Effect of different materials, trays and procedures on the accuracy of impression techniques: a clinical-laboratory study

Abstract

The success and longevity of fixed dental prostheses (FPDs) are related to an adequate and accurate marginal fit, which depends on the quality of impression technique, impression material, and gypsum cast obtained. Thus, the purpose of this study is to clinically evaluate the accuracy of impression techniques for FDPs, using different materials and trays. Initially, an estimated sample of 10 patients will be selected for the pilot study. For each patient, 20 impressions from teeth 13 to 16 will be made, according to five 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 a small movement of the tray and (5) two-step/reline - without relief. Impression techniques will be made with metal stock or individual acrylic resin partial trays, using condensation-polymerized silicone (Speedex; Coltène/Whaledent Inc.) or addition-polymerized silicone (Express XT; 3M ESPE). To standardize the passive seating position of the trays during impression procedures on the oral cavity, a position guide will be fabricated in acrylic resin (Pattern Resin LS, GC America) on the surfaces of the lateral incisor and second molar. Impressions will be made by a single calibrated operator and disinfected with 0.5% sodium hypochlorite for 10 minutes. After that, it will be expected for 10 and 110 minutes to pour with type IV gypsum (GC Fuji Rock EP) the condensation and addition silicone impressions, respectively. Three lateral photographs of the patient will be obtained using a tripod and a customized radiographic positioner, being the images imported into the Image Tool software (version 3.0). The total area of the buccal surface of the canine-first molar region will be measured. A 4.0% coefficient of variance will be a criterion for using these measurements as a standard baseline for comparison with those obtained on gypsum working casts. The gypsum casts will be photographed and analyzed using the same photograph procedures and imaging software used for the clinical images. The total mean area (mm2) value for each group will be compared with the initial area obtained from the patient's photographs. Regardless of the patient used, the results of each technique will be tabulated and grouped in the same experimental condition with the aim to compare the effect of the factors technique, material, and tray on the gypsum cast reproduced dimensions. (AU)