This retrospective cohort study was approved by Medical Ethics Committee of the Fourth Military Medical University, School of Stomatology (Approval Number: IRB-REV-2022052) and registered in Chinese Clinical Trial Registry (ChiCTR2200062113) and conducted in accordance with the Helsinki Declaration as revised in 2013. Informed consent was obtained from all participants prior to their inclusion in the study.
2.1 Patients and study design
In our retrospective cohort study, we included patients who underwent KM augmentation at osseointegrated bone level implant sites in the posterior mandible based on risk assessment. Patients were excluded if they didn’t have regular follow-up after KM augmentation. The patients were divided into three groups: APF alone, APF plus FGG, and APF plus CM depending on dental records.
2.3 Surgical procedures (Fig.1)
KM augmentation technique option was discussed and reach a consensus based on the risk assessment before surgery (Table 1) by the studying team. The studying team was consisted of three investigators (D.H.L, C.X, and H.B.W), who have a more than ten-year experience in implant surgery.
The surgical protocol was performed by the same experienced surgeon (D.H.L). Local infiltration anesthesia was administered. In the APF alone group, a split-flap horizontal incision at the alveolar ridge crest and bilateral vertical incisions was prepared using a #15C stainless steel blade. It was better to design the incision within the keratinized mucosa, 1-2 mm away from adjacent natural teeth. A supraperiosteal partial flap was elevated by sharp dissection in order to create a periosteal bed free of any muscle attachment, moved roughly 10-12 mm apically. The resulting flap was stabilized at the base of the newly created vestibule and the incision margin within the mobilized mucosa was sealed to periosteum by firmly suturing using non-resorbable sutures (5/0 Vicryl, Ethicon J&J Medical GmbH, Norderstedt, Germany). The previous inserted bone-level implants were still covered. In APF plus FGG group, FGG of an appropriate size were harvested from the palate on the same side, thinned to a uniform thickness, apposition sutured to the corresponding periosteum with interrupted sutures, and sticked the FGG graft to the recipient bed using cross stitch. In APF plus CM group, the collagen matrix (Mucograft®, Geistlich Pharma AG, Wolhusen, Switzerland) was trimmed to appropriate size and sutured in the same way with the spongious layer to the recipient bed. The depth of the newly created vestibule was measured from the horizontal incision at the alveolar ridge crest to the region of the sutured mucosal flap with a periodontal probe.
Postsurgical instructions were as follows: (1) an ice-compress for 4 h after surgery; (2) cefuroxime axetil 250 mg every 12 h for 3 days and loxoprofen sodium 60 mg in case of pain; (3) a soft-food diet for 10 days; (4) appropriate oral hygiene for 14 days (twice daily tooth brushing, except in the augmented region, and twice daily antiseptic rinsing with chlorhexidine 0.2%).
Suture were removed after 2 weeks, expect those of apically positioned flap at the newly created vestibule after 4 weeks. After 4 weeks of healing, flapless design was performed in the second stage surgery and healing abutments were inserted.
The follow-up measurements were taken at 3, 12 months, 60 months after surgery.
2.3 Clinical outcomes
The clinical outcomes were measured at baseline before KM augmentation (T0) and immediately post-operatively (T1), at the second stage surgery (T2) or crown after a surgery time of 3 months (T3), 12 months (T4), and 60 months (T5).
The clinical effective outcome of KM augmentation was considered as remaining >=2 mm KM [27, 28]. Implant success was evaluated using the modified criteria: allowing for 1.0 mm bone loss during the first year of function followed by a maximum 0.2 mm of bone loss for the following years[29]. Prosthesis survival was considered as remaining in function and not needing to be substituted[30]. Smoke should be based on self-report, by means of questionnaires following the same guidelines used in a recent study [31].
Peri-implant parameters as follows:
(1) KMW, measured from the mucogingival junction to mid-crestal line before KM augmentation (T0), immediately post-operatively (T1); or to the free mucosal margin at the buccal midpoint of implant abutment at the second stage surgery (T2) or crown after a surgery time of 3 months (T3), 12 months (T4) and, 60 months (T5) with a periodontal probe.
(2) Marginal bone loss (MBL), radiographic crestal bone levels changes measured at the mesial and distal sites of all implants after 12 months (T4) and 60 months (T5).
(3) Probing depth (PD), at the four sides of the implants (mesio-labial/buccal, mid- labial/buccal, disto-labial/buccal, and palatal/lingual) after 12 months (T4) and 60 months (T5).
(4) Gingival index (GI), assessed at four sites of each implant (mesio-labial/buccal, mid- labial/buccal, disto-labial/buccal, and palatal/lingual) after 60 months (T5);
(5) Plaque Index (PI), assessed at four sites around the implant after 60 months (T5);
(6) Bleeding on probing (BOP), yes/no after 60 months (T5).
2.4 Statistical analysis
Statistical analysis was performed using SPSS version 23.0. ANOVA (for continuous variables) and Pearson c2 (ordinal variables) was used to assess the correlation between the anatomy factors and KM augmentation techniques at baseline. Fisher’s exact test was used to compare the KM augmentation outcomes, GI, PI, and BOP among different factors. Spearman coefficient was used to analyze the association between keratinized mucosa augmentation outcomes and different factors. General linear model (GLM) for multiple linear regression was carried out to assess the association between keratinized mucosa width at T5 and different factors. p < 0.05 was considered significant.