The aim of this study is to analyse contiguous dental implants and its peri-implant status depending on different inter-implant distances.

The present research project has been designed as a multicentric cross-sectional study. The STROBE guidelines have been followed in the design of this observational study.

Ethical issues The study will be performed after the approval of the Ethics Committee of the Universitat Internacional de Catalunya (UIC) and Faculdade de Medicina Dentária da Universidade de Lisboa (FMDUL) and will be intended to be registered in clinicaltrials.gov. Also, this research will be conducted according to the principles outlined in the Declaration of Helsinki (revised, amended, and clarified in 2013). In addition, all study participants will provide written informed consent before participating in the study.

Study population Subjects will be selected from an electronic database collected at the CUO (UIC) / CU (FMDUL) and composed of patients treated with at least two contiguous dental implants before 2021. This investigation will use a stratified random sampling based on year of implant placement to select a representative sample of subjects with implant-supported restorations conducted during the period 2001-2020 at the CUO and CU. Then, two examiners (I.T. / F.F.) will contact patients by telephone once every 2 days (with a maximum of 3 attempts) for an evaluation by the same investigators (I.T / F.F).

Criteria for subject selection will be as follows:

• The patient must be ≥18 years of age and systemically healthy;
• Partially edentulous patients, rehabilitated until 2020 (i.e., a minimum of 3 years of function time) with at least two contiguous implant-supported restorations in the maxilla or mandible;
• Screw or cemented-retained prosthesis;
• Fixed dental prosthesis (i.e., single crowns and partial prosthesis);
• No implant mobility.

Moreover, the exclusion criteria will be the following:

• Pregnant and lactating women;
• Patients who have taken systemic antibiotics during the 3 months prior to the examination;
• Patients being treated with drugs that may induce a gingival overgrowth;
• Patients undergoing orthodontic treatment (since it can influence periodontal diagnosis);
• Patients who have received mechanical debridement during the 3 months prior to the exam;
• Patients who have received surgical treatment for peri-implantitis;
• Psychophysical inability to carry out study procedures.

Inclusion Criteria:

• The patient must be ≥18 years of age and systemically healthy;
• Partially edentulous patients, rehabilitated until 2020 (i.e., a minimum of 3 years of function time) with at least two contiguous implant-supported restorations in the maxilla or mandible;
• Screw or cemented-retained prosthesis;
• Fixed dental prosthesis (i.e., single crowns and partial prosthesis);
• No implant mobility.

Exclusion Criteria:

• Pregnant and lactating women;
• Patients who have taken systemic antibiotics during the 3 months prior to the examination;
• Patients being treated with drugs that may induce a gingival overgrowth;
• Patients undergoing orthodontic treatment (since it can influence periodontal diagnosis);
• Patients who have received mechanical debridement during the 3 months prior to the exam;
• Patients who have received surgical treatment for peri-implantitis;
• Psychophysical inability to carry out study procedures.

Subjects will be selected from an electronic database collected at the CUO (UIC) / CU (FMDUL) and composed of patients treated with at least two contiguous dental implants before 2021. This investigation will use a stratified random sampling based on year of implant placement to select a representative sample of subjects with implant-supported restorations conducted during the period 2001-2020 at the CUO and CU. Then, two examiners (I.T. / F.F.) will contact patients by telephone once every 2 days (with a maximum of 3 attempts) for an evaluation by the same investigators (I.T / F.F).