The biomechanical definition of spinal segmental instability has not been clarified sufficiently, because of the great personal and level differences 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 relationship between the disc geometry and the disc deformation was confirmed in the normal intervertebral discs. The degree of spinal segmental instability could be evaluated statistically by calculating a regression residual from the regression line of the normal discs. Strain distributions of an intervertebral disc in the sagittal plane also were investigated to make sure of the differences in the deformation behaviors between normal spines and unstable lumbar spines. Although normal lumbar spines showed the consistent deformation pattern, the patterns of the unstable lumbar spines were different.