Observational Comparative Study for Surgical Outcomes of One- or Two-Level Lumbar Fusion Surgery Between Transforaminal Lumbar Interbody Fusion and Lateral Lumbar Interbody Fusion

<b>Background/Objectives:</b> Transforaminal lumbar interbody fusion (TLIF) and lateral lumbar interbody fusion (LLIF) are widely utilized techniques for degenerative lumbar diseases. This study compared radiological and clinical outcomes of LLIF and TLIF in patients undergoing lumbar fusion. <b>Methods:</b> This non-randomized prospective observational study enrolled 117 patients (LLIF: n = 17; TLIF: n = 100), with an inherent imbalance in group sizes, who underwent one- or two-level lumbar interbody fusion. Primary outcome was segmental lordotic angle (SLA) at the operated level. Secondary outcomes included disc height, lumbar lordotic angle, sagittal vertical axis, and patient-reported outcomes. Assessments were conducted at baseline, 3, 6, 12, and 24 months. Linear mixed models analyzed longitudinal data. <b>Results:</b> Mean SLA improvement was not significantly different between the groups (LLIF: 3.04° vs. TLIF: 3.18°, <i>p</i> = 0.782). No significant differences were observed for disc height (<i>p</i> = 0.518), lumbar lordotic angle (<i>p</i> = 0.718), or sagittal vertical axis (<i>p</i> = 0.866). Patient-reported outcomes improved significantly in both groups. Linear mixed model analysis revealed no significant between-group effects for Oswestry Disability Index (<i>p</i> = 0.335) or low back pain (<i>p</i> = 0.069). TLIF showed higher rates of dural tears and wound complications, while LLIF had more sympathetic chain injuries and transient psoas weakness. Overall complication rates were comparable (<i>p</i> > 0.05). <b>Conclusions:</b> TLIF and LLIF demonstrate comparable radiographic and clinical outcomes at 24-month follow-up. Surgical technique selection should be individualized based on patient-specific anatomical and clinical factors, considering distinct approach-specific complication patterns.