Change of CPAK class does not affect functional outcomes in robotic arm‐assisted total knee arthroplasty performed with functional alignment

Abstract

PurposeThe purpose of this study was to assess the impact of post‐operative coronal plane alignment of the knee (CPAK) class change on functional outcomes and determine the rate and type of CPAK class change after image‐based robotic arm‐assisted (RA)‐total knee arthroplasty (TKA) performed with functional alignment (FA) at two different centres.MethodsThe present retrospective, observational, multicentre study included 201 patients treated with RA‐TKA between October 2020 and April 2022 at two different centres. The radiographic CPAK classification was adapted using CT images to achieve pre‐ and post‐operative knee categorization into CPAK classes. At a minimum of 1 year post‐operatively, patients were administered the Forgotten Joint Score‐12 (FJS‐12) and surveyed about their post‐operative satisfaction level using a 5‐level Likert scale (5‐LLS).ResultsThe most common preoperative overall CPAK classes were: Types II, I, III, IV and V. Implant positioning after RA‐TKA with FA within the alignment boundaries, determined distribution in the CPAK classification, predominantly maintaining classes I, II, IV, and V. No statistically significant FJS‐12 differences were detected between subjects who maintained and changed their preoperative CPAK class. The mean 5‐LLS for satisfaction in patients where the preoperative CPAK class was maintained intraoperatively was 4.4 ± 1.1 (range = 1–5), while subjects having the CPAK class changed had a mean 5‐LLS of 4.4 ± SD 1.0 (range 1–5).ConclusionIn the setting of image‐based RA‐TKA with FA, CPAK can be changed within a ‘functional safe‐zone’, without compromising functional outcomes. Good functional outcomes are the result of a stable and balanced knee with soft‐tissue preservation, regardless of the maintenance of the preoperative CPAK class.Level of EvidenceLevel III.