Patient selection: Patients at the Department of Oral Implantology, Peking University School and Hospital of stomatology, Bei Jing, China, who seek for implant restoration for two continous posterior teeth are potentially recruited. Inclusion criteria are generally healthy (ASA score I), non-smoking, periodontally healthy, and sufficient bone volume to place an implant without augmentation procedure. Patients were excluded in cases of medical and/or psychiatric contraindications, local infection, pregnancy or lactating, poor oral hygiene, tissue deficiency, or not willing to participate in this study.

Randomization: The two implant sites of each subject are randomly assigned to two different group with a predefined randomization tables. In order to reduce the chance of unfavorable splits between groups in terms of key prognostic factors,the randomization process will take into account the following variables: patient's gender, age, presence of adjacent teeth, distal extension sites and site location in the dental arch. Assignment will perform using a sealed envelope.

Surgical protocol: Before the surgical procedure, prophylactic antibiotics of 1g amoxicillin will prescribed to the patient 1 h before surgery, and continued with 2 g/day for 6 days. After crestal incision, full thickness buccal flap was raised, whereas lingual part was not elevated. Thickness of soft tissues was measured with 1.0-mm marked periodontal probe on the top of bone crest in the center of the two future implant sites. This ensured direct visibility of mucosal thickness during measurement. After measurements, full-thickness lingual flap is raised and subsequent implant osteotomy is finished, Two dental implants (Dentsply Implants Manufacturing GmbH, Mannheim, Germany) with diameter of 3.5 mm are ready to torch into the bone cavities. In one site (group 1), the platform of implants are placed at the crestal level (flush to the alveolar ridge) . In the other site (group 2), the implants are placed 1mm below the alveolar ridge. narrow healing abutment are connected to all the implants. The flap are repositioned and closed with interrupted single sutures. Implants are allowed for a un-submerged healing. 3 month after healing, the implant level impressions are taken, splinted restorations of two neighbouring posterior teeth are delivered 2 weeks later.

Radiographic and Clinical Examination: All the patients are required for recall visit at 3, 6,12 months and 2 years post surgery. A digital peri-apical radiograph was taken at the 1 year and 2year follow-up examination for bone level evaluation using a film holder to aim the x-ray beam perpendicular to the implant threads, and the implant length is used as calibration for the measurement. Using a image analysis software (ImageJ, version 1.47, NIH, Bethesda, MD, USA), the investigators are able to have an accurate assessment of the crest of bone and to analyze the bone changes over period of time. Bone loss was defined as a positive value and bone gain as a negative one. Mean values, standard deviations, and maximum and minimum values were calculated. Furthermore, periodontal parameters \[bleeding on probing (BOP), probing pocket depth (PPD), modified plaque index on adjacent teeth and implants\] were assessed.