Reliability and Agreement of Four Radiographic Methods for Measuring Posterior Tibial Slope on Lateral Knee Radiographs

Objective: The posterior tibial slope (PTS) is a critical anatomical parameter influencing knee biomechanics and ligamentous stability. Despite its clinical relevance, no consensus exists on the most reliable radiographic method for measuring PTS. This study aimed to evaluate the inter- and intra-observer reliability and agreement of four radiographic techniques for PTS measurement on standard lateral knee radiographs. Material and Method: A retrospective analysis was conducted on 70 adult patients with high-quality true lateral knee radiographs. Four commonly used methods were applied to measure the PTS: the fibular shaft axis (FSA), anterior cortical line (ACL), posterior cortical line (PCL), and proximal anatomical axis (PAA). Two experienced observers performed all measurements twice, 15 days apart, using a standardized protocol. Intraclass correlation coefficients (ICCs) were calculated to assess reliability, and Pearson correlation coefficients were used to evaluate the agreement between measurements and demographic characteristics. Results: All four methods showed excellent intra-observer reliability (ICC, range 0.916–0.975) and good-to-excellent inter-observer reliability (ICC, range 0.813–0.968). Mean PTS values differed by method: ACL 12.4° ± 3.2°, PCL 7.2° ± 3.1°, FSA 10.1° ± 3.2°, PAA 9.6° ± 3.0° (p = 0.001). Pairwise correlations were strong for all methods (p < 0.001). PTS showed no significant association with age, sex, height, weight, or body mass index. Conclusions: Although reliability was high for all techniques, differences in absolute values indicate that methods are not interchangeable. Standardized reference axes should be specified to ensure consistent reporting across clinical and research settings.