Evaluation of laboratory scanner accuracy by a novel calibration block for complete-arch implant rehabilitation

Effect of using scan body accessories and inter-implant distances on the accuracy of complete arch implant digital impressions: An in vitro study

PURPOSE

To introduce a novel design for scan body accessory parts that are reusable, easy to attach and detach without permanent change of the scan body, and can be used with different inter-implant distances to enhance the accuracy of complete arch implant scans.

MATERIALS AND METHODS

A maxillary polymethylmethacrylate (PMMA) model with a soft tissue replica was fabricated with four implant analogs located at tooth positions 17, 13, 22, and 27 with 18, 25, and 30 mm inter-implant distances. Four scan bodies (SBs) were attached to the implants. The model was scanned with a laboratory scanner to be used as a reference scan. A total of 40 scans were made with the same intraoral scanner and they were divided equally into two groups. Group A: Complete arch implant scans without scan body accessories (n = 20), and Group B: Complete arch implant scans with scan body accessories (n = 20). Intraoral scans were exported and superimposed on the reference scan using reverse engineering inspection software to be evaluated for 3D deviations, angular deviations, and linear deviations. Statistical analysis was performed with student t-test and analysis of variance (ANOVA) with repeated measures followed by post hoc adjusted Bonferroni test. The level of significance was set at P = 0.05.

RESULTS

The scan body accessories decreased both the 3D and linear deviations, with a statistically significant difference at SB4 for the 3D deviation (P = 0.043) and the linear inter-implant discrepancies between SB1-SB2 and SB3-SB4 (P = 0.029 and < 0.001), respectively. However, there was no statistically significant difference in angular deviation between the study groups. Implant positions had significant differences within each group.

CONCLUSIONS

 A significant improvement in the accuracy of the complete arch implant digital impression was achieved by using the scan body accessories, particularly in reducing the 3D and linear deviations at the most distant implant positions.

Effect of printing layer thickness on the trueness of 3-unit interim fixed partial dentures

STATEMENT OF PROBLEM

Three-dimensional printing has facilitated the fabrication processes in dentistry. However, knowledge on the effect of layer thickness on the trueness of 3D printed fixed partial dentures (FPDs) is lacking.

PURPOSE

The purpose of this in vitro study was to compare the effect of printing layer thickness on the trueness of 3-unit interim FPDs fabricated by using additive manufacturing with that of those fabricated by subtractive manufacturing.

MATERIAL AND METHODS

The right first premolar and first molar teeth of a dentate mandibular model were prepared for a 3-unit restoration and then digitized by using an intraoral scanner. A 3-unit interim FPD was designed to fabricate 40 restorations by using either the additive (NextDent C&B MFH) with layer thicknesses of 20 μm (n=10), 50 μm (n=10), and 100 μm (n=10) or subtractive manufacturing technique (Upcera) (milled, n=10). After fabrication, the interim FPDs were digitized by using the same intraoral scanner and were superimposed over the design data by using a 3D analysis software program. Root mean square (RMS) was used to analyze the trueness of the restorations at 4 different surfaces (external, intaglio, marginal area, and intaglio occlusal) and as a complete unit (overall). Data were analyzed with the Kruskal-Wallis and Wilcoxon tests with Bonferroni correction (α=.05).

RESULTS

The 100-μm-layer thickness interim FPDs showed the greatest overall (P≤.015), external (P≤.021), and intaglio occlusal (P≤.021) deviations, whereas the milled interim FPDs showed the lowest (P=.001). No significant differences were found among the test groups for marginal RMS (P≥.108). The differences between the 50-μm-layer thickness and 100-μm-layer thickness interim FPDs for the intaglio surface deviations (P=.064) and between the 20-μm-layer thickness and 50-μm-layer thickness interim FPDs for each surface tested were not statistically significant (P≥.108).

CONCLUSIONS

The printing layer thickness had a significant effect on the trueness of the additively manufactured interim FPDs. However, subtractively manufactured interim FPDs presented higher trueness than those additively manufactured, regardless of the printing layer thickness.

Effect of a Novel 'Scan Body' on the In Vitro Scanning Accuracy of Full-Arch Implant Impressions

OBJECTIVE

This in vitro study aimed to determine whether a newly designed arcuate scan body can improve intraoral scanning accuracy for implant rehabilitation of edentulous jaws.

