Investigating mechanical and acoustic properties of the thoracolumbar fascia in non-specific low back pain

Background and Motivation: Nonspecific low back pain (NSLBP) represents a major global health burden, yet its underlying tissue-level mechanisms remain poorly understood and objective biomarkers are still lacking. Unlike other conditions, NSLBP lacks identifiable causes, complicating diagnosis and treatment evaluation. Traditionally considered inert, the thoracolumbar fascia (TLF) is now recognized for its rich innervation and potential involvement in proprioception and nociception, suggesting its contribution to NSLBP. Existing ultrasound (US) research on the TLF has provided insights into its thickness, echogenicity, and mobility; however, standardized quantitative approaches capable of jointly characterizing its mechanical behavior and microstructural properties in vivo remain limited. By shifting the analytical focus from displacement to shear strain (ShS), a more direct biomechanical descriptor of interlayer sliding, the present work aims to provide a more mechanistic characterization of fascial function. Furthermore, non‑invasive assessment of tissue‑level microstructural alterations, such as fibrosis or inflammation, remains challenging. Advanced US techniques may enable quantitative evaluation of TLF mechanical and microstructural properties as objective markers associated with NSLBP. Objectives: This thesis aimed to quantitatively characterize the mechanical and microstructural properties of the TLF in NSLBP using advanced US techniques, to evaluate their measurement properties including reliability, discrimination, and sensitivity to change, and to investigate their sensitivity to change following therapeutic interventions. Methods and Results: Three complementary studies were conducted. In the first study, TLF ShS was quantified using the Lagrangian speckle model estimator (LSME) in patients with NSLBP and asymptomatic controls, and its association with pain and disability was examined. Contrary to prior findings focused on displacement, NSLBP patients exhibited significantly greater TLF ShS than asymptomatic controls, and these parameters were positively associated with self‑reported pain and disability. A brief standardized massage did not induce immediate changes in TLF ShS. ShS by LSME showed high intra‑examiner reliability. The second study employed quantitative ultrasound (QUS) with Homodyned K‑distribution (HKD) modeling to assess TLF microstructure. NSLBP patients showed significantly lower values of the reciprocal scatterer‑clustering parameter (1/α) and the diffuse‑to‑total signal power ratio (1/(κ+1)) than controls, indicating microstructural differences; μn did not differ. QUS HKD parameters demonstrated moderate-to-good reliability. The third study was a randomized controlled trial (n = 60) of massage, acupuncture, and chiropractic therapies in a three-arm parallel-group design, followed by a within-subject massage phase in the former waitlist group. Changes in TLF mechanical and microstructural parameters and in clinical outcomes were analyzed using linear mixed-effects models. The trial found that massage significantly reduced both cumulative and maximal TLF ShS compared with baseline within-group, whereas chiropractic therapy significantly reduced cumulative ShS only; acupuncture did not produce significant changes. None of the interventions induced significant between-group or within-group changes in QUS HKD microstructural parameters. Clinically, chiropractic therapy was associated with a statistically significant improvement in disability. Conclusion: This research demonstrates that NSLBP patients exhibit altered TLF mechanical and microstructural characteristics detectable by advanced ultrasound techniques. While massage and chiropractic interventions can modulate TLF mechanical characteristics, microstructural changes were not observed within the study period. Taken together, these results establish a quantitative framework for assessing fascial properties in vivo and support the use of ultrasound-based TLF measures as objective markers associated with NSLBP.