Rotational Variability in Arthritic Knee Morphometry Predominantly Arises From the Hip and May Warrant Preoperative Consideration Prior to Total Knee Arthroplasty

Background: Preoperative planning in total knee arthroplasty (TKA) has relied on local knee landmarks without considering proximal ones. Although balancing requires triplanar (coronal, sagittal, axial) anatomy, the literature emphasizes 2-dimensional imaging that ignores axial-plane rotation clarified by computed tomography scans. This study of broad arthritic knee anatomy aimed to (1) identify the source of greatest rotational variation between hip and knee, and (2) determine if local knee anatomy morphometrically approximates hip rotation or if the hip independently impacts knee morphometry. Methods: A computer vision pipeline analyzed 1352 TKA candidate computed tomography scans to extract 5 hip and 10 knee angular measurements; among 422 anatomical outliers, variability was quantified via dispersion metrics and principal component analysis, while regression models assessed if knee morphology predicts hip morphology. Results: Hip-related features showed significantly greater variability than knee-related features among anatomical outliers (P < .05), particularly in the axial plane, suggesting that most lower-limb rotational variability originates from the hip. Knee-based measurements failed to predict hip morphometry, identifying hip rotation as an independent factor in lower-limb presentation of arthritic knee anatomy. Conclusions: Despite the dogma of intraoperatively balancing using local knee landmarks, the hip—a major source of lower-limb anatomical heterogeneity not captured by standard knee imaging—contributes greater rotational variability than the knee and independently influences observed variation in arthritic knee anatomy. While specific clinical protocols are not yet defined, the proximal femur may represent an important area for further investigation to determine if its consideration can improve TKA rotational alignment, flexion balancing, and patellofemoral stability.