## 4 Discussion  
The present systematic review integrated data from thirteen human long-term prospective clinical trials and provided weighted averages for clinical and radiographic outcomes after surgical treatment of peri-implantitis. Studies were grouped based on treatment modality and it was found that more favorable bone gain was noted with regenerative approaches, despite successful outcomes for PD reduction and implant survival rate being achieved in all treatment modalities. Previous systematic reviews on the topic have reported similar findings as the regenerative treatment of peri-implantitis was followed by 2.1 mm of bone fill in [PERSON] et al. [38] and by 2.4 mm in [PERSON] et al. [39], which is very similar to the 2.3 mm of RBG reported in the present systematic review. As most of the peri-implantitis defects have infrabony components [40], regenerative therapies appear advantageous to obtain bone regeneration, and gain peri-implant attachment while minimizing probing depth. [PERSON] et al. obtained a PD reduction of 3.4 mm and about 2 mm RBG using a regenerative protocol with bovine bone and connective tissue graft [9]. A recent study by [PERSON] et al. used bone allograft and acellular dermal matrix membrane after decontamination with Erbium YAG (Er:YAG) laser and reported 2.25 mm of PD reduction and 1.18 mm of RBG [8]. [PERSON] et al. implemented bovine bone with the use of PDGF or EMD and reported that 91% of the treated implants had 5.1 mm of PD reduction and 1.77 mm of RBG [29]. Regenerative approaches seem advantageous to reconstruct the lost supporting bone in infrabony defects. On the contrary, resective and flap approaches are best indicated for suprabony defects in non-esthetic areas. [PERSON] et al. treated 95 implants with open flap debridement together with limited osteoplasty and reported a PD reduction of about 2.5 mm. The estimated probabilities of further bone loss related with residual PD so that the probability of no further bone loss was 83% for postsurgical PD shallower than 5 mm [10]. The same group reported how implants with minimally rough surfaces have 45% more probability to achieve treatment success after resective treatment if compared with moderately rough surfaces [41]. [PERSON] et al. advocated for open flap debridement without any bone recontouring or implant surface modification. As reported, a saline-soaked gauze was used to rub against the implant surfaces along with copious saline irrigation; in addition, a combination of two adjunctive antimicrobials (amoxicillin and metronidazolezo) was prescribed as an adjunctive therapy. Mean PD reduction was 2.1 and half of implants had less than 5 mm residual PD, no BoP and no further bone loss [30]. Flap and resective treatment modalities stress the importance of a post-surgical shallow probing depth; however, whether or not residual pockets after therapy represent a risk factor for recurrence of peri-implantitis remains unclear. [PERSON] et al. concluded that residual pathological probing depth is an unfavorable prognostic factor [33]; however, [PERSON] et al. and [PERSON] et al. reported that a stable peri-implant condition was maintained without correlation with pockets deeper than 5 mm [30; 31]. It could be speculated that the presence of plaque and inflammation in the loci of deep pockets could be a prognostic factor for disease progression more accurate than PD alone; however, data from a 5-year randomized clinical trial (RCT) failed to demonstrate any association between peri-implantitis progression and clinical variables including plaque, BoP, and PD [31]. In the present analysis, direct comparisons among treatment groups were voluntarily avoided due to the high heterogeneity of available articles for inclusion criteria and re-treatment in the case of disease recurrence in the follow-up period. As for the inclusion criteria, most of the studies were designed before the publication of the 2017 World Workshop on the Classification of Periodontal and Peri-Implant Diseases and Conditions.