Flexion gap discrepancy drives femoral rotational adjustment in robot-assisted TKA: a CPAK-Based phenotypic analysis

This study investigates the correlation between femoral component rotational adjustment strategies and coronal alignment parameters in robot-assisted total knee arthroplasty (TKA), and explores differences among Coronal Plane Alignment of the Knee (CPAK) subtypes. A retrospective analysis of 100 patients with varus gonarthrosis who underwent robot-assisted total knee arthroplasty (TKA) was conducted. Intraoperative navigation data quantified medial/lateral flexion-extension gaps, physiological external rotation, and femoral rotational adjustments. Preoperative CT-based 3D reconstruction, CPAK classification (types I/IV/VII), and radiographic parameters (HKA, lateral distal femoral angle [LDFA], medial proximal tibial angle [MPTA]) were analysed using one-way ANOVA and multivariate linear regression to identify predictors of rotational adjustments and subtype differences. Post-adjustment femoral external rotation angles exhibited a unimodal distribution (4.41°±2.28°), with 52% of cases concentrated in the 3°–5° range. This represented a significant increase from preoperative physiological external rotation (3.49°±2.17°, Δ = 1.39°, p < 0.001). No differences in final external rotation were observed among CPAK subtypes (p = 0.356), but type I required greater adjustments than type VII (Δ = 1.46°, p = 0.020). Intraoperative flexion gap discrepancy strongly predicted rotational adjustments (β = 0.342, p < 0.01). Adjusted cases exhibited smaller preoperative MPTA (83.37°±5.17 vs. 85.49°±3.40, p = 0.018) and larger preoperative flexion gap discrepancies (2.56 ± 3.33 vs. 0.86 ± 1.30 mm, p < 0.01). Robot-assisted TKA optimizes femoral rotational alignment through dynamic gap balancing, with flexion gap discrepancy serving as the primary determinant for rotational compensation. CPAK classification provides preoperative identification of phenotypes requiring specific compensatory strategies, particularly type I deformities which necessitate greater external rotation adjustments. Robotic systems minimize alignment outliers and reduce soft-tissue releases, supporting the adoption of personalized, gap-balancing protocols in TKA.