MATERIAL AND METHODS

A master model containing 4 implant abutment replicas was fabricated and digitized with different scan bodies using an intraoral scanner. Four types of scan bodies were evaluated: original scan bodies (group OS), computer-aided design and computer-aided manufacturing (CAD/CAM) scan bodies without extension (group CS), CAD/CAM scan bodies with straight extension (group CSS), and CAD/CAM scan bodies with arcuate extension (group CSA). Conventional splinted open-tray impressions (group CI) were used as controls. The master model and the poured casts were digitized using a laboratory scanner. Impressions were repeated 10 times each in 5 groups. Scans in standard tessellation language format were exported to reverse engineering software and root mean square (RMS) values were used for trueness and precision assessments. In each group, 45 RMS values were acquired for precision evaluation and 10 RMS values were obtained for trueness assessment. Statistical evaluation was performed with the Kruskal-Wallis test and Dunn-Bonferroni test (α = 0.05).

RESULTS

The median trueness values were 41.40, 55.95, 39.80, 39.75, and 22.30 μm for group OS, CS, CSS, CSA, and CI, respectively. CI showed better trueness than OS (P = .020), CS (P < .001), and CSS (P = .035). The median precisions for group OS, CS, CSS, CSA, and CI were 47.40, 51.50, 43.90, 25.20, and 24.60 μm. respectively. The precision of CSA and CI were higher than OS (P < .001), CS (P < .001), and CSS (P < .001). Between CI and CSA, there was no significant difference (P = 1.000).

CONCLUSIONS

For full-arch implant rehabilitation, the scan body with arcuate extension could improve the intraoral scanning precision and showed similar 3-dimensional discrepancy compared to conventional splinted open-tray impressions.

Accuracy of different digital acquisition methods in complete arch implant-supported prostheses: An in vitro study

STATEMENT OF PROBLEM

Digital methods such as intraoral scanners for recording the location of implants supporting complete arch prostheses have limitations. Photogrammetry devices should be able to digitize implant positions accurately, but standardized comparisons between different digital acquisition methods are lacking.

PURPOSE

The purpose of this in vitro study was to compare the repeatability of different digital acquisition methods for complete arch prostheses supported by 6 and 4 implants.

MATERIAL AND METHODS

A master cast was created with 6 and 4 dental implants with multiunit abutments to obtain the master digital casts. The evaluated devices were the industrial high-resolution 12-megapixel scanner (reference) Atos Compact Scan 12M (GOM), the laboratory scanners D2000 (3Shape A/S) and S900 Arti (Zirkonzahn), the photogrammetry devices iCam (iMetric4D) and PIC (PIC Dental), and the intraoral scanners TRIOS 3 (3Shape A/S) and iTero Element 5D (Align Technology). The resulting files were imported to a computer-aided design software program (exocad GmbH) to obtain the implant replicas as standard tessellation language (STL) files. These files were imported into a software program (Geomagic Control X) and superimposed per group through the best-fit algorithm to determine repeatability, defined as the closeness of agreement between each group's scanned results as root mean square (RMS) values. The normality of distribution was tested by the Shapiro-Wilk normality test, and the Kruskal-Wallis test with adjustment with the Bonferroni correction method was used accordingly (α=.05).

RESULTS

The repeatability means and 95% confidence intervals for the 4 implant scans were: 1.07 µm (0.86; 1.29) for GOM, 2.05 µm (1.89; 2.21) for D2000, 3.61 µm (3.23; 3.99) for S900, 7.01 µm (6.11; 7.91) for iCam, 5.18 µm (4.6; 5.76) for PIC, 20.52 µm (18.33; 22.72) for TRIOS3, and 20.5 µm (17.37; 23.63) for iTero. Statistically significant differences were found between devices, except for iCam versus PIC, GOM versus S900, iCam versus D2000, PIC versus D2000, and TRIOS3 versus iTero. The repeatability means and 95% confidence intervals for the 6 implant groups were: 1.36 µm (1.08; 1.65) for GOM, 3.17 µm (3.01; 3.33) for D2000, 2.15 µm (2.04; 2.25) for S900, 8.67 µm (8.06; 9.28) for iCam, 13.88 µm (12.62; 15.14) for PIC, 40.32 µm (36.29; 44.36) for TRIOS3, and 38.86 µm (34.01; 43.71) for iTero. Statistically significant differences were detected between devices, except for S900 versus GOM, PIC versus iCam, and iTero versus TRIOS 3.

CONCLUSIONS

The results suggest that photogrammetry could be a suitable alternative for recording implant locations of complete arch prostheses supported by 4 or 6 implants, with better repeatability than intraoral scanners. Increasing the number of implants decreased the repeatability of every device tested except the laboratory scanners.

The cumulative effect of error in the digital workflow for complete-arch implant-supported frameworks: an in vitro study

PURPOSE

To investigate the error accumulation and distribution through various stages of the digital workflow for complete-arch implant-supported framework fabrication.

