Flexion-extension x-rays are almost exclusively employed to analyze cervical spine kinematics. Yet, 2Dradiographic measures of key vertebral metrics used to evaluate cervical stability are limited by x-ray source beamdivergence, magnification errors and off-axis image acquisition. Three-dimensional CT images can be used to accuratelymeasure these parameters, however flexion-extension CT scans are rarely acquired and do not provide information on thekinematics of the loaded cervical spine. This study evaluates the ability of an open source 2D-3D intensity-based imageregistration algorithm (xSePT) to create accurate flexion-extension 3D CT quality data from 2D flexion-extensionradiographs and a single neutral 3D CT scan. Off axis 2D digitally rendered radiographs (DRRs) were generated from aset of ‘gold standard’ flexion-extension CT images of a single patient. The xSePT algorithm used the DRRs and a neutralCT to generate 3D flexion-extension CT images. Cervical vertebral metrics of subluxation, intervertebral disc height andinterspinous process gap distance were compared between the ‘gold standard’ flexion-extension CT images, the 2Dflexion-extension DRRs and the 3D output of the xSePT algorithm. The xSePT 2D-3D registration tool successfullyaligned to the original flexion-extension CT data within 1° rotation and 0.5mm translation. The algorithm rapidlycalculated values for the vertebral metrics equivalent to those based on the original flexion-extension CT data. Futureevaluation of cervical pathology and kinematics under load may be possible through the application of this algorithm togenerate 3D loaded flexion-extension data based on 2D standing flexion-extension x-rays and a single neutral unloadedCT scan.
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