P14. Analysis of physiological stress shielding within the lumbar musculoskeletal system as a consequence of mechanical adaptations associated with bilateral and unilateral low back pain: a finite element study

Abstract

<h3>BACKGROUND CONTEXT</h3> Low back pain (LBP) is a public health crisis with nearly 85% of cases considered idiopathic. With the emergence of research analyzing the role of the thoracolumbar fascia (TLF) in spinal biomechanics, it is possible that the fascia may play a role in the progression of LBP. Previous studies have demonstrated changes to the TLF thickness and shear strain, as well as stiffness and morphological changes within lumbar spine-stabilizing muscles (eg, erector spinae [ES] and multifidus [MF]) in LBP patients. Such modifications to lumbar soft tissues suggest the plausibility of stress shielding, defining a load allocation bias, within the lumbar spine's soft tissues. <h3>PURPOSE</h3> The aim of this study is to develop and compare finite element models representing healthy, along with both bilateral and unilateral LBP-affected lumbar musculoskeletal systems to determine the potential for stress shielding within the lumbar spine from soft tissue property augmentation associated with LBP. <h3>METHODS</h3> Using a finite element platform, three models depicting the lumbar musculoskeletal system undergoing trunk flexion were created and compared. The first model represented the tissues of a healthy lumbar spine. The remaining models depicted lumbar soft tissues reflective of bilateral LBP (bLBP) and unilateral LBP (uLBP) patients. Each model was composed of vertebrae, intervertebral discs (IVDs) and soft tissues (including the MF, ES, TLF and tendons) from L1-S1. Material properties were selected from published literature. For the bLBP model, tissues on either side of the vertebral column were modified to reflect the changes in morphology and stiffness in the MF, and TLF as documented in literature involving bLBP patients. For the uLBP model, the MF and TLF located laterally to the right of the vertebral column ("symptomatic tissues") were augmented to reflect changes found within clinical studies involving uLBP patients. Remaining uLBP tissues were unchanged. Relevant model context of use validation preceded testing. <h3>RESULTS</h3> Validation was achieved as results demonstrated agreement in IVD pressure and intervertebral rotation with clinical data and in silico spine models. Compared to the healthy model, the cumulative average tension exhibited by soft tissues in the bLBP and uLBP model increased by 15.6 and 9.24%, respectively. The average tension exhibited by the bLBP model's MF, ES, and TLF changed by 13.0, -4.03, and 15.6%, while the uLBP model's symptomatic MF, ES, and TLF exhibited changes of 19.0%, -10.4%, and 16.1%, respectively. <h3>CONCLUSIONS</h3> The bLBP and uLBP models exhibited an increase in soft tissue tension by 18.4kPa and 10.9kPa relative to the healthy model soft tissues. In each LBP model, 99.8% of the overall stress increase, relative to the healthy model, was distributed towards the TLF. This demonstrates potential for the TLF in both the bLBP and uLBP scenarios to withstand the majority of increased tension, indicating a stress allocation bias within the lumbar soft tissues. This may initiate stress shielding whereby the TLF prevents adjacent spine-stabilizing soft tissues, such as the MF and ES, from receiving loading. Tissues may thus undergo atrophy and become less capable to withstand loading. To compensate, patients may depend on additional soft tissues, leading to irregular muscle activation and force balances, trapping the soft tissues in cyclical stress shielding. Ultimately, stress shielding within lumbar soft tissues may potentially lead to the progression of LBP. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.