Due to the limited access of CMMs, industrial 3D scanners, or dental laboratory scanners into the oral cavity. It is generally impossible to establish reference data in real patients. Indeed, there is no in vivo study that has investigated the trueness of the digital impression for dental implant and all of the following reviewed in vitro studies are laboratory-based.
Linear and angle error evaluation using CMM (Table 2)
Studies that evaluated digital impression compared to conventional methods
Gintaute et al. evaluated the trueness of digital impressions and conventional impressions using four types of reference models with different inter-implant distances and inter-implant angles: (1) two straight, (2) four straight, (3) two straight and two tilted, and (4) six straight dental implants [29]. The inter-implant distances and inter-implant angles of the reference models were measured as reference data using CMM. As test groups, digital impressions of the reference models that were acquired using TDS and STL data were analysed using 3D evaluation software. Polyether and vinyl polysiloxane impressions were utilised for the conventional impressions, and stone casts were made from the impressions and subsequently measured using CMM.
Regarding implant orientations (1), (3), and (4), digital impressions showed significantly lower inter-implant distance errors than conventional impressions. In terms of inter-implant angle error, the digital impressions exhibited significantly higher trueness than the conventional impression in all four reference models. However, the inter-implant distance and inter-implant angle errors were within 100 μm and 0.5°, respectively, which the authors judged to be clinically acceptable (Table 2).
Ajioka et al. evaluated the trueness of the digital impression by COS and the influence of the height of the abutments on the angle error [14]. A reference model with two implants in a partially edentulous model (#35 and #36) was prepared. Conventional models made of plaster were fabricated from a reference model using a silicone impression. For the distance measurements, two ball abutments were connected to the implants, and the distance between the centres of the balls of the abutments was measured. For the angle measurements, pairs of healing abutments that were 5 mm or 7 mm tall were connected, and the angulation between the healing abutments was measured. The reference model and conventional models were measured using CMM. The distance errors of the digital impressions were slightly greater than those of the conventional impressions. The angulation error was also greater for the 5-mm digital impressions but was not significantly different from the conventional method when 7-mm abutments were connected. Suggesting that a longer abutment or scan bodies may improve the trueness of digital impressions (Table 2).
Chia et al. evaluated the trueness of digital impressions for a three-unit bridge supported by two implants with three different inter-implant angles [31]. Three reference models with buccolingual inter-implant angulations of 0°, 10°, and 20° were fabricated. The scanned bodies connected to the reference models were scanned using IOS. The conventional impressions of each reference model were made using polyether impression materials, and conventional plaster models were fabricated. The reference and conventional models were measured using CMM. The impression technique (p = 0.012) and implant angulations (p = 0.007) had a significant effect on the linear error. In terms of the angle effect, the digital impression group showed consistent linear and angle errors, irrespective of inter-implant angulation. In addition, digital impressions tended to replicate the implant position more apically than the actual position (Table 2).
Menini et al. compared the trueness of digital impressions and conventional impressions using a full-arch edentulous reference model with four implants [39]. CMM was used to measure the implant angulation and inter-implant distances in the reference model as well as on the conventionally fabricated casts. Conventional impression data and digital impression data were compared with the reference data measured using the reference model. The trueness of the conventional group, as evaluated by the linear error, was inferior to that of the digital impression data (Table 2).
Tan et al. compared the trueness of digital impressions using two IOSs (Trios and TDS) to conventional impressions [10]. They used two reference models with edentulous maxillary arches with six or eight implants. The inter-implant distances were approximately 20 mm in the six implant models and 13 mm in the eight implant models. The centre positions at the implant platform level on the reference models were detected using the CMM. The results of this study showed that narrower inter-implant distances might decrease IOS linear errors. In addition, TDS showed a greater linear error than Trios (Table 2).
Alikhasi et al. investigated the trueness of digital impressions by Trios using two maxillary edentulous reference models with different internal or external implant connections, with two anterior straight and two posterior angulated implants [19]. Conventional plaster models were fabricated from silicone impressions using an open tray or closed tray. The conventional and reference models were measured using an optical CMM. STL datasets from the digital impression were superimposed on the reference data to assess the angle and linear errors. Digital impressions demonstrated superior outcomes compared to conventional methods. While the trueness of digital impressions was not affected by the type of connection and angulation, conventional impressions were significantly affected by these factors (Table 2).
Studies that exclusively evaluated digital impressions
Giménez et al. conducted two studies evaluating the trueness of a digital impression by COS using a reference model with six implants (#27, #25, #22, #12, #15, and #17). The implant at #25 was mesially inclined by 30°, the implant at #15 was distally inclined by 30°, and the implants at #22 and #12 were placed 2 mm and 4 mm subgingivally, respectively [18, 40]. Two experienced and two inexperienced operators performed the scans. The CMM was used to measure the reference model, and the linear error was calculated. The angulation (p = .195) and depth of the implant (p = .399) measured by digital impression did not deviate significantly from the true values. Additionally, the experience of the operator significantly influenced the trueness of digital impressions (Table 3).
Sami et al. evaluated the trueness of digital impressions from four IOSs (TDS, TRIOS, Omnicam, and Emerald Scanner) [30]. An edentulous reference mandible model with six implants was fabricated and measured using four IOSs and an optical CMM. Data from the four IOSs were superimposed on the reference data, and the discrepancy between them was evaluated. The results indicated no statistical or clinical differences among the IOSs (Table 3).
