Controversy over the importance of keratinized mucosa (KM) for the long-term success of dental implants began in the late 1990s. Much academic debate has ensued since then. For example, in 1999, at the 3rd European Workshop on Periodontology (Lang and coworkers 1999), one group concluded that in the presence of good oral hygiene, the nature of the mucosa may have little influence on the long-term survival of implants, suggesting clinicians did not need to consider soft-tissue grafting. During the same meeting, another group found that suboptimal oral hygiene may lead to greater tissue damage around implants emerging through alveolar mucosa (AM) than around implants within KM and that proper oral hygiene may also be facilitated by an adequate band of KM; this created the perception that, under certain clinical conditions, soft-tissue augmentation could be beneficial (Lang and coworkers 1999). It should be noted that at that time, most dental implants had a smooth machined surface that minimized the accumulation of biofilm, unlike the microroughened surfaces commonly used today, particularly when soft-tissue recession has occurred.
Nine years later, the 6th European Workshop on Periodontology reached the consensus that although scientific evidence is in the most part lacking, soft-tissue augmentation at implant sites may need to be considered to obtain satisfactory esthetic results. “Studies are required to evaluate the value and importance of keratinized/non-mobile soft tissue in maintaining peri-implant health” (Palmer and Cortellini 2008).
In February 2012, the 3rd EAO Consensus Conference presented its conclusions that “despite methodological problems and heterogeneity of studies, 40% to 50% of the included studies demonstrated statistically significant associations between lack of “adequate” keratinized tissue and higher plaque and bleeding scores. The studies that did not demonstrate an association usually included well-maintained populations” (Sicilia and Botticelli 2012).
In the last few years, several meta-analyses of recent trials have been published to assess the possible statistically significant differences in plaque index, modified gingival index, mucosal recession, and attachment loss, in the absence of KM (Lin and coworkers 2013; Gobbato and coworkers 2013; Brito and coworkers 2014; Wu and coworkers 2015). Lin and coworkers (2013) presented the following research question: “Does a minimal width of KM around dental implants have a beneficial effect on the health of peri-implant soft and hard tissues?” Their review included eleven studies and concluded that “based on current available evidence, a lack of adequate KM around endosseous dental implants is associated with more plaque accumulation, tissue inflammation, mucosal recession, and attachment loss.”
Gobbato and coworkers (2013) attempted to determine the effect of width of keratinized mucosa (KMW) on clinical parameters of peri-implant health and stability based on eight studies. The authors concluded that “reduced KMW around implants appears to be associated with clinical parameters indicative of inflammation and poor oral hygiene. However, based on the selected evidence, the predictive value of KMW is limited.”
Brito and coworkers (2014) presented the following research question: “Is KM around the dental implant necessary to maintain proper health of peri-implant tissues?” After analyzing seven studies, they concluded that “the presence of an adequate zone of keratinized tissue may be necessary because it was shown to be related to better peri-implant tissue health. Further randomized controlled trials are necessary to support this statement.”
All three of these systematic reviews are based on several common articles and come to similar conclusions. On the other hand, shortly afterwards, Wu and coworkers (2015) investigated “the effects of various techniques and biomaterials on peri-implant KM augmentation in terms of KMW and peri-implant tissue health.” They selected six papers, none of which were included in the three previously presented systematic reviews, and came to a different conclusion: that “although all included studies reported satisfactory results regarding KMW, a definitive conclusion could not be achieved owing to the lack of well-designed studies and appropriate methods of studying soft tissue.”
As a result, the question about the relationship between the KMW and peri-implant health apparently remains open. Many systematic reviews have been published, each delivering a different interpretation on a specific topic of interest to health professionals and influencing their clinical decision-making. The issue of a possible correlation between KM and peri-implant health is no exception.
To overcome this problem, Moraschini and coworkers (2017) performed a study to assess the methods, quality, and outcomes of systematic reviews on the importance of KM for the maintenance of peri-implant tissue health. Only systematic reviews (with or without a meta-analysis) that focused on the influence of KM on peri-implant tissue health were included in this study. According to the authors, a quality analysis of the four selected studies was performed to increase the precision and reduce the possibility of bias, using two analysis tools: AMSTAR (Shea and coworkers 2007) and the checklist proposed by Glenny and coworkers (2003). All systematic reviews reported a positive association between the presence of a KMW ≥ 2 mm and peri-implant health. Nevertheless, since none of the systematic reviews achieved the maximum score in the two quality analyses, the authors concluded that “there is still insufficient data on the long-term survival and success rates of dental implants. Prospective studies evaluating the importance of KM for the long-term maintenance of dental implants are needed.”
It is interesting to note that the negative scores were partly related to the fact that the authors did not search for unpublished literature, that only English-language articles were selected, and that the assessment of heterogeneity was not complete. Indeed, the actual importance of these factors in the overall analysis of the topic does not seem significant. Furthermore, it is worthwhile to observe that the vast majority of the studies included well-maintained populations, while up to 60% of all patients are either erratic recall attenders (< 2 times/year) or not compliant in supportive maintenance, and of these, roughly one out of five develop peri-implantitis (Zeza and coworkers 2017; Monje and Blasi 2019).
