Tibial Slope Variation Across Coronal Plane Alignment of the Knee Phenotypes: A Three-Dimensional Computed Tomography-Based Analysis of Osteoarthritic Knees

Abstract

The Coronal Plane Alignment of the Knee (CPAK) classification system categorizes nine phenotypes based on constitutional limb alignment and joint line obliquity (JLO). Understanding relationships between CPAK phenotypes and tibial slope (TS) could streamline total knee arthroplasty planning. This study investigated the correlations between CPAK classification and medial TS in patients with osteoarthritis. A retrospective analysis of 622 cases in 535 patients with osteoarthritis undergoing primary total knee arthroplasty was conducted. Three-dimensional computed tomography imaging with MyPlanner software determined mechanical lateral distal femoral angle, mechanical medial proximal tibial angle, and medial TS. Statistical analysis included multiple linear regression, Pearson correlation, and one-way analysis of variance (ANOVA) with Tukey post hoc testing. Outliers were removed using interquartile range criteria, resulting in 581 knees for final analysis. Multiple linear regression revealed minimal correlation between coronal alignment and TS (TS = 26.35 − 0.1045 arithmetic hip–knee–ankle [aHKA] − 0.1004 JLO; r ² = 0.0233). aHKA angle and JLO explained only 2.33% of TS variance. Contour mapping demonstrated no discernible patterns in data distribution. Despite weak correlations, ANOVA identified statistically significant differences between CPAK groups for TS (F = 2.97; p = 0.003). Tukey post hoc analysis revealed significant differences between CPAK group I and groups V and VII, with mean differences ranging 1.39 to 2.06 degrees. No clinically meaningful relationship exists between CPAK classification and TS in osteoarthritic knees. While statistical significance differences were observed between certain CPAK groups, the extremely low correlation coefficient and small effect sizes indicate these differences fall within measurement variability and are substantially smaller than TS variations that influence total knee arthroplasty outcomes. CPAK classification cannot reliably predict sagittal plane morphology, necessitating an independent, comprehensive three-dimensional assessment of coronal and sagittal characteristics during surgical planning.

Level of Evidence

Level III—Retrospective Comparative Study.

Keywords

knee phenotypes - Coronal Plane Alignment of the Knee - tibial slope - total knee arthroplasty

Data Availability Statement

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Contributors' Statement

V.L.M., J.M.A.: conception and study design. V.L.M., J.H.A., and M.L.L.: data collection and data analysis. V.L.M., J.H.A., M.L.L., J.M.A., P.S.U.: interpretation of data. V.L.M., M.L.L., P.S.U.: preparation of this manuscript. F.L.M., F.S.M., J.M.A.: revising the draft critically and final approval of the version. All the authors read and approved the final version of the manuscript for submission.

Ethical Approval

This study was approved by our institutional review board (protocol number: CEIC-HMM-16/19).

Informed Consent

Not applicable. The study was granted exemption from requiring informed consent for publication since the complete and irreversible anonymization of the images did not involve data processing.

Publication History

Received: 15 August 2025

Accepted: 24 January 2026

Accepted Manuscript online:
30 January 2026

Article published online:
10 February 2026

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