Etiology and Treatment of Peri-Implant Soft Tissue Dehiscences: A Narrative Review

Implant soft tissue dehiscences compromise not only the aesthetics of the supported restorations but implant survival in the long run. The aim of this narrative review was to briefly present the causative factors of buccal peri-implant soft tissue dehiscences (PSTDs), how these are classified, and the current therapeutic approaches. Implant malposition and the thin peri-implant phenotype are the two major determinants for the occurrence of PSTDs, but other risk factors have also been identified. The most common surgical procedure for treating PSTDs is the split-thickness coronally advanced flap combined with either a connective tissue graft or acellular dermal matrix materials. However, depending on the class and subtype of the dehiscence, the combination of surgical techniques with modifications in the restoration may further ameliorate the final result. In general, within a five-year follow-up period, most techniques lead to a satisfactory aesthetic result, although full coverage of the implant/abutment surface is not always achievable, especially in more extended lesions.


Introduction
Dental implants have been proven to be a reliable solution for the successful rehabilitation of missing or nonrestorable teeth with favorable long-term survival rates [1,2]. However, today, implant survival is not the sole objective of implant dentistry. Optimal aesthetics and high patient satisfaction are considered the game changers in clinical practice, and therefore, the need for measurable outcomes/parameters is obvious [3,4]. For this reason, different indices, such as the pink and the pink/white aesthetic scores, have been proposed to help clinicians evaluate, in a more objective manner, the aesthetics of the peri-implant soft tissues and of the implant-supported prostheses. The facial contour and the level of the soft tissue margin are significant variables of these indices [5][6][7]. Recently, Zucchelli et al. introduced a new index (IDES) specifically for PSTD, in which the evaluation begins at least six months after the surgical attempts for coverage [8]. Specifically, four parameters are evaluated, which are the level of the soft tissue margin (STM), the peri-implant papillae height (PPH) compared with the homologous tooth, the peri-implant mucosa color, and the peri-implant mucosa appearance (PMA). The highest score that can be acquired is 10, and after examining a total of 51 cases with four different evaluators, the authors concluded that there was a high degree of agreement among different clinicians and among different time points of evaluation by the same clinician. Therefore, the index constituted an objective and reproducible means of evaluation of PSTD cases. Furthermore, the subjective perception of aesthetics by the patient is an important parameter of patient-reported outcomes measures [4,7]. Thus, a greyish hue visible through the mucosa, exposure of the abutment or of the implant surface, and discrepancies on the soft tissue in a retrospective study, assessed the alterations of peri-implant soft tissues in 90 implants placed at different times postextraction, and almost half of them presented a combination of guided bone regeneration (GBR) and connective tissue graft (CTG) [20]. A 5.6% incidence of midbuccal mucosa recession was found after a mean 4.5-year follow-up of implant loading, while at 1, 3, and 5 years, the percentage of recession-free implants was 98.9%, 97%, and 89.6%, respectively. On the contrary, in their review, Chen and Buser claimed that PSTDs following immediate implant placement should be considered as a common finding ranging from 9 to 45% of the sites, while the overall mean frequency of the sites with >1 mm of buccal mucosa recession was 21% [21]. This rate seemed to be lower for implants with early placement protocol.
The aim of this narrative review was to present the main factors associated with greater risk for the development of PSTDs and describe the main surgical techniques for the coverage of these deficiencies.

Risk Factors for PSTD
Different parameters such as implant position, peri-implant biotype, width and thickness of keratinized mucosa, peri-implant bone crest height and thickness, implant design, and surgical implant placement protocol have been evaluated as possible risk factors for the development of PSTDs. The degree to which some of these parameters affect the peri-implant soft tissue stability remains controversial in the literature [16,18,20,[22][23][24][25][26][27][28][29][30].
