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

Purpose: 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).

Methods: 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.

Results: 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.

Conclusion: The CPAK classification prior to surgery affected TIR outcomes regardless of the alignment method used, as measured via AR-CT navigation.