A local and systemic path in L4 degenerative spondylolisthesis: independent roles of paraspinal muscle fat infiltration and dyslipidemia.

PURPOSE: To investigate the associations of paraspinal muscle fat infiltration (FI) and dyslipidemia with slippage severity in L4 degenerative spondylolisthesis (DLS). METHODS: This retrospective cross-sectional study analyzed 151 patients with L4 DLS. The slip rate was measured on lateral radiographs. FI of the multifidus (MF), erector spinae (ES), and psoas major muscles (PS) at L3/4-L5/S1 levels was quantified using T2-weighted MRI. A Comprehensive Fat Infiltration Index (FI-C), representing the average FI across all three muscles and levels, was calculated. Fasting blood lipids were collected and lumbar disc degeneration was graded. Associations were assessed using Spearman’s correlation, beta regression, mediation, and sex interaction analyses. RESULTS: Spearman correlation analysis showed significant positive associations between total paraspinal FI and slip rate at all lumbar levels (all p < 0.001). In multivariable beta regression, total paraspinal FI (β = 2.79, p < 0.001), high total cholesterol (≥ 6.2 mmol/L; β = 0.20, p = 0.048), and severe disc degeneration (β = 0.34, p = 0.003) were independent predictors of higher slip rate. Mediation analysis showed that the effect of total cholesterol on slip rate was not mediated through muscle FI or disc degeneration (all indirect effects p > 0.05). Sex interaction analysis revealed that PS FI was associated with slip rate in females (slope = 0.36, p = 0.008) but not in males, suggesting a potential sex-specific role. CONCLUSION: A dual-pathway model is supported, wherein paraspinal muscle FI serves as a local factor and dyslipidemia as a systemic factor for L4 DLS severity. Dyslipidemia does not drive DLS slippage via disc or muscle FI changes, suggesting alternative mechanisms. A potential sex-specific role of PS FI in females suggests that greater attention to this muscle group may be warranted in clinical assessment of DLS.