# Accuracy of Extraoral Digital Impressions with Multi-Unit Implants
## 2 Materials and Methods  
Six Straumann bone-level 4.1 implants, each with an internal hexagonal connection (Institut Straumann AG, Basel, Switzerland) and the corresponding screw-retained abutments (SRA) (Institut Straumann AG, Basel, Switzerland), were placed in an artificial mandibular with artificial gingiva, simulating a clinical situation. A 12-megapixel reference scanner was used to scan the master cast (Gom Atos Compact Scan 12 M, GOM Metrology, Braunschweig, Germany). The scan was stored in an STL file and considered the digital master cast (Figure 1). Figure 1: Digital master cast used in this study. Based on the master cast, impressions were taken using two silicones with open trays and splinted copings using the double-mix technique. In Group A, 10 impressions were taken using Light Body Type III and Heavy Body Type I silicones (Hydrorise Implant, Zhermack, Rovigo, Italy). In Group B, 10 impressions were taken using Light Body Type III and Heavy Body Type I silicones (Coltene, Altsatten, Switzerland). The trays were made using 3D printing and pre-coated with a silicone adhesive (Coltene, Altsatten, Switzerland) prior to taking the impressions. The workflow is shown in Figure 2. The copings were tightened at 15 Ncm into the SRAs and splinted with methylmethacrylate (Figure 3). After 24 h of polymerization, the splint was sectioned, and additional resin was added in small amounts. After polymerization, the copings were unscrewed, and the impression was removed from the master cast. Subsequently, any excess impression material and the buccal surface of the impression were carefully trimmed with a scalpel to allow for exposure to light. The impressions were then stored for 8 h at 23 \({}^{\circ}\)C. All steps were performed by a single experienced operator. For the digitalization of the master cast impressions with extraoral scanners, ScanAnalogs (dynamic abutment solutions; Lleida, Spain) were screwed into the copings. TiO\({}_{2}\) particles  
Figure 3: Master cast with copings splinted with methylmethacrylate. Figure 2: Stages of the study workflow. were sprayed to obtain a coating of a minimum thickness on the impressions, according to the ISO-12836 [26]. To digitalize the impressions, two laboratory scanners were used: a blue-LED scanner (Identica T500; Medit, Seoul, Republic of Korea) and a structured-light optical scanner (S600 ARTI; Zirkonzahn, South Tyrol, Italy). The images obtained were stored in STL files (Figure 4). Before pouring the gypsum, the ScanAnalogs were removed, and the multiunit analogs were screwed into the transfers. Subsequently, the impressions were cleaned and dried, and artificial gingiva were placed on the impressions (Gingifast elastic, Zhermack). Type IV plaster (Elite Rock; Zhermack) was used for casting, with a water/powder ratio of 20 mL:100 g, according to the manufacturer's recommendations. The impressions were poured under constant vibration. After 2 h, the impression and cast were separated by unscrewing the transfers. For digitalization with extraoral scanners, ScanBodies (Institut Straumann AG, Basel, Switzerland) were placed, and the stone casts were sprayed with TiO\({}_{2}\) particles. Scanning was carried out using the two described scanners, and the scans were saved in STL files. All scans were performed in a controlled environment. A total of eight digital casts were obtained for the study groups (Table 1). The scans obtained were edited in Exocad to place digital replicas corresponding to the ScanBodies and ScanAnalogs used. After this process, the files were exported as