Factors affecting dynamic foraminal stenosis in the lumbar spine

Background contextLumbar foraminal stenosis is a common clinical problem and a significant cause of lower extremity radiculopathy. Minimal in vivo data exists quantifying changes in foraminal area (FA) as the spine moves from flexion to extension in the lumbar spine or on the relationship between FA and lumbar segmental angular motion, translational motion (TM), or disc bulge migration.PurposeTo use kinetic magnetic resonance imaging (kMRI) to evaluate changes in dimensions of lumbar neural foramina during weight bearing in neutral, flexion, and extension positions. To evaluate the relationship between foraminal stenosis and lumbar segmental angular motion, TM, and disc bulge migration.Study designA retrospective radiographic study.Patient sampleForty-five patients with a mean age of 44 years undergoing kMRI for symptoms of low back pain or radiculopathy.Outcome measuresMagnetic resonance imaging measurements of FA, angular motion, TM, and disc bulge migration.MethodsKinetic magnetic resonance imaging of the lumbar spine was reviewed in 45 patients with low back pain or radiculopathy, and parasagittal images were evaluated for changes in neural foraminal dimensions in various degrees of motion with weight bearing. The changes in foraminal dimension were correlated to the amount of segmental angular motion, TM, and disc bulge migration at each level. Neural foramina were also assessed qualitatively by Wildermuth criteria. Only those foramina that were clearly visualized with well-defined anatomic boundaries in all three positions were taken into consideration. Patients with previous surgery, tumor, and scoliosis were excluded from the study.ResultsThere was a significant decrease in the FA from flexion to neutral (p<.05) at all levels except L5–S1 and from neutral to extension at all levels (p<.05). The average percent decrease in FA was 30.0% with the greatest decrease from flexion to extension occurring at L2–L3 (167–107 mm2) and the smallest change occurring at L5–S1 (135–106 mm2) (p<.05). The magnitude of change in FA increased as angular motion at a segment increased. The mean change in FA was 32.3 mm2 when angular motion was less than 5° and was 75.16 mm2 when angular motion exceeded 15°. The extent of disc bulging posteriorly in the neural foramen was also correlated with the reduction in the FA from flexion to extension, but TM had no effect.ConclusionsForaminal area decreased significantly in extension compared with flexion and neutral on MRI. Lumbar disc bulge migration and angular motion at each level contributed independently to the decrease in FA in extension, whereas TM had no effect on FA.