Since the introduction of the CAF technique, a variety of different root coverage procedures has been proposed. Nowadays, there is a large amount of evidence that the CAF + subepithelial connective tissue graft (SCTG) can be considered the gold standard for the treatment of recession-type defects [
40,
41]. The outcome of this procedure is affected by several prognostic factors, such as patient-related factors, tooth/site-related factors, and technique-related factors [
42]. Amongst all these factors, the soft tissue handling can be considered a key factor that is related to the skills of the operator. This includes the design of the flap, the tension of the flap, and the positioning of the flap [
42]. Indeed, the postoperative position of the gingival margin in relation to the CEJ is considered an important factor in achieving complete root coverage following a CAF procedure [
43,
44]. Moreover, a tight marginal flap adaptation is considered essential for promoting wound healing and blood clot stabilization [
45,
46]. Most commonly, sutures are used for wound closure and stabilization of the wound margins since they provide adequate mechanical support to the healing of the tissues. As an alternative, cyanoacrylate-based tissue adhesives have been proposed for intraoral wound closure [
27]. The results of the present study clearly demonstrate the ability of CTA to improve marginal flap stability following CAF when compared to suturing alone. This was found when CTA was used alone and when applied as an adjunct to suturing. A possible explanation for this finding is that after polymerization of CTA, a strong bonding and stabilizing effect is obtained over the entire extent of the flap compared to only punctual fixation with sutures. Indeed, cyanoacrylate tissue adhesives demonstrated to have strong bonding capabilities to biologic tissues when compared with other adhesives [
47]. Moreover, the ability of CTA to adhere to the tooth surface might have contributed to an additional stabilization in the marginal tissue area in the present study. Thus, when referring only to MFS, the use of CTA alone seems to be sufficient for atraumatic CAF stabilization. This finding is further supported by previous research showing that CTA can be used as an alternative to sutures for stabilization following different flap procedures [
48,
49]. Furthermore, several studies reported that the use of CTA compared to standard suture wound closure offers several advantages such as ease of use, faster application time, better hemostasis, aid in initial wound healing, less intraoperative and postoperative discomfort to the patients, antimicrobial properties, and decreased wound infections [
33,
37,
50,
51]. Due to the hemostatic and anti-infective properties, the use of CTA seems also beneficial in patients with an increased risk of bleeding, diabetes, and immunodeficiency. From the clinicians’ point of view, the biggest advantage of CTA is that compared to suturing, traumatization of the tissue is avoided, thereby reducing the risk of tearing the flap and compromising vascularization. On the other hand, one must be aware that the present results obtained from a pig mandible model cannot be extrapolated to the clinical situation in humans in general. Thus, the soft tissue conditions used in the present pig jaw model may differ from the clinical situation in humans with respect to hydration, vascularization, tissue thickness, elasticity of the tissues, and muscle tension [
6]. Moreover, the results of the present study are based on MFS measurements without a clinical setting evaluating the wound healing and the clinical outcomes. Thus, one might question if the high MFS obtained in the present study could be maintained over time in a clinical environment. It has been demonstrated that the strength of flap attachment to the root surface is weak during the first 2 weeks of healing [
52,
53]. Therefore, it is necessary to ensure stability of the wound during this period until flap attachment to the root surface reaches a clinically adequate strength level. In this context, it has been shown that early suture removal (<10 days postoperatively) can negatively influence root-coverage outcomes following a CAF procedure [
54]. It can be speculated if the sole use of CTA would be sufficient to ensure MFS over a 2-week period, and therefore, the combination with a suitable suturing technique seems reasonable. In the present study, the sling and tag (SAT) suture technique was used either alone or in combination with CTA. Tavelli et al. reported in a study on human cadaver heads with the SAT technique the highest MFS when compared with sling and simple interrupted sutures [
6]. The obtained MFS of ~5 N with the SAT technique is much higher compared to the values in our study (0.88 N). This can be attributed primarily to the use of 5-0 sutures compared to 6-0 sutures in the present study. Further factors that may account for the difference include the used suturing material, bite size, tension of sutures, type and number of knots, cadaver model, and the type of tensile testing machine.
The analysis of data in the present study found no significant correlation between GT and initial MFS. Initial GT is considered a critical factor associated with complete root coverage with CAF procedures [
55]. Thus, it was demonstrated that sites with an initial gingival thickness (≥ 1.2 ± 0.3 mm) demonstrated a higher chance of complete root coverage than those with GT < 1.2 mm. A thicker flap tissue may facilitate marginal adaptation in the CEJ area and provide more stability in the early healing phase. The uniform flap thickness (2–3 mm) in the present study might have precluded the finding of an interaction of GT and MFS. Similarly, no significant correlation between KT and MFS was found in the present study. The amount of KT apical to a recession defect is considered a critical factor for the selection of the root-coverage surgical procedure [
56]. Moreover, a narrow band of KT was found to be associated with inferior treatment outcomes following surgical root coverage [
57]. Indeed, the absence of apical KT makes flap handling, positioning, and fixation more difficult compared to clinical situations with the presence of at least 2 mm of KT. In the present study, a wide band of KT was present that might have contributed to flap manipulation and adaptation and therefore could be relevant for the overall treatment outcome in a clinical setting.
From an economic point of view, the cost-effectiveness of CTA compared to suture material must be discussed. Material costs (per unit) of the suture material (synthetic monofilament 6-0 sutures) and CTA used in this study are similar. In this context, further clinical studies must clarify whether on the one hand increased costs for using a combination of sutures and CTA are justified by improved clinical outcomes and on the other hand whether secondary costs, e.g., by postoperative complications, can be prevented by using a combination of sutures and CTA.
Apart from cyanoacrylate-based tissue adhesives, several other oral wound dressing materials have been proposed to accelerate healing, reduce postoperative pain, and prevent infection [
58]. These include palatal stents, surgical sponges, platelet concentrates, zinc oxide–based dressings, collagen membranes, and hyaluronic acid [
59,
60]. To what extent periodontal dressings are beneficial for intraoral wound healing and the impact of periodontal dressing composition has been the subject of debate [
61]. Thus, it has been demonstrated that the use of different zinc oxide–based periodontal dressings was associated with increased plaque accumulation, irritation of the soft tissues, and more pain and swelling [
61]. Saito et al. (2008) showed that non-eugenol zinc oxide–based periodontal dressings may induce an intense inflammatory response [
62]. On the other hand, a most recent study demonstrated that the use of a surgical stent made from thermoplastic zinc-containing polymer granules provided significant benefits for wound healing parameters and patients’ postoperative morbidity in the early phase of palatal wound healing [
63].
Since this is an ex-vivo study, some relevant clinical aspects cannot be taken into account, such as wound healing in an oral environment, muscle traction, long-term stability, and much more. The results and the CAF stability shown in this study can only be transferred to the clinical situation with limitations. To provide a more comprehensive evaluation, the results of this study should be re-evaluated in an in-vivo study and long-term clinical follow-up data should be generated to assess the persistence of benefits in clinical settings.