A major determinant for the occurrence of PSTD has been proven to be the buccolingual malposition of the implants [17,18,22,24]. Implants with a buccal position of the implant shoulder relative to the line drawn between the cervical margins of the adjacent teeth had three times more recession compared with implants of which the shoulder was placed lingually to this line (1.8 mm and 0.6 mm, respectively) [22]. For this reason, Buser et al. recommended that the implant shoulder should be placed 1-2 mm palatal to the line of emergence of the adjacent teeth [25]. In a cross-sectional observational study, implants positioned >1 mm outside the alveolar envelope, as determined by cone beam computed tomography (CBCT), were 34 times more likely to present PSTD (odds ratio (OR) 34.65, p < 0.001) [18]. The association between the buccally located implant shoulder and the midfacial recession was also demonstrated in a retrospective cohort study by Cosyn et al. with an OR 17.2 [24]. Similarly, Romandini et al. identified excessive buccal implant position as a risk factor for buccal peri-implant soft tissue dehiscence [17]. On the contrary, in another cross-sectional study, the implant position was not found to significantly affect the facial marginal mucosal level [23]. In this study, the authors claimed that the implant fixture angle was a considerable factor that increased the risk for PSTD in an inverse way. Thus, implants with more proclined fixture position are expected to demonstrate a more apical displacement of the facial marginal mucosal level.
The thin peri-implant biotype has also been identified as a significant risk factor for the presence of PSTD [16,17,20,22,23]. Specifically, peri-implant sites with thin biotype exhibited significantly greater risk for development of PSTD than sites with thick biotype (OR 18.8, p = 0.01) [23]. In a prospective study, Kan et al. reported significant differences in the mean facial gingival changes between sites with thick and thin biotype at 1 year post-implant placement (−0.25 mm vs. −0.75 mm, respectively). This difference was further increased when implants were assessed at the most recent follow-up (−0.56 mm vs. −1.50 mm, respectively) [16]. In their study Evans and Chen observed a higher frequency of recession of >1 mm after 19 months of immediate implant placement in sites with thin rather than thick biotype [22]. Therefore, sites with thin tissue biotype should be considered as more prone to PSTD, especially when implants are also placed in a buccal position [17,22].
The clinical significance of the presence of an adequate band of keratinized mucosa around dental implants has also been widely investigated in the literature, with controversial results [31,32]. The protective effect of a keratinized mucosal zone of >2 mm against the occurrence of PSTD was reported by Sanz et al. [18]. In addition, Roccuzzo et al. observed significantly less soft tissue recession for implants surrounded by keratinized mucosa than those with alveolar mucosa after a 10-year follow up (0.16 ± 0.39 mm vs. 2.08 ± 0.71 mm, p = 0.0001) [30]. Tavelli et al. also recently found that both inadequacy of mucosal thickness and reduced keratinized mucosa width were significantly correlated with PTSD occurrence [19]. These results were in concordance with the findings of a meta-analysis wherein the inadequate width of keratinized mucosa was correlated with higher values of mucosa recession and loss of attachment [33].
The effect of the height and thickness of the buccal bone on the development of PSTDs appears to be ambiguous in the available studies. In a CBCT study, Benic et al. observed that implants with absence of the buccal bone at 7 years after immediate placement demonstrated 1 mm more apical displacement of the marginal mucosa level than implants with intact buccal bone [26]. Nisapakultorn et al. noted also that the facial crest level significantly affected the buccal marginal mucosa level, while the facial crest thickness did not seem to have a significant effect [23]. Tavelli et al. also identified increased buccal bone distance (BBD) as a significantly associated parameter to PTSD development [19]. However, contradictory findings came from another CBCT study, in which the buccal bone thickness was found to have a significant negative correlation to both buccal vertical bone loss and soft tissue recession. For this reason, the authors suggested that a minimum labial bone thickness of 2 mm should be present at the time of implant placement with both immediate and two-stage protocol in order to maintain long-term tissue stability [29].
In an RCT study, 28 implants with an osseous dehiscence defect (≤5 mm) at implant placement were randomized to a guided bone regeneration (GBR) or a spontaneous healing (SH) group. Despite the fact that the SH group demonstrated significantly greater vertical buccal bone loss at the re-entry surgery 6 months after implant placement and at 18 months after implant loading, there were no significant differences at the marginal mucosa level between the two groups after 18 months [28]. This was in agreement with the findings of two clinical studies in which buccal bone dehiscence was presented without a concomitant recession of the buccal mucosa level [18,34].
