Study design

This retrospective observational study was approved by the ethical committee of Nagasaki University (No. 1512). The cases of all of the patients who underwent dental implant treatment with a TPS-surfaced solid-screw implant and whose prosthesis was set in the years 1984–1990 at seven private practices were analyzed. All inserted implants were either a TPS-type (TPS-type, Institute Straumann, Basel, Switzerland) implant or a BONEFIT 45° shoulder-type (S-type, Institute Straumann) implant. We identified a total of 223 implants inserted into 92 patients.

Medical record assessment

Medical records were reviewed, and the patient-related parameters of age, gender, smoking habit, the date of implant surgery, and the date of the prosthesis setting were collected. The information of implant (length, diameter, type), site of implantation, width of keratinized mucosa, and additional pre- and/or post-implant surgery (i.e., bone augmentation, soft tissue management) was also collected. The types of prostheses were classified into implant-supported fixed prostheses, tooth implant-supported fixed prostheses, and implant-supported overdentures.

The endpoint of this study was set at December 31, 2015. Episodes of implant failure, biological complication (i.e., peri-implantitis with suppuration), and mechanical complications (i.e., component or laboratory-fabricated suprastructure’s failure) were recorded.

Statistical analysis

JMP Pro software (ver. 11.2.0, SAS, Cary, NC, USA) was used for the statistical data analyses. The cumulative implant survival rate, the cumulative incidence of peri-implantitis, and the cumulative “complication-free” survival rate of implant-supported restorations were analyzed using the Kaplan-Meier survival estimator method. The cumulative “mechanical complication-free” survival rate of implant-supported restorations was estimated by a restoration-based analysis. The influence of the following variables on the implant survival rate and the incidence of peri-implantitis were determined by a single factor in univariate analyses (Kaplan-Meier) and multivariate analyses (Cox proportional hazards regression analysis): patient gender, smoking habit, implant type (S-type or TPS-type), implant position (three categories: maxilla, anterior mandible, posterior mandible), presence of additional soft tissue management (i.e., free-gingival graft, vestibular extension, and frenectomy), and the width of keratinized mucosa around implant (> 2 mm). The influence of patient gender and type of prosthesis on the complication-free survival rate of implant-supported restorations was determined. The results were considered statistically significant at p < 0.05.