Restoring the native knee or designing the ‘optimal prosthetic’: Alignment, phenotypes and AI‐powered personalization in total knee arthroplasty

摘要

Few questions in modern total knee arthroplasty (TKA) generate as much debate as whether the surgeon's goal should be to restore the native pre-arthritic knee, with all its inherent characteristics, or to aim for what could be described as an ‘optimal’ prosthetic knee, independent of the patient's original anatomy. This discussion touches the very core of our understanding of knee arthroplasty: are we restoring a joint, or are we engineering a new one? The pre-arthritic knee is the product of a patient's unique anatomy, morphology, ligament balance and kinematic behaviour. Attempts to restore this through concepts such as kinematic alignment and/or individualized implants aim to maintain as much as possible the patient's original joint line orientation, ligament tension and motion patterns [17] (Figure 1). Proponents suggest that this approach may enhance the natural prosthetic joint's function, perception, proprioception, and overall patient satisfaction. Conversely, critics argue that replicating the knee's inherent ligament laxities could compromise both short- and long-term outcomes, favouring instead a more ‘functional’ alignment target [14]. However, the pre-arthritic knee may also harbour features, such as excessive coronal or rotational malalignment, or ligament imbalance, that contributed to osteoarthritis in the first place. Restoring such features without modification might risk reproducing the poor biomechanical conditions that led to joint degeneration, raising concerns of premature implant failure [4]. A contrasting philosophy is to implant a knee prosthesis in what is considered an ideal position for fixation, load distribution, wear minimization and implant longevity, regardless of the patient's original alignment or phenotype. This ‘biomechanically friendly’ approach has historically been represented by mechanical alignment, and has more recently evolved through various forms of adjusted or restricted kinematic alignment, aiming to combine the theoretical biomechanical advantages with a degree of individualization [18]. Advocates point out that conventional (off-the-shelf) implants have fixed geometric properties and limited ability to replicate the complex three-dimensional (3D) kinematics of the native joint. In this view, the ‘optimal prosthetic knee’ is one that functions best as a prosthetic construct, not necessarily as a replica of the original anatomy. Bridging these two philosophies requires a deeper understanding of the variability of native knees. Here, the concept of functional knee phenotypes [2, 8] offers a powerful framework. By classifying knees based on coronal alignment of the femur and tibia, joint line obliquity, and other parameters, this system (recently published by Hirschmann et al. [11]) allows surgeons to identify the normal, neutral, deviant and aberrant knee phenotypes and describe optimal individual patterns preoperatively. When applied to TKA planning, functional knee phenotyping supports truly patient-specific alignment strategies. For some patients, particularly those with mild deviations from neutral, restoring their native phenotype may be both feasible and beneficial [3] (Figure 1). For others with extreme or dysplastic phenotypes, a shift towards an optimized, prosthesis-driven alignment may be more appropriate. That is the reason why functional alignment (functional positioning in modern 3D terms [13]) appears to integrate these considerations, using enhanced technology assistance to adjust implant positioning with the aim of balancing the restoration of native metrics and mechanical optimization. This philosophy uses the patient's soft tissue envelope and bony anatomy [7] to guide the final positioning with no rigid targets and sometimes modifying the original phenotype to reach a desired better functional balance (Figure 2). In addition, functional positioning incorporates sagittal and axial planes into the analysis [19], thus including the patellofemoral joint, and it un