Under the assumption that the pelvic incidence influences the development of an abnormal curvature, this study examined the relationship between the pelvic incidence and the development of an abnormal curvature.

Twenty-two adults with a normal curvature and seventy-nine adults with an abnormal curvature were enrolled in this study. All subjects were older than fifty years of age at the time of the study. The patients were classified based on the alignment of the spine and the strategic vertebrae as follows: extension of the lordosis of the lumbar spine to the upper part of the thoracolumbar transitional portion (Group 1), kyphosis on the lower lumbar area but an extension of the lordosis from the upper lumbar area to the upper part of the thoracolumbar transitional portion (Group 2), extension of the kyphosis from the thoracic spine to the lower lumbar area (Group 3), the presence of lordosis on the lower lumbar portion but an extension of the kyphosis from the upper lumbar portion to the upper portion of the thoracolumbar area (Group 4). In each group, the interrelationship between the sagittal alignment, pelvic tilt, sacral slope and pelvic incidence were evaluated, and the changes in the numerical figures were compared and analyzed.

In the normal group the average pelvic incidence was 52.5 °. The average pelvic incidence was higher in groups 1 and 2 (61.6 and 58.5 °, respectively) than in groups 3 and 4 (44.5 and 47.2 °, respectively) (p<0.001). The average sacral slope was 31.5 in the normal group, 34.5 in group 1, 25.1 in group 4, 20.1 in group 2, and 9.2 in group 3. In groups 1 and 2, group 2 showed a lower sacral slope compared with group 1 (p<0.001). In groups 3 and 4, group 4 had a larger sacral slope than group 3 (p<0.001). The average pelvic tilt was 21 in the normal group. Groups 2 and 3 showed a larger pelvic tilt (38.4 and 35.3, respectively) than that of groups 1 and 4 (27.1 and 22.1, respectively).

In the elderly with an abnormal lordotic curve, the sagittal alignment of the spine is associated with the pelvic tilt, sacral slope and pelvic incidence, in a similar manner to that in normal adults with a normal lordotic curve. Therefore, measuring these figureswould help in making an estimate of the perspective changes in the sagittal alignment of spine.