# Static Bacterial Leakage in Different Conometric Connections: An In Vitro Study
## 1 Introduction  
Biological factors such as aging, social, and cultural, as well as genetic, trauma, and even iatrogenic factors, lead to tooth loss that requires functional, aesthetic, and phonetic rehabilitation to restore the individual's health [1; 2]. In addition to single prostheses or conventional prosthetic appliances, osseointegrated implants can be used, which can also offer masticatory stability with the retention of rehabilitations in the stomatigment system. However, in addition to the stability obtained with osseointegration, implants, usually with at least three components, can also be lost even with a high success rate that reaches 97% of cases [3]. Failure in prosthetic rehabilitation treatment may denote a number of factors, one of the most complicated of which is bone loss around the dental implant. Factors such as fixture and abutment design, surgical trauma, occlusal overload, peri-implantitis, autoimmune host reaction, and bacterial microleakage contribute to bone and implant loss [2; 4; 5]. Implant-abutment microleakage has been studied since the 1990s and refers to bacterial microleakage between the interface of the pieces, caused by the presence of a microgap [6]. The size of this microgap can range from 1 to 49 \(\upmu\)m, according to the different connections. The passage of bacteria and endotoxins can occur freely through the microspace between the connections, since the size of the microorganisms can vary from nanometer to a few micrometers, depending on the type [4; 6; 7; 8]. Although one-piece dental implants showed encouraging biological results, two-piece implants continue to be the preferred method because they are also easy to manage in the prosthetic phase [9]. A microgap between the abutment and the implant as well as between the dental prosthesis and the abutment seems inevitable for two-part implant systems, despite the fact that both are tightly connected with one-piece implants [10]. The formation of these microgaps and the presence of bacteria may be responsible for peri-implant inflammation and consequent bone loss adjacent to the implant-abutment [11; 12]. As a result, internal and marginal adaptation at the implant-abutment interface are important aspects because they are directly connected to biological integrity, structural stiffness, and the maintenance of healthy peri-implant tissue [13]. Therefore, the relationship between the geometry of the implant-abutment connection can have a great influence on microgap formation and microbial infiltration. Yet, according to the literature, Morse-Cone connections usually have low levels of contamination compared to other geometries [4; 14; 15]. An important factor is represented by the rotational tolerance that determines the stability of the implant/abutment connection [16; 17]. The connections currently found are internal hexagon (IH) and external hexagon (EH). It has been shown that if the rotational tolerance of EH is less than 2 degrees, it could resist to a mean of 6.7-million loading cycles before loosening, while if this angle is greater than 5\({}^{\circ}\), there will be a reduction of 67% to the resistance of screw-loosening [17]. Based on the geometric conformation of the inner part of the fixture platform, internal hexagon can be further classified in clearance-fit, conical, and combined connections [18]. In order to reduce friction between the components, clearance-fit connectors include parallel walls on the interior of the fixture and the abutment with a small gap. To prevent the rotation of the prosthetic components, geometric features called \"index\" of different shapes can be added to these connections, with an anti-rotational function [18]. In conical connection, the inner part of the implant's platform has a conical shape that creates a tapered interface with the corresponding portion of the abutment without the presence of an inner space. Anti-rotational geometric characteristics can also be introduced at the apical region of the abutments in these connections [18].