The level of implant positioning at the apicocoronal dimension could also have a significant effect on the buccal soft tissue level [23,25]. Nisapakultorn et al. reported that increased distance between the contact point and the implant platform increased the risk of buccal marginal mucosa recession with an OR of 2.3 (p = 0.005) [23]. Moreover, the greater distance between the contact point and the bone-to-implant level, as well as the interproximal crest level, was, the higher the risk for PSTD was. The correlation between midfacial mucosa recession and papilla level was found to be irrelevant [23]. Nevertheless, in a study by Garabetyan et al., the authors claimed that alterations in papilla height could also induce changes at the buccal marginal mucosa level [20].
The immediate implant placement protocol has also been associated with midfacial recession [7,21,29,[35][36][37][38], especially in the presence of risk indicators such as thin tissue biotype, facial implant malposition, and thin buccal bone wall [21,36,38]. In a systematic review, immediate implant placement demonstrated a higher median frequency of recession of >1 mm of the midfacial mucosa (range 9-41%, median 26% of the sites) in comparison with early implant placement after soft tissue and partial bone healing (no sites with recession > 1 mm) [36]. On the other hand, in a case series study, advanced midfacial recession (>1 mm) was found only in 7% of immediate implants, while the respective percentage for conventional installed implants was 43%. This difference could be attributed to the flapless procedure that was performed in 9 of the 16 immediately placed implants, which noted a significantly lower midfacial recession of 0.89 mm at 52 weeks compared with the flap approach [39]. In addition, immediate provisionalization enabled the preservation of the soft tissue contour after tooth extraction and immediate implant placement [35]. In an RCT, De Rouck et al. observed a 2.5-to 3-fold larger amount of midfacial recession for immediate single tooth implants with delayed restoration (provisionalization 3 months after implant placement) than those with immediate restoration [40]. The mean difference between the two groups at 12 months was 0.75 mm and was statistically significant (1.16 mm for delayed vs. 0.41 mm for immediate restoration group, p = 0.005). On the contrary, in another RCT, in which implants were placed at healed sites, there were no differences in the midfacial soft tissue level between immediate and conventional restoration groups [41].
With regard to the implant design, a recent case-control study associated one-piece implants with higher risk for PSTD [18]. However, in a prospective randomized controlled clinical study, the authors did not find differences at the peri-implant soft tissues between one-and two-piece implants 1 year after loading [27]. Buser et al. noted the significance of the proper selection of implant platform and neck in relation to the mesiodistal and buccolingual dimensions in order to achieve better aesthetic outcomes [25]. In an RCT study, immediately placed implants restored with the platform switching technique demonstrated significantly more favorable results in terms of midbuccal mucosa recession in comparison with implants restored with platform matching technique 2 years after loading [42]. In contrast to these findings, Zuiderveld et al. claimed that the platform switching technique was not a determinant for the midbuccal mucosa level [43]. Interestingly, the time of function of an osseointegrated and loaded implant and the presence of an adjacent implant are two factors that were also recently found to be associated with buccal soft-tissue dehiscence in a statistically significant manner [19].
Finally, behavioral factors such as smoking have also been associated to soft-tissue dehiscence around implants. Indeed, Raes et al. demonstrated the impact of smoking on midbuccal recessions on 95 patients, concluding that smoking contributed to greater recession [44].
Zucchelli et al. proposed the most appropriate treatment protocol for each of the four classes of dehiscence according to their classification [13]. Thus, in cases wherein the height of both papillae is ≥3 mm (subclass a), the coronal advancement and suturing of the flap with CTG is sufficient for the treatment of Class I and II. However, a combined prostheticsurgical approach [10] with or without abutment replacement was recommended for Class IV and III, respectively, to augment the interproximal soft tissues and increase the vascular supply to the flap. In cases wherein the height of at least one papilla is <3 mm, a prosthetic-surgical approach was proposed for all Classes except Class IV, in which soft tissue augmentation with submerged healing was required. Soft tissue augmentation with submerged healing was also recommended to be performed in Classes II and III with concomitant absence of at least one papilla; on this occasion, as in Class IV, the implant should be removed.
