This study using digitized radiographs and custom software demonstrates that patients with spondylolysis and low-grade spondylolisthesis have increased Pelvic and L5 Incidence as well as a more vertically oriented L5-S1 intervertebral disc than patients without radiographic abnormality of the spine. We propose that shear across the more vertical L5-S1 disc may underlie the etiology of spondylolysis when Pelvic Incidence is high, while a “nutcracker” mechanism may be involved when Pelvic Incidence is low.

The purpose of this study was to assess whether differences exist in sagittal alignment between normal controls and patients with spondylolysis or low-grade isthmic spondylolisthesis.

Standing PA and lateral spine radiographs from eighty-two consecutive patients with spondylolysis or low-grade spondylolisthesis (Average age nineteen, range 15–44) were retrospectively compared with those from one hundred and sixty normal volunteers. The films were digitized with a VIDAR scanner and key landmarks were determined. Customized software was then used to measure geometric indices. Pelvic Incidence (PI), Sacral Slope (SS), Pelvic Tilt (PT), and L5-S1extension angle were compared between seventy-two patients with high PI (> 45°) versus ten patients with low PI (< 45°). Average high-PI vs. low-PI values were, respectively: PI (67.32° vs. 43.13°), SS (51.08° vs. 38.05°), PT (16.23° vs. 5.08°), and L5-S1ext (−8.69° vs. −9.57°). Furthermore, the range of values for L5-S1extension in the low-PI subgroup was much narrower (−17.81° to 0.93°) than that for the high-PI subgroup (−31.58° to 38.12°).

This study demonstrates that patients with spondylolysis and low-grade spondylolisthesis have increased Pelvic and L5 incidence, a more vertically oriented L5-S1 intervertebral disc, and less segmental extension between L5 and S1 than patients without radiographic abnormality of the spine. We propose that different mechanisms underlie the etiology of spondylolysis depending on the magnitude of the Pelvic Incidence. These data highlight the importance of seeing localized lumbosacral spine disorders in the context of global alignment of the entire spine and pelvis.

Funding: This research was assisted by support from the Spinal Deformity Study Group

This research was funded by an educational/research grant from Medtronic Sofamor Danek