## 4 Discussion
However, there was a controversial result indicating that the tunnel technique demonstrated comparable primary and secondary outcomes to those achieved with connective tissue grafting in adults with gingival recessions [52]. Moreover, the type of graft (connective tissue or acellular dermal matrix) did not impact the overall findings. This study showed that the utilization of the coronally advanced flap with collagen matrix may yield comparable results in terms of the percentage of complete root coverage. The use of a modified coronally advanced tunnel combined with collagen matrix revealed inferior results when compared with the subepithelial connective tissue. The previous report indicated that subepithelial connective tissue grafts achieved excellent outcomes in terms of keratinized tissue gain [53]. Moreover, a prior study revealed a tendency towards better clinical outcomes for subepithelial connective tissue grafts concerning keratinized tissue gain as opposed to collagen matrix [36]. Conversely, previous results prove that new collagen matrix was as effective and predictable as the connective tissue graft for attaining a band of keratinized tissue in patients with fixed prosthetic restorations, but its use was associated with significantly lower patient morbidity [54]. The findings of this research demonstrate that subepithelial connective tissue typically exhibits superior performance in terms of keratinized tissue. Furthermore, the utilization of the modified coronally advanced tunnel technique with collagen matrix yields inferior results when compared to subepithelial connective tissue graft in regards to keratinized tissue. However, previous research indicated that the collagen matrix, when employed as a soft tissue substitute to augment the width of keratinized tissue or mucosa during the second implant surgery, demonstrated comparable effectiveness and predictability to the use of free gingival grafts [55]. The utilization of the collagen matrix may yield comparable results when compared with subepithelial connective tissue in terms of probing depth. Similarly, a previous report showed that collagen biomatrix was effective in reducing the probing depth [56]. It was also shown that the probing depth was sustained at optimal levels throughout the follow-up period, which lasted up to 24 months, with the employment of matrix [46]. Despite the rigorous approach of our meta-analysis, which involved a comprehensive search strategy and strict compliance with PRISMA guidelines, several limitations need to be acknowledged. The primary limitations stem from the studies included in the qualitative and quantitative assessments. Additionally, blinding of participants may have been unfeasible due to the nature of the intervention, which required the creation of a second surgical site to obtain autogenous subepithelial connective tissue [57]. Nine of the included studies exhibited concerns related to bias, particularly in the selection of reported results, potentially affecting the objectivity and generalizability of the findings. The statistical analysis also indicated substantial heterogeneity among the included studies. Sensitivity analysis revealed that one study [22] significantly impacted heterogeneity in a specific outcome, but it did not affect the heterogeneity of the other outcomes, so it was not excluded. This suggests that the high heterogeneity observed in these analyses may be attributable to broader discrepancies among the studies. High heterogeneity in a meta-analysis warrants careful consideration, as it can undermine the external validity of the pooled effect estimates and may reflect underlying differences between the studies. Our results suggest that further analysis and randomized clinical trials with rigorous inclusion criteria are necessary to validate these findings [58]. Variations in follow-up periods and differences in patient populations must also be considered when applying these findings in clinical practice.