Use of expandable lateral lumbar interbody devices to restore disc height and segmental lordosis: a cadaveric study

Lateral lumbar interbody fusion (LLIF) is a well-established and efficient technique for treating various thoracolumbar spine pathologies. However, in some scenarios, the amount of segmental lordosis achieved is insufficient. Two main strategies have been used to address this: disruption of the anterior longitudinal ligament (ALL) via anterior column realignment and, more recently, the use of expandable interbody cages that allow intraoperative adjustment of segmental lordosis without requiring ALL release.

Twelve expandable lateral interbody devices were placed in seven fresh-frozen cadaveric specimens in the prone position. Segmental lordosis (degrees), anterior disc height, and posterior disc height (both in arbitrary units normalized to the L4 vertebral body height) were assessed before and after cage expansion. Because of the small number of clusters, generalized estimating equations (GEE) with Mancl-DeRouen small-sample correction were used as the primary analysis; disc height ratios were modeled on the log scale and the lordosis angle on a Gaussian scale.

Seven specimens were included in the study. Two patients underwent three-level instrumentation, one underwent two-level instrumentation, and four underwent one-level instrumentation, totaling 12 instrumented levels. The mean segmental lordosis increased from 4.5° (± 4.3°) pre-expansion to 11.8° (± 4.2°) post-expansion. Small-sample-corrected GEE models confirmed significant postoperative improvements across all three outcomes: segmental lordosis increased by a mean of 7.25° (post-pre; 95% CI, 1.6-12.9; p = 0.020), posterior disc height by 61% (post/pre ratio, 1.61; 95% CI, 1.30-1.92; p < 0.0001), and anterior disc height by 122% (post/pre ratio, 2.22; 95% CI, 1.31-3.12; p = 0.001). Complete ALL rupture occurred in two of the 12 levels (17%).

Within the limitations of a small cadaveric feasibility study, the placement of an expandable lateral interbody device in the prone position was associated with immediate increases in segmental lordosis and both anterior and posterior disc heights. These findings should be considered exploratory and hypothesis-generating in nature. Because the study design cannot separate the effect of cage expansion from that of prone positioning and because no static cage control was included, adequately powered prospective clinical studies are required before any claim of superiority can be made.