Use of Xenogenic Collagen Matrices in Peri-Implant Soft Tissue Volume Augmentation: A Critical Review on the Current Evidence and New Technique Presentation

Plastic peri-implant surgical procedures aiming to increase soft tissue volume around dental implants have long been well-described. These are represented by: pedicle soft tissue grafts (rotational flap procedures and advanced flap procedures) and free soft tissue grafts (epithelialized, also called free gingival graft (FGG), and non-epithelialized, also called, connective tissue graft (CTG) or a combination of both. To bypass the drawback connected with autologous grafts harvesting, xenogenic collagen matrices (XCM)s and collagen-based matrices derived from porcine dermis (PDXCM)s have been introduced, as an alternative, in plastic peri-implant procedures. Aim: This review is aimed to evaluate and to critically analyze the available evidence on the effectiveness of XCMs and PDXCMs in soft tissue volume augmentation around dental implants. Moreover, a clinical case with a new soft tissue grafting procedure technique (Guided Soft Tissue Regeneration, GSTR) is presented. Material and Methods: An electronic search was performed on the MEDLINE database, SCOPUS, Cochrane Library and Web of Science. The electronic search provided a total of 133 articles. One hundred and twenty-eight not meeting the inclusion criteria were excluded. Seven articles of human randomized clinical trials were selected. A total number of 108 patients were treated with CTG, and 110 patients with XCM. Results: in peri-implant soft tissue augmentation procedures, XCMs seem an effective alternative to CTGs, associated with lower patient morbidity and lower operative times.


Introduction
Soft tissues around implants differ from those around teeth regarding the amount of blood supply, the direction of connective tissue fibers, the number of fibroblasts and collagen fibers, and the permeability of junctional epithelium [1]. To clinically describe the morphologic and dimensional features of soft tissue components around dental implants, recently, it has been introduced in literature the term "peri-implant phenotype" [2,3]. This includes four parameters: (1) the peri-implant keratinized mucosa width (PKMW), (2) the peri-implant soft tissue thickness (PST), (3) the peri-implant supra-crestal tissue height (PSTH), and (4) the peri-implant bone thickness (PBT). The PKMW represents the vertical height of the keratinized gingiva that goes from the free gingival margin to the mucogingival line [2]. Even if there are conflicting opinions in the literature [4], it is generally accepted that 2 mm of PKWM is needed to maintain optimal bacterial plaque control and limit the risk of mucosal recessions [5][6][7][8][9][10][11][12][13]. The PST represents the thickness of the periimplant soft tissue. It is measured horizontally at the base of the peri-implant sulcus or at the most coronal part of the implant shoulder [2]. This horizontal measurement, defined in the past as "mid-facial peri-implant mucosa", has been used for the esthetic evaluation around dental implants [14][15][16][17][18][19]. As for the PKMW, also for this parameter, there is a general  Focused questions (based on PICO criteria): What are the clinic relative to soft tissue augmentation methods (C) on improving PKMW around dental implants (P)?

