Multifidus fatty infiltration and vertebral translation are key factors associated with movement-related low back pain in L5 isthmic spondylolisthesis

INTRODUCTION: In isthmic lumbar spondylolisthesis (ILS), persistent pain is a major indication for surgical intervention, yet it correlates poorly with vertebral slippage. Therefore, this study analyzes the influence of translation, segment degeneration and paraspinal muscle morphology on pain in patients with isolated ILS at the L5 level. METHODS: This cross-sectional study retrospectively enrolled 78 patients (46 females, 32 males). Fatty infiltration (FI) of the multifidus (MF) and erector spinae (ES) muscles, as well as the psoas index, were assessed. Segmental degeneration was graded for intervertebral discs and facet joints. Relative vertebral translation was assessed using standing and dynamic radiographs. Pain intensity at rest and during movement was measured using a Numeric Rating Scale (NRS). Multivariable linear regression models were adjusted for age, sex, and body mass index. RESULTS: Median pain at rest was 3 (interquartile range [IQR]: 2-6), and median pain during movement was 6 (IQR: 4.25-7) on the NRS. After adjusting for demographic confounders, pain during movement was significantly associated with FI of the MF (η² = 0.060, p = 0.026) and vertebral translation (η² = 0.054, p = 0.037). No significant correlations were found for ES, psoas index, disc or facet joint degeneration, dynamic translation, or pain at rest. DISCUSSION: Movement-related pain in L5 ILS appears to be primarily associated with FI of MF and translation, rather than with disc or facet pathology, indicating a potential relationship with segmental stability. FI may represent a key parameter for individualized treatment planning.