Fukazawa et al. evaluated the trueness of the inter-implant distance on the surface data captured by several IOSs and a laboratory scanner and compared these to measurements acquired by CMM as references. They prepared two reference models with missing teeth at #35, #36, #45, #46, and #47. Model A had two neighbouring implant analogues at #35 and #36, whereas model B had implant analogues at #45 and #47. They found that the IOS error values were greater than the errors of the laboratory scanner. The linear error tended to be greater with longer inter-implant distances (model B) (Table 3).
Di Fiore et al. compared the trueness of the digital impression from 8 IOSs (TDS, Trios, Omnicam, 3D progress, CS3500, CS3600, Planmeca Emelard, and Dental Wings) in a full-arch implant-supported FPD [42]. An acrylic model of an edentulous mandible with six implants was used as the reference model. They evaluated the 3D position of the scan bodies and inter-implant distances captured by the IOSs in comparison to those captured by the CMM. The deviations of the 3D positions of the scan bodies were calculated using the best-fit algorithm. The distances between all combinations of the six scan bodies (15 pairs) were calculated from the STL data using analysis software and were compared to the reference data measured by CMM. The 3D position results of the implants, as measured by each IOS, showed that the TDS and Trios showed the best trueness among the IOSs, followed by Omnicam and CS3600 with average performance; CS3500 and Planmeca Emelard presented a middle-low performance, while the 3D progress and Dental Wings showed the lowest performance. The inter-implant distance analysis showed that shorter inter-implant distances corresponded to better trueness when using the True Definition and CS3600 devices (Table 3).
Summary of the results of studies that utilised CMM for trueness evaluation
Except for one study, digital impressions showed superior trueness to conventional impressions. A longer inter-implant distance tended to deteriorate trueness. Three studies found a difference in trueness among manufacturers of IOS, while one study did not. The experience of operators in digital impressions positively affected the trueness of digital impressions. A longer scan body seemed to contribute to better trueness. The inter-implant angle and the difference in platform configuration (internal or external) did not affect the trueness of digital impressions.
Linear and angle errors by industrial 3D scanners
Studies that evaluated digital impression compared to conventional methods
Amin et al. evaluated the trueness of digital impressions from two IOSs (Omnicam and TDS) using a full mandibular edentulous reference model with five implants [23]. The three median implants were parallel to each other. The far-left and far-right implants were inclined by 10° and 15° distally, respectively. A splinted open-tray technique was used for conventional polyether impressions to fabricate conventional models. The reference and conventional models were scanned using an industrial 3D scanner. The digital impression data from the reference model that was captured by the IOSs and the data from the conventional model captured by the industrial 3D scanner were superimposed with the reference data and evaluated using the best-fit algorithm. The full-arch digital impression using TDS and Omnicam showed significantly higher trueness than the conventional impressions using the splinted open-tray method (Table 4).
Studies that exclusively evaluated digital impression
Van der Meer et al. evaluated the trueness of three IOSs using dentate reference models with three implant analogues (#36, #41, #46) [13]. They measured the inter-implant distances and inter-implant angles of #36–41 and #36–46. An industrial 3D scanner and engineering software were used to obtain the reference data. The inter-implant distances and inter-implant angles captured by the IOSs were compared with the reference data, and the trueness of each scanner was evaluated. The distance discrepancies between the IOS data and reference data varied depending on the IOS and scanning range. An increase in distance and/or angle errors were associated with a larger scanning range but this trend was not statistically significant (Table 4).
Imburgia et al. compared the trueness of four IOSs (CS3600, Trios3, Omnicam, TDS) using a partially edentulous model with three implants and a fully edentulous model with six implants. The reference data were acquired using an industrial 3D scanner, which was superimposed with the scanned data from each IOS [27]. Trueness differed among IOSs. For all scanners, the trueness values obtained from the partially edentulous model were significantly better than those obtained from the fully edentulous model (Table 4).
Arcuri et al. evaluated the influence of implant scan body materials on digital impressions using an IOS (Trios3) [26]. An edentulous maxillary model with six internal connection implants was scanned using an industrial 3D scanner to acquire the reference data. Scanned bodies made of three different materials (polyetheretherketone (peek), titanium, and polyetheretherketone with a titanium base (peek-titanium)) were scanned by three operators using the IOS. These data were superimposed on the reference data using a best-fit algorithm. Linear and angle errors were assessed, and a significant influence of the type of material was identified (p < 0.0001), where the peak showed the best results in terms of both linear and angular measurements, followed by titanium and the peek-titanium (Table 4).
Kim et al. evaluated the trueness of digital impressions by five IOSs using a partially edentulous model [28]. A 3D printed partially edentulous mandible model made of Co-Cr, with six bilaterally positioned implants in the canine, second premolar, and second molar area served as the reference model. Reference data were acquired with an industrial 3D scanner, and the test data were obtained from five IOSs (Omnicam, CS3600, i500, iTero Element, and TRIOS3). For data from each IOS, the XYZ coordinates of the implants were obtained, and the deviations from the reference data were calculated.
The linear and angle errors differed depending on the implant position and the IOS. Regardless of the IOS type, the implants positioned on the left second molar, nearest to the scanning start point, showed the smallest linear error. The error generally increased further away from the scanning start point towards the right second molar (Table 4).