Background and Objectives This study was performed to ascertain whether the position of the intercrestal line changes as a result of flexion of the lumbar spine.

Methods Previously taken lumbar spine x-rays of 103 patients in the neutral and full-flexed positions were reviewed. In the lateral flexion images to compensate for the sagittal rotation of the pelvis during lumbar flexion and for the possible difference in the level between the two sides of the ilium when taking the lateral images, the intercrestal line was drawn as follows: a potential line, crossing the midpoint of the highest points of both iliums, should be perpendicular to the tangential line at the point of intersection of the potential line with the skin. The position of the intercrestal line in relation to the spinous process was determined on an imaginary line moved 1 cm toward the vertebral body from the tangential line on the 2 successive spinous processes, and the interspinous distance of L3-4 was measured on this imaginary line.

Results With full-flexion of the lumbar spine, the position of the intercrestal line in relation to the spinous process [median (25th to 75th percentiles)] changed slightly from L4 (L4-L4-5) into L4-5 (L4-L4-5) (P < .001), but it remained at the same level in 58.3% of the patients (60/103). In no case was a change of more than 1 level observed. The interspinous width (mean ± SD) of L3-4 increased from 6.5 ± 2.4 mm to 13.2 ± 4.4 mm (P < .001).

Conclusion When compared with the neutral position, the position of the intercrestal line usually does not change with full flexion of the lumbar spine, and even in cases in which change occurs, it does not move beyond the next level.