Coronal Knee Alignment in Adult Spinal Deformity Patients: Implications for Total Knee Arthroplasty Planning and Coronal Plane Alignment of the Knee Classification Reliability

<h2>Abstract</h2><h3>Introduction</h3> Kinematic alignment in total knee arthroplasty has been increasingly investigated for its potential improved functional outcomes. The coronal plane alignment of the knee (CPAK) classification was developed to better define native coronal alignment (to act as a target) using joint line obliquity (joint line apex [distal, neutral, or proximal]) and the arithmetic hip-knee-ankle angle (varus, neutral, or valgus). Since the literature is limited in addressing how spinal deformity influences coronal plane alignment of the knees, this study examined the distribution of CPAK types in patients who had adult spinal deformity and evaluated whether surgical correction of the spinal deformity alters this distribution. <h3>Methods</h3> A total of 264 patients (528 knees) from a prospectively maintained multicenter database were included based on the availability of full-body biplanar radiographs both before spinal realignment surgery and at one-year follow-up. The CPAK classification was assigned at each timepoint and compared to the distribution reported in a healthy population. Statistical analyses included <i>Chi</i>-square goodness-of-fit testing, univariate correlation analyses, and multivariate regressions. <h3>Results</h3> Preoperative and postoperative CPAK distributions differed significantly from the healthy population (<i>P</i> < 0.001), but the overall distribution of the cohort did not change significantly following surgery. However, on an individual level, 36% of knees showed a change in CPAK classification. These patients were older and had higher body mass indexes and greater changes in global spinal alignment, pelvic shift, and knee flexion. Multivariate analysis identified changes in lumbar lordosis, pelvic shift, and sagittal knee angle as independent predictors of change in arithmetic hip-knee-ankle angle. <h3>Conclusion</h3> Patients who had adult spinal deformity demonstrate a distinct CPAK profile. Though global distribution remains stable following spinal realignment, substantial individual variability exists, with some patients experiencing major changes to coronal knee alignment postoperatively. Clinicians should interpret CPAK classification cautiously in this population, especially in those at risk for or undergoing spinal realignment surgery.