Bilateral Minimally Invasive Transforaminal Interbody Fusion at L5/S1 – a Case Series of Patients with Lumbar Degenerative Disease

Introduction: Unilateral approaches are commonly employed for placement of interbody instrumentation during minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF). Compared to unilateral MIS-TLIF, bilateral interbody placement via a biforaminal approach has the theoretical advantage of allowing for improved lordotic corrections, mimicking a Schwab Grade 2 osteotomy, while decreasing subsidence risk through increased total interbody surface area. These features make the bilateral MIS-TLIF an attractive technique at the L5-S1 level in particular given its high physiologic segmental lordosis and large axial loading forces. However, this technique has not been described in the literature. We sought to address this knowledge gap by presenting a series of bilateral L5-S1 MIS-TLIFs.

Methods: We performed a retrospective review of bilateral MIS-TLIF surgeries performed at L5-S1 for lumbar degenerative disease by a single surgeon (AJC). All patients had at least 6-weeks follow up. The primary outcome variables were segmental lordosis at L5-S1, L4-S1, and L1-S1. For patients in whom both pre- and post-operative standing radiographs were available, C7 sagittal vertical axis (SVA) was also analyzed. Pre- and post-operative radiographic parameters were compared using the Wilcoxon signed rank test with an alpha level of 0.05 used to determine significance.

Results: Nine patients were included in the series. The mean increase in segmental lordosis was 4.9degrees at L5-S1 (p=0.004) and 6.1degrees across L4-S1 (p=0.004). L1-S1 lordosis (p=0.81) was not significantly affected by surgery. When considering the absolute difference in C7 SVA from 0cm, the reduction in post-operative SVA trended towards significance (pre-op absolute mean=7.4cm vs. post-op absolute mean=2.7cm, N=5, p=0.06) but statistical power was limited by incomplete radiographic data for some patients. No durotomy, nerve root injury, or interbody subsidence was observed.

Conclusion : Bilateral L5-S1 MIS-TLIF offers reliable lordotic correction of lower lumbar lordosis and may represent a useful technique for the MIS surgeon to avoid subsidence or iatrogenic flat back.