practical cookbook for state-of-the-art phenotyping
Total knee arthroplasty (TKA) has historically pursued a uniform neutral mechanical axis, an approach justified by early implant constraints but increasingly challenged by evidence that neutral alignment is not the constitutional norm. The recognition that meaningful personalisation requires prior systematic characterisation of the individual knee has driven the emergence of phenotyping as a structured framework integrating morphology, alignment, laxity, and kinematics.
This article provides a practical, stepwise cookbook for state-of-the-art knee phenotyping in TKA, intended to guide surgeons through the application of currently available classifications within a coherent decision-making sequence.
Step 1 addresses the bony Functional Knee Phenotype (B-FKP), describing how to measure the hip-knee-ankle angle, femoral mechanical angle, and tibial mechanical angle from weight-bearing long-leg radiographs to derive the two-dimensional phenotype, and how to extend the classification to a three-compartment model incorporating the posterior condylar angle and anterior trochlear angle when CT imaging is available. The phenotype obtained then informs the choice between mechanical, kinematic, functional, or fully personalised alignment strategies.
Step 2 introduces the laxity Functional Knee Phenotype (L-FKP), explaining how standardised intraoperative varus and valgus stress testing at full extension and 90° of flexion, performed after dialling in patient-specific bony parameters on robotic platforms, generates a nine-group matrix that directly links laxity patterns to a structured management algorithm of bony cut adjustment, soft-tissue release, and constraint escalation.
Step 3 presents the Dynamic Alignment of the Knee (DyAK) classification, which captures coronal alignment across the functional arc and addresses the limitations of static measurements, given that fewer than 15% of knees maintain consistent alignment throughout range of motion. Together, these layered phenotypes provide a reproducible framework in which alignment philosophies become tools matched to individual anatomy rather than competing doctrines, supporting safer and more personalised reconstruction.
