• Background Patients experiencing problems with their conventional maxillary denture can benefit from implant- retained overdentures. Bar-retained implant overdentures can be supported by 4 or 6 implants, splinted by a bar. Evidence on maxillary implant overdentures has been mostly short to medium term, non-comparing or retrospective. Even more, 15-years data of comparative studies are lacking. It is important to evaluate treatment outcomes, such as implant survival, peri-implant bone changes and patient satisfaction at the long-term.
• Main research question The aim of this 15-years study is to evaluate a patient group treated 15 years ago in a prospective study with a maxillary overdenture on 4 or 6 implants in an edentulous maxilla. The primary objective of the study is to analyze marginal bone level changes by radiological assessments at 15-years follow-up. Secondary objectives are implant and overdenture survival, condition of peri- implant mucosa and patients' satisfaction.
• Design (including population, confounders/outcomes) The study design is an observational study of a group of patients which were treated 15 years ago with dental implants and an overdenture in the maxilla because of problems with retention and stability with their conventional denture. Outcomes: primary outcome is the change in marginal peri-implant bone level 15 years after placing the overdenture. Secondary outcome measures will be implant and overdenture survival, peri-implant mucosa health and patients' satisfaction using a questionnaire.
• Expected results Stable peri-implant bone levels, high implant and overdenture survival rate and satisfied patients.

• Introduction and rationale In patients who present persistent complaints with respect to their maxillary denture, providing implant support improves comfort, denture satisfaction and overall quality of life considerably and high implant survival rates are reported. Implant position, the number of implants and bone volume are all factors that presumably influence success of both the maxillary overdenture and the implants.

Four- and 6-implant bar-supported maxillary overdenture studies report successful short and medium-term results, indicating that these treatment modalities are comparable and favourable. However, there is considerable heterogeneity with respect to patient and treatment variables within studies reporting on maxillary overdentures in general and four versus six implants in particular, with a lack of high quality long term data as well.

The aim of the present study is to report long term results of bar-supported maxillary implant overdentures on four and six anterior implants for which various clinical surgical and prosthetic outcome measures and subjective, patient-reported parameters are described and compared. Evidence on maxillary IOD attachment systems has been mostly short to medium term, non-comparing or retrospective and therefore inconclusive. This underlines the need for studies comparing different attachment systems with a longer follow-up.

The study design is an observational study to compare treatment outcomes of fully edentulous patients with maxillary IODs, supported by four or six implants. Patients were treated 15 years ago with dental implants and an overdenture in the maxilla because of problems with retention and stability with their conventional denture. Outcomes: primary outcome is the change in marginal peri-implant bone level 15 years after placing the overdenture. Secondary outcome measures will be implant and overdenture survival, peri-implant mucosa health and patients' satisfaction using a questionnaire.

Inclusion Criteria:

• Fully edentulous patients or patients with an overdenture in the mandible, who were referred to the Department of Oral and Maxillofacial Surgery (University Medical Center Groningen, the Netherlands) 15 years ago, because they suffered from a lack of retention and stability of the upper denture;

At the time of treatment:

• Edentulous in the maxilla for at least one year;
• Sufficient bone volume in height in the anterior or posterior region of the maxilla to place the implants;
• The patient was 18 years or older;
• Sufficient interocclusal space for placement of an overdenture with attachment system;
• The patient was capable of understanding and giving informed consent.

Exclusion criteria at the time of treatment:

• Patients with American Society of Anesthesiologists score (ASA score) ≥ III;
• Patients who were smoking;
• Patients with a history of radiotherapy in the head and neck region;
• Patients with a history of pre-prosthetic surgery or previous implant placement in the maxilla.

Exclusion Criteria:

None

Patients who were treated 15 years ago with dental implants and an overdenture in the maxilla because of problems with retention and stability with their maxillary conventional denture.