How successful are maxillary implant-retained overdentures on two or four implants?

Design A randomised crossover clinical trial was designed to determine survival and success of four-implant overdentures in the edentulous maxilla, in participants with mandibular two-implant overdentures. Participants received four implants in the maxilla, two anteriorly and two posteriorly, then were randomly allocated into two treatment groups. The prosthetic phase included alternate loading of anterior and posterior implants for three months each, followed by loading of all four implants for another three months. Participants were then followed up in the recall phase for a mean period of 2.2 years.Case selection Four participants were taken from the authors' previous clinical study, meeting the inclusion criteria for a two-implant overdenture in the mandible. Another 21 participants were recruited from study calls after a widening of inclusion criteria. These patients were all pre-treated in accordance with the previous study protocol, so there was standardisation before maxillary implant placement.Data analysis A sample size calculation was performed and a Kaplan-Meier curve was used to demonstrate implant survival over time.Results Twenty-four patients were included in the study. Three implants were lost during the prosthetic phase and two during the recall phase; all were anterior implants. Implant survival after loading was 93.8% after a mean period of 3.1 years. There were 27 maxillary prosthetic complications overall. Denture success was 95.8% after a mean period of 2.2 years.Conclusions Implant survival of four asynchronously loaded implants in implant-supported overdentures in the edentulous maxilla was good and is a recommendable treatment option for patients with two-implant mandibular overdentures. Implant and prosthetic complications are common but mostly straightforward to manage. Two posterior implants are not superior to two anterior implants, but the majority of patients prefer the four-implant maxillary overdenture.