This study used a partially edentulous area Kennedy class IV model to compare both groups. The sample size was calculated using the Sample Calculator (https://www.calculator.net/sample-size-calculator.html), with the lower discrepancy value between groups, and the number of samples required for the test was 10. Therefore, the surgical guide was reproduced ten times for the evaluations.
The same model selected for the study was used to create all surgical guides was divided into two distinct groups: the MILLED GROUP, comprising 10 milled surgical guides and 3D PRINTING GROUP, comprising 10 3D printing surgical guides.
Production of surgical guides
The model used in this study was digitized using an intraoral scanning system (Cerec AC®; Sirona Dentsply, Bensheim, Germany). The scanning process generated a projection in SSI language that enabled virtual planning of the ideal position for inserting the implant. After that, an image was developed in DXD language, allowing file import to inLab 15 software (Sirona Dentsply, Germany), making it possible to create the appropriate design of the surgical guide.
The surgical guide design started by defining the ridge boundaries and length of the surgical guide (Fig. 1a) and determining the ring's position and size responsible for guiding implant insertion (Fig. 1b). After this step, a preview of the surgical guide design was generated (Fig. 1c). After verification and approval of the planned guide, the project was ready for manufacture.
For the milling guides, the file was sent to the MCXL milling machine (Sirona Dentsply, Germany) for production using polymethyl methacrylate (PMMA) according to the manufacturer’s standardized parameters.
3D printing guides were produced after converting the DXD archive into STL extension, which was then sent to a 3D printer (Perfactory P4K Life Series, EnvisionTEC, Germany). This printer uses DLP technology with a 4-M pixel projector and a UV wavelength of 385 nm. The printer was calibrated following the manufacturer’s instructions before the beginning of the printing process (detailed instructions can be found on page 31 of the printer’s technical guide). The resin used was EnvisionTEC's E-Guide Tint (Dearborn, USA), which is a biocompatible Class I certified material. The guides were positioned at a 45° angle. After printing, the guides were immersed in isopropyl alcohol to perform surface cleaning, and then light curing was performed using the manufacturer’s parameters.
Once the surgical guides were completed, individual digitization of each surgical guide was performed using a Data Sheet camera (stereoSCAN3D R8; 8.0 megapixel, Germany), thereby creating a mathematical model (STL) so that the guides could be superimposed overlapping the virtual master model using the software Optocat (Breuckmann, Heiligenhaus, Germany). The sequence is illustrated in Fig. 2.
Once the best fit alignment between images was obtained, the superimposed models were evaluated and the areas of misalignment were identified. The data obtained with the superimposed files were evaluated between the groups for the precision in obtaining the guides from the master model (intergroup evaluation). Additionally, the guides of the same group were compared, obtaining the trueness to verify the reproducibility of the guides using both fabrication processes (intragroup evaluation).
The minimal and maximum values of misalignment for each group, the average of a mismatch for each sample, and the standard deviation between these misalignments in each model were recorded.
Data analysis
The data were evaluated using GraphPad software. The Kolmogorov–Smirnov test was performed to test the normality of the data for precision (> 0.1000 and 0.0637 for the milled and 3D printing groups, respectively) and trueness (0.571 and > 0.1000 for the milled and 3D printing groups, respectively). After passing the normality test, the data were submitted to the parametric evaluation of Student’s t-test. The alpha level for significance was set at 5%.
