Material and methods 2.1 Selection of study samples This study was approved by the Research Ethics Committee of our university. Patients who underwent implant fixed restoration and follow-up evaluations for 3-36 months from January 2015 to December 2018 at the Oral Hospital of our university were recruited for this study. Informed consent was obtained from all patients. Three dental implant systems with diameters of 3.6-5 mm, lengths of 7-13 mm, and a repair procedure for a single or consecutive crown were purchased from ITI (Straumann AG, Waldenburg, Switzerland), Bego (Bremen, Germany), and Dentium Co. (Seoul, Korea). Patient samples were selected based on the following inclusion criteria: 1. Implant procedure and prostheses had already been performed. 2. Implant prostheses were only supported by implants. 3. Implant status was regularly reviewed by clinicians and patients were in good compliance. 4. Good proximal contact and healthy adjacent teeth were confirmed after the implant procedure. The exclusion criteria were as follows: 1. evidence of severe periodontal disease; 2. edentulous jaw with implant