## 4 Discussion
may have also played a role in determining such discrepancies between these two experimental approaches. A possible explanation, for instance, can be probably found in, but not limited to, the sheltering effect that rough surfaces offer ex vivo to the microbes against the shear removal forces of the oral environment [18]. Only a few studies in the literature have investigated bacterial adhesion on Ti surfaces of different grades, with contrasting results [14; 15]. In the study of [PERSON] et al. (2014) [14] commercially pure Ti (cp-Ti) and grade 5 (Ti-6 Al-4V alloy) disks were inoculated with _P. ginginals_ in artificial saliva at different pHs, and bacterial adhesion evaluated after 6 h. Different from our results, significantly more bacterial adhesion to the cp-Ti disks than to the Ti-6 Al-4V alloy disks was shown. In the study of [PERSON] et al. (2016) [15], grade 4 and grade 5 Ti disks were incubated with _P. gingivalis_ for 48 h and the biofilm biomass spectrophotometrically quantified. In this case, as for our study, a significantly higher biofilm formation was detected on grade 5 than grade 4 surfaces when machined and laser-treated specimens were tested, whereas no effect of Ti composition on biofilm biomass formation for sandblasted surfaces was found. Such discrepancies and similarities among those studies and the present one must be interpreted with caution due to the different design, bacteria and materials used. The use of an ex vivo model to reproduce conditions closer to the clinical reality, for instance, represents a significant difference and novelty of the present study compared with the above-mentioned ones. Surface properties, in particular surface roughness, of implant surfaces are directly correlated with bacterial adhesion, as demonstrated in both in vitro and ex vivo experimental models in the present study, as well as in the existing literature. [PERSON] et al. (2015) [22] measured initial biofilm growing (48 h after inoculation) on four Ti surfaces different for texture or chemical composition. Machined surfaces were reported to favor a slower biofilm formation and maturation than the roughened surfaces. The higher tendency of rough surfaces to accumulate and retain more mature plaque can be found in their ability to provide locations for initial adhesion in which bacteria are sheltered against shear forces [6]. Although a number of studies have tried to correlate surface roughness parameters with bacterial adhesion [23; 24], this association is still far from being fully determined. The profile of areal average roughness (Ra or Sa) remains the most extensively investigated roughness parameter for implant surfaces, and a strong correlation between bacterial adhesion with the Sa values of differently micro-roughened surfaces has been shown [6; 18; 23; 24]. [PERSON] et al., in their studies, have proposed a threshold value of roughness (Sa = 0.2 \(\upmu\)m) below which we cannot expect further reduction of bacterial adhesion [23; 24]. Our findings, however, cannot confirm this aspect [25], because we found a significantly lower biofilm formation on T surfaces (Sa about 0.13 \(\upmu\)m) compared to MA ones. The existing positive correlation between surface roughness and bacterial adhesion has a direct influence on the producing process and design of dental implants and related components; transmucosal implant parts and components are conventionally mirror-like polished, whereas in cases where the intraosseous rough surface is exposed for some reason to the oral environment, bacterial adhesion and, possibly, peri-implant disease (mucositis and peri-implantitis) could be promoted. However, the correlation between implant surface characteristics and the incidence of peri-implant disease is still far from being clearly defined. Available literature data are very heterogeneous, and it is difficult to extrapolate in vitro and in vivo findings to the clinical field, also due to the multifactorial nature of the disease [26; 27; 28; 29].