Title: Xenogeneic Matrices in Peri-Implant Soft Tissue regeneration: Case Series Authors: Sérgio Matos, Pedro Nolasco, Cristiano Pereira Alves, Tony Rolo. Background: The role of keratinized tissue in the longevity of dental implants, based on the current knowledge of the literature, is still controversial. Several techniques are described in the literature in the attempt to enhance keratinized tissue width, which includes the use of collagen matrices (CM) isolated or with autogenous grafts harvested from the palate. Aim: Describe two different approaches in the correction of mucogingival defects around dental implants using xenogeneic collagen matrices (XCM) in a monotherapy or combined with a strip free gingival graft (SFGG) and, additionaly, to present a systematic review about its clinical efficacy. Material and Methods: In this case series, both patients were systemically healthy, non-smoking females with inadequate attached and keratinized tissue ( 2mm) and lower vestibule. The surgical technique applied included, for case 1, the use of a xenogeneic dermal matrix and, for case 2, a combination of XCM with an apical SFGG harvested from the palate. The systematic review was structured according to the PRISMA statement and a PICOT questions was structured. Results: The patient experienced no postoperative complications, such as intense pain, infection or bleeding. All treated sites exhibited an increased gain in KTW (between 1 to 5mm). Xenogeneic collagen matrix presented some shrinkage. The literature evidence emphasizes for surgery time and patient morbidity more favorable results achieved for collagen matrices, despite less keratinization and higher tissue contraction (more than 50%) compared with autogenous grafts. The application of CM is associated with high variability of gain in KTW (between 1.5 and 10 mm). Conclusions: All techniques showed efficacy in improving peri-implant KTW, nevertheless limited level of evidence prevents definitive guidelines. Apically positioned flap plus autogenous grafts seems to favor better outcomes related to gain in KTW and volume. XCM demonstrated less gain in KTW and higher volume contraction, but can be an alternative in specific indications due to less patient morbidity and surgery time. The association with a SFGG seems to diminish tissue shrinkage. However, this positive results should be considered carefully and must be tested in well-designed clinical trials. Keywords: gingivoplasty/methods, keratinized tissue, free gingival graft, collagen matrix, gingival autograft, attached gingiva, soft tissue augmentation, dental implant