Identification of Studies via Databases and Registers
In order to increase the quality and transparency of the study, t was followed. (Table 1)  Finally, the references of all selected full-text articles were searc cles. In Table 1 are reported the outcomes of the selected clinical trials use of XCM vs. CTG. Seven randomized clinical trials were selected i arms, one treatment arm using CTG and in the other using XCM [2 time of RCTs ranged from 3 to 12 months (mean 6 months). Patients' number of 108 patients were treated with CTG, and 110 patients wi were periodontally healthy, smoking < 10 cigarettes/day. In 44 pat techniques were performed after implant placement, and in the crown delivery (Table 2). In addition, in the present critical review, re tive cohort studies on use of XCMs [27,28], and of seven clinical studi were also reported (Tables 2-5). Focused questions (based on PICO criteria): W relative to soft tissue augmentation methods (C) on around dental implants (P)?
In order to increase the quality and transpare was followed. (Table 1)  Finally, the references of all selected full-text cles. In Table 1 are reported the outcomes of the sel use of XCM vs. CTG. Seven randomized clinical tri arms, one treatment arm using CTG and in the ot time of RCTs ranged from 3 to 12 months (mean 6 m number of 108 patients were treated with CTG, an were periodontally healthy, smoking < 10 cigaret techniques were performed after implant placem crown delivery (Table 2). In addition, in the presen tive cohort studies on use of XCMs [27,28], and of s were also reported (Tables 2-5). Systemic, Periodontally healthy After plac Focused questions (based o relative to soft tissue augmentati around dental implants (P)?
In order to increase the qua was followed. (Table 1)  Finally, the references of all cles. In Table 1 are reported the o use of XCM vs. CTG. Seven rand arms, one treatment arm using C time of RCTs ranged from 3 to 12 number of 108 patients were tre were periodontally healthy, smo techniques were performed aft crown delivery (Table 2). In addi tive cohort studies on use of XCM were also reported (Tables 2-5). Finally, t cles. In Table 1 use of XCM vs arms, one trea time of RCTs r number of 10 were periodo techniques w crown deliver tive cohort stu were also repo Focused questions (based on PICO criteria): What are relative to soft tissue augmentation methods (C) on improvi around dental implants (P)?
In order to increase the quality and transparency of th was followed. (Table 1) Finally, the references of all selected full-text articles w cles. In Table 1 are reported the outcomes of the selected clin use of XCM vs. CTG. Seven randomized clinical trials were s arms, one treatment arm using CTG and in the other using time of RCTs ranged from 3 to 12 months (mean 6 months). P number of 108 patients were treated with CTG, and 110 pa were periodontally healthy, smoking < 10 cigarettes/day. techniques were performed after implant placement, and crown delivery ( Table 2). In addition, in the present critical r tive cohort studies on use of XCMs [27,28], and of seven clin were also reported (Tables 2-5). Focused questions (based on PICO criteria): What are the clinical effects of relative to soft tissue augmentation methods (C) on improving PKMW, PST and P around dental implants (P)?
In order to increase the quality and transparency of the study, the PRISMA was followed. (Table 1) Finally, the references of all selected full-text articles were searched for rele cles. In Table 1 are reported the outcomes of the selected clinical trials published f use of XCM vs. CTG. Seven randomized clinical trials were selected including two arms, one treatment arm using CTG and in the other using XCM [20][21][22][23][24][25][26]. The f time of RCTs ranged from 3 to 12 months (mean 6 months). Patients' characteristi number of 108 patients were treated with CTG, and 110 patients with XCM. All were periodontally healthy, smoking < 10 cigarettes/day. In 44 patients [20,21] techniques were performed after implant placement, and in the remaining 6 crown delivery (Table 2). In addition, in the present critical review, results of two tive cohort studies on use of XCMs [27,28], and of seven clinical studies on use of P were also reported (Tables 2-5).  15,3937 Focused questions (based on PICO criteria): What are relative to soft tissue augmentation methods (C) on improvi around dental implants (P)?
In order to increase the quality and transparency of th was followed. (Table 1) Table 1. PRISMA flow chart.

