Influence of Impression Techniques and Second Pouring on Cast Accuracy

Objectives: To evaluate the influence of impression techniques and second pouring on the dimensional accuracy of gypsum casts in a clinical-laboratory study.
Methods: Ten patients were selected and for each one, 5 partial impressions were obtained from the maxillary right canine to the first molar, following one of the impression techniques: (1) single-step; (2) two-step/reline-relief with PVC film; (3) two-step/reline-relief with Maxicut bur and scalpel blade; (4) two-step/reline-relief with small movement of the tray; and (5) two-step/reline-without relief (negative control). Impressions were made with custom acrylic resin partial trays using vinyl polysiloxane (Express XT; 3M ESPE). After 110 minutes, the first pouring was performed with type IV gypsum (GC Fuji Rock EP). The second pouring was made after 120 minutes from the removal of the first cast. Three lateral photographs of each patient were obtained using a tripod and a customized radiographic positioner, and imported into the ImageJ software. The total area of the buccal surface from the maxillary right canine to the first molar was blinded measured in triplicate. A 4.0% coefficient of variance was a criterion for using these measurements as a standard Baseline for comparison with those obtained on gypsum casts after the first and second pouring. The gypsum casts (N=100) were photographed and analyzed using the same standardization and procedures used for the clinical images. The mean area values (mm²) from the differences between the Baseline and the gypsum casts for each impression technique were analyzed by repeated measures one way-ANOVA (α=0.05).
Results: No significant differences were observed for Impression technique (P=0.525), Pouring time (P=0.281), and for their interaction (P=0.809).
Conclusions: Regardless of the pouring time, the single-step and the different two-step impression techniques produced gypsum casts with similar accuracy,