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Fast-running surrogate computational models (simpler computational models) have been successfully used to replace time-intensive finite element models. However, it is unclear how well they perform in accurately and efficiently replicating complex, full human body finite element models. Here we survey several surrogate modeling techniques and assess their accuracy in predicting full strain fields of tissues of interest during a highly dynamic behind armor blunt trauma impact to the liver. We found that coupling dimensionality reduction on the high-dimensional output space (principal component analysis or autoencoders) with machine learning techniques (Gaussian Process Regression or multi-output neural networks) provides a framework capable of accurately and efficiently replacing complex full human body models. It was found that these surrogate models can successfully predict the strain fields (<10% average strain error) of select tissues during a nonlinear impact event but careful consideration should be given to element parsing and modeling technique. [ABSTRACT FROM AUTHOR]

INTRODUCTION: Fighter pilots and military helicopter aircrew experience chronic neck and back pain at rates significantly higher than civilians. Their unique loading conditions, including high-G exposures and whole-body vibration, likely contribute to accelerated degenerative changes that can alter the response of the spine to external loads and contribute to spine-mediated pain. This study aimed to characterize mechanical changes to the annulus fibrosus (AF) associated with repeated loading modeling in the military aviation environment. METHODS: Fresh porcine AF specimens were used in this study which have been previously identified as representative of human tissue. Test specimens were harvested from the anterior superficial region of the AF and exercised in tension using a protocol that quantified pre- and post-damage stiffness and viscoelastic (i.e., time/rate dependent) properties. Tissue damage was induced using repeated subfailure tension loading with one of nine pre-defined groups with a combination of different strain magnitudes (11%,28%,44%) and cycle counts (400,1600,6400). Damage loading time ranged from 1 to 81 minutes. Following damage and post-damage material characterization, specimens were quasi-statically pulled to failure. RESULTS: Stiffness and viscoelasticity of AF tissues decreased significantly after damage loading for all damage groups and changes were correlated with both the magnitude of applied loading and the total number of cycles. Despite significant dose-dependent changes to the viscoelastic and elastic properties, the ultimate properties during quasistatic distraction to failure did not change. This indicates the mechanical response during low-strain magnitudes is significantly affected, but not the high-strain mechanical response. DISCUSSION: This study quantified changes in AF mechanics caused by tensile damage loading and found decreases in viscoelastic and elastic properties post-damage. These changes reduce the ability of the intervertebral disc to absorb energy associated with external loads and can contribute to disc laxity, which can lead to spine segmental instability. These changes can increase the loading of the surrounding apophyseal joints and may contribute to discogenic and radicular pain symptoms. These findings may partially explain a mechanism of acute flight-related pain symptoms in military aircrew, highlighting a need to reduce the magnitude of cyclic loads an AF is exposed to during flight. Learning Objectives 1. The audience will learn how the mechanical response of the annulus fibrosus changes in response to damage caused by cyclic loading. 2. The audience will be able to understand why changes in the mechanical response of the annulus fibrosus may contribute to discogenic and radicular pain symptoms. [ABSTRACT FROM AUTHOR]

At its most basic level, a polymer is a molecule consisting of a long, repeating chain of smaller monomers typically four to ten atoms in size. Polymers can be straight [...]

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