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Director of Orthodontics, Department of Surgery and Medical-Surgical Specialties, Area of Orthodontics, Medical and Dental School, Instituto Asturiano de Odontologia, University of Oviedo, Oviedo, Spain
Intraoral scanners (IOSs) provide a digital alternative to conventional implant impression techniques. However, the effect of the supramucosal height of the scan body and implant angulation on the accuracy of IOSs remains unclear.
Purpose
The purpose of this in vitro study was to measure the impact of the supramucosal height of the scan body and implant angulation on the accuracy (trueness and precision) of intraoral digital implant scans in partially edentulous models.
Material and methods
Two maxillary partially edentulous casts with 4 implant analogs were fabricated, 1 with 4 parallel implants (P-groups) and 1 with 2 implants distally inclined 18 degrees (A-groups). An implant scan body was positioned on each implant analog (CARES RC Mono Scanbody). For each cast, 3 subgroups were determined based on the soft tissue moulage fabricated for each reference cast exposing 3 mm (P-3 and A-3 subgroups), 5 mm (P-5 and A-5 subgroups), and 7 mm (P-7 and A-7 subgroups) of the implant scan bodies. The 2 reference casts were registered by using a coordinate measurement machine and desktop scanner (7 Series Dental Wings) and then scanned using an IOS (TRIOS 4) (n=15). Linear and angular discrepancy values and root mean square (RMS) error values between the implant scan bodies measured on the reference and experimental scans were computed with an inspection software program (Geomagic). Mann-Whitney U tests with Bonferroni correction were applied for planned comparisons (α=.05/9 ≈ .006).
Results
For linear discrepancies, statistically significant differences were found between groups P-3 and A-3 (P=.004) and between P-7 and A-7 (P=.005). For angular discrepancies, statistically significant differences were found between groups A-3 and A-5 (P=.002) and between P-7 and A-7 (P=.003). The RMS error analysis found no statistically significant differences among the groups.
Conclusions
Implant angulation of 18 degrees did not significantly affect the accuracy of the intraoral scans in terms of 6 of the 9 planned comparisons, although the angled groups had lower mean values. Also, the supramucosal height of the scan body did not significantly affect the accuracy of the intraoral scans in terms of 17 of the 18 planned comparisons. Results may vary with different implant scan body designs.
Clinical Implications
The reduced supramucosal height of the implant scan body might not be a critical factor for the accuracy of intraoral digital scanning, eliminating the need for longer scan bodies when their clinical visibility is reduced. Intraoral scanning seems to be a valid option for angled implants.
With the introduction of computer-aided design and computer-aided manufacturing (CAD-CAM) systems, many studies have compared the accuracy of conventional implant impressions with the digital alternative in completely edentulous
Comparison of conventional, photogrammetry, and intraoral scanning accuracy of complete-arch implant impression procedures evaluated with a coordinate measuring machine.
Accuracy of printed casts generated from digital implant impressions versus stone casts from conventional implant impressions: a comparative in vitro study.
Intraoral digital scans—Part 1: influence of ambient scanning light conditions on the accuracy (trueness and precision) of different intraoral scanners.
often reporting conflicting results. The accuracy of an intraoral scanner (IOS) has been defined in the International Organization for Standardization (ISO) 5725-1 standard by its trueness and precision.
Trueness refers to the ability of the IOS to register the oral conditions as closely as possible to their true form, and precision refers to the closeness or variability between the repeated measures made with the IOS under the same conditions.
A recent systematic review and meta-analysis compared conventional impressions and digital scans in completely and partially edentulous arches, concluding that the implant angulation decreased the accuracy of the digital scans compared with the conventional impressions only in partially edentulous arches; however, the difference was not statistically significant.
However, another study compared the conventional impressions and digital scans of 4 casts with 2 implant analogs placed parallel and at 3 different angles (15, 30, and 45 degrees). Interestingly, the authors reported that the digital scans showed better results when the interimplant angulation was greater than 30 degrees.
Effect of implant divergence on the accuracy of definitive casts created from traditional and digital implant-level impressions: an in vitro comparative study.
