Keratinized mucosa, which is composed of free gingiva and attached gingiva, is a barrier against bacterial invasion in oral cavity and provides good tissue sealing for periodontal environment. When the keratinized mucosa is insufficient, it is difficult to maintain the long-term stability of the implant, which is not conducive to the peri-implant health. It is generally believed that enough keratinized mucosa width is beneficial to reduce plaque accumulation and reduce the incidence rate of peri-implant diseases. In recent years, clinicians have gradually recognized that the thickness of keratinized mucosa plays an important role in maintaining the peri-implant health. Some researchers found that the thickness of mucosa is very important to maintain the aesthetic effect of implant restoration, and when the thickness of mucosa around the implant is less than 2mm, alveolar bone absorption will occur to maintain a stable biological width. A meta-analysis showed that when the thickness of the keratinized mucosa around the implant was greater than 2mm, the amount of marginal bone loss was significantly reduced (- 0.8mm, P < 0.0001).

It is considered that autogenous soft tissue graft is the most reliable technology to augment keratinized mucosa. Subepithelial connective tissue graft (SCTG) is a mucogingival surgery that transplant autologous free connective tissue under the pedicled semi thick flap to augment keratinized mucosa. It can effectively increase the thickness of soft tissue, cover the exposed implant, and reconstruct the interdental papilla. It is the gold standard for peri-implant soft tissue agumentation. Keratinized mucosal thickening surgery may be done prior to the surgical phase, after the surgical phase, before loading, or even after loading. It is believed that keratinized mucosal thickening at the same time of implantation can effectively reduce the possibility of mucosal recession after implantation, reduce the amount of marginal bone absorption in the process of osseointegration, which is conducive to maintaining the long-term stability of the implant. For the sake of clear clinical vision and convenient operation, clinicians often choose to thicken the keratinized mucosa during the secondary operation, and also obtain good postoperative effect. However, after the completion of the final repair, the keratinized mucosa thickening surgery increases the difficulty of operation and the technical requirements for the operator. In clinical practice, it is rarely selected to perform keratinized mucosal thickening at this time. At present, the effectiveness of timing on the outcome of soft tissue augmentation is still debated, and, most importantly, a direct comparison between simultaneous and staged procedures remains underexplored.

Therefore, this clinical trial is to prospectively compare the clinical efficacy of simultaneous versus delayed timing of soft tissue augmentation by SCTG placement around single implants, by evaluating the peri-implant marginal bone level change and soft tissue change, so as to provide reference for the formulation of clinical treatment plan and the selection of the best operation time.

Inclusion Criteria:

• No systemic diseases or pregnancy;
• The thickness of keratinized mucosa where the posterior tooth is missing is less than 2 mm;
• Full-mouth plaque score (FMPS) < 20% (measured at four sites per tooth) and bleeding on probing score (BOP%) ≤25% (measured at six sites per tooth);
• Periodontal condition is stable, no probing depths ≥5 mm;
• No previous soft tissue augmentation procedure at experimental site;
• The patient has signed informed consent and accepts the operation of subepithelial connective tissue transplantation.

Exclusion Criteria:

• Patients with uncontrolled hypertension;
• Patients with uncontrolled diabetes;
• Patients with insufficient oral hygiene and Untreated periodontitis;
• Pregnant or lactating women;
• Smoker;
• Patients with long-term (> 6 months) use of glucocorticoids;
• History of malignancy, radiotherapy, or chemotherapy for malignancy within the past 5 years;
• Patients with severe cardiovascular problems;
• Patients with uncontrolled infectious or metabolic diseases;
• Patients with substance abuse.