The influence of CPAK classification on the pivot motion between kinematic and mechanical alignment total knee arthroplasty.

Abstract

This study investigated whether the preoperative coronal plane alignment of the knee (CPAK), categorized as type I or II, has an effect on postoperative tibial internal rotation (TIR) relative to the femur in patients who received a medial pivot-cruciate retaining (MP-CR) total knee arthroplasty (TKA). We retrospectively analyzed 62 patients who underwent MP-CR TKA with the GMK Sphere implant (Medacta International, Switzerland) from January 2024 to May 2025. Based on their CPAK classification, patients were divided into two groups (type I or II). Intraoperative kinematic assessments were performed using augmented reality (AR) CT-guided navigation (NextAR, Medacta), evaluating the dynamic elongation of medial and lateral collateral ligaments during either kinematic (KA) or mechanical alignment (MA) procedures. The primary measure was TIR at 30°, 60°, 90°, and 120° of knee flexion.Univariate analyses identified significant differences, which were further assessed using multi-way ANOVA to determine interactions with CPAK classification. Notable differences in TIR were found between CPAK types I and II at 60° (2.1° ± 4.8° vs. -2.0° ± 4.2°, p = 0.009), 90° (7.0° ± 4.3° vs. 2.1° ± 5.6°, p = 0.002), and 120° (13.1° ± 5.2° vs. 7.5° ± 4.9°, p = 0.002). Additionally, at 90°, sex and alignment method (KA vs. MA) showed significant interaction with CPAK classification (F = 6.4; p = 0.015), while the alignment method alone had no significant effect on TIR. The CPAK classification prior to surgery affected TIR outcomes regardless of the alignment method used, as measured via AR-CT navigation.