Radiological Outcomes and Approach-Related Complications in Oblique Lateral Interbody Fusion at the Upper Lumbar Level

<b>Background/Objectives:</b> Despite recent advances in minimally invasive extrapleural lateral approaches, oblique lateral interbody fusion (OLIF) at the upper lumbar level is often difficult and limited to optimal reconstruction. We aimed to compare the radiological outcomes and approach-related complications of OLIF between the upper (L1-2 or L2-3) and lower (L3-4 or L4-5) levels. <b>Methods:</b> This study is a retrospective review of OLIF in the upper (n = 63) and lower (n = 60) lumbar level groups. Radiological parameters included the anterior/posterior disc height, coronal/sagittal disc angle, cage position, cage subsidence, and fusion rate at a postoperative 1-year follow-up. Approach-related complications including pleural/peritoneal lacerations, neurovascular injury, and other organ injuries were examined. <b>Results:</b> The baseline radiological parameters were similar between the two groups (all <i>p</i> > 0.05). At 1-year postoperatively, the anterior disc height (ADH) was significantly greater in the lower-level group (<i>p</i> = 0.031), while no significant differences were observed in the posterior disc height, coronal/sagittal disc angle, cage anterior position, or cage subsidence rate (all <i>p</i> > 0.05). The fusion rates were 97.9% and 95.0% at the upper and lower lumbar levels, respectively (<i>p</i> = 0.146). During OLIF at the upper lumbar level, chest tube insertion due to pleural laceration was observed in 11 (17.5%) cases. One case (1.2%) of segmental artery injury and two cases (3.2%) of pseudo-hernia were attributed to iliohypogastric nerve injury. <b>Conclusions:</b> Although the extrapleural approach in OLIF at the upper lumbar level is often limited, the radiological outcomes were comparable to those of OLIF at the lower lumbar level.