Investigation of physiological stress shielding within lumbar spinal tissue as a contributor to unilateral low back pain: A finite element study

INTRODUCTION The pathomechanism of low back pain (LBP) remains unknown. Unilateral LBP patients have demonstrated ipsilateral morphological and material property changes within the lumbar soft tissues, potentially leading to asymmetric tissue loading. Through the comparison of healthy and unilateral LBP validated finite element models (FEMs), this study investigates potential stress shielding consequential of spinal tissue property augmentation. METHODS Two FEMs of the musculoskeletal system - one demonstrating healthy and unilateral LBP conditions - were developed undergoing 30-degree flexion. FEMs included the vertebrae, intervertebral discs, and soft tissues from L1-S1. Material properties selected for the soft tissues were retrieved from published literature. To reflect unilateral LBP, the paraspinal morphology was atrophied, while the tissue moduli were increased. The symptomatic thoracolumbar fascia (TLF) was uniformly increased. Validation of the models preceded testing. RESULTS Model validation in spinal flexion was accomplished through comparison to literature. Compared to the healthy model, the unilateral LBP multifidus (MF), longissimus thoracis (LT), and TLF exhibited average tension changes of +7.9, -5.1, and +9.3%, respectively. Likewise, the symptomatic MF, LT, and TLF exhibited tension changes of +19.0, -10.4, and +16.1% respectively, whereas the asymptomatic MF, LT, and TLF exhibited -4.0, -2.0, and +0.4% changes in tension, respectively. CONCLUSION Relative to the healthy tissues, the symptomatic LBP soft tissues demonstrated a 19.5 kPa increase in stress, with 99.8% of this increase distributed towards the TLF, suggesting a load allocation bias within the symptomatic unilateral LBP tissues. Consequentially, symptomatic paraspinal muscles may be unable to withstand loading, leading to stress shielding.

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