MATERIALS AND METHODS

A resin model of edentulous maxilla with 6 dental implants was scanned using an intraoral scanner for 10 times (Complete-digital group). Ten conventional gypsum casts were made and digitized by a laboratory scanner (Analogue-digital group). Five implant frameworks were designed and milled using CAD-CAM technique for each workflow. Inter-implant distances and angles of the resin model (reference) and frameworks were measured by a coordinate measuring machine, while the scans and virtual frameworks were examined by an inspection software. Effect of type of workflow and the individual stage on the accuracy of the frameworks were analysed by Two-way ANOVA.

RESULTS

The expanded uncertainty of both workflows was ~150 μm and ~0.8°. The accuracy of the CAD stage was the highest. In the complete-digital workflow, the greatest distortion was found in the data acquisition stage, while in the analogue-digital workflow, it was found in the CAM stage. Compared to the analogue-digital group, the complete-digital group showed a significant higher precision in first-quadrant but lower trueness in second-quadrant in data acquisition, and a significantly lower precision in second-quadrant at the CAD stage.

CONCLUSIONS

Linear distortions of the complete-digital and analogue-digital workflows were clinically acceptable, while angular distortions were not. Distortions were generally derived from data acquisition and CAM stage. The CAD precision depended on the distortions derived from data acquisition. The complete-digital workflow was not as accurate as the analogue-digital in complete-arch implant rehabilitation.

In Vitro Accuracy of Digital and Conventional Impressions for Full-Arch Implant-Supported Prostheses

The aim of this study was to evaluate the accuracy of full-arch digital impressions when compared to conventional impressions, when performed on the abutment or implant level. Methods: One resin cast with six implants and another cast with six abutments were scanned with Primescan v5.1 (PS51), Primescan v5.2 (PS52), Trios 3 (T3), and Trios 4 (T4). Additionally, conventional impressions (A) were made, poured in gypsum, and digitized using a lab scanner (IScan D104i). A coordinate machine (Atos, GOM, Braunschweig, Germany) was used to generate the reference scan of both casts. For all scans, the position of the implants was calculated and matched with the reference scan. Angular and coronal measurements per implant were considered for trueness and precision. Results: For the implant-level model, PS52 performed significantly better in terms of trueness and precision compared to all other impressions, except for the angular trueness of A (p = 0.072) and the coronal trueness of PS51 (p = 1.000). For the abutment-level model, PS52 also performed significantly better than all other impressions, except for the coronal trueness and precision of A (p = 1.000). Conclusions: Digital impressions for full-arch implant supported prostheses can be as accurate as conventional impressions, depending on the intra-oral scanner and software. Overall, abutment level impressions were more accurate compared to implant level impressions.

Does the geometry of scan bodies affect the alignment accuracy of computer-aided design in implant digital workflow: an in vitro study?

OBJECTIVES

To compare 2 implant scan bodies with different geometry on the accuracy of the virtual alignment process in the digital workflow.

MATERIALS AND METHODS

A master model of the edentulous maxilla with 6 implants and multi-unit abutments (MUA) inserted was fabricated. Six dome-shaped and cuboidal scan bodies were mounted on the MUAs respectively and consecutively scanned by a laboratory scanner 10 times. The original scans were imported to a dental-specific CAD software and virtually aligned with the default CAD model in the implant library. Thus, 10 aligned models were created. Both the original scans and the aligned models were evaluated by an inspection software for deviation of the scan body surfaces, the centroids of scan body and MUA, the scan body centre-axis and the inter-MUA distances/angles. The two-sample T-test/Mann Whitney U test were used to analyse the data with the level of significance set at 0.05.

RESULTS

The cuboidal group showed significant greater deviations of the model surface (13.9 µm vs 10.7 µm) and the MUA centroids (31.7µm vs 22.8 µm) but smaller deviation of the inter-MUA angle (0.047° vs 0.070°) than those of the dome-shaped group (P<0.05). No significant differences in the deviation of scan body centroids, centre-axis and the inter-MUA distances between the 2 groups were found.

CONCLUSIONS

Virtual alignment of implant scan body affected the accuracy of the digital workflow for complete-arch implant-supported prostheses (up to ~30 µm/0.09°). Different geometries of the implant scan body could also influence the transfer accuracy in the CAD process.

Comparison of the virtual techniques in registering single implant position with a universal-coordinate system: An in vitro study

OBJECTIVES

The aim of this in-vitro study was to compare the virtual techniques for registering single-implant position to the physical gold standard using a universal-coordinate system.

