Redefining coronal knee alignment: A population-specific centre-of-mass framework beyond coronal plane alignment of the knee thresholds

Minor variations in radiographic measurements of variables underlying the coronal plane alignment of the knee (CPAK) classification can alter its categorisation. Accordingly, this study introduces a population-referenced framework for describing coronal knee alignment, based on the centre of mass (COM) of the arithmetic hip-knee-ankle angle (aHKA)-joint line obliquity (JLO) distribution. This study is presented as a descriptive and methodological contribution. It does not include clinical validation and does not propose an alternative alignment strategy for immediate clinical implementation. A geometric-statistical model was developed in which the COM of the aHKA-JLO distribution represents the population-referenced geometric reference point of central tendency. Each knee was characterised by its radial distance from the COM (magnitude of deviation,) and its angular orientation relative to the COM (direction of deviation,). Two concentric regions, encompassing 68% and 95% of the distribution, were defined based on radial distances for each value in the distribution. This model was subsequently applied to four previously published datasets comprising 2494 knees from Belgian, Spanish, Turkis, and Korean populations. The COM of the aHKA-JLO distribution and dispersion parameters differed across populations, indicating that neutral alignment is population-specific rather than universal. In the healthy reference cohort (500 knees), the COM was located at aHKA = -0.87° and JLO = 174.94°. The proposed framework allows each knee to be described according to its distance (magnitude of deviation) and orientation (direction of deviation) relative to this reference. Coronal knee alignment phenotypes may be described relative to the COM of a population-specific aHKA-JLO distribution, rather than by fixed angular thresholds. This population-referenced framework enables quantitative definition of both the magnitude and direction of alignment deviation, thereby preserving the geometric context of the distribution. The potential influence on classification stability, measurement-related variability and clinical applicability has yet to be determined and necessitates formal validation in future studies. N/A.