Records identified from
PubMed searching: Finally, the references of all selected full-text articles w cles. In Table 1 are reported the outcomes of the selected clin use of XCM vs. CTG. Seven randomized clinical trials were s arms, one treatment arm using CTG and in the other using time of RCTs ranged from 3 to 12 months (mean 6 months). P number of 108 patients were treated with CTG, and 110 pa were periodontally healthy, smoking < 10 cigarettes/day. techniques were performed after implant placement, and crown delivery ( Table 2). In addition, in the present critical r tive cohort studies on use of XCMs [27,28], and of seven clin were also reported (Tables 2-5). Focused questions (based on PICO criteria): What are the clinical effects of relative to soft tissue augmentation methods (C) on improving PKMW, PST and P around dental implants (P)?
In order to increase the quality and transparency of the study, the PRISMA was followed. (Table 1) Finally, the references of all selected full-text articles were searched for rele cles. In Table 1 are reported the outcomes of the selected clinical trials published f use of XCM vs. CTG. Seven randomized clinical trials were selected including two arms, one treatment arm using CTG and in the other using XCM [20][21][22][23][24][25][26]. The f time of RCTs ranged from 3 to 12 months (mean 6 months). Patients' characteristi number of 108 patients were treated with CTG, and 110 patients with XCM. All were periodontally healthy, smoking < 10 cigarettes/day. In 44 patients [20,21] techniques were performed after implant placement, and in the remaining 6 crown delivery (Table 2). In addition, in the present critical review, results of two tive cohort studies on use of XCMs [27,28], and of seven clinical studies on use of P were also reported (Tables 2-5). Focused questions (based on PICO criteria): What are relative to soft tissue augmentation methods (C) on improvi around dental implants (P)?
In order to increase the quality and transparency of th was followed. (Table 1) Finally, the references of all selected full-text articles w cles. In Table 1 are reported the outcomes of the selected clin use of XCM vs. CTG. Seven randomized clinical trials were s arms, one treatment arm using CTG and in the other using time of RCTs ranged from 3 to 12 months (mean 6 months). P number of 108 patients were treated with CTG, and 110 pa were periodontally healthy, smoking < 10 cigarettes/day. techniques were performed after implant placement, and crown delivery ( Table 2). In addition, in the present critical r tive cohort studies on use of XCMs [27,28], and of seven clin were also reported (Tables 2-5). Focused questions (based on PICO criteria): What are the clinical effects of XCM (I) relative to soft tissue augmentation methods (C) on improving PKMW, PST and PSTH (O) around dental implants (P)?
In order to increase the quality and transparency of the study, the PRISMA checklist was followed (Table 1).
Finally, the references of all selected full-text articles were searched for relevant articles. In Table 1 are reported the outcomes of the selected clinical trials published following use of XCM vs. CTG. Seven randomized clinical trials were selected including two parallel arms, one treatment arm using CTG and in the other using XCM [20][21][22][23][24][25][26]. The follow-up time of RCTs ranged from 3 to 12 months (mean 6 months). Patients' characteristics: a total number of 108 patients were treated with CTG, and 110 patients with XCM. All patients were periodontally healthy, smoking < 10 cigarettes/day. In 44 patients [20,21] surgical techniques were performed after implant placement, and in the remaining 64 before crown delivery ( Table 2). In addition, in the present critical review, results of two prospective cohort studies on use of XCMs [27,28], and of seven clinical studies on use of PDXCMs were also reported (Tables 2-5).

Assessments of the Risk of Bias
The risk of bias analysis was performed by one reviewing author (R.G), using the Cochrane Collaboration's tool for assessing RCT risk of bias. In Table 3, parameters used for analysis of risk of bias (low, medium and high) are reported.

Statistical Analysis
The continuous outcomes were expressed as mean difference (MD), with a confidence interval (CI) of 95%. Chi-square tests were used to assess the heterogeneity of RCT. Values ≤ 25% = low heterogeneity, values > 25 ≤ 50% = moderate heterogeneity, values ≥ 50% = high heterogeneity. The random effect model was used when heterogeneity was found (p < 0.10).

Results
Compared to XCM, the CTG yielded an increment difference in PKMW of 0.19 mm (−0.03, 0.41), but the difference was not statistically significant. The increase in PST was reported by 2 trials: compared to XCM, the CTG yielded an increment difference of 0.07 (−0.39, 0.53), without a statistical difference. Compared to CTC, XCM yielded an overall PKMW gain difference of −0.06, but this was not statistically significant, regardless the surgical technique used (apically positioned flap vs. bilaminar technique). The difference in postsurgical discomfort, evaluated with the visual analog scale was 1.98 (0.63, 3.33) in favor of XCM, with a statistically significance. A significant longer treatment time (15.46 min.) was associated with CTG, compared to XCM (Table 4). Table 4. General overview of the results reported by selected RCT which compared CTG (control) versus XCM (test).
The XCM was as effective and predictable as the CTG for attaining a band of keratinized tissue, but its use was associated with a significantly lower patient morbidity.