Contrarily, a randomized in vitro study reported that angulation could negatively affect the accuracy of the digital scans from a linear point of view.
Several in vitro studies conducted by the same group with an edentulous maxillary model with 6 implants studied the effect of implant depth, among other factors, with different scan systems and reported conflicting results.
Effect of implant divergence on the accuracy of definitive casts created from traditional and digital implant-level impressions: an in vitro comparative study.
One study reported that partially visible scan bodies showed significantly higher angle deviations than the fully visible scan bodies and recommended placing longer scan bodies when scanning deep implants.
An in vitro study of factors influencing the performance of digital intraoral impressions operating on active wavefront sampling technology with multiple implants in the edentulous maxilla.
Accuracy of a digital impression system based on active triangulation technology with blue light for implants: effect of clinically relevant parameters.
Accuracy of a digital impression system based on active wavefront sampling technology for implants considering operator experience, implant angulation, and depth.
Accuracy of a digital impression system based on parallel confocal laser technology for implants with consideration of operator experience and implant angulation and depth.
Impact of different scan bodies and scan strategies on the accuracy of digital implant impressions assessed with an intraoral scanner: an in vitro study.
Impact of different scan bodies and scan strategies on the accuracy of digital implant impressions assessed with an intraoral scanner: an in vitro study.
However, the influence of the implant depth, the supramucosal height of the scan body, and the implant angulation on the accuracy of registrations with an IOS remain unclear.
The objectives of this in vitro study were to assess the effect of the implant angulation and the supramucosal height of the scan body on the accuracy of intraoral digital implant scans in a partially edentulous arch. The null hypothesis was that no difference would be found in the accuracy of intraoral digital implant scans in partially edentulous models with different implant angulations and different supramucosal heights of the scan bodies.
Material and Methods
Two reference definitive implant casts were fabricated with Type IV die stone (New FujiRock IMP; GC Corp) representing a maxillary partially edentulous arch with anterior teeth present and bilateral posterior edentulous areas (Fig. 1). Each maxillary reference implant cast contained 4 implant analogs (RC implant analog; Institut Straumann AG) on the first molar and first premolar positions. In one of the reference casts, the implant analogs were positioned parallel (P group). In the other reference implant cast, the implant analogs in the first molar positions were angled 18 degrees distally (A group). A new ISB (CARES RC Mono Scanbody; Institut Straumann AG) was positioned on each implant analog of both reference implant casts and tightened to 15 Ncm with a torque control device (Torque control device and Ratchet; Straumann AG). The implant scan bodies were not removed or replaced during the process.
Figure 1Reference definitive implant casts. A, Parallel group (P group). B, Angled group (A group).
Both reference definitive implant casts were registered by using 2 different methods: a coordinate measurement machine (CMM) and a laboratory scanner. As a result, 2 different reference data were obtained to be compared with the experimental intraoral digital scan data. The comparison was performed using 2 different discrepancy measurement methods: linear and angular measurements (based on 2-dimensional references) and root mean square (RMS) error discrepancies (based on 3-dimensional [3D] references).
For the CMM registration, an industrial CMM (Contura G2 10/16/06 RDS; Carl Zeiss Industrielle Messtechnik GmbH) was used. The nominal linear accuracy of the CMM machine was within 1 μm on all axes.
Accuracy of a digital impression system based on active wavefront sampling technology for implants considering operator experience, implant angulation, and depth.
For the laboratory scanning registration, each reference implant cast was scanned with a desktop scanner (7 Series; Dental Wings) without the soft tissue moulage. Three subgroups were determined based on the supramucosal height of the ISBs tested (3, 5, and 7 mm) (Table 1), which were obtained by varying the height of the soft tissue moulage (Gi-Mask; Coltène).
