• Background Patients experiencing problems with their conventional maxillary denture can benefit from implant- retained overdentures. Several retention systems are available. These retention systems can be roughly divided into bar-retained implant overdentures, in which multiple implants are splinted and the overdenture is attached through retentive clips, and into retention systems using non-splinted solitary attachments (locators). Evidence on maxillary implant overdenture attachment systems has been mostly short to medium term, non-comparing or retrospective. Even more, 10-years data of comparative studies are lacking.
• Main research question To compare treatment outcomes of fully edentulous patients with maxillary overdentures, supported by four implants, retained by either bars or locators . The primary objective of the study is to analyze marginal bone level changes by radiological assessments at 10-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 10 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 10 years after placing the overdenture. Secondary outcome measures will be implant and overdenture survival, peri-implant mucosa health and patients' satisfaction using a questionnaire.

• Introduction and rationale Patients experiencing problems with their conventional maxillary denture can benefit from implant-retained overdentures (IODs). Several retention systems are available. These retention systems can be roughly divided into bar-retained IODs, in which multiple implants are splinted and the overdenture is attached through retentive clips, and into retention systems using non-splinted solitary attachments. Bar-retained IOD's provide good retention, require little maintenance, but are more expensive than solitary attachments. Medium term results of bar-retained IODs are promising with high implant and overdenture survival and low incidence of complications and may be considered the gold standard. Solitary attachments serve as an alternative to bars. These attachments are more economical and are easy to clean by the patient, but wear more easily, which can cause lack of retention. However, replacement of these attachments can often be done chair side. A recent review reported that, when a maxillary IOD is supported by four implants, both types show equal implant survival, overdenture survival and patient satisfaction. However, these conclusions are based on a limited amount of randomized controlled trials. Moreover, most studies reporting on solitary attachments were retrospective. 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 implants, retained by either bars or Locators. Patients were treated 10 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 10 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) 10 years ago, because they suffered from a lack of retention and stability of the upper (and lower) denture;
• Edentulous in the maxilla for at least one year;
• Sufficient bone volume in height in the anterior 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:

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.