Diffusion tensor tractography of the lumbar nerves before a direct lateral transpsoas approach to treat degenerative lumbar scoliosis

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OBJECTIVE

The purpose of this study was to determine the relationship between vertebral bodies, psoas major morphology, and the course of lumbar nerve tracts using diffusion tensor imaging (DTI) before lateral interbody fusion (LIF) to treat spinal deformities.

METHODS

DTI findings in a group of 12 patients (all women, mean age 74.3 years) with degenerative lumbar scoliosis (DLS) were compared with those obtained in a matched control group of 10 patients (all women, mean age 69.8 years) with low-back pain but without scoliosis. A T2-weighted sagittal view was fused to tractography from L3 to L5 and separated into 6 zones (zone A, zones 1–4, and zone P) comprising equal quarters of the anteroposterior diameters, and anterior and posterior to the vertebral body, to determine the distribution of nerves at various intervertebral levels (L3–4, L4–5, and L5–S1). To determine psoas morphology, the authors examined images for a rising psoas sign at the level of L4–5, and the ratio of the anteroposterior diameter (AP) to the lateral diameter (lat), or AP/lat ratio, was calculated. They assessed the relationship between apical vertebrae, psoas major morphology, and the course of nerve tracts.

RESULTS

Although only 30% of patients in the control group showed a rising psoas sign, it was present in 100% of those in the DLS group. The psoas major was significantly extended on the concave side (AP/lat ratio: 2.1 concave side, 1.2 convex side). In 75% of patients in the DLS group, the apex of the curve was at L2 or higher (upper apex) and the psoas major was extended on the concave side. In the remaining 25%, the apex was at L3 or lower (lower apex) and the psoas major was extended on the convex side. Significant anterior shifts of lumbar nerves compared with controls were noted at each intervertebral level in patients with DLS. Nerves on the extended side of the psoas major were significantly shifted anteriorly. Nerve pathways on the convex side of the scoliotic curve were shifted posteriorly.

CONCLUSIONS

A significant anterior shift of lumbar nerves was noted at all intervertebral levels in patients with DLS in comparison with findings in controls. On the convex side, the nerves showed a posterior shift. In LIF, a convex approach is relatively safer than an approach from the concave side. Lumbar nerve course tracking with DTI is useful for assessing patients with DLS before LIF.

ABBREVIATIONS AP = anteroposterior; DLS = degenerative lumbar scoliosis; DTI = diffusion tensor imaging; DWI = diffusion-weighted imaging; FOV = field of view; lat = lateral; LIF = lateral interbody fusion; LL = lumbar lordosis; LS = lumbar scoliosis; PI = pelvic incidence; PT = pelvic tilt; SS = sacral slope; SVA = sagittal vertical axis; TK = thoracic kyphosis; VB = vertebral body; VRA = vertebral rotational angle.

Article Information

Correspondence Yawara Eguchi: Shimoshizu National Hospital, Chiba, Japan. yawara_eguchi@yahoo.co.jp.

INCLUDE WHEN CITING Published online January 25, 2019; DOI: 10.3171/2018.9.SPINE18834.

Y.E. and M.N. contributed equally to this work.

Disclosures The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

© AANS, except where prohibited by US copyright law.

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Figures

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    Vertebral body anteroposterior partition (A) and morphological assessment of the psoas major (B and C): (B) rising psoas sign at the L4–5 level, and (C) determination of the anteroposterior diameter (AP) to lateral diameter (lat, indicated by L in image) ratio (AP/lat ratio). Axial T2-weighted image of L4–5. The VB was divided into 4 zones of equal AP length within the VB itself, plus an anterior zone and a posterior zone, for a total of 6 zones (zone A, zones 1–4, zone P). A rising psoas sign was diagnosed if the most posterior aspect of the psoas major was anterior to a horizontal line defining the most posterior aspect of the disc or VB. If the AP diameter was more than twice the lateral diameter (AP/lat > 2), the psoas major was considered to be extended. Figure is available in color online only.

