Assessment of different techniques and materials for dental impression on implants by optical microscope

Introduction: In implant prosthodontics, 50% of the errors generally encountered relate to the techniques used in implant impression and cast contruction. The remaining 50% of errors are generated by inaccurate laboratory procedures. The current dental literature reports conflicting results regarding the best material and the best technique to obtain a faithful reproduction of the position of the implants in an edentulous arch. Objectives: The aim of this in vitro study was to evaluate by an optical microscope the accuracy of the dental impressions made with 7 different techniques in full-arch rehabilitation dental implant rehabilitations. Materials and methods: A cast metal framework was realized using a master cast representing a superior jaw in which 4 implants were inserted at th elevel of the canines and of the first molars. Then implant nalogues were screwed to the metal framework and a simplified master cast was realized. This master cast was used for all the tests subsequently described. Impressios of the master cast have een taken using different materials and techniques: Impregum (polyether) + open tray technique (OTI); Impregum (polyether) + closed tray technique (CTI); Impregum (polyether) + open tray splintied technique (OTIS); Ramitec (polyether) + open tray technique (OTR); Ramitec (polyether) + closed tray technique (CTR); Ramitec (polyether) + open tray splinted technique (OTSR); BF plaster (plaster dental impression) + open tray technique (PLASTER). For each of these techniques 5 impressions of the master cast have been taken. Standard plastic impression trays provided with rimming were used. A special device was used to standardaze the force exerted during the impression and the direction of the impression tray. Casts have been realized connecting abutment analogues into the impressions. The accuracy of the framework was evaluated by the "one screw test" or Sheffield's test, screwing the metal framework previously realized on the 35 casts. An optical microscope (Smartscope MVP) with a 120x magnification was used to measure the accuracy of the interface between the abutment analogs incorporated in the casts and the metal framework. For each cast 8 measurements were taken: 4 screwing the framework at the level of the implant 26 and 4 screwing the framework at the level of the implant 16. Statistical analysis: For each of the 35 models acerage values of deviation compared to the master cast were obtained when screwing the stiff framework according to the Sheffield's test. This information was subsequently compared with the respective average values of the master model using a T-test to a sample (one sample T-test). Results: The casts made with the techniwues PLASTER (0.077±0.033μm, p=0.221), OTI (0.095±0.042μm, p=0.111) and OTSR (0.140±0.080μm, p=0.078) did not show a significant difference compared to the master model (0.056±0.047μm). Considering both the mean values compared to the master cast and the standard deviation of these three techniques, respectively V=0.43, V=0.44, V=0.57 the models obtained with plaster resulted the most similar to the master cast. Conclusion: 1) Closed tray techniques were the least reliable; 2) The splinting of impression copings with acrylic resin did not improve accuracy; 3) The association of an open tray technique with a stiff material (PLASTER) exhibited the best accuracy. Keywords: impression materials, impression techniques, implant rehabilitations