The primary objective of this study will be to study the correlation between implant position (malposition), evaluated through periapical radiographs of peri-implantitis vs healthy implants, and the occurrence of peri-implant biological complications such as peri-implantitis, as evaluated after at least 6 months from the placement of the prosthesis. The clinical records of all subjects treated with implants during the period between January 1st, 2005, and June 30th, 2021 (last follow-up included: 6 month visit on December 2021) in the Dental Clinic of the IRCCS Istituto Ortopedico Galeazzi (Milan, Italy) will be screened for inclusion.

Will be included 45 implants with peri-implantitis and 135 healthy implants regardless of the corresponding number of patients . Descriptive statistics will be provided by means of mean values and standard deviations. Correlation between baseline parameters and outcomes will be measured through regression analysis, being the independent variable the occurrence of peri-implantitis. Multilevel analysis will be performed.

For all the analysis, the level of significance was set at p<0.05.

For sample size calculation the investigators have considered alpha = .05, power = 80% and a proportion between controls and cases of 3:1. The investigators hypothesized a detection of an effect of malposition with a proportion of 0.25 of exposes in control group and 0.5 in test group. the investigators decided to include 45 implants with peri-implantitis and 135 healthy implants, after augmenting the numbers hypothesizing a maximum of 10% dropouts. The proportion of exposed are estimated on the basis of the study published by Yi et al. in 2020 .

The calculation was performed using the method proposed by Schoenfeld.

The primary outcome was the occurrence of peri-implantitis described as:

• presence of signs of inflammation (bleeding and / or suppuration after probing);
• radiographic bone loss in addition to crestal bone resorption caused by initial remodeling or, in the absence of radiography at one year, bone level positioned ≥ 3 mm apically to the most coronal portion of the intraosseous portion of the implant body;
• presence of increased probing depth as compared to previous measurements, if available.

Implant malposition will be defined on the basis of the following parameters:

• criteria proposed by Buser and coworkers in 2004: at least 1.5 - 2 mm between implant neck and adjacent tooth (mesio-distal); ii) at least 1 mm of apico-coronal distance between implant neck and the Cemento-enamel Junction (CEJ) of adjacent teeth (no more than 2.5 mm); iii) at least 3 mm between the necks of two adjacent implants.
• inclination of the implant axis as compared the axis of adjacent teeth (both mesial and distal if present)
• characteristics of the prosthesis (emergency profile, presence of cantilevers, imprecise implant-abutment connection)

The following parameters will be collected from clinical records:

• implant-related factors: implant type, length, diameter, and type of implant-abutment connection, implant surface, vertical position of the implant (distance between implant neck and the bone level at the time of intervention (I-BC) (periapical radiograph)), distance between the implant neck margin and adjacent teeth (on periapical radiographs) (I-MT, I-DT [I-MI, I-DI]), distance between implant neck and the projection of CEJ of adjacent teeth (periapical radiographs) (I-MT-CEJ, I-DT-CEJ), the angle between the projection of the implant axis and the axis of adjacent teeth or implant.
• patient-related factors: age, gender, smoking status, systemic disease, medications, presence of periodontal disease at the time of intervention (severity), history of periodontal disease, bruxism;
• site-related factors: implant position / site, bone quantity / quality following the Lekholm and Zarb classification;
• prosthesis-related factors (to be evaluated using the radiographs taken after placement of the prosthesis): single crown, partial fixed denture, ceramic- or resin-made, time of loading, fixation methods (screwed or cemented), platform switching, crown height (I-CH), extension of prosthetic cantilever (only for single-tooth restorations) (mesial and / or distal) (C-M, C-D), angle of emergency profile (mesial and distal) (C-M-EP, C-D-EP), presence of misfit of the prosthesis.

Clinical data will be completely anonymized through the association of each subject to one identification code, and the elimination of the document containing the link between them.

Inclusion Criteria:

• subjects aged more than 18 years old, treated with implants with moderately rough surface.
• subjects treated with dental implants at Istituto Ortopedico Galeazzi during the period between January 1st, 2005, and June 30th, 2021
• subjects without any systemic disease that could be an impact on bone metabolism (e.g. diabetes mellitus, osteoporosis, neoplasms)
• single and partial rehabilitations (without cantilever extension): single tooth restorations (one implant between two teeth or one single implant supporting single-tooth prosthesis with one adjacent tooth); multiple tooth restorations (one tooth per implant or bridges, splinted or not).
• complete information about patient status (systemic diseases, smoking status, medications, age, gender)
• complete description of the surgical and prosthetic protocol that was adopted.
• at least one periapical radiograph of good quality taken at the end of the intervention and a sufficient number of follow-up visits (at least one per year); the quality of the images will be assessed by the Guidance Notes for Dental Practitioners on the Safe Use of X-Ray Equipment ((UK) 2018), accepting Grade 1 and Grade 2 images.
• having clinical and / or radiographic records to assess periodontal status at the time of intervention.

Exclusion Criteria:

• Incomplete data (e.g. absence of periapical radiographs).

The clinical records of all subjects treated with implants during the period between January 1st, 2005, and June 30th, 2021 (last follow-up included: 6 month visit on December 2021) in the Dental Clinic of the IRCCS Istituto Ortopedico Galeazzi (Milan, Italy) will be screened for inclusion.