The surgical-prosthetic approach was described by Zucchelli et al. in 2013 [10]. According to this approach, the implant prosthetic crown is removed at least one month prior to surgery, and the abutment is reduced and polished in order for a new finishing line to be created. Consequently, provisional restorative crowns are placed, allowing proper postsurgical soft tissue healing. After the mechanical treatment of the exposed implant surface with diamond burs, the dehiscence is treated with a CAF and CTG, the latter of which derives from the de-epithelization of a free gingival graft harvested from the palate. A 1-year follow-up after final restoration revealed complete dehiscence coverage in 75% of the cases and significant improvement in patients' aesthetics in terms of VAS and PES/WES scores. At the 5-year follow-up, the percentage of complete coverage as well as patients' aesthetics scores remained stable. In addition, the soft tissue thickness and keratinized tissue height increased significantly both at 1-and 5-year follow-up compared with baseline. Also very important was the absence of peri-implant mucositis, which was attributed to the brushing technique and maintenance care program [10,48]. The successful outcome of this technique derived from the augmented interdental tissues, which provided greater interdental beds for the graft and for the surgical papillae [10,48]. However, the surgical-prosthetic technique increases the time and cost of treatment. In another prospective study by Burkhardt et al., 10 patients were treated for soft tissue dehiscence with CAF and CTG [45]. In the 6-month postoperative evaluation, the mean dehiscence coverage was 66%. Despite the satisfactory degree of coverage, the authors concluded that total coverage around implants could rarely be achieved. It is possible that the inferior coverage outcomes reported by Burkhardt et al. could be attributed to the use of deep palate CTG, which is richer in fatty and granular tissue than de-epithelialized free gingival graft, which contains denser collagen fibers and is consequently more stable and less prone to contraction [57,58].
In an RCT, Anderson et al. compared the clinical outcomes of CAF with those of CTG and ADM for the treatment of PSTDs in a group of seven (control group) and six patients (test group) [46]. After an observational period of 6 months, the control group demonstrated 40% recession coverage, while the respective percentage for the test group was 23%. Although the differences in terms of clinical and aesthetic outcomes were not statistically significant between the two groups, the control group presented a more uneventful healing. The authors concluded that it was the underlying bone morphology and not the soft tissue biotype that determined the soft tissue treatment outcomes. Partial coverage of a baseline 3 mm recession was also reported in a case report study in which a triangular-shaped incision was performed for a CAF combined with ADM [49].
In a prospective study of Roccuzzo et al., 16 patients were treated for single implant dehiscence with a split-thickness envelope flap and CTG derived from the maxillary tuberosity [9]. This technique was proposed to be performed for the treatment of shallow dehiscence (2.0 ± 0.7 mm) with intact interproximal tissue, as the authors avoided the use of vertical incisions for greater coronal flap movement. The mean reduction in the dehiscence at the 1-year follow-up was 1.7 mm (range 1.4-2.0 mm), which was statistically significant. Complete coverage was achieved in 56.3% of the cases. The favorable results remained stable for the 13 patients who attended the 5-year follow-up, while complete coverage was still present at 62% of the implants [47].
An envelope (pouch) technique in conjunction with a subepithelial connective tissue graft (SCTG) was also used for the treatment of two peri-implant midfacial recessions of 1.5 and 2 mm. Both cases were left with a residual dehiscence of 0.5 mm after both 3 and 9 months [51]. In another prospective study, Schallhorn et al. performed a pouch flap with collagen matrix at 35 implant sites with soft tissue deficiencies [50]. Although the soft tissue thickness and the KT height were increased significantly at a 6-month follow-up, the authors did not observe any significant change in the mean recession.
Frisch et al., in a retrospective case series study, evaluated the clinical outcomes of a new surgical approach using partially epithelialized connective tissue grafts (PECTGs) [52]. The aim of this technique was to augment the KT height and thickness, as well as to cover peri-implant soft tissue recessions with a mean depth of 2.4 mm. The authors prepared a split thickness flap, and a PECTG was harvested from the palate. Subsequently, the graft was sutured with the KT portion towards the local KT tissue, and the connective tissue portion was covered by the mucosal flap. At a mean 5-year follow-up, all 22 implants demonstrated significant recession coverage of 88%, while complete coverage was observed in 64% of them. In addition, significant gains in terms of KM width and thickness were achieved. The PECTG technique was proven to be effective for the treatment of PSTDs, although a larger sample size is needed for more solid conclusions.
