# Accuracy of Extraoral Digital Impressions with Multi-Unit Implants
###### Abstract  
The aim of this in vitro study was to evaluate the accuracy of impressions made using two different silicones and corresponding stone casts in full-arch implant rehabilitation, using two laboratory scanners. A master model with six dental implants was created, scanned with a 12-megapixel scanner and used as digital master model. Ten implant impressions were obtained via two silicone impressions systems--Coltene(r) and Zhermack(r)--using the open-tray technique and poured gypsum. Two extraoral scanning systems (S600 ARTI Zirkonzhan(r) and Identica T500 Medit(r)) were used to scan the impressions and stone casts. Best-fit superimpositions were conducted between the master model and the virtual models obtained. A significance level of \(p<0.05\) was considered. The accuracy of the Medit(r) scanner was 82.26 [53.18; 111.34] \(\upmu\)m for Coltene(r) silicone and 87.43 [72.00; 102.86] \(\upmu\)m for Zhermack(r) silicone, and the accuracy of the Zirkonzhan(r) scanner was 69.90 [62.37; 77.43] \(\upmu\)m for Coltene(r) silicone and 80.67 [65.77; 95.57] \(\upmu\)m for Zhermack(r) silicone, without significant differences between scanners. When comparing the two silicones, no significant differences were found for the Medit(r) (\(p=0.112\)) or Zirkonzhan(r) scanners (\(p=0.162\)). The Zhermack(r) scanner showed better accuracy than the corresponding stone casts in the two scanners, Medit(r) (\(p=0.019\)) and Zirkonzahn(r) (\(p=0.002\)). The extraoral digitalization of impressions in dendulous dental arches with six implants proves to be a valid technique.  
type=color=black!10,outer=black!10,outer=black!Extraoral scanners were introduced in the 1980s to overcome the shortcomings of intraoral scanners at the time, which were considered too slow to be used in office and had limited accuracy. Extraoral scanners were initially developed to scan stone casts, but currently, more advanced systems also allow for the scanning of dental impressions [10; 11; 12; 13; 14]. In the indirect digitization of both impressions and models, the initial step is an analog impression.  
Various factors can affect the accuracy of complete-arch implant impressions, such as the characteristics of the impression material, the impression technique, the splinting of the impression copings, the depth and angulation of the implants or the connection of the implants [4; 6; 9; 15; 16; 17; 18; 19].  
According to a systematic review published in 2018, no guidelines can be found to recommend a particular technique and material to achieve the most adequate accuracy in impressions with multiple implants [4]. Achieving an accurate transfer of implant position and angulation is a critical factor in obtaining a satisfactory prosthesis with an accurate fit [7; 20; 21]. According to several studies and considering the accuracy factor, the use of an intraoral impression with an optical scanner is not recommended in cases of multiple implants [22; 23; 24].  
Although IOSs are increasingly used, they still require a learning curve and a considerable initial investment. In addition, in many clinical situations, using an IOS is less accurate than extraoral scanning [21]. Furthermore, the scanning of impressions benefits from eliminating the model fabrication procedure, which is time-consuming and a potential source of error [25]. For these reasons, scanning conventional impressions may be an interesting option in oral rehabilitation with implants [8; 21].  
The objectives of this in vitro study are to evaluate the accuracy of digital implant impressions in full-arch implant rehabilitation through the use of two silicones, corresponding stone casts and two laboratory scanners. The null hypotheses are that (1) the scanner, (2) silicone and (3) pouring dental cast do not affect the trueness of the impressions.