## 4 Discussion
The study in the present manuscript was performed according to PRISMA guidelines and a standardized tool for the risk of bias assessment was applied. [PERSON] et al. [7] mentioned the risk of bias in the methods section but it was not reported in the final results and [PERSON] et al. [36] did not perform a risk of bias evaluation. [PERSON] et al. [8] used a non-standardized risk of bias, while [PERSON] et al. [18] applied the Newcastle-Ottawa modified scale. In the present review, the Joanna Briggs tool was chosen as it includes two questions focused on the presence of confounding factors and strategies for their management. In particular, referring to KM significance, we considered the following as major confounding factors: smoking habits, a history or actual presence of periodontal disease, the type of implant surface (rough or machined), the type of implant-supported reconstruction, the implant position and the maintenance protocol. Only a few papers reported all this information [25; 35]; apart from when data were given at the implant level, patient-related variables were impossible to apply to the analysis. [PERSON] et al. [25] provided data on smoking habits but very few smokers were included to allow a powerful statistical analysis. [PERSON] et al. [35] performed a univariate and multivariate regression analysis adjusted for confounding factors (oral hygiene, smoking status, history of chronic periodontitis, implant prosthesis type, and diabetes). This type of analysis could be useful for data interpretation. The type of maintenance was rarely reported [25] even if it represents a key aspect of peri-implant health. In our manuscript, a sensitivity analysis was performed, excluding the paper by [PERSON] & Blasi [23] in which only erratic maintenance patients were included. In this group of patients, the presence of KM demonstrated a crucial role. On the other side, [PERSON] et al. [35] concluded that no association was observed between keratinized mucosa width and plaque accumulation, mucosal inflammation, and interproximal bone level in a population that adhered to implant maintenance therapy and demonstrated optimal oral hygiene. Only [PERSON] et al. [25] provided detailed information on the maintenance care protocol. We believe that the maintenance protocol after implant positioning is a crucial point for implant survival. When strict maintenance schedules and oral hygiene were consistently adhered to by patients, no association between keratinized mucosa width and peri-implant diagnostic parameters were observed in many studies [12; 38; 39; 40]. [PERSON] et al. [36] included only studies with more than 12 months of follow-up, while [PERSON] et al. [18] examined studies with at least 4 months of follow-up. We decided to include studies with at least 6 months of follow-up, in accordance with [PERSON] et al. [7] and [PERSON] et al. [8], since this allowed the inclusion of a larger number of studies, given that few studies have long follow-ups. Fifteen studies were included in the final analysis while only eight studies had a follow-up lasting longer than 3 years [11; 23; 24; 28; 29; 32; 33; 34] and only one study had a 10-year follow- up [25] so data on implant survival were difficult to explore. A long follow-up was deemed necessary to adequately answer the review question and in particular to evaluate one of the most important primary outcomes of this review that is implant failure. According to the data in the literature, indeed, the likelihood of complications increases after 5 years from loading. If we could analyze data from studies with follow-ups of more than 5 years, we could perhaps achieve results that are very different and more significant than those reported in this review. PROMs have become a crucial endpoint of clinical studies and should be included in every implant-related clinical study to capture patient satisfaction and quality of life [41]. None of the included studies evaluated the quality of life of patients with or without KM and none reported on esthetic perception.