## 2 Materials and Methods  
### Digital and Operative Flow-Chart  
Eleven acrylic resin models, reproducing completely edentulous mandbles with different alveolar shapes, were scanned with a CBCT. The models reproduced typical situations to be managed without grafting techniques to avoid dehiscence or fenestrations. DICOM data were imported in a dedicated software (B&B Dent SL GS, B&B Dental Implant Company, Bologna, Italy). The placement of eight implants 4 mm wide and 11 mm long (Duravit 3P, B&B Dental implant company, Bologna, Italy) in each model, uniformly distributed in posterior and anterior locations, was planned for a fixed full-arch rehabilitation according to the bone volumes and shapes simulated by the models. The virtual implant position was identified through the DICOM data, which were then converted into STL-files for the models' segmentation and the guide design and printing, according to the implant positions. The first dentist (GP), the more experienced operator, accomplished the planning phase in collaboration with the digital designer.  
The surgical guides were projected according to the supporting bone modality. Since there was no occlusal correlation with an opposite maxilla, the plan could not start from actual prosthetic demands. Three vestibular fixing pins to stabilize the guides and eight sleeves of 4.2 and 5 mm in diameter and length, respectively, were designed. The sleeves were made in poly-ether-ether-ketone (PEEK), instead of using metallic ones, to avoid friction and overheating during the implant site preparation. Five models and a half, with the applied guides, were randomly assigned to each of the two operators and fixed to the dental chair headboard to simulate the clinical ergonomics (Figure 1). The guides were fixed to all the models by the more experienced operator (GP), who was unaware of the subsequent casual operator's assignment.  
The first dentist (GP) had had more than fifteen years of experience in freehand implantology and more than ten years of experience in s-CAI. The second one (FD) was a post-graduate dentist, an utter novice in general dental practice. The implants were placed independently in different sittings dedicated to accomplishing each model procedure, and the time taken was recorded. For each model a new kit of burs was used to eliminate the bias of the tool's wear. The operative timings were recorded in seconds and statistically elaborated.  
After that, the implanted models were CBCT scanned once again, and the DICOM data was imported in the same software mentioned above (method n.1). The \"image substitution analysis\" matched the pre- and post-CT session without the segmentation of the implants and the STL data conversion.  
Optical scanning of the resin models was taken with a laboratory scanner (EGS dscan3) and an intra-oral scanner (3 Shape Trios, Copenhagen, Denmark) after connecting the scan bodies with the implants as in methods n.2 and n.3.  
The flow-chart is shown in Figure 2.  
The data relating to the external surface of the resin clones from the three techniques, exported as STL data, were volumetrically analyzed, and the pre- and postoperative virtual models and implants were aligned by the _GOM inspect_ software. The alignment of the pre- and postoperative surfaces was singularly exported and loaded onto _Meshmixer_ software, which was able to remove the superimposing surfaces of the resin copies while maintaining those of the implants (Figure 3).  
Figure 1: Surgical guide fixed to the acrylic resin model.  
Figure 3: Pre- (**A**) and post- (**B**) operative virtual models and their alignment superimposing the external surfaces (**C**). Planned (**D**) and actual (**E**) implant positions after removing the models’ superimposing surface.  
Figure 2: The scheme of the flow-chart.