Postoperative CPAK type I alignment combined with restoration of native posterior tibial slope is associated with superior patient-reported outcomes after medial unicompartmental knee arthroplasty

Previous studies investigating the association between postoperative maintenance of the Coronal Plane Alignment of the Knee (CPAK) alignment pattern and clinical outcomes following medial unicompartmental knee arthroplasty (UKA) have reported inconsistent findings, possibly due to heterogeneity among preoperative CPAK phenotypes and the lack of inclusion of sagittal alignment changes. This study aimed to evaluate whether postoperative maintenance of the CPAK type I alignment pattern and restoration of posterior tibial slope (PTS) were associated with postoperative outcomes following medial UKA. A retrospective analysis of 162 knees in 157 patients with preoperative CPAK type I who underwent medial UKA was conducted. Knees were classified according to postoperative CPAK type I maintenance and PTS restoration. PTS was considered restored when − 2° ≤ ΔPTS ≤ + 2° and not restored when ΔPTS < − 2° or > + 2°. Combining these two factors yielded four coronal–sagittal alignment groups. The primary outcome was the 2-year Forgotten Joint Score (FJS). Secondary outcomes included the Oxford Knee Score (OKS), Knee Society Score–Knee (KSS-K), Knee Society Score–Function (KSS-F), visual analogue scale (VAS), and knee range of motion (ROM). Knees with postoperative maintenance of CPAK type I had significantly higher 2-year FJS, OKS, and KSS-K scores than knees in which CPAK type I was not maintained. The highest FJS was observed in knees with both CPAK type I maintenance and PTS restoration. The fully adjusted general linear model showed a significant interaction between CPAK type I maintenance and PTS restoration in relation to FJS. Postoperative maintenance of CPAK type I combined with PTS restoration was associated with higher postoperative outcome scores among knees with an intact medial UKA at 2 years. These findings suggest that both coronal and sagittal alignment may be relevant when developing individualized alignment strategies for patients with preoperative CPAK type I. Retrospective cohort study.