion-criteria to only allow abutment-free designs with CAD/CAM frameworks on external flat-to-flat connections with identical platform greatly reduced the number of potential subjects.However, by excluding abutments, confounding effects could be avoided from multiple tolerances of implant to abutment, abutment to scanbody/impression-coping, and abutment to framework.
Furthermore, the specific third-party scan-bodies and the 3D print analogues used in this study were certified and distributed by the implant manufacturer.The components are based on technical drawings of the actual implants.Not all manufacturers of scan-bodies have access to technical drawings.Instead, they rely on reverse-engineering, measuring the implants which may lead to varying tolerances and misfit.
The rationale to use only flat-to-flat connections was to limit the risk for increased axial displacement of scan-bodies, impression copings and analogues seen in conical connections [3].This allowed for hand-tightening with maintained axial fit.
This study did not investigate reseating of scan-bodies, impression copings or analogues.It can be expected that reseating will lead to a horizontal displacement in flat-to-flat connections depending on tolerance and material properties of the components.
Although rotational displacement occurs [4], this study did not investigate this parameter as IFD on multiple implants do not include an anti-rotational feature.
One factor that could impact the trueness is the angulation and interimplant distance.It is the authors' opinion that the variations seen in inter-implant distance and angulation are within clinical expectation and clinical reality, (Fig. 6).The implants are relatively well clustered and there are no extreme angulations present.However, the superimposition displays a greater variation in maxillary width of implant positions.It is unclear if this has an effect in vivo.
The presented deviations of DeltaZ, which has been of higher concern because of biomechanical stress in the implant's axis showed the overall lowest deviations in this study based on flat-to-flat implant design (Table 4).This is comparable to results in previous studies.It should be noted that depending on alignment method, negative deviations are allowed using the methodology in this study [2,11].A method taking the physical boundaries in consideration would be complex, as it should also take in consideration the tolerances of all components.
There was no statistically significant difference between MOD1 taken by three different specialists on which the restoration was fabricated, stored between 33 and 73 months and subsequently re-scanned, or MOD2, which was taken by a different specialist as part of the study.Although dimensional changes generally occur in gypsum when setting or in extreme humid conditions, there is no scientific or manufacturer data suggesting long-term storage deformations of the specific type IV dental stone used in MOD1 and MOD2.
The total deviations between REF and BRIDGE were generally twofold or higher when compared with MOD1 and MOD2 for Resultant, DeltaX and DeltaY (Table 4).These results are in line with previous findings, addressing the framework manufacturing process being the factor that contributes the most to the total misfit [1].The results further indicate that in vitro manufacturing trueness for BRIDGE when MOD1 acted as a reference showed similar range as in vivo trueness of BRIDGE.Hence, errors introduced when taking impressions, fabricating and scanning models are not additive to the full workflow.
Other studies may not be fully comparable because of differences in manufacturing process, implant system, number of implants, different Global Alignments, and analysis methods.However, the numerical values serve as a good indication of investigated misfit.
Although not within the primary scope of this study, none of the existing IFDs displayed any clinical misfit using the alternate finger technique or screw-resistance test when assessed by an experienced prosthodontist.Because of the platform being subgingival, Sheffield test, visual inspection or tactile probing test was not conducted.
The results in the present pilot study show a misfit of the IFD which is half that of the standing clinically relatable limit of 150 µm [13].Similarly, the results are favourable when compared to a newly proposed 7-graded classification based on CAD/CAM restorations with a maximum acceptable deviation of 100 µm for full-arch restorations [12].However, the results from this study are limited to the maxilla.Unless the mandible can be fixated, the reference measurement is not suitable for acquisition because of mandibular micro-movements during the reference scan.
Further in vivo studies are required to validate the results in this study and to address if in vitro research provides equivalent results as in vivo data.

Conclusions
The described method in this pilot study can be applied in vivo for trueness studies in maxillary full-arch implant treatments.
In vivo impressions in maxillary full-arch implant treatments provides trueness similar to previously published in vitro studies.
IFDs manufactured through CAD/CAM show deviations twice that of impression-based models, however, errors introduced from impressions, fabrication and model scans were not additive to the entire workflow.
CAD/CAM fabricated IFD in the maxilla presents in vivo deviation twice that of impressions.
In vivo IFD misfit was below 100 µm.