Discussion
Comparative results in PST and PKMW at 6 months have been reported by Cairo et al. [24] who, in a randomized clinical study, performed soft tissue augmentation at 60 implants in 60 patients during implant uncovering. In the CTG group the authors found a final PST increase of 1.2 ± 0.3 while in the XCM group it was 0.9 ± 0.2, with a significant difference (0.3 mm; p = 0.0001). However, both procedures resulted in similar final PKMW amount with no significant difference between treatments. Comparative similar results in the final PST increase were indeed obtained in a RCT by Thoma et al. [22], who treated 20 patients obtaining with XCM a mean soft tissue thickness increase at 90 days post-surgery of 1.4 ± 1.4 mm (occlusal) of 1.1 ± 1.4 mm (buccal) and of 0.9 ± 1.9 mm (apical). The corresponding values obtained with CTG were 0.8 ± 1.8 mm, 0.8 ± 2.2 mm and 1.6 ± 2.6 mm, respectively. Sanz et al. [20] performed soft tissue volume augmentation of 20 randomized implants which presented <1 mm of keratinized tissue. Ten patients received CTG, and 10 patients received XCM. At 6 months, the CTG group showed a mean width of keratinized tissue of 2.6 (SD 0.9) mm, while in the XCM group it was 2.5 (SD 0.9) mm, these differences being insignificant. A 60% and 67% of volume contraction was recorded in the CTG and XCM group, respectively, without variations of periodontal parameters between groups. Compared to CTG group, a lower patient morbidity and a reduced surgery time was recorded in the XCM group. Difference in outcomes between the study by Cairo et al. [24], Thoma et al. [22] and Sanz et al. [20] may be linked to the type of XCM used. Unlike what was performed by Cairo et al. [24], who used a double layer XCM, Thomas et al. [22] and Sanz et al. [20] used a three-dimensional stable XCM. The structure of this XCM consists of two functional layers: a cell occlusive layer consisting of collagen fibers in a compact arrangement and a porous layer. The porous layer is thicker in order to achieve more keratinized tissue by inducing a space-creating effect and by favoring blood clot formation. The same XCM was also used in two RCTs by Lorenzo et al. [21] and Huber et al. [26]. Lorenzo et al. [21] at 6 months post-surgery in the CTG group attained a mean PKMW of 2.75 mm, while in the XCM group the mean PKMW was 2.8 mm, the inter-group differences not being statistically significant. Moreover, in both groups a similar esthetic result and a similar significant increase in the vestibular depth as a result of the surgery was recorded. Huber et al. [26] obtained at 1-year post-surgery a mean PST value of 3.0 mm for XCM, and of 2.8 mm for CTG without statistically significant differences within and in between the groups.
None of the RCTs selected by the current review report results relating to PSTH and PBT after treatment with XCMs vs. CTG. In peri-implant soft tissue augmentation procedures, an increase in PSTH should still be within the therapeutic goals as it has been shown that a thickness > 2 mm could have a protective effect on peri-implant marginal bone resorption [2]. In addition, even a minimum thickness of 1.5 mm of PST should always be present to reduce peri-implant bone remodeling [2].
Although results of the selected randomized clinical trial proved that XCM, when used as a soft tissue substitute aiming to increase the peri-implant tissue volume was as effective and predictable as the CTG with a significantly lower patient morbidity, they indicated a higher shrinkage rate of XCM compared to CTG over time. Clinically, threedimensional tissue alterations following soft tissue thickening around dental implants were quantified in a prospective study by Schmitt et al. [29] using a digital method able to superimpose the baseline model with the one obtained after surgery. The results indicated a contraction of the initially augmented soft tissue volume of 81.76% in the XCM group and 56.39% in the CTG group after 6 months. Histological studies showed that, during healing, CTG is encapsulated [30], while XCM undergoes to remodeling process without encapsulation [31,32]. This could justify the higher shrinkage rate of XCM compared to CTG after soft tissue thickening [33]. Another factor which could favor the higher shrinkage rate of XCMs vs. CTGs is the inflammatory response and the foreign body reaction connected to cross-linking molecules present in XCM [34]. Moreover, the difference in loss of volume between groups treated with CTG vs. XCM could be explained by a different revascularization process which occurs in the graft [16,17,33]. A better blood perfusion may lead improved graft integration and less soft tissue resorption. The current literature review also identified two prospective studies reporting results of XCMs in per-implant soft tissue augmentation. Pompa and Papi [27], using a Mucoderm ® XCM indicated at 12 months post-surgery a mean PKMW gain of 4.32 ± 1.22 mm, while Schallhorn et al. [28] at 6 months, using a Mucograft ® XCM indicated a mean PST and PKMW gain of 2.2 ± 0.9 mm and 2.1 ± 1.0 mm, respectively.
