Four vs. Six Implant Full-Arch Restorations-A Direct Comparative Retrospective Analysis in a Large Controlled Treatment Cohort.

The potential impact of the number of implants (four vs. six) on the implant survival of implant-supported fixed complete dentures (IFCDs) remains inconclusive and subject to ongoing debate. This study compared the implant survival of immediately loaded four vs. six IFCDs, delivered using a patient-centered systematic treatment plan, in a large patient cohort assessed with consistent diagnostic methodology and comprehensive longitudinal follow-up. This retrospective study included records of 943 patients receiving 5989 implants with an average follow-up of 5.0 ± 3.2 (range 0-17) years delivered using a systematic clinical decision support system (CDSS) based on a defined alveolar atrophy classification (CC). Implant survival was analyzed considering predictor variables comprising treatment and anatomic and systemic health-related factors at the overall, maxillary, and mandibular levels using Kaplan-Meier analysis, log-rank tests, and single-predictor and multilevel Cox proportional hazard analysis derived from causal direct acyclic graph methodologies. The 2- and 5-year cumulative survival rates (CSRs) of four and six IFCDs were comparable (2-year: 98.6% vs. 98.4%, = 0.362; 5-year: 98.8%, 98.7% = 0.346). The differences between four and six IFCDs were more pronounced at the maxillary levels, specifically after 2 years (97.7% vs. 98.6% = 0.084), and at the mandibular level after 5 years (98.6% vs. 99.4%, = 0.136.). Multilevel Cox proportional hazard models at overall and jaw levels adjusted for confounding factors indicated that implant loss was correlated to jaw type and age at overall and age alone at the mandibular level. Alveolar atrophy (CC) defined within the adopted CDSS was not robustly associated with implant loss. Under the guidance of a systematically applied patient-centered CDSS, four and six IFCDs demonstrated high and comparable mid-to-long-term implant survival rates irrespective of the level of analysis or statistical model used to adjust for confounding factors. Prosthetic and technical complications were not evaluated and were, therefore, beyond the scope of this study.