Extraforaminal entrapment of the fifth lumbar spinal nerve by nearthrosis in patients with lumbosacral transitional vertebrae.

Lumbosacral transitional vertebrae (LSTV) often have nearthrosis between the L5 transverse processes and the sacral ala; this causes the formation of new bone and synovial-like tissue, which can entrap L5 nerve root. The present study aimed to examine the role of nearthrosis in L5 nerve root compression in patients with LSTV. From 2008 to 2018, 65 patients were surgically treated for severe leg pain/numbness caused by L5 extraforaminal stenosis. The patients were assessed regarding the presence of LSTV, radiographic features of nearthrosis, operative/radiological findings, and clinical outcomes. CT/MRI were used to classify the patients into three groups: group A had L5 nerve root compression that was not related to nearthrosis, group B had L5 nerve root impingement due to nearthrosis with new bone formation, and group C had L5 nerve root impingement due to nearthrosis with synovial-like tissue. The relationships between the type of LSTV (based on the Castellvi's classification) and these three groups were investigated. Although 26 of 65 patients had LSTV (40%), four were excluded because of less than 1-year follow-up. The 22 patients with LSTV were classified as type IA (n = 2), IIA (n = 13), and IIB (n = 7). In accordance with the radiological findings, there were eight patients in group A, six in group B, and eight in group C; the LSTV morphology did not significantly differ between groups. L5 nerve root was compressed by nearthrosis in 64% of symptomatic patients with LSTV; this region should be carefully assessed in all symptomatic patients with LSTV.