## 1 Introduction  
Since the beginning of the contemporary era, dental impressions of oral hard and soft tissues have played a central role in dental practice because they represent the main communication medium between clinician and technician to fabricate a dental prosthesis. Historically, the first dental impression material used was bees wax, and a few years later the field benefited from the invention of the first dental tray [1]. The most crucial turning point was the discovery of alginate in 1947, an irreversible hydrocolloid derived from marine algae that is associated with good dimensional stability [2]. Alginate is still today the most used dental material in everyday practice. In order to improve accuracy and dimensional stability, dental industries developed another material that replaced alginate in precision impressions, especially in fixed prosthetic rehabilitation: elastomers Elastomers could be divided into polyethers, polysulfides, and silicones, which provide better accuracy, elasticity, and dimensional stability than previous impression materials [3]. The materials mentioned above are used for conventional analogic impressions, but another impression technique is available nowadays: the optical impression. In 1983, dr. [PERSON] was the first clinician to fabricate a dental crown with an optical impression taken by an intraoral scanner (IOS) [4]. Digital impression provides many advantages: it is better tolerated by patients, allows faster communication with the dental technician, provides a high accuracy level, and is highly powerful marketing tool because it is less invasive and has reduced chair time [5; 6]. Optical impressions are still under debate because their accuracy could be affected by technical errors depending on clinical conditions (such as the presence of blood and saliva during scanning and clinician experience) and differs between each IOS [7]. This simple formula defines Accuracy as Accuracy = Trueness \(\pm\) Precision, where Trueness indicates how far a measurement is from the reference value and Precision indicates how far measurements are far from each other [8]. Few comparative studies between conventional and digital impressions have been conducted with discrepant findings, considering that most were in vitro [9]. Impression accuracy remains the most critical factor in dental impressions, especially in multiple implant-prosthetic rehabilitation, because an accurate impression could guarantee a passive fit of a definitive prosthetic crown [10]. Passive fit, defined as the minimal gap at the framework-implant surface without tension, is the result of the best accuracy level that could have an impact on the final outcome of implant-based prosthesis, but misfit values are still under debate [11]. Many methods were proposed to evaluate the exact correspondence of implant positions between the definitive cast and the real clinical position, allowing the passive fit of future rehabilitations. These methods include polymeric, metal, and resin devices, but as suggested by [PERSON] et al. even Gypsum devices could be used to verify the implant position [12]. This comparative study aims to test a simple way to perform a clinical evaluation of digital impression reliability, through the outcomes of a gypsum check screwed on implant abutments. Moreover, it aims to propose a threshold value of clinical tolerance misfit, considering the hypothesis of the correlation between the fracture of gypsum check and a misfit above 100 \(\upmu\)m, which does not allow the passive fit of prosthetic frameworks.