In a cross-sectional study with 60 patients, Ueno and coworkers (2016) investigated whether KMW is associated with the health status of the tissue surrounding dental implants and the contralateral teeth. Implants in the premolar or molar region with a KMW < 2 mm demonstrated significantly greater pocket depth, plaque accumulation, and BOP than implants with a KMW ≥ 2 mm. Moreover, implant sites had a higher rate of BOP compared with the contralateral natural teeth. Inadequate KM impeded access for proper oral hygiene at the implant sites and increased mucosal inflammation. It was suggested that plaque accumulation at implant sites causes a more pronounced inflammatory response compared with the contralateral natural teeth.
More recently, a lack of KM was found to be positively associated with discomfort during toothbrushing, which could affect patients’ desire and capability to perform proper plaque control (Roccuzzo and coworkers 2016; Souza and coworkers 2016; Perussolo and coworkers 2018; Monje and Blasi 2019). The issue is of substantial importance, as there is evidence that plaque accumulation around implants results in more inflammatory infiltrate, characterized by the abundant presence of lymphocytes and plasma cells compared to the situation around teeth (Berglundh and coworkers 2011).
In contrast, a recent study with a follow-up limited to six months failed to support the association between an absence of KM and discomfort during brushing around implants (Bonino and coworkers 2018). This controversy may be attributed to patients’ different pain thresholds, brushing techniques, mucosal thickness, and vestibular depth.
Indeed, in twenty-four well-maintained patients with good oral hygiene and low bleeding on probing, there was no difference in discomfort during brushing or in clinical parameters at large. However, patients reported greater satisfaction with the esthetics of implants in the presence of KM. The authors concluded that to improve patients’ esthetic satisfaction, it is recommended to achieve a band of keratinized mucosa around the implants. Furthermore, they acknowledged the need for extended follow-up studies to confirm whether the results from a six-month study would persist in the longer term.
Figures 1a-b show examples of reported discomfort during brushing associated with a lack of KM.
It is commonly assumed that the possible protective role of KM is similar in all areas of the oral cavity. This general conclusion is based on the perception that there is no difference, for example, between the anterior mandible and the posterior maxilla. In reality, each research study only presents information regarding the specific conditions or sites prevailing in that study.
For example, Boynueğri and coworkers (2013) selected all implants placed in the interforaminal mandibular region. All implants were of the same size and diameter, and in order to prevent possible effects of the microflora of the remaining teeth on dental implants, the patients recruited had been edentulous in both jaws for at least two years.
Conversely, Roccuzzo and coworkers (2016) recruited only patients who presented a site with an implant in the posterior mandible as the most distal element that supported either a single crown or a fixed dental prosthesis. The implant could be in the position of a molar or a premolar, but no natural dentition could be present distally to it, and no distal cantilevers were allowed. Therefore, the question about which of these criteria is more representative of the general population remains open.
The proceedings of the 2nd Consensus Meeting of the Osteology Foundation based on the systematic review of Thoma and coworkers (2018a) concluded that the clinician may consider the use of autologous soft-tissue grafting to promote peri-implant soft-tissue health and marginal bone levels at implant sites with insufficient soft-tissue dimensions. It is anticipated that plaque control can be better in the presence of > 2 mm of keratinized tissue. When an increase in the zone of keratinized tissue is desired around an implant, the clinician should consider performing a free gingival graft (Giannobile and coworkers 2018).
Very recently, in a cross-sectional study on 231 dental implants in 52 patients, Grischke and coworkers (2019) found that a reduced KMW around dental implants is a risk indicator for the severity of peri-implant mucositis. The overall tendency of the results indicated that a given specific KMW may help reduce the risk for peri-implantitis and its severity.
In contrast, in a retrospective five-year analysis, Lim and coworkers (2019) investigated the influence of the KM on peri-implant health or disease and to identify a threshold value for KMW for peri-implant health.
The study was designed as a non-interventional five-year follow-up clinical trial based on two prospective studies. The total dataset was subsampled; one implant was randomly chosen per patient. In 87 patients, data were extracted at baseline (prosthesis delivery) and five years. Depending on the definition of peri-implant diseases, the prevalence of peri-implantitis ranged from 9.2% to 24.1%. The prevalence of peri-implant mucositis was similar, irrespective of the definition. The degree of the association between the buccal KMW and other parameters (MB change, and BOP, PD, and PI at five years) was negligible and not statistically significant. Moreover, none of the parameters showed any conspicuous pattern in relation to the buccal KMW. No threshold value was found for the mid-buccal KMW in relation to peri-implant health.
One possible explanation why the study found no robust and unequivocal evidence is that the same definition of “peri-implant mucositis” included implants that exhibited just “spot” bleeding on one site with otherwise tight and healthy mucosa as well as implants with positive bleeding on probing, and a high degree of edemas, swelling, and ulceration. Even though the clinical difference between the two scenarios is substantial, the statistical differences are not easy to detect. Another limitation is that the strict maintenance care program followed by all included patients was able to influence the outcomes, as all patients were provided with individually designed maintenance schedules and follow-up visits. Moreover, the influence of the width of KM on oral hygiene could not be evaluated due to the lack of PROMS.