## 4 Discussion
None of the included studies reported on the vestibule depth. A shallow vestibule, often present in the lower jaw and particularly in patients who underwent tooth extraction for periodontal reasons, should be taken into consideration as it hampers oral hygiene procedures resulting in plaque accumulation and inflammation. Future studies should take into account the presence or absence of attached keratinized mucosa and report on the depth of the vestibule to better define the role of KM in peri-implant health. Interestingly, no studies clearly indicated implant failure and complications as outcome variables. Even if failure is a rare occurrence, it should be analyzed and reported. Two studies [30; 32] reported on complications but they were recorded as drop outs, which could alter the real results. In cases without adequate KM, four complications linked to infection (two in [PERSON] et al. [33], two in [PERSON] et al. [30]), two failures for infection and one failure due to fracture [32] were present, while in the group with KM \(>\)2 mm only one implant was lost due to fracture and one complication was due to infection [30]. In one study in the cross-sectional groups, biological complications were reported in 51.4% of cases without KM and in 12.7% of cases with KM [25]. Other systematic reviews [7; 8; 18; 36] on this topic reached conclusions similar to the ones reported in the current manuscript, although differences in the inclusion criteria, methods of analysis, outcomes analyzed and risk of bias assessments were present. In this review, only RCT, prospective cohort and cross-sectional studies were considered, excluding studies with a retrospective design (for example, [15; 17]). These strict eligibility criteria were set to improve the quality of the included studies. The original plan, as described in the statistical analysis, was also to only include studies that performed analysis at the patient level, but since only two papers fulfilled this criteria [25; 27], we decided to include implant-level analysis studies and meta-analysis was conducted at the implant level. This is a clear limitation of both the included studies and of the current review. All the studies in which the analysis was conducted at the implant level were judged to be at0 least at moderate risk of bias. In the review by [PERSON] et al. and [PERSON] et al. [7; 18], recessions and bone level were not analyzed as they recognized difficulties in standardizing measurement methods. We agree with their considerations, but since we believe that both parameters are important to evaluate, we have decided to analyze these data. Bone level can be considered a surrogate outcome of implant failure, which requires a longer follow-up. Since none of the included papers reported on implant failure, it appeared reasonable to analyze bone stability over time. Interestingly, no differences were found in terms of bone level in patients with or without KM. In accordance with [PERSON] et al. [18], to increase the statistical power of the analysis, we pooled outcomes that measured the same clinical parameters as a group (GI, mGI, BI, mBI and PI, mPI). Since we recognized that the indices were different, we normalized them to be able to perform the statistical analysis. We observed that definitions were often used incorrectly and or associated with a wrong reference [26; 27; 29; 33]. We advise a uniform use of indices. A dichotomous index indicating the presence or absence of marginal inflammation, BoP and plaque would be easier to apply and more useful for the comparison of different studies. Data presented as percentages would be easier to interpret clinically. [PERSON] et al. [37] performed an overview to assess the methods, quality, and outcomes of the systematic reviews on the importance of KM. The major limitations that were highlighted consisted of the lack of standardization methods and the absence of evaluation of possible confounding factors.