Recently, collagen-based matrices derived from porcine dermis (PDXCMs) have been introduced in dentistry as a substitute for CTG in peri-implant plastic surgery. Due to the preservation of their mechanical stability these collagen matrices allow cells adhesion, and proliferation and blood vessel in-growth [34,35], developing into a fully functional tissue [36,37].
The literature search did not allow to identify randomized clinical trials comparing PDXCMs vs. CTGs. However, some prospective pilot cohort studies reported data on in peri-implant soft tissue augmentation using PDXCMs (Table 5). Papi et al. [27] documented a PKMW mean increase of 6.51 and 5.67 mm at 5 months and at 1-year post-surgery, respectively. At the last follow-up control (12 months) a mean shrinkage of 29% was observed. In another prospective study the same authors performed soft tissue peri-implant augmentation combining the use of PDXMs with synthetic bone [43]. Both PKMW and PST showed an increase from pre-surgery to the first month, a decrease during the first 12 months, and a stabilization from 12 to 24 months. Compared to the baseline, after 24 months PST and PKMW gained 1.94 ± 0.05 mm and 1.60 ± 0.11 mm, respectively. Similar results on PST increase have been reported also by Zafiropoulous et al. [38], who using PDXCMs in 27 patients and found at 6 months after surgery a significant increase of 1.06 mm. Stefanini et al. [39] in 10 patients used a coronal advanced flap surgical technique combined with PDXCMs obtaining at 1-year post-surgery 1.2 ± 0.18 mm gain of PST. Papi & Pompa. [40], using digital linear and volumetric measurements reported in 12 patients treated with PDXCMs a volumetric gain in PST of 51,501 mm 3 with a mean shrinkage of 23.31%. Comparative clinical data between CTGs and PDXCMs in peri-implant plastic surgery with a 3D analysis have been reported also by Schmitt et al. [41]. Authors at 6 months post-surgery found a mean volume gain of 19.56 ± 8.95 mm 3 and 61.75 ± 52.69 mm 3 for PDXCM and CTG, respectively, and a mean increase of PST of 0.30 ± 0.16 mm for PDXCM versus 0.80 ± 0.61 mm for CTG. Aragoneses et al. [42] evaluated in female white pigs (Sus scrofa domestica) the clinical and histological differences in peri-implant soft tissue volume augmentation using PDXCM vs. CTG. Three months post-surgery the mean volume increase was 1.53 mm, and it decreased 6 months later of 0.51 mm due to shrinkage and PDXCM resorption. At 45 days, the biopsies corresponding to PDXCMs documented a complete epithelial healing with the presence of a well keratinized layer. At 90 days all sites treated with PDXCM showed a matured keratinized squamous stratified epithelium sustained by a connective tissue with correctly organized collagen fibers and normal vascularization. However, at 90 days post-surgery, sites treated with CTG, compared to those treated with PDXCM, showed a statistically significant higher thickness (approximately 60%). The variability of the results reported by the over mentioned studies might depend by the different surgical techniques and materials used [44][45][46]. The use of a surgical bilaminar technique with the PDXCM placement under a split-thickness buccal flap could favor the blood perfusion of the matrix, which has been described as one of the factors affecting the result. Other factors influencing clinical outcomes might be also connected to the different suturing techniques and materials (i.e., the needle's characteristics, bite size, suture position, location of knots tied, etc.), and early suture removal (<10 days). Another important consideration is that commercially available PDXCMs have a standard thick, while after the withdrawal, CTG can be prepared to the desired thickness. To overcome some of the above-mentioned limitations (Table 6) authors of the current paper present preliminary results of a RCT in which a new inlay technique named Guided Soft Tissue Regeneration (GSTR) performed using a new PDXCM (NovoMatrix TM ; BioHorizons, Birmingham, AL, USA) has been compared to CTG. In Figures 1a, 2a, 3, 4a, 5a and 6a is presented a clinical case treated with PDXCM and GSTR technique. In Figures 1b, 2b, 4b, 5b and 6b is presented a clinical case treated with CTG.

