Impact of different alignment techniques in total knee arthroplasty (TKA): a biomechanical patient-specific analysis of a well-aligned knee during walking

The determination of the optimal alignment strategy for Total Knee Arthroplasty (TKA) remains a subject of ongoing debate. To date, the concept of Mechanical Alignment (MA), in which both the femoral and tibial components are positioned perpendicularly to the mechanical axis of their respective bones, has been the gold standard. As an alternative, Kinematic Alignment (KA) and Inverse Kinematic Alignment (iKA) represent innovative patient-specific strategies aimed at restoring pre-arthritic patient alignment, with the hypothesis of promoting a more physiologic knee kinematics and enhancing functional outcomes. The aim of this work is to analyze the biomechanical effects induced on the same knee joint by three different alignment strategies (MA, KA and iKA), considering a patient-specific case of TKA during walking. Computed Tomography (CT) scans were performed on the left knee of the patient (76 years old, male), which shows a deformity of 1.5 degrees varus. The CT images were imported into image processing software to extract the geometries and generate 3D models of the following structures: distal femur, proximal tibia, fibula and collateral ligaments. Following the surgical procedure of each TKA strategy, the proper surgical cuts were identified and performed on the bone models. For each configuration, a Genus Fixed-Bearing (FB) Posterior-Stabilized (PS) implant (Adler Ortho, Cormano, Italy) was virtually implanted. Finite Element Method (FEM) models were created and FEM analysis were carried out to simulate walking. The numerical results obtained were evaluated in terms of tibio-femoral kinematics (tibial internal-external rotation, femoral anterior-posterior translation) and kinetics (contact area and pressure, stress on the insert and on the tibial bone, and ligament isometry). The results of this study demonstrate that, regardless of the alignment strategy selected, there are no significant differences between the three models, since the alterations in joint line inclination on the coronal plane are almost negligible. This arises from the fact that the patient suffers from a very mild varus deformity, which can be approximated to an almost neutral knee phenotype. Nevertheless, even if the three alignment techniques resulted in similar biomechanical outputs, minor discrepancies can be observed in the contact pressure and area of polyethylene, in the kinematics, and in the collateral ligament isometry. This study shows that in a patient with a mild varus deformity, a range of distinct alignment strategies can be chosen, but at the same time long-term implant survival could be further investigated. In conclusion, this work, along with other patient-specific models, can be useful in selecting the most appropriate alignment strategy based on the patient-specific anatomy and degree of deformity, to develop a pre-operative plan, and in predicting the post-operative joint kinematics.