The restricted 4D inverse kinematic alignment workflow: A patient‐specific approach integrating both bony and laxity phenotypes for total knee arthroplasty planning

PURPOSE: Two standardized total knee arthroplasty (TKA) balancing goals exist-equal gaps or a larger lateral flexion gap. Applying either without considering preoperative laxity phenotypes may alter native laxity and post-operative kinematics. This study proposes a restricted inverse kinematic alignment (iKA)-based technique to integrate bony and laxity phenotypes in three dimensions and evaluates its performance using a TKA simulator. METHODS: The restricted 4D-iKA technique and its boundaries were described and validated in 80 knees using the Knee-CAT simulator (MDA). Preoperative MPTA, LDFA, HKA and PTS were entered into the simulator and reevaluated after TKA. All simulated resections, including posterior condylar offset (PCO), were measured in millimetres. The individual laxity phenotype was defined by first recreating the bony functional knee phenotype (B-FKP) and then classifying each knee into one of nine laxity phenotype types (L-FKP). L-FKP restoration and correction were assessed, together with the reconstruction of CPAK and B-FKP types. RESULTS: = 2.6° [95% CI = 0.9-4.3°]; p < 0.01), consistent with the predefined boundaries. PCO exceeded the predefined ±2 mm reconstruction limit in 45% of cases, compared to the preoperative state. The predefined target L-FKP IV or V was achieved in all cases, except one. In 60% of knees, the lateral flexion gap was larger than the medial flexion gap, while in 40% of cases the gaps were equal. CONCLUSION: This first restricted iKA-based technique incorporates patient-specific L- and B-FKP to personalize TKA planning. However, balancing flexion and extension required increasing PCO by ≥2 mm in 45% of cases. This suggests that knee anatomical adjustments should be expected when laxity targets are considered. LEVEL OF EVIDENCE: Level IV, retrospective, non-comparative study.