Presentation of education
The permission for this study was given by the University Ethics Committee of Semmelweis University (SE TUKEB number: 61/2016). Dental students from 6 and 10 semesters with no experience of digital impressions took part in this study. Students represented an average student attending to graduate education of Semmelweis University. The study was preceded by education with theoretical and practical parts.
During the theoretical part of the education, a presentation was held by a dentist experienced in scanning, and an educational video (which was made by the research team) was viewed. The presentation was about the types, structure, operating principles, and indication areas of digital scanners. The Trios 3® intraoral scanner was introduced in detail, as it was used in the study. The video focused on the practical application of the scanner. In the video, the process of taking a digital impression was introduced step by step. This was followed by practical training. Each student took a digital study impression with a Trios 3® intraoral scanner of the lower and upper jaw models in an articulator, with occlusion recording.
Participants
“Participant” phrase was used for all the people who took part in our study. Ten dental students were involved in intraoral scanning in pairs assisting each other. Students had no previous experience with intraoral scanning. During scanning, supervision was granted by a dentist who was experienced in digital impression taking. Each examiner student took 10 scans according to the following: they scanned eight different dental students (who were volunteers). Furthermore, there was a university employee, who was scanned two times (first and last impression) by each examiner students. Patients’ different individual factors such as saliva flow or mouth opening can affect the speed of digital scanning procedure. For standardization, the first and the last volunteers was the same person (the university employee). The scans were performed separately from each other. The whole data collection procedure was performed across from June 2016 to 2017 September. The inclusion criteria of volunteers were the following: full dentition (except the missing third molar), good oral hygiene, aged at least eighteen years, intact hard and soft tissue (no decay or teeth extraction socket), normocclusion (Angle I). The exclusion criteria were undergoing orthodontic treatment, dental implants, any prosthetic treatment (inlays/onlays or crowns), gingivitis or periodontitis.
Every scan was made by two examiner students: one of them took the digital impression and the another assisted. The scanning student was on the right side and the assisting student was on the left side. The scans were performed with patients in a supine position. Every scan was performed with the help of a retractor (Optragate, Ivoclar Vivadent) to ensure the best accessibility. Dental light was turned off during scannings.
Intraoral scanner
As to the intraoral scanner, each time the same Trios 3® was used. In this study according to a scanning protocol based on the manufacturer’s instructions, students took digital impressions with the USB version Trios 3®, which can be connected to a high- performance laptop with a pen grip of (2). The software and hardware of Trios 3® have the capability to capture fully colored model. This scanner is a powder-free scanner which operates on the confocal principle with the video-recording method (2, 30). Before starting to scan the scanner was calibrated using the respective calibration device. The software version 3Shape Trios Classic 1.3.4.6 was used.
Digital impression taking
For standardization, the scanning device was calibrated before impression taking. It was done using supplementary tips and the calibration box. Diagnostic scan was taken after selecting the “Study model” icon. At first, patient data and the digital order form were completed. Scanning procedures were performed according to the manufacturer's instructions and previous education. It was started with the upper jaw, followed by the lower one. Scanning was always started on the right second molar and ended on the left second molar. The scanning strategy on the upper arch is the following: occlusal surface followed by buccal surface and, finally, palatal surface. Scanning strategy on the lower jaw starts with the occlusal surface, then the lingual surface and, finally, the buccal surface (31). The next step was bite registration in intercuspidal position on both sides. During bite scan the scanner tip was inserted at molar region, the buccal side of the teeth and slowly moved in mesial direction. After the scanning of the upper and lower arches, the virtual cast appeared on the screen. The virtual casts of upper and lower arches were accepted if included accurate scans of all the surfaces of every teeth with 2-3 mm of gingiva margin and no crack lines were found. The quality of the scans was satisfactory if the software could attach the arches to each other based on bite registration scan (16). (Fig. 1) If a crack line appeared on the virtual cast, the procedure was repeated from the beginning (the virtual cast was deleted and new intraoral scan was taken). In case of missing data (e.g. missing parts of the teeth’ surfaces) the virtual cast was not deleted but additional images were taken. Irrelevant areas such as palatal soft tissue were removed.
Registered data
Scanning time was measured with a stopwatch. Total scanning time was measured from the first step of recording patients’ data to sending the case to the lab. Total impression taking time included time of data recording, scanning time required for complete scanning of the upper and lower arches, bite registration on the right and left sides and processing time of the scan. Image numbers of upper and lower arches and bite registration were also recorded. Image number is the count of images made by intraoral scanner during scanning process. Number of images appeared automatically in the upper left corner of the screen after scanning. The present study focused on the total required scanning time and the total amount of images.
Learning curve
A learning curve is the representation of the rate of learning something over time or repeated experiences in a graph form. We know many types of learning curves, but the classical type is a sigmoid shaped ascending curve starting from zero learning level. The curve can be divided into three parts. During the positive growing period, the learning speed of the testers is increasing constantly, followed by the middle section where the pace of learning is uniform. During the negative period, the learning speed of the testers decreases and, finally, the curve ends in a flat phase (Fig. 2).
If the number of measurements is represented on the x-axis and the y-axis represents the output, i.e. the required time or number of images, we get an inverse learning curve (17) (Fig. 3). In statistical evaluation, we can draw a reverse learning curve of the examined students from the point of view of total scanning time and number of images.
Statistical analyses
Statistical evaluation was carried out using the Stata package to fit random-effects generalized least-squares regression models of outcome variables (total scanning time and image count data) against the sequential number of measurements, a continuous explanatory variable analogous with learning stage. Relationship curvature was allowed by adding a squared term for measurement number if its effect was significant at α = 0.05. Hausman’s specification test was used to assess whether fitting a fixed-effects model was justified. Outcome variables were natural log-transformed to improve normality.