## 1 Introduction  
The peri-implant bone and soft tissues preservation over the long-term is extremely important for the implant success rate [1]. However, in some cases, it is possible to observe a resorption of the bone. The causes of this are not yet completely known, but are in all probability related to a multitude of different factors, i.e., reformation of the biological width, presence of a bacterial infiltration at the implant-abutment micro-gap level, implant overloading, peri-implantitis, relative micromovements, peri-implant bone density, peri-implant soft tissue phenotype, surgical trauma during implant insertion, inter-implant distance, surface structure, and implant neck or abutment characteristics and macro-design [2; 3; 4; 5; 6; 7; 8; 9; 10]. A relevant role is certainly held by the presence of a bacterial reservoir at the level of the voids present in the implant-abutment junction (IAJ) after the insertion of the abutment into the implant [9; 10; 11]. Furthermore, in the literature, it is known that not all implant-abutment connections behave in a similar way. For example, external and internal implant-abutment assemblies have been shown to be much more permeable to bacterial colonization than Cone-Morse or conical connection assemblies [11; 12]. In fact, the conical contact between the implant and the abutment has been demonstrated to allow a better distribution of masticatory forces and a reduction of mechanical stress on the surrounding bone. This is especially important to promote bone health and prevent its resorption. In addition, factors such as the type of load considered and bone quality play a role both on primary stability, but above all, in preventing bone resorption. In addition, the collar area of the implant plays a crucial role as it reaches higher stress values than the apical area. Alternatively, the crestal and subcrestal placement of dental implants refers to the depth at which the implant is inserted into the bone tissue of the jaw. Crestal positioning is the most common technique, in which the implant is inserted in the site of tooth extraction or in a previously defendulous area, until it reaches the level of the alveolar bone margin. In this way, the neck of the implant is at the same level of the gum or slightly below it. This technique is commonly used when the level of the alveolar bone is adequate enough to ensure a good primary stability of the implant and a correct aesthetics for the final result. On the other hand, subcrestal implant placement involves inserting the implant so that the implant neck is positioned below the level of the alveolar bone margin. This can be done when the quantity of the alveolar bone is insufficient in height or thickness [5; 6; 7]. The goal is to cover the implant neck with the gum tissue, creating a \"shield\" effect that can contribute to the long-term stability of the implant and the preservation of gingival aesthetics. Both methods present some advantages and disadvantages. The crestal positioning could offer greater surgical ease and better accessibility for cleaning or maintenance of the implant. However, it may require an adequate volume of alveolar bone to achieve a sufficient primary stability. Subcrestal implant placement could prove to be useful in cases where bone volume is reduced or in order to improve gingival aesthetics, but the surgical procedure may become more intricate and necessitate heightened focus on gum management during the healing process [5; 8]. The choice of where to position the implant will depend on various factors, including the patient's anatomical features, desired aesthetics, bone stability, and implant surgeon preferences. Inserting an external or internal connection below the alveolar crest would mean to apically move the micro-gap colonized by bacteria with a possible resultant higher resorption rate [12].