Previous in vitro studies evaluating IOS and IMPR vary considerably for study design, but also regarding material properties of the reference-model [15, 18]. Nevertheless, extensive literature reviews show results which are clinically satisfactory for both digital and analogue impressions, as did the full manufacturing flow of single restorations and shorter fixed prosthesis [18, 22, 23].
However, we have noticed when working clinically with multiple IOS in parallel, that there are large variations in distinctness of the finish line of the acquired scans, and that IOS and desktop scanners display unique system-specific mesh appearances. The aim of the present in vitro study was to evaluate any differences of finish line distinctness and finish line accuracy of IOS and IMPR, a critical component in prosthodontics which has not been investigated previously. Furthermore, the descriptive method aimed at visualizing the effect of other parameters, such as mesh resolution, tessellation, topography, and the effect of color.
The results of this study do not support the null hypothesis that there were no sizeable differences between IOS and IMPR regarding finish line distinctness and finish line accuracy.
This in vitro-study adopts a test model where the digital and conventional impressions were taken under the best conditions without interference from extrinsic adverse factors, such as gingival crevicular fluid, blood, or displaced retraction cords. The preparation, with supragingival finish line and two areas simulating subgingival finish lines, was selected to investigate the IOS limitations as it imposes a great challenge for successful identification of the finish line .
TRIOS, with the highest triangle count, displayed the highest level of finish line distinctness and together with CS3600, the highest finish line accuracy and surpassed IMPR. DWIO and PLAN on the other hand displayed a generally low level of finish line distinctness and finish line accuracy. Together with 3M, deviations in local subgingival areas reached deviations above ±100 μm. This deviation in IOS relates by at least a two-fold factor to that seen in studies on margin fit of final restorations, which also take in consideration all contributing factors, such as the milling of the restoration and the seating . Hence, the deviations should be considered sizeable in relation to the full workflow.
PLAN showed the lowest finish line accuracy of all IOS, as well as the lowest overall accuracy, and contrary to other IOS, held negative deviations. A positive deviation may result in a restoration being short onto the preparation and with a potential larger spacing. A large negative deviation may result in a restoration having premature contact in specific hotspots, thus leaving greater spacing in other areas. It appears that the acquisition method by laser and triangulation technology used in PLAN suffers the same deficiencies previously found in the preceding E4D system .
Deviations above ±50 μm reached varying distances from the periphery in 3M, DWIO and predominantly PLAN. The size of deviations in combination with the extent of the deviations from the finish line may play an important part in the long-term success of final restorations.
Resolution varied between the evaluated systems, with PLAN showing the lowest triangle count, as well as a low level of finish line distinctness and the lowest finish line accuracy. DWIO on the other hand, with the second highest resolution, also showed a low level of finish line distinctness and low finish line accuracy. The highest triangle count was found in the TRIOS system, which also had the highest level of finish line distinctness. However, the scanner with the second lowest triangle count, CS3600, showed a similar degree of local finish line accuracy as TRIOS. Thus, overall resolution appears to not have a direct relation to the finish line distinctness and finish line accuracy, but may depend on localized finish line resolution. These findings are in agreement with previous studies [9, 25].
The effect of low resolution was however visible in the cropped area of the finish line with a higher level of jaggedness. It is unclear how proprietary software as well as different dental CAD software treats the jaggedness through post-processing and possible triangle subdivision when plotting the finish line. The system with the highest level of smoothness was TRIOS.
Tessellation of any 3D mesh derives from both the specific scanning technology and from active engineering choices when designing software algorithms. Although 3M and the DWIO had different mesh appearances, a consistent higher uniformity was present in the tessellation at the finish line when compared to other IOS and IMPR, (Fig. 3). This may not have an impact on larger surface accuracy, but can be perceived when evaluating the finish line distinctness which holds a low resolution and lacks the capacity to clearly depict the undulating transition area. Poor depiction of the finish line may lead to an unnecessary over- or undersized contour of the final restoration.
Topography describes the variations in height of a surface. A previous study has shown that earlier systems based on coating displayed a smoother topography, whilst non-coating systems with a higher resolution produced a surface with greater noise . Although many of the scanners in this study belong to a newer generation, the non-coating TRIOS system displays some minor noise not seen among the other non-coating IOS and is dependent on the specific technology. However, the deviation spectrum in TRIOS was within the nominal range and most likely lacks any clinical effect in later processing and manufacturing. 3M displayed minor areas outside the nominal threshold.
The introduction of color among IOS systems, may improve the detection of the finish line due to the visible contrast between tooth and soft tissue as seen in Fig. 6 when compared to the monochrome renderings in Fig. 2. TRIOS and OMNI, and to some extent CS3600 showed a clear and distinct color rendering. PLAN using laser for measurements and RGB LED for color mapping, had low congruence with color bleed from the tooth onto the adjacent surface, thus not increasing the identification of the finish line.
Several described factors may influence the finish line distinctness and identification of the finish line. However, a parameter not simulated is the possibility to rotate the model in the 3D space and through variations of inter-facet angulations visualize variations in the 3D rendering. This rendition created with artificial lighting, generates glare, light reflection of high to low intensity as well as full cut-off, and can assist the operator in identifying a distinct finish line. Furthermore, tools in proprietary software and third party dental CAD/CAM solutions can facilitate and automate the recognition of the finish line, and use 3D imaging snapshots to enhance the manual identification . However, these tools only enhance existing features of the 3D mesh and does not replace a high-quality scan.
There are several limitations within this study. First, the need for coating the translucent preparation model with titanium dioxide to allow for a reference-scan. Even though thickness of the coating material was minute, a material buildup could occur . To limit this negative impact, the reference scanning was outsourced to a specialized entity, with extensive experience of scanning for the industry in general, and for research and development within leading dental companies. Specific airbrush technology with fine-adjustable air-pressure was used to deliver the thin coating at control beyond that of aerosols or powder dispenser used in the field of dentistry.
Second, as only one file (the tenth scan/repetition) for each IOS was investigated regarding finish line accuracy, the deviations may fluctuate both in severity and distance from the finish line. However, it is beyond the scope of this descriptive pilot study to assess the full intra-system range of such deviations or the precision of each system.
Last, the used in vitro model cannot fully simulate hard and soft-tissue interaction, and it excludes adverse factors known to negatively affect the quality of impressions. Thus, the clinical reality may prove to be more challenging than conditions in this study.
From a clinical perspective, it is essential that IOS can perform well in all scenarios, with similar, or better results than conventional impressions. This study shows that some investigated IOS can provide finish line distinctness and finish line accuracy that is higher than IMPR in vitro. However, not all IOS performed equally well.