Evaluation of the Coronal Plane Alignment of the Knee (CPAK) classification in arthritic knees following robotic total knee arthroplasty

BACKGROUND: The Coronal Plane Alignment of the Knee (CPAK) classification describes knee morphology using arithmetic hip-knee-ankle angle (aHKA) and joint line obliquity (JLO). Currently there is uncertainty about the relevance and restoration of coronal alignment following total knee arthroplasty (TKA), particularly among Asian populations. This study examined changes in coronal alignment following robotic TKA using functional alignment, along with concordance between CPAK JLO and knee joint line obliquity (KJLO) – this would help develop strategies to optimize outcomes of TKA among Asian patients. METHODS: A retrospective analysis was undertaken for 333 knees that received cruciate-retaining robotic TKA during 2019–2023. Pre- and 6-week postoperative radiographs were used to assess aHKA, JLO, CPAK phenotypes, and KJLO. Maintenance of constitutional CPAK alignment and crossover in aHKA and JLO categories were estimated. The concordance between CPAK JLO and KJLO at pre- and post-operative levels were also estimated. RESULTS: Preoperatively, 72% showed varus aHKA, 82% apex distal JLO, and CPAK type I was predominant (63%). Postoperatively, varus alignment persisted in 50% and CPAK type IV in 32%. Constitutional aHKA and JLO was maintained in 46% and 42% knees, respectively. Less than 7% patients crossed over to a category beyond neutral considering aHKA or JLO. The most frequent phenotypic change was from CPAK type I to IV. Concordance between CPAK JLO and KJLO increased from 14% to 41% postoperatively, with highest agreement for apex proximal knees and lowest for apex distal knees. CONCLUSIONS: While constitutional CPAK was maintained in less than 50% patients, overcorrection in CPAK was minimal. Therefore, functional alignment with robotic assistance facilitates optimization by limiting extreme adjustments to constitutional CPAK following TKA.