From Imaging to Instrument: A CT-Based Classification of the Nasofrontal Beak and Frontal Ostium Clearance with Prospective Surgical Correlation

Background/Objectives: Frontal sinus surgery remains one of the most challenging procedures in endoscopic sinus surgery due to the complex and variable anatomy of the frontal recess. While several radiological parameters have been proposed to guide surgical planning, there is no standardized anatomical classification that informs both approachability and techniques. This study aimed to develop a reproducible CT-based classification of the nasofrontal beak (NFB) and frontal ostium clearance (FOC), and to validate its surgical applicability through a two-phase design. Methods: A combined retrospective–prospective cohort study was conducted. In Phase 1, CT scans from 1383 patients (2595 sinuses) were analyzed to measure NFB and FOC lengths via a novel technique. The cutoff points were defined through histograms, kernel density estimation, and K-means clustering. In Phase 2, 100 patients (191 sinuses) who underwent frontal sinus surgery were prospectively assessed. Radiological classifications were compared with intraoperative findings, and instruments were selected on the basis of anatomical classes. Agreement between radiological and surgical classifications was analyzed, and surgical outcomes were recorded. Results: NFB and FOC were each classified into three groups: Class A (≤6 mm), Class B (6–12 mm), and Class C (≥12 mm) for NFB, and Class A (≥12 mm), Class B (6–12 mm), and Class C (≤6 mm) for FOC. There was excellent agreement between radiological and intraoperative NFB classes. Class-specific techniques included no modifications for Class A NFB, frontal sinus punches for Class B, and straight/curved drill for Class C. Only one intraoperative lamina papyracea injury and two cases of postoperative neo-ostium narrowing were reported. Significant sex-based anatomical differences and minor side-to-side asymmetries were also observed. Conclusions: This novel classification provides a reproducible, internally validated system for stratifying frontal sinus anatomy preoperatively, with good radiological–surgical concordance for the nasofrontal beak. By mapping the anatomical class onto instrument selection, it offers a practical planning adjunct. As a single-center derivation without an external cohort or comparator arm, it demonstrates anatomical–surgical concordance and feasibility rather than clinical superiority, and warrants multicenter external validation.