Table 1Experimental group descriptions
Group
Subgroup
Implant Analog Positions
Scan Body Supramucosal Height
P
3
Parallel with less than 2 degrees of divergence between implants
3 mm of implant scan body visible (5 mm covered by soft tissue moulage)
5
5 mm of implant scan body visible (3 mm covered by soft tissue moulage)
7
7 mm of implant scan body visible (1 mm covered by soft tissue moulage)
A
3
18 degrees of distal angulation of implant analogs in the first molar positions, with respect to the implant analogs in the first premolar positions
3 mm of implant scan body visible (5 mm covered by soft tissue moulage)
5
5 mm of implant scan body visible (3 mm covered by soft tissue moulage)
7
7 mm of implant scan body visible (1 mm covered by soft tissue moulage)
The maxillary reference implant casts were fixed to the upper plate of a dental mannequin (1560 Series Dentoform; Dentalez) opposing a completely dentate typodont with a soft outer plastic mannequin head (Nasco SB23464; Nasco). The mannequin was attached to the head rest of a dental chair to replicate the clinical environment, and the IOS (TRIOS 4 wireless, v. 20.1.3; 3Shape A/S) was positioned on the left side of the chair.
The illuminance of the room on the definitive implant casts was recorded with a light meter (LX1330B Light Meter; Dr. Meter Digital Illuminance) and adjusted to 1000 lux to maximize the accuracy of the IOS.
A room with no windows was chosen, and the chair light was turned off. The IOS was calibrated at the start and after every 15 scans to minimize errors, and 15-minute breaks were taken between the different groups to reduce operator fatigue. The intraoral digital scans were obtained by a prosthodontic graduate resident (E.S.) with 6 years of experience using IOSs by following an occlusal-buccal-palatal scanning strategy rotating around the ISBs.
For the P-3 subgroup, the soft tissue moulage exposing 3 mm of the ISBs was positioned on the corresponding reference definitive implant cast. Fifteen maxillary intraoral digital scans were consecutively acquired by following the scanning protocol recommended by the manufacturer. Subsequently, the intraoral digital scans were postprocessed, and the standard tessellation language (STL) files were exported.
For the P-5 subgroup, the soft tissue moulage exposing 5 mm of the ISBs was positioned on the corresponding reference definitive implant cast. The same data acquisition procedures performed on the P-3 subgroup were followed until the 15 STL files were exported. For the P-7 subgroup, the soft tissue moulage exposing 7 mm of the ISBs was placed on the corresponding reference definitive implant cast. The same data acquisition procedures were followed until the 15 STL files were exported. For the A-3, A-5, and A-7 subgroups, the maxillary reference implant cast with angled implant analogs was selected. The same data acquisition protocol was followed until 15 STL files were exported for each subgroup.
A total of 90 STL files were obtained (n=15). For the linear and angular measurement discrepancies, each STL file was imported into an inspection software program (Geomagic Control X; 3D Systems), the cylindrical shape of each ISB was selected, and a cylinder of the same dimensions was created. The horizontal portion of the most coronal part of each ISB was identified, and a plane was created. The axis for each cylinder was calculated, reflecting the axis of each ISB. The point of intersection between each cylinder’s axis and the respective horizontal plane was found.
For each STL file, 6 linear measurements were calculated between all the points, and 6 angular measurements were computed between all the axes (Fig. 2). This process was repeated for the 90 files. For each reference definitive implant cast, an Initial Graphics Exchange Specification (IGES) file was obtained containing for each ISB a vector representing its axis and a plane representing the horizontal portion of the coronal part of the ISB. Six linear and 6 angular measurements were made as previously described (Fig. 3).
Figure 2Positioning of cylinders, planes, vectors, points in inspection software program and representative linear measurements between all points from each STL file. STL, standard tessellation language.
Figure 3IGES file obtained from CMM registration with linear measurements between all points from each IGES file. CMM, coordinate measurement machine; IGES, Initial Graphics Exchange Specification.
The linear and angular measurements obtained from the 6 groups were subtracted from their respective reference values. The resulting differences were identified as discrepancies. Six linear discrepancy values and 6 angular discrepancy values were calculated for each specimen. All values were transformed into absolute values to avoid positive and negative values compensating for each other. For each specimen, the 6 linear discrepancy values were averaged into 1 overall linear discrepancy and the 6 angular discrepancy values were averaged into 1 overall angular discrepancy. The overall discrepancy values were used for the statistical analysis. Trueness (linear and angular) was defined in terms of the average linear and angular discrepancy values, while precision was defined in terms of standard deviation (SD).