MATERIALS AND METHODS

Thirty dentate maxillary resin models with a dental implant inserted in the incisor region were prepared. On each model, a tooth-supported acrylic stent with a 1 cm x 1 cm x 1 cm cubic-corner (CC) was prepared. The Cartesian x,y,z-coordinate of the implant neck and apex were measured physically by a coordinate-measuring machine (CMM) with reference to this CC and served as the gold-standard. The resin models were scanned by a benchtop scanner (Group BS), cone-beam computed tomography (Group CBCT), and intraoral scanner (Group IOS). Stone casts, poured from open-tray polyether impression of the resin models, were scanned by the benchtop scanner (Group BS-cast). The implant neck and apex coordinates with reference to the CC were measured and the differences in the coordinates (∆x, ∆y, ∆z) and distance r from the gold standard were calculated. The data were analyzed by one-sample t-test and one-way ANOVA/Kruskal-Wallis test with the level of significance set at 0.05.

RESULTS

The implant neck and apex positions of Group BS were statistically different from that of the CMM, r>0 (p<0.001). Group IOS showed a significant less ∆z and r at the implant neck than Group BS-cast (p = 0.006). No significant difference was found in the coordinates and distance at implant apex among Groups BS, CBCT, IOS and BS-cast.

CONCLUSIONS

The physical measurements could be adopted as the gold standard in assessing the single-implant positions. The IOS was more accurate in registering the single-implant neck positions than scanning of the cast.

CLINICAL SIGNIFICANCE

A universal-coordinate system defined by the cubic-corner allows comparing the virtual techniques in registering single-implant positions to the physical gold standard.

Improving intraoral implant scanning with a novel auxiliary device: An in-vitro study

OBJECTIVES

To develop a novel auxiliary device for improving the accuracy of intraoral implant scanning in the complete-edentulous arch.

MATERIALS AND METHODS

A standard model of edentulous maxilla with six dummy implants was prepared. Scan bodies were attached to the model, which was scanned by a laboratory scanner. A simulated mucosa (Group 0), a resin base (Group 1), a resin base with a cuboidal reference block and 4 fiduciary spheres (Group 2) or artificial teeth (Group 3) in between the implants were mounted on the model, respectively. Each group were consecutively scanned using an intraoral scanner (n = 10). The scans were analysed for trueness and precision in inter-implant distances and angles by inspection software. Effects of the auxiliary device and different quadrants on the accuracy of complete-arch intraoral scanning were analysed by two-way ANOVA.

RESULTS

Significant effects of the auxiliary device and quadrant were found on both linear and angular accuracies. The lowest linear accuracy was found in group 0. Group 1 and group 3 showed the best linear accuracy in quadrant 1 and quadrant 2, respectively. Group 2 showed the least angular precision among the three groups.

CONCLUSIONS

The three designs of auxiliary devices significantly improved the accuracy of complete-arch intraoral implant scanning. The base-only design showed good scanning accuracy in a single quadrant, while the base with tooth-shaped landmarks design showed better accuracy in cross-arch. The fiduciary spheres might compromise the precision of scanning.

Use of measuring gauges for in vivo accuracy analysis of intraoral scanners: a pilot study

PURPOSE

The purpose of this study is to present a methodology to evaluate the accuracy of intraoral scanners (IOS) used in vivo.

MATERIALS AND METHODS

A specific feature-based gauge was designed, manufactured, and measured in a coordinate measuring machine (CMM), obtaining reference distances and angles. Then, 10 scans were taken by an IOS with the gauge in the patient's mouth and from the obtained stereolithography (STL) files, a total of 40 distances and 150 angles were measured and compared with the gauge's reference values. In order to provide a comparison, there were defined distance and angle groups in accordance with the increasing scanning area: from a short span area to a complete-arch scanning extension. Data was analyzed using software for statistical analysis.

RESULTS

Deviations in measured distances showed that accuracy worsened as the scanning area increased: trueness varied from 0.018 ± 0.021 mm in a distance equivalent to the space spanning a four-unit bridge to 0.106 ± 0.08 mm in a space equivalent to a complete arch. Precision ranged from 0.015 ± 0.03 mm to 0.077 ± 0.073 mm in the same two areas. When analyzing angles, deviations did not show such a worsening pattern. In addition, deviations in angle measurement values were low and there were no calculated significant differences among angle groups.

CONCLUSION

Currently, there is no standardized procedure to assess the accuracy of IOS in vivo, and the results show that the proposed methodology can contribute to this purpose. The deviations measured in the study show a worsening accuracy when increasing the length of the scanning area.