## 4 Discussion
Different values for the minimum gingival thickness to achieve complete root coverage are presented by several authors; >1.1 mm [49] > 1.44 mm [50]. Before the collection of a CTG from the palatal masticatory mucosa, its thickness needs to be assessed. If the soft tissue at the donor site shows suitable thickness, an acceptable connective tissue graft can be prepared, without fragments of epithelial tissue, bearing in mind that its' presence in the graft could lead to graft rejection [11]. When a thin mucosal phenotype is identified at the potential donor site, the free gingival grafting technique is used in combination with its extracorporeal de-epithelialisation [28]. Another solution involves preoperative augmentation of the palatal mucosa using a lyophilized xenogeneic collagen sponge, after which a suitable graft is harvested to cover the gingival recession [15; 16]. [PERSON] et al. [15] used a lyophilized bovine collagen sponge to thicken the thin palatal mucosa, by placing it subperiodestally at the potential donor site. After 8 weeks, they collected a subepithelial connective tissue graft of appropriate thickness from this site, which ensured effective usage in the treatment of gingival recessions. [PERSON] [11] suggested a minimum mucosal thickness of 3 mm at the potential palatal donor site to assure the collection of an adequate connective tissue graft for utilization in periodontal plastic surgery. [PERSON] et al. [51] performed in 26 patients an augmentation of the palatal mucosa with an initial thickness of less than 3 mm, measured at the mid-palatal points of teeth with a distance of 6 mm from the gingival margin. As a result, they achieved a thickening of the palatal mucosal tissue from 2.03 mm at baseline to 3.57 mm, 8 weeks after augmentation. [PERSON] et al. [16] performed in 10 patients a bilateral augmentation of thin palatal mucosa, using two collagen materials in a single procedure. From one side they used a lyophilized collagen sponge Biokol\({}^{\text{\textregistered}}\). The average thickness of the mucous membrane was measured at two points, the first one placed 4 mm below (PGT1--palatal gingival thickness 1) and the second, 8 mm below the gingival margin (PGT2--palatal gingival thickness 2). PGT1 and PGT2 values were measured in this group at 86 sites, demonstrating the following values: PGT1 2.27 \(\pm\) 0.25 mm, PGT2 2.53 \(\pm\) 0.21 mm at baseline and 3.71 \(\pm\) 0.41 mm and 4.08 \(\pm\) 0.56 mm at 8-10 week after augmentation procedure respectively. Significant thickening was achieved. In order to enable a histological evaluation of the connective tissue, biopsies were taken at the potential donor sites with prior augmentation and without augmentation. The histological images of the grafts, taken from the augmented sites, exhibited characteristics of mature fibrous connective tissue. Compared with the biopsies taken from the nonaugmented sites, the biopsies from the augmented tissue with collagen material, did not reveal the presence of any adipose cells. Neither did any of the collagen-augmented sites present any xenogeneic collagen residues, concentrations of inflammatory cells, swelling, necrotic focus or the presence of epithelial cells. An important parameter used to determine the predictability of gingival recession coverage is the percentage of complete root coverage achieved. The closer this value is to 100%, the more the assessed technique meets the modern clinical requirements and effectiveness for such treatment. [PERSON] et al. [33] evaluated in a systematic review the efficiency of periodontal plastic procedures in the treatment of multiple gingival recessions. In nine studies with a minimum 6 months observation period, a total of 858 multiple gingival recessions were treated in 208 patients, demonstrating a CRC of 24-89%. [PERSON] et al. [34] compared the clinical and aesthetic results in 50 patients for 5 years after gingival recession coverage, using a coronally positioned flap (CPF) together with a connective tissue graft and CPF alone. [PERSON] et al.