For the RMS error discrepancies analysis and using the same inspection software program, each of the 90 STL files was superimposed with a best-fit algorithm on their respective reference file, based on the areas of the 3D mesh corresponding to the ISBs, and RMS error values were obtained for these same areas. In this case, trueness was also defined as the mean RMS error values and precision in terms of SD.
The statistical analysis was conducted according to the recommendations of Ruxton and Beauchamp,
who suggested comparisons be conducted, whenever practical, to encourage biologically meaningful results. In the present study, the 6 groups could be compared in 15 different pairs for each of the 3 outcomes (overall linear deviations, overall angular deviations, and RMS values). However, of these 15 comparisons, only the 9 in which each pair of groups shared either the implant angulation condition or the supramucosal height of the ISB condition were considered to have scientific value (Fig. 4).
For each outcome, statistically significant nonnormality was found (P<.05) via the Shapiro-Wilk test. Therefore, nonparametric tests were used. Ruxton and Beauchamp
recommended not performing the global test in cases where there were planned comparisons. Therefore, 9 Mann-Whitney U tests were performed by controlling the experiment-wise Type I error rate (EER) maintained via the Bonferroni correction (P<.05/9 ≈ .006) instead of the Kruskal-Wallis test. A statistical software program (IBM SPSS Statistics, v26 for Windows; IBM Corp) was used in the analysis.
A power calculation was conducted with the module for the Mann-Whitney U test of a software program (PASS 13; NCSS) in terms of overall linear discrepancy, which was considered the primary outcome of the study. The Type I error rate was set at α=.05/9 ≈ .006. The effect sizes for the differences between the groups were based on the results of Gintaute et al
The calculation determined that with a sample size of n=15, the power was at least 96% for each of the 9 planned comparisons.
Results
Side-by-side boxplots of the overall linear measurement discrepancies are presented in Figure 5. The greatest linear trueness was found in the A-3 subgroup, as this subgroup had the lowest mean ±SD linear measurement discrepancy of 55.3 ±21.1 μm. The least linear trueness was found in the P-7 subgroup, which had the greatest mean ±SD linear measurement discrepancy mean value of 88.3 ±33.9 μm. In the 9 planned comparisons, statistically significant differences were found between groups P-3 and A-3 (P=.004) and P-7 and A-7 (P=.005). In both cases, the angled groups exhibited lower medians. No other significant differences among groups were found.
Figure 5Side-by-side boxplots of overall linear discrepancies. ∗Data subjected to planned comparisons through Mann-Whitney U tests with Bonferroni correction for trueness analysis. Significant differences found between groups P-3 and A-3 (P=.004) and P-7 and A-7 (P=.005).
Side-by-side boxplots of the overall angular discrepancies are presented in Figure 6. The A-3 group had best angular trueness with the lowest mean ±SD of 0.90 ±0.22 degrees. The A-5 group had the worst angular trueness with the highest mean ±SD value of 1.12 ±0.14 degrees. The 9 Mann-Whitney U tests indicated a significant difference only between groups A-3 and A-5 (P=.002) and P7 and A7 (P=.003). Side-by-side boxplots of the overall RMS error values are presented in Figure 7. No significant differences among groups were found.
Figure 6Side-by-side boxplots of overall angular discrepancies. ∗ Data subjected to planned comparisons through Mann-Whitney U tests with Bonferroni correction for trueness analysis. Significant differences found between groups A-3 and A-5 (P=.002) and P-7 and A-7 (P=.003).
Figure 7Side-by-side boxplots of RMS error values. RMS, root mean square. ∗ Data subjected to planned comparisons through Mann-Whitney U tests with Bonferroni correction for trueness analysis. No significant differences found.
