Clinical Outcomes at 2 Years and Early Radiographic Changes Following Transcrestal Sinus Floor Elevation in Sites with Residual Bone Height Below or Above 4 mm: A Retrospective Comparative Cohort Study

Background/Objectives: Maxillary sinus floor elevation is a well-established procedure for enabling implant placement in the posterior maxilla when residual bone height is insufficient. Minimally invasive transcrestal techniques, including the use of the Crestal Approach Sinus (CAS) kit, have been developed to reduce surgical morbidity while maintaining predictable clinical outcomes. The aim of this retrospective comparative study was to evaluate the clinical and radiographic outcomes of transcrestal sinus floor elevation performed using the CAS kit in patients presenting with residual bone height below or above 4 mm. Methods: Forty-five partially edentulous patients who received single implant-supported rehabilitations in the posterior maxilla between September 2021 and October 2024 were included. Patients were allocated to a test group (residual bone height < 4 mm; n = 20) and a control group (residual bone height ≥ 4 mm; n = 25). All procedures were performed using a standardized CAS-kit protocol with hydraulic membrane elevation and xenogeneic bone grafting. The primary outcomes were implant and prosthetic survival rates and the occurrence of biological or technical complications. Secondary outcomes included the radiographic evaluation of residual bone height, final bone height, vertical bone gain, and bone height above the implant apex at implant placement, prosthetic delivery (9 months) and at the 2 years follow-up. Results: No implant or prosthetic failures and no biological or technical complications were recorded, resulting in 100% implant and prosthetic survival rates. Residual bone height was significantly lower in the test group (2.8 ± 0.8 mm) than in the control group (5.3 ± 1.0 mm) (p < 0.001). Despite significantly greater vertical bone gain in the test group (10.9 ± 1.6 mm vs. 8.2 ± 0.6 mm; p < 0.001), comparable final bone heights were observed immediately after augmentation (13.7 ± 1.7 mm vs. 13.5 ± 1.5 mm; p = 0.742) and at prosthetic delivery (12.6 ± 1.6 mm vs. 12.9 ± 1.6 mm; p = 0.556). Bone height above the implant apex was also similar between the groups at both time points. A modest but statistically significant reduction in augmented bone height was observed during healing in both groups. Conclusions: Within the limitations of this retrospective study, transcrestal sinus floor elevation using the hydraulic CAS-kit technique showed favorable clinical and radiographic outcomes in both groups, including sites with residual bone height below 4 mm. These preliminary findings suggest that the crestal approach may represent a treatment option in carefully selected patients with limited residual bone height. However, larger prospective controlled studies with standardized radiographic assessments and longer follow-up periods are needed to confirm these findings.