Measurement of lumbar spinal flexion-extension kinematics from lateral radiographs: simulation of the effects of out-of-plane movement and errors in reference point placement.

Flexion-extension of the spine is defined in the sagittal plane, which divides the body into right and left halves, and can be measured from lateral radiographs, including those obtained by videofluoroscopy. The effects of axial rotation and lateral bending, which lead to movement out of the sagittal plane, were investigated by generating a three-dimensional computer model of two adjacent vertebrae and projecting it on to the sagittal plane. The projected model was measured as if it were a radiograph, allowing the effects of out-of-plane movement and errors in reference point placement to be calculated. Reference points were defined as the corners of a quadrilateral which fully enclosed the two-dimensional image of each vertebral body, and their projected locations were randomly displaced by 0.5 mm in order to simulate the measurement process. A combination of lateral bending with 1 degree of flexion can lead to a root mean squared (RMS) variation in the calculated position of the instantaneous centre of rotation (ICR) of 292 mm, reducing to 5 mm for 10 degrees of flexion. When the reference point quadrilaterals were divided into two triangles, the RMS variation in the calculated position of the centroid of one of the triangles was only 0.42 mm under the same conditions, remaining constant over the same flexion range. Similar results were obtained when axial rotation was combined with flexion. The calculated centroid provides a robust reference point for kinematic calculations.