###### Abstract  
Two critical factors that influence the accuracy of an impression include the proper manipulation of the impression materials and the technique used to make the impression. The purpose of this study was to clinically evaluate the effect of different mixing techniques on the accuracy of vinyl polysiloxane (VPS) impressions by assessing metal framework fit of fixed partial restorations. The study included 92 consecutive patients diagnosed with partial edentulism and treated with fixed partial denture restorations. The mixing technique was one of the two following mixing methods: hand mixing technique (45 patients), with the putty material mixed according to the manufacturer's instructions; or mechanical mixing technique (47 patients), with the putty material mixed by a Pentamix device. Under both mixing methods, vinyl polysiloxane was used as the impression material. Two impression techniques were randomly used by the operators (One/Two-stage puty -wash impression techniques). The accuracy of the metal framework restorations was tested clinically and radiologically, resulting in significant statistical difference (\(p=0.04\)) between different mixing techniques. The mechanical mixing produced more accurate restorations (metal framework misfit only in 14.9% of patients vs. hand mixing 31.1%). Regarding the impression techniques, the two-stage impression technique was found to be significantly more accurate (\(p=0.04\)), resulting in 14.6% ill-fitted metal frameworks vs. 31.8%, in the one-stage technique. It can be concluded that mechanical mixing yields more accurate impressions leading to more accurate restorations, especially when combined with two-stage impression technique. v 17 June 2021  
23 August 2021  
26 August 2021  
[[http://www.mdpi.com/journal/applsci](http://www.mdpi.com/journal/applsci)]([http://www.mdpi.com/journal/applsci](http://www.mdpi.com/journal/applsci))  
Previous studies have shown that different mixing techniques have a substantial effect on the quality of impressions and castings obtained [19; 25]. The working time of impression material can also be affected by the mixing technique [26]. Generally, the mixing techniques can be divided into two major categories: mechanical mixing and hand mixing techniques. One of the major problems with using hand mixing is that the air bubbles incorporated into the impression material cause void formation on the impression surface. Although it is impossible to achieve bubble-free mixing, the frequency and size of bubbles and voids can be reduced by automatic mixing. Additionally, due to the more uniform blending of the base with the catalyst, machine mixing enhances the polymerization reaction and generates homogeneity of the impression material with superior physical properties, especially when using high viscosity materials [27; 28; 29; 30]. Automatic mixing also allows for the maintenance of hygiene and avoidance of contamination of the impression material [31; 32; 33; 34; 35]. Likewise, there is a clear preference for automatic mixing among medical staff (clinicians and assistants) due to the convenience of the machine method [36]. Furthermore, automatic mixing is considered to be more economical compared to hand mixing thanks to minor waste of the impression material [37; 38; 39; 40]. Various impression techniques have been proposed to improve the accuracy of VPS impressions. The putty-wash impression can be made using two principal techniques: the putty-wash one-step technique and putty-wash two-step technique. Some laboratory studies have suggested that the impression accuracy does not depend on the impression technique, Refs. [41; 42] while many other in vitro studies concluded that the impression technique is an important factor affecting impression accuracy, mainly because it controls the wash bulk thickness (1-2 mm), which is considered to be the major determining factor [13; 14; 15; 16; 19; 20; 21].