Single- vs. Two- Implant retained overdentures: systematic review and meta-analysis.

Objective: The aim of this systematic review was to compare the outcomes of mandibular single- versus two- implant retained overdentures (IODs). Method: Electronic database (PubMed, EMBASE and CENTRAL) and hand searches were performed to identify publications reporting on mandibular IODs anchored by unsplinted attachments. Studies were included if they were prospective human studies reporting on two-piece micro-rough surface implants with a diameter ≥3mm. Two investigators extracted data independently. An overall inter-investigator kappa score was calculated for different parameters extracted (κ=0.86-1.00). A meta-analysis was performed on the included comparative studies, and a descriptive analysis for the remaining prospective reports. Result: From the searched articles (PubMed=8031; EMBASE=1490; CENTRAL=2922), 30 studies were included for data extraction and analysis. These included two randomized controlled trials (RCTs), comparing single- versus two- IODs, and 28 prospective reports. The meta-analysis of the RCTs revealed contradictory results; one study favored single-IODs (RD:0.08, 95%CI:0.01, 0.14) while the other favored two-IODs (RD:-0.04, 95%CI:-0.27, 0.19). However, the overall random effects model did not reveal a significant risk difference (RD) for early implant failure between the two interventions (I2=36.6%, p=0.209; RD:0.05, 95%CI:-0.07, 0.18). Descriptive analysis of studies with lower evidence revealed reported implant survival rates of 91.7%-100% and 90.4%-100% for single- and two- IODs, respectively. Conclusion: This meta-analysis provides evidence that the 1-year post-loading survival rates of single-IODs may be comparable to conventional two-IODs. However, before recommending the single-IOD as a treatment modality, long-term observations are needed and a larger range of functional, prosthodontic and patient-centered outcome measures should be considered.