Patient population

Between January 2013 and October 2015, 217 patients (88 males and 129 females) were included in this study. All of the included patients had pre-operative anteroposterior and lateral films, CT scans, and MR images. L5 nerve root was compressed at L4–5 lateral access, and the symptoms were identified at L5 nerve root, needed to be performed L4–5 decompression and interbody fusion. However, parts of them were suspected of having foraminal stenosis of L5–S1, which also compressed the L5 nerve root, and cannot be pre-operatively differentiated if the symptoms were caused by the compression at L4–5 lateral access (single site) or both L4–5 lateral access and foraminal stenosis of L5–S1 (double sites). The foraminal stenosis of L5–S1 was assessed according to the Lee et al. classification (14) (Figure 1):

Schematic diagrams illustrating Lee et al. classification to assess the foraminal stenosis of L5–S1. (A) Grade 0: normal; (B) grade 1: mild degree of foraminal stenosis, perineural tissue obliteration surrounding nerve root in transverse direction, without the morphologic change of the nerve root; (C) grade 1: mild degree of foraminal stenosis, perineural tissue obliteration surrounding nerve root in vertical direction, without the morphologic change of the nerve root; (D) grade 2: moderate degree of foraminal stenosis, perineural tissue obliteration surrounding nerve root in both transverse and vertical direction, without the morphologic change of the nerve root; (E) grade 3: severe degree of foraminal stenosis, nerve root collapse or morphologic change due to severe perineural tissue obliteration surrounding the nerve root. VB, vertebral body; IVD, intervertebral disc; NR, nerve root; FL, flavum ligamentum; FJ, facet joint.

Grade 0: normal;

Grade 1: mild degree of foraminal stenosis, perineural tissue obliteration surrounding nerve root in the transverse direction or vertical direction, without the morphologic change of the nerve root;

Grade 2: moderate degree of foraminal stenosis, perineural tissue obliteration surrounding nerve root in both transverse and vertical direction, without the morphologic change of the nerve root;

Grade 3: severe degree of foraminal stenosis, nerve root collapse or morphologic change due to severe perineural tissue obliteration surrounding the nerve root.

All of the pre-operative MRIs were assessed by two independent senior spine surgeons. If they had any disagreement, the image will be discussed at the group research meeting.