ObjectivesThis 5-year multicenter randomized clinical trial assessed the non-inferiority of maxillary implant-supported fixed complete dentures (FCDs) with four (4-I) compared to six implants (6-I) in terms of radiographic marginal bone level (MBL) changes after 5 years in function.Materials and MethodsIndividuals were randomly assigned to the 4-I or 6-I groups. Follow-ups occurred at 1, 3, and 5 years. At each visit, FCDs were unscrewed, clinical parameters (plaque index, bleeding on probing, pocket depth, and keratinized mucosa width) were recorded using a periodontal probe, periapical radiographs were obtained, and maintenance care was performed. MBL changes, incidence of techincal, prosthetic, and biological complications, treatments costs, and satisfaction were evaluated.ResultsOf 47 patients rehabiliated with 233 implants, survival rates were 99.3% for 6-I and 100% for 4-I (one early failure; 6-I group). No significant MBL differences were observed at 5 years between and within the groups. Both groups showed a decrease in keratinized mucosa width and experienced prosthetic and biological complications, but no peri-implantitis. The 4-I exhibited a significantly higher incidence of technical complications (16.6% vs. 0%) Cost analysis favored 4-I for initial and total costs. Clinician and patient satisfaction varied, with 4-I preferred aesthetically and 6-I functionally, particularly in speaking ability at earlier follow-ups.ConclusionThe use of FCDs supported by four implants is non-inferior to six implants in terms of radiographic MBL changes after 5 years in function. Both groups demonstrated comparable survival rates and incidence of biological and prosthetic complications. The 4-I was associated with higher technical complications and reduced overall treatment cost.Trial RegistrationClinicalTrials.gov identifier: NCT02405169