CTG
low shrinkage after the healing period is completely incorporated histologically more effective in generating attached tissue the palate is healed by secondary intention and requires a dressing for 10 to 14 days, which is uncomfortable for most patients inability to harvest large grafts, high morbidity rates after surgery, poor aesthetics due to differences in texture and color from adjacent areas.
-High risk of complications

CTG
low shrinkage after the healing period is completely incorporated histologically more effective in generating attached tissue the palate is healed by secondary intention and requires a dressing for 10 to 14 days, which is uncomfortable for most patients inability to harvest large grafts, high morbidity rates after surgery, poor aesthetics due to differences in texture and color from adjacent areas.
-High risk of complications

Advantages Disadvantages
CTG low shrinkage after the healing period is completely incorporated histologically more effective in generating attached tissue the palate is healed by secondary intention and requires a dressing for 10 to 14 days, which is uncomfortable for most patients inability to harvest large grafts, high morbidity rates after surgery, poor aesthetics due to differences in texture and color from adjacent areas.
-High risk of complications Preliminary in vitro studies showed that the proprietary tissue processing of Novo-Matrix TM , maintains tissue stability, allows a rapid blood vessel in-growth and fibroblast adhesion and proliferation with a minimal inflammatory response [47][48][49].
Unlike the other commercially available PDXCMs, this matrix is made up of a single layer that can be sectioned and superimposed to obtain the desired thickness. Moreover, the bilaminar technique used, with a split thickness flap, allows to keep periosteum and muscular insertion in order to maintain periosteal vascularization of the bone and to have soft tissue available to suture the matrix.
Limitations: The electronic search of the present critical review allowed to select only seven RCTs that compare the use of XMCs vs. CTGs in plastic peri-implant surgery, while it did not allow to identify RCTs comparing PDXCMs vs. CTGs. All the selected studies reported results of PKMW and PST, but no results are related to PSTH and PBT. Given the limitation of the peri-implant phenotypic parameters described and the limited number of samples, results should be interpreted with caution. Moreover, not all the surgical procedure were performed at the same time points. Another limitation can de due to the fact that all RCTs were supported by companies that produce the XCMs. Many studies do not report information about the inclusion of smoking participants. Tobacco smoking affects the healing potential of the periodontal tissues, and it could therefore represent a confounding factor [50]. This, and other confounding factors, such as periodontal phenotype, type of implant, type of abutment, etc., should be also standardized in future studies with the aim to reduce bias.

Conclusions
Results of the present literature review suggest that the use of XCMs and PDXCMs is effective in increasing PKMW and PST around dental implants. Nevertheless, further studies are required to evaluate the long-term outcomes. Furthermore, RCTs are also required to evaluate the effectiveness of PDXCMs compared to CTG.