## 1 Introduction  
Currently, osseointegrated implants have a high long-term predictability associated to fixed oral rehabilitation [1]. The essential condition to support the functional loading of the dental implant is the creation of an intimate and dynamic relationship between the device surface and the vital surrounding bone [2; 3]. This aspect is directly associated to the fixture primary stability, the bone tissue quantity and quality, the implant macro- and micro-design, the surface characteristics, and the thread profile and pitch, which represent key factors for implant osseointegration [2; 3; 4; 5; 6]. Bone density seems to be a critical factor for implants' primary stability. [PERSON] et al. classified bone density into four different categories, based on the perception of the subjective operator during osteotomy perforation (D1 is the hardest and D4 the softest). It is a non-quantitative, subjective, and non-reproducible system [7]. In the literature, it was reported that the high predictability associated with oral implant rehabilitation is directly correlated to obtaining fixture primary stability and a successful functional healing of the osteotomy site. Many different dental implant and thread profiles have been proposed to achieve a favourable primary stability in low bone density [8]. The assessment of the quality of the bone structure must be considered as an essential procedure before the implant placement in order to avoid early bone failure and increase the success of the surgical procedure [9]. Posterior dedutulous maxillary ridges are often associated with a lack of bone quantity and quality due to the proximity of the maxillary sinus. In this scenario, a regenerative approach is often required. The primary stability of the dental implant is considered in the literature as a key factor for the long-term predictability of endosseous implants under functional loading [5]. This concept is correlated to the absence of micromobility of the fixture when in contact with the bone substrate [10]. The primary stability is clinically determined by the mechanical relationships and interface engagement of the fixture with the cortical bone [11]. In fact, the bone-implant interface is influenced by the surgical technique [12] and the size of the implant, design, and micro-/macrotopography [13].  
In this way, many different approaches have been described to improve the primary stability in critical density and bone availability: under preparation techniques [6; 14], manual and stepped osteotomies [10], optimized implant micro- and macro-geometry, and thread profiles [15]. It is very important to quantify the stability of the implant at various times for a long-term prognosis. Therefore, the use of simple and clinically applicable non-invasive tests to evaluate implant stability and osseointegration can be considered useful [10; 16; 17; 18; 19; 20]. In this way, many different non-destructive protocols have been reported in the literature for the primary stability assessment, for example insertion torque (IT) [21] and resonance frequency analysis (RFA) [22]. Polyurethane blocks in the form of standardized densities have been proposed for in vitro tests to simulate the consistency and the density of the bone [23].  
The aim of the present study was to evaluate through a laboratory simulation the biomechanical behaviour of an implant with a special design proposed for a one-stage sinus augmentation compared to a standard implant.