The demand for dental implants continues to grow because more providers and patients are viewing dental implants as a predictable therapeutic treatment option. With more patients receiving dental implant treatments, treating clinicians are observing more complications and adverse events as well. Therefore, the long-term efficacy of dental implants must be carefully assessed. Criteria such as marginal bone level, patient comfort level, and sulcus depth have been used to define the long-term success of implant therapy.
Although osseointegration has traditionally been considered the most important factor in maintaining implant stability, the roles of transmucosal healing and health around the implant are also being considered as determining factors that influence long-term implant success [3, 4]. Peri-implant mucosa that forms during the wound healing process following implant placement is the corresponding term for biologi- cal width around natural teeth [5, 6]. The basic function of the peri-implant mucosa is to protect the underlying bone that supports the implant [7]. The average peri- implant mucosal dimension is 3 mm, which is 1 mm longer than the biologic width of natural teeth [8]. Although it has been controversial for many years, the prepon- derance of evidence suggests that a minimum dimension of peri-implant mucosa is crucial in maintaining long-term implant health. Invasion of the peri-implant mucosa results in tissue inflammation and potential bone loss [8–11], and the break- down and loss of peri-implant tissue can compromise the long-term success and survival of the implant [7]. The soft tissue attachment apparatus around dental implants is significantly differ- ent from that of natural teeth [12, 13]. In natural teeth, the root of the tooth is covered by a root cementum from which collagen fibers run perpendicular to the long axis of the tooth and attach to the surrounding hard and soft tissues. An implant has no cemen- tum, and the collagen fibers are oriented parallel to the long axis of the implant without direct anchorage [12–14]. Therefore, the peri-implant soft tissues are more fragile dur- ing the inflammatory process [15, 16]. Clinical soft-tissue parameters such as tissue phenotype, amount of keratinized mucosa, and tissue thickness have been considered important factors that influence long-term implant success [17–20]. According to a study that examined peri-implant mucosal dimension using bone sounding around the implants, a peri-implant mucosal dimension at the facial aspect of less than 3 mm was associated with a thin biotype. A thick biotype was observed when there was a peri- implant mucosal dimension of more than 4 mm at the facial aspect. Implant restoration may be prone to papilla loss when the distance from the tip of the papilla to the bone of the adjacent tooth is greater than 4 mm in a thin biotype.
To maintain long-term stable implant health, a minimum peri-implant dimension is necessary. Violation of peri-implant mucosa causes peri-implant tissue inflamma- tion and marginal bone loss, and angular defects may develop.
Peri-Implant Phenotype
The peri-implant phenotype is defined as “the morphologic and dimensional fea- tures characterizing the clinical presentation of the tissue that surround and support osseointegrated implants” [21]. The components of the peri-implant phenotype are keratinized mucosa width (KMW), mucosal tissue thickness (MTT), and supra- crestal tissue height (STH). The peri-implant bone thickness makes up the osseous component of the peri-implant phenotype [21] (Fig. 3.1). This description applies to the lingual and facial aspects of the implants. The peri-implant phenotype is site- specific and may change because of external factors [21]. The thick labial peri- implant phenotype is present when there is a thick buccal plate [17]. Furthermore, the peri-implant soft tissue phenotype may be modified with surgical interventions [22]. Therefore, clinicians may consider intervention by means of bone augmenta- tion before or during implant placement when a thin buccal plate is present 
Keratinized Mucosa Width
Keratinized mucosa extends from the margin of the peri-implant mucosa to the oral mucosa [23]. The peri-implant KMW is the most coronal part of the peri-implant soft tissues [21]. The need for adequate width of keratinized mucosa around dental implants has been studied by many researchers and remains somewhat controversial [20, 24–31]. At present, the consensus seems to be that an adequate KMW is associ- ated with low plaque accumulation and improved soft tissue health (Fig. 3.2).
