# (Bio)Tribocorrosion in Dental Implants: Principles and Techniques of Investigation
## 4 Dental Implant
### Implant Structure  
The implant is formed of a fixture that is a cylindrical screw, acting as an artificial root in contact with the bone; the crown or dental prosthesis, corresponding to the upper zone, represents the visible area of the total implant and the abutment, which connects the last two. In addition, the implant can provide, to fix beyond the components, a junction screw or can be cemented. The former is, currently, preferred because of the likely destruction of the cement layer and colonization of bacteria and because of less stress distribution to the implant [58]. Moreover, it can be monophasic or biphasic according to the number of operations realized as follows: for the first, all the implants will be installed in one operation, whereas for the second, the fixture and the remaining parts are collocated in different periods of time. The choice depends on the patient as well as on the distribution of the deformations [59]. Regarding the geometry, instead, the implants can assume the following many configurations:  
In Figure 5, a biphasic implant with a head diameter of 4.4 mm and a length of 13 mm with the presence of a cutted zone is shown. The cutting area is an interesting zone because the incision has the function of facilitating the formation of the bone and reinforcing its stability. The surface topography [60] and the specific dimensions of the thread, as well as its type (buttress, reverse buttress, V-shape, sinusoidal, square), play a key role, as well as the upper diameter and the total length of the implant, in the contact conditions and stress/strain field [61]. The last impact on the life span of a prosthesis is as follows: the magnitude of the transmitted forces and the resulted contact pressures should be investigated in the different areas of the implant and in the surrounding bone and comparing the values with the ones coming from the adopted failure criteria. More precisely, the value of the forces should be not too high to avoid a potential overload and the rupture of implant/bone, and not too low according to the cited above Wolff law. Indeed, the bone remodels in response to the external load as follows: it means that if the forces are not big enough, stress shielding will occur with the reduction of bone density, causing issues such as inhibition of bone development, inflammation, or, in the worst case, the failure of the prosthesis [62]. Regarding, instead, the topographical properties, numerous studies confirmed the relationship between \(R_{a}\) (arithmetical roughness) and osseointegration as follows: an _ideal_ range for the implantable surfaces was found to be between 1 and 2 \(\upmu\)m [63; 64]. Hence, a not completely smooth surface could be preferred, as follows: treatments such as acid etching, shot blasting and plasma spraying are all valid processes impacting the topography of the surface [65]. On the contrary, greater roughness determines a lower real contact area [66] and consequently higher pressures in localized zones. In addition, the probability of bacterial adhesion is higher [67] than that which can develop a biofilm, i.e., a complex structure of microbiological cells enclosed essentially in a polysaccharide matrix [68], with variable chemical nature.