MRI evaluation of lumbar foraminal stenosis: correlation between a new quantitative evaluation and the qualitative Lee's classification.

PURPOSE

Lumbar foraminal stenosis (LFS) accounts 8-11% of leg radiculopathy cases, typically assessed by MRI with qualitative classifications and more recently, quantitative ones. However, a combined surface area assessment of the foramen and exiting nerve root is lacking. We aimed to correlate the Nerve root/Foramen ratio (N/F ratio), a novel quantitative assessment for LFS, with Lee's classification.

METHODS

We studied lumbar spine MRI images of 36 patients eligible for degenerative lumbar spine surgery. Inclusion criteria comprised T2-weighted 3D images (120-210 slices) from L1 to S1 without artifacts. LFS severity was assessed using the N/F ratio and Lee's classification. Image analysis utilized sagittal reconstructions of T2-weighted 3D axial sequences using 3D slicer software. Quantitative analysis of 360 foramens was conducted using manual segmentation to calculate the N/F ratio from the cross-sectional areas of the foramen and exiting nerve root (higher score indicates more severe LFS). Qualitative LFS analysis was based on Lee's classification (grade 0 to 3).

RESULTS

LFS severity (N/F ratio and Lee's grade) increased in the craniocaudal direction. Stratified by level and side, the N/F ratio was significantly correlated with Lee's grade, except for the right L2L3 foramen. Applying a linear mixed model, a positive and significant association was found between Lee's classification and the N/F ratio (β = 0.10 [95%CI: 0.09 ; 0.10]; p < 0.001).

CONCLUSION

The N/F ratio emerges as an objective quantitative measure for LFS correlated to Lee's classification, integrating evaluation of perineural intraforaminal elements and exiting nerve root. Future automating manual segmentation could facilitate rapid LFS evaluation in daily clinic.