Several studies have shown that in subjects with KMW a greater than 2 mm, the Modified Plaque Index and Modified Gingival Index were higher and the Plaque Index and Gingival Index were lower [20, 27, 31–36]. When probed, implants with a narrow zone of keratinized mucosa tend to bleed more than implants with a wider zone of keratinized mucosa [30, 37, 38]. It has been revealed that implants with a KMW less than 2 mm showed more plaque accumulation and that they were more likely to sustain tissue breakdown, leading to earlier loss of attachment [15, 16, 30, 37, 39]. Pain during home care was reported more frequently by the patients with inadequate KMW at implant sites. This could be caused by mechanical irritation resulting from the mobility of non-keratinized tissue when stimulated by a tooth- brush [30]. However, it has also been observed that peri-implant health can be sus- tained in the presence of inadequate KMW (less than 2 mm) in patients who practice meticulous oral hygiene [28, 29]. Dental implants without adequate KMW, com- bined with high levels of plaque accumulation, had a greater incidence of peri- implantitis [28, 37–39]. Furthermore, it was found that gingival recession was greater around implants that were not surrounded by adequate KMW compared to the implants with adequate KMW (Fig. 3.3) [31, 32, 36, 38]. Keratinized mucosa width can be a key parameter to measure in the timely identification of mucosal recession of dental implants [32, 40]. However, it has been suggested that inade- quate KMW may be a consequence of recession, rather than the precipitating factor for worsening of a site [32, 38, 41]. In conclusion, although an adequate KMW may not be a prerequisite for long-term implant health, the presence of adequate KMW may reduce the occurrence of peri-implant tissue inflammation and the breakdown of peri-implant mucosa
Mucosal Tissue Thickness
Peri-implant mucosal tissue thickness (MTT), the horizontal dimension of soft tis- sue, can be measured with a probe at approximately 1.5 mm apical to the soft tissue margin at the base of the probing depth. MTT extends from the external soft tissues to the internal hard surfaces. Peri-implant MTT may or may not be keratinized and does not necessarily correspond to the height of the keratinized tissue. The soft tis- sue thickness is closely related to the peri-implant phenotype. The dimension may vary from location to location (e.g., facial versus lingual) [21, 42]. The midfacial aspect of the tissue thickness is affected by the implant’s shoulder location [43].
The thickness of the peri-implant tissue plays a significant role in the long-term success of implant therapy, as well as functional and esthetic outcomes [21, 44–46]. Implants with thin crestal mucosa tend to have more marginal bone loss than implants with adequate crestal mucosa [47, 48]. Mucosal tissue thickness plays a more significant role in minimizing marginal bone loss with implants that were placed supracrestally compared to implants that were placed at the level of the crest [49, 50]. This could be because of the different locations of the interface between the implant fixture and the implant abutment. Implants with crestal placement have a microgap that is very close to the bone, and marginal bone loss may occur because of potential bacterial microleakage [49].
Implants with mucosal tissue thickness of more than 2 mm, as measured hori- zontally and perpendicularly from the crest, have low marginal bone loss; therefore, it is suggested to perform autogenous or allogenic tissue grafting to minimize early marginal bone loss [48, 51, 52]. Soft tissue grafting procedures to increase mucosal tissue thickness minimize interproximal marginal bone loss overtime [53]. Additionally, successful outcomes have been achieved when soft tissue augmenta- tion was attempted to enhance esthetics and compensate for an underlying osseous defect. The procedure can be performed before or after the loading of the implants [54, 55]. With the effect of the abutment shade on the peri-implant mucosa, the negative visual consequence was minimized in sites with a minimal mucosal tissue thickness of approximately 2 mm [45, 46].
In conclusion, mucosal tissue thickness must be carefully evaluated by clinicians prior to implant placement because implants placed on the sites with thicker peri- implant soft tissue tend to sustain less marginal bone loss. Furthermore, thin muco- sal tissue is a risk factor for esthetic complications because it compromises the appearance of the shade of the abutment [56]. When the thin tissue is noted, the use of soft tissue augmentation with allograft or autogenous graft should be considered to minimize peri-implant marginal bone loss and improve the esthetic outcome. Clinicians should make every effort to achieve and maintain a minimum of 2 mm of mucosal tissue thickness in daily clinical practice.
Supracrestal Tissue Height
The supracrestal tissue height (STH) is the “vertical dimension of the soft tissue,” which runs from the mucosal margin to the bone crest; this is assessed “circumferen- tially” around an implant, at both proximal sites and buccal and lingual aspects [21]. This characteristic can be measured by transmucosal sounding with a periodontal probe [56]. “Vertical soft tissue thickness on crestal bone” is another term that is used to refer STH [51]. The sulcular epithelium, the junctional epithelium, and the supra- crestal connective tissue are engirdled in the peri-implant STH. The supracrestal con- nective tissue is not directly attached to the implant abutment surface [21]. The dimension of the peri-implant STH is different from its counterpart, the supracrestal tissue attachment of the tooth, in that it is taller by 1 to 1.5 mm in all buccal/lingual and proximal aspects. It is typically greater in interproximal areas [23].
An inadequate STH at the time of implant placement surgery is a predisposing factor for early marginal bone loss [47, 48, 51], regardless of the type of implant (e.g., bone level implant, tissue level implant, or implant with platform switching) [21]. This is probably due to the physiologic organization of implant-supporting elements during the initial healing period [21]. It has been suggested that a practical cut-off value to distinguish between favorable and unfavorable STH should be 2–3 mm in clinical practice [21, 50, 57, 58].
