Adequate cage placement for a satisfactory outcome after lumbar lateral interbody fusion with MRI and CT analysis

Introduction: Through an extreme lateral retroperitoneal and transpsoas approach to intervertebral disc and fusion surgery, a large lordosis cage can be placed for solid and stable intervertebral fusion and to provide strong anterior support, disc height restoration, favorable alignment, and indirect nerve decompression. However, appropriate placement of the inter-body cage remains insufficiently researched. We sought to determine both appropriate cage placement as well as other factors affecting nerve decompression in extreme lateral interbody fusion (XLIF) surgery. Methods: We included 53 consecutive patients suffering from lumbar degenerative diseases with an indication for XLIF. Radiographic analysis using a sagittal computed tomography (CT) and axial magnetic resonance imaging (MRI) views was conducted to determine intervertebral disc height and angle, degree of disc bulging and thickness of the flavum, the area of the dural tube, cage height, pre-and postoperative disc bulging, change of disc bulging after surgery, cage subsidence, and cage placement at the rostral and caudal endplates. Results: Intervertebral disc height and angle were significantly increased at all levels (L2/3, 3/4, 4/5) (p < 0.05). The area of the dural tube was significantly increased (p < 0.05), whereas the degree of disc bulging and thickness of the flavum were significantly decreased at all disc levels (p < 0.05). The enlarged area of the dural tube showed significant correlation with increased disc height (p = 0.019), preoperative flavum thickness (p = 0.008), change of flavum thickness (p < 0.0001), and cage placement at the rostral endplate (p = 0.014). Conclusions: A decrease in flavum buckling is more important than disc protrusion as a consideration for obtaining indirect decompression. Central placement may be advantageous for indirect decompression.