The null hypothesis was partially rejected. Of the 27 planned comparisons performed, only 4 significant differences, with conflicting results, were found in the accuracy of intraoral digital implant scans in partially edentulous models with different implant angulations and different supramucosal heights of the ISBs. A consensus on the specific design features of the ISBs and how these may affect the accuracy of the digital intraoral scans is lacking. One design feature is the supramucosal height of the ISB, ISB visibility or ISB exposure, which can be determined by the depth of the implant or the thickness of the soft tissue and also by the length of the ISB. Regarding the effect of implant angulation on the accuracy of the IOSs, conflicting conclusions have been reported.
Intraoral digital scans—Part 1: influence of ambient scanning light conditions on the accuracy (trueness and precision) of different intraoral scanners.
An in vitro study of factors influencing the performance of digital intraoral impressions operating on active wavefront sampling technology with multiple implants in the edentulous maxilla.
Hence, this study was designed to compare the accuracy of intraoral scans with different degrees of implant angulation and different supramucosal heights of the ISB.
Other studies that assess the accuracy of intraoral scans have reported linear and angular deviations with no superimposition involved,
Effect of implant divergence on the accuracy of definitive casts created from traditional and digital implant-level impressions: an in vitro comparative study.
An in vitro study of factors influencing the performance of digital intraoral impressions operating on active wavefront sampling technology with multiple implants in the edentulous maxilla.
Comparison of conventional, photogrammetry, and intraoral scanning accuracy of complete-arch implant impression procedures evaluated with a coordinate measuring machine.
Comparison of conventional, photogrammetry, and intraoral scanning accuracy of complete-arch implant impression procedures evaluated with a coordinate measuring machine.
Impact of different scan bodies and scan strategies on the accuracy of digital implant impressions assessed with an intraoral scanner: an in vitro study.
after using a best-fit algorithm or iterative closest point algorithm for superimposition between the reference and test files. The present study combined 2 different ways of measuring deviations, reporting linear and angular overall deviations and using a best-fit algorithm for superimposition of the reference files obtained with a desktop scanner and the test files, to obtain RMS error values. The first method identified discrepancies among the groups that the RMS error method failed to recognize.
Based on the results of this study, the overall linear discrepancy means were smaller for the angled groups with respect to their equivalent parallel groups, indicating that the angled groups had improved linear trueness. However, the statistical analysis only identified as significant the difference between groups P-3 and A-3 and P-7 and A-7. During the scanning procedure, when the ISB is positioned straight and perpendicular to the occlusal plane, if the IOS first registers the occlusal surface of the ISB and the soft tissue, without visualizing the axial walls properly, a stitching error may have been introduced. However, when the ISB is angled, the IOS detects the occlusal surface of the ISB, part of the axial wall, and the underlying soft tissue, thereby improving the stitching, which could explain the lower linear mean values of the angled groups. The difference was consistent with that of a previous study that concluded that the digital method produced more accurate definitive casts when the implant divergence was greater.
Effect of implant divergence on the accuracy of definitive casts created from traditional and digital implant-level impressions: an in vitro comparative study.
A similar in vitro study performed in completely edentulous arches also reported lower discrepancies in the groups with angled implants as compared with the groups with parallel implants.
In terms of precision, the biggest differences in the present study were observed in the overall angular measurement discrepancy analysis between the 3-mm ISB height groups when compared with the 5-mm and 7-mm ISB height groups; this suggests that a reduced ISB height may translate into lower scanning precision. However, tests of statistical significance were not performed.
Further research is needed to assess whether the findings of the present study in terms of linear trueness and precision might translate into a clinical misfit. Some studies evaluating different degrees of misfit refer to a vertical gap or rotational deviation in the interface between the implant and the prosthesis.
However, these studies reported linear discrepancies that were almost perpendicular to the implant axis and might have created a horizontal misalignment between the implant and the intaglio surface of the prosthesis. Hence, other factors such as the machining tolerance of the implants and the implant components play an essential role.
