The surgical procedure was done by one experienced surgeon. Before surgery, patients were given to rinse the mouth with 0,12% clorhexidine gluconate for 1 minute. Following the local anesthesia utilizing Artikaine 4% with 1:100000 epinephrine the elevation of split-thickness flap was started. Incision was done at the level of MGJ from maxillary tuberosity on one side to tuberosity of the opposite side. Initially, sharp dissection was carefully done in an apical direction to leave exposed periosteum without tearing. Further, blunt dissection was used to advance periosteum exposure and muscle fibres detachment. For apically positioned flap group the split thickness flap was apically positioned as much as possible and sutured to the periosteum utilizing T-mattress sutures. Regarding free gingival graft strip group, after the split thickness flap was sutured apically, free gingival graft in the form of strip, harvested from the palate, was sutured to the apical end of the recipient bed leaving the rest of periosteum to heal by secondary epitelization. Free gingival graft strip stabilized with 6-0 resorbable sutures by means of two single interrupted and cross-mattress sutures. The dimensions of free gingival graft strip was 30-35mm in length (depending on the length of the apical part of recipient bed), 2-3mm in width and 1-1.5mm in thick. The palatal donor site was sutured using cross-mattress sutures. Patients were given postoperative instructions which included antibiotics regime (amoxicillin 500mg, 3x1 for 5 days or in case of penicillin allergy- clindamycin 600mg, 2x1 5 days) and rinsing with chlorehexidine solution 0.12% two times a day for 14 days.

To control for bias, measurements were carried out by one examiner not involved in the surgical procedure. Measurements were taken from regions representing tooth #16 to tooth #26. The colour-coded periodontal probe (University of North Caroline Probe CP15 PCPUNC156, Hy Friedy) utilized to measure the distance from depth of newly formed vestibule to the edge of vertical cut made on the GM representing position of certain tooth. Measurement for each tooth were summarized in three regions: anterior group (central and lateral incisor region)- AG, middle group (canine, first and second premolar region)- MG and posterior group (first molar region)- PG.

First measurement was done immediately after surgery (T0) and represented the distance from depth of newly formed vestibule to the edge of vertical cut made on the GM (for apically positioned flap group) and from apical margin of free gingival graft strip to the edge of vertical cut made on the GM (for free gingival graft strip group). Follow up measurements were taken at 30 (T1), 60 (T2) , 90 (T3) and 180 (T4) days after surgery.

Primary outcome was wound contraction rate representing the percentage of keratinized tissue reduction between time point immediately after surgery (T0, 100%) and follow-up time points (T1,T2,T3 and T4). Secondary outcomes were 1) patient-reported outcomes such as patient discomfort and pain which was evaluated utilizing visual analogue scale (VAS) at 1, 2, 7 and 10 days after surgery and the amount of ibuprofen taking within 10 days after surgery, 2) the surgery time which represented the time from first incision to the last suture with a digital timer rounded to the whole minutes, 3) wound infection (yes/no), and wound bleeding (yes/no).