An adequate STH is associated with greater papillary volume and is determined by the connective tissue adhesion level at the adjacent interproximal tooth surface [5, 59, 60]. It is about 3 mm between two adjacent implant restorations, and it is extremely unpredictable and challenging to regenerate [56]. Therefore, an inter- proximal space morphology should be carefully assessed prior to the implant place- ment to plan an ideal esthetic outcome
In conclusion, the STH reestablishes an anatomical space for the implant- supporting apparatus, and adequate STH dimension is crucial for long-term peri- implant health. Both functional and esthetic outcomes can be improved by careful examination and possible modification of the STH architecture
Peri-Implant Soft Tissue Phenotype Modification and Its Influence on the Long-Term Success of the Implant
Peri-implant phenotype influences the esthetic and functional long-term outcomes of implant treatment [21, 23, 27, 28, 46, 51, 53, 57]. A thin peri-implant mucosa, as measured by the supracrestal tissue height, is correlated with greater marginal bone loss than a thick tissue phenotype [47, 48, 51]. In clinical practice, horizontal and vertical ridge resorption following tooth extraction is commonly observed [62, 63]. Clinicians often perform bone augmen- tation procedures along with implant surgery; this may induce a coronal displace- ment of the mucogingival junction, which has a negative effect on peri-implant soft tissue dimension [63, 64]. The following protocols have been suggested as interven- tion points for soft tissue augmentation: prior to implant placement, at the time of implant surgery, after implant placement but prior to second stage surgery (Fig. 3.4), at the time of stage two (implant uncovering) surgery (Fig. 3.5), and after the implant is already uncovered. Intervention after an implant is restored is not recommended unless intervention is required due to peri-implant tissue inflammation [63].
Several techniques to increase the peri-implant soft tissue phenotype, including KMW, MTT, and STH, have been suggested [21, 46, 53]. Autogenous free gingival graft and autogenous connective tissue graft have historically been the first therapy of choice because of their proven efficacy around natural dentition [22, 65]. Free gingival grafts not only increase KMW but also deepen the vestibule during the recipient bed preparation. Due to this additional benefit, the free soft tissue graft technique is the preferred therapy when planning an implant-assisted prosthesis, such as implant-supported overdentures [66]. When using a free gingival tissue graft, an apically positioned flap technique should be employed rather than a bilami- nar technique because it has been reported that KMW did not significantly increase following the bilaminar technique with free gingival graft [22, 67]. In a study on dogs, free soft tissue autograft (i.e., free gingival graft) placed at an implant site resulted in a greater KMW gain than an apically positioning flap or xenogenic col- lagen matrix [66, 68]. Connective tissue grating increases mucosal tissue thickness in addition to supracrestal tissue height [22, 66]. This grafting procedure is particu- larly helpful because it improves the esthetic outcome as well as peri-implant tissue health [22, 65, 67].
To minimize patient morbidity and eliminate a second surgical site for the donor tissue, different allograft and xenograft techniques are also utilized [22, 67, 69]. An acellular dermal matrix can be used to increase keratinized mucosa width and mucosal tissue thickness [22, 67, 70–72]. Although a connective tissue graft is con- sidered the gold standard for the grafting material of choice when augmenting peri- implant soft tissue, an acellular dermal matrix is reported to accomplish comparable mucosal tissue thickness gain as well as increase supracrestal tissue height [22, 47, 67, 70, 73]. A xenogenic collagen matrix has also been used as an alternative for autogenous connective tissue grafting. This material has been known to increase keratinized mucosal width and mucosal tissue thickness [22, 70–72]. When a bilam- inar approach is utilized, peri-implant soft tissue phenotype modification with a xenogenic collagen matrix can stabilize marginal bone levels [22, 74]. Therefore, both allogenic and xenogenic soft tissue grafts are considered adequate and safe alternatives, and they may improve treatment acceptance by reducing patient con- cerns for donor site discomfort and the need for a second surgical site [66].
Concluding Remarks
The role of peri-implant soft tissue on the long-term health of the implant has historically been considered somewhat controversial. However, most evidence suggests that a thick peri-implant phenotype, wide band of keratinized mucosa, adequate mucosal tissue thickness, and adequate supracrestal tissue height may reduce peri-implant tissue inflammation and future functional and esthetic com- plications [17, 56, 66]. Clinicians should carefully examine peri-implant pheno- types when planning implant therapy and, if necessary, should create a favorable peri-implant architecture by employing peri-implant soft tissue phenotype modification.
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