Loss of regularity in the curvature of the thoracolumbar spine: a measure of structural failure.

Abstract

Departure from regularity (smoothness) in the curvature of the spine was quantified and correlated with the number of fractures, deficits in height, BMD, and identified women with vertebral fractures.Differences in anterior and posterior vertebral heights (VHs) form the thoracolumbar curvature needed for stability in bipedal gait. Modest differences in VHs within and between adjacent vertebrae allow the spine curve to change its trajectory gently. Large differences in VHs, as occur following a fracture, produce abrupt changes in the direction of the curve, producing a departure from regularity (i.e., irregularity or loss of smoothness).VHs and BMD were measured using DXA in 697 Lebanese women 20-87 years of age. Regularity of the spinal curvature was measured by comparing the ratio of the anterior to the posterior VHs of one vertebra to this ratio of adjacent vertebrae. If these ratios are similar, there is a smooth transition in the trajectory of the spinal curve. Departure from this regularity (smoothness) was measured at each pair of adjacent vertebrae in each individual and expressed as the spinal curvature irregularity index (SCII) for the entire thoracolumbar spine.In premenopausal women, the mean SCII was 8.5% (range, 4-15%); that is, regularity was 91.5%. Only 0.8% of women had a SCII >17%. In postmenopausal women, the mean SCII was 10% (range, 4-36%) and was correlated with age (r = 0.25), height (r = -0.21), BMD (r = -0.13), and the number of deformities assessed by quantitative vertebral morphometry (QVM; r = 0.31-0.60; all p < 0.001). About 5% of women had an SCII >17%, and this group had 3- to 9-fold more deformities (as defined by QVM) than women with SCII <17%, reduced lumbar spine BMD (-1.01 SD), and 2- to 4-fold greater height deficits (-0.5 SD) than women with deformities (by QVM). The SCII is a robust method of identifying structural failure that is easy to compute and does not require controls.