Accuracy of conventional and digital impressions at different span lengths of missing teeth : (In-Vitro study).

Abstract

Impression taking is a crucial step in prosthodontics, as the quality of the final prosthesis and its long term survival depends on the accuracy of this process. Conventional impressions are the most common in the clinical practice; conventional workflows and CAD/CAM technologies can be combined through indirect digitization. However, new digital techniques allow the full digitization of the workflow, by the use of IOSs. The main feature to be evaluated in an intraoral scanner is accuracy. According to ISO 12836, the accuracy of an impression technique is defined in terms of trueness and precision. Trueness is defined as the difference in measurement between the reference model and the scan model and precision is the difference in measurement between digital models created using the same impression technique. This in-vitro study was designed to evaluate the accuracy of three different impression techniques at three different span length bridges. For full arch prostheses and FPDs with more than 5-units, digital impressions do not seem as accurate as conventional impressions. Therefore, the aim of this study was to assess the accuracy in terms of trueness and precision of conventional and digital scanning (direct and indirect) techniques on different span length bridges. The bridge preparations were done on acrylic typodont models (Nissin, Kyoto, Japan) with the aid of a dental surveyor. Three different impression techniques were used, a conventional PVS impression material (Elite HD+ putty soft and light body consistencies), an intraoral scanner (CEREC Primescan) and an extraoral scanner (Medit Identica t300). The three groups (3,4 and 6-unit bridges) were divided into 3 subgroups according to the impression technique received (PVS, Primescan and Medit t300). For the trueness measurement, the three different bridge types were scanned using a desktop scanner (inEos X5) which was used as the reference scanner to obtain the reference datasets (REF STL files). The different impression techniques were used to record five different impressions (n=5) for each bridge span type and with the help of a reverse engineering 3D analysis software (Geomagic Control X), the digitized measurement models were superimposed on the reference to calculate the amount of deviation or RMS value of error. For the precision measurement, the calculations were done within each subgroup. Each scan was considered as the reference superimposing the remaining 4 scans in pairs to calculate the amount of deviation or RMS value of error. Color difference maps and reports were generated for all of the test groups. Data was recorded, tabulated and analyzed. Statistical work was done using the two-way ANOVA test. Results Regarding the different span length bridges, the best trueness and precision values were recorded for the 3-unit posterior followed in descending order by the 4-unit posterior and 6-unit anterior bridges for all three impression techniques. Regarding the impression technique, the best trueness values were recorded by Primescan followed by PVS and Medit t300. The best precision values were recorded by Medit t300 followed by Primescan and then PVS, all of which showed statistically significant difference.