## 4 Discussion  
In this retrospective clinical study, full-arch implant prosthetic rehabilitation supported by six axial implants or four implants placed according to the All-on-Four protocol were analyzed clinically and radiographically to assess implant survival rate, peri-implant parameters, marginal bone loss, bone cortical thickness, possible muscular, articular and prosthetic complications and patient satisfaction with the procedure. The overall implant survival rate at the six-year follow-up was 98.09%. Similar results have been reported by other authors. [PERSON] et al., in their retrospective clinical study of full-arch rehabilitation, found a 5-year implant survival rate of 97.9% [38]. [PERSON] et al., when evaluating immediate loading rehabilitations with a reduced number of implants, reported a 4-year implant survival rate of 96.9% [39], while [PERSON] et al. reported a survival rate of 97.5% at a longer follow-up [40]. Concerning peri-implant parameters, the plaque index, the bleeding index and the thickness of the keratinized component of the mucosa were evaluated. The Chi-square test revealed a significant association between implant site and plaque index and between implant site and bleeding index and a positive relationship was identified between both parameters and implant site in the diathoric position. In their study, [PERSON] et al. evaluated bleeding and plaque index; BoP was detected around 6 implants (3.7%) and 16 implants showed a superficial amount of plaque resulting in a plaque score of 10% [41]. [PERSON] et al., when analyzing plaque accumulation and peri-implant soft tissue inflammation in patients with immediately loaded implants supporting fixed full-arch prostheses, recorded a mean PI and BOP of 61.7% and 21.1%, respectively, in congruence with the results of this study [42]. As reported by several authors, and in accordance with the protocol adopted in the present study, home and professional hygiene maintenance could be significant in reducing plaque, bleeding and consequently any peri-implant complications, having a favorable impact on the tested parameters [43; 44; 45]. Concerning the width of keratinized tissue, the Chi-square test revealed a significant association between a KTW value \(\geq\) 2 and the anterior implant site. As pointed out by [PERSON] et al., the proportion of keratinized mucosa on peri-implant soft tissue could influence health and stability around implants; in the case of a thickness of less than 2 mm it could favor lingual plaque accumulation and bleeding [46]. Similar results were obtained by other authors, who stated that the lack of keratinized mucosa around the dental implants was significantly associated with more plaque accumulation, mucosal recession, inter-proximal bone level \(\geq\) 3 mm and peri-implantitis [47]. Regarding this concept, the literature might illustrate conflicting results; the presence of more than 2 mm of KM width, although it might be associated with mucosal recession, might not affect other peri-implant parameters [48; 49]. Regarding marginal bone loss, the results obtained are similar to those of other authors. [PERSON] et al., in their prospective clinical study, evaluated crestal bone loss around tilted and straight implants supporting immediate-loading full-arch rehabilitations and reported that the crestal bone level was 0.5 \(\pm\) 0.4 mm for straight implants and 0.6 \(\pm\) 0.4 mm for tilted implants (\(p\) > 0.05). Straight and tilted implants seemed to have similar behavior after immediate loading rehabilitations. After functional loading, straight and tilted implants did not differ significantly in clinical outcome [50]. [PERSON] et al., in their retrospective clinical study of immediate rehabilitation of the dentulous and potential dentulous jaws with implant-supported full-arch prostheses, found that the average marginal bone loss at 1 and 5 years was (0.7 \(\pm\) 0.2) mm and (1.1 \(\pm\) 0.3) mm [51].