A Functional-Aggregate Method for Repeatable Determination of Vertebral Coordinate Systems

During in vitro spine testing, local vertebral coordinate systems (CSs) must be established to measure intervertebral motion. Anatomical CS rely on landmark collection from individual vertebrae, which are subject to observer variability and not easily reproduced. This study aims to present a novel functional-aggregate method for establishing reproducible local CS in the spine, where axis orientations are derived from an aggregate of anatomical landmarks across the whole spine, and origins are calculated as a function of kinematics data obtained during pure-moment loading. In this study, three observers collected anatomical landmarks on a full spine model (C2-L5), and observer variation in vertebral CS axis orientation was calculated for anatomical and aggregate CS groups. Variation in axis orientation significantly reduced with the aggregate method for lateral bending (LB), axial rotation (AR), and flexion/extension (FE) axes. To understand the effects CS definitions have on kinematic response variability, three observers collected anatomical landmarks on three cadaveric lumbar spines (T12-L5), and each specimen underwent 18 loading conditions, where intervertebral kinematics were calculated using anatomical and functional-aggregate CS. Observer variation in kinematic response was calculated. Average kinematic variation significantly reduced with functional-aggregate CS, compared to anatomical, by 1.22 deg ± 0.72 deg, 0.97 deg ± 0.53 deg, and 1.20 deg ± 1.26 deg, in LB, AR, and FE, respectively. The functional-aggregate method for defining CS in the spine is less influenced by observer differences in collecting anatomical landmarks and provides a more reproducible solution than traditional approaches.