Regarding the angular accuracy in assessing the effect of implant angulation, only 1 significant difference was found between groups P-7 and A-7, with a difference of 0.11 degrees between groups. Regarding the RMS values, no significant differences were found. The findings showed that a lower level of ISB exposure might not be a detrimental factor while making a digital intraoral scan in a partially edentulous patient, since of the 18 planned comparisons (6 for each outcome), only 1 showed a significant difference, while 17 were not significant. Such results contradict the findings from a previous in vitro study that concluded that a lower amount of visible ISB reduces the accuracy of the intraoral scan and recommended using longer ISBs when scanning deep implants. However, a different study design (based on active wavefront sampling) in an edentulous maxilla was used.
An in vitro study of factors influencing the performance of digital intraoral impressions operating on active wavefront sampling technology with multiple implants in the edentulous maxilla.
Also, studies from the same group with the same reference cast but with different scanning technologies concluded that the implant depth did not affect the accuracy of the IOSs.
Accuracy of a digital impression system based on active triangulation technology with blue light for implants: effect of clinically relevant parameters.
The present results were consistent with those of a more recent in vitro study that compared the accuracy of intraoral scans between conventional ISBs and scannable healing abutments which were exposed approximately 3 mm above the soft tissue. Both components were made of polyetheretherketone (PEEK), and, although they differed in some design features, the main difference between them was the clinical height. The authors concluded that there was no difference in the intraoral digital scan with a conventional ISB and with the short scannable healing abutments.
Lastly, a similar in vitro study performed on edentulous casts reported similar results in the parallel implant groups, as no statistically significant differences in accuracy were reported with different supramucosal heights of the ISBs. However, in the angled groups, the casts with reduced supramucosal height of the ISBs had lower intraoral scanning accuracy, although the angulation was higher than in the present study.
Limitations of the present study included the in vitro design that did not replicate intraoral conditions. Furthermore, only 1 type of scanning technology, 1 design of ISB, and 1 operator were tested, and the long span of the complete-arch scans can be a confounding factor. Additionally, the selected ISB presented the antirotational feature in the most coronal 3 mm. A different ISB design that incorporates the antirotational feature more apically might be affected differently by the supramucosal height of the ISB.
Conclusions
Based on the findings of the present investigation, the following conclusions were drawn:
1.
Implant angulation or divergence of 18 degrees did not significantly affect the accuracy of the intraoral scans in terms of 6 of the 9 planned comparisons for the overall linear deviations, overall angular deviations, and RMS values. Only 3 comparisons were statistically significant, with conflicting results.
2.
The supramucosal height of the ISB did not significantly affect the accuracy of the intraoral scans in terms of 17 of the 18 planned comparisons for the overall linear deviations, overall angular deviations, and RMS values. Only 1 comparison was statistically significant.
Acknowledgments
The authors thank the Prosthodontics Department of Tufts University (Boston, USA) for supporting the project, and Mr Adrián Hernández for his assistance on the registrations with the coordinate measurement machine.
References
Abduo J.
Elseyoufi M.
Accuracy of intraoral scanners: a systematic review of influencing factors.
Comparison of conventional, photogrammetry, and intraoral scanning accuracy of complete-arch implant impression procedures evaluated with a coordinate measuring machine.
Accuracy of printed casts generated from digital implant impressions versus stone casts from conventional implant impressions: a comparative in vitro study.
Intraoral digital scans—Part 1: influence of ambient scanning light conditions on the accuracy (trueness and precision) of different intraoral scanners.
Effect of implant divergence on the accuracy of definitive casts created from traditional and digital implant-level impressions: an in vitro comparative study.
An in vitro study of factors influencing the performance of digital intraoral impressions operating on active wavefront sampling technology with multiple implants in the edentulous maxilla.
Accuracy of a digital impression system based on active triangulation technology with blue light for implants: effect of clinically relevant parameters.
Accuracy of a digital impression system based on active wavefront sampling technology for implants considering operator experience, implant angulation, and depth.
Accuracy of a digital impression system based on parallel confocal laser technology for implants with consideration of operator experience and implant angulation and depth.
Impact of different scan bodies and scan strategies on the accuracy of digital implant impressions assessed with an intraoral scanner: an in vitro study.
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