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    In the DLS group, the AP/lat ratio of the psoas major at the L4–5 level was significantly greater on the concave side than on the convex side (2.1 ± 0.6 vs 1.2 ± 0.6, respectively; p < 0.05). The mean values for the control group were 1.1 ± 0.2 on the right and 1.2 ± 0.2 on the left, with no significant right–left difference (p = 0.39). Figure is available in color online only.

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    Morphological assessment of the apical vertebrae and psoas major on coronal (A and C) and axial (B and D) T2-weighted MR images. As seen on the coronal images, the upper apex (apical vertebra at L2 or higher) accounted for 75%, and the psoas major was extended on the concave side (arrowheads). As seen on the axial images, the lower apex (apical vertebrae at L3 or lower) accounted for 25%, and the psoas major was extended on the convex side (arrowheads). Figure is available in color online only.

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    Graphs showing nerve distribution in healthy volunteers (A) and patients with DLS (D); fusion images of tractography and T2-weighted MRI obtained in a healthy volunteer (B and C) and a patient with DLS (E and F). (Lateral views are shown in B and E and AP views in C and F.) The entire nerve distributions from the L3–4 to the L5–S1 level on both sides were evaluated in patients in the DLS group and the control group. In the control group (A), at the L3–4 level, all nerves were in the posterior quarter of the VB (zone 4). At the L4–5 level, all nerves were posterior to the center of the VB, with 10% in zone 3 and 90% in zone 4 or zone P. At the L5–S1 level, approximately 15% were anterior to the center of the VB (3% in zone 1, 12% in zone 2). In the DLS group (D), at the L3–4 level, 17% of the nerves were in zone 3 (p = 0.012). At the L4–5 level, 11% of the nerves were in zone 2 (p = 0.014), and approximately 50% were anterior to the center of the VB (12% [p = 0.004] in zone A, 16% [p = 0.009] in zone 1, and 21% [p = 0.173] in zone 2). The lumbar nerves showed an anterior shift at all intervertebral levels compared with findings in the control group. Figure is available in color online only.

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    Nerve distribution at each vertebral level on the side of the extended and flexed psoas muscle in patients with DLS. A: L3–4 level. B: L4–5 level. C: L5–S1 level. A significant anterior shift of nerves in the psoas major extended side was noted at all vertebral levels. The x-axis indicates the zones, as described in Methods. The blue arrows indicate the significant difference of nerve distributions between groups. Figure is available in color online only.

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    Nerve distribution at each vertebral level on the concave and convex sides in patients with DLS. A: L3–4 level. B: L4–5 level. C: L5–S1 level. At the L3–4 level, nerves showed a significant posterior shift on the convex side of the scoliotic curve. Although the shift was not significant at the L4–5 or L5–S1 levels, we noted a tendency for the nerves to be in a more posterior position on the convex side at those levels as well. Figure is available in color online only.

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    Schematic illustrations of the course of the psoas major. A: Normal spinal alignment, sagittal view. B: Lumbar lordosis, sagittal view. In the DLS group, all patients showed a rising psoas sign. The psoas muscle had shifted anteriorly together with the lumbar nerves. C: Normal spinal alignment, coronal view. In the control patients, the psoas major course was symmetrical. D: DLS with apex at L2 or higher. The majority of our DLS patients (75%) had an upper apex, and in all of these patients, the psoas major was flexed on the convex side and nerves had shifted to the posterior. This is attributed to shortening of the distance between the L2 apical vertebrae and lesser trochanter near the origin of the psoas major. E: In the lower apex cases (25% of our DLS patients; apex at L3 or lower), we considered that the center of the psoas major tract corresponding to the L3 VB protruded, and the psoas major on the convex side was extended. The asterisk corresponds to the apical vertebrae. Arrows indicate the direction of extension. Figure is available in color online only.

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    Fusion of tractography and T2-weighted images obtained in a representative patient with DLS. A: Anteroposterior view. B: Right lateral view (convex side). C: Left lateral view (concave side). Nerves on the convex side shifted posteriorly, and in LIF, an approach from the convex side is considered to be relatively safer than an approach from the concave side. The arrows indicate the direction of the lumbar nerve shift. Figure is available in color online only.

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