Posterior tibial slope varies across functional tibial phenotypes but not CPAK categories: A radiographic analysis from the FP‐UCBM Knee Study Group

Abstract Purpose The relationship between the coronal and sagittal planes of the knee remains largely unexplored. The aim of this study was to evaluate the correlation between Posterior Tibial Slope (PTS), Hirschmann's functional tibial phenotypes, CPAK categories, and their defining parameters, hypothesising that coronal and sagittal tibial parameters are interrelated. Methods A retrospective analysis was conducted on preoperative radiographs from 204 patients (217 knees) undergoing primary TKA for end‐stage osteoarthritis between January 2023 and September 2023. PTS was measured on weight‐bearing true lateral radiographs with strict quality criteria. Coronal alignment variables—including arithmetic Hip‐Knee‐Ankle Angle (aHKA), Joint Line Obliquity (JLO) and Tibial Mechanical Angle (TMA)—were derived from full‐length standing radiographs, and CPAK categories and functional tibial phenotypes were assigned accordingly. Statistical analysis included Kruskal–Wallis tests and Spearman's correlations. Statistical significance was set at p < 0.05. Results 6No significant differences in PTS were observed across CPAK categories ( p = 0.501), aHKA categories ( p = 0.103), or JLO categories ( p = 0.906). In contrast, PTS differed significantly among functional tibial phenotypes ( p = 0.004), with the highest values in VAR TMA 6 (8.2° ± 3.4), VAL TMA 6 (8.0° ± 2.7), and NEU TMA 0 (7.7° ± 2.8), and lowest values in VAR TMA 3 (5.7° ± 2.6) and VAL TMA 3 (6.2° ± 2.1). Conclusion PTS varies significantly across functional tibial phenotypes but not CPAK categories, suggesting that functional tibial phenotypes may better capture sagittal alignment variability. Incorporating PTS into phenotype‐based preoperative planning could help refine patient‐specific strategies in total knee arthroplasty. Level of Evidence Level IV.