36. Assessment of static spine stability and relative implications of trunk muscles, thoracolumbar fascia and intra-abdominal pressure

Abstract

<h3>BACKGROUND CONTEXT</h3> The spine is an inherently unstable structure due to the numerous degrees of freedom it possesses. Yet, its static spinal stability is believed to be maintained by the actions of muscle activations combined with their intramuscular pressure (IMP), intra-abdominal pressure (IAP) generated by the abdominal muscles, and the passive engagement of the thoracolumbar fascia (TLF). Nonetheless, previous studies provide little information about the relative contribution of these tissues. <h3>PURPOSE</h3> The purpose of this research was to leverage a novel, validated and fully representative finite elements (FE) model of the spine to explore the relative actions of spinal muscles, TLF, IMP and IAP, as well as their collective efforts to achieve equilibrium static spinal stability. <h3>STUDY DESIGN/SETTING</h3> A novel, validated and comprehensive FE spine model was previously developed by the authors, which was applied to the current study. The model put forth includes accurate continuum models of all thoracic and lumbar vertebral bodies (VBs), intervertebral discs (IVDs), major spine muscles inclusive of their IMP (modelled as fluid-structure continuum), TLF tissue and a model of the IAP (modeled strategy like IMP). <h3>METHODS</h3> Equilibrium static stability was assessed as the ability of select tissues to restore the spine's initial position after an external perturbation is applied. For this reason, a previously validated spinal perturbation of 350N flexion force was applied at the first thoracic vertebra. Case studies including each tissue on its own, as well as combinations, were investigated. For each scenario, in-planar lumbar VBs final displacements, after tissue activations and applying the perturbation, were recorded in efforts of quantifying equilibrium static stability. Muscle activation was based on validated EMG muscle forces while IAP was introduced as an increasing pressure of 67 mmHg (9 kPa) maximum value in an abdominal cavity. <h3>RESULTS</h3> With the upright spine position being the initial stable position, the application of the 350N perturbation on the model inclusive of only the VBs and IVDs was identified as the most unstable position. Thereafter, activating the IAP, muscles, and TLF contributed individually to 24%, 53.8%, and 77% equilibrium static stabilities, respectively. Furthermore, combined tissues stability results fell between 60% and 93%, with 93% being the result of all tissues activated together to their maximum physiological capability. Another worthwhile finding was the approximately 46% drop in IMP developed by spinal muscles on the passive engagement of the TLF, suggesting its role in stabilizing and preventing excessive muscles pressurization. <h3>CONCLUSIONS</h3> This research concluded that the included muscles, combined with their IMP, IAP and TLF, are major spine stabilizers, contributing to 93% of the spine's equilibrium static stability, as defined in this paper. The TLF in particular, one of the strongest passive tissues in the back, showed to be capable of storing excessive loads, potentially as a protective measure. This was supported by the observed resultant forces at the TLF-VBs connection points, which summed up to around 244N acting opposite to applied perturbation. Lastly, results of this study may assist to better understand the notion of equilibrium static spinal stability as literature and previous studies are still pressing for more representative spine models able to assess other modes of stability. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.