## 4 Discussion  
The present systematic review aimed at assessing the role of KM on peri-implant tissue health, esthetics and patient-related variables. The presence of KM \(\geq\) 2 mm was  
Figure 14: Sensitivity analysis excluding high-risk studies reporting on MBL ([PERSON] et al. [24]) and [11; 23; 28; 29; 33; 34; 35]. Figure 15: Cross-sectional studies reporting on REC included in the meta-analysis [25; 26; 28; 33; 35]. associated with less marginal inflammation (GI, mGI, BI, and msBi), plaque accumulation (PI, mPI), recession, biological complications and less sorens or discomfort during oral hygiene procedures, while no differences were found for implant failures, PPD and bone levels. BoP resulted in being significantly lower in the KM \(\geq\) 2 mm groups in the three prospective studies [30; 31; 32]. These results should be considered with caution. Only one RCT study with 18 months [30] of follow-up could be included and we decided to analyze studies with data given at the implant level since very few studies used the patient level. Only two studies [25; 27] at low risk of bias were included, while studies presented high heterogeneity and few had long follow-ups. Even if some differences between groups were statistically significant, their clinical importance should be determined, since small differences between groups were found. These differences were even smaller when excluding one study [23] in which patients with erratic compliance were included. In particular, no differences were found for PPD and bone loss. The mean values reported for these variables showed that the population examined presented shallow PPD and little bone loss, so the results should be applied to patients with generally healthy peri-implant tissue. The only exception was presented by [PERSON] et al. [23] where pockets deeper than 4 mm were present in the group with KM \(<\) 2 mm and its results suggest that the presence of KM could be important in patients in which compliance is suboptimal. A reason behind the small differences found between groups could be the choice of the cut-off for sorting a patient into one group or another. Most of the studies used the presence of at least 2 mm of KM as the criteria for describing one group [7; 25]. This measure could be difficult to record and similar cases could be assigned to different groups, altering the results. Additionally, using this definition, cases with a presence of KM were pooled with cases with an absence of KM. A minimal presence of KM could have different clinical significance with respect to its complete absence. Only three studies considered the presence or absence of KM as the case definition [24; 25; 28]. The meta-analysis conducted without these studies did not alter the results. Moreover, the presence of attached mucosa was never analyzed and this could be an important issue when evaluating the results. If KM plays a role in maintaining a peri-implant soft tissue seal that contributes to maintaining the state of health, it ideally means that KM should have characteristics of stability and that it must adhere to the underlying hard tissues. KM consists histologically of a dense, collagen-rich connective tissue covered by a keratinized epithelium. Adherent KM has a lamina propria firmly connected to the underlying perigsteum, while free KM lacks this connection. This can be due to tghe implant position or bone resorption leaving supracreal peri-implant soft tissue of an adequate thickness to avoid recession but without attachment to the underlying perigsteum. This lack of attachment could negatively affect the soft tissue seal around implants. Therefore, measuring a vertical dimension of keratinized tissue without defining its characteristics (free or adherent) may not be sufficient. From a clinical point of view, in fact, it may be sufficient to have 1 mm of adherent KM while perhaps the presence of \(>\)2 mm of non-adherent KM may not be adequate. The presence of KM should be evaluated also in relation to other clinical conditions.