Fig. 1 .
Fig. 1.Full workflow overview.1,acquisition with reference scanner (REF) and implantsupported fixed denture (BRIDGE) using the same reference scanner, and impressions (MOD2).2, mesh of virtual models with REF cropped and original model used to manufacture IFD (MOD1).3, Datum Alignment of Virtual Scan-Body (VSB) with indirect alignment of Virtual Analogue (VAN) for REF, and direct alignment of VAN for virtual 3D print models (MOD1 and MOD2), and BRIDGE.4, ensuing alignment file Aligned Virtual Analogues (AVAN) from REF, MOD1, MOD2 and BRIDGE.5, consistent Global Alignment using geometries.6, Example of RPS Alignment with deviations for each position, Resultant, DeltaX, DeltaY an DeltaZ.
ATOS Industrial-grade reference scanner (ATOS Core 80 5MP; GOM) AVAN Aligned Virtual Analogues, file containing six aligned VAN BRIDGE Scan of IFD with attached analogues CAD-AN CAD file of analogue (modified platform diameter) CAD-SB CAD file of scan-body IFD Implant-supported fixed dentures IGS Initial Graphics Exchange Specification -file format MOD1 Original model used to fabricate IFD based on polyether impression MOD2 New model based on polyether impression REF Reference scan RPS Reference Point System Alignment STL Stereolithography -file format STP Standard for the Exchange of Product -file format VAN Virtual Analogue (defeatured CAD-AN) VSB Virtual Scan-Body (defeatured CAD-SB) the sequence was manually discarded, and the scan sequence repeated.Each sequence was cropped to remove any measurement data beyond the surface of the scan-bodies.Post-processing was performed in the software through best-fit transformation of sequences and polygonization detail was set at highest detail for the three exported STL files (REF-S1, REF-S2 and REF-S3), the S1, S2 and S3 suffix denoting the scan iteration.

Fig. 2 .
Fig. 2. A, Inter-aligned CAD Scan-body (CAD-SB) (lower body) with visible faceted plane and original CAD Analogue (upper body).B, view of CAD-SB and modified diameter of CAD analogue interface.C, modified and defeatured CAD-SB and final CAD-Analogue (CAD-AN) interface used in alignments.

Fig. 3 .
Fig. 3. Surfaces to construct Virtual Scan-Body (VSB) Datums, (blue color), in Reference Data.A, VSB-Vector constructed from the axis of highlighted cylinder.B, VSB-Plane1.C, VSB-Plane2.Equivalent geometry was constructed for each scan-body in Measured Data.

Fig. 4 .
Fig. 4. Surfaces to construct Virtual Analogue (VAN) Datums, (blue color), in Reference Data.A, VAN-Vector constructed from the axis of highlighted cylinder.B, VAN-Plane 1. C, VAN-Plane 2. Point 1 is created at the intersection of VAN-Vector and VAN-Plane1 and Point2 is created at the intersection of VAN-Vector and VAN-Plane2 (not shown).Equivalent geometry was constructed for each analogue in Measured Data.

Fig. 6
Fig. 6 shows a superimposition of all subjects' REF-AVAN-S1, displaying the implant distributions and relative angulations after Global Alignment.

Fig. 5 .
Fig. 5. Aligned Virtual Analogues (AVAN) after Global Alignment with Constructed Geometries.A, view of XZ plane (frontal view).B, view from XY plane (occlusal view).Created circle numerical order for Position 1 to Position 6.

Table 1
List of abbreviations

Table 3
Mean MOD1 and MOD2 was statistically significantly different from REF Resultant and REF DeltaX.BRIDGE was statistically significantly different from REF Resultant, DeltaX and DeltaY, (Table difference of precision, (Diff), with 95% confidence interval in micrometres, (µm), and p value, between REF and scanning methods.A negative Diff and a statistical significance, (p < 0.05), correspond to scanning Method 2 having a higher precision than Method 1.3.3.Trueness REF vs MOD1, MOD2 and BRIDGE

Table 5
Cross-comparison of trueness with respective p values supporting Fig. 7.