A Novel Radiographic Classification of the Proximal Tibiofibular Joint

Background and Objectives: We aim to develop a simple radiographic classification for describing the morphological inclination of the proximal tibiofibular joint (PTFJ) based on composite measurements obtained from standard anteroposterior and lateral knee radiographs, and to evaluate its interobserver and intraobserver reproducibility. Materials and Methods: A retrospective analysis was performed on 100 standard anteroposterior and lateral knee radiographs from skeletally mature individuals. Proximal fibular articular-surface inclination was measured in both projections, and a composite index value was calculated and expressed in degrees. Based on percentile thresholds derived from reference measurements, joints were classified into three categories: low inclination (<29.4°), intermediate inclination (29.4°-35.5°), and high inclination (≥35.6°). Six observers, blinded to reference measurements, independently assessed all radiographs by performing angular measurements and assigning classifications. Data distribution was assessed using the Kolmogorov-Smirnov test. Comparisons were performed using the Chi-square, Wilcoxon, and Friedman tests. Intra- and interobserver agreement for measures were evaluated using the Intraclass Correlation Coefficient (ICC). For comparisons involving two ordinal angular classifications, both inter- and intraobserver agreement were assessed using the Weighted Cohen's Kappa coefficient (KW). For comparisons involving more than two ordinal angular classifications, interobserver agreement was assessed using Gwet's AC2 coefficient (GAC2). A p value < 0.05 was considered significant. Results: The final sample comprised 59 female (59.0%) and 41 male (41.0%) radiographs. According to the proposed classification, 31% of joints were categorized as low inclination, 35% as intermediate, and 34% as high, demonstrating a balanced distribution across categories. Interobserver agreement was higher for the composite index and final classification than for isolated anteroposterior or lateral measurements, supporting the internal consistency of the system. Conclusions: This radiographic classification provides a simple and reproducible method for standardized assessment of proximal fibular articular-surface inclination using routine knee radiographs. The findings establish a morphologic framework with excellent measurement reliability. However, external validation in independent cohorts and correlation with clinical outcomes are required before broader clinical application or prognostic use can be established.