Influence of Modic Changes on Lateral vs. Central Fusion Features in Stand-Alone Oblique Lumbar Interbody Fusion: A Retrospective Analysis of Outcomes, Imaging, and Histology.

Traditional lumbar interbody fusion evaluation emphasizes central bony bridging between endplates. However, Extra-Vertebral Bony Bridge (EVB) formation extending beyond the epiphyseal ring is frequently observed after Stand-Alone Oblique Lumbar Interbody Fusion (SA-OLIF), particularly in segments with Modic changes (MCs). This study aims to characterize this unique fusion pattern and investigate the underlying biological mechanisms. A retrospective review of SA-OLIF procedures (July 2017-May 2023) was conducted. Surgical levels were stratified by the presence of MCs. Fusion characteristics, cage subsidence, and clinical outcomes (ODI, VAS) were compared. Annulus fibrosus samples underwent histology, immunohistochemistry, and RT-qPCR to evaluate osteogenic activity. Of 132 patients initially reviewed, 105 met the inclusion criteria, and 98 patients (147 segments) were included in the final analysis. MCs were present in 63 segments. Overall lateral fusion rate was 50.3%, higher in MCs (61.9%) versus non-MCs (41.7%) (P = 0.015), with earlier fusion at 3 months (P < 0.001). Lateral fusion correlated with preoperative osteophyte (P < 0.001) and high endplate HU (P < 0.001). Cage subsidence rates differed between MCs and non-MCs (P = 0.018), and between fusion types (P < 0.001). ODI and VAS scores improved significantly at all postoperative time points (P < 0.001). MCs showed increased BMP-2 and OCN expression, and elevated mRNA levels of OCN, BMP-2, ALP, RUNX2, OPG, COL1A1, and FN1. SA-OLIF provides favorable outcomes with a high lateral fusion rate, particularly in segments with MCs. The early and frequent lateral fusion may be mediated by osteogenic activity in the annulus fibrosus, supporting SA-OLIF as an effective option for patients with normal BMD and MCs.