Sagittal Alignment of the Spine and Pelvis in the Presence of L5-S1 Isthmic Spondylolysis and Low-Grade Spondylolisthesis

• a - Medtronic Sofamor Danek, Spinal Deformity Study Group - Spondylolisthesis Section

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

Methods: Standing long cassette PA and lateral spine radiographs from 82 consecutive patients with spondylolysis or low-grade spondylolisthesis (Average age 19, range 15-44) were retrospectively compared with those from 160 normal volunteers. The films were digitized with a VIDAR scanner and key landmarks were determined. Customized software was then used to determine the data listed in Table 1. In addition, PI, SS, PT, and L5-S1ext were compared between 72 patients with high PI (>45º) versus 10 patients with low PI (<45 º). Average high-PI vs. low-PI values were, respectively: PI (67.32º 43.13º), SS (51.08º 38.05º), PT (16.23º 5.08º), and L5-S1ext (-8.69º -9.57º). Furthermore, the range of for in subgroup was much narrower (-17.81º to 0.93º) than that (-31.58º 38.12º).

Summary: 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.

Discussion: These results document that patients with spondylolysis or low-grade spondylolisthesis have higher Pelvic and L5 incidence than radiographically normal patients. High Pelvic Incidence (>45°) and vertical orientation of the sacral endplate increase the shear forces acting across the L5-S1 intervertebral disc, resulting in increased traction on the L5 pars. If a pars defect is already present, this vertical orientation of L5- S1 may increase the likelihood of anterior L5 displacement. In contrast, when PI is low (<45 °) and L5-S1 is more horizontally oriented, the L5 pars experiences less tension due to shear. Spondylolysis in this setting may result from a “nut-cracker” effect wherein articular processes of L4 S1 repetitively impinge on extension. This concept supported by observation that segmental lordosis (L5-S1ext) patients with PI <45° had much narrower range compared for>45°. Thus, we speculate that the mechanism underlying the evolution of isthmic spondylolysis may depend on the magnitude of the Pelvic Incidence. Accordingly, native sagittal alignment may influence whether or not other risk factors such as increased slip angle, female sex, or early onset of symptoms will contribute to development of spondylolysis and progression of spondylolisthesis.

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