Quantification of lumbar intradiscal deformation during flexion and extension, by mathematical analysis of magnetic resonance imaging pixel intensity profiles

Study Design. A magnetic resonance imaging study of the internal kinematic response of normal lumbar intervertebral discs to non-weight-bearing flexion and extension. Objectives, To quantify the pattern of magnetic resonance imaging pixel intensity variation across discs, and noninvasively monitor displacement of the nucleus pulposus during sagittal-plane movements. Summary of Background Data, Invasive techniques used to study intradiscal movements of the nucleus pulposus have suggested that it moves posteriorly during flexion and anteriorly during extension. A noninvasive study based on magnetic resonance images gave similar results for normal young women. Quantification has been problematic, and the invasive procedures may have altered disc dynamics. Methods. Ten male subjects (age, 21-38 years) with healthy backs were positioned in a magnetic resonance imaging portal with their lumbar spine stabilized in flexion and extension by supporting pads. For each disc, a T2-weighted image was obtained, as was a computer-generated profile of pixel intensities along a horizontal mid-discal transect. Mathematical curve-fitting regression analysis was used to characterize the shape of the intensity profile and to compute the point of maximum pixel intensity. Results. A single equation fitted the profile for all normal discs. The intensity peak shifted posteriorly during flexion, anteriorly during extension. Conclusions, Automated mathematical modeling of magnetic resonance imaging pixel data can be used to describe the fundamental shape of the pixel intensity profile across a normal lumbar disc, to determine the precise location of the site of maximum pixel intensity, and to measure the movement of this peak with flexion and extension. This technique may be of value in recognizing incipient degenerative changes in lumbar discs.