Prosthetic Retention Mode and Peri-Implant Marginal Bone Level at a Minimum Five-Year Follow-Up After Maxillary Sinus Floor Elevation: A Retrospective Clustered Cohort Study

Background/Objectives: Evidence comparing cement- and screw-retained implant-supported restorations remains heterogeneous, particularly in augmented posterior maxillae. This study evaluated the association between definitive retention mode and peri-implant marginal bone level at a minimum 5-year follow-up after maxillary sinus floor elevation. Methods: This retrospective clustered cohort included 248 patients and 528 bone-level implants; radiographic measurements were available for 522 implants from 247 patients. Marginal bone level was assessed by CBCT immediately after implant placement and at least 60 months later. The primary adjusted analysis used generalized estimating equations with patient-level clustering and robust standard errors. Results: Mean marginal bone level at final follow-up was 0.133 mm for cement-retained and 0.105 mm for screw-retained restorations; the unadjusted difference was not statistically significant (Mann–Whitney U = 31,366; p = 0.064). In the primary GEE model, which excluded provisional retention because of its strong collinearity with definitive retention, definitive screw retention was not associated with marginal bone level at follow-up (B = −0.0028 mm; 95% CI: −0.0165–0.0108; p = 0.684). The time × definitive retention interaction was also not significant (B = −0.0069 mm; 95% CI: −0.0187–0.0049; p = 0.249). Conclusions: Definitive retention mode was not significantly associated with marginal bone level in the primary adjusted analysis, and longitudinal marginal bone change did not differ significantly between retention strategies. Sensitivity analyses indicated that estimates were influenced by the strong collinearity between provisional and definitive retention. These observational findings do not demonstrate superiority or equivalence of either retention strategy.