Introduction: Chronic low back pain (LBP) is a pervasive musculoskeletal disorder with a substantial impact on individuals and society. Despite advancements in diagnostic technologies, the etiology of LBP remains unknown in the majority of cases, hindering effective treatment and prevention strategies. Lumbar multifidus muscle (LMF) alterations have been implicated in LBP, but a comprehensive understanding of the structure-function relationship is lacking. This observational case-control study aimed to explore the bivariate and multivariate relations between LMF composition (e.g., fatty infiltration) and LMF function among individuals with and without LBP.
Methods: A total of 25 patients with chronic nonspecific LBP and an equal number of healthy controls, matched for age and sex were included. Inclusion criteria for LBP patients included 1) chronic nonspecific LBP, 2) moderate to severe disability on the Oswestry Disability Index, and 3) seeking current care for LBP. Participants underwent MRI assessment for LMF morphology and composition, utilizing IDEAL fat-water images. Ultrasound measures were used to evaluate LMF function, including shear-wave elastography (SWE) for stiffness/elasticity and thickness change during rest and submaximal contraction task. All measurements were acquired at the L4/L5 and L5/S1 spinal levels, bilaterally. Bivariate and multivariate relations between morphology and function were explored with correlational and linear regression analyses, respectively. To quantify the increase in shear elastic modulus resulting from contraction, the contraction ratio, as outlined by Botanlioglu et al. (2013), was computed for the LMF. This ratio was determined by dividing the shear modulus at rest by the mean shear modulus observed during contraction (absolute values). Age, sex, body max index (BMI), physical activity levels and group status were explored as possible covariates in the models.
Results: Fifty participants were included (26 female) with overall mean age of 39.22+ 11.67. Although BMI was negatively correlated with LMF function at both levels, age, sex, BMI, group status and physical activity level demonstrated positive significant relations with LMF fat infiltration and LMM function. Greater % LMF fat was associated with greater LMF SWE ratio at L4/L5 level at (P=0.002). There were no other significant bivariate or multivariate relations between LMF composition and LMF function.
Discussion: This comprehensive study addresses the structure-function relationship of lumbar LMF in chronic LBP, utilizing imaging techniques. The findings provide crucial insights into the prognostic value of imaging biomarkers, enhancing clinical assessment, and guiding targeted rehabilitation for individuals with LBP. Issues specific to LMF measurement and recommendations for future research are discussed.