The Restoration of the Prearthritic Joint Line Does Not Guarantee the Natural Knee Kinematics: A Gait Analysis Evaluation Following Primary Total Knee Arthroplasty

Background: Unrestricted kinematic alignment (uKA) in total knee arthroplasty (TKA) has the theoretical advantage of reproducing patients' constitutional alignment and restoring the pre-arthritic joint line position and obliquity. However, modifications of the original uKA technique have been proposed due to the potential risk of mechanical failure and instability. Given the significant variability in soft tissue behavior within the same bony morphology group, uKA pure knee resurfacing could be occasionally detrimental. This study aimed to kinematically compare the outcomes of uKA TKA with those of a robotic-assisted KA TKA technique based on specific soft-tissue boundaries. Methods: In this retrospective gait analysis study, 24 TKA patients and 12 healthy controls were recruited. Inclusion criteria were a 9-month minimum follow-up from successfully, primary medial-pivot or medially-congruent TKA performed for isolated degenerative joint disease. Preoperatively, patients were randomly assigned to two surgical groups: A) uKA (#12) and B) robot-assisted (#12), KA (hybrid-kinematic) with boundaries (±3° from hip-knee-ankle neutral axis) and a slight intercompartmental gap asymmetry (max 2 mm lateral-opening). The gait analysis was performed using instrumented treadmills equipped with 3D cameras. Results: Sagittal knee kinematic data: during the early-stance phase of gait, the uKA group showed a less consistent weight-acceptance phase and a less efficient transition between the first knee-flexion peak and mid-stance-extension plateau with respect to the hybrid-kinematic alignment group. Spatiotemporal and overall gait quality data: no significant differences were found between the two TKA groups regarding walking speed (P = .51) and step length (P = .8534). Control group patients walked more efficiently compared to TKA groups, showing inferior trunk flexion and inferior variation in step length (P < .0001). Conclusions: This study showed that restoring the pre-arthritic joint line, as advocated by surgeons following the uKA philosophy, does not guarantee a closer-to-normal knee kinematics.