Influence of psoas major morphology on clinical outcomes following L4-5 oblique lumbar interbody fusion: a retrospective study of 191 patients.

BACKGROUND: Oblique lumbar interbody fusion (OLIF) has emerged as a widely adopted minimally invasive surgical technique in recent years. However, the relationship between the positional characteristics and morphological configuration of the psoas major (PM) relative to the vertebral body and the specific influence of psoas major morphology on surgical outcomes remains incompletely characterized. This study aims to investigate the impact of the elevated and apposed PM on the efficacy of OLIF. METHODS: A retrospective analysis was conducted on 191 consecutive patients who underwent L4-5 OLIF between January 2022 and January 2025. All the patients had a minimum follow-up duration of 12 months. A novel parameter, the interior elevation (IE), was introduced. Based on the IE index, patients were divided into two groups: the elevated type and the apposed type. The relevant key parameters in accordance with the expressions below: The operative window (OW), psoas major sagittal diameter (PMSD), major transverse diameter (PMTD), psoas major cross-sectional area (PMCSA), psoas major hematoma area (PMHA), central spinal canal area (CSCA) were compared. Binary logistic regression analysis was conducted to identify imaging-related predictors of clinical outcomes following OLIF. RESULT: This study enrolled a total of 191 patients, all of whom were followed up for a 1-year period. Patients with the apposed-type PM demonstrated a larger post-operative PMCSA (1448.52 ± 269.97 vs. 1242.42 ± 331.01, p = 0.007), a higher degree of PM swelling percentage (38.42 ± 7.11% vs. 14.38 ± 4.70%, p = 0.005) and a higher incidence of transient post-operative complications (20.80% vs. 9.50%, p = 0.012). Moreover, these patients exhibited higher VAS scores at 1 week post-operatively (VASL: 4.71 ± 0.81 vs. 3.82 ± 0.75, p < 0.001), (VASB: 4.32 ± 0.47 vs. 3.67 ± 0.80, p = 0.024). In the Univariate regression analysis of multiple variables, the incidence of transient post-operative complications was significantly associated with OW (odds ratio, 0.641; p = 0.004) and IE (odds ratio, 0.882; p < 0.001). CONCLUSION: We propose a novel classification method for the psoas major muscle based on IE. The apposed-type PM, characterized by a smaller IE and a narrower OW, is associated with a higher incidence of post-operative complications.