Beyond the pars defect: multifidus degeneration as an independent predictor of vertebral slippage severity in isthmic lumbar L5 spondylolisthesis.

INTRODUCTION: Isthmic lumbar spondylolisthesis (ILS) results from a defect of the pars interarticularis, leading to anterior vertebral displacement. We hypothesize that paraspinal muscle atrophy contributes to the degree of vertebral slippage in symptomatic ILS. METHODS: This retrospective cross-sectional study included 78 patients (46 females, 32 males). Fat infiltration (FI) and height-adjusted functional cross-sectional area (HI) of the multifidus (MF), erector spinae (ES), and psoas (PS) muscles were quantified applying the Otsu thresholding method. Multivariable linear regression was used to analysis the association between the paraspinal musculature and vertebral slippage. Confounding variables such as sex, age, body mass index (BMI), disc (Pfirrmann classification) and facet joints degeneration (Weishaupt classification) were included. RESULTS: The median age was 48.5 (interquartile range [IQR]: 31.5–55) years, median BMI was 26.4 (IQR: 22.8–29.4) kg/m², and the median translation was 33 (IQR: 24–47) %. After adjustment for demographic confounders and degeneration, the MF atrophy showed significant association with vertebral slippage (FI: η² = 0.164, p < 0.001; HI: η² = 0.052, p = 0.028). In contrast, no significant associations were observed for the ES (FI: η² = 0.012, p = 0.315; HI: η² = 0.000, p = 0.936) or PS (FI: η² = 0.001, p = 0.742; HI: η² = 0.000, p = 0.979) muscles. CONCLUSION: The degree of vertebral slippage in isthmic lumbar spondylolisthesis was specifically associated with multifidus atrophy, while other paraspinal muscles showed no such relationship. These results support the distinct biomechanical role of the multifidus in segmental stability.