Multivariate Analysis of the Survival Rates and Risk Factors of One-Piece Zirconia Implants Supporting Single Crowns or Fixed Dental Prostheses: A Retrospective Cohort Study with Follow-Up Periods of up to 8 Years

Background/Objectives: Titanium implants remain the gold standard in implant dentistry. However, growing interest in metal-free alternatives has led to increased use of zirconia implants. Despite encouraging short-term outcomes, evidence regarding the medium- to long-term survival of one-piece zirconia implants (O-PZIs) and associated risk factors remains limited. The aim of this retrospective cohort study was to evaluate the survival of O-PZIs over follow-up periods of up to 8 years and to explore variables potentially associated with implant failure. Methods: This retrospective observational cohort study was conducted at a private dental clinic (Madrid, Spain). A total of 307 O-PZIs placed in 196 patients between 2017 and 2021 were analyzed. Implant survival was assessed using Kaplan–Meier analysis, while associations between clinical variables and implant failure were explored using chi-square tests and multivariate Cox regression models (p < 0.05). The mean follow-up period was 61.37 ± 2.25 months. Results: After a mean follow-up of 61.37 ± 2.25 months (range: 39–96 months), 42 failures were recorded, resulting in a cumulative survival rate of 86.32% (CI 95%: 79.28–92.96%). Most failures (64.29%) occurred before prosthetic loading. Kaplan–Meier analysis revealed significantly lower survival for tapered implants (p < 0.001) and among smokers (p < 0.001). Multivariate analysis indicated that only simultaneous guided bone regeneration (GBR) was independently associated with implant failure (Exp(B) = 3.191; 95% CI: 1.299–7.840; p = 0.011). However, this association should be interpreted with caution due to the retrospective design, potential confounding, limited number of events, and lack of adjustment for clustering at the patient level. The discrepancies observed between statistical methods highlight the importance of time-to-event analyses in implant research. Conclusions: Within the limitations of this study, O-PZIs demonstrated acceptable medium- to long-term survival. Simultaneous GBR may be associated with increased risk of failure. However, these findings should be considered exploratory. Further prospective studies are required to confirm these results and to better define risk factors in ceramic implant therapy.