The Morphometric Analysis of the Extraforamen in the Lumbosacral Spine: Magnetic Resonance Imaging and Computed Tomography Study

Objective: The goal of this study is to establish the anatomical criteria of the normal and stenotic lumbosacral extraforaminal tunnel, and also to determine the effect of the pathologic intervertebral disc on the size of extraforaminal tunnel in the lumbosacral spine. Methods: MRI and CT scans were reviewed and classified into two groups: (1) 40 patients with normal discs at L5- S1 (Group 1) and (2) 43 patients that had undergone successful decompression surgery for extraforaminal entrapment at the lumbosacral region(Group 2). In these two groups, the following parameters were compared are compared: the distance between the disc margin and the ala (lumbosacral tunnel) on the axial MRI, and the posterior disc height at L5-S1 on the mid-sagittal MRI. Results: In the group 1, the mean distance of the lumbosacral tunnel on the axial MRI was $10.1{\pm}2.2mm$. The mean posterior disc height at L5-S1 was $7.4{\pm}1.7mm$ on the mid-sagittal MRI. In the group 2, the mean distance between the disc margin and the ala (costal process) was $1.6{\pm}1.3mm$ on the axial MRI. The average posterior disc height was $4.4{\pm}1.5mm$ on the mid-sagittal MRI. The posterior disc height and the size of the lumbosacral tunnel between the two groups were statistically different on the paired t-test (p<0.0001). However, the posterior disc height was not positively correlated with the size of the extraforaminal tunnel for group 2 (p=0.909). Conclusion: The extraforaminal stenosis was correlated to pathologic disc. However, the posterior disc height was not correlated to the size of the of the extraforaminal tunnel.