Computer analysis of spinal segment motion in degenerative disc disease with and without axial loading.

The center of rotation of the spinal motion segment is an indicator of spinal instability. However, the motion segment does not move about a single fixed axis of rotation, but rather a locus of instantaneous axes of rotation, the centrode. This paper studies centrodes in various stages of degenerative disc disease comparing them with normal spines. Computer analysis is undertaken in 47 cadaveric spines, 22 of which also were evaluated with axial loading. The normal centrode fell within the posterior half of the disc space and averaged 21 mm in ten specimens. In the earliest stages of degenerative disc disease, the centrode lengths increased significantly (average 116 mm). Specimens with moderate disc degeneration also migrated inferiorly into the L5 vertebra. Axial loading did not appear to influence the centrode lengths or position. This technique is highly sensitive, detecting 94% of the abnormal spines as compared with only 25% detected by means of measuring an excessive range of motion on flexion and extension radiographs. This method provides a highly reliable and quantifiable method of detecting early changes in spinal motion in degenerative disc disease prior to the well recognized radiographic abnormalities.