Abstract
BACKGROUND: Multifidus muscle may be reflexively inhibited by painful co-existing structural degenerative features, affecting spinal loading and control in patients with chronic low back pain (cLBP). This study aimed to investigate the relationship between magnetic resonance imaging (MRI) features of lumbar spine degeneration and their influence on clinical outcomes of multifidi neurostimulation for cLBP. METHODS: This is a secondary analysis of a Food and Drug Administration (FDA) randomized controlled trial testing the efficacy of an implantable neurostimulator targeting the L2 dorsal rami medial branch nerves innervating the deep multifidi in 204 subjects. Degenerative anatomical features of the lumbar discs, vertebrae, facets, and paraspinal muscles were graded using standardized criteria. Patient-reported outcomes (PROs), including visual analog scale (VAS), Oswestry Disability Index (ODI), and the five-level version of the EuroQol five-dimensional descriptive system (EQ-5D-5L), were collected at baseline, 12, and 24 months. Mixed-design analysis of variance (ANOVA) and post hoc pairwise t-tests were performed to evaluate changes in outcomes in relation to individual demographic variables and MRI features. RESULTS: Included 204 subjects [110 females, 94 males; age: mean ± standard deviation (SD) =47±9 years; body mass index (BMI): mean ± SD =28±4 kg/m(2)] at baseline had, on average, cLBP for 14±11 years, average VAS of 7.3±0.7 cm, ODI of 39.1±10.3 points, EQ-5D-5L of 0.585±0.174, and mean percentage of days with cLBP in the year before enrollment was 97%±8%. In comparing PRO responses with MRI features, there was no significant finding (P>0.05) explaining improvements in pain, disability, and quality of life for subjects with multifidus atrophy and fatty infiltration, facet arthropathy, disc herniation, annular tears, Modic changes, or lumbar degenerative disc disease. Mixed-design ANOVA revealed statistically significant pre- to post-implantation differences for VAS and ODI for patients with stable, grade I spondylolisthesis (P=0.009, n(p) (2)=0.031 and P=0.040, n(p) (2)=0.021). Differences for those with stable, grade I spondylolisthesis were statistically significant and clinically relevant at 12 months in VAS, ODI, and EQ-5D-5L (P=0.006, P=0.040, P<0.001; Cohen's d =0.539, d =0.396, d =-0.689) and these comparative improvements remained significant for ODI and EQ-5D-5L at 24 months (P=0.03 and P=0.02; Cohen's d =0.420, d =-0.441). CONCLUSIONS: Results indicate that, in a population of cLBP patients, existing lumbar spinal degeneration did not influence the effect of the neurostimulation therapy on PROs. Thus, the presence of mild and moderate lumbar spinal degeneration does not appear to be a contraindication for implantable multifidus neurostimulation, including patients with grade I spondylolisthesis.