[The Importance of the Dynamic Leg Axis: Biomechanical Influences on Total Knee Arthroplasty and Joint Load]

In routine clinical practice, static radiographic imaging has become the standard method for assessing the leg axis in the frontal plane. With increasing use of functional analyses, such as motion analysis, the focus is shifting more towards the dynamic leg axis. The dynamic axis of the knee joint plays a critical role in the wear of total knee arthroplasty (TKA) components, as it influences load distribution and movement patterns in the femorotibial joint. In particular, the knee adduction moment (KAM) affects the inlay during medial loading, which exhibits wear at high loading frequencies. Studies indicate that KAM increases with varus alignment, leading to uneven load distribution and greater stress on the medial compartment. In-vitro and post-mortem analyses quantify the annual material loss in the inlay, with prosthesis design and alignment influencing wear rates. The dynamic leg axis and gait patterns can further modify wear; for example, a "toe-in" gait pattern significantly reduces KAM and may therefore represent a non-surgical option to mitigate wear. More recent approaches, such as kinematic alignment, aim for a more natural positioning of the prosthesis, which does not necessarily increase wear but, compared with mechanical alignment, may preserve a pre-arthritic varus or valgus axis. In summary, a dynamic evaluation of the leg axis may contribute to preoperative planning and wear reduction in TKA, while static radiological axis assessment will remain clinically relevant.