Biplanar radiographic analysis of knee alignment: a stepwise approach for phenotype classification and knee arthroplasty planning

This review presents a standardized, stepwise method for biplanar radiographic analysis of knee alignment, integrating both coronal and sagittal measurements for use in arthritic and non-arthritic knees. It critically compares leading classification systems, including the coronal plane alignment of the knee (CPAK) and the functional knee phenotype classifications. While CPAK provides a simplified 2D coronal model, the functional phenotype system offers a more granular, 3D approach that includes segmental deformities and has recently been expanded to incorporate laxity parameters. Sagittal plane parameters – including posterior tibial slope and femoral component flexion/extension – are essential for comprehensive alignment assessment. These factors influence implant positioning, knee kinematics, and postoperative function. The review outlines preferred measurement techniques, highlighting the value of long-leg weight-bearing radiographs and discussing the limitations and variability of 2D versus 3D imaging approaches. Incorporating both alignment and soft tissue behavior provides a more individualized approach to total knee arthroplasty planning and may lead to improved outcomes by better replicating native knee biomechanics.