A BIOMECHANICAL DEFINITION OF SPINAL SEGMENTAL INSTABILITY TAKING PERSONAL AND DISC LEVEL DIFFERENCES INTO ACCOUNT

The biomechanical definition of spinal segmental instability has not b een clarified sufficiently, because of the great personal and level di fferences in intervertebral disc deformation. This article proposes a new method of judging spinal segmental instability regardless of these differences, using lateral functional radiographs. The linear relatio nship between the disc geometry and the disc deformation was confirmed in the normal intervertebral discs. The degree of spinal segmental in stability could be evaluated statistically by calculating a regression residual from the regression line of the normal discs. Strain distrib utions of an intervertebral disc in the sagittal plane also were inves tigated to make sure of the differences in the deformation behaviors b etween normal spines and unstable lumbar spines. Although normal lumba r spines showed the consistent deformation pattern, the patterns of th e unstable lumbar spines were different.