Other surgical [53], prosthetic [54], and combined surgical-prosthetic techniques [55,56] have also been described for the management of PSTD, mainly in case report studies. Lee et al. suggested an envelope technique with a modified vestibular-incision tunnel approach (VISTA technique) and a connective tissue graft [56]. The authors found very satisfactory results with regard to tissue height and width. Ueno et al. used a semilunar coronally positioned flap with an SCTG for the treatment of a PSTD around two posterior implants in the maxilla [53]. They reported successful coverage of the PSTD, which was maintained after 9 months. In a recent case report, Yang et al. corrected the compromised aesthetics of an anterior buccally malpositioned implant with midfacial recession [55]. For this purpose, they applied digital prosthodontics using the one-step zirconia coping technique and a tunneling surgical technique with a SCTG. The gingival margin was repositioned 2.9 mm coronally, and only a 0.5 mm recession was observed 3 years later. In their case, Schoenbaum et al. replaced the initial abutment and implant crown with a new provisional screw-retained composite resin and titanium interim restoration with an undercontoured emergence profile [54]. After 3 months, the midfacial mucosa migrated 1 mm coronally, and the final restoration was placed.
Taking into account all the aforementioned studies, it is clear that the treatment of mucosal recessions around dental implants requires advanced surgical skills and does not always guarantee the complete coverage of implant dehiscence. Moreover, the results presented should be examined with caution, since several limitations follow these studies. In particular, all investigations addressed shallow recessions, up to 3 mm [45][46][47][48]52]. In addition, among studies, different reference points to evaluate the peri-implant recession, as well as various indices for the assessment of the aesthetic outcome, have been used. Therefore, direct comparison of the clinical results is not always easy or meaningful [8,12]. In order to overcome this problem in future studies, Zucchelli et al. introduced a new Implant soft tissue Dehiscence coverage Esthetic Score (IDES) for a more objective evaluation of the treatment outcome that incorporates and standardizes both clinical and aesthetics outcomes, as mentioned above [8].
Another point worth discussing is the timing of the intervention. Since these surgical procedures are relatively unpredictable, it is reasonable to advocate the notion that prevention is always better than treatment. Therefore, it has been supported that it is preferable to perform soft tissue augmentation procedures before or at the time of implant placement or at the second stage surgery (re-entry) rather than after implant loading [59,60]. This is even more critical in immediate implant placement, since the dimensional changes of the alveolar ridge after tooth extraction are not prevented [61] and result in a higher incidence of midfacial recession [21]. Therefore, especially in cases with thin biotype or buccal bone dehiscence, immediate implant placement should be performed with concomitant bone augmentation and soft-tissue thickening in order to compensate for bone remodeling and the subsequent soft tissue contraction after tooth extraction [35,62]. This is in accordance with the findings of a meta-analysis wherein CTG with immediate implant placement had a significant protective effect against the occurrence of midfacial recession [63].
The long-term stability of peri-implant soft tissues after surgical intervention has been assessed by few studies with a mean follow-up of 5 years [47,48,52]. At this time, point soft tissue parameters has remained stable with minor nonsignificant changes. The maintenance care phase is always a very important aspect to be taken into consideration by clinicians. Modern, minimally invasive means of implant cleaning, such as air-abrasive decontamination with erythritol powder, damage neither the implant surface nor the soft tissues and remove all bacterial biofilms, contributing to the long-term stability of implant restorations [64].
In conclusion, buccal peri-implant soft tissue deficiencies developed after implant loading could compromise patients' aesthetics. After immediate implant placement, the possibility for PSTD development is increased, especially when other risk factors such as implant malposition and thin biotype coexist. The most studied surgical technique for the treatment of these deficiencies is the coronally advanced flap with a CTG. However, complete implant coverage is not always feasible. The only available RCT, by Anderson et al., included a rather small sample size, while the assessment of the outcome was made within the first six months after the operation [46]. Therefore, there is a lack of randomized controlled trials that evaluate the effectiveness of different surgical approaches and biomaterials used. More studies with larger sample sizes and comparable outcome measures are necessary to evaluate other surgical approaches.