Posterior tibial slope is independent of coronal plane knee alignment and needs to be assessed to determine knee phenotype

Introduction Personalized total knee arthroplasty (TKA) alignment has gained popularity due to its perceived advantages. Results are usually reported using the Coronal Plane Alignment of the Knee (CPAK) system. However, CPAK takes into account only the coronal plane alignment and ignores posterior tibial slope (PTS), a key determinant of sagittal plane alignment impacting knee biomechanics and clinical outcomes. It is not known if the distribution of PTS is similar across different CPAK types and if there is a variation across ethnicities. This study investigates the relationship between PTS and CPAK classifications. Materials and Methods A retrospective analysis was conducted on long-leg anteroposterior and lateral radiographs from 420 patients (747 legs) of Japanese, Chinese, and Indian origin. Coronal alignment was classified using CPAK, and sagittal alignment was assessed via PTS. Results Weak or no correlations between CPAK and PTS was identified across all groups, indicating that coronal and sagittal alignments are largely independent.Significant ethnic differences were observed in PTS with Indian patients showing the steepest PTS and Japanese the flattest. Sex differences in PTS were significant only among Chinese patients, while no age-related variations were detected in PTS. Conclusion PTS varies significantly amongst different ethnicities and has no correlation to individual CPAK types. The independence of sagittal and coronal alignments underscores the need for a biplanar approach in knee phenotype assessment and personalized TKA strategies.