Analysis of CPAK change in robotic functional alignment TKA: a new simplified classification

The Coronal Plane Alignment of the Knee (CPAK) classification method describes knee phenotypes. The rise in robotic-assisted total knee arthroplasties (RA-TKA) has enabled surgeons to fine-tune bony cuts, minimizing soft tissue release while prioritizing balanced gaps rather than predetermined alignment targets, a technique known as functional alignment (FA). As a patient’s preoperative CPAK changes when using FA, our aim was to assess which preoperative CPAK phenotypes are maintained post-TKA and further define this change with a simplified classification. We retrospectively reviewed 1,028 primary RA-TKA cases performed using functional alignment (FA) technique from 2023 to 2024. Arithmetic hip-knee-ankle (aHKA) and joint line obliquity (JLO) angles were obtained using robotic software, with boundaries in accordance with CPAK. Demographics, CPAK phenotypes, and planned resections were collected and analyzed. Most cases were in CPAK 1 (34.1%), 2 (30.5%), or 3 (19.5%) preoperatively and 4 (30.2%) or 5 (51.2%) postoperatively. Postoperatively, 63.5 and 27.1% of cases retained their preoperative aHKA and JLO alignments, respectively. Of 427 cases that changed CPAK, most varus and neutral cases retained preoperative aHKA alignment, otherwise becoming neutral and varus respectively, while most valgus became neutral. Preoperative varus and valgus cases that retained aHKA had larger native deformities compared to those that became neutral. Most preoperative neutral JLO cases remained neutral, while a majority of apex distal cases became neutral. Cases that changed CPAK, compared to retained CPAK cases, were more neutral (aHKA) and apex distal preoperatively (P < 0.001). Based on results, we classified preoperative alignment according to final functional coronal alignment that would reflect tibia and aHKA angles and propose a new, simplified Functional Coronal Alignment (FCA) classification composing of four categories. This study highlighted the clinical usefulness of robotics for FA and described the FCA classification system to guide surgeons